Water Activity in Foods Page 1 Safefood 360° is the provider of food safety management software for industry leading businesses. WHITEPAPER Water Activity (aw) in Foods CONTENTS SUMMARY 1. Introduction 2. The Structure of Water 3. Water In Foods 3.1 Water and Food Safety 3.2 Water Activity (aw) 3.3 Effects of Reduced aw on Food Safety Water is the most abundant 3.4 To Reduce aw constituent of food and in terms of food safety the most significant. Its pres- 4. Measure of Water Content and aw ence, quantity and nature 4.1 Relationship of Water Activity and Water determines many chemical 4.2 Analysis of Water Content and biochemical processes important for the control of 4.3 Analysis of Water Activity product safety and quality. 4.4 Water Activity as a CCP A better understanding of water activity aw can assist in developing robust and scientifically supported food safety plans. Safefood 360, Inc. 2014 Part of Our Professional Whitepapers Series Water Activity in Foods Page 2 1. Introduction At standard temperature and pressure water is a liquid. It is also tasteless and Water is the most abundant constituent odorless. The molecule itself is non-linear of food and in terms of food safety the and polar with an electrical dipole most significant. Its presence, quantity movement. It is a good polar solvent and and nature determines many chemical and is typically referred to as the universal biochemical processes important for the solvent. control of product safety and quality. Hydrophilic (water-loving) substances In many HACCP plans, water is frequently dissolve readily while hydrophobic (water- referred to as an intrinsic parameter fearing) substances are immiscible. At required in product safety and while most standard temperature and pressure the of us understand its importance we often boiling point of water is 100 °C (212 °F). fail to understand the reasons why. A better understanding of water, and in The density of liquid water is 1,000 kg/m3 particular water activity (aw), can assist (62.43 lb/cu ft) at 4 °C. Ice has a density of in developing robust and scientifically 917 kg/m3 (57.25 lb/cu ft). supported food safety plans. 3. Water In Foods 2. The Structure of Water Water in food is an important parameter in the field of food science and safety. Due The chemical structure of water is H2O. It to its unique role in various chemical and does not exist in an absolutely pure form biochemical reactions in a food matrix an in nature due to its chemical properties. understanding of water is crucial. One molecule of water is composed of two 3.1 Water and Food Safety hydrogen atoms covalently bonded to a single oxygen atom. It commonly occurs One of the oldest forms of preserving foods in all three states, i.e. liquid, solid and gas. is drying or desiccation. When first applied there was little understanding of why removing moisture from food had the effect of extending its shelf life. The table on the following page shows the typical water content of certain food product and categories: Safefood 360, Inc. 2014 Part of Our Professional Whitepapers Series Water Activity in Foods Page 3 Food % Water Food % Water Lettuce, tomatoes 95 Cabbage, broccoli 92 Carrots, potatoes 90 Citrus fruit 87 Apples, cherries 85 Raw poultry 72 Raw lean meat 60 Cheese 37 White bread 35 Salami 30 Preserves 28 Honey 20 Dried fruit 18 Butter 18 Wheat flour 12 Dried pasta 12 Milk powder 4 Beer 90 Fruit juices 87 Milk 87 Over time, an understanding emerged and To understand this more fully, we must today we have a solid scientific recognise that much of the water in food understanding of how water impacts on is bound water, i.e. bound to ions as water product quality and safety. We now know of hydration, or bound to surfaces of large that the water content is less important molecule or cell structures. This water is than the specific behaviour of the water or not free to support microbial growth, or Water Activity. Water activity is related to to participate in or support chemical or water content in a non-linear relationship enzyme reactions and spoilage processes. which is represented using as a moisture The total amount of bound water in a food sorption isotherm curve. has no relation to food stability. 3.2 Water Activity (aw) The free or available water in a food supports microbial growth, and This is the most important parameter participates in and supports chemical and of water in terms of food safety. Water enzymatic reactions and spoilage processes. activity or aw is the partial vapor pressure It is this amount of free water which is of water in a substance divided by the called water activity, aw, and it is more standard state partial vapor pressure of important for food stability, chemical and water. microbial, than total water content. In the field of food science, the standard The factors that reduce the mobility of state is most often defined as the partial water in a food also reduce its tendency vapor pressure of pure water at the to evaporate and its vapour pressure and same temperature. Using this particular this provides a means of defining and definition, pure distilled water has a water measuring it. activity of exactly one. Safefood 360, Inc. 2014 Part of Our Professional Whitepapers Series Water Activity in Foods Page 4 Therefore, we can generally define Water Another chemical reaction effect is Activity (aw) as an indication of the amount Maillard Browning which is maximum at of Free Water in a food. 0.6-0.7. Most enzymes are inactivated at <0.85. At less than 0.75 bacterial growth Specifically Water Activity (aw) is: is inhibited but some yeast and moulds may grow. At less than 0.6 all growth is = Actual Equilibrium v.p. of air-space inhibited. In terms of nutrients, reduced over the food aw reduces losses of Vitamins C, E, B1. The following table indicates the aw of Equilibrium v.p. of pure water at certain foods. same temperature Food Typical aw Alternatively we can define it using Fresh meat 0.98 Relative Humidity (RH): Cheese 0.97 Preserves 0.88 = Actual v.p. water vapour in air x 100 Salami 0.83 Dried fruit 0.76 Equilibrium v.p. of Pure Water at Honey 0.75 Same Temperature Pasta 0.50 Water Activity Table: Water activity values of foods = Relative Humidity of Airspace In general, bacteria require higher Over the Food / 100 values of aw for growth than fungi, with The ideal solution may be written as : gram-negative bacteria having higher requirements than gram-positive. Aw = P = %ERH Organism group a Po 100 w Most spoilage bacteria 0.90 Most spoilage yeasts 0.88 Most spoilage moulds 0.80 3.3 Effects of Reduced wa on Food Safety Halophilic bacteria 0.75 Xerophilic moulds 0.61 The rate of certain chemical and Osmophilic yeasts 0.61 biochemical processes are effected by the amount of available water. One of the effects of reducing the aw in a food product Table (above): Minimum aw values for is to reduce the rate of these reactions. The growth of microorganism groups in food exception is the oxidation of fats where Table: Minimum a values for growth of the rate decreases down to 0.4 to 0.5, then w specific microorganisms in food increases. Safefood 360, Inc. 2014 Part of Our Professional Whitepapers Series Water Activity in Foods Page 5 Organism aw Organism aw Clostridium botulinum, type E 0.97 Candida scottii 0.92 Pseudomonas spp. 0.97 Trichosporon pullulans 0.91 Acinetobacter spp. 0.96 Candida zaylanoides 0.90 Escherichia coli 0.96 Staphylococcus aureus 0.86 Enterobacter aerogenes 0.95 Alternaria citri 0.84 Bacillus subtilis 0.95 Penicillium patulum 0.81 Clostridium botulinum, types A and B 0.94 Aspergillus conicus 0.70 Candida utilis 0.94 Aspergillus echinulatus 0.64 Vibro parahaemolyticus 0.94 Zygosaccharomyces rouxii 0.62 Botrytis stolonifer 0.93 Rhizopus stolonifer 0.93 Mucor spinosus 0.93 For example, where a product is sold by 3.4 To Reduce aw weight, controlling its water content may Reducing the aw of a food product may be be important to meet legal and commercial achieved through a number of methods. requirements. Water activity is more The most obvious is by partial removal of significant for qualitative considerations water in the food product using a variety such as product stability, shelf life (e.g. of unit operations or processes. The microbiological & enzymatic stability, concentration of water can also be reduced aroma retention), handling characteristics, by the addition of other substances physical properties and chemical stability. including salt and sugar. Many of these methods have been used long before the Water activity and water content can concept of water activity was understood. be related by a graph called a Sorption Isotherm (see diagram), so if the user has the ability to measure both 4. Measure of Water Content parameters, the relationship can be and aw defined and each parameter derived from the other (interpolation). 4.1 Relationship between Water Activity and Water In practice, the sorption isotherm may be impractical to use because, not only does There is often confusion between water the relationship between Aw and moisture activity and water content measurements. content change with temperature of In many sectors, water content is used to measurement, but also any variations in control the amount of water present in a the material composition has a modifying product for quantitative reasons.