The plasmin enzyme system in non-fat dry milk: Effects of heat treatment, cysteine addition, and somatic cell count
Abstract
Non-fat dry milk (NFDM) is a large commodity in the United States with uses in nearly every branch of the food industry, such as processed cheese, infant formula, desserts, and soups. As with most dairy products, proteolysis, especially due to the plasmin (PL) enzyme system, is a great concern, because of its impact on quality. However, little research has been done on the PL system as it relates to NFDM. The overall objective of this research was to establish a baseline for PL system activity in NFDM of differing heat classifications and understand how it relates to proteolysis and changes in powder functionality. Results clearly showed that the PL system within NFDM is active and capable of hydrolyzing β-casein. While no PL activity was present in medium or high heat powders, activity remained in instantized and low heat powders. Plasminogen activation in reconstituted NFDM was observed for the first time. Reconstituted low heat NFDM underwent extensive proteolysis of β-casein, especially upon the addition of urokinase-type PG activator (uPA), while this was not observed in medium heat NFDM. Plasmin activity was highly correlated with γ-casein concentration, the product of PL-induced hydrolysis of β-casein. Further experimentation showed that somatic cell count (SCC) significantly affected PL activity in low heat NFDM, while both SCC and post-pasteurization storage significantly decreased viscosity of acidified milk gels made with low heat NFDM, which was highly correlated with β-casein. The addition of cysteine was shown to control proteolysis by reducing PL activity and PG activation in low heat samples. The overall conclusion is that PL remains active in low heat powders, thus leading to the β-casein hydrolysis during storage of reconstituted NFDM. Small changes in the PL activity of powders were enough to cause significant alteration of product functionality. In products where proteolysis is not desired, cysteine and heat could be used to control PL activity, thereby increasing the quality of products that utilize NFDM. This research significantly contributes to the dairy industry, as low heat NFDM is frequently used in applications such as processed cheeses and puddings in which proteolysis poses a threat to product quality.
Degree
Ph.D.
Advisors
Hayes, Purdue University.
Subject Area
Food science
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.