Dairy Foods: Microbiology

Heat coagulation of milk results from a complex series of physicochemical reactions that eventually lead to precipitation or gelation. Viscosity of milk tends to increase upon heating as it approaches the point of coagulation of the proteins, which is the basis for producing high viscosity in super-heated condensed milk. Although viscous property may have been studied extensively in cow fluid milk, relatively little information is available on viscosities of concentrated milks and comparison of those of different species milks. Commercial cow and goat fluid milks and their evaporated products were purchased from retail outlets to study characteristics of viscosity and physico-chemical stability of bovine milks compared to caprine counterparts stored at 4 and 13oC refrigeration for 4 weeks. The respective initial viscosity values for bovine fluid (BF), evaporated (BE), and caprine fluid (CF), evaporated (CE) milks were: 1.54, 1.55, and 1.72, 2.03 centipoises, indicating that goat milk products had higher viscosity values than cow counterparts. The viscosity of BF was gradually increased until day 12 at 4oC, and the increase continued to day 28 during which gelation occurred. The BE was stable till day 28 at 4oC storage and did not undergo gelation during this period. The viscosity of CF increased slightly under refrigeration until day 7, then decreased gradually to the lowest at day 11. A sharp increase in viscosity occured from day 12, and continued to day 28 of storage, during which gelation of the CF milk occured. However, CE did not show a noticeable decrease on day 9 and remained physically stable until day 28 of storage without any gelation. The refrigeration temperatures had minimal effect on the viscosities of the tested milks. The evaporated milks had greater physical stability under refrigerated storage relative to fluid milks. This may be attributable to the fact that heat treated milk products are actually electrodialysed before evaporation, whereby the partial removal of soluble salts by dialyzing milk against water increases the heat stability when the milk is subsequently concentrated.