PPT series on Food and Nutrition..@2 Description of Water and its role in foods as a part of nutrients....

1. By NK
WATER
as a nutrient in foods

2. Goals
• Definition
• Structure of water and ice
• Types of water
• Water activity and sorption
• Water activity and shelf life
• Water activity and packaging

3. Definition of water in food
• It is a chemical compound composed of two hydrogen atoms and one
oxygen atom.
• Liquid state – water, solid state – ice and gaseous state – steam.
• The physical and chemical stability as well as growth of microbes in a
food can be controlled by optimizing the water within foods by
various methods.
• Water content is measured by drying, infrared or nuclear magnetic
resonance techniques.
• The measurement of water activity is more relevant in understanding
the quality and safety aspects of water.

4. Structure of water and ice
• Physical properties of water:-
• High melting point of 00C & boiling point of 1000C.

5. Cont..
• Each molecule of water is composed of 2 H-atoms & 1 O2-atom.
• Each H-atom is bound to central oxygen atom by a pair of e-s, covalent bond.
• Two electrons out of 6 in outer shell of a water molecule engaged in covalent
bonding while remaining e-s arranged far away providing tetrahedral
geometry.
• H-atoms and O2- atoms arranged at 104.50 angle.
• Ice has a well defined structure where each H2O molecule is surrounded by
four neighbouring water molecules.
• 2 molecules are bound to O2 atom directly & other two are bound to H-
atoms. This bridging breaks when ice melts leading to the formation of water.

7. Types of water
• Bound water – the part of the total water bound by water soluble
compounds such as sugar & salt or substrates that can forma matrix thus
making it unavailable to any physical, chemical & microbial activity. Major
proportion. Its measurement is important in predicting reactive nature of
water in foods.
• Free water - the proportion pf water inside foods that is available for such
chemical reactions and growth of microbes. Its measurement is important
for understanding and analyzing the chemical and microbial stability of a
food than its total water/moisture content.
• Water content in food can be measured by gravimetric method where food
is dried till moisture content is evaporated fully followed by measurement
of the dry matter.

8. Water activity and sorption phenomena
• Water activity is defined as the ratio of the vapour pressure of water in a
substance to the vapour pressure of pure water.
Formula, aw = p/p0
aw – water activity of a food, P – partial pressure of water
Po – vapour pressure of pure water in the same temperature.
• Water activity ranges from 0 to 1 where 1 is water activity of pure water.
• Water activity defines the efficacy of water in the food to take part in
chemical reactions.
• Higher the proportion of bound water lower will be the water activity.

9. Cont..
• It depends on temperature, boiling & freezing point, equilibrium relative
humidity and osmotic pressure.
• Relative humidity (RH) of a food product is the humidity of air surrounding
food.
• When this is in equilibrium with the environment, k/a equilibrium relative
humidity (ERH) at which food product does not gain nor loss water
• RH > ERH, gains moisture & RH < ERH, loss moisture.
• aw=ERH/100
• Adsorption :- product – wet, gains moisture
• Desorption :- product – dry, loss moisture

10. Sorption isotherm for a typical food product..

11. Cont..
• Water sorption isotherms usually have a sigmoid shape curve which is attributed
to three types of water:-
I – strongly bound monolayer of water molecules,
II – weak but hard to remove multiple layers of water,
III – loosely bound multilayered water condensed in the capillaries and pores of
the food material.
• Hygroscopic products – steep sorption isotherm & non hygroscopic – flat
isotherm.
• Water activity can predict safety and stability w.r.t. microbial growth, chemical &
biochemical reaction rates, and physical properties.
• Measurement can be done by using a resistive electrolytic hygrometer,
capacitance hygrometer or a dew point hygrometer.

12. Water activity and shelf life
• Moisture content and water activity affect te progress of chemical
and microbiological spoilage reactions in foods which are:-
1. Microbial growth – microbes growth can be checked by controlling
moisture, optimum pH, right temperature and nutrients; and by
increasing shelf life.
• Foods with high aw are highly perishable.
• Dried or freeze dried foods – moisture 5-15% -, good shelf life
• Intermediate moisture foods – 20-40% moisture & aw > 0.5
• Reactions such as gas production, syneresis and changes in color or
viscosity can indicate changes in the stability of the product.

13. Cont..
2. Chemical deterioration of foods – can be due to three major
reactions:-
a. Oxidation of lipids
b. Enzymatic degradation
c. Non-enzymatic browning.

14. Water activity and packaging
• The shelf life of a packaged food is determined by the rate of
exchange of moisture through the package and rate of change in aw.
• Hygroscopic products – packaging material moisture proof.
• Non Hygroscopic products – polyethylene containers as reactions do
not occur at normal storage conditions.

15. Thank you
for
attention!