Growth of Psychrotrophic Bacteria in Solids Fortified Skim Milk

The effect of fortifying skim milk with non-fat dried milk on growth and proteolysis of psychrotrophic bacteria was determined. Raw skim milk of 8.7% total solids was fortified to 10% and 12% total solids and pasteurized. Growth rates of proteolytic psychrotrophic bacteria were determined in these milks during incubation at 4°C. Proteolysis was determined by measuring the concentration of free amino groups throughout the incubation period. Seven of nine psychrotrophic isolates grew to greater numbers in the fortified milks during the early log phase of growth than they did in the unfortified milks. Growth stimulation was slight in most cases, however, one isolate exhibited a ten-fold increase in numbers when growing in the 10% solids fortified milk. The greatest increases in growth were observed when comparing unfortified milk to milk at 10% solids. Early log phase growth in 12% solids milk was often less than in 10% solids milk but still greater than in unfortified milk. All isolates exhibited increased proteolysis in the fortified milks within 48 h of incubation. These results indicate that increasing the solids content of skim milk with non-fat milk powder produces a microbiological growth medium more suitable for growth and protease production for selected psychrotrophic bacteria.