INFLUENCE OF PSYCHROTROPHIC BACTERIAL-GROWTH IN RAW-MILK ON THE SENSORY ACCEPTANCE OF UHT SKIM MILK

The influence of the growth of psychrotrophic bacteria in raw milk on the acceptability and resultant shelf life of ultra-high temperature ( UHT) processed skim milk was investigated. Five batches of raw milk wi th different psychrotrophic counts were UHT processed in a commercial dairy plant. The processed milk of each batch was divided into three l ots which were stored for 139 d at 20, 30, and 40-degrees-C, respectiv ely. Changes in the extent of proteolysis, lipolysis, and sensory attr ibutes of the five batches of UHT milk stored at the various temperatu res were determined at fixed intervals of storage. The psychrotrophic counts in the raw milk were strongly correlated with the extent of pro teolysis (r = 0.9476) but not with the extent of lipolysis (r = 0.18) in the stored UHT milk. Correlations between the extent of proteolysis and bitterness scores in the stored UHT milk were high at 30-degrees- C (r = 0.916) and at 40-degrees-C (r = 0.896) but not at 20-degrees-C (r = -0.233). Correlations between the extent of lipolysis and rancidi ty scores were not significant in the stored UHT milk. It appeared tha t the proteinases played a more important role in the loss of taste an d acceptability of the UHT skim milk than did the lipases. This invest igation suggested that storage time and storage temperature had a grea ter influence on the sensory acceptance and resultant shelf life of th e UHT skim milk than the psychrotrophic count of the raw milk.