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- 1. See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/327876409 Raisin Processing: Physicochemical, Nutritional and Microbiological Quality Characteristics as Affected by Drying Process Article in Food Reviews International · September 2018 DOI: 10.1080/87559129.2018.1517264 CITATIONS 0 READS 1,511 3 authors, including: Some of the authors of this publication are also working on these related projects: Changes in the physicomechanical characteristics of a ceramic paste during drying View project Daoued Mihoubi Research and Technology Centre of Energy 44 PUBLICATIONS 535 CITATIONS SEE PROFILE All content following this page was uploaded by Ramla Khiari on 09 October 2018. The user has requested enhancement of the downloaded file.
- 2. Full Terms & Conditions of access and use can be found at http://www.tandfonline.com/action/journalInformation?journalCode=lfri20 Food Reviews International ISSN: 8755-9129 (Print) 1525-6103 (Online) Journal homepage: http://www.tandfonline.com/loi/lfri20 Raisin Processing: Physicochemical, Nutritional and Microbiological Quality Characteristics as Affected by Drying Process Ramla Khiari, Hassène Zemni & Daoued Mihoubi To cite this article: Ramla Khiari, Hassène Zemni & Daoued Mihoubi (2018): Raisin Processing: Physicochemical, Nutritional and Microbiological Quality Characteristics as Affected by Drying Process, Food Reviews International, DOI: 10.1080/87559129.2018.1517264 To link to this article: https://doi.org/10.1080/87559129.2018.1517264 Published online: 25 Sep 2018. Submit your article to this journal View Crossmark data
- 3. Raisin Processing: Physicochemical, Nutritional and Microbiological Quality Characteristics as Affected by Drying Process Ramla Khiaria,b,c , Hassène Zemnic , and Daoued Mihoubib a Higher School of Food Industries of Tunis (ESIAT) - 58 Avenue Alain Savary, 1003 Tunis El Khadra, University of Carthage, Tunisia; b Laboratory of Wind Energy Management and Waste Energy Recovery, Research and Technology Center of Energy (CRTEn) - B.P. N°95, Hammam-Lif, Tunisia; c Laboratory of Molecular Physiology of Plants, Center of Biotechnology of Borj-Cedria (CBBC) - B.P. 901, Hammam-Lif, Tunisia ABSTRACT Processing and conservation of grapes by suitable techniques has been a major challenging issue for a long time. Optimization of drying and pretreatment operations of this fruit have been exten- sively studied. However, in order to achieve the production of high- quality raisins and reach consumers’ acceptance, special attention for quality attributes should be taken into account. Quality characteris- tics of grapes such as color, texture, vitamins, phytochemicals, aroma profile and microbial stability are of paramount importance since they could vary throughout the dehydration procedure and would directly determine quality perception and consumer choice. This paper presents a comprehensive review of the physicochemical, nutritional and microbiological characteristics of dried grapes as affected by the drying process. In addition, it investigates the changes of different grapes quality attributes (mainly nutritional and aromatic proprieties) during processing, which enables profes- sionals and scientists to better choose and optimize grape processing to deliver the highest raisin quality to consumers. KEYWORDS Drying process; grapes; quality; raisins Introduction Grapes are one of the main prevalent agricultural crops; they have been cultivated since prehistoric times. The global grape production currently amounts to more than 75.8 million tons (Mt) according to Food and Agriculture Organization[1] and International Organization of Vine and Wine (OIV)[2] data for 2016. The world’s five largest grape producers are: China (about 14.5 Mt), Italy (about 7.9 Mt), United States of America (about 7.1 Mt), France (about 6.4 Mt) and Spain (about 6.0 Mt).[1] Around 71% of this production is destined for wine making, while the remainder is consumed fresh as table grapes and juice or dried as raisins.[3] Due to their high moisture and sugar contents, grapes are very perishable and even stored under best refrigerated conditions, they still remaining highly susceptible to con- tamination with spoilage and pathogenic microorganisms. That is why these fruits should be consumed or converted to other derived products within few weeks after harvest CONTACT Daoued Mihoubi daoued.mihoubi@crten.rnrt.tn Laboratory of Wind Energy Management and Waste Energy Recovery, Research and Technology Center of Energy (CRTEn) - B.P. N°95, Hammam -Lif 2050, Tunisia Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/lfri. FOOD REVIEWS INTERNATIONAL https://doi.org/10.1080/87559129.2018.1517264 © 2018 Taylor & Francis
- 4. otherwise, their marketability would be jeopardized, which could result in conspicuous economic losses.[4] The improvement of grape management, processing and marketing is required which, in turn, emphasizes the need for the adoption of more effective preserving approaches. In this respect, drying, as the oldest food-preservation method, should be one of these techniques that may fulfill this need by enhancing derived grapes product quality, widening their availability and diversifying their trade.[3] Drying basically removes the excess of water until an appropriate moisture level is reached that inhibits the growth of bacteria, molds, and yeasts, slows down the enzyme degradation and inactivates the majority of the physical and biochemical reactions.[5] It is believed that the first raisins were produced in the near east by simply burying the grapes in the sand. The aim of grape drying was to extend their shelf life and, due to their high sugar content, to provide excellent sources of energy for workers executing hard tasks.[6] Preserving grapes in the form of raisins has other advantages including reduction of weight and bulk, which contribute to the lessening of packing, storage and transportation costs, as well.[7] World production of dried grapes (Raisins, Sultanas and Currants) reached 1.24 million metric tons during the season 2016–2017. Turkey was the major producer, accounting for 310,000 tons (25%), followed by the United States with 297,738 tons (24%), China with 185,000 tons (15%) and Iran with 170,000 tons (14%). The four countries together account for 78% of the world production, according to the latest report of the United States Department of Agriculture (USDA).[8] Commonly, dried grapes are used as an ingredient in baking, snacks, breakfast cereals and confectionery industry.[8] In 2016, about 1.2 million tons of raisins were consumed in the world, an increase of 17% from 2000. With more than 250,000 tons consumed, the United States and Turkey are the leading domestic markets, accounting for one quarter of global consumption. China, with 203,100 tons of dried grape consumed in 2016, was in third place.[1] World raisin consumption is in fact steadily increasing due to their nutri- tional quality well recognized by consumers.[8] Many reviews concerning grapes phytochemical characteristics and their health benefits have been published.[9–12] Grape drying has been well studied and the literature is rife with very interesting examples, but we noticed the lack of a synthesis review dealing with grape dehydration and its effect on quality attributes. However, when compared to other food materials, grape processing is a more complicated bioprocess because the grape berries should undergo a pretreatment prior to dehydration. Predrying treatments help to soften grapes’ waxy skin, which prevents the moisture diffusion and restrains the procedure of drying.[13] Chemical and physical pretreatments have been extensively proven to accelerate the grape drying process and improve the quality of the final dried samples.[14,15] Chemical predrying treatments consist of putting grape berries in some chemical preparation, which enables dissolution of the wax layer and affects the drying kinetics in addition to procuring some antimicrobial properties.[16–18] According to Deng et al.[19] , chemical pretreatments could be categorized into two types: chemical solution (hyperosmotic, alkali, sulfite and acid, etc.) and gas (sulfur dioxide, carbon dioxide and ozone) treatments. While physical pretreatment entails the use of thermal or nonthermal methods including blanching (hot water, steam, superheated steam impingement, ohmic and microwave heating, etc.), superficial abrasion, ultrasound, pulsed electric fields and 2 R. KHIARI ET AL.
- 5. high hydrostatic pressure, which often targets the waxy fruit skin forming small crevices to speed the drying process.[19–21] Several drying operations have also been widely studied. Sun and solar drying are the two methods traditionally used for drying of commercial raisins. However, these processes are very slow and depend mainly on weather conditions, which can induce microbial and insect contamination of the resulting dried fruits and hence, lower their quality.[22] More recently, advanced drying techniques such as oven drying, microwave drying, vacuum pulsed drying, infrared drying and many others have been employed in order to enhance the dehydration rate and guarantee a better quality of the raisins.[7] Nevertheless, in order to achieve the production of high-quality raisins and reach consumers’ acceptance, special attention of all processing phases should be taken into account. The variation of the quality characteristics during processing should be explored.[23] Additionally, it is widely known that drying may affect the quality attributes of the raw materials either positively or negatively, and at the same time, partially or totally.[5] Therefore, a better understanding of grape-drying operations and their impact on these quality properties may help to bring new insights to deliver raisins of the highest quality to consumers and satisfy their demand for this commodity. The aim of this review is to present an overview on the effects of drying methods on the quality characteristics of grapes emphasizing the physicochemical, nutritional and micro- biological aspects. Processing of grapes into raisins At the ripening stage, grapes are harvested and transported to consumption or processing. Ripening is characterized by the formation of a waxy layer on berry skin, softening and development of the specific grape variety color as well as changing of taste from sour to sweetness.[24] In the case of drying, the berries are first cleaned, washed, selected and sorted, and then subjected or not to a pretreatment that accelerates the dehydration process. After drying, the resulting raisins are sorted once again before being packaged and marked (Figure 1). With a waxy cuticle, grape berries are normally protected from biotic and abiotic stresses. Indeed this skin layer plays an important role in controlling the drying process because the waxes laid on the skin surface are hydrophobic and serve as impermeable barriers to moisture movement through the cuticle.[25] With the intention to decrease skin resistance, facilitate water evaporation and hence ameliorate drying kinetics, the use of pretreatments prior to grape dehydration is highly recommended.[26] Pretreatments of grapes As far as is currently known, there are two types of pretreatments that have been frequently applied in raisins processing: chemical and physical treatments.[26] Both of these pretreatments have been the subject of numerous investigations and have as main objectives the modification of the grape skin permeability, enhancement of moisture rates, swiftness of the drying process as well as improvement of raisins’ quality.[13] Some examples of grape pretreatments described in the literature and their relevant findings are summarized in Table 1. FOOD REVIEWS INTERNATIONAL 3
- 6. Chemical pretreatments Chemical pretreatments involve treatment of grapes with sulfur dioxide (SO2) or dipping berries into certain chemical solutions. These include sodium carbonate (Na2CO3), potas- sium carbonate (K2CO3), sodium hydroxide (NaOH) and oil emulsion (e.g. ethyl oleate and olive oil) at different concentrations and times.[15,42] It is worth noting that in the grape industry, the sulfiting technique (i.e. gas SO2 fumigation or immersion in SO2 solutions) was by far the commonly-used method for bleaching grapes during storage and drying since sulfur dioxide treatment was known to ameliorate food color development and conserve their quality.[42] However, excessive SO2 usage may alter the quality of the processed raisins and could cause potential environ- mental problems such as air pollution[42,43] or certain health concerns about reactions associated with some types of asthma.[44] In order to alleviate these incidents, many authors have assessed other alternative chemical pretreatments. Figure 1. Diagram of grapes drying. 4 R. KHIARI ET AL.
- 15. The effect of different pretreatments (C1: Dipping in 0.5% sodium hydroxide (NaOH) solution; C2: Dipping in 2% ethyl oleate (EO) + 0.5% NaOH solution; C3: Dipping in 3% EO + 0.5% NaOH solution and C4: Dipping in 3% EO + 2.5% potassium carbonate (K2CO3) solution) has been studied by Tulasidas et al.[28] on drying rates, color and appearance attributes of Thompson seedless grapes. Their results showed that C2 and C3 pretreatments exhibited shorter drying times and resulted in good quality raisins com- pared to C1 and C4 predrying treatments. C1 pretreatment was judged to generate raisins of inferior quality in terms of color and appearance (dull and pale raisins). This could be attributed to the specific effects of each treatment.[45] Dipping in NaOH solution causes solubilization of the waxy surface and physical damage of the skin, hence accelerating only the first stage of drying. However, dipping in EO alkali solution results in both decreasing the resistance of skin tissues and increasing internal diffusion, thus allowing moisture to more readily evaporate from grapes in addition to reducing browning and other degra- dative reactions affecting quality attributes. A survey was conducted by Doymaz[32] in which the influence of various dipping pretreatments was tested including treatment of black grapes with EO or olive oil plus potassium carbonate (K2CO3), potassium hydroxide[46] and sodium carbonate (Na2CO3) solutions on drying time and drying kinetics of raisins. EO plus K2CO3 pretreatment enhanced drying rate to a greater extent and displayed shorter drying times than the other pretreatments (K2CO3 plus olive oil, ethyl oleate plus KOH, ethyl oleate plus Na2CO3 and untreated samples). The use of alkaline emulsion of EO, commercial emulsion (alkaline emulsion of ‘‘PAKSAN’’ oil) and hot water (HW) has been examined by Esmaiili et al.[18] as pretreat- ments before drying seedless grapes. They found that the three treatments strongly affected raisin moisture diffusivities (average effective moisture diffusivities ranged from 3.34 to 8.46 × 10−10 m2 s−1 at 50°C). On the other hand, mass transfer coefficients at a given moisture content and different temperatures for the EO pretreated raisins were revealed to be two times greater than HW-pretreated ones during drying. With reference to the study of Doymaz and Altıner[33] , the effect of POTAS solution (composed of 4% potassium carbonate + 1% olive oil) pretreatment on drying and color characteristics of Sultana seedless grapes was investigated. The authors indicated that when applying POTAS pretreatment, a reduction of the resistance to the moisture move- ment associated with an increase of the drying rate were noted. Regarding color attributes, POTAS solution was found to influence significantly the color characteristics of seedless grapes by producing lighter raisins in comparison with untreated samples. This might be ascribed to the action of potassium carbonate by saponification of fatty acids such as oleic, stearic and oleanolic acids, that are known as constituents of grape wax.[47] These fatty acids and their esters, characterized by the presence of lipophilic and hydrophilic groups on the same molecule, were suggested to yield light-colored raisins.[48] The effect of dipping Thompson seedless grapes in potassium carbonate and EO solutions for different durations (1, 2 and 3 min) at several temperatures (30, 40, 50 and 60°C) on drying rate and color kinetics was studied by Bingol et al.[34] Grapes dipped into the solution at 60°C for 2 and 3 min had the fastest drying rate. Regardless of the dipping time and temperature applied, all raisins had varying degrees of brown coloring. As Grncarevic and Hawker[49] suggested, this could be explained by the loss of the grape cell integrity, which was dependent on pretreatment conditions. Cell integrity was found FOOD REVIEWS INTERNATIONAL 13
- 16. to be maintained for about 50% of weight loss and up to this time browning could be initiated. Regarding the work of Mandal and Thakur[36] , various pretreatments (P1: soda dip, P2: golden bleach and P3: sugar syruping method) for the processing of raisins from two grape varieties (Thompson Seedless and Perlette) have been tested. In general, the effective pretreatment was P3, which resulted in raisins, obtained from the Perlette variety, with higher yield (283.51 g. kg−1 ), titrable acidity (0.60%) and organoleptic proprieties (attrac- tive and golden-yellowish raisins according to sensory analysis). However, it required more time than the two other predrying treatments, which could explain the increase in the acidity levels. On the other hand, rising sugar concentrations due to sugar syruping pretreatment could result in the inhibition of polyphenol oxidase and thereby reduction in browning as reported by Grncarevic and Hawker[49] and Radler.[48] Physical pretreatments Physical pretreatment is the second predrying method that has been successfully used to ameliorate grape drying. It encompasses the employment of thermal or nonthermal techniques such as blanching, superficial abrasion, microwave or ohmic heating, pulsed electric fields, and carbonic maceration.[7,19,50] The use of a physical pretreatment (superficial abrasion of the grape peel) has been proposed by Di Matteo et al.[16] instead of a chemical one (traditional EO dipping) for improving the drying rate of seedless grapes. The results of their study showed that both pretreating methods manifested a quite similar drying behavior. The mass transport coeffi- cient for physically pretreated grapes was 4 times superior than that estimated for untreated samples (drying time about 35 h). However, physical predrying method gave darker raisins than chemical processed ones. In fact, while chemical pretreatment inhibited the activity of the polyphenol oxidase (PPO), the physical pretreatment allowed its activation because it is mainly located in the peel. In the presence of oxygen, PPO catalysis leads to the formation of brown pigments and thus, enhances the degree of grape browning. Similarly, Senadeera et al.[37] and Adileltta et al.[15] conducted experiments on the effect of abrasive pretreatment on the drying rate and the quality of dried grapes. Better drying characteristics and quality were recorded in raisins pretreated by peel abrasion in com- parison to those pretreated chemically (dipping in EO solution) or untreated ones. The use of ohmic pretreatment before drying seedless red grapes has been assessed in the study of Salengke and Sastry.[38] Ohmic heating affected the drying process by increasing the dehydration rate, which was largely attributed to breaking action on the skin of the treated samples. In turn, this affected their permeability and increased the moisture diffusion rate. In addition, a shift in the sorption isotherm of the raisins produced was observed, which was possibly attributed to a leaching of a small amount of solute sugar from the grape berries during ohmic pretreatment, thus reducing the amount of water required for sugar dissolution during adsorption at high water activities. Since the shift was to the right, this indicated that the equilibrium moisture contents of the treated samples were less than that of the untreated samples at the same water activity levels. With reference to the work of Kostaropoulos and Saravacos,[39] the feasibility of microwave heating as a predrying technique of Sultana seedless grapes has been evaluated. Their results revealed that microwave pretreated grapes dried nearly two times faster than untreated ones. This might be assigned to the absorption of microwave energy by water 14 R. KHIARI ET AL.
- 17. molecules in the interior of the berries, which resulted in rapid evaporation, causing partial puffing. Thus, the moisture diffusivity during drying may increase considerably, due to increased porosity of the treated grapes. The color and appearance of the pretreated grapes were similar to commercial raisins presumably due to a partial inactivation of browning enzymes by the absorption of microwave energy. The effects of microwave, pulsed electric fields (PEF) and conventional chemical pretreatments has been investigated by Dev et al.[40] on grape drying rate and quality. Drying rate increased significantly due to PEF and microwave pretreatments to the same extent, but the highest dehydration rate was registered in chemically pretreated grapes. This could be ascribed to the different effects of each pretreatment: while chemical treatments enable the breakdown of the waxy skin, physical pretreatments act by forming small cracks in the grape peel so that the drying rate will be accelerated. On the other hand, the appearance and market quality of PEF and microwave pretreated samples were better than the chemical treated and the untreated samples because of their soft texture and shiny eye appeal, which may be due to increased porosity. A new predrying method, carbonic maceration (CM), has been recently investigated for raisin making.[41] CM involves grapes setting in a closed tank with a carbon dioxide-rich atmosphere, which allows an intracellular fermentation and a CO2 impregnation in the fruits and vegetables under the rich CO2 anaerobic conditions. In this manner, the plant tissues loosen while keeping the fruit intact, thus enhancing the drying rate.[50] The effects of CM, dipping in alkaline emulsion of EO solution (AEEO) and dipping in AEEO then freezing at −18°C for 12 h (AEEO + Freezing) on infrared drying kinetics of red grapes were studied.[41] CM-treated raisins had the shortest production time, the highest total phenol content, the best oxidation resistance ability and the best rehydration ratio. In fact, CM could decrease the pH of the cytoplasm, decompose the cell structure, and increase the cell wall and membrane permeability so that high polymers are broken down into smaller ones releasing water, and thus, improving the drying kinetics as well as the quality of the dried grapes.[50] Water blanching has been also used to soften the grape waxy skin and to increase mass transfer during dehydration. This pretreating method is traditionally the most adopted technique in the food industry because it is a simple, easy and cost-effective technology.[51] The effects of high-humidity hot air impingement blanching (HHAIB) on drying kinetics and color of seedless grapes at different temperatures (90, 100, 110 and 120°C) and times (30, 60, 90 and 120 s) have been assessed.[21] HHAIB not only reduced the drying time but also effectively inhibited enzymatic browning as the obtained raisins had desirable green- yellow or green color. During blanching, the grape skin will generate microcrevices when the pressure inside the grapes is greater than the ambient pressure. These crevices could greatly reduce resistance to water diffusion from the peel to the drying air. In addition, blanching could expel the intercellular air entrapped inside the sample tissues and reduce the resistance of cell membranes and cell walls to water diffusion by structure softening. As regards browning prevention, HHAIB acts by decreasing the PPO residual activity. Drying methods Numerous drying methods have been employed for raisin making with the main objectives to prolong their shelf life, to produce high-quality dried grapes as well as to diminish postharvest losses. Currently, the most frequently used grape drying FOOD REVIEWS INTERNATIONAL 15
- 18. techniques include sun and solar dehydration, oven and microwave drying, vacuum drying and infrared drying. Some examples of the most assayed techniques for grape drying are outlined in Table 2. Drying kinetics The best way to characterize the behavior of a food product during drying is to evaluate experimentally its drying kinetics. The drying kinetics are also used as a tool to select for the suitable drying techniques and for controlling, optimizing and engineering the food dehydration process.[63] The drying characteristic expresses generally the moisture ratio as a function of time, which can be illustrated by either moisture content of grapes ‘M’ versus time ‘t’, drying rate ‘dM/dt’ versus time ‘t’ or drying rate ‘dM/dt’ versus moisture content ‘M’.[64] While the drying curve (plot of ‘M’ versus time ‘t’) outlines the dehydration kinetics as a function of time, the drying rate curve (plot of ‘dM/dt’ versus time ‘t’) describes the rate of moisture content change over the time. Both of these representations are used to get an overview of moisture content changes during drying. On the other hand, when plotting drying rate ‘dM/dt’ versus moisture content ‘M’ at different times, a curve illustrating the drying cycle is obtained. The typical drying cycle consists of three stages: heating the food to drying temperature, evaporation of moisture from product surface (constant rate period) and falling of drying rate (falling rate period).[65] As regards grape drying, the constant-rate drying period was generally absent in all drying methods. The dehydration took place principally during the falling drying rate period where internal diffusion mechanisms dominate.[15,37,52] In order to describe the kinetics of the drying process, many thin-layer drying models have been developed. These models can be classified into three categories: theoretical, semitheoretical and empirical.[54] Mathematical modeling of drying agricultural products is important in understanding the heat and mass transfer phenomena involved in the production and processing of dried foods. Mathematical modeling and numerical simula- tion not only reduce the need for expensive and repetitive experimentation, but also help to design new and improve existing commercial drying processes. They are used to estimate drying time of several products and also to generalize drying curves.[66] As pertains grape drying, many authors have developed mathematical models based on the diffusion theory to predict the drying kinetics of moist substances in thin layers, as shown in Table 3. The constants in the fitted models have been found to be functions of air temperature, air velocity, heat of sorption and so forth. Model fit is generally evaluated based on statistical parameters including the correlation coefficient (R2 ), the sum squared errors (SSE) and the root mean square error (RMSE). The higher R2 and the lower SSE and RMSE values represent the goodness of model fitting and provide a better prediction of the drying characteristics of grapes.[64] Traditional drying methods Historically, sun drying grapes into raisins dated back to 1490 B.C.[78] To date, such process is by far the most traditionally used method for preserving food and agricultural crops, especially in the developing countries owing to its low cost and ease of handling.[60] Natural open sun drying consists of exposing grape bunches directly to sunlight (with or without cover) until thorough dehydration.[79] It is an easy-to-use and practical drying 16 R. KHIARI ET AL.
- 23. method that does not require high capital cost. Nevertheless, despite its simplicity, feasibility and cost effectiveness, sun drying has certain drawbacks including long duration of biopro- cessing, reliance on weather conditions, and risk of mold and insect contamination.[22] Therefore, solar dehydration, which involves the harnessing of solar radiation as source of energy, has been developed as an alternative technique to sun drying. Basically, there are three types of solar dryers: direct type, indirect type and mixed type.[78] In the direct type of solar dryer, grape berries are subjected to solar radiation that would be converted through a transparent material into heat and then directly absorbed by the fruits. In the indirect mode of solar dryer, the sun’s radiation is collected in a solar collector where the air is heated then passed into the drying cabinet to dehydrate the grapes.[79] The mixed mode of solar dryer combines the use of both the direct and the indirect types. It implies the fruit materials dehydration by two ways: through the direct absorption of incident solar rays by the grapes in addition to the preheated air flow emanating from the solar air collector.[80] Each of the aforementioned procedures has been extensively investigated in many studies for grape drying. The study of Mahmutoğlu et al.[29] was conducted to evaluate the effect of two drying methods: sun (on concrete ground, on polypropylene canvas sheets and on wooden racks) versus solar drying and different pretreatments on the quality of Sultana grapes (Table 2). Drying rates were classified for the tested dehydration techniques: solar dying > sun dying on concrete ground > sun dying on wooden racks, or on polypropylene canvas sheets. The color of the pretreated dried grapes was lighter than that of the untreated ones. Drying on polypropylene canvas sheets or on wooden racks or by solar drying gave a relatively lighter product compared to drying on concrete ground due to reduced drying times. In addition, the overheating of the concrete results in the darkening of the bottom layers of the grapes. The use of sun and solar drying methods to dehydrate Sultana seedless and black seedless grapes (currents) was also examined by Karathanos.[52] This study showed that solar drying obviously improved the drying rates of both varieties and exhibited shorter drying times than sun dehydration. Sun drying of Sultana raisins took 31 days (without pretreatment) and 7.5 days (when they were pretreated with an emulsion of salt and olive Table 3. Mathematical thin-layer models used in fitting grapes drying kinetics. Model name Model equation Reference Lewis MR = e(-k.t) [67] Handerson and Pabis MR = a e(-k.t) [68] Modified Handerson and Pabis MR = a e(-k.t) + b e(-g.t) + c e(-h.t) [69,70] Logaritmic MR = a e(-k.t) + c e(-k.t) [15,71,72] Two term MR = a e(-k0.t) + b e(-k1.t) [15,73] Two term exponential MR = a e(-k.t) +(1-a) e(-k.a.t) [15,74] Verma et al. MR = a e(-k.t) +(1-a) e(-g.t) [75] Approximation of diffusion MR = a e(-k.t) +(1-a) e(-k.b.t) [73] Page MR ¼eðÀk:tn Þ [15,30,32] Modified Page MR ¼eðÀk:tÞ n [54] Midilli et al. MR ¼eðÀk:tn Þ þ b:t [34,54] Parabolic MR = a + b.t + c.t2 [33] Wang and Singh MR = 1 + a.t + b.t2 [76] Guggenhein, Anderson and De Boer (GAB) MR ¼ MeqCkaw ð½ð1ÀkawÞð1ÀkawþCkawÞŠÞ [77] Brunauer, Emmett and Teller (BET) MR ¼ ABaw ð½ð1ÀawÞð1þðAÀ1ÞawÞŠÞ [77] t: Drying time (s); MR: Moisture Ratio; a, b, c, g, h; n: dimensionless constant for drying; k, k1, k0: Drying velocity constant; aw: water activity; A, B, C: Estimated parameters of the models applied to experimental sorption data. FOOD REVIEWS INTERNATIONAL 21
- 24. oil), whereas solar drying lasted only 2 days. The drying of currants (characterized as natural currants with no addition of any emulsifier), on the other hand, required much shorter times (10 days and 1 to 2 days for sun and solar drying, respectively). The discrepancy in the drying behavior between the two grape varieties may be due to the thin skin of currants compared to the thicker one of Sultanas as well as the application of the pretreatment, which would speed the process of drying. With regard to the study of El-Sebaii et al.,[55] the usage of an indirect type natural convection solar dryer for drying some vegetables and fruits (including seedless grapes) has been investigated. The required time to achieve the equilibrium moisture content for seedless grapes was 60 h and 72 h when the system was used with and without a storage material (sand), respectively. The combination of a chemical pretreatment (dipping samples into boiling water containing 0.4% olive oil and 0.3% NaOH for 60 s) drastically reduced the drying time to 8 h for the system with the storage material. The authors concluded also that the designed dryer was capable of dehydrating 10 kg of chemically pretreated grapes during 20 h of sunshine. Fadhel et al.[53] analyzed the drying of the Sultanine grape variety by three different solar processes: natural convection solar dryer, solar tunnel greenhouse drying and open sun drying. Grapes in the solar dryer were dried within about 4 days, those dehydrated in the greenhouse required around 5 days, while with open sun drying the fruits took more than 10 days to dry. The solar tunnel greenhouse drying was satisfactory and competitive to the natural convection solar drying process (in terms of energy efficiency and opera- tional cost); besides, it produced raisins with the best quality due to the increase of their drying rate as well as their protection from pests, rain and dust. The work of Rathore and Panwar[56] suggested the development of a natural convection walk-in type solar tunnel dryer for the dehydration of Thompson seedless grapes. They proposed the use of the designed drying system not only for grape but also for other agricultural and horticulture product drying. The solar tunnel was adequate to transform the Thompson grapes into raisins within almost 7 days and the best moisture removal rate was recorded at 65°C temperature. The effect of two drying methods (polythene tunnel type rack systems solar dryer and in direct sunlight drying) as well as four different dipping alkali solution (1%, 2%, 3% and 5% of sodium hydroxide + 1% olive oil) on V. vinifera cv. “Sultanina” processing have been studied.[57] Tunnel drying was found to be satisfactory and competitive to traditional drying method (direct sun drying) since grapes in the solar drying tunnel dried faster and had better color quality than samples dehydrated with direct sunlight. Advanced and combined drying methods With the progress of science and technology, novel drying techniques have been used for raisins production and are considered as key ways in the concept of sustainable engineer- ing processes. The improvement of these methods contributes to lift some of the conven- tional drying technologies limitations such as reducing drying time and energy consumption as well as improving final dried product quality and safety.[81] Over the years, oven and microwave drying have received considerable attention due to their rapid processing rates, short drying times, instantaneous and precise electronic control in addition to being as clean heating processes.[82] While oven drying (OD) uses thermal energy to directly dehydrate the food product, microwave drying (MWD) 22 R. KHIARI ET AL.
- 25. involves the use of an electromagnetic radiation that would be converted into thermal energy for evaporating the moisture from the grapes.[81] Another drying method, vacuum drying (VD), has been successfully used in raisin processing, as well. VD is a process in which moist materials are dried under subatmospheric pressure. The reduced pressure by vacuum increases the mass transfer of water between the food and its surroundings, which lowers the heat needed for rapid drying and procures a high quality product.[83] Recently, infrared drying (ID) has become among the popular drying techniques in the food industry owing to its diminished drying time, the acceptable quality of the final dried product, and its greater energy savings ability, in addition to its cost-effectiveness com- pared to MWD and VD methods.[84] ID implies the use of infrared radiation, which is an electromagnetic wave that can be classified into three regions based on its wavelength: the near-infrared (NIR; 0.78–1.4 mm), the middle-infrared (MIR; 1.4–3 mm) and the far- infrared (FIR; 3–1000 mm).[85] At the NIR wavelength, the infrared radiation is trans- ported through water, whereas the FIR wavelength assures its absorption at the surface. In general, NIR radiation is advantageous for processing thick products, while FIR radiation is favorably absorbed by food with thin layers.[84] Advanced processes. Several papers have reported the feasibility of the above cited methods (OD, MWD, VD and ID) either alone or assisted to other techniques for raisin drying. A mathematical model was developed by Tulasidas et al.[58] to describe MWD of grapes and suggested its viability at commercial scale. The simulation was found to describe the diffusion equation quite well as demonstrated by a good fitting with the experimental data since it took into account shrinkage, which occurs drastically in grapes, as well as the changes in physical and dielectric properties that occur throughout the process due to changes in moisture content and temperature. The proposed numerical model was based on a semitheoretical approach and therefore could be adapted to scale- up purposes. Their findings affirmed also that MWD was energy efficient since it required low specific energy, decreased the drying time in addition to producing raisins with acceptable quality. The work of Çağlar et al.[62] was carried out to determine the moisture diffusion coefficient of seedless grapes under an infrared heating system. They investigated the effects of grape drying and temperature on the moisture diffusion coefficient and the thermal diffusivity. Nine different derived models including the effect of moisture content and drying temperature were evaluated in order to predict moisture diffusivity, thermal diffusivity and drying rate. These models differed from each other based on the form of the empirical equation without physical foundation. The choice of the best model was based on statistical parameters. The model with the highest correlation coefficient (R2 ) and the lowest chi-square (χ2 ) was considered as the best model to choose. The results indicated that both moisture diffusivity and thermal diffusivity increased exponentially with increase in temperature. The studied model equations were significant for use not only in calculation and design related to drying but also in calculation of required parameters in design of other heating processes such as thermal processing, and cool- ing/freezing related to seedless grape. The effects of two drying methods (greenhouse and oven drying) as well as different chemical pretreatments (pretreatment I: dipping in olive oil and K2CO3 solution and pretreatment II: dipping in NaOH solution) on the physicochemical, phytochemical and FOOD REVIEWS INTERNATIONAL 23
- 26. microbiological quality parameters of Muscat Italia grapes were studied.[3] Oven drying at 50°C was found to have shorter drying time and resulted in raisins with better quality characteristics (in terms of appearance and phytochemical attributes) than greenhouse drying. This might be attributed to the shortened drying time, in the case of oven drying, which allowed for a reduction in raisin color degradation as well as an accumulation of phenolic compounds. Wang et al.[59] analyzed the effects of ripeness on the physicochemical properties and pulsed vacuum drying (PVD) kinetics of Thompson seedless grapes, and found that the total phenol concentration, soluble solid content and pH increased considerably while the titratable acidity content decreased, which could be related to the concentration of sugars and the degradation of organic acids (mainly malic acid) during maturation of grape berries and upon exposure to high drying temperature. The drying rate and effective moisture diffusivity also dramatically rose with increasing ripeness due to the softening of the fruit cell wall and the increase of its pericarp permeability, which facilitates moisture transport according to magnetic resonance imaging analysis. During PVD processing, most of the drying time occurs under vacuum, which partly hinders the browning reaction and results in more desirable raisins according to color and texture quality attributes. Indeed, during PVD, the pulsed vacuum atmosphere engenders an oxygen deficient environment, which can reduce some biochemical reactions, such as oxidation deteriora- tion, browning reactions and hence, improve the quality characteristics of dried grapes. Combined processes. In recent studies, there has been substantial progress in using combined drying methods since the combination of such techniques have shown an increase in the dehydration process efficiency as well as an improvement in the quality of the final dried products. Among the many possible combinations, only two combined processes have been assayed for grape drying, combining convection with microwaves and vacuum with microwaves. Combined hot air forced convection and microwave drying (MWD) was studied by Tulasidas[86] to assess the best operating conditions that might enhance the color quality of Thompson seedless raisins. MWD (using a modified microwave oven apparatus at 2450 mHz) was found to be energy efficient with a low specific energy requirement because it presented shorter drying times when compared to hot air convective drying. However, with reference to their previous citations[87] and[88] , a selection of suitable levels of temperature and power (by means of response surface models) were needed to achieve a desired level of quality in the finished product. Clary et al.[60] investigated the drying of grapes using a microwave vacuum dehydration (MWVD) technique versus sun-dried grapes. The MWVD process provided a discrete, real-time control of the process to produce dried grapes with better retention of fresh character, including nutritional composition compared to sun-dried raisins. With refer- ence to a temperature control system, the MW power was controlled to ensure the grapes did not exceed the set treatment temperature. When the surface temperature of the grapes approached the set point, the MW power was automatically reduced. The results showed that MWVD of grapes using temperature monitoring controlled MW power improved product quality compared to using fixed levels and incrementally staged MW power applications. Vitamin A was found in the MWVD raisins but not detected in the sun- 24 R. KHIARI ET AL.
- 27. dried ones, and vitamin C, thiamine and riboflavin were also higher in the MWVD grapes than in the sun-dried raisins. Kassem et al.[61] evaluated the drying characteristics of Thompson seedless grapes using combined microwave oven and hot air cabinet dryer. The hot air cabinet alone as a drying method required more time to dry grapes while within a certain microwave power range (75–900 W), the increase in microwave power resulted in speeding up of the drying process, thus shortening the drying time. To select the best drying method, which has the highest selection percentage, a number of criteria have been chosen including rehydration ratio, total soluble solids, drying ratio, final drying time, final moisture content and specific energy consumption. When grapes were dried in the microwave oven followed by hot air cabinet drying method, 78% of the optimum selection percentage (100%) was achieved, which was considered as reasonable (in comparison with hot air cabinet followed by microwave oven drying or hot air cabinet dryer alone methods with respective percentages of 67% and 56%). It is noteworthy that there are also many other advanced drying techniques (e.g., freeze drying, swell drying, ultrasonic drying, etc.) that have been effectively employed for dehydrating some fruits and vegetables. However, to the best of our knowledge and with reference to the available data, these methods have only been studied for drying grape by-products from wine and juice production (i.e., grape seeds, skins, pomace, stems and stalks) and not for raisin processing. Effect of drying on raisin quality characteristics Appearance, texture and flavor (taste and aroma) along with nutritional value are parti- cularly the most intriguing quality attributes that influence consumers’ acceptance of agri- food products.[89] The quality parameters of raisins, as for other dried fruits and vege- tables, can be substantially affected during the dehydration process, depending on the operating and pretreatments conditions as well as the applied drying technology.[90] Effect of drying on the physicochemical characteristics Visual appearance parameters (i.e., color, size and shape) are intrinsic quality attributes highly associated with consumers’ quality expectations and their choice of purchasing a product, since these attributes are the first characteristics used to assess food quality, to some extent.[91] Color Raisin color seems to be one of the most important quality attributes relevant to market acceptance. Depending on the grape cultivar, pretreatment conditions as well as drying method used, raisins may be found in a variety of sizes and colors including green, yellow, brown and black.[92] The brown color of dried grapes is formed consequently due to the effects of enzymatic reaction (action of the enzyme polyphenol oxidase (PPO)) and nonenzymatic browning (Maillard reaction) taking place during the dehydration process.[93] The appraisal of color can be achieved via destructive or nondestructive methods. Destructive method consists of quantifying the color pigments present in dried foods FOOD REVIEWS INTERNATIONAL 25
- 28. spectrophotometrically or using high performance liquid chromatography.[90] On the other hand, the nondestructive method describes the food color in terms of CIELAB parameters (L*a*b*).[94] CIELAB color notation locates a color in a three-dimensional space defined by lightness (L*) and the chromaticity coordinates a* (redness/greenness) and b* (yellowness/blueness). L* represents light-dark spectrum with a range from 0 (black) to 100 (white). a* is the red-green spectrum with a range from −60 to + 60, and b* indicates yellow-blue spectrum with a range from −60 (blue) to + 60 (yellow).[59] Color properties of Sultana seedless grapes as affected by two pretreatments, POTAS (traditional potassium carbonate solution) and AEEO (Alkaline emulsion of EO) solutions as well as different drying temperatures, were studied by Doymaz and Pala.[17] The use of AEEO solution led to better color (lighter and brighter raisins as indicated by the higher L and lower a/b values, respectively) in comparison with POTAS or untreated samples. When grapes were pretreated with AEEO and dried at 60°C, lightest raisins’ hue was observed. However, further increase in drying temperature resulted in undesirable darkening. The appearance and market quality of dried grapes were evaluated by Dev et al.[40] in response to MW, PEF and conventional chemical pretreatments. The appearance and market quality of PEF and MW-treated samples were superior (higher aesthetic appeal according to their shiny appearance) to the chemical treated and the untreated samples. Statistical analysis revealed that MW-treated raisins had a significant difference in light- ness in comparison with the other applied treatments. PEF-treated raisins were signifi- cantly darker than MW but of close lightness value with untreated and chemical treated ones. This could be attributed to the effect of MW pretreatment, which has been reported to prevent enzymatic browning in some fruits and vegetables and thus, enhance the color of the dried products.[51] Bingol et al.[34] investigated the effect of chemical pretreatment (dipping into 2% ethyl oleate and 5% K2CO3 solution) at different temperatures and times of dipping of Thompson seedless grapes on color kinetics. The drying was carried out in a tray dehydrator at 60°C. Slight color changes occurred immediately after dipping; for example, the grapes’ green color changed to greenish/yellowish at 40°C/3 min, 50°C/2 min, and 50° C/3 min. However, at 60°C dipping temperature and 2- and 3-min dipping times, the color of the grapes was visibly yellowish following the dipping pretreatment and the final color of the raisins was lighter brown. This could be correlated with the inactivation of the PPO enzyme at higher temperatures, which led to the minimizing of browning.[48] The effect of high-humidity hot air impingement blanching (HHAIB) at different temperatures and times was studied on the color changes of seedless grapes.[21] Increasing the blanching time and temperature and decreasing the drying temperature effectively inhibited enzymatic browning and resulted in raisins with brighter color (desirable green-yellow or green color). It was concluded also that the color changes caused by enzymatic browning were more severe than nonenzymatic browning. Wang et al.[41] analyzed the color characteristics of infrared dried red grapes upon dipping in alkaline emulsion of ethyl-oleate (EO) solution (AEEO), AEEO dipping + freezing and carbonic maceration (CM) treatments. CM pretreatment resulted in the most desirable color of raisins (characterized with the lowest color difference (ΔE)) followed by AEEO dipping + freezing treatments then AEEO dipping solution. These 26 R. KHIARI ET AL.
- 29. two latter pretreatments were found to have prolonged drying time, which induces the darkening of the final dried fruits. According to Guiné et al.,[95] the color of raisins from Crimson seedless cultivar was affected by the drying method assessed (in solar greenhouse, in convective chamber at 50° C and at 60°C). Convective-dried grapes were lighter than those dried in the solar greenhouse. This could be attributed to the longer drying time, which could result in severe browning (drying time = 47 h, 101 h and 721 h for convective at 60°C, at 50°C and greenhouse, respectively). As Zemni et al.[3] reported, the color of raisins from Muscat of Italy variety was highly influenced by the predrying treatment used. Oven drying of grapes that were dipped in NaOH solution produced raisins with light brown shade in comparison with greenhouse- dried grapes (dark brown colored). Alternatively, olive oil and K2CO3 emulsion dipping resulted in reddish-brown raisins. This could be attributed to the sensitivity of the PPO to the higher temperature of dipping (90°C) in the case of caustic soda (NaOH) pretreatment in comparison to the potash (K2CO3) pretreatment (50°C). Indeed, the exposure of PPO to temperatures of 70–90°C at certain times is known to destroy their catalytic activity and prevent browning of processed products.[96] Shrinkage One of the most important physical changes that biological materials also endure during dehydration is the reduction of their volume or size, often called shrinkage. Shrinkage is caused by structural collapse, which is generally associated with water removal.[90] Shrinkage assessment can be carried out using common methodologies including direct measurement of product dimensions or volume displacement technique.[97] Several mod- els and equations predicting volume changes during the procedure of drying have been widely tested for agri-food materials, as well.[15,37,98–102] Novel methodologies based on image analysis and computer monitoring have been proposed to estimate the size reduc- tion (shrinkage) and shape change (deformation) during food dehydration. Shrinkage deformation characteristics determined by these methods were successfully applied in the assessment of water diffusivities and drying simulation of food products and results were comparable with other studies where different methodologies have been used.[97] These innovative technologies create the opportunity for multidimensional quality analysis; in other words, they allow the simultaneous measuring of different quality parameters including shrinkage, deformation, color and texture.[103] In the case of raisins, both of these approaches (modeling and new technologies) have been studied for investigating shrinkage throughout the process of drying. Gabas et al.[104] evaluated the effect of a chemical pretreatment (a solution of 2% CaCO3 with 0–3% EO) on the physical properties of the grapes of Italy variety. They found that the shrinkage increased with drying temperature between 40 and 80°C and decreased with increasing concentration of EO pretreatment. The temperature effect on shrinkage can be attributed to the temperature dependence of elastic and mechanical properties of cell wall structures. On the other hand, EO acts on the grape skin by dissolving the waxes and resulted in less collapsed structure, which, in turn, led to better-dried grapes. Azzouz et al.[105] compared two models of diffusion developed to evaluate the effective diffusivity: a simplified one based on Fick’s law and a second one that FOOD REVIEWS INTERNATIONAL 27
- 30. accounted for the shrinkage as a fundamental parameter by modeling the movement of the solid skeleton. Shrinking was ideal since the eliminated water was replaced by the solid, there was no appearance of pores (ε = 0) and a linear correlation between volume changes and water content was discerned for both varieties of grapes. The same conclusion was also ascertained by Bennamoun and Belhamri[106] , who found that the necessity of introducing shrinkage was clear and neglecting it brought about erroneous results (in the evolution of moisture content, which was under estimated without considering shrinkage). According to many researchers, a linear relationship between shrinkage and moisture content and temperature is generally observed during drying operations.[15,107–110] Indeed, the drying process leads to changes of foods at microstructural level and consequently, it affects their macroscopic characteristics. Loss of water and the segregation of components occurring during drying, result in damage and disruption of the cellular walls, and even collapse of the cellular tissue. These changes are generally associated with volume reduction of the product. The work of Bingol et al.[34] investigated the quality parameters of Thompson seedless grapes as affected by drying conditions (dipping temperature and time in a tray dehy- drator). Regarding shrinkage, their findings disclosed that as drying proceeds, grapes shrink due to the loss of moisture content and the decrease in porosity. Senadeera et al.[37] expressed the shrinkage behavior as average diameter variation during drying in a convective oven at 50°C, and found that at the end of drying, treated grapes by peel abrasion showed lower shrinkage changes than untreated ones. The diameter change of untreated white grapes was about 35% and only 19% for treated ones. For red grapes, the diameter changed from about 40% for untreated samples to 30% after abrasive pretreatment. It seems that physical pretreatment of grapes causes a loss in their initial moisture content, which reduces the drying time and thereby, decreases shrinkage. The influence of physical (peel abrasion) and chemical (EO dipping) pretreatments on drying characteristics and quality of Red Globe grapes has been studied by Adiletta et al.[15] Peel abrasion combined with optimal drying temperature (50°C) was not only effective in shortening the drying time, but also reducing the shrinkage of grapes. In fact, the decrease of grape volume has been observed to be proportional to the water content decrease during drying. Among the common used empirical models that correlate shrinkage with moisture content, the quadratic model showed an acceptable fit to experimental data (according to the values of statistical parameters mainly R2 ) for all the samples and temperatures studied. Machine vision (MV) was used by Behroozi Khazaei[103] to measure grape shrinkage through the drying process. An artificial neural network (ANN) was developed for dynamic modeling of Sultana grape drying in a hot air dryer. The ANN was qualified as multilayer feed forward neural network (MFNN) and was found to have better perfor- mance than multiple linear regression (MLR) model (value of R2 = 0.99952 and 0.99830, respectively) for predicting the moisture content and shrinkage of grapes during the process of dehydration. The MFNN model proposed by the MV methodology might be useful for developing an automatic control system for hot air dryers. Texture Raisins textural parameters of importance include hardness, stickiness, moistness, springi- ness (elasticity), cohesiveness and chewiness. These often can be measured by texture profile analyses (TPA).[110] 28 R. KHIARI ET AL.
- 31. Grapevine cultivars, Fiesta and Selma Pete, dried by two drying methods, dry-on-vine (DOV) and tray-dry (TD) were analyzed for texture.[92] Moistness, stickiness and chewi- ness of DOV samples were higher than TD samples; however, the grittiness of DOV raisins was lower than TD ones. The grittiness is generally due to the formation of sugar crystals, a phenomenon known as “sugaring.” Sugaring occurs often with stored raisins that are overly moist or that are mechanically damaged. Since the TD raisins were less moist than the DOV raisins, so presumably the higher grittiness noted with these raisins was owing to mechanical damage during the tray-drying process. They concluded also that raisins being the most sticky or chewy were generally the least moist. The effect of air impingement drying at different temperatures (50, 55, 60 and 65°C) on texture (expressed in terms of hardness) of Monukka seedless grapes were evaluated by Xiao et al.[111] Drying temperatures (from 55 to 65°C) significantly increased the hardness during the drying process. This observation was explained by the fact that high tempera- ture allowed the water removal from the surface better than the migration of water from the interior, which led to the formation of a hard layer on the surface. According to Adiletta et al.,[110] the hardness and chewiness of dried white grapes (Regina) were higher than those of red raisins (Red Globe). This may be explained by the difference in the size and thickness of the berries, which could be determining factors, since both varieties were compared at the same final moisture content. However, for each cultivar, untreated and treated dried samples were similar in terms of hardness and chewiness. With respect to springiness, no difference was detected between both cultivars. The texture of Italia grapes dried under different temperature and time combinations was examined in the work of Rybka et al.[112] Firmness was negatively correlated to consumer’s overall acceptance. In fact, the longer drying time or higher temperature resulted usually in harder texture, which, in turn, influences the consumer’s acceptability and alters the mouthfeel for the worse. The texture attributes of Crimson seedless cultivar under different drying methods was studied by Guiné et al.[95] They concluded that grapes dried in a convective chamber at 60°C had higher hardness and springiness but less chewiness than raisins obtained by solar greenhouse dehydration and convective drying at 50°C. This might be assigned to the difference in their final moisture content (at the end of drying), which was lower in the case of convective drying compared to solar greenhouse drying. In addition, the drying temperature could also influence these texture proprieties. The temperature in the con- vective drying chamber was constant and higher; however, it was variable and lower in the greenhouse solar drying. Aroma Flavor is also deemed an important quality feature that influences the marketability of a product in addition to its appearance and shape. Flavor is generally determined by taste and aroma.[94] It can be also defined as a blend of several volatile and nonvolatile compounds with a wide range of physicochemical characteristics.[113] While the volatile components impact both taste and aroma, the nonvolatile ones influence only the taste.[114] Flavor is probably the most elusive and subjective quality parameter because it is difficult to measure and relies typically on consumer preference.[113] In the present review, we are not going to emphasis the taste determination but rather aroma identification in dried grapes since taste perception is often accomplished by FOOD REVIEWS INTERNATIONAL 29
- 32. sensory analysis. Indeed, sensory evaluation encompasses taste assessment along with other organoleptic attributes such as color, texture and aroma, so that taste is usually judged as a part of raisins’ overall appreciation by panelists as reported in.[30,95,112] Only a few papers have focused on the characterization of raisins’ aroma profile[115–119] although grape volatile compounds have been extensively studied.[113,120–123] It is worth mentioning that almost all the other earlier researches on raisins’ aroma have been interested in the identification of the volatile components in sun or air-dried grapes and no information is available on the aroma profiles of their corresponding fresh counter- parts, except the work of.[115] Volatile compounds are low molecular weight substances (usually < 300 Da) that volatilize rapidly with the contact of air. These volatile molecules are present in food in low concentra- tions ranging from milligram per kilogram to nanogram per kilogram.[124] Raisins aroma constituents exist as both free and glycosidically bound-form volatile compounds.[118] These odoriferous components come generally from three origins: the majority of them derived from the fresh grapes, other metabolites including furans and pyrazines are produced during the process of drying as a result of the Maillard reaction[117,118,125] , while aliphatic compounds such as acids and aldehydes are generated from the oxidative degradation of unsaturated fatty acids.[117,118] The evaluation of aromatic constituents includes usually four steps: extraction, con- centration, chromatographic separation and detection.[126] In the former studies of raisins’ aroma[115,116] , steam distillation and simultaneous distillation extraction were used as isolation methods. The solid phase microextraction (SPME) is another technique recently applied[117–119] for the extraction of raisin volatiles since it is considered as a simple, inexpensive, solvent-free and sensitive method. For study and characterization of volatiles in raisins, instrumental analysis (Gas Chromatography-Mass Spectrometry, GC-MS) as well as sensory evaluation (olfactometry) have been frequently used. The combination of these two methods is also called Gas Chromatography-Olfactometry (GC-O). GC-O allows the detection and the estimation of each aroma-active compound based on its odor activity value (OAV); hence, a volatile that has an OAV above 1 is considered as a flavor contributor.[127] Currently, a total of 91 odorants have been distinguished in raisins belonging typically to six categories: floral, fruity, green, roasted, fatty and chemical aromas.[117,118] The common volatile compounds found in the aroma profiles of raisins include some aliphatic acids, aldehydes, terpenoids, furans and pyrazines. The aliphatic acids such as hexanoic, heptanoic, octanoic, nonanoic and decanoic acids along with the aliphatic aldehydes pentanal, heptanal, hexanal, (E)-2-heptenal, (E)-2-octenal, (E)-2-non- anal and (E,E)-2,4-nonadienal were reported to be present at high concentration in raisins.[117,118] These aliphatic compounds are all well known to be derived from unsatu- rated fatty acid oxidative degradation and to generate especially the dried grapes’ green aroma. Terpenoids (geraniol, linalool and β-damascenone) were also recorded recently as potent volatile compounds since they strongly contribute to raisins characteristic floral, fruity and fatty aromas. Furans (2-pentylfuran and furfural), pyrazines (3-Ethyl-2,5- dimethyl pyrazine and 2,6-Diethyl pyrazine) as well as benzene-acetylaldeyde were also identified in high levels. These compounds have been pointed out to come from the Maillard reaction and to contribute to the marked fruity, roasted and floral aromas in raisins, respectively.[117,118] 30 R. KHIARI ET AL.
- 33. As mentioned above, most of the raisins volatiles have been reported to originate from the fresh grapes. The process of drying contributes additionally to the generation of other compounds through the unsaturated fatty acid auto-oxidation (UFAO) and Maillard reac- tions. Based on these data, we propose in Figure 2 a schematic representation of the probable pathways leading to the formation of raisins volatile compounds adapted from previous references investigating the metabolic aroma biosynthesis in grapes and wine.[121,123,130–132] Many factors may affect raisins’ volatile composition including the grape variety genotype, the climatic, edaphic and agronomic conditions during production, the harvesting date, the postharvest handling, the pretreatment conditions, the drying process and storage conditions.[130] To date, we have a limited understanding of the involvement of these factors in the volatile composition and the resulting flavor of raisins; hence, future challenges need to focus more on this subject.[128] The volatile compounds ascertained in the above cited investigations dealing with dried grapes’ aroma profile[115–119] are delineated in Table 4. These studies are discussed below. The study of Ramshaw and Hardy[115] evaluated the aroma-active components present in fresh and dried sultana grapes previously treated with a commercial solution. Their results showed that the application of a pretreatment prior to drying highly influenced raisins’ aroma Figure 2. Probable pathways for the metabolism of raisins aroma (adapted from Wen et al.[128] ; van Boekel[129] ; Dunlevy et al.[ 122] , with modifications). AACT: acetoacetyl-CoA thiolase; HMGS: 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) synthase; HMGR: 3-hydroxy-3-methylglutaryl-CoA reductase; MVA: mevalonate; MK: mevalonate kinase; MVP: Mevalonate-5P; PMK: phosphomevalonate kinase; MVPP: mevalonate-5PP; MPDC: diphosphomeva- lonate decarboxylase; 3GP: D-glyceraldehyde 3-phosphate; DXS: 1-deoxy-D-xylulose-5-phosphate (DXP) synthase; DXR: DXP reductoisomerase; MEP: 2-C-methyl-D-erythritol 4-phosphate; MCT: MEP cytidyltransferase; CDP-ME: 4-(cytidine 5ʹ-diphospho)-2-C-methyl-D-erythritol; CMK: CDP-ME kinase; CDP-MEP: 2-Phospho-4-(cytidine 5ʹ-diphospho)-2-C-methyl-D-erythritol; MCS: 2-C-methyl-D-erythritol 2,4-cyclodiphosphate (ME-2,4cPP) synthase; HDS: 1-hydroxy-2-methyl-2-butenyl 4-diphosphate (HMBPP) synthase; HDR: HMBPP reductase; IPPI: isopentenyl diphosphate (IPP,C5) Delta-isomerase; DMAPP (C5): dimethylallyl diphosphate; GPS: geranyl diphosphate (GPP,C10) synthase; FPS: farnesyl diphosphate (FPP,C15) synthase; GGPS: geranylgeranyl diphosphate (GGPP,C20) synthase; TPS: Terpenoid synthase; ABA: abscisic acid; UGT: Uridine diphosphate (UDP)-dependent glycosyl transferases. FOOD REVIEWS INTERNATIONAL 31
- 34. Table 4. Volatile compounds reported in raisins. Compounds Reported by Concentration of free-form (µg/L)1 Concentration of bound-form (µg/L)1 Acids 2-Methyl-propanoic acid W1, W2 2.70–281 nd Pentanoic acid W1, W2 43.3–171 70.8–76.8 Hexanoic acid B, W1, Bh, W2 271–675 tr 2-Ethyl-hexanoic acid W1 122 71.2–75.1 Heptanoic acid B, W1, W2 9.77–198 41.6–59.1 Octanoic Acid B, W1, W2 11.8–182 59.2–59.7 Nonanoic acid B, W1, Bh, W2 36.0–150 80.0 −109 Decanoic acid B, W1, W2 28.5–133 265–275 Dodecanoic acid W1, W2 25.4–137 3.97–5.56 Tetradecanoic acid W1, W2 122–129 4.64–7.51 n-Hexadecanoic acid W1, W2 25.5–124 tr Aldehydes Pentanal R, W1, W2 6.7–102 nd Hexanal R, B, W1, W2 8.79–344 nd Heptanal B, W1, W2 5.98–87.1 nd (E)-2-Hexenal B, W1, W2 16.6–30.5 nd Octanal B, W1, Bh, W2 2.56–54.5 nd (E)-2-Heptenal R, B, W1, W2 21.8–86.1 nd Nonanal B, W1, W2 5.35–29.5 nd (E)-2-Octenal B, W1, W2 16.7–23.1 nd (E,E)-2,4-Heptadienal B, W1, Bh, W2 62.1–257 nd Decanal B, Bh, W2 0.10–86.5 nd Benzaldehyde R, B, W1, Bh, W2 6.60–18.5 nd (E)-2-Nonenal B, W1, W2 9.3–12.7 nd Benzeneacetaldehyde R, B, W1, W2 10.9–398 nd (E,E)-2,4-Nonadienal B, W1, W2 54.3–60.2 nd Esters Ethyl acetate W1, Bh, W2 97–395 nd Ethyl hexanoate W1, W2 3.94–28.5 nd Ethyl octanoate R, W1, Bh, W2 1.13–2.77 nd Butyrolactone R, W1, W2 3.00–6.24 nd Ethyl decanoate Bh, W2 nd - 0.11 nd Diethyl succinate W1, W2 0.60–2.03 nd Benzyl acetate W1, W2 1.01–1.16 nd Methyl salicylate W1, W2 0.80–794 0.77–12.5 Phenethyl acetate W1, W2 1.04–4.11 nd γ-Nonalactone W1, W2 2.30–48.1 nd Methyl hexadecanoate W2 nd - 1.27 nd Ethyl hexadecanoate W1, W2 0.15–3.99 nd Alcohols 3-Methyl-1-butanol W1, W2 0.52–463 302–969 1-Pentanol R, W1, W2 0.60–5.40 5.21–20.7 3-Methyl-2-buten-1-ol W1, W2 0.13–0.28 0.44 −157 1-Hexanol W1, W2 12.7–141 38.3–215 1-Octen-3-ol B, W1, Bh, W2 2.47–28.2 1.16–10.4 Heptanol W1, W2 1.50–26.8 2.20–6.72 (Continued) 32 R. KHIARI ET AL.
- 35. Table 4. (Continued). Compounds Reported by Concentration of free-form (µg/L)1 Concentration of bound-form (µg/L)1 6-Methyl-5-hepten-2-ol W1, W2 0.19–0.36 0.17–2.41 2-Ethyl-1-hexanol W1, W2 0.37–1.90 0.96–10.4 1-Octanol B, W1, W2 0.30–7.47 0.73–7.06 (E)-2-Octen-1-ol W1, W2 1.17–26.2 0.35–1.33 1-Nonanol B, W1, W2 0.08–1.27 0.10–9.50 Benzyl alcohol R, W1, W2 4.33–77.0 126–2931 Phenylethyl alcohol R, W1, Bh, W2 10.1–397 41.3–159 Ketones 2,3-Butanedione R, W2 52.9–88.3 nd 2,6-Dimethyl-4-heptanone W1, W2 1.65–16.9 nd Acetoin R, W1, W2 15.8–2684 18.8–1922 6-Methyl-5-hepten-2-one W1, Bh, W2 1.73–34.1 nd 6-Methyl-3,5-heptadiene-2-one B, W1, W2 1.59–66.6 nd Acetophenone R, W2 4.13–10.1 nd Terpenoids Limonene W2 6.33–19.7 7.43–8.19 γ-Terpinene W2 nd - 17.0 7.40–8.37 (E)-β-Ocimene W1, W2 10.0–21.8 2.87–3.46 p-Cymene W1, W2 1.07–27. 2 nd Terpinolene W2 7.50–19.4 7.72–7.90 Rose oxide W2 0.30–54.3 5.05–5.72 Alloocimene W2 7.0–17.3 nd 3-Ethyl-2-methyl-1,3-hexadiene W1, W2 16.4–22.1 nd Nerol oxide Bh, W2 0.43–156 5.35–29.2 Linalool W1, Bh, W2 0.83–184 1.78–10.2 α-Terpinenol W2 nd - 0.60 nd Hotrienol W1, W2 0.87–1071 0.56–0.79 p-Menth-1-en-9-al W1, W2 0.20–99.9 5.06–5.59 α-Terpineol R, B, W1, W2 0.30–23.6 0.24–8.59 γ-Terpineol W2 nd - 3.33 nd Lilac alcohol W1, W2 0.14–0.43 nd Neral Bh, W2 1.70–31.2 46.1–51.8 cis-Pyran linalool oxide W1, W2 3.30 −12.4 0.45–3.19 β-Citronellol W1, W2 0.27–3.22 0.91–2.25 Nerol W1, W2 2.30–206 2.36–170 β-Damascenone W1, W2 1.30–13.4 1.20–127 Geraniol W1, W2 0.10–558 0.16–60.5 Geranylacetone B, W1, W2 1.42–1.85 3.25–16.3 Geranic acid W1, W2 45.3–1091 180–250 Furans 2-Pentyl furan B, W1, Bh, W2 9.99–256 nd Furfural R, B, W1, W2 82.9–3209 nd 2-Acetylfuran R, B, W1, W2 9.75–194 nd 5-Methyl-2-furfural R, B, W1, W2 4.38–57.8 nd Pyrazines 2-Ethyl-6-methyl pyrazine W1, W2 1.97–86.7 nd 2,6-Diethyl pyrazine W1, W2 19.2–157 nd 3-Ethyl-2,5-dimethyl pyrazine W1, W2 22.0–106 nd (Continued) FOOD REVIEWS INTERNATIONAL 33
- 36. profile. In whole, 34 compounds were identified, among them the fresh grape volatiles mainly furfural, methyl furfural, 2-hydroxybutan-3-one and diacetyl were found in considerable amounts in the dipped sultanas but at far less rates in the undipped raisins. These compounds, commonly ascribed to nonenzymatic browning, seemed to be responsible for the malty, caramel like aroma of the undipped sultanas and their production was presumably inhibited by the predrying treatment. On the other hand, the dipped sultanas contained several carbonyl compounds that were not observed in the undipped ones. Buttery et al.[116] recorded a total of 38 components in the volatile oil of sun-dried grapes obtained from the Thompson seedless variety. They concluded that the process of drying substantially affected the raisins’ aromatic composition. The major components identified included the aliphatic acids octanoic, nonanoic, hexanoic, heptanoic and decanoic acids as well as 2-hexyl-3-methylmaleic anhydride, nonanal, phenylacetaldehyde and 2-pentylfuran. In this study, two unusual metabolites (2-hexyl-3-methylmaleic anhy- dride and 1-octen-3-one) were detected for the first time in raisins. The former belongs to anhydrides, which are usually hydrolyzed by water so that the loss of moisture owing to drying favored its generation. Alternatively, the latter odorant had been reported to be the main characteristic aroma of mushroom and was found in cooked mushroom and artichoke. Wang et al.[117] analyzed the volatile compounds in air-dried raisins from Flame Seedless, Thompson Seedless and Crimson Seedless varieties. In total, 77 volatiles were identified of which 37 had never been reported as raisin volatiles before. The aroma characters of the three varieties were quite similar except for some discrepancies in the concentration of each aroma character. The major free-form volatiles were ethyl acetate, hexanoic acid, (E,E)-2,4-heptadienal and geraniol, with β-damascenone, 3-ethyl-2,5- dimethylpyrazine, 1-octen-3-ol and hexanal making up the highest contribution to the aroma. Fruity and floral were the main characteristics of the free-form fragrances in raisins. The major bound-form (glycosidically bound) metabolites were benzyl alcohol and acetoin, with β-damascenone being responsible most for the bound-form aromas, Table 4. (Continued). Compounds Reported by Concentration of free-form (µg/L)1 Concentration of bound-form (µg/L)1 Phenols Phenol W1, W2 0.44–14.1 0.40–69.5 4-Vinylguaiacol W1, W2 1.28–8.59 0.35–27.9 Aromatic compounds Toluene W2 2.53–22.4 nd Naphthalene W1, W2 3.65–10.1 nd 2-Methyl-naphthalene W1, W2 1.06–2.36 nd R: Reported in[115] for the study of sun-dried sultanas. B: Reported in[116] for the study of sun-dried Thompson seedless raisins. W1: Reported in[117] for the study of sun-dried raisins (Thompson Seedless, Flame Seedless and Crimson Seedless) cultivated in China. Bh: Reported in[119] for the study of sun-dried raisins (cultivars: Chriha, Raseki, Assli, and Meski) cultivated in Tunisia. W2: Reported in[118] for the study of sun-dried raisins (Thompson Seedless, Crimson Seedless and Zixiang Seedless) cultivated in China. 1: Min-Max concentration values reported for the free and bound-form volatile compounds as reported by.[118, 119] tr: trace. nd: not detected in raisins. 34 R. KHIARI ET AL.