Role of water and water activity in foods

• The shelf-life of foods is limited by the activity of microorganisms.

• Food preservation operations such as concentration, dehydration and freezing reduce the availability of water to microbiological activity.

• Other preservation processes like adding solutes (sugars or salts) also reduce the availability of water to microorganisms.

• Water also has a profound influence on the physical and chemical processes, which influence shelf life.

• However, extensive studies on food properties and reactions in foods have established that water content as such (per se) is not an adequate predictor of food stability.

Importance of water in foods

• The concept of water activity (aw) is more than 50 years old.

• William James Scott showed in 1953 that microorganisms have a limiting aw level for growth.

• It is now generally accepted that aw is more closely related to the microbial, chemical, and physical properties of foods than is total moisture content.

• Next to temperature, aw is considered one of the most important parameters in food preservation and processing (van den Berg 1986).

• The aw of a food describes the energy status of the water in that food, which is an indicator of the availability of water to act as a solvent and participate in the chemical/bio-chemical reactions.

Significance of water activity

• Water activity determines the shelf-life of the product.

• Aw affects the enzyme activity and stability of foods. Most enzymatic reactions are slowed down at water activities below 0.8.

• Aw also affects the stability, flow and caking of milk powder and other powdery food materials during storage.

• Aw also affects the textural properties of foods.

• Water activity is defined by:aw =P/Pw

where P =equilibrium partial vapour pressure of water in food at a given temperature

Pw= vapor pressure of pure water at the same temperature.

Thus, water activity is a thermodynamic entity.

Product Aw at 25ºCDried milk products 0.1-0.3Butter, unsalted 0.99Butter, salted 0.91-0.93Sweetened condensed milk 0.77-0.85Hard cheese 0.86-0.97Soft cheese 0.96-0.98Cream 0.99Frozen desserts 0.98-0.99Fermented milk products 0.97-0.99Milk and whey 1.0Khoa/Pedha 0.96Paneer 0.99Kunda 0.86

Approximate water activities of selected dairy products

Figure. Food stability map(Adopted from Barbosa-Cánovas et al 2007)

Food Stability Diagram

• The effects of aw on growth of microorganisms and food stability can be described using “stability maps” showing relative rates of deteriorative changes against aw.

• It is obvious that water is required for the growth of microorganisms in addition to required environmental parameters and nutrients.

• It is evident that some reactions exhibit extremely low rates at low water activities, but their rates increase above a critical aw as diffusion becomes enhanced.

• However, the reaction rates decrease at high water activities,possibly as a result of dilution.

• These reactions also are affected by the glass transition of amorphous foods because the molecular mobility is affected by the glass transition.

• Food stability maps also can relate glass transition-dependent changes with aw.

• However, the rates of glass transition-dependent changes are affected by food composition rather than the presence the presence of moisture.

• They may occur with no water present in the solids or at water activities corresponding to the critical aw of the particular material.

• This Tg-water sorption plot is useful for the selectionof storage conditions for low- and intermediate-moisture foods.

• Therefore, it is essential to establish the stability maps showing critical values for aw allowing sufficient molecular mobility to result in structural transformation, increased rates of deteriorative reactions and component crystallization.

Aw Bacteria Yeasts Molds0.97-0.98 Clostridium botulinum, Pseudomonas - -0.96 Lactobacillus, Proteus,

Flavobacterium, Shigella- -

0.95 Clostridium perfrigens, Enterobacter, Escherichia, Salmonella

- -

0.94 Bacillus, Microbacterium, Streptococcus

- Stachybotrys

0.93 B. stearothermophilus, Micrococcus - Botrytis, MucorRhizopus

0.92 - Saccharomyces -0.90 B. subtilis, Staphylococcus aureus0.88 - Candida Cladosporium0.86 S. Aureus (aerobic), Vibrio Hansenula,

Saccharomyces-

0.84 - - Alternaria, Aspergillus0.83 Staphylococcus Debaryomyces Pencillium0.75 Halobacterium, Halococcus Aspergillus0.70 - - Aspergillus

Chrysosporium0.62 - Saccharomyces Eurotium

0.61 - - Monascus

Minimal Aw requirement for multiplication of microorganisms

Adopted from the compendium on “Advances in packaging of dairy and food products”, NDRI, Karnal.