Relative humidity Water activity

Desiccation of plant tissues presents a shift of the water from the liquid to the vapour phase (Sun, 2002). Temperature influences evaporation, as well as the partial water vapour pressure in the air and the energy status of water in plant tissue, both in dry and hydrated plant tissue. An increase in temperature results in a decrease in the equilibrium water content at a given relative humidity (water activity) or an increase in the equilibrium water activity for a given tissue water content (Fig. 1). Water activity can be described as the effective water content, which is thermodynamically available for various physiological processes in cells. The temperature dependence of the isotherm shift is described by the Clausius-Clapeyron Equation ... 

Fig. 29. The water diffusion in 6-polyamide at 80 °C at function of the rel. humidity. The activation energy of Dmax is 13.7 1.5 kcal/mole for 20 °C T = 80 °C149 ...

Where k is Henry s law constant and in the case of moisture absorption, the activity relates to relative humidity as activity is p/po and relative humidity is (p/po) x 100. The symbols p and po represent the vapor pressure and saturated vapor pressure of water, respectively. Henry s law does not allow for polymer-penetrant chemical interaction and is generally only applicable at low concentrations or in systems in which the penetrant and polymer have little affinity for each other. 

As mentioned in Section IX-2A, binary systems are more complicated since the composition of the nuclei differ from that of the bulk. In the case of sulfuric acid and water vapor mixtures only some 10 ° molecules of sulfuric acid are needed for water oplet nucleation that may occur at less than 100% relative humidity [38]. A rather different effect is that of passivation of water nuclei by long-chain alcohols [66] (which would inhibit condensation note Section IV-6). A recent theoretical treatment by Bar-Ziv and Safran [67] of the effect of surface active monolayers, such as alcohols, on surface nucleation of ice shows the link between the inhibition of subcooling (enhanced nucleation) and the strength of the interaction between the monolayer and water. 

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). 

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. 

Because of their selectivity, molecular sieves offer advantages over silica gel, alumina or activated charcoal, especially in their very high affinity for water, polar molecules and unsaturated organic compounds. Their relative efficiency is greatest when the impurity to be removed is present at low concentrations. Thus, at 25° and a relative humidity of 2%, type 5A molecular sieves adsorb 18% by weight of water, whereas for silica gel and alumina the figures are 3.5 and 2.5% respectively. Even at 100° and a relative humidity of 1.3% molecular sieves adsorb about 15% by weight of water. 

Figure 2.4 shows the equilibrium relationships of biological materials between the water content and the water activity, at constant temperatures and pressures. These data were first published in 1971, but did not find much attention in the RM field until now. At equilibrium the water activity is related to the relative humidity cp of the surrounding atmosphere (Equation 2.3) where p is the equihbrium water vapor pressure exerted by the biological material and po the equilibriiun vapor pressure of pure water at the same temperature. 

In this area the change of water content, Aw.c., as a function of the change of relative equilibrium humidity, Acp, as a function of water activity (a 100 = rp), Aw.c./Acp, is at a minimum. This also minimizes the potential error in a certified value by water taken up from the surrounding area. Based on these findings, it appears absolutely necessary that during the preparation of each material, water activity as well as water content must be determined and adjusted to achieve optimal stability and thus also a long shelf hfe of the final product. 

The practical measurement of water activity nowadays is usually performed by dew point determination or a change of electrical properties of the material depending on the relative humidity. 

Sharma et al. [153] have devised a gentle accelerated corrosion test using a kinetic rate equation to establish appropriate acceleration factors due to relative humidity and thermal effects. Using an estimate for the thermal activation energy of 0.6 eV and determining the amount of adsorbed water by a BET analysis on Au, Cu and Ni, they obtain an acceleration factor of 154 at 65°C/80% RH with respect to 25 °C/35-40% RH. 

How much water is present and what is the corresponding water activity (approximated by relative pressure or percent relative humidity/100) ... 

Thus, the water activity of a food is equal to the relative vapor pressurepv/p°. The relative vapor pressure is also related to percentage relative humidity (%RH) divided by 100. It is critical to bear in mind the assumptions underlying the development and thus the use of Eq. (5) for determining the aw of a food. These assumptions are examined in detail in the next section. 

Norrish, R.S. 1966. An equation for the activity coefficients and equilibrium relative humidities of water in confectionery syrups. J. Food Technol. 1, 25-39. 

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