You might also like: Factors That Affect Growth of Harmful Microorganisms The specie Staphylococcus aureus is a notable exception, which can grow at 0.86 a w. Pathogenic bacteria, the ones that cause foodborne illnesses, do now grow well below 0.93 a w. Most of these, particularly spoilage bacteria, grow at above 0.90 a w. They are the main reason of spoilage in fresh vegetables and meat. Generally, bacteria require a higher water activity than most yeasts and molds. While some types of microorganisms require an environment with high amount of unbound water, others can survive in an environment of low water activity. The water activity requirement for microorganisms to thrive vary as well. See below for the list of water activity values of some common foods. And the water activity of dried or dehydrated foods ranges between 0.60 to 0.66. Perishable food products such as fresh vegetables, fruits, and meats have a water activity of 0.98 to 1.00. Pure water has a water activity of 1.0, all is readily available for use. The values of water activity range from 0 to 1.0. You might also like: 5 Main Functions of Salt (Sodium Chloride) In Food For example, sugars and salts are more capable of lowering the vapor pressure than proteins and starch (larger molecules.) It is why sugars (jams) and salts (dried fish) are effective antimicrobial agents. The amount of pressure depends on the water available, the food composition and temperature. Hence, water activity is the available water for many chemical, physical, enzymatic, and more specifically microbiological reactions.Īnother definition of water activity is the ratio of the amount of vapor pressure exerted by food ( P s) to the saturated vapor pressure of pure water at the same temperature (P w). While water activity is the amount of unbound or free water. Moisture content refers to the amount of water in the food. More often, students think water activity and moisture content are the same thing. But before we get to that, let’s define first what water activity is. Water activity (a w) is one of the few important parameters that dictate the shelf life stability of food-a food safety concern.
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