PHYSICAL, CHEMICAL AND MICROBIOLOGICAL CHARACTERIZATION OF POLYMER AND SOLUTE BOUND WATER

Water in a food may be divided into two broad categories: free and bound. Free water shows the properties of bulk water while bound water shows a reduced vapor pressure, lower freezing point, etc. Unfortunately, bound water is not a single entity and may be present in a number of physical states such as water of hydration, capillary water, etc.;A working equation was developed for calculating moisture content of a formulated food system giving the desired water activity (Aw). A simple mass balance relation was shown to be valid for mixtures of macromolecules and solutes over Aw 0.33 to 0.95. This mass balance was then combined with a linear sorption isotherm equation resulting in a model which gives Aw for mixtures from knowledge of the total moisture content of the mixture and ingredient isotherms.;A model was developed for calculating energy of water binding by a mixture of ingredients. The model must be applied at constant Aw and calculated values were within 3% of experimental.;Sorption data plotted as moisture content vs log (1-Aw) gave linear isotherms over the range 0.30 to 0.95. Two distinct shapes were shown depending on the sorbent; macromolecules showed a low slope and a positive intercept while solutes showed a very large slope and negative intercept. It was concluded that macromolecules bind one type of water, termed "polymer" water and that solutes bind a different type of water, termed "solute" water. A mixture of these shows a broken isotherm line.;This concept of different forms of bound water is of great importance in preservation by freezing, evaporation or dehydration as well as thermal processing. The state of water present undoubtedly governs the energy required to thermally process a food. It is of crucial importance to the preservation of intermediate moisture foods (IMF) against microbial and chemical changes. Therefore, a characterization of each form of bound water present in the 0.30 to 0.95 Aw range will enable the food scientist to elucidate the role of water in IMF food, and thus place their formulation on a scientific basis. This will lead to improvements in the food supply of this nation and the world.;Water bound by polymers in the range 0.95 to 0.99 was shown to have the same sorption characteristics as porous glass. In addition, pulsed NMR studies showed the signal from macromolecules to be similar to that from the porous glass. It was concluded that macromolecules contain "capillary" water at the high Aw.;The effect of temperature on water sorption by sugars and sodium chloride was determined. The saturation Aw, saturation moisture content and slope of the line increased with decreasing temperature.;Pulsed NMR studies showed the T(,1) relaxation times were negligible for polymer water and very large for solute water. These NMR data coincided with linear sorption isotherm data previously presented. T(,2) relaxation times from NaCl indicated less water structure than in free water while signals from sucrose solutions showed more. Upon dilution, T(,2) approached that of free water. Polymer water gave no T(,2) signal.;The chemical characterization of polymer and solute waters was done by determining the rate of atmospheric oxidation of ascorbic acid on starch and on sugar. Starch showed only 12% less in 84 hours while the sucrose solution at the same Aw (0.946) showed only 12% remaining in 24 hours. At 0.910 Aw, only the sucrose solution showed an appreciable rate.;The microbiological characterization of polymer and solute waters was done by studying germination of Aspergillus parasiticus conidia on starch and on sugar. It was concluded that these conidia will germinate only in the presence of capillary or solute waters, not on polymer water at any Aw.