水分吸附等温线Moisture Sorption Isotherms

1、Moisture Sorption Isotherms Moisture Sorption IsothermsDefinition and ZonesTemperature DependenceHysteresisDefinition and Zones A plot of water content (expressed as mass of water per unit mass of dry material) of a food versus p/p0 at constant temperature is known as a moisture sorption isotherm (M

2、SI). Information derived from MSIs are useful (a) for concentration and dehydration processes, because the ease or difficulty of water removal is related to RVP, (b) for formulating food mixtures so as to avoid moisture transfer among the ingredients, (c) to determine the moisture barrier properties

3、 needed in a packaging material, (d) to determine what moisture content will curtail growth of microorganisms of interest, and (e) to predict the chemical and physical stability of food as a function of water content (see next section). Shown in Figure 18 is a Shown in Figure 18 is a schematic MSI f

4、or a high-moisture schematic MSI for a high-moisture food plotted to include the full food plotted to include the full range of water content from range of water content from normal to dry. This kind of plot is normal to dry. This kind of plot is not very useful because the data of not very useful b

5、ecause the data of greatest interest those in the greatest interest those in the low-moisture regionare not low-moisture regionare not shown in sufficient detail. shown in sufficient detail. Omission of the high-moisture Omission of the high-moisture region and expansion of the low-region and expans

6、ion of the low-moisture region, as is usually done, moisture region, as is usually done, yields an MSI that is much more yields an MSI that is much more useful (Fig. 19).useful (Fig. 19).FIGURE 18Schematic moisture sorption isotherm encompassing abroad range of moisture contents Several substances t

7、hat have Several substances that have MSIs of markedly different MSIs of markedly different shapes are shown in Figure shapes are shown in Figure 20.These are resorption(or 20.These are resorption(or adsorption) isotherms adsorption) isotherms prepared by adding water to prepared by adding water to

8、previously driedpreviously dried samples.Desorption isotherms samples.Desorption isotherms are also common.Isotherms are also common.Isotherms with a sigmoidal shape are with a sigmoidal shape are characteristic of most foods.characteristic of most foods.FIGURE20 Resorption isotherms for various foo

9、ds and biological substances. As an aid to understanding the meaning and usefulness of sorption isotherms it is sometimes appropriate to divide them into zones as indicated in Figure 19. As water is added (resorption), sample composition moves from Zone I (dry) to Zone III (high moisture) and the pr

10、operties of water associated with each zone differ significantly. Water present in Zone I of the isotherm is most strongly sorbed and least mobile. This water associates with accessible polar sites by water-ion or water-dipole interactions, is unfreezable at -40C, has no ability to dissolve solutes,

11、 and is not present in sufficient amount to have a plasticizing effect on the solid. It behaves simply as part of the solid. The high-moisture end of Zone I (boundary of Zones I and II) corresponds to the “BET monolayer” moisture content of the food. The BET monolayer value should be thought of as a

12、pproximating the amount of water needed to form a monolayer over accessible, highly polar groups of the dry matter. In the case of starch, this amounts to one HOH per an hydroglucose unit. Zone I water constitutes a tiny fraction of the total water in a high-moisture food material. Water added in Zo

13、ne II occupies first-layer sites that are still available. This water associates with neighboring water molecules and solute molecules primarily by hydrogen bonding, is slightly less mobile than bulk water, and most of it is unfreezable at - 40C. As water is added in the vicinity of the low-moisture

14、 end of Zone II, it exerts a significant plasticizing action on solutes, lowers their glass transition temperatures, and causes incipient swelling of the solid matrix. This action, coupled with the beginning of solution processes, leads to an acceleration in the rate of most reactions. Water in Zone

15、s I and Zone II usually constitutes less than 5% of the water in a high- moisture food material. In gels or cellular systems, bulk-phase water is physically entrapped so that macroscopic flow is impeded. In all other respects this water has properties similar to that of water in a dilute salt soluti

16、on. This is reasonable, since a typical water molecule added in Zone III is “insulated” from the effects of solutes molecules by several layers of Zone I and Zone II water molecules. The bulk-phase water of Zone III, either entrapped or free, usually constitutes more than 95% of the total water in a

17、 high-moisture food, a fact that is not evident from Figure 19.Temperature Dependence As mentioned earlier, RVP is temperature dependent; thus MSIs must also be temperature dependent. An example involving potato slices is shown in Figure 21. At any given moisture content, food p/p0 increasing with i

18、ncreasing temperature, in conformity with the Clausius-Clapeyron equation.FIGURE 21 Moisture desorption isotherms for potatoes at various temperaturesHysteresis An additional complication is that an MSI prepared by addition of water (resorption) to a dry sample will not necessarily be superimposable on an isotherm prepared by desorption. This lack of superimposability is referred to as “hysteresis,” and a schematic example is shown in Figure 22. Typically, at any given p/p0 , the water content of the sample will be greater during desorption than during resorption. MSIs of polymers,