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Water activity6/13/2023 ![]() Pre-rigor state, which gives the meat greater water holding and binding capacity.Īdding acids such as lactic acid (fermented milk) and citric acid (lemon juice) not only give products flavour, but also increase shelf life. The addition of phosphates to meat corrects the pH to a The non scientific meaning of pH is the “potential hydrogen” or the concentration of hydrogen ions in a given solution the stronger the solution the smaller the number ProductĪs you can see a small difference in the pH number indicates a large change in its concentration. The pH is a measurement of acidity in a given solution. This is why there are specific guide-lines for product like summer sausage having a pH of less than 5.2 and a Aw of less then 0.91. Food can be made safe to store by lowering the water activity to a point that will not allow dangerous pathogens such as Clostridium botulinum and Staphylococcus aureus to grow in it. This can effectively increase the product’s stability and make it possible to predict its shelf life under known ambient storage conditions. Water activity (Aw) has its most useful application in predicting the growth of bacteria, yeasts and molds.įor a food to have a useful shelf life without relying on refrigerated storage, it is necessary to control either its acidity level (pH) or the level of water activity (Aw) or a suitable combination of the two. For example a commercial shelf life over twelve months might be required for confectionery in these circumstances an aw below 0.6 would be required.Water Activity – Why do we need to control it? Ma11:05 am A Close Look at Water in your Products While this will not ensure complete freedom from microbial spoilage, those few yeasts and moulds which do grow at lower water activities need only to be considered when special shelf life conditions must be met. Most moulds cease to grow at a water activity level below about 0.8, but since some moulds will grow slowly at this aw, it is usually recommended that products of this type do not have an aw greater than 0.75. When these foods spoil, it is usually the result of surface mould growth. These foods can be described as semi-moist and include fruit cakes, puddings and sweet sauces such as chocolate and caramel. However many foods can be successfully stored at room temperature by proper control of their water activity (a W). These include most fresh foods and many processed foods such as soft cheeses and cured meats. Semi-moist foodsįor foods with a relatively high water activity correct refrigeration is always necessary. Foods which may support the growth of this bacterium include cheese and fermented sausages stored above correct refrigeration temperatures. Staphylococcus aureus, a common food poisoning organism, can grow down to this relatively low water activity level. The risk of food poisoning must be considered in low acid foods (pH > 4.5) with a water activity greater than 0.86 aw. We also know that Clostridium botulinum, the most dangerous food poisoning bacterium, is unable to grow at an aw of. For example we can see that food with a water activity below 0.6 will not support the growth of osmophilic yeasts. The diagram illustrates the water activity (a W) levels which can support the growth of particular groups of bacteria, yeasts and moulds. This can effectively increase the product's stability and make it possible to predict its shelf life under known ambient storage conditions.įood can be made safe to store by lowering the water activity to a point that will not allow dangerous pathogens such as Clostridium botulinum and Staphylococcus aureus to grow in it. ![]() Water activity (a W) has its most useful application in predicting the growth of bacteria, yeasts and moulds.įor a food to have a useful shelf life without relying on refrigerated storage, it is necessary to control either its acidity level (pH) or the level of water activity (a W) or a suitable combination of the two. Thus a food with a water activity (a W) of 0.7 would produce an ERH of 70%. If we multiply this ratio by 100, we obtain the equilibrium relative humidity (ERH) that the foodstuff would produce if enclosed with air in a sealed container at constant temperature. ![]() The water activity (a W) represents the ratio of the water vapour pressure of the food to the water vapour pressure of pure water under the same conditions and it is expressed as a fraction. Water activity is in practice usually measured as equilibrium relative humidity (ERH). The water activity scale extends from 0 (bone dry) to 1.0 (pure water) but most foods have a water activity level in the range of 0.2 for very dry foods to 0.99 for moist fresh foods.
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