Control of water activity

1. OBT554
Unit 3
Dr K.Geetha
Associate Professor, Dept of
Biotechnology, Kamaraj College of Engg
& Tech, Madurai

4.  The influence of food water content on
perishability has been known since ancient
times.
 Between 15,000 and 10,000 BC our ancestors
began to preserve excess fish, meat and fruit
by drying in the wind and sun.

5. *The unbound or free water
molecules in the food is
referred to water activity.
*This unbound water
molecules aid in the
growth of microbes-
bacteria, fungi etc..
*It’s value ranges from 0 to
1.

7. The food get spoiled on
microbial growth over the
fruits and vegetables having
water activity greater than
0.95.

8. The moisture condition of a product can be measured as the equilibrium
relative humidity (ERH) expressed in percentage or as the water
activity expressed as a decimal.
*
*

9.  aw is equal to f/fo, where f is the fugacity of
water in the system (i.e., the escaping
tendency of water from solution) and fo is
the fugacity of pure water at the same
temperature.
 Water activity depends on water
concentration as shown by equation:
 Where is the mole fraction and is the
activity coefficient of the water

11.  The optimum aw for growth of the majority of
microorganisms is in the range 0.99±0.98.
 Every microorganism has limiting aw values below which it
will not grow, form spores, or produce toxic metabolites.
 Minimum aw for growth is always equal or lower than
minimum aw for toxin production.
 Water activity reduction through curing, conserving,
drying, evaporation is one important method for food
preservation.

13.  Vapour pressure manometers
 Dew point hygrometer
 Freezing point depression methods
 Electric hygrometers
 Fibre hygrometer

15.  aw can be influenced in at least three ways
during the preparation of dried, intermediate
and high moisture foods:
1. Water can be removed by a dehydration,
evaporation or concentration process.
2. Additional solute can be added.
3. Combining 1. and 2. when the food pieces are
infused with the solutes and additives and then
partially dried.

17.  At various stages of the food distribution
chain, it is used during storage, processing,
and/or packaging.
 Used for obtaining long shelf-life products.
 Used to obtain high moisture novel foods

18. The spoilage due to water activity can be
controlled by,
*Drying
*canning
*Freezing

20. The canned food has good shelf
life time.

22.  The free water molecules in the fruits are
removed.
 Suitable for juicy fruits like grapes.
 Grapes are dried to get raisin.

24.  The food is cooled to temperature at which
microbial growth is deterioted.
 Unbound water molecules get inactive.
 Example : freezing of sea food to avoid
spoilage during transportation.