2. CONTENTS
• INTRODUCTION
• Methods of concentration
• The purposes of concentration
• Evaporation
• Preservative Effects of Concentration
• Methods of Concentration
• Changes During Concentration
•
3. INTRODUCTION
• When a part of the water is removed, which results in
concentrated solutions or dispersions or in semisolid products
with water contents in excess of 20%. are called concentration
processes.
• Concentration reduces weight and volume and results in
immediate economic advantages.
•
4. • It may be used as an economical preparatory step for
subsequent dehydration, eg spray-dried tea or freeze-dried coffee.
• It may be used to reduce the bulk of materials to be preserved by
freezing or by sterilization eg frozen orange juice or evaporated
milk.
• It may also be used as a method of preservation
5. Methods of concentration
• Evaporation
• Freeze concentration
• Membrane separation
• Osmotic dehydration
• Solvent extraction
osmotic dehydration and solvent separation are carried out with the
conjugation of various other concentration method
6. Purpose of concentration.
• Concentration reduces weight and volume and results in immediate
economic advantages.
• Moisture can be more economically removed in highly efficient evaporators
than in dehydration equipment
• Increased viscosity from concentration often is needed to prevent liquids
from running off drying surfaces or to facilitate foaming or puffing.
• Some concentrated foods are desirable componentsof diet in their own right
eg. jellies
• The more common concentrated foods include evaporated and sweetened
condensed milks, fruit and vegetable juices and nectars, sugar syrups and
flavored syrups,
7. Evaporation
• Evaporation is the removal by vaporization of part of the solvent from a
solution or dispersion of essentially nonvolatile solutes.
• Evaporation is distinguished from crystallization and drying by the fact that
the final product is a concentrated dispersion or solution rather than
precipitated solids.
• An evaporator consists of a heat exchanger capable of maintaining the
liquid at its boiling point and a device to separate the vapor phase from the
liquid
8. Food Properties and Evaporator Performance
• In food applications, a number of considerations pertaining to
properties of the foods assumes a dominant role.
• They are:
• Viscosity and Consistency , Fouling , Foaming , Corrosion
• Entrained Liquid , Flavor , Heat Sensitivity etc
9. Viscosity and Consistency
• Food products subjected to evaporative concentration become
quiteviscous
• Viscosity affects not only the rate of heat transfer but also
pumping requirements and other material handling
considerations.
• In processing of very viscous fruit juices, special treatments may
be necessary to reduce viscosity.
10. Fouling
• Fouling is often critical in food evaporation.
• Proteins and polysaccharides in particular are capable of forming
deposits that are difficult to remove and which adversely affect
efficiency of heat transfer.
11. Foaming
• The presence of various natural surface-active components,
including proteins, is often the cause of extensive foaming, and
this reduces heat transfer efficiency and may produce problems in
removal of product and vapor from the evaporator.
• Use of antifoam agents can be of some value, but their choice is
limited to those acceptable as food additives.
12. Corrosion
• Some foods, especially fruit juices, contain components which are
corrosive to heat-exchange surfaces.
• This damages the equipment and results in undesirable transfer of
metals to the evaporated product.
13. Entrained Liquid
• Separation of entrained liquid from the vapor stream is often
difficult.
• Efficient separation is, however, important in order to avoid
economically intolerable losses of solid matter.
14. Flavor
• Desirability of food depends on various volatile flavor
components which often are lost during vaporization of water.
• Design of evaporation processes is often directed either toward
decreasing such losses or toward recovering volatile flavor
components from the vapor phase.
15. Heat Sensitivity
• Heat sensitivity is a problem of particular concern since it affects
the quality of food.
• The operational factors, time and temperature, affect the degree
of damage
caused by heat.
16. Preservative Effects of Concentration
• The levels of water in virtually all concentrated foods are in
themselves more than enough to permit microbial growth
• Removal of water by concentration also increasesthe level of food
acids in solution.
• Preservation is quite satisfactory provided there is no moisture
condensation from the air onto interior tank surfaces.
• The principal reason for most food concentration is to reduce
food weight and bulk.
17. Methods of Concentration
• Solar Concentration
• Open Kettles
• Flash Evapomtors
• Thin-Film Evapomtors
• Vacuum Evaporators
18. Solar Concentration
• Simplest methods of evaporating water is with solar energy.
• Solar evaporation is very slow and is suitable only for
concentrating salt solutions.
19. Open Kettles
• Some foods can be satisfactorily concentrated in open kettles that
are heated by steam
• In case for some jellies and jams and for certain types of soups.
• Thickening and burn-on of product to the kettle wall gradually
lower the efficiency of heat transfer and slow the concentration
process.
• High temperatures and long concentration times damage most
foods.
20. Short Tube Evaporator
• In this evaporator the solution boils inside vertical tubes with the
steam condensing in a chest through which the tubes pass.
• The steam chest is doughnut shaped, allowing easy downward
flow through the large central opening. Circulation past the
heating surface is induced by boiling in the tubes, each of which is
usually 5–7 cm in diameter and 1–2 m in length.
• This evaporator is quite satisfactory for evaporation of relatively
low-viscosity, noncorrosive liquids that are resistant to high
temperatures.
21.
22. Flash Evapomtors
• Clean steam superheated at about 150°C is injected into food
which is pumped into an evaporation tube where boiling occurs.
• The boiling mixture then enters a separator vessel in which the
concentrated food is drawn off at the bottom and the steam plus
water vapor from the food is evacuated through a
separate outlet.
• Because temperatures are high, foods that lose volatile flavor
constituents will yield these to the exiting steam and water vapor.
These can be separated from the vapor by essence-recovery
equipment on the basis of different boiling points between the
essences and water.
23.
24. Thin-Film Evapomtors
• Food is pumped into a vertical cylinder which has a rotating element that
spreads the food into a thin layer on the cylinder wall.
• Water is quickly flashed from the thin food layer and the concentrated food
is simultaneously wiped from the cylinder wall.
• The concentrated food and water vapor are continuously discharged to an
external separator, from which product is removed at the bottom and water
vapor passes to a condenser.
• In some systems the water vapor temperature is raised by mechanical vapor
recompression to yield steam for reuse to save energy. Product temperature
may reach 85°C or higher, but since residence time of the concentrating food
in the heated cylinder may be less than a minute, heat damage is minimal.
25. Vacuum Evaporators
• Heat-sensitive foods are most commonly concentrated in low-temperature
vacuum evaporators.
• Several vacuum vessels is constructed in series so that the food product
moves from one vacuum chamber to the next and more concentrated to the
progressive stages
• The successive stages are maintained at progressively higher degrees of
vacuum, and the hot water vapor arising from the first stage is used to heat
the second stage, the vapor from the second stage heats the third stage, and
so on .Such system of evaporator are called multiple effect evaporator
26.
27. Freeze Concentration
• When a solid or liquid food is frozen, some of the water which firstly forms
ice crystals in the mixture. The remaining unfrozen food solution have high
solids concentration
• Separation of the initially formed ice crystals is through the centrifugation of
the partially frozen slush through a fine-mesh screen. The concentrated
unfrozen food solution passes through the screen while the frozen water
crystals are retained and can be discarded.
• Freeze concentration has been applied commercially to orange juice.
28. • Freeze Concentration consits of two steps
• In first stage fruit juice is super cooled below its freezing point to
allow water to separates as ice crystals
• In second stage ice crystals are separated from concentrated fruit
juices
29. Food applications of freeze concentration
• Concentration of wine in order to increase alcohol content without
adding pure alcohol, a procedure which is illegal in many countries
• Concentration of beer to improve stability, reduce distribution costs,
and develop new higher alcohol beverages.
• Production of high quality fruit juice concentrates.
• Concentration of cider and production of vinegar with up to 40% acetic
acid.
• Coffee has been concentrated prior to freeze drying, and its
concentration prior to spray drying was reported. The concentrations
achieved were up to 45 wt% solids
30. Advantages of FC over evaporative methods
• The energy needed to freeze a unit of water is much less.
• The low process temperature prevents undesirable chemical and
biochemical reactions (minimum color change, non-enzymatic
browning and vitamin losses
• As vacuum is not involved, the losses of low-boiling flavor and
aromatic esters are completely avoided
• The flavor profile is better.
31. Drawbacks of Freeze concentration
• Major problem is the loss of soluble solids of the juice in the
separated ice.
• The final concentration of the concentrated juice is as low as 40-
55% dry matter, due to steep increase in the viscosity of ice-
concentrated mixture.
32. MEMBRANE PROCESSES FOR
CONCENTRATION
• High quality product is obtained through membrane cocentration
• The principle involved is the interposition of a membrane
between the feed stream and a waste or transfer stream, and the
establishment of conditions providing a driving force for
transport of water across the membrane from the feed to the
transfer stream
33. Driving Forces for Membrane Processes
• 1. Concentration difference
2. Pressure difference
3. Difference in electric potential
4. Temperature difference
34. Ultrafiltration and Reverse Osmosis
• These applications are largely dependent on membrane properties
such as water permeability rate, solute and macromolecule
rejection rates, and length of useful membrane life.
• Different membranes are required for different liquid foods.
Synthetic membranes are manufactured from cellulose acetate,
polyamide, and other materials, with considerable control over
their physical and chemical properties.
35. • Ultrafiltration membranes are generally "less tight" than reverse osmosis
membranes; that is, they restrict macromolecules such as proteins but with
moderate pressure allow smaller molecules such as sugars and salts to pass
through.
• Reverse osmosis membranes are "tighter," and with greater pressure will permit
the passage of water but hold back various sugars, salts, and larger molecules.
• Various applications in the dairy industry, these membrane processes are being
used
to concentrate fruit juices, coffee and tea extracts, egg white and whole egg, soy
proteins, enzymes, and other materials.
36. Advantages of Ultra Filtration
• Produces juice of desirable quality at low cost of operation and
with greater speed.
• In a single step, it performs juice clarification and fining.
• Lower energy consumption
• Increased flavor and aroma retention
37. Reverse osmosis (RO)
• It is basically a concentration process. Pressure is applied
to fruit juice that is greater than its osmotic pressure. This
pressure forces the water out of the juice.
• It limits the upper concentration level at about 28Brix.
38. Advantages of RO
• Considerable amount of aroma retention at a cost
competitive with evaporation, without undue loss of
solids.
• Concentration without phase change or thermal damage.
39. Changes During Concentration
• Concentration processes that expose food to 100°C or higher temperatures
for prolonged periods cause major changes in organoleptic and nutritional
properties.
• Cooked flavors and darkening of color are two ofthe more common results.
• In caramel production, sugar-milk mixtures are intentionally concentrated at
high temperature.
• All sugars have an upper limit of concentration in water beyond which they
are not soluble.
• when concentration is done under vacuum, many bacterial species not only
survive the low temperatures but
multiply in the concentrating equipment.
40. Advantages of concentration
• Provides microbiological stability
• Permits economy in packaging, transportation and distribution of finished
product
• Provides a preservative effects
• Reduce the bulk of materials it helps transportation and packaging
