Monolayers moisture value

By plotting the natural logarithm of activity versus the reciprocal of absolute temperature at constant moisture values, straight lines are obtained with a slope of -QJR (Figure 1-14). The values of Qs obtained in this way for foods having less than full monolayer coverage are between about... 

Figure 27. Sorption isotherms predicted by BET sorption theory for various values of n (broken lines), compared with the experimental isotherm (solid line). Also shown is the monolayer moisture content Mj (10). (Reproduced with permission from Ref 10. Copyright 1972, Syracuse University Fress.)...

The monolayer moisture content (Xm) for freeze-dried natural camu-camu pulp can be compared with the values found for other freeze-dried fruits, such as persimmon (X = 0.131) (Telis and Sobral, 2001), pineapple (X = 0.072) (Sobral et al., 2001), grape, fig, and apricot (X = 0.125, 0.117, and 0.151) (Maroulis et al., 1988) at 25°C. The monolayer moisture content obtained for freeze-dried camu-camu pulp with 30% maltodextrin DE 20 was close to the value found by Roos (1993) for pure maltodextrin DE 20. 

CWC was of 3% (g water/100 g product) and CWA was of 0.22. If the CWC is compared with the monolayer moisture content (wq = 12.35%), the greater value of the last one is observed, such as has been reported by other authors (Roos, 1993 Moraga et al., 2004). According to Roos (1993), the rates of the quality changes increase notably after plasticization promoted by temperature or water, even below the monolayer value. 

For most woods the initial differential heat of sorption (at 0% moisture content) is approximately 1250 J g . This equates to 22.5 kJ mol (18 g of water in one mol) which is typical for hydrogen bonding. By the time a complete monolayer has formed, by about 4-5% moisture content, the differential heat of wetting is around half the initial value (9-10 kJ moF ). This value would be amongst the weakest hydrogen bond values reported. The first water molecules to be adsorbed on the oven-dry ground-wood have a complete choice of accessible cell wall hydroxyls and... 

Mobility of water in cellulose has been studied by solid-state and high-resolution NMR as a function of moisture content within the unfreezable moisture range (0-19% dry basis).Measurements of relative mobilities were based on relative intensities, transverse and longitudinal relaxation times and line shape analysis. At 2-16% moisture content (dry basis), water molecules reoriented anisotropically, suggesting an interaction with cellulose fibers. At moisture content below the monolayer value (2.8%, dry basis), 90% of the protons were immobile and no liquid deuterium signal was detected. A sharp increase in liquid or mobile intensity (accompanied by a decreased LW) and increases in NMR Ti and T2 relaxation times were observed as moisture increased above 9% (dry basis). 

NEB and almost all chemical reactions in foods are thought to cease in systems that are below a of 0.23 to 0.43, the typical range of the moisture monolayer value (Labuza et al., 1969 Rockland and Nishi, 1980). In Figure 20.5b, this trend appeared as expected for the control formulation, which contained no humectant. However, upon the addition of hydrophilic glycols as humectants (which were liquids at the test temperature and thus miscible in water), the at which the maximum reaction rate occurred shifted to a much lower value. Propylene glycol showed a maximum rate at a of about 0.2, with a rate equal to the maximum rate of the control at a a of 0.8. 

The Brunauer-Emmett-Teller (BET) and Guggenheim-Anderson-deBoer (GAB) sorption isotherm models were used to obtain experimental steady-state moisture contents in dry basis by linear regression analysis according to Kouassi and Roos (2002). These equations provide the value of monolayer water content, which is an important parameter in food deterioration studies. 

Moisture content is a very important parameter influencing the stability of dehydrated foods. It has been suggested that the optimal amount of water for long-term storage corresponds in most dehydrated foods to the Brunauer-Emmett-Teller (BET) monolayer value. On the other hand, items such as freeze-dried spinach, cabbage, and orange juice were reported to be more stable at a zero moisture content, whereas items like potatoes and corn had maximum stability at the monomolecular moisture content. It appeared that optimal moisture content could not be predicted with precision on the basis of theoretical considerations. 

Adsorbed moisture can be held very tenaciously by natural fibers. Nuclear magnetic resonance studies have shown that adsorbed water on green and remoistened wood can exist in two states, with an immobile monolayer bonding directly to the cell walls of the fibers [19]. For most woods the differential heat of sorption is about half that of vaporization, falling to about one quarter of that value when a complete monolayer is formed at about 4%-5% moisture content [15]. 

Another important quality defect of fried snacks is the reduction of crispness during storage due to absorption of moisture. In addition, moisture gain accelerates rancidity. Data concerning the effect of water activity (a ) on oxidation rate are presented by Labusa [39]. As a increases above the monolayer, the reaction is slowed up to the value where intermediate moisture foods begin (i.e., approximately 0.4). A further moisture increase tends to increase the rate of oxidation. Data on water activity that results in loss of crispness to an extent where the chips are perceived as not acceptable are not available, but Labusa comments that it probably occurs at an of 0.4-0.5. 

Additionally, many experimental studies have shown that, for various protein systems, the storage stability improves as the residual humidity decreases and that the optimal final water content is generally around 1% or less. Water contents above the monolayer content increase the mobility of the water that becomes more available for protein degradation reactions with the other solutes of the concentrated phase. Moreover, water is a plasticizer of the amorphous phase that decreases the Tg value, which leads to more difficult storage conditions. Finally, it should be noted that the moisture content is generally not uniform within the vial, the top and the wall zones of the cake being the driest zones and these non-uniformities in the spatial distribution of the water content generally increase with the size of the vial. 

---