Sterilization of yak milk by direct steam injection (DSI) and effects on milk quality

Abstract

To facilitate development of new yak milk products with high added value, sterilization effects of direct steam injection (DSI) were evaluated. The Z value (temperature increase necessary to reduce inactivation time by 90%) of Geobacillus stearothermophilus spores (ATCC 7953; 106 CFU mL−1) in yak milk was 5.88 °C. Heating of the milk above 153.86 °C for 0.116 s achieved complete inactivation of ATCC 7953 spores; i.e., commercially-acceptable sterility. The native lactoferrin and β-lactoglobulin (β-Lg) contents of the milk decreased by 40 and 30% after DSI treatment, respectively, whereas the α-lactalbumin (α-La) content did not differ significantly; the overall extent of denaturation of heat sensitive proteins was lower than in UHT milk. The furosine and lactulose contents in DSI treated yak milk were ≤1.11 mg/10 g of protein and 0.368 mg/10 mL, respectively, similar to those in pasteurized milk. The pH, Hunter color values and aroma were significantly influenced by DSI treatment, but the overall sensory acceptability of DSI treated yak milk was assessed as good. These findings indicate that DSI has great potential for producing yak milk and processed yak milk products with considerable added value.