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Food Preservation: Water Activity Control

Methods for stabilizing food systems by controlling water activity include drying, freezing, and adding sugar or salt. Drying methods like natural dehydration, traditional drying, freeze-drying, and spray-drying work by removing moisture to lower water activity and prevent microbial growth. Freezing halts bacterial growth without eliminating bacteria. Adding sugar above 68% or salt above 12% also controls water activity through osmosis or plasmolysis, inhibiting microbial growth. These various methods extend shelf life by restricting the availability of water for pathogens and spoilage organisms.

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414 views1 page

Food Preservation: Water Activity Control

Methods for stabilizing food systems by controlling water activity include drying, freezing, and adding sugar or salt. Drying methods like natural dehydration, traditional drying, freeze-drying, and spray-drying work by removing moisture to lower water activity and prevent microbial growth. Freezing halts bacterial growth without eliminating bacteria. Adding sugar above 68% or salt above 12% also controls water activity through osmosis or plasmolysis, inhibiting microbial growth. These various methods extend shelf life by restricting the availability of water for pathogens and spoilage organisms.

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Food & Health
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WATER:

Methods for stabilization of food systems by control of water activity:


Water activity in food has been controlled by drying, addition of sugar or salt and freezing.
These methods prevent spoilage and maintain food quality. The following methods can be
adopted:
1. Natural Dehydration- Natural dehydration is a low-cost method and water is removed by
the heat of sun. It is used to dry grains, fruits, vegetables, fish, meat, and milk especially
in warmer countries. Such drying process is slow and depends on the air conditions.
2. Drying (Traditional, Freeze-Dry, Spray-Dry) Dehydrated foods have a long shelf life
because removal of moisture lowers the water activity to below 0.50 so that spoilage
organisms cannot grow. Fruits and vegetables can be dried and sold as is, or they can be
used in other dry products that have a long shelf life, such as cereal or granola bars.
Traditional drying of foods utilizes heat, air, and time in various processes that allow
removal of moisture to a desired level.
Freeze-drying is a form of dehydration in which the product is frozen and water is
removed as vapor.
Spray-drying is a method that rapidly dries liquid slurry by spraying small droplets into a
heated chamber. Milk that undergoes spray-drying is sold as powdered milk that can be
reconstituted. The reduction of moisture content by heat treatment in drying can be
expensive, depending on the time required. Additionally, there is usually some loss of
product quantity and quality associated with any drying method.
3. Freezing: Freezing can be used to preserve a number of food products. Freezing is a
common food preservation processes that is used to extend the shelf life of foods because
it renders water unavailable for pathogenic or spoilage bacteria. Freezing halts bacterial
growth, but does not eliminate bacteria.
4. Addition of sugar and salt: Sugar in high concentrations acts as a preservative due to
osmosis. Sugar attracts all available water and water is transferred from the
microorganisms into the concentrated sugar syrup. The microflora is dehydrated and
cannot multiply further. The concentration of sugar in sugar preserved products must be
68 per cent or more, which does not allow microorganisms to grow. Lower
concentrations may be effective but for short duration unless the foods contain acid or
they are refrigerated. The critical concentration of sugar required to prevent microbial
growth varies with the type of microorganisms and the presence of other food
constituents. With the use of sugar, the water activity cannot be reduced below 0.845.
This level of water activity is sufficient to inhibit mesophillic bacteria and yeast but does
not check mould attack. Salt acts as preservative when its concentration is increased
above 12 per cent. Salt levels of about 18 to 25 per cent in solution generally will prevent
all growth of microorganisms in foods. However, this level is rarely tolerated in foods
except in the case of certain briny condiments. Salt exerts its preservative action by
plasmolysis of microbial cells due to high osmotic pressure, drawing moisture from
microbes, ionizing to yield chloride ion, which is harmful to microorganisms, reducing
the solubility of oxygen to water, sensitizing the cells against carbon dioxide and
interfering with the action of proteolytic enzymes.

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