No abstract

Recent studies have shown that dielectric properties of raw potato can be predicted over the range of 300–3000 Mhz and 5–65°C by a noninteractive Distributive model derived from lumped circuit analysis by a two‐phase approximation which treats the potato as a binary system consisting of an inert solid phase and an active liquid phase. Dielectric behavior was seen to result primarily from water and ion activities of aqueous regions but subject to appreciable modification by a mechanism of volume exclusion due to effects of colloidal solids. Cell‐free and whole potato extract measurements showed cation binding and complexing effects, resulting in considerably lower effective salts concentrations than implied by ash content. In addition, intracellular cation and biochemical constituent levels, were significantly higher than extracellular levels. However, dielectric behavior of aqueous regions of the potato appeared to be based on bulk average fluid properties subject to displacement by colloidal solids. Low‐frequency measurements of raw potato showed other regions of relaxation and conductivity effects rather than free water and bulk conductivity at low frequencies. But these appeared not to contribute to high‐frequency dielectric response of the potato since observed relaxations were of small magnitude or occurred at frequencies well below the ultrahigh and microwave regions, suggesting that surface properties of solid foods may not be of much significance at high frequencies. Preliminary analysis of solid food measurements by other workers suggests the feasibility of modelling solid food behavior by two‐phase approximations of Distributive, Maxwell or Rayleigh model behavior based on physical‐chemical properties. For example, raw beef measurements were predicted closely by a two‐phase model in Rayleigh form, suggesting modelling characteristics similar to the potato but with specific model behavior due to differencesin biological structure of beef and potato. A general physical‐chemical model is proposed for high‐frequency dielectric behavior of solid foods based on observed mechanisms of interaction between water and the biochemical constituents of foods.

Levels of oxygen and carbon dioxide in the gas environment were found to influence pigment production significantly and growth to a lesser extent in solid‐state fermentations with Monascus purpureus on rice. Maximum pigment yields were observed at 0.5 atm of oxygen partial pressure in closed pressure vessels. However, high carbon dioxide partial pressures progressively inhibited pigment production, with complete inhibition at 1.0 atm. In a closed aeration system with a packed‐bed fermentor, oxygen partial pressures ranging from 0.05 to 0.5 atm at constant carbon dioxide partial pressures of 0.02 atm gave high pigment yields with a maximum at 0.50 atm of oxygen, whereas lower carbon dioxide partial pressures at constant oxygen partial pressures of 0.21 atm gave higher pigment yields. Maximum oxygen uptake and carbon dioxide evolution rates were observed at 70–90 and 60–80 h, respectively, depending on the gas environment. Respiratory quotients were close to 1.0, except at 0.05 atm of oxygen and 0.02 atm of carbon dioxide partial pressures. Optimum conditions for pigment formation were generally not the same as those for growth.

The gas environment is solid‐substrate fermentations of rice significantly affected levels of biomass and enzyme formation by a fungal species screened for high amylase production. Constant oxygen and carbon dioxide partial pressures were maintained at various levels in fermentations by Aspergillus oryzae. Control of the gas phase was maintained by a “static” aeration system admitting oxygen on demand and stripping excess carbon dioxide during fermentation. Constant water vapor pressures were also maintained by means of saturated salt solutions. High Oxygen pressures stimulated amylase productivity significantly. On the other hand, amylase production was severely inhibited at high carbon dioxide pressures. While relatively insensitive to oxygen pressure, maximum biomass productivities were obtained at an intermediate carbon dioxide pressure. High oxygen transfer rates were obtained at elevated oxygen pressures, suggesting, in view of the stimulatory effect of oxygen on amylase production, a stringent oxygen requirement for enzyme synthesis. Solid‐substrate fermentations were highly advantageous as compared with submerged cultures in similar gas environments. Not only were amylase productivities significantly higher, but the enzyme was highly concentration in the aqueous phase of the semisolid substrate particles and could be extracted in a small volume of liquid. Results of this work suggest that biomass and product formation in microbial processes may be amenable to control by the gas environment. This is believed to offer an interesting potential for optimizing selected industrial fermentation processes with respect to productivity and energy consumption.