SUMMARY: Survivor curves for spores of Bacillus subtilis were determined in wet and dry heat over a wide range of temperatures. Wet heat tests were determined using a thermoresistometer and thermal death time cans. Dry heat tests were conducted in a closed system using thermal death time cans. There were major differences in the shape of the wet vs. dry heat survivor curves. Wet heat resulted in convex curves at low temperatures, but a straight line at higher temperature. Dry heat resulted in concave curves at all temperatures. These results suggest that physiological differences exist between wet and dry heat destruction of bacteria. Several possible explanations for the difference in the shape of the survivor curves were discussed.

Spores of Bacillus subtilis were dried in vacuo for use in dry-heat thermal destruction tests. Survivor curve tests were conducted in a specifically designed dry-heat oven. This oven provided accurate temperature control and permitted air or nitrogen to be passed over the spores during the lethal treatment. Experiments were carried out at various flow rates of the two gases (air and nitrogen) and various temperatures, and the data were expressed as survivor curves from which the decimal reduction time (D value) was obtained. Linear regression analysis methods were used to compute the slope of the survivor curves. The results indicated that as the flow rate of gas is increased, the effect of temperature on the destruction rate of the spores is lessened, the z value becoming very large. It is believed that the higher flow rates of dry gas cause greater dehydration of the spores and that spore moisture loss is one of the major factors in determining the dry-heat thermal destruction rate of bacterial spores.

Modifications of a commercial 2,450-megahertz microwave oven were made so that 6 ml of microbial suspension could be exposed to the microwave field for various periods of time. The microorganisms were contained in the central tube of a modified Liebig condenser positioned in the approximate geometric center of the oven cavity. Kerosene at-25 C was circulated through the jacket of the condenser during microwave exposure permitting microwaves to reach the microbial suspension. Flow rates of the kerosene were varied to permit the temperature of the suspension to range from 25 to 55 C during microwave exposure. Conductive heating experiments using similar temperatures were also conducted. A thermocouple-relay system was employed to measure the suspension temperature immediately after the magnetron shutoff. Continuous application of microwaves to suspensions of 108 to 109 Streptococcus faecalis or Saccharomyces cerevisiae per ml appeared to produce no lethal effects other than those produced by heat. Respiration rates of microwave-exposed S. cerevisiae were directly related to decreases in viable count produced by increased microwave exposure times.

Modifications of a commercial 2,450-megahertz microwave oven were made so that 6 ml of microbial suspension could be exposed to the microwave field for various periods of time. The microorganisms were contained in the central tube of a modified Liebig condenser positioned in the approximate geometric center of the oven cavity. Kerosene at -25 C was circulated through the jacket of the condenser during microwave exposure permitting microwaves to reach the microbial suspension. Flow rates of the kerosene were varied to permit the temperature of the suspension to range from 25 to 55 C during microwave exposure. Conductive heating experiments using similar temperatures were also conducted. A thermocouple-relay system was employed to measure the suspension temperature immediately after the magnetron shutoff. Continuous application of microwaves to suspensions of 10 8 to 10 9 Streptococcus faecalis or Saccharomyces cerevisiae per ml appeared to produce no lethal effects other than those produced by heat. Respiration rates of microwave-exposed Scerevisiae were directly related to decreases in viable count produced by increased microwave exposure times.

Limited quantities of dried juice sacs have been produced commercially in Florida since the late 1950’s. Several citrus processors have been involved in this production at one time or another and one well known processor is currently producing relatively small but significant quantities. Workers at the University of Florida’s Agricultural Research and Education Center at Lake Alfred, as well as at the USDA’s Utilization Research Labs at Winter Haven, Florida and Pasadena, California have been advocating this speciality by-product citing the tangible advantages of the higher return as compared to that of dried orange pulp for animal feed. At the same time they point out the intangible benefits of removing one of the major sources of air pollutants from the stack emissions of feed mills. Paper published with permission.