Various aspects of the bacteriology of inedible rendering have been investigated in order to establish a solid basis for future decisions concerning an up-to-date and flexible legislation on rendering. Thermal death (TD)-graphs for spores of B. cereus and Cl. perfrigens, PA 3679 (Fig. 3), and heat transmission equations for animal tissues have been determined. By using the heat transmission data for bones and the TD graphs for the spores it is possible to predict the decimal reductions of spores in the centre of the largest pieces present during a given rendering process, thus establishing conditions for bacteriological safe processes. The calculations show that predrying for 45 min followed by cooking at 125 degrees C for 15 min and final drying ensures destruction of non-sporeforming bacteria and Bacillus anthracis spores even in the centre of 70 mm bone particles while heat resistant spores of clostridia are virtually unaffected. By reducing the particle size to less than 40 mm, the same process will result in a reasonable reduction of heat resistant clostridia spores, too (Table 4). In order to verify such theoretically calculated effects a new technique has been developed in which steel tubes containing a paste inoculated with spores were inserted in bones. These were treated in a cooker, were caught during discharge and examined. The results confirmed the calculations (Table 5). Most modern rendering systems (Carver-Greenfield, Stork-Duke, Wet Pressing) are continuous without pressure cooking and a common feature is a fine mincing minimizing the problem of heat penetration. In order to obtain information regarding the thermal sterilizing effect in such systems investigations were made in a pilot cooker using inoculated meat-and-bone meal mixed with water and/or fat. Regardless of whether fat was added or not sterility was found for samples containing water when the temperature during drying reached 110-120 degrees C, whereas cooking in fat only drastically increased the heat resistance of spores of both strains. Sterility was only obtained at temperatures of the order of 140 degrees C, a fact of minor importance for rendering, where thermal treatment usually takes place with moisture present. The decimal reductions actually found were compared to calculated ones and the former were all substantially higher than the latter (Table 6). Thorough investigation of sterilization in the wet pressing system has confirmed the conclusion that inactivation of pathogenic microorganisms during drying is obtained when temperatures reach 110 degrees C (Table 7 and 8).(ABSTRACT TRUNCATED AT 400 WORDS)