Sensory and enzymatic factors associated with defects in low fat UHT milk

Abstract

Ultra-high temperature processing of milk renders a product that is bacteriologically stable for several months at ambient temperature. Various factors have been reported to induce changes in UHT milk during storage and thereby limit the shelf life of the milk. Consequently, changes in the sensory and microbiological properties of low fat UHT milk were assessed over time at different temperatures to develop a model whereby the shelf life of the milk can be predicted in a short time-period. UHT milk was stored at 25°C and accelerating temperatures of 35°C and 45°C and evaluated for changes over 195 days (d). A multivariate accelerated shelf life test (MASLT) was applied to the descriptive sensory data and allowed the successful prediction of the shelf life at various temperatures, with the shelf life of milk stored at 25°C estimated at 211 d. Higher storage temperatures negatively affected the shelf life of the milk, with estimations of 73 and 27 d for milk stored at 35°C and 45°C, respectively. The shelf life obtained from the MASLT was validated using survival analysis where the acceptance or rejection of samples by consumers gave an estimated shelf life of 214 d. The acceptability of UHT milk depends on the sensory quality of the milk. Consumer perception and physico-chemical properties of low fat UHT milk of various ages were evaluated to determine the parameters associated with the deterioration of the milk. As the consumer liking for aroma, appearance, taste and overall liking decreased over time, the detection of positive attributes in the milk decreased, while the detection of negative attributes increased. Parameters associated with the deterioration of UHT milk, including increased titratable acidity, Maillard reaction products and enzymatic reactions, decreased pH and changes in the colour of the milk, increased over storage time. Although sufficient heat treatment and packaging with light and oxygen barriers prevent microbial and oxidative spoilage of UHT milk, there are no means of inactivating heat-stable enzymes in UHT milk. Subsequently, the ability of different protease inhibitors to reduce the activity of a native milk enzyme (plasmin) and enzymes produced by bacterial contaminants (Pseudomonas fluorescens, Bacillus licheniformis and B. lentus) was evaluated. Protease inhibitors extracted from soybeans (Glycine max (L.) Merr), marama beans (Tylosema esculentum (Burch) A. Schreib) and cowpeas (Vigna unguiculata (L.) Walp) were evaluated with regards to their ability to inhibit these enzymes in a buffer system and in low fat UHT milk. The legume protease inhibitors were effective in reducing the activity of plasmin and proteases produced by Bacillus spp., while it showed low inhibitory activity towards P. fluorescens proteases in a buffer system. In UHT milk, the same protease inhibition was observed, however, to a lesser extent as compared to inhibition in the buffer system. Overall, the results indicate that the sensory properties of low fat UHT milk can be used to predict the shelf life of UHT milk in accelerated storage using the MASLT. Survival analysis can also be successfully employed to determine the shelf life of low fat UHT milk. Increased storage time of low fat UHT milk is associated with a reduction in the liking and positive attributes associated with the UHT milk, and an increase in negative attributes and physico-chemical and enzymatic parameters related to the deterioration of milk. Legume protease inhibitors show great potential in preventing or reducing proteolytic activity of Bacillus proteases and plasmin that may cause both sensory and consistency defects in the UHT milk during storage.