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 with different psychrotrophic counts were UHT processed in a commercial dairy plant. The processed milk of each batch was divided into three lots which were stored for 139 d at 20, 30, and 40°C, respectively. Changes in the extent of proteolysis, lipolysis, and sensory attributes of the five batches of UHT milk stored at the various temperatures were determined at fixed intervals of storage. The psychrotrophic counts in the raw milk were strongly correlated with the extent of proteolysis (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°C (r = 0.916) and at 40°C (r = 0.896) but not at 20°C (r = -0.233). Correlations between the extent of lipolysis and rancidity scores were not significant in the stored UHT milk. It appeared that the proteinases played a more important role in the loss of taste and acceptability of the UHT skim milk than did the lipases. This investigation suggested that storage time and storage temperature had a greater influence on the sensory acceptance and resultant shelf life of the UHT skim milk than the psychrotrophic count of the raw milk.