Abstract
The relation between age gelation and proteolytic activity was investigated in this study, as proteolysis is considered to be one of the principal factors that cause gelation. Two different methods for measuring the proteolytic activity of milk samples were applied; the measurement of absorbance at 280 nm and the trinitrobenzene sulfonic acid (TNBS) method. The milk samples used were raw and UHT cow’s and goat’s milk. Proteolysis was also induced in UHT milk by inoculating cells of four different strains of Pseudomonas fluorescens into the milk and the changes taking place were observed. It has been found that the proteolytic activity of raw milk was not affected by a refrigerated storage for 10 days and only after this period it was gradually increased. The higher proteolytic activity of goat’s milk in comparison with cow’s milk during storage and its increased susceptibility to gelation were also established. Furthermore, the importance of storage temperature and the different effect of the four strains of P. fluorescens on proteolytic activity and pH of the UHT milk were shown. Finally, it has been demonstrated that there is a certain level of proteolytic activity in milk that leads to gelation when it is exceeded.
Highlights
One major concern about UHT milk is age gelation.After weeks to months storage, there is a sudden sharp increase in viscosity accompanied by visible gelation and irreversible aggregation of the micelles into long chains forming a three-dimensional network
As mentioned by Datta and Deeth (2001), the gel which forms in UHT milk is a threedimensional protein matrix formed by the whey proteins, especially β-lactoglobulin, interacting with casein, mainly κ-casein, of the casein micelle
These interactions, which are favoured by the high temperature during the UHT process, result in the formation of β-lactoglobulin-κ-casein complexes
Summary
One major concern about UHT milk is age gelation.After weeks to months storage, there is a sudden sharp increase in viscosity accompanied by visible gelation and irreversible aggregation of the micelles into long chains forming a three-dimensional network. As mentioned by Datta and Deeth (2001), the gel which forms in UHT milk is a threedimensional protein matrix formed by the whey proteins, especially β-lactoglobulin, interacting with casein, mainly κ-casein, of the casein micelle. The formation of βκ-complexes alters the conformation of the κ-casein in the casein micelle and the association between κ-casein and other caseins (e.g. αs1-casein) gets weaker The disruption of these interactions, with enzymatic and nonenzymatic mechanisms, results to the release of κcasein along with the attached β-lactoglobulin. These released βk-complexes aggregate into a three- dimensional network of cross-linked proteins and when a critical concentration of them is
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