Production of yoghurt from sheep's milk which had been concentrated by reverse osmosis and stored frozen. 1. Physicochemical, microbiological and physical stability characteristics of concentrates

Abstract

Some physicochemical, microbiological and physical stability changes in sheep's milk as a result of concentration by reverse osmosis (RO) and long-term frozen storage were investigated. Skim milk was concentrated by RO to 24–26% total solids (TS), and mixed with cream to obtain recombined concentrates with 31.6–35.2% TS. A similar RO concentrate (32.1 % TS) was prepared from whole milk. The concentrates were frozen and stored at −20 °C for up to 6–8 months. No significant ( P > 0.05) difference in milk lipolysis (acid degree value) was observed between the control milk and the fresh or frozen RO milk concentrates. Fat oxidation (peroxide value) of milk was not affected by the RO process, and increased with storage time. The RO process resulted in significant ( P ≤ 0.05) increases in the total bacterial and coliform counts, which subsequently decreased during frozen storage. The physical stability of the thawed and reconstituted concentrates, especially those prepared from the whole milk, was excellent even after 6 months frozen storage.