Fundamentals of Water Activity
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ater is the main ingredient in many foods. It has a significant effect on a food’s microbial, chemical, and structural stability. To achieve consistent, reliable quality in every batch and product, food manufacturers need a scientifically accurate way of measuring water relationships. In many cases, water activity is the best measurement to use. Fundamentals of Water Activity T his paper discusses the scientific origins and attributes of water activity, reviews the regulations which specify water activity as a measurement, and details some of the most common applications using water activity. u

Water Activity and Growth of Microorganisms in Food* Range of aw Microorganisms Generally Inhibited by Lowest aw in This Range Foods Generally within This Range Pseudomonas, Escherichia, Proteus, Shigella, Klebsiella, Bacillus, Clostridium perfringens, some yeasts Highly perishable (fresh) foods and canned fruits, vegetables, meat, fish, milk, and beverages Salmonella, Vibrio parahaemolyticus, C. botulinum, Serratia, Lactobacillus, Pediococcus, some molds, yeasts (Rhodotorula, Pichia) Some cheeses (Cheddar, Swiss, Muenster, Provolone), cured meat (ham), bread, tortillas Many yeasts...

Water Activity: Measuring Energy n 1953, William James Scott showed that microbial growth in food is governed not by water content, as most people thought, but by water activity. Four years later, he established the concept of a minimum water activity for microbial growth. Water activity is now routinely used by food manufacturers to determine whether or not a product is susceptible to microbial proliferation. Scott’s work is relevant to every product from fresh tree nuts and wheat berries to processed cheese and pharmaceuticals. The microbial growth limits he and his colleagues established...

Table A. Interaction of pH and aw for control of spores in food heat-treated to destroy vegetative cells and subsequently packaged. * PHF: Potentially Hazardous Food ** TCS: Time/Temperature Control for Food Safety *** PA: Product Assessment Required Table B. Interaction of pH and aw for control of vegetative cells and spores in food not heat-treated or heat-treated but not packaged. aw Values PHF: Potentially Hazardous Food ** TCS: Time/Temperature Control for Food Safety *** PA: Product Assessment Required 2009 Food Code Definition of Potentially Hazardous Foods Water activity is the only...

What is Water Activity? et’s use a thought experiment to better understand water activity. Take a glass of water, and a dry sponge. Dip the corner of the sponge into the glass of water. The water will, of course, move from the glass into the sponge. What is the difference between the water in the glass and the water in the sponge? The answer is that the water in the glass is free, while that in the sponge is, to some extent, bound. It has a lower energy state than the water in the glass. We know that, because to retrieve the water from the sponge we need to do work on it (squeeze the...

The ratio p /p0 is called the water activity, aw, when we are talking about the water in the sponge, or water in foods or other solids or liquids. We call it the relative humidity when we apply it to water in the air, and sometimes multiply it by 100 to express it as a percent. The ratio U/n is the energy per mole of water and is called the water potential, with the symbol y. Water potential has units of Joules/mole. With this substitution we finally arrive at the equation relating the energy of the water in the sponge and its water activity ψ The equation tells us that we can express the...

Measurement of How do we measure water activity? The equation we just derived also provides a convenient way of measuring water potential or water activity. If we enclose a sample in a sealed container the relative humidity of the head space will equilibrate with the water activity of the sample. At equilibrium the two will be equal, and we can measure the relative humidity of the head space to know the water activity of the sample. Early water activity meters used this method. Primitive hygrometers used changes in length of hair or swelling and shrinking of specially prepared membranes to...

Chemical Stability hemical reactions can affect the taste, appearance, and nutritional value of food products. They can also reduce the effectiveness of active ingredients in pharmaceuticals. Water can affect these chemical reactions in several ways. It can be a reactant, it can act as a solvent, and it can affect the rate of diffusionlimited reactions by changing the molecular mobility of the reactants. Chemical reactions can be complex and the mechanisms are not always completely understood. However, a food’s chemical stability often correlates better with water activity than with...

Physical Stability ater plays a critical role in the stability of foods near glass transition. In these foods, changes in water can affect both structure and texture. The result of structural degradation can be seen in phenomena like crystallization, caking, deliquescence, and collapse. Textural degradation results in effects such as loss of crispness, hardening, and stickiness. It is difficult to separate the effects of water content, water activity, and temperature on the physical stability of foods. Physical stability is best understood through a moisture sorption isotherm—a graph of the...

How To Use Water Activity Data PREDICT AND MAXIMIZE SHELF LIFE, MAKE PACKAGING DECISIONS, AND AVOID DEGRADATIVE REACTIONS. STABILITY Microbial REACTIONS ending shelf life Pathogenic Bacteria Yeasts Mold Vitamin Degradation Loss of Active Ingredient Maillard Browning Reaction Lipid Oxidation Enzyme Stability INPUTS modeling data OUTPUTS model prediction Microbial counts under different environmental conditions including water activity. Days until growth or growth rate. Concentration data of measurable attribute related to the reaction taken at different temperatures and water activities....