Production of yoghurt from sheep's milk which had been concentrated by reverse osmosis and stored frozen. 2. Compositional, microbiological, sensory and physical characteristics of yoghurt

Abstract

The effects of concentration of sheep's milk by reverse osmosis (RO) followed by long-term frozen storage, thawing and reconstitution on the compositional, microbiological, organoleptic and physical characteristics of yoghurt were determined. 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. In another series of experiments, a RO concentrate containing 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) differences in the composition and microbiological quality were observed between the control yoghurt (made from fresh whole sheep's milk) and the yoghurts made from the frozen RO concentrates. Generally no significant ( P > 0.05) differences in the sensory characteristics were observed between the yoghurts made from RO concentrates of whole milk stored frozen for up to 8 months and the control milk yoghurt. Yoghurts from frozen RO concentrates of whole milk had higher curd consistency and apparent viscosity values than the control milk yoghurt, although both displayed similar syneresis values. However, yoghurts made from frozen recombined RO concentrates of skim milk were of inferior quality compared with the control milk yoghurt, mainly because of the formation of a thick “cream layer” at their surface. In addition, these yoghurts had a slightly grainy texture and displayed higher syneresis values than the control milk yoghurt.