Dried water activity

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 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). 

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. 

Alumina used in TLC can be recovered by washing in ethanol for 48h with occasional stirring, to remove binder material and then washed with successive portions of ethyl acetate, acetone and finally with distilled water. Fine particles are removed by siphoning. The alumina is first suspended in 0.04M acetic acid, then in distilled water, siphoning off 30 minutes after each wash. The process is repeated 7-8 times. It is then dried and activated at 200° [Vogh and Thomson Anal Chem 53 1365 1981],... 

Commercial silica gel has also been purified by suspension of 200g in 2L of 0.04M ammonia, allowed to stand for 5min before siphoning off the supernatant. TTie procedure was repeated 3-4 times, before rinsing with distilled water and drying, and activating the silica gel in an oven at 110° [Vogh and Thomson, Anal Chem 53 1345 198]]. 

Thus, in the observation of the drying process of biological materials, the water activity is at least as valuable as the water content of a material. As water activity measurements are complete in about 3 min, they represent a useful tool for control of the drying process. 

Drying out can be prevented in one of two ways, either by packaging or by lowering the water activity of the product. Starch retrograda-tion can be inhibited by using a starch complexing emulsifier with or without the addition of fat. Mould growth is inhibited by a low water activity. 

These solve the shelf life problem by having a very low water activity aided by the quantity of dried fruit that they contain. The fat content and the low water activity inhibit starch retrogradation. 

The storability of the dried product depends to a large extend on the selected type, e. g. strawberries, carrots and green beans [4.7]. For meat, the fat content can be important. Karel [4.8] studied the influence of the water content in stored dried food, and found that not only was the amount of water of influence, but also the kind of bond to the solids. This link can be described by adsorption isotherms, as shown in Fig. 4.1. In food technology, the bond of water is often given by the term water activity, aw ... 

Briefly, you mix the adsorbant with water, spread the mix on the microscope slide in a thin layer, and let it dry, then activate the coating by heating the coated slide on a hot plate. Then you spot or place your unknown compound on the plate, let an eluent run through the adsorbant (development), and finally examine the plate (visualization). 

FIG. 17 Water activity (or relative vapor pressure) and moisture content (%, wb) values plotted for a variety of food materials ( as is ), resulting in a composite (or universal) food isotherm. Potato chip and nonfat dry milk (NFDM) data are from van den Berg (1986). All other data are from the author s laboratory (at 20 or 25 1 °C). Water activity and moisture content values for the breakfast cereal and fruit and vegetable categories had the largest ranges (expressed here as standard deviation) 0.311 0.073 aw and 2.94 1.55% me (wb) and 0.995 0.004 aw and 89.8 3.90% me (wb), respectively. The curve is to guide the eye—it is not a fitted line. 

Roos, Y.H. 2002. Importance of glass transition and water activity to spray drying and stability of dairy powders. Lait 82, 475-484. 

Soluble solids uptake due to osmotic dehydration, in addition to improving color, aroma, and vitamin stability during both air drying and frozen storage, could also play a very important role in the preparation of new types of ingredients at reduced water activity (Torreggiani et al., 1988). Due to the... 

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