Effect of rendering on meat meal protein quality and in vitro assessment of meat and blood meal protein quality : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Applied Science in the University of Canterbury [Lincoln College]

Abstract

The different types of rendering processes and the conditions each type applies to raw material to produce meal and tallow are reviewed. The effect different types of raw material have on the composition and nutritive value of meat meals is also reviewed. Some of the likely beneficial and detrimental effects of the rendering process are discussed and the nutritional effect of rendering using the traditional batch dry renderer was compared with the effect of a semi continuous wet (Centrimeal) process and the MIRINZ low temperature wet rendering (ML TR) system. The comparative nutritive values were determined using rat nitrogen balance trials when the test sample was fed as the sole source of protein (8%) in a semi synthetic diet which was nutritiously adequate in all other respects. Both the Centrimeal and MLTR processes produced better quality meal and had significant advantages over the traditional batch dry method of rendering. In addition individual amino acid digestibilities determined using both the ileal digesta of rats and faeces of precision fed intact cockerels confirmed the nitrogen balance results. Jn vitro methods for determining soluble nitrogen as an index of in vivo determined protein quality were reviewed. Two methods, a pepsin and pronase method were evaluated for both meat and blood meals. Good correlations with in vivo values were achieved for meat meals using both methods although changes to the methods used were suggested. Blood meals in vitro values generally did not correlate well with in vivo values. It was thought that part of the poor correlation was due to poor in vivo values. An alternative method for determining in vivo values for blood meals using rat nitrogen balance trial diets supplemented with iso leucine was shown to be more effective than supplementing the semi-synthetic diets with albumin. Quick in vitro digestibiliy methods for evaluating in vivo were reviewed covering the pH drop and pH-stat methods. The pH-stat method was used on 4 progressively heat treated meals and showed excellent correlations with in vivo values after digesting for 10 minutes using Alcalase enzyme and correcting for base line drift. The results of these experiments show that the pH-stat method used is capable of quickly and accurately evaluating heat damage for the same type of sample.