Water activity

Water activity is the moisture content of the food which is available for microbial growth.By controlling water activity the food can be preserved for longer duration

1. SUBMITTED TO: Er . Vikas bansal SUBMITTED BY: AAKRITI SINGH SU13147 WATER ACTIVITY

2. WATER IN FOOD All foods contain at least some water Free water Held inside cells Maintains properties of free water May be removed by pressure Bound water Is part of molecule structure Reduced mobility Does not retain properties of free water

3. WATER ACTIVITY water activity is the amount of free or available water in a product as opposed to bound water. water activity is the measure of the energy status of the water in a system. Ratio of the vapor pressure of water in a food at a specified temperature to the vapor pressure of pure water at same temperature.

4. WHY WATER ACTIVITY IS IMPORTANT Water activity (aw) is one of the most critical factors in determining quality and safety of foods. Water activity affects the shelf life, safety, texture, flavour, and smell of foods. While temperature, pH and several other factors can influence if and how fast organisms will grow in a product, water activity may be the most important factor in controlling spoilage. Most bacteria, for example, do not grow at water activities below 0.91, and most molds cease to grow at water activities below 0.80.

5. Contd.. By measuring water activity, it is possible to predict which microorganisms will and will not be potential sources of spoilage. Water activity--not water content--determines the lower limit of available water for microbial growth. In addition to influencing microbial spoilage, water activity can play a significant role in determining the activity of enzymes and vitamins in foods and can have a major impact their color , taste, and aroma.

6. WATER ACTIVITY MEASUREMENT Resistive Electrolytic Hygrometers Capacitance Hygrometers Dew Point Hygrometers

7. Resistive electrolytic hygrometers C onsist of potentiometer, sample holder, and sensor with immobilized electrolyte (e.g., lithium chloride); C hanges in ERH are reflected in changes in conductance of current through sensor (Beckman, Rotronic); T ypically slow and requires routine calibration with standards

8. Capacitance hygrometers Capacitance hygrometers consist of two charged plates separated by a  polymer membrane dielectric. As the membrane adsorbs water, its ability to hold a charge  increases and the capacitance is measured. This value is roughly proportional to the water activity as determined by a sensor-specific  calibration. Capacitance hygrometers are not affected by most volatile  chemicals. They do not require cleaning, but are less accurate than dew point hygrometers (+/- 0.015 a w ).

9. Dew point hygrometers Use a cooled mirror as condensing surface; mirror is cooled Condensation occurs Temperature = dew point; ERH is derived from psychrometric chart (automatically) V ery fast and accurate C alibration is not needed

10. FACTORS EFFECTING Drying:  Water activity is decreased by physically removing water (Ex: beef jerky). Solutes:  Water activity is decreased by adding solutes such as salt or sugar (Ex: jams, cured meats). Freezing:  Water activity is decreased by freezing (Ex: water is removed in the form of ice). Combination:  One or more of the above can be combined for a greater influence on water activity (Ex: salting and drying fish).

11. Moisture Content Water content  or  moisture content  is the quantity of water contained in a material. Water activity is related to water content in a non-linear relationship known as a moisture sorption isotherm curve. These isotherms are substance and temperature-specific. Isotherms can be used to help predict product stability over time in different storage conditions.

12. MICROBIAL GROWTH Microorganisms Aw value Gram negative bacteria 0.91 Gram positive bacteria 0.86 Yeast 0.88 Production of mycotoxins 0.80 Molds 0.70 Osmophillic yeast 0.62 Xerophillic Molds 0.61 Absolute limit for all growth 0.60 No microbial proliferation 0.50

13. Minimum aw for some microorganisms BACTERIA Staphylococcus aureus halophilic bacteria 0.86 Halobacterium spp. 0.75 MOLDS Aspergillus flavus 0.78 Chrysosporium fastidium 0.69 Xeromyces bisporrus 0.61 YEASTS Debaryomyces hansenii 0.83 Torulopsis spp. 0.70 Zygosaccharomyces bailii 0.80 Z ygosaccharomyces rouxii 0.62

14. Examples of aw values of several foods fresh, raw fruits, vegetables, meat, fish >0.98 Coked meat, bread 0.91-0.95 Cured meat, cheeses 0.91-0.95 Fermented sausages 0.83-0.87 jams 0.75-0.80 Honey 0.75 Dry cereals 0.65-0.75 Pastry fillings 0.65-0.71 Candies 0.60-0.65 Sugars, syrups 0.60-0.75 Cake and pastries 0.60-0.90 Dried fruits 0.60-0.75 Powdered milk, dried pasta, spices 0.20-0.60

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