Water is often the major constituent in foods. Even relatively ‘dry’ foods like bread and cheese usually contain more than 35% water. The state of water in a food can be most usefully described in terms of water activity (ay). Water activity of a food is not the same as its moisture content. Water activity is a measure of the energy status of ¥ ater r the degree to which water is bound). Thus water’ measure of the amount of water that is free to to act vent _and_ participa chemical and biochemical and growth of. microorganisms. | On the other hand moisture content is a measure of both the free and bound water in a system. Foods may have the same moisture content and yet have quite different water activities and therefore stabilities and vice- verse. For eg. wheat with a moisture content of about 18% has the same water activity (about 0.7) as rapeseed- which has a ire content of only 8%. However in gencral, for any given > water activity increases as the moisture contentie ¥ AEASDREET OF BAER ACTIVITY Water activity of a food is the ratio between the vapour Pressure of the food, when in a completely undisturbed balance with the surrounding air, and the vapour pressure of pure water under identical conditions. Water activity, in practice, is measured as Equilibrium Relative Humidity(ERH) and is given by the formula: WATER ACTIVITY(a,) = ERH / 100 In the laboratory, %ERH of a food is measured by keeping a sample of the food ina small container. The container is placed inside a chamber that seals the sample from outside environment. A sensor inside the chamber measures the relative humidity of the air above food. After a period of time this relative n ent remains constant due to establisliment of ir and the food. The final reading is n technology f water activity has becomefoods er activity applications of wate, < maintaining the + min Spontancous autocataty + prolong the + optimize the phy: aS However the m Water activity in predic g the food saf in be used to and quality. Shell va aera T activity include: chemical stability of foods, is an important Property that ca: food safety, Stability oak Qe os « ize non enzymatic as browning reactions and tic lipid oxidation reactions desired activity of enzymes and Vitamins in Steal properties of foods such as texture, ost important application is the role of fety and stability with icrobial activity in foods.Microorganisms have an absolute demand for water, for without water no growth can occur. This water requirement is best explained in terms of available water or water activity. Lower is.the water aetivity of the foods, more unfavourable will be the conditions for microbial growth. Thus restricting the water activity in foods is an excellent way to make the foods microbiologically safe and stable. 3 ‘Throughout history, the importance of controlling water in foods to preserve and control food quality has been recognized. This phenomenon is also seen in nature. Many plants produce seeds which will keep for a long time because their moisture content is so low that microbial activity is impossible. In fact the rise of our civilization was based on the ability to store wheat and other grains that resist spoilage by virtue of their low levels of vity in the range of 6.2 IF moist fresh foods. aes 2 food has a water activity value of Onsicicred as non-hazardous, This isbecause below a water the pathogens such exception is Staphy activity of 0.91, most bacteria including as Clostridium botulinum cannot grow. But an lococcus aureus which can be inhibited by water activity value of 0.91 under anaerobic conditions but under aerobic conditions. , it requires a minimum water activity value of 0.86. Most molds and yeasts can grow at a minimum water a y value of 0.80. Thus a dry food like bread is generally spoiled by molds and not bacteria. In general, the water activity requirement of microorganisms decreases in the following order: es Bacteria > Yeast > Mold In addition to the relationship between microbial growth and water activity, a number of other aspects of food microbiology are influenced by water activity. The effect of water Y on sporulation, germination and mycotoxin Production of microorganisms is complex. Generally, for yeasts, a higher water activity level is required for sporulation than for spore Sermination. The minimum Water activity level for toxin Production is generally higher than the minimum water activity necessary for growth, ies... sa : eeeeecen eee) Below 0.60, dried foods Tike reater she no microbiological growth is possible, milk powder, cookies, If stable and safe as compared Thus the biscuits ete are much to moist or semi-moistoe? ’* °% TABLE SHOWING LOWEST WATER ACTIVITY ¥ ALUES PERMITTING THE GROWTH OF SPOILAGE MICROORGANISMS [MICROBIAL [MINIMUM iGrowrn | WATER ACTIVITY EXAMPLES Salmonella spp., Clostridium botulinum _ Torulopsis spp.SEAG WORS ay COREG NC YAY SS’ GROORGAME Factors — that affect water activity requirements of microorganisms include the following: +KIND OF SOLUTE ADDED : For many organisms , especially molds, the lowest water activity for growth is Practically independent of the kind of solute used. Other organisms, however, have lower limiting water activity values with some solutes than with others. Potassium chloride, for eg, is less toxic than sodium chloride, and it in turn is less inhibitory than sodium sulphate. +NUTRI VALUE OF CULTURE MEDIUM: In better the medfum for growth. The lower the. > of water activity. : Most organisms have the greatest ictivity at about optin 1 temperatures. ': Growth of aerobes take: place at a ‘the presence of air than ii absence, arrows the range ms,ese SVRSCT OF Avy OM VICROBING GROYT Ts Each microorganism has a maximal, optimal and minimal water activity for growth. Water acts as an essential solvent that is needed for most biochemical reactions by the microorganisms. Under normal conditions, each microbial cell exists in a state dency of microorganism to maintain 2 internal environment Reduction of water s hening ofWER ACTIVE aD Now days, consumers are demanding fresher and more natural products without added preservatives. This prompts the food manufacturer to use milder preservation techniques. One such technique is controlling the water activity of foods. Reduction of water activity is regarded as one of the hurdle technologies used in food preservation since under certain conditions it acts synergistically with other environmental Parameters such as ph or temperature to retard or prevent the microbial growth and under other conditions, it will be the sole parameter responsible for food preservation. For foods with a relatively high water activity, correct refrigeration is always necessary, since these are easily spoiled by microorganisms, ‘These include most fresh foods like meat, fish, fruits, milk ete. However many foods can be successfully stored at room temperature by proper control of their water activity. A wide variety of foods are preserved by restricting their water a ity. These include : nal water activity between eg. Dried egg powder, milk powder, cr: roducts are stored at room temperature method of preservation. These areshelf stable and do not spoil as longer as moisture content is kept low. + INTERMEDIATE MOISTURE FOODS: These foods contain between 15% and 50% moisture content and have a water activity betwéen 0.60 and 0.85. These foods normally require added protection by secondary methods such as pasteurization, ph control, refrigeration, preservatives, but they can also be stored at room temperature. These include dried fruits, cakes, pastries, fruit cake, jams, syrups and some fermented sausages. These products are usually spoiled by surface mold growth.= SHANODS BE REDUCING . WATER ABTA There are several methods by which water activ foods can be reduced so as to make the food microbiologically stable and safe. These methods can be divided into two catego ies: 1) in which the free water is converted to bound water. - a) ADDITION OF SOLUTES: Solutes and ions tie up water in a solution. Therefore an increase in the concentration of dissolved solutes such as sugars, salts, glycols and amino acids will help in lowering the amount of available or free water. Addition of such solutes will, in particular, prevent the growth of bacteria, since bacteria will grow well only in a medium with a water activity approaching 1 ie. they grow best in a low concentration of solute: s little as 3 to Y% sugar and 1 to 2% salt may inhibit some bacteria. ethod of reducing water activity is used in many ike pickles, jams, jellies, candied fruits, salted OF “HYDROPHILIC COLLOIDS: olloids form gels with water thus making x unavailable for microbial growth. As little as 4% agar in 2 medium may prevent growth by leaving too iittle available moisture. c) WATER OF CRYSTALLIZATION OR HYDRATION: These forms of water are usually —ABLE! unavailable to microorganisms. Water crystallized as ice , no longer can be used cells. Thus freezing is used as a method of pr reducing water activity. itself, When by microbial eservation by TI) In which the free wate ris removed a) COOKING b) DRYING AND DEHYDRATION: eg. egg powder, pasta c) CONCENTRATION: eg. Condensed milkBLE SHOWING WATER ACTIVITY OF SOME FOODS AND SUSCEPTIBILITY TO SPOILAGE BY MICROORGANISMS WATER ACTIVITY 1.00 — 0.95 0.95 —0.91 | 091 MICROORG ANISMS GROW AT P THIS _ Aw AND ABOVE FOOD EXAMPLES some y | Salmonella, Pediococcus; Candida, enula; Pseudomonas, E.coli, | Proteus, Shigella, Bacillus, ridium perfringens Vibrio parahaemolyticus, | Clostridium botulinum, | juice concentrates; bread; | verratia, Lactobacillus, some _| molds and yeats Many yeasts like Torulopsis, Highly perishable fresh |foods & canned fruits, veg., meat, fish, milk, eggs; foods containing up to 40%(w/w) | sucrose or 7% NaCl. ome cheeses(cheddar,swiss) Cured meats; some fruit high moisture prunes; foods containing 55% (w/w) sucrose or 12% NaCl Fermented sausages; sponge cakes; dry cheese; margarine; foods containing 65% (w/w) sucrose (saturated) or 15% NaCl Most fruit juice concentrates, sweetened condensed milk, flour, rice, pulses containing 15 — 17%, moisture, salami | | —WATER | MICROORGANISMS FOOD | ‘ACTIVITY || GROW AT THIS EXAMPLES 0.80 — 0.75 | Most halophilic |Jam, Marmalade, Glace bacteria, Mycotoxigenic | fruits, Soy sauce Aspergilli os = | 0.75 - 0.65 Xerophilic molds, | Rolled oats containing 10% | Saccharomyces bisporus | moisture; Fudge; Marshmallows; Jelly; Some dried fruits; Nuts, Peanut ieee eared 22S). _ | Butter 0.65 - 0.60 | Osmophilic yeasts, Few | Dried fruits containing 15 — molds 20% moisture; Honey 0.50 No microbial | P: containing 12% proliferation moisture; Spices containing eee | «10% moisture ae 0.40 No microbial | ' Whole egg powder proliferation | containing 5% moisture 0.30 No microbial | Cookies, biscuits crackers, proliferation | bread crusts etc: containing a 3 — 5oimoistine gees No microbial Whole milk powder proliferation | containing 2 — 3% moisture, : Dried vegetables easy (5% moisture, Corn flakes | containing 5% moisture, ee Instant coffee