Abstract
Proteolysis of UHT milk during storage is one of the problems affecting the dairy industry worldwide. Native enzymes have been implicated as being the main cause of spoilage of such milk. In the current study, reverse phase high pressure chromatography (RP-HPLC) method was used to detect proteolysis by native enzymes in high temperature heated milk. The aim of this research was to assess susceptibility of milk to proteolysis by native enzymes, after being subjected to various temperature-time processing conditions. Samples of raw and heated milk were clarified prior to analysis. Clarification was carried out to obtain pH 4.6 and 6% TCA soluble extracts, which were injected into RP-HPLC after filtration. This method confirmed that raw milk and milk processed at 85°C /15 s were the most proteolysed, indicating that the high temperatures employed during this study (110, 120, 130 and 142°C for 2s) inactivated the native enzymes. The RP-HPLC method is a useful method for the detection of proteolysis in milk. Key words: Reverse phase-high performance chromatography (RP-HPLC), proteolysis, milk, plasmin, heat-treatments.
Highlights
Heat treatment of milk is used for pasteurisation/sterilization and to promote desirable physical characteristics in the protein or food system (Raikos, 2010)
The current study described the effect of high temperature processing of milk samples and its susceptibility to proteolysis during storage
Milk heated at higher temperatures (>85°C) had lower proteolysis than raw milk and samples heated at 85°C possibly due to destruction of the plasmin enzyme, which is a heat resistant enzyme
Summary
Heat treatment of milk is used for pasteurisation/sterilization and to promote desirable physical characteristics in the protein or food system (Raikos, 2010). Chemical, physical and biochemical reactions take place These changes are significant because they influence nutritional, sensorial and microbiological aspects of milk. Plasmin (EC 3.4.21.7), occurs in the highest amount and is one of the most heat resistant enzymes in milk (Datta and Deeth, 2003) It is part of a complex system consisting of plasminogen (PG), plasminogen activators (PA), plasminogen activator inhibitors (PAI), plasmin (PL) and plasmin inhibitors (PI) (Crudden and Kelly, 2003). The rate and extent of proteolysis is determined by the types and activities of the proteolytic enzymes present (Pereda et al, 2008) These proteases have been studied from the perspective of their physiological significance in milk, their effect on the processing of dairy products, and their nutritional and antimicrobial roles (Aslam and Hurley, 1998). Proteolytic enzymes are of great importance to the dairy industry because they are responsible for imparting desirable or undesirable properties to dairy products through changes in flavour and texture (Nilsen, 2002)
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