Water Activities

Water activity (u ) is defined as the ratio between the water vapour pressure exerted by the water in a food system (p) and that of pure water (p ) at the 

Due to the presence of various solutes, the vapour pressure exerted by water in a food system is always less than that of pure water (unity). Water activity is a temperature-dependent property of water which may be used to characterize the equilibrium or steady state of water in a food system (Roos, 1997). 

For a food system in equilibrium with a gaseous atmosphere (i.e. no net gain or loss of moisture to or from the system caused by differences in the vapour pressure of water), the equilibrium relative humidity (ERH) is related to by  

under ideal conditions, ERH is the % relative humidity of an atmosphere in which a foodstuff may be stored without a net loss or gain of moisture. Water activity, together with temperature and pH, is one of the most important parameters which determine the rates of chemical, biochemical and microbiological changes which occur in foods. However, since presupposes equilibrium conditions, its usefulness is limited to foods in which these conditions exist. 

Water activity is influenced by temperature and therefore the assay temperature must be specified. The temperature dependence of is described by the Clausius-Clapeyron equation in modified form  

Water activity a ) is another critical factor. It is defined as the accessible or available water for the growth of the microorganism. It gives the amount of unbounded water available in the immediate surroundings of the microorganism. It is closely related, but not equal to, the water content. It is represented as 

It is well known that a has a profound influence on the growth and metabolism of microorganisms in SSF. At a low a., fungi generally accumulate polyols, such as glycerol, erythritol, arabitol, or mannitol, to prevent water loss from the cells. 

The solid substrate used in SSF exerts a buffering effect due to its complex chemical composition. It is difficult to monitor pH changes in SSF. To control the pH in SSF, ammonium salts have been used in combination with urea or nitrate salts so that the effect of acidification and alkalization can be neutralized. Filamentous fungi and yeasts with a broad range of pH for growth can be exploited to prevent bacterial contamination by using a low pH. 

Substrates that provide sohd support and act as carbon and energy source in SSF have a common basic macromolecular structure for example, cellulose, starch, pectin, lignocelluloses, fibers, and so on. Based on the nature of the substrate, certain preparative and pretreatment steps are necessary to convert the raw substrate in to a suitable form, which include the following  

Several factors influence the selection of substrates for SSF. Among them, cost, availability and heterogeneity of the substrates are the crucial factors. Based on the 

Another method of calculating water activities was presented by Meissner (MIO) in 1980  

Here again the superscript denotes pure solution, and the odd numbered 

The water activities calculated in this manner could then be combined using equation (5.31). 

The mixed solution water activity could also be calculated by substituting the q in equations (4.46) with qij.mix- 

Used in conjunction with equation (5.52), for each dectrolyte ij in the solution, a hypothetical water activity, Aj j, would be calculated. These would then be 

Labuza (1971) has described the complex relationship between water activity (uw) and lipid oxidation, with a minimum observed at intermediate aw ( 0.4) 

Routine procedures to assay the extent of oxidation in lipids and lipid-containing foods should be simple, reliable and sensitive. Results from routine procedures should ideally correlate well with results obtained from sensory taste panels. St. Angelo (1996) has described volatile compound profiles formed during lipid oxidation in different groups of food products. However, because of the complexity of lipid oxidation, no single test can be equally useful at all stages of the oxidative process. The methods should be capable of detecting autoxidation before the onset of off-flavor. This is particularly true in the case of milk products where a low level of oxidation can lead to off-flavor. 

Measurement of hydroperoxides is the classical method for quantifying lipid oxidation and a variety of assay procedures are available. The oxidation of ferrous to ferric iron by hydroperoxides in the presence of ammonium thiocyanate to produce ferric thiocyanate, which can be quantified spectrophotometrically at 505 nm, has been used extensively to study lipid oxidation in milk (Loftus-Hills and Thiel, 1946). Newstead and Head-ifen (1981) recommend that extraction of fat from whole milk powder be carried out in the dark when using this procedure to avoid artefactually high 

King (1962) showed that the TBA method correlates well with the intensity of oxidized flavor in liquid milk. Downey (1969) suggested that the TBA procedure of Dunkley and Jennings (1951) is more applicable than that of King (1962) for determining the extent of the off-flavor. 

Other traditional methods available for monitoring the extent of lipid oxidation include the Anisidine value, the Kreis test (Mehlenbacher, 1960), methods based on the carbonyl content of oxidized fats (Henick et al., 1954 Lillard and Day, 1961), and measurement of oxygen uptake either by manometry or polarography (Tappel, 1955 Hamilton and Tappel, 1963). 

In 1952, Scott came to the conclusion that the storage quality of food does not depend on the water content, but on water activity (a ), which is defined as follows  

P = partial vapor pressure of food moisture at temperature T 

The relationship between water content and water activity is indicated by the sorption isotherm of a food (Fig. 0.3). 

At a low water content ( 50%), even minor changes in this parameter lead to major changes in water activity. For that reason, the sorption isotherm of a food with lower water content is shown with an expanded ordinate in Fig. 0.3b, as compared with Fig. 0.3a. 

Foods with aw values between 0.6 and 0.9 (examples in Table 0.4) are known as intermediate moisture foods (IMF). These foods are largely protected against microbial spoilage. 

Before proceeding further it is important to note that the moisture content of food is often expressed in terms of water activity Uw which is defined as the partial pressure of water vapour above the food surface divided by the pure component vapour pressure of wafer p, at the same temperature as the sample, and therefore 

The partial pressure p, canbe obtained from Raoult s law which relates the partial pressure of wafer vapour above the surface of an aqueous solution held at a constant temperature, the mole fraction of wafer in the solution and the pure component vapour pressure of water, i.e. 

Thus for both an ideal system and for high moisfure confenfs, wafer acfivify is effecfively equal fo the mole fraction of wafer in fhe liquid phase wifhin fhe food. There are difficulfies in using equafion 4.5 because acfivify coefficients are complex functions of both temperature and moisture content and are not readily determined. Now, relative humidity is defined by 

If the food is in contact with, and at thermal equilibrium with, the surrounding air then, from equafion 4.1, water activity is equal to the fractional relative humidity. That is 

This now forms fhe basis of fhe experimental measurement of water activity. 

---

Water Activity. The rates of chemical reactions as well as microbial and en2yme activities related to food deterioration have been linked to the activity of water (qv) in food. Water activity, at any selected temperature, can be measured by determining the equiUbrium relative humidity surrounding the food. This water activity is different from the moisture content of the food as measured by standard moisture tests (4). 

When water activity is low, foods behave more like mbbery polymers than crystalline stmctures having defined domains of carbohydrates, Hpids, or proteins. Water may be trapped in these mbbery stmctures and be more or less active than predicted from equiUbrium measurements. As foods change temperature the mobiUty of the water may change. A plot of chemical activity vs temperature yields a curve having distinct discontinuities indicating phase... 

Dehydration Processing. Dehydration is one of the oldest means of preserving food. Microbes generally do not grow below a minimum water activity, of 0.65 defined as the equiHbrium relative humidity surrounding food ia a sealed container at a given temperature, ie, no microbes can... 

Each food or food ingredient shows a characteristic equiHbrium relative humidity at a given moisture content and temperature. Thus as a food is dried and its moisture content is reduced from its fresh value where water activity is generally 1.0, to lower and lower values, the equiHbrium water activity of the food decreases as a complex function of residual moisture. The shape of the equiHbrium relative humidity—moisture content curve is set by the chemistry of the food. Foods high ia fmctose, for example, biad water and thus show lower water activities at high moisture contents. Dried pmnes and raisias are examples. Drying can be terminated at any desired moisture content and hence any water activity. 

Foods high ia sucrose, proteia, or starch (qv) tend to biad water less firmly and must be dried to a low moisture content to obtain microbial StabiHty. For example, grain and wheat flour can support mold growth at moisture contents above 15% (wet basis) and thus are stored at moisture contents below 14%. Stored grains and oil seeds must be kept at a water activity below 0.65 because certain molds can release aflatoxias as they grow. Aflatoxins are potent carciaogens (see Food toxicants, naturally occurring). 

The immersion of glass electrodes in strongly dehydrating media should be avoided. If the electrode is used in solvents of low water activity, frequent conditioning in water is advisable, as dehydration of the gel layer of the surface causes a progressive alteration in the electrode potential with a consequent drift of the measured pH. Slow dissolution of the pH-sensitive membrane is unavoidable, and it eventually leads to mechanical failure. Standardization of the electrode with two buffer solutions is the best means of early detection of incipient electrode failure. 

Reaction can be initiated by several means, aH of which depend on deHvery of heat at a relatively high temperature to a starting cone. Cartridge-actuated and electric match units are usuaHy used. The former is in the majority. A water-activated unit has been described (12). The heat generated by the starting device initiates reaction in a cone, which is a smaH amount of candle that is higher in fuel content, eg, 30 wt % iron. Compared to... 

A variety of methods have been devised to stabilize shales. The most successful method uses an oil or synthetic mud that avoids direct contact between the shale and the emulsified water. However, preventing direct contact does not prevent water uptake by the shale, because the organic phase forms a semipermeable membrane on the surface of the wellbore between the emulsified water in the mud and the water in the shale. Depending on the activity of the water, it can be drawn into the shale (activity lower in the shale) or into the mud (activity higher in the shale) (95—97). This osmotic effect is favorable when water is drawn out of the shale thus the aqueous phase of the oil or synthetic mud is maintained at a low water activity by a dding a salt, either sodium chloride or more commonly, calcium chloride. The salt concentration is carried somewhat above the concentration required to balance the water activity in the shale to ensure water movement into the mud. 

Addition of a salt can transform the shale by cation exchange to a less sensitive form of clay, or reduce the osmotic swelling effect by reducing the water activity in the mud below that which occurs in the shale. These effects depend on the salt concentration and the nature of the cation. Salts containing sodium, potassium, calcium, magnesium, and ammonium ions ate used to varying degrees. 

Other factors also impact the type of crystals formed upon cooling of hot soap. Water activity or moisture content contribute to the final crystal state as a result of the different phases containing different levels of hydration. Any additive that changes the water activity changes the crystallization pathway. For example, the addition of salt reduces the water activity of the mixture and pushes the equiUbrium state toward the lower moisture crystal stmcture. Additionally, the replacement of sodium with other counter cations influences the crystallization. For example, the replacement of sodium with potassium drives toward the formation of 5-phase. 

The inhibitory activity of sorbates is attributed to the undissociated acid molecule. The activity, therefore, depends on the pH of the substrate. The upper limit for activity is approximately pH 6.5 in moist appHcations the degree of activity increases as the pH decreases. The upper pH limit can be increased in low water activity systems. The following indicates the effect of pH on the dissociation of sorbic acid, ie, percentage of undissociated sorbic acid at various pH levels (76,77). 

Food apphcations of sorbates expanded rapidly after issuance of the original patents in 1945 (92). The first uses were based on their excellent fungistatic properties and thus involved foods with low pH and/or low water activity in which yeasts and molds are the primary spoilage agents. More recent appHcation research has been directed toward utilizing the bacteriostatic properties of sorbates. 

Sorbate combined with mild heat has a synergistic effect with regard to microbial destmction thus, in the presence of 0.025—0.06 wt % sorbate, products such as apple juice, peach and banana sHces, fmit salads, and strawberries can be treated with less severe heat treatments to extend shelf life (119,120). Sorbates increase the heat sensitivity of various spoilage fungi under varying conditions of pH and water activity (121—124). A similar synergistic effect has been reported for the combination of sorbate with irradiation (125). 

Pet Foods and Commercial Animal Feeds. Eor many years, it has been known that stable, long-shelf-life, intermediate-moisture pet foods can be prepared through the use of 0.1—0.3 wt % sorbates. In these products, the antimicrobial effectiveness of sorbates is enhanced by a combination of moderate heat treatment, pH adjustment, and reduced water activity via humectants such as propylene glycol, or by adjusting sugar and salt content. These techniques have been reviewed extensively (138,139). 

Sucrose is often used as a decorative agent to impart a pleasing appearance to baked goods and confections (36). In jams and jeUies, sugar raises osmotic pressure and lowers water activity to prevent spoilage (18). Sucrose is a fermentation substrate for lactic acid in cultured buttermilk (40) and lowers the freezing point of ice cream and other frozen desserts to improve product mouthfeel and texture. 

The rate of aspartame degradation in dry mixes is more dependent on the water activity than on the temperature (23). In dry mixes, aspartame may also engage ia Maillard reactions with the aldehyde moieties of flavoting agents, resulting ia the loss of sweetness and flavor. Use of the corresponding acetals of the flavor compounds to avoid this reaction has been reported (24). 

Reserve batteries have been developed for appHcations that require a long inactive shelf period foUowed by intense discharge during which high energy and power, and sometimes operation at low ambient temperature, are required. These batteries are usually classified by the mechanism of activation which is employed. There are water-activated batteries that utilize fresh or seawater electrolyte-activated batteries, some using the complete electrolyte, some only the solvent gas-activated batteries where the gas is used as either an active cathode material or part of the electrolyte and heat-activated or thermal batteries which use a soHd salt electrolyte activated by melting on appHcation of heat. 

In this process the addition of water vapor to the sweep stream can be controlled so that the water activity of the gas phase equals that of the beverage. When this occurs, there is no transport of water across the membrane. The water content of both the beverage feed and the sweep stream is kept constant. These conditions must be maintained for optimum alcohol reduction. The pervaporation system controls the feed, membrane, airstream moisture level, and ethanol recovery functions. An operational system has been developed (13). 

Sucrose is widely used in the food industry to sweeten, control water activity, add body or bulk, provide crispness, give surface glaze or frost, form a glass, provide viscosity, and impart desirable texture. It is used in a wide variety of products from bread to medicinal symps. 

Microbiol Stability. Microbial growth is hindered by reducing water activity and adding preservatives. An overview is available (30). Reduction in water activity is typically obtained by including approximately 50% of a polyalcohol such as sorbitol or glycerol. Furthermore, 20% of a salt like NaCl has a pronounced growth inhibiting effect. 

Some alcohols, eg, propylene glycol, not only lower water activity but also have an additional preservative effect caused by the way they interfere with the ceU membrane transport system of the contaminating microorganisms. Surfactants (qv) may show a similar effect. 

The actual name dry scrubbing was first publicized by Teller [U.S. Patent no. 3,721,066 (1973)]. He worked both with classical Army-type soda-lime and with his patented water-activated form of the alkaline feldspar nepheline syenite as a flow agent and feedstock sorbent for HF and SO9 in hot, sticky fumes from glass melting furnaces. He claimed capture of more than 99 percent of 180 ppm HF and SO9 for more than 20 hours in a packed bed of 200 X 325 mesh hydrated nephehne syenite at 42,000/hr. 

---