One of the most important environmental parameters which affect the growth, biochemical activities and death of microorganisms in foods is water activity (aw). By appropriate adjustment of aw in the food system the food scientist can secure its extended preservation and minimize the possibility of a food-borne disease problem. This paper deals with the following subjects with respect to aw: the effect of aw on the growth and the biochemical activities of microorganisms; the parameters which affect the water requirement of microorganisms; the effect of aw on the survival of microorganisms at low and high temperature; the technology of aw adjustment in foods and its food safety implications and finally a discussion of some recent developments, trends and problems in the application of the aw principle in food preservation.

One dimensional unsteady heat conduction was studied on soybean curd as a water-protein-fat food system. This heterogeneous soybean curd could be treated as homogeneous material for the unsteady heat conduction by "effective" thermal diffusivity. Measurements were made with an apparatus designed and constructed by the author and also made with ice to establish the reliability, the reproducibility and the accuracy of the apparatus. The effective thermal diffusivity of soybean curd was substantially dependent on the fraction of protein and fat as well as water

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The bioaccumulation of sup(233,234)U, 238U, 238Pu, sup(239,240)Pu, 241Am, and 244Cm in both native and introduced biota was studied at Pond 3513, a former low-level radioactive waste settling basin (0.4 ha surface area) at Oak Ridge National Laboratory. This system, which was decommissioned in 1976 after more than 30 years' use, contains approximately 5 Ci of sup(239,240)Pu; inventories of other actinide isotopes are considerably less. Significantly higher concentrations of actinides in fish that were allowed access to sediments indicated that sedimentary particulates may be the primary source of transuranics to biota in shallow fresh-water ecosystems. The study determined habitat, in particular the degree of association of an organism with the sediment/water interface, to be the primary factor in controlling transuranic concentrations in aquatic biota. In most of the biological samples analysed, excluding samples suspected of being contaminated by sediment, 241Am/239Pu, 244Cm/239Pu, and 238U/239Pu ratios were greater than the respective ratios in sediment while sup(233,234)U/238U, and sup(239,240)Pu/238Pu (aquatic biota only) ratios were not different from the respective ratios in sediment. The relative uptake of actinides from contaminated sediment by aquatic and terrestrial biota at this site was UCm=AmPu. The relative extractability of actinides from shoreline sediment (using 1 M HNO3 and 0.01 M HO1) was UCm approximately equal to AmPu. Concentrations of transuranics in water, terrestrial vegetation, and vertebrate carcasses were less than 10% of the recommended public exposure maximum permissible concentration (MPC) of the ICRP, even in this highly contaminated system, which has radioactive inventories atypical of any other fresh-water site now in use or planned for future use in association with nuclear power technologies. (author)