Abstract
Native enzymes play a significant role in proteolysis of milk during storage. This is significant for heat resistant native enzymes. Plasmin is one of the most heat resistant enzymes found in milk. It has been reported to survive several heat treatments, causing spoilage during storage. The aim of this study was to assess susceptibility of high temperature heated milk to proteolysis by native enzymes. The trinitrobenzene sulphonic acid (TNBS) method was used for this purpose. Raw milk was heated at 110, 120, 130,142°C for 2 s and 85°C for 15 s and milk processed at low temperature (85°C /15s) was selected to mimic pasteurisation. TNBS method confirmed that raw milk and milk processed at 85°C /15s were the most proteolysed, whereas treatment of milk at high temperatures (110, 120, 130 and 142°C for 2 s) inactivated the native enzymes. It may thus be concluded that high temperature processing positively affects proteolysis by lowering its susceptibility to spoilage during storage. Key words: Plasmin, proteolysis, trinitrobenzene sulphonic acid (TNBS), heat treatment, isoelectric precipitation.
Highlights
Proteolysis in milk may be a positive or negative attribute depending on the processing purposes and conditions (Nielsen, 2002)
It is important for cheese ripening through development of desirable changes in flavour and texture, but is undesirable when it results in gelation and bitterness as observed in ultra high temperature (UHT) milk (Datta and Deeth, 2001)
Effect of storage time on proteolysis of milk heated at high temperatures
Summary
Proteolysis in milk may be a positive or negative attribute depending on the processing purposes and conditions (Nielsen, 2002) It is important for cheese ripening through development of desirable changes in flavour and texture, but is undesirable when it results in gelation and bitterness as observed in ultra high temperature (UHT) milk (Datta and Deeth, 2001). It is caused by either bacterial enzymes or naturally occurring enzymes of which plasmin is significant (Grufferty and Fox, 1988). Gelation of UHT milk during storage (age gelation) is a major factor limiting its shelf life (Chavan et al, 2011)
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