The objective of this study was to evaluate the effectiveness of cold microfiltration (MF), alone or in combination with heat treatment, in extending the shelf life of skim milk. Raw skim milk underwent MF at 6 ± 1°C with a ceramic membrane of 1.4-μm pore size, at a transmembrane pressure of 75.8 kPa and a crossflow velocity of 7 m/s. Samples of raw skim milk; MF skim milk; high-temperature, short-time (HTST)-pasteurized milk; and MF+HTST-pasteurized skim milk were stored at 6°C for 92 d. During the shelf-life study, the total bacterial count and degree of proteolysis were evaluated weekly. The study was replicated 3 times. Cold MF was very effective in reducing the microbial load in skim milk, and an average of 3.4 log reduction in vegetative bacteria was obtained. Although HTST pasteurization reduced the bacterial load by ∼2 log, the MF+HTST process resulted in near complete elimination of vegetative microflora, with a total of ∼4 log reduction. A 9-member sensory panel found no significant differences between skim milk samples processed with or without MF. The MF+HTST skim milk had only minor microbial growth after 92 d at 6°C, but its proteolytic shelf life was limited by plasmin activity. A reduction of plasmin activity and a slower rate of proteolysis were obtained by increasing the heat treatment temperature to 85°C. The results of this study can be used to make decisions regarding processing strategies that lead to increased skim milk shelf life.
Keywords: cold microfiltration; pasteurization; plasmin; proteolysis; shelf life; skim milk.
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