2.1.1 Fundamental Principles
The extent to which seeds and grain are susceptible to spoilage fungi depends, above all, on the presence of water in the system. In fact, many authors have written about the fundamental requirement for water in microbial growth
(Cooke and Whipps 1993; Griffin 1981; Scott 1957). Most stored products such as seeds and grain are hygroscopic in nature, i.e., they will either take up or release moisture until they reach a dynamic equilibrium with the surrounding environment. It is straightforward to express this water in terms of the percentage moisture content, based on the ratio of the dry weight to the wet weight (expressed either on a wet weight or dry weight basis). This, however, implies that the stored material consists of dry matter plus a certain amount of "attached" liquid water. In fact the situation is more complex than this. This is principally because the water in these substrates is not all equivalent, but held in a number of different "states." Broadly speaking, three main states of water can be identified in a hygroscopic material: (a) constitutive water; water that is chemically linked to the substrate material and forms part of its structural makeup, (b) adsorbed water, which is closely linked with the substrate surface by physical interactions, and (c) absorbed water, which is loosely associated with the substrate surface (Pixton 1967). In practice, probably there is no clear demarcation between these different states; the relative proportions of each type will depend on the overall moisture content of the material. The absorbed water fraction will become more weakly bound as further layers of water molecules accumulate on the surfaces (i.e., as the substrate becomes wetter). It is the loosely held water that provides the "free" or "available" water, since it is the fraction that is most readily available for microbial growth, although its overall availability will be influenced by the presence of solutes. The important implication here is that microorganisms, including spoilage fungi, respond not to water content but to water availability. Therefore, the concept of water availability is of fundamental importance in relation to microbial growth and spoilage.
Water availability itself can be expressed in a number of ways, one of the most convenient being water activity (aw). The free water in a system is the proportion of the total water that is immediately available to reach equilibrium with the surrounding atmosphere. This will be reflected in the equilibrium vapor pressure or equilibrium relative humidity (ERH) exerted by the system. Water activity is defined as the ratio of the vapor pressure of the water in a substrate to that of pure water at the same temperature and pressure (Scott 1957). It is, therefore, directly linked to the ERH by the expression:
where p is the vapor pressure of water in solution or solid substrate, P0, vapor pressure of pure water at experimental temperature and pressure, and ERH (%) is equilibrium relative humidity at which a solution or solid substrate neither gains nor loses moisture to the atmosphere.
Water activity can, therefore, be seen to be a measure of the ability of water to evaporate from a substrate and humidify the immediate environment, and is measured in the range 0-1.0 with 1.0 representing the aw of pure water. Most importantly it should be noted that aw is a function of temperature, and for a given substrate and moisture content, aw will increase with increasing temperature. This is primarily the consequence of the general increase in thermal motion (Multon 1988).
Water availability may also be expressed as water potential (C). Water potential approaches the concept of water availability in terms of the thermodynamics of systems, and considers the movement of water along gradients of potential energy. The term "water potential" is actually an abbreviation of "potential energy of water" and is defined as "the free energy of water in a system relative to the free energy of a reference pool of pure free water having a specified mass or volume," and is measured in J/m3 or Pa (Papendick and Mulla 1985). The reference state of pure free water is assigned zero. Water which is "constrained" in a system, i.e., the constitutive, adsorbed and (to a lesser extent) absorbed water is, therefore, at a lower (negative) water potential, and any microorganism must expend energy to lower internal water potential values relative to the exterior to make water available. The numerical value of water potential may be related to water activity using the formula:
where R is gas constant, T, absolute temperature (K), and Vw is molecular weight of water.
Although C and aw are interconvertible by this expression, measurement of the separate components that make up total water potential cannot be carried out using water activity measurement, since water activity is not a sufficiently sensitive parameter.
The relationship between water content and aw in a given substrate may be usefully expressed by moisture sorption isotherms. These are curves produced by plotting water content against water activity, or ERH at constant temperature (hence, the term "isotherm"). They are highly specific to substrate type and condition but generally sigmoidal in shape show a steep rise in the curve above 80% ERH. The importance of moisture sorption isotherms is that they establish the relationship between moisture content and water availability for specific product types and, therefore, allow water availability to be set to safe levels from knowledge of moisture content alone. For most stored seed products safe moisture levels will typically correspond to water activity levels of around 0.7 water activity or 70% ERH. The actual moisture content which it corresponds to will vary greatly between seed types, and particularly between starchy and oily types. Moisture sorption isotherms are complicated by a hysteresis effect that influences the position of the curve depending on whether the material is being wetted or dried as each equilibrium point is plotted. This effect may account for a difference of as much as 0.5% (Pixton 1967) and always shows a higher water activity for a given moisture content where water is being sequentially added. This indicates that the "history" of the material is important and probably reflects irreversible and permanent changes occurring after wetting.
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