Evaluation of heat sterilization of commercial rat diets for use in FDA toxicological studies.

Certified commercial rat diets, control and fortified, in the form of pellets and meal, were evaluated in a simulated subchronic rat feeding study. The diets were analyzed before and after autoclaving to determine nutrient integrity and loss, as well as the efficiency of autoclaving for removal of microbiological contaminants. Sterilization reduced the level of heat-labile vitamins, but protein level was minimally reduced. Sterilization eliminated most of the bacterial contaminants and virtually all the mold and yeast colonies. Male and female Osborne-Mendel rats (3-4 wk old) were fed control or sterilized diet for 6 wk. Both males and females consumed more pelleted chow than meal chow. This apparent difference in consumption may be due to wastage of pellets, because there were no differences in male or female growth during the 6-wk study. At necropsy, no gross pathology was noted, and organ weights did not differ significantly among the groups for either sex. Testicular weights were also similar among the groups. Blood serum proteins were analyzed by electrophoresis to screen for possible effects on various target organs. Gamma globulin levels for female rats fed sterilized meal were significantly reduced compared to levels for rats fed the control diet. These results suggest that either nutritional factors or heat inactivation of the microbes affects basal levels of humoral immunity, possibly by reduction of gut-mediated immune responses.