Controlled batch dry rendering in a model cooker: the separate influence of offal type and processing variations on the nutritive value of meat meal

Despite considerable research into the factors affecting the nutritive value of meat meal, the separate effects of raw material type as distinct from processing influence have not been unequivocally established. Following a survey conducted into rendering practice in the Australian meat industry, a commercial cooker was extensively instrumented and used to record the details of heat and mass transfer as they occur in dry-rendering. The knowledge of the process so obtained was employed in the study reported here, which was undertaken using a scale model of a commercial cooker in which process conditions could be set to simulate commercial practice. The experimental design incorporated meat meal production from a range of typical offal materials cooked either under standardized conditions or under specified different conditions. Raw materials high in bone are high in collagenous material which has a low proportion of essential amino acids so that meat meals manufactured from these materials are low in nutritive value. Dilution of material high in essential amino acids with collagenous material results in poor quality meal. High qualtiy meals can be prepared by rendering washed soft offal materials separately from hard offals. In practice a certain amount of total lysine becomes unavailable in the rendering process. However, wide variations in the time of normal processing and severe over-processing do not cause loss of available lysine or decrease in nutritive value. Extended holding periods before rendering allows bacterial degradation sufficient to cause loss in nutritive value of the meat meal to occur