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Water Activity
Vitamin Metabolism see Metabolic Pathways: Metabolism of Minerals and Vitamins
W Water Activity K Prabhakar, Sri Venkateswara Veterinary University, Tirupati, India EN Mallika, NTR College of Veterinary Science, Gannavaram, India Ó 2014 Elsevier Ltd. All rights reserved.
Water activity (aw) is a thermodynamics parameter. The water requirements of microorganisms are described in terms of aw. W.J. Scott (1953) and his associates first established that it is the aw and not the water content that is correlated with the microbial growth. aw describes the amount of water available for interaction and cross-reaction with other molecules and solutes. When water interacts with solutes, it is not available for other interactions, and hence, water required for growth of microbes is not available and they get suppressed. Water in food not bound to molecules supports growth of microbes. aw measures availability of this free water that is not bound to molecules. The availability of free water also contributes to diminished or altered chemical and enzymatic reactions involving hydration. aw is defined as the ratio of vapor pressure of food substrate to the vapor pressure of pure water when both are measured at same temperature. It is an indicator of the potential for chemical or physicochemical interaction between water and the rest of the components in food and is used widely as an indicator of food stability as it correlates with microbial growth and rate of chemical reactions. The concept of aw is related to equilibrium relative humidity (ERH) in the following way. ERH ¼ aw 100 The relative humidity of air in equilibrium with a food component or sample is called the ERH.
Minimum Water Activity Values for Growth of Microorganisms aw is related to water content in a nonlinear relationship known as the moisture sorption isotherm curve. This can be used to predict food product stability over a period of time in different storage conditions. The aw value of pure water is 1.00. Most fresh foods have values close to 1.00 or above 0.99. Higher aw values are required for the growth of bacteria when compared with fungi. Gram-negative bacteria have higher requirements than Grampositive organisms. Many microorganisms prefer aw values of
Encyclopedia of Food Microbiology, Volume 3
0.99. Most spoilage bacteria need aw higher than 0.91, whereas spoilage molds can grow even at 0.80 aw. Most of the spoilage bacteria require a minimum aw of 0.90 for growth in foods. Most spoilage fungi, for example, Sacchromyces species and Debaryomyces, require a minimum aw value of 0.80 for growth in foods with 15–17% moisture. Many types of yeasts like Candida, Torulopsis, Haurenula, and Micrococcus can grow even at a aw level of 0.80 in foods like fermented sausage, sponge cake, dry cheese, and margarine or in products with 65% sucrose or 15% salt. The approximate minimum aw values for growth of Clostridium botulinum type E and Pseudomonas is 0.97, whereas Acinetobacter species and Escherichia coli require a minimum aw value of 0.96. Enterobacter aerogenus and Bacillus subtilis can grow even at 0.95 and above. Clostridium botulinum type A and B, Candida utilis, and Vibrio parahaemolyticus require 0.94, whereas Botrytis cinerea, Rhizopus stolonifer, and Mucor spinosus require 0.93 and higher. Halophytes (salt-loving) can grow at the lowest aw value of 0.75, whereas xerophilic (dry-loving) molds and osmophilic (preferring high osmotic pressure) yeasts have been reported to grow at a low aw of 0.65 and 0.61, respectively. When salt is employed to control aw, an extremely high level is necessary to achieve aw values below 0.80. The water activities values of certain common foods as reported by several authors ( Shelly J. Schmidt and Anthony J. Fontana Jr, 2008) are presented in Table 1. The minimum aw values required for optimum growth of certain microorganisms are indicated in Table 2.
Importance of Water Activity in Foods Control of aw influences various aspects of food product design, processing, distribution, and consumption.
Indicator of Food Stability Food products processed to specific aw levels that do not encourage spoilage and pathogenic bacteria, molds, and fungi
http://dx.doi.org/10.1016/B978-0-12-384730-0.00435-3
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Water Activity
Table 1 aw of certain common foods as reported by several authors Type of food
aw
Beef Lamb Whole milk Chicken Coke Sausages Eggs Fruit juices Bacon Cheddar cheese Jams and jellies Sauces Chocolate syrup Semidry sausages Salami dry Cakes Wheat Biscuits Rice Oatmeal cookies Potato chips
0.990 0.990 0.998 0.979 0.978 0.975 0.970 0.970 0.968 0.950 0.94–0.82 0.81–0.98 0.862 0.880 0.875 0.720–0.944 0.700–0.675 0.630–0.605 0.531 0.517–0.553 0.165–0.267
Table 2 Minimum aw values required for optimum growth of certain microorganisms Name of the bacteria
Minimum aw value required
Escherichia coli Enterococcus faecalis Pseudomonas fluorescens, Yersinia enterocolitica, Shigella Clostridium perfringens, Bacillus cereus Bacillus subtilis, Salmonella newport Enterobacter aerogenes, Mycobacterium, Vibrio parahaemolyticus Lactobacillus viridescens Micrococcus roseus, Staphylococus Lactobacillus, Pediococcus Staphylococus aureus Listeria monocytogenes Halophilic bacteria Yeasts Molds
0.99 0.98 0.97 0.96 0.95 0.94 0.93 0.91 0.90 0.86 0.83 0.75 0.86–0.93 0.60–0.88
are safe for consumption. Most spoilage organisms do not grow below 0.91. aw also influences chemical reactions and enzymatic reactions especially those involving hydration. Lower aw can control spoilage attributable to such changes.
Designing of Foods In food products in which crispness and crunchiness are important, aw values below 0.65 usually are maintained. aw values contribute to limit moisture migration in composite food products. It can also be used to predict the migration of moisture that affects food products. aw can be decreased with an increase in temperature and increase in pressure.
Shelf Stability of Foods The shelf life of foods can also be prolonged with suitable manipulation of aw levels in food products. Addition of certain agents or solutes (humectants) like glycerol, sucrose, sodium chloride, and so on can lower aw and enhance storage period. These agents bind most of the available water molecules and prevent access to microorganisms for growth purpose. Exacting requirements for packaging and the preservation of stored foods can also be moderated with proper manipulation of aw, which offers greater flexibility in food-processing and distribution-marketing operations. Growth of most bacteria, molds, and yeasts can be controlled effectively in foods processed to aw values of 0.80 and below.
Consequences of Lowering the Water Activity Lowering of aw increases the duration of the lag phase of growth of microbes, thereby declining the growth rate and finally the numbers of the population. The lower aw harmfully influences all of the metabolic activity of the microbes as all chemical reactions of the cells require an aqueous environment. Environmental parameters like pH, temperature, and oxidoredox potential can also affect the aw levels. For survival and growth, bacteria require a positive turgor pressure. When they experience aw stress, the cells lose water due to osmosis, which results in the shrinkage of the cell and sometimes plasmolysis. To neutralize and survive aw stress, bacteria have evolved a physiological response that includes changes to the cell membrane, protein synthesis, and adjusting their cytoplasmic aw. A rise in the proportion of negatively charged phospholipids of the cell membrane leads to increased levels of solute transport protein. Synthesis of certain protein in response to stress and intracellular accumulation of compatible solutes results with lowered aw. Bacteria adjust their cytoplasmic aw using one of two stages – that is, salt-in-cytoplasm type and the organic osmolytic-in-cytoplasm type. Halophytes maintain the concentration of KCl in their cytoplasm equal to that of the suspending menstruum, which is referred as the saltin-cytoplasm response. Nonhalophytes accumulate compatible solutes (osmolytes) in a biphasic manner. In this, inorganic salts are excluded, while organic solutes are synthesized or accumulated in the cytoplasm from the environment. As they are compatible with enzyme, the organic osmolytes are known as compatible solutes. Compatible solute molecules have low molecular weight and polar functional groups with no net charge at physiological pH. They are highly soluble, facilitating their accumulation to higher intracellular concentration. The most common compatible solutes in most bacteria are carnitine, glycine, betain, and proline. Proline is accumulated to high levels by Gram-negative bacteria. The possibilities of survival of some important pathogenic strains as indicated should be kept in mind while producing shelf-stable food products.
Methods of Measuring Water Activity No device can be put into a product to directly measure the aw. The aw of a product can be determined, however, from the
Water Activity
relative humidity of the air surrounding the sample when the air and the sample are at equilibrium. Therefore, the sample must be in an enclosed space where this equilibrium can take place. Once this occurs, the aw of the sample and the relative humidity of the air are equal. The measurement taken at equilibrium is called the ERH. Two different types of aw instruments are commercially available. One uses chilledmirror dew-point technology and the other measures relative humidity with sensors that change electrical resistance or capacitance. Each has advantages and disadvantages. The methods vary in accuracy, repeatability, speed of measurement, stability in calibration, linearity, and convenience of use. The major advantages of the chilled-mirror dew-point method are accuracy, speed, ease of use, and precision. The range may be from 0.030 to 1.000 aw, with a resolution of 0.001 aw and accuracy of 0.003 aw. Measurement time is typically less than 5 min. Capacitance sensors have the advantage of being inexpensive, but they typically are not as accurate or as fast as the chilled-mirror dew-point method. Capacitive instruments measure over the entire aw range – 0 to 1.00 aw, with a resolution of 0.005 aw and accuracy of 0.015. Some commercial instruments can measure in 5 min, while other electronic capacitive sensors usually require 30–90 min to reach ERH conditions.
Chilled-Mirror Theory In chilled-mirror dew-point instruments, a sample is equilibrated within the headspace of a sealed chamber containing a mirror, an optical sensor, an internal fan, and an infrared temperature sensor. At equilibrium, the relative humidity of the air in the chamber is the same as the aw of the sample. A thermoelectric (Peltier) cooler precisely controls the mirror temperature. An optical reflectance sensor detects the exact point at which condensation first appears. A beam of infrared light is directed onto the mirror and reflected back to a photo detector, which detects the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror accurately measures the dew-point temperature. The internal fan is for air circulation, which reduces vapor equilibrium time and controls the boundary layer conductance of the mirror surface. Additionally, a thermopile sensor (infrared thermometer) measures the sample surface temperature. Both the dew-point and sample temperatures are then used to determine the aw. During aw measurement, the instrument repeatedly determines the dew-point temperature until vapor equilibrium is reached. Since the measurement is based on temperature determination, calibration is not necessary, but measuring a standard salt solution checks proper functioning of the instrument. If there is a problem, the mirror is easily accessible and can be cleaned in a few minutes.
Capacitive Sensor Theory Some aw instruments use capacitance sensors to measure aw. Such instruments use a sensor made from a hygroscopic polymer and associated circuitry that gives a signal relative to the ERH. The sensor measures the ERH of the air immediately around it. This ERH is equal to sample aw only as long as the temperatures of the sample and the sensor are the same. Since
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these instruments relate an electrical signal to relative humidity, the sensor must be calibrated with known salt standards. In addition, the ERH is equal to the sample aw only as long as the sample and sensor temperatures are the same. Some capacitive sensors need between 30 and 90 min to come to temperature and vapor equilibrium. Accurate measurements with this type of system require good temperature control.
Role of Humectants The aw of the material has to be decreased to a certain level to inhibit the growth of contaminating microorganisms. Watersoluble compounds like sodium chloride and sugar can be used to lower the aw of foods. Lower amounts of salt than sugar is necessary for adequate reduction of aw because of the lower particle mass of NaCl. NaCl is used in meat products and in some sour vegetables while sugar is added to fruits and candies. During aw reduction, the following facts should be taken into account: 1. Characteristic composition of the end product (water, protein, salt, other soluble materials, insoluble materials) 2. aw to be reached In reducing the aw, water may be removed either by adsorption or desorption. Sorption isotherm of material is a plot of the amount of water adsorbed as a function of the relative humidity or activity of the vapor space surrounding the material. Solution of glycerol, NaCl, sucrose, potassium sorbate, and so on can be added to reduce aw of the product. Lactose and sugar derivatives can be used in foods as sweeteners and humectants. The aw depressing property of sodium chloride was the most effective, followed by that of glycerol and propylene glycol. Glucose was reported to be not as effective as glycerol or propylene glycol but was superior to sucrose as an aw-lowering solute. Dried, dehydrated low-moisture foods like traditional dried foods have generally low aw below 0.60. Intermediate moisture foods (IMFs) contain 15–50% moisture with aw between 0.60 and 0.85. At aw values of 0.80–0.85, spoilage occurs readily by fungi in 1–2 weeks. Spoilage is delayed at aw values of 0.75 and may not occur during prolonged holding at aw of 0.70. Even though spoilage cannot occur at aw level less than 0.65, some fungi are known to grow slowly at 0.60–0.62, for example, osmophilic yeasts such as Zygosaccharomyces rouxii, or Aspergillus glaucus group and Xeromyces bisporus, which are the predominant spoilage molds of dried foods. The maximum browning rates in fruits and vegetable products occur in the aw range of 0.65–0.75, whereas nonfat dried milk browning occurs at 0.70. IMF have aw values ranging between 0.60 and 0.85. Dried foods, cakes and pastry, sugar syrup, fruit cakes, honey, fruit juice concentrates, jams, and condensed milks have aw value in the range between 0.60 and 0.84. All these lowered aw values are achieved by desorption, adsorption, or addition of permissible additives, such as salts and sugars. Fruits and fruit pieces are often used as the necessary materials in many composite foods. In such systems, the aw must be controlled to avoid moisture resettlement. The fruit aw
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Water Activity
usually is reduced by controlled dehydration but sometimes excess hardening may occur. In such cases, an alternative is to decrease the aw by an osmotic treatment – that is, by soluble solid intake rather than by loss of water. The combination of osmosis and a limited air-dehydration is reported to be the best choice. Generally, small diminution of aw is sufficient to prevent the growth of some important spoilage microorganisms, for example, Pseudomonas species that nurture at high aw and swiftly mess up foods such as fresh meat stored in air. Cured meats generally have aw suitably reduced to ensure longer shelf lives. Slow souring caused by lactic acid bacteria occurs instead. In salamis and dry-cured meat products, slow spoilage may occur due to low aw-tolerant micrococci. Of the food poisoning microorganisms, Staphylococcus aureus is the most tolerant, with a low aw limit for growth of about 0.86. Shelf-stable dried foods generally are formulated around aw 0.3, where lipid oxidation and other chemical changes are minimal. Manipulation of aw, as one of the antibacterial hurdles, offers great prospects in development of shelf-stable food products storable at ambient temperature or with minimum refrigeration requirements. Foods processed to specific aw values offer consumer health protection and contribute to maximizing food-processing operations.
See also: Dried Foods; Fermented Foods: Origins and Applications; Fermented Meat Products and the Role of Starter Cultures; Hurdle Technology; Intermediate Moisture Foods; Traditional Preservatives: Sodium Chloride; Permitted Preservatives: Nitrites and Nitrates; Water Quality Assessment: Routine Techniques for Monitoring Bacterial and Viral Contaminants; Water Quality Assessment: Modern Microbiological Techniques; Microbial Spoilage of Eggs and Egg Products; Spoilage of Animal Products: Seafood.
Further Reading Alberto, S., Dimitrios, F., Marco, S., 2012. Water activity in biological systems – a review. Journal of Food and Nutrition Science 62, 5–13. Angelides, A.S., Smith, G.M., 2003. The transportation mediate uptake of glycine bacteria and carminative on L. monocytogenes in response to hyper osmotic stress. Applied Environmental Microbiology 69, 1013–1022.
Baird-Parker, T.C., 2000. The production of microbiologically safe and stable foods. In: Lund, B.M., Baird-Parker, T.C., Gould, G.W. (Eds.), The Microbiological Safety and Quality of Food. Aspen Publishers, Inc., Gaithersburg, pp. 3–18. Campbell-Platt, G., 1995. Fermented meats – a world perspective. In: CampbellPlatt, G., Cook, P.E. (Eds.), Fermented Meats. Blackie, Glasgow, p. 39. Chang, S.F., Huang, T.C., Pearson, A.M., 1996. Control of the dehydration process in production of intermediate-moisture meat products: a review. Advances in Food and Nutrition Research 39, 71–160. Davidson, P.M., 1997. Chemical preservatives and natural antimicrobial compounds. In: Doyle, M.P., Beuchat, L.R., Montville, T.J. (Eds.), Food Microbiology: Fundamental and Frontiers. ASM Publications, Washington, DC, pp. 520–556. Fernandez-Salguero, J., Gomez, R., Carmona, M.A., 1993. Water activity in selected high-moisture foods. Journal of Food Composition and Analysis 6, 364–369. Garden, R., Duche, O., Leroy-strin, S., European Listeria genome comortium and J. Labadie, 2003. Role of ctc from L. monocytogenes in osmotolerance. Applied and Environmental Microbiology 69, 154–161. Gram, L., Huss, H.H., 2000. Fresh and processed fish and shellfish. In: Lund, B.M., Baird-Parker, A.C., Gould, G.W. (Eds.), The Microbiological Safety and Quality of Foods. Chapman & Hall, London, pp. 472–506. Horner, K.J., Aragnostopoulus, J.D., 1973. Combined effects of water activity, pH and temperature on the growth and spoilage potential of fungi. Journal of Applied Microbiology 36, 427–436. Labuza, T.P., 1980. Water activity: physical and chemical properties. In: Linko, P., Melkki, Y., Olkku, J., Larinkari, J. (Eds.), Food Process Engineering. Applied Science Publishers, London, p. 320. Leistner, L., 1995. Principles and applications of hurdle technology. In: Gould, G.W. (Ed.), New Methods of Food Preservation. Blackie, Glasgow, p. 1. Leistner, L., 1995. Stable and safe fermented sausages world-wide. In: CampbellPlatt, G., Cook, P.E. (Eds.), Fermented Meats. Blackie, Glasgow, p. 160. Leistner, L., Rodel, W., 1976. The stability of intermediate moisture foods with respect to micro-organisms. In: Davies, R., Birch, G.G., Parker, K.J. (Eds.), Intermediate Moisture Foods. Applied Science Publishers, London, p. 120. Morris, E.O., 1962. Effect of environment on microorganisms. In: Hawthorn, J., Leitch, J.M. (Eds.), Recent Advances in Food Sciences, vol. 1, pp. 24–36. Roos, Y.H., 1993. Water activity and physical state effects on amorphous food stability. Journal of Food Processing and Preservation 16, 433–447. Scott, W.J., 1953. Water relations of Staphylococcus aureus at 30 C. Australian Journal of Biological Sciences 6, 549–564. Shelly, J., Schmidt, Anthony, J. Fontana Jr., 2008. Water activity values of select food ingredients and products. Water Activity in Foods: Fundamentals and Application. Wiley online library, 407–420. Sleator, R.D., Hill, C., 2001. Bacterial osmoadaptation: the role of osmolytes in bacterial stress and virulence. FEMS Microbiology Reviews 26, 49–71. Van den Berg, C., 1984. Description of water activity of food for engineering purpose by means of the GAB model of sorption. In: McKenna, B.M. (Ed.), Engineering and Food. Elsevier, London, pp. 311–321. Wodzenski, R.J., Frazer, W.C., 1961. Moisture requirements of bacteria –II. Influence of temperature, pH, and malate concentration on requirements of Acetobacter Aerogenes. Journal of Bacteriology 81, 353–358. Yun-chan lo, Froning, G.W., Arnold, R.G., 1983. The water activity lowering properties of selected humectants in eggs. Poultry Science 62, 971–976.
W Water Activity K Prabhakar, Sri Venkateswara Veterinary University, Tirupati, India EN Mallika, NTR College of Veterinary Science, Gannavaram, India Ó 2014 Elsevier Ltd. All rights reserved.
Water activity (aw) is a thermodynamics parameter. The water requirements of microorganisms are described in terms of aw. W.J. Scott (1953) and his associates first established that it is the aw and not the water content that is correlated with the microbial growth. aw describes the amount of water available for interaction and cross-reaction with other molecules and solutes. When water interacts with solutes, it is not available for other interactions, and hence, water required for growth of microbes is not available and they get suppressed. Water in food not bound to molecules supports growth of microbes. aw measures availability of this free water that is not bound to molecules. The availability of free water also contributes to diminished or altered chemical and enzymatic reactions involving hydration. aw is defined as the ratio of vapor pressure of food substrate to the vapor pressure of pure water when both are measured at same temperature. It is an indicator of the potential for chemical or physicochemical interaction between water and the rest of the components in food and is used widely as an indicator of food stability as it correlates with microbial growth and rate of chemical reactions. The concept of aw is related to equilibrium relative humidity (ERH) in the following way. ERH ¼ aw 100 The relative humidity of air in equilibrium with a food component or sample is called the ERH.
Minimum Water Activity Values for Growth of Microorganisms aw is related to water content in a nonlinear relationship known as the moisture sorption isotherm curve. This can be used to predict food product stability over a period of time in different storage conditions. The aw value of pure water is 1.00. Most fresh foods have values close to 1.00 or above 0.99. Higher aw values are required for the growth of bacteria when compared with fungi. Gram-negative bacteria have higher requirements than Grampositive organisms. Many microorganisms prefer aw values of
Encyclopedia of Food Microbiology, Volume 3
0.99. Most spoilage bacteria need aw higher than 0.91, whereas spoilage molds can grow even at 0.80 aw. Most of the spoilage bacteria require a minimum aw of 0.90 for growth in foods. Most spoilage fungi, for example, Sacchromyces species and Debaryomyces, require a minimum aw value of 0.80 for growth in foods with 15–17% moisture. Many types of yeasts like Candida, Torulopsis, Haurenula, and Micrococcus can grow even at a aw level of 0.80 in foods like fermented sausage, sponge cake, dry cheese, and margarine or in products with 65% sucrose or 15% salt. The approximate minimum aw values for growth of Clostridium botulinum type E and Pseudomonas is 0.97, whereas Acinetobacter species and Escherichia coli require a minimum aw value of 0.96. Enterobacter aerogenus and Bacillus subtilis can grow even at 0.95 and above. Clostridium botulinum type A and B, Candida utilis, and Vibrio parahaemolyticus require 0.94, whereas Botrytis cinerea, Rhizopus stolonifer, and Mucor spinosus require 0.93 and higher. Halophytes (salt-loving) can grow at the lowest aw value of 0.75, whereas xerophilic (dry-loving) molds and osmophilic (preferring high osmotic pressure) yeasts have been reported to grow at a low aw of 0.65 and 0.61, respectively. When salt is employed to control aw, an extremely high level is necessary to achieve aw values below 0.80. The water activities values of certain common foods as reported by several authors ( Shelly J. Schmidt and Anthony J. Fontana Jr, 2008) are presented in Table 1. The minimum aw values required for optimum growth of certain microorganisms are indicated in Table 2.
Importance of Water Activity in Foods Control of aw influences various aspects of food product design, processing, distribution, and consumption.
Indicator of Food Stability Food products processed to specific aw levels that do not encourage spoilage and pathogenic bacteria, molds, and fungi
http://dx.doi.org/10.1016/B978-0-12-384730-0.00435-3
751
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Water Activity
Table 1 aw of certain common foods as reported by several authors Type of food
aw
Beef Lamb Whole milk Chicken Coke Sausages Eggs Fruit juices Bacon Cheddar cheese Jams and jellies Sauces Chocolate syrup Semidry sausages Salami dry Cakes Wheat Biscuits Rice Oatmeal cookies Potato chips
0.990 0.990 0.998 0.979 0.978 0.975 0.970 0.970 0.968 0.950 0.94–0.82 0.81–0.98 0.862 0.880 0.875 0.720–0.944 0.700–0.675 0.630–0.605 0.531 0.517–0.553 0.165–0.267
Table 2 Minimum aw values required for optimum growth of certain microorganisms Name of the bacteria
Minimum aw value required
Escherichia coli Enterococcus faecalis Pseudomonas fluorescens, Yersinia enterocolitica, Shigella Clostridium perfringens, Bacillus cereus Bacillus subtilis, Salmonella newport Enterobacter aerogenes, Mycobacterium, Vibrio parahaemolyticus Lactobacillus viridescens Micrococcus roseus, Staphylococus Lactobacillus, Pediococcus Staphylococus aureus Listeria monocytogenes Halophilic bacteria Yeasts Molds
0.99 0.98 0.97 0.96 0.95 0.94 0.93 0.91 0.90 0.86 0.83 0.75 0.86–0.93 0.60–0.88
are safe for consumption. Most spoilage organisms do not grow below 0.91. aw also influences chemical reactions and enzymatic reactions especially those involving hydration. Lower aw can control spoilage attributable to such changes.
Designing of Foods In food products in which crispness and crunchiness are important, aw values below 0.65 usually are maintained. aw values contribute to limit moisture migration in composite food products. It can also be used to predict the migration of moisture that affects food products. aw can be decreased with an increase in temperature and increase in pressure.
Shelf Stability of Foods The shelf life of foods can also be prolonged with suitable manipulation of aw levels in food products. Addition of certain agents or solutes (humectants) like glycerol, sucrose, sodium chloride, and so on can lower aw and enhance storage period. These agents bind most of the available water molecules and prevent access to microorganisms for growth purpose. Exacting requirements for packaging and the preservation of stored foods can also be moderated with proper manipulation of aw, which offers greater flexibility in food-processing and distribution-marketing operations. Growth of most bacteria, molds, and yeasts can be controlled effectively in foods processed to aw values of 0.80 and below.
Consequences of Lowering the Water Activity Lowering of aw increases the duration of the lag phase of growth of microbes, thereby declining the growth rate and finally the numbers of the population. The lower aw harmfully influences all of the metabolic activity of the microbes as all chemical reactions of the cells require an aqueous environment. Environmental parameters like pH, temperature, and oxidoredox potential can also affect the aw levels. For survival and growth, bacteria require a positive turgor pressure. When they experience aw stress, the cells lose water due to osmosis, which results in the shrinkage of the cell and sometimes plasmolysis. To neutralize and survive aw stress, bacteria have evolved a physiological response that includes changes to the cell membrane, protein synthesis, and adjusting their cytoplasmic aw. A rise in the proportion of negatively charged phospholipids of the cell membrane leads to increased levels of solute transport protein. Synthesis of certain protein in response to stress and intracellular accumulation of compatible solutes results with lowered aw. Bacteria adjust their cytoplasmic aw using one of two stages – that is, salt-in-cytoplasm type and the organic osmolytic-in-cytoplasm type. Halophytes maintain the concentration of KCl in their cytoplasm equal to that of the suspending menstruum, which is referred as the saltin-cytoplasm response. Nonhalophytes accumulate compatible solutes (osmolytes) in a biphasic manner. In this, inorganic salts are excluded, while organic solutes are synthesized or accumulated in the cytoplasm from the environment. As they are compatible with enzyme, the organic osmolytes are known as compatible solutes. Compatible solute molecules have low molecular weight and polar functional groups with no net charge at physiological pH. They are highly soluble, facilitating their accumulation to higher intracellular concentration. The most common compatible solutes in most bacteria are carnitine, glycine, betain, and proline. Proline is accumulated to high levels by Gram-negative bacteria. The possibilities of survival of some important pathogenic strains as indicated should be kept in mind while producing shelf-stable food products.
Methods of Measuring Water Activity No device can be put into a product to directly measure the aw. The aw of a product can be determined, however, from the
Water Activity
relative humidity of the air surrounding the sample when the air and the sample are at equilibrium. Therefore, the sample must be in an enclosed space where this equilibrium can take place. Once this occurs, the aw of the sample and the relative humidity of the air are equal. The measurement taken at equilibrium is called the ERH. Two different types of aw instruments are commercially available. One uses chilledmirror dew-point technology and the other measures relative humidity with sensors that change electrical resistance or capacitance. Each has advantages and disadvantages. The methods vary in accuracy, repeatability, speed of measurement, stability in calibration, linearity, and convenience of use. The major advantages of the chilled-mirror dew-point method are accuracy, speed, ease of use, and precision. The range may be from 0.030 to 1.000 aw, with a resolution of 0.001 aw and accuracy of 0.003 aw. Measurement time is typically less than 5 min. Capacitance sensors have the advantage of being inexpensive, but they typically are not as accurate or as fast as the chilled-mirror dew-point method. Capacitive instruments measure over the entire aw range – 0 to 1.00 aw, with a resolution of 0.005 aw and accuracy of 0.015. Some commercial instruments can measure in 5 min, while other electronic capacitive sensors usually require 30–90 min to reach ERH conditions.
Chilled-Mirror Theory In chilled-mirror dew-point instruments, a sample is equilibrated within the headspace of a sealed chamber containing a mirror, an optical sensor, an internal fan, and an infrared temperature sensor. At equilibrium, the relative humidity of the air in the chamber is the same as the aw of the sample. A thermoelectric (Peltier) cooler precisely controls the mirror temperature. An optical reflectance sensor detects the exact point at which condensation first appears. A beam of infrared light is directed onto the mirror and reflected back to a photo detector, which detects the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror accurately measures the dew-point temperature. The internal fan is for air circulation, which reduces vapor equilibrium time and controls the boundary layer conductance of the mirror surface. Additionally, a thermopile sensor (infrared thermometer) measures the sample surface temperature. Both the dew-point and sample temperatures are then used to determine the aw. During aw measurement, the instrument repeatedly determines the dew-point temperature until vapor equilibrium is reached. Since the measurement is based on temperature determination, calibration is not necessary, but measuring a standard salt solution checks proper functioning of the instrument. If there is a problem, the mirror is easily accessible and can be cleaned in a few minutes.
Capacitive Sensor Theory Some aw instruments use capacitance sensors to measure aw. Such instruments use a sensor made from a hygroscopic polymer and associated circuitry that gives a signal relative to the ERH. The sensor measures the ERH of the air immediately around it. This ERH is equal to sample aw only as long as the temperatures of the sample and the sensor are the same. Since
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these instruments relate an electrical signal to relative humidity, the sensor must be calibrated with known salt standards. In addition, the ERH is equal to the sample aw only as long as the sample and sensor temperatures are the same. Some capacitive sensors need between 30 and 90 min to come to temperature and vapor equilibrium. Accurate measurements with this type of system require good temperature control.
Role of Humectants The aw of the material has to be decreased to a certain level to inhibit the growth of contaminating microorganisms. Watersoluble compounds like sodium chloride and sugar can be used to lower the aw of foods. Lower amounts of salt than sugar is necessary for adequate reduction of aw because of the lower particle mass of NaCl. NaCl is used in meat products and in some sour vegetables while sugar is added to fruits and candies. During aw reduction, the following facts should be taken into account: 1. Characteristic composition of the end product (water, protein, salt, other soluble materials, insoluble materials) 2. aw to be reached In reducing the aw, water may be removed either by adsorption or desorption. Sorption isotherm of material is a plot of the amount of water adsorbed as a function of the relative humidity or activity of the vapor space surrounding the material. Solution of glycerol, NaCl, sucrose, potassium sorbate, and so on can be added to reduce aw of the product. Lactose and sugar derivatives can be used in foods as sweeteners and humectants. The aw depressing property of sodium chloride was the most effective, followed by that of glycerol and propylene glycol. Glucose was reported to be not as effective as glycerol or propylene glycol but was superior to sucrose as an aw-lowering solute. Dried, dehydrated low-moisture foods like traditional dried foods have generally low aw below 0.60. Intermediate moisture foods (IMFs) contain 15–50% moisture with aw between 0.60 and 0.85. At aw values of 0.80–0.85, spoilage occurs readily by fungi in 1–2 weeks. Spoilage is delayed at aw values of 0.75 and may not occur during prolonged holding at aw of 0.70. Even though spoilage cannot occur at aw level less than 0.65, some fungi are known to grow slowly at 0.60–0.62, for example, osmophilic yeasts such as Zygosaccharomyces rouxii, or Aspergillus glaucus group and Xeromyces bisporus, which are the predominant spoilage molds of dried foods. The maximum browning rates in fruits and vegetable products occur in the aw range of 0.65–0.75, whereas nonfat dried milk browning occurs at 0.70. IMF have aw values ranging between 0.60 and 0.85. Dried foods, cakes and pastry, sugar syrup, fruit cakes, honey, fruit juice concentrates, jams, and condensed milks have aw value in the range between 0.60 and 0.84. All these lowered aw values are achieved by desorption, adsorption, or addition of permissible additives, such as salts and sugars. Fruits and fruit pieces are often used as the necessary materials in many composite foods. In such systems, the aw must be controlled to avoid moisture resettlement. The fruit aw
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usually is reduced by controlled dehydration but sometimes excess hardening may occur. In such cases, an alternative is to decrease the aw by an osmotic treatment – that is, by soluble solid intake rather than by loss of water. The combination of osmosis and a limited air-dehydration is reported to be the best choice. Generally, small diminution of aw is sufficient to prevent the growth of some important spoilage microorganisms, for example, Pseudomonas species that nurture at high aw and swiftly mess up foods such as fresh meat stored in air. Cured meats generally have aw suitably reduced to ensure longer shelf lives. Slow souring caused by lactic acid bacteria occurs instead. In salamis and dry-cured meat products, slow spoilage may occur due to low aw-tolerant micrococci. Of the food poisoning microorganisms, Staphylococcus aureus is the most tolerant, with a low aw limit for growth of about 0.86. Shelf-stable dried foods generally are formulated around aw 0.3, where lipid oxidation and other chemical changes are minimal. Manipulation of aw, as one of the antibacterial hurdles, offers great prospects in development of shelf-stable food products storable at ambient temperature or with minimum refrigeration requirements. Foods processed to specific aw values offer consumer health protection and contribute to maximizing food-processing operations.
See also: Dried Foods; Fermented Foods: Origins and Applications; Fermented Meat Products and the Role of Starter Cultures; Hurdle Technology; Intermediate Moisture Foods; Traditional Preservatives: Sodium Chloride; Permitted Preservatives: Nitrites and Nitrates; Water Quality Assessment: Routine Techniques for Monitoring Bacterial and Viral Contaminants; Water Quality Assessment: Modern Microbiological Techniques; Microbial Spoilage of Eggs and Egg Products; Spoilage of Animal Products: Seafood.
Further Reading Alberto, S., Dimitrios, F., Marco, S., 2012. Water activity in biological systems – a review. Journal of Food and Nutrition Science 62, 5–13. Angelides, A.S., Smith, G.M., 2003. The transportation mediate uptake of glycine bacteria and carminative on L. monocytogenes in response to hyper osmotic stress. Applied Environmental Microbiology 69, 1013–1022.
Baird-Parker, T.C., 2000. The production of microbiologically safe and stable foods. In: Lund, B.M., Baird-Parker, T.C., Gould, G.W. (Eds.), The Microbiological Safety and Quality of Food. Aspen Publishers, Inc., Gaithersburg, pp. 3–18. Campbell-Platt, G., 1995. Fermented meats – a world perspective. In: CampbellPlatt, G., Cook, P.E. (Eds.), Fermented Meats. Blackie, Glasgow, p. 39. Chang, S.F., Huang, T.C., Pearson, A.M., 1996. Control of the dehydration process in production of intermediate-moisture meat products: a review. Advances in Food and Nutrition Research 39, 71–160. Davidson, P.M., 1997. Chemical preservatives and natural antimicrobial compounds. In: Doyle, M.P., Beuchat, L.R., Montville, T.J. (Eds.), Food Microbiology: Fundamental and Frontiers. ASM Publications, Washington, DC, pp. 520–556. Fernandez-Salguero, J., Gomez, R., Carmona, M.A., 1993. Water activity in selected high-moisture foods. Journal of Food Composition and Analysis 6, 364–369. Garden, R., Duche, O., Leroy-strin, S., European Listeria genome comortium and J. Labadie, 2003. Role of ctc from L. monocytogenes in osmotolerance. Applied and Environmental Microbiology 69, 154–161. Gram, L., Huss, H.H., 2000. Fresh and processed fish and shellfish. In: Lund, B.M., Baird-Parker, A.C., Gould, G.W. (Eds.), The Microbiological Safety and Quality of Foods. Chapman & Hall, London, pp. 472–506. Horner, K.J., Aragnostopoulus, J.D., 1973. Combined effects of water activity, pH and temperature on the growth and spoilage potential of fungi. Journal of Applied Microbiology 36, 427–436. Labuza, T.P., 1980. Water activity: physical and chemical properties. In: Linko, P., Melkki, Y., Olkku, J., Larinkari, J. (Eds.), Food Process Engineering. Applied Science Publishers, London, p. 320. Leistner, L., 1995. Principles and applications of hurdle technology. In: Gould, G.W. (Ed.), New Methods of Food Preservation. Blackie, Glasgow, p. 1. Leistner, L., 1995. Stable and safe fermented sausages world-wide. In: CampbellPlatt, G., Cook, P.E. (Eds.), Fermented Meats. Blackie, Glasgow, p. 160. Leistner, L., Rodel, W., 1976. The stability of intermediate moisture foods with respect to micro-organisms. In: Davies, R., Birch, G.G., Parker, K.J. (Eds.), Intermediate Moisture Foods. Applied Science Publishers, London, p. 120. Morris, E.O., 1962. Effect of environment on microorganisms. In: Hawthorn, J., Leitch, J.M. (Eds.), Recent Advances in Food Sciences, vol. 1, pp. 24–36. Roos, Y.H., 1993. Water activity and physical state effects on amorphous food stability. Journal of Food Processing and Preservation 16, 433–447. Scott, W.J., 1953. Water relations of Staphylococcus aureus at 30 C. Australian Journal of Biological Sciences 6, 549–564. Shelly, J., Schmidt, Anthony, J. Fontana Jr., 2008. Water activity values of select food ingredients and products. Water Activity in Foods: Fundamentals and Application. Wiley online library, 407–420. Sleator, R.D., Hill, C., 2001. Bacterial osmoadaptation: the role of osmolytes in bacterial stress and virulence. FEMS Microbiology Reviews 26, 49–71. Van den Berg, C., 1984. Description of water activity of food for engineering purpose by means of the GAB model of sorption. In: McKenna, B.M. (Ed.), Engineering and Food. Elsevier, London, pp. 311–321. Wodzenski, R.J., Frazer, W.C., 1961. Moisture requirements of bacteria –II. Influence of temperature, pH, and malate concentration on requirements of Acetobacter Aerogenes. Journal of Bacteriology 81, 353–358. Yun-chan lo, Froning, G.W., Arnold, R.G., 1983. The water activity lowering properties of selected humectants in eggs. Poultry Science 62, 971–976.