Effect of temperature, pressure and alkaline treatments on meat meal quality

The recommendation of the 96/449/EC directive for processing mammalian animal waste does not ensure complete inactivation of transmissible spongiform encepalopathy (TSE) agents. The safety of the process might be ensured by adding alkaline treatments to the heating step. Evaluating the in vivo and in vitro nutritive value of meat meals produced according to 11 heat, time and pressure treatments, either with or without addition of alkali was completed. Meat meals were obtained from fresh soft bovine meat in a 20 kg stirred displacement autoclave, resembling the standard equipment used in Italian rendering plants. Operating temperature and pressure were monitored continuously. Meat meals were analysed for crude protein and ash contents and pH was measured. The protein fraction was analysed for amino acids and lysinoalanine (LAL). Protein quality was evaluated in vivo with the rat-based protein efficiency ratio (PER) bioassay and in vitro by pepsin digestibility, the essential amino acids composition (DC-PER) and from both essential amino acid composition and enzyme digestibility (C-PER) of the protein. Compared to the standard procedure in use before 96/449/EC directive (MM8), implementing of the EC treatment did not reduce the nutritive value of the meat meal (MM), whereas negative effects were observed when the length of treatment was increased to 180 min or when adding an alkaline step to the MM preparation. In particular, addition of alkali reduced the nutritive value of the MM as suggested by the PER bioassay, and increased the (LAL) content of the protein fraction. Data suggests that LAL content of the protein fraction could be used as an indicator of protein quality of meat meals. In vitro and in vivo results demonstrate the inadequacy of the in vitro technique (pepsin digestibility, C-PER and DC-PER) for evaluating the nutritive value of meat meals.