Consider the solubility of different food
products.
Eg: Non fat and Full cream milk powder
Non fat milk powder dissolves better as it has
high amount of constituents which can form
hydrogen bonds with water.
Spray dried milk powder and freeze dried milk
powder are considered, in freeze dried milk
powder changes to constituents forming
hydrogen bonds are less (Low temperature)
Relationship between water activity & moisture
content:
There is no linear relationship as the degree of
tenacity of water binding in different food
systems are not the same.
Sorption isotherms are employed, for most
foods sigmoidal isotherms are observed.
Moisture sorption isotherm (MSI)
• A plot of moisture (water) content versus relative
vapor pressure (P/P0 , aw) at a constant temperature
Information derived from MSI are important in,
- Concentration and dehydration process
- Formulating food mixtures avoiding moisture
transfer among ingredients
- To determine moisture barrier properties needed for
packing material
- To determine the moisture content related to growth
of microorganisms
- To predict chemical and physical stability of foods as
a function of water content
Zone 1- aw < 0.25 (Non freezable water)
Zone 2- aw < 0.25 to 0.8 (Water molecules with restricted mobility)
Zone 3- aw < 0.8 to 0.98 (Mobile water molecules within the tissues
surrounded by membranes)
Desorption and Resorption
isotherms
• Ideally desorption and resorption
isotherms should be superimposable.
• Practically it doesnot happen due to
varying degrees of interaction between
water and food molecules.
Hysteresis
• Moisture sorption isotherms (MSI) can be obtained
by either adsorption or desorption protocols.
• But resorption (addition of water to a dry sample)
and adsorption isotherms of some food samples
are not superimposable.
• The lack of superimposability is recognized as
“hysteresis”.
• Magnitude of hysteresis, the shape of the curves
(hysteresis loop) can vary depending on, nature of
the food, temperature, rate of desorption, degree
of water removed during desorption and physical
changes taking place when water is removed or
added to a food.
• Lack of superimposable nature of
desorption and resorption isotherms is
known as “Hysteresis”.
• This is due to the food constituent holding
an additional amount of moisture during
resorption than corresponding desorption.
• More related to nature and state of food
constituents, which changes structure and
conformation providing exposure of polar
sites.
Hysteresis is important in
(i) Designing and optimization of
equipment for;
• Better drying
• Packaging
(ii) Predicting conditions to improve shelf
life
(iii) Calculating moisture changes during
storage
Water activity and stability of food
Rejection of a food by consumer could be due to,
- Microbial spoilage/contamination/toxin production
- Unfavorable physical nature (Eg. Browning)
- Reduced nutritional quality (Eg. Lipid oxidation)
All are related to water activity.
Water activity and related microorganisms
Type of food Water
activity
Microorganism
Fresh vegetables,
milk, meat, fish
0.97 Salmonella,
Clostridium sp.
Cheese, sugar syrup 0.90 Most bacteria
Cake, rice, cereals 0.85 Most yeast
Jam, pickle 0.75 Most moulds
Dried fruits,
dehydrated foods
0.6 Some fungi
Decreasing water activity only for increasing
keeping quality doesnot provide quality food
as water activity affects the texture of food.
Instead as control measures other
parameters such as temperature, light,
chemical preservatives and addition of other
ingredients are used in combination.
• Other modes of controlling microbial growth
changes minimum water activity.
Eg. addition of sugar/preservatives increases
minimum water activity.
• Manipulation of water activity is not advisable to
improve stability of food because it affects textural
properties.
• Water activity modifies the sensitivity of
microorganisms to heat/light/chemicals.
(When the environment is unfavorable for
microorganisms high water activity minima is
observed.)
.
Eg. 1. Sausages with(a) /without(b) chemical
preservatives.
(a) will have a better texture due to increased
water activity.
2. Toxin production in red chilies.
Two situations: growth of fungi (a) and
production of toxin(b).
(b) is an additional metabolic activity, therefore
toxin production is observed at a higher water activity
than fungal growth.
Effect of water activity on
chemical/biochemical changes in food
1. Enzymatic hydrolysis of lipids.
Water is in the substrate for enzymatic
reactions, enzyme gets diffused into the
substrate.
Diffusivity of the substrate decreases with
decreasing water activity.
Functionality of lipids can be maintained by
decreasing water activity.
2. Non enzymatic browning (Maillard
browning)
• Reducing component (a sugar) and amino
groups are needed for the reaction.
Water acts as a reactant in the beginning of
Maillard browning and is produced during the
reaction.
If water gets accumulated the concentration
of main ingredients of the reaction (NH2 )
decreases, reducing the reaction rate.
3. Non enzymatic lipid oxidation
Metal catalysts are used during oxidation.
Water helps stabilize the hydroperoxides
formed during the reaction.
Water can contribute to swelling of dry food
matrix, thereby contribute to increased oxygen
uptake.
Saturated lipids are less likely to oxidise in
comparison to unsaturated lipids.
(Stearic and Oleic acids)
Decomposition of hydroperoxides reduce the
molecular size. Molecules become volatile,
resulting in rancid odor of food.
Water on the surface make H-bonds with
hydrogen peroxide.
Metal catalysts are getting hydrated or
become hydroxides.
Reduced rate of oxidation.
Swelling of dry matrix, improves oxygen
uptake.
Diffusivity of metal catalysts increases when
they are not hydrated.
Increased rate of oxidation
** Indicates a need of balanced situation.
4. Non enzymatic oxidation of ascorbic acid
Water acts as a medium for both reactants
and products. Favors the reaction. Therefore
at high water activity more ascorbic acid
should be added to the food product.
Ascorbic acid is used as an antioxidant in
many foods.
If oxidation is taking place, sensory attributes
(aroma, flavor) gets changed.
Role of freezing on stability of foods
During freezing textural changes can take
place.
• During freezing temperature gets reduced.
Entropy of system gets decreased.
Reactants get concentrated, rates of non
enzymatic reactions can be increased.
• Enzymatic reactions need an optimum
temperature, reaction rates are affected by
freezing. Cell rupture may increase
reactions
Water becoming ice, increases volume
causing cell rupture.
Intermixing of enzymes and substrate takes
place increasing reaction rates.
(Freeze induced dislocation of enzymes)
• Freezing can be slow (forming large ice
crystals) or fast (forming small ice crystals).
• Freeze concentration
Water gets transformed into pure ice, solute
co-exists with decreasing solvent water,
properties such as pH, Titratable acidity
change significantly.
1.Briefly state the reasons for,
(a) High boiling point and heat capacity of water
(b) Non superimposable nature of desorption and
adsorption isotherms of most food products
(c) Hard texture observed in refrigerated cooked rice
(d) Need of high amount of preservatives for foods
having high water activity
(e) Slight increase in enzyme reaction rates during
freezing
(f) Effective use of water as a solvent in most food
products