1. Factors affecting microbial growth in food
(a) Intrinsic factors:
oThey include:
pH, water activity, oxidation reduction potential, nutrient
content, antimicrobial contents, biological structure.
(b) Extrinsic factors:
oAre factors external to the food that affect microbial
growth.
Temperature,Concentration of gases in the environmentand
RH
2. Intrinsic factors:
o These are inherent in the food.
-They include:
Hydrogen ion concentration (pH) will see this
Moisture content Wa two factors
Nutrient content of the food
Antimicrobial substances
Biological structures
3. Hydrogen ion concentration (pH)
• Most bacteria grow best at neutral or weakly
alkaline pH usually between 6.8 and 7.5.
• Some bacteria can grow within a narrow pH
range of 4.5 and 9.0, e.g. Salmonella
4. cont
• Other microorganisms especially yeasts and
molds and some bacteria grow within a wide
pH range, e.g. molds grow between 1.5 to
11.0, while yeasts grow between 1.5 and 8.5.
5. Introduction
What is PH?
o It is measure of the acidity or alkalinity of a
solution in water.
o The acidity or alkalinity of a water solution is
determined by the relative number of hydrogen
ions (H+) or hydroxyl ions (OH-) present.
6. Cont
• Acidic solutions have a higher relative number
of hydrogen ions, while alkaline (also called
basic) solutions have a higher relative number
of hydroxyl ions.
• Acids are substances which either dissociate
(split apart) to release hydrogen ions or react
with water to form hydrogen ions.
7. cont
o Bases are substances that dissociate to release
hydroxyl ions or react with water to form
hydroxyl (OH)ions.
10. cont
o Increasing the acidity of foods, either through
fermentation or the addition of weak acids, has
been used as a preservation method since
ancient times.
o In their natural state, most foods such as meat,
fish, and vegetables are slightly acidic while
most fruits are moderately acidic.
11. cont
o The pH is a function of the hydrogen ion
concentration in the food:
i.e. pH = -log10 [H+]
12. pH values of some food products
Food type Range of pH values
Beef 5.1 - 6.2
Chicken 6.2 – 6.4
Milk 6.3 – 6.8
Cheese 4.9 - 5.9
Fish 6.6 - 6.8
Oyster 4.8 - 6.3
Fruits < 4.5 (most < 3.5)
Vegetables 3.0 – 6.1
13. cont
• Microorganisms that are able to grow in acid
environment are called acidophilic
microorganisms.
• These microorganisms are able to grow at pH
of around 2.0
• Yeasts and molds grow under acidic this env’t.
14. cont
• Other microorganisms such as Vibrio cholerae
are sensitive to acids and prefer alkaline
conditions.
• Most bacteria are killed in strong acid or
strong alkaline environment except
Mycobacteria.
15. Minimum and maximum pH for growth of some
specific microorganism
Microorganism Minimum Maximum
Escherichia coli 4.4 9.0
Salmonella
enterica serovar
typhi
4.5 8.8
All bacteria 4.0 9.0
Molds 1.5 11.0
Yeast 1.5 8.5
16. cont
o Examples of high-acid foods include
jams and jellies, pickles and most fruits.
o This type of foods has PH value less or equal
to 4.5.
o Low acidic foods include:
vegetables, legumes, beas,peas,carrot,corn,onion and
egg white
o which has greater or equal to 4.5.
17. Types of Water in Food
Most natural foods contain water up to 70% of their
weight or greater unless they are dehydrated.
Fruits and vegetables contain water up to 95% or greater.
Water in foods and biological materials can be grouped
in three categories:
free water
entrapped water, and
bound water
18. cont
o Free water- is easily removed from foods or
tissues by cutting, pressing, squeezing or
centrifugation.
• Entrapped water is- immobilized within the
lattices of large molecules, capillaries, or cells,
but it is released during cutting or damage, it
flows freely.
19. cont
• It may be entrapped in foods such as pectin
gels, fruits, vegetables, and so on.
o Entrapped water- although not free flowing,
does have the properties of free water.
o Free and entrapped water together may be
considered bulk water.
20. cont
• Entrapped water has properties of free water and
no properties of bound water.
• Bound water usually is defined in terms of the
ways it is measured; different methods of
measurement give different values for bound
water in a particular food.
21. cont
o This water will behave almost like pure water
during food processes.
o It is easily removed by drying, easily
converted to ice during freezing, and available
as a solvent.
22. Some characteristics of bound water include:
– It is not free to act as a solvent for salts and sugars.
– It can be frozen only at very low temperatures
(below freezing point of water).
– It exhibits essentially no vapor pressure.
– Its density is greater than that of free water
23. Moisture content
o The amount of free water in a food medium.
o The amount of free water is important for
growth of microorganisms.
24. cont
• If there is lack of free water microorganisms
will not grow.
• Water activity is defined as the vapour
pressure of a food substance to that of water at
the same temperature.
• (Aw = VPFood/VPWater)or Aw = P/P0
25. cont
o The water activity of pure water is equal to 1.0
o Food products have a water activity of less than 1.0.
o A saturated salt solution has a water activity of 0.75.
o Salting and drying reduces the water activity of a food
product
26. Water activity of some food products
Food Product Water activity
Raw meat and milk 0.99- 1.0
Luncheon meat 0.95
Boiled ham, sliced bacon 0.90
Dried grains 0.80
27. cont
• Growth of microorganisms is greatly affected by
the level of water activity (Aw) in the food.
• Inhibition of growth occurs if the water activity
for food is lowered beyond an organism’s
minimum level of water activity that is
necessary for growth.
28. cont
• Microorganisms have varying minimum water
activity requirements that supports their
growth in food.
29. Minimum water activity that supports growth of
some microorganisms
Microorganism Water activity
Clostridium botulinum,
Bacillus cereus,
Pseudomonas aeruginosa,
Salmonella spp.
0.95
0.95
0.95
0.95
Staphylococcus aureus (anaerobic),
Candida spp., Saccharomyces
0.90
Staphylococcus aureus (aerobic) 0.86
Penicillium spp. 0.82
Most spoilage yeast 0.88
Most spoilage molds 0.80
Osmotic yeast 0.70
30. cont
o Water activity is a measure of how efficiently the
water present can take part in a chemical or
physical reaction.
o If half the water is so tightly bound to a protein
molecule that it could not take part in a hydrolysis
reaction the overall water activity would be
reduced.
31. cont
o The tightly bound water has no tendency to
escape from a food as a vapor and therefore
exerts no partial pressure and has an effective
water activity of zero.
32. cont
• Concentration and dehydration processes are
conducted primarily for the purpose of:
– decreasing the water content of a food
– simultaneously increasing the concentration of
solutes and
– decreasing perishability.
33. cont
o Water activity refers to the water in the food
that is available (free) to support microbial
growth.
o It is measured with a water activity meter in a
scale from 0 to 1.
34. cont
o Foods with values below 0.85 are non-
hazardous regardless of their acidity, because
they do not support the growth of harmful
bacteria.
Examples are dried and semidried products.
36. cont
• A critical factor that determines the stability or
shelf life of foods.
• Most bacteria, for example, do not grow at
water activities below 0.91, including pathogens
such as Clostridium botulinum.
37. cont
• Below 0.80 most molds cannot be grown and
below 0.60 no microbiological growth is
possible.
• However, there remain a number of food
spoilage microbes that can grow within the
range 0.8 - 0.6
38. cont
• E.g. Staphylococcus aureus, a common food
poisoning organism, can grow down to this
relatively low water activity.
• Intermediate-moisture foods, which have aw
values between 0.6 and 0.9, have drawn
considerable attention because they are
palatable without the need to rehydrate them.
39. Table: typical growth limits function of wa
aw= 0.91-0.95 = most bacteria
aw = 0.88 = most yeast
aw = 0.80 = most mushroom
aw = 0.75 = halophile bacteria
aw = 0.70 = osmiophil yeast
aw = 0.65 = xerophile mushroom
40. cont
• Foods which have high level of water activity, the
shelf life is limited mainly by microbiological
activity.
• Products with aw levels below about 0.70 may well
be stable microbiologically and consequently have
a longer shelf life, but enzyme related breakdown
processes is occurred.
41. cont
• It is mainly determine chemical reactions that
affects the quality and stability of these foods.
• Water activity control is an important factor for
the chemical stability of foods.
42. cont
• Most foodstuffs contain carbohydrates and proteins
and are therefore subject to non-enzymatic browning
reactions (Maillard reaction).
• The Maillard reaction gets stronger at increasing aw
values and reaches its peak at aw = 0.6 to 0.7 with
further increase of aw this reaction gets rapidly
weaker.
43. cont
• Most enzymatic reactions are slowed down at aw
values below 0.8.
• Some of these reactions occur even at very low
aw values.
• However, as many foodstuffs are thermally treated
during their processing, enzymatic spoilage is
usually of very little importance.
45. SORPTION PHENOMENA
• Below moisture content of about 50 % the
water activity decreases rapidly and the
relationship between water content and relative
humidity (water activity) is represented by the
sorption isotherms.
46. cont
• Sorption is a physical and chemical process by which
one substance becomes attached to another.
• Absorption – the incorporation of a substance in one
state into another of a different state (e.g., liquids being
absorbed by a solid or gases being absorbed by a
liquid).
47. cont
• The adsorption isotherms are required for the
observation of hygroscopic products and the
desorption isotherms are useful for
investigation of the process of drying.
• The adsorption and desorption processes are
not fully reversible.
49. cont
• Sorption isotherms usually have a sigmoid shape and
can be divided in to three areas that correspond to
different conditions of the water present in the food.
• As water is added (resorption), sample composition
moves from Zone I (dry) to Zone III (high moisture)
and the properties of water associated with each zone
differ significantly.
.
50. Note:
– Water present in Zone I of the isotherm is most
strongly sorbed and least mobile
– Water present in zone II is slightly less mobile than
bulk water
– Zone III water is freezable, available as a solvent,
and readily supports the growth of microorganism.
– It is referred to as bulk-phase water.
51. Hysteresis
• The SI prepared by addition of water (resorption or
adsorption) to a dry sample will not necessarily be super
imposable on an isotherm prepared by desorption.
This lack of superimposability is referred to as
“hysteresis”.
• SIs of polymers, glasses of low molecular- weight
compounds, and many foods exhibit hysteresis.
52. cont
o The magnitude of hysteresis, the shape of the curves, and
the inception and termination points of the hysteresis loop
can vary considerably depending on factors such as:
– nature of the food,
– the physical changes it undergoes when water is
removed or added,
– temperature,
– the rate of desorption, and
– the degree of water removal during desorption.
54. aw and food stability
• aw is:
– A critical factor that determines the stability or shelf
life of foods.
– Most bacteria, for example, do not grow at water
activities below 0.91, including pathogens such as
Clostridium botulinum.
– Below 0.80 most molds cannot be grown and below
0.60 no microbiological growth is possible.
55. cont
– However, there remain a number of food spoilage
microbes that can grow within the range 0.8 - 0.6.
Eg. Staphylococcus aureus, a common food poisoning
organism, can grow down to this relatively low aw.
– By measuring aw, it is possible to predict which mos will
and will not be potential sources of spoilage.
– Further, aw can play a significant role in determining the
activity of enzymes and vitamins in foods and can have a
major impact on their color, taste, and aroma.
56. Note:
–Many preservation processes attempt to
eliminate spoilage by lowering the
availability of water to mos.
–Reducing the amount of free--or unbound--
water also minimizes other undesirable
chemical changes that occur during storage.
57. cont
– The processes used to reduce the amount of free
water in consumer products include techniques like
concentration, dehydration and freeze drying
(lyophilization).
– Freezing is another common approach to controlling
spoilage.
– Water in frozen foods is in the form of ice crystals
and therefore unavailable to mos and for reactions
with food components.
