SUBMITTED TO:ER. VIKAS BANSAL

SUBMITTED BY:AAKRITI SINGH

SU13147

WATER ACTIVITY

WATER IN FOOD

All foods contain at least some waterFree 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

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.

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.

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.

WATER ACTIVITY MEASUREMENT

Resistive Electrolytic HygrometersCapacitance HygrometersDew Point Hygrometers

Resistive electrolytic hygrometers

Consist of potentiometer, sample holder, and sensor with immobilized electrolyte (e.g., lithium chloride);

Changes in ERH are reflected in changes in conductance of current through sensor (Beckman, Rotronic);

Typically slow and requires routine calibration with standards

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 aw).

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) Very fast and accurate Calibration is not needed

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).

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.

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

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

Zygosaccharomyces rouxii 0.62

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

THANK YOU