Reconstituted UHT-treated milk: Effects of raw milk, powder quality and storage conditions of UHT milk on its physico-chemical attributes and flavour

Reconstituted, UHT-processed milk was produced from whole milk powders that were manufactured from fresh (control) or stored (4 +/- 1 degrees C, 48 +/- 2 h) raw milk and stored for different periods at 25 +/- 1 degrees C. Lipolytic and proteolytic activities, physico-chemical and flavour changes and gelation in UHT milk during storage at 3 +/- 1 degrees C or 25 +/- 1 degrees C for up to 6 months were investigated in relation to the biochemical status of raw milk and whole milk powder. The green-red and blue-yellow components of colour and the non-protein nitrogen content of UHT milk increased, while pH and colour lightness decreased with storage; the rate of change being greater at higher storage temperature. Sediment increased with longer storage period, but was independent of storage temperature. With longer storage at both 3 +/- 1 degrees C and 25 +/- 1 degrees C, greater sediment and lower pH were observed in UHT milk processed from older milk powders. Rates of enzymatic and oxidative reactions appeared greater in UHT milk stored at 25 +/- 1 degrees C and in those processed from older powders and contributed to the development of off-flavours in UHT milk with a prolonged storage period. Gelation was observed only at 25 +/- 1 degrees C. Lipases and proteinases were reactivated during storage and their activity was greater in UHT milk processed from powder manufactured from stored raw milk. The taste of reconstituted UHT milk was affected more by lipolysis than by proteolysis. (C) 1997 Elsevier Science Ltd.