Water in food

• Unusual properties of water

• Different types of water (Bulk water and

bound water)

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