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Abstract
The double-overlay method for the isolation of lipase producing psychrotrophs (Alford and Steinle, 1967) yielded slightly greater numbers of lipolytic bacteria, but greater visual clarity was achieved with the single-layered media containing 1% peptone. The four lipase producing psychrotrophic isolates were identified as two different strains of Pseudomonas fluorescens, P. cepacia, and P. putida. Using response surface methodology, the optimum environmental conditions for lipase synthesis by Pseudomonas fluorescens were 25$\sp\circ$C, environmental pH of 8.07, and minimal media aeration (65 rpm). Heat stabilities of ten different extracellular lipases subjected to a simulated high temperature short time (HTST) heat treatment ranged from 0-23.5%. All emulsifiers (2%) were potent inhibitors (100%) of lipase activity. Their effectiveness as lipolysis inhibitors decreased with decreasing emulsifier concentration and, with the exception of sodium oleate (SO), with increasing incubation temperature. SO demonstrated significant (P $<$ 0.05) lipolysis inhibition at a 1% concentration regardless of temperature or initial lipase concentration by 45.3% through competitive inhibition. At 25$\sp\circ$C, only the most hydrophobic sucrose fatty acid ester (SE), F-10, significantly (P $<$ 0.01) inhibited lipolysis by 15.7% through competitive inhibition. Inhibitory effectiveness decreased as the degree of hydrophobicity of the SE decreased and increased with increasing temperature (40$\sp\circ$C). At a 2% concentration, the antioxidants, l-ascorbyl 6-palmitate (AP) and d-alpha-tocopherol (AT) inhibited lipolysis by 100% and 80%; through non-competitive and uncompetitive inhibition, respectively; but their effectiveness decreased at 40$\sp\circ$C. SO and AT were the only additives that did not stimulate proteolytic activity. Proteinase and lipase activities in non-fat milk were decreased by 72.2% and 72.5% solely by UHT processing. Solitary SO and AT significantly (P $<$ 0.01) decreased lipase activity in the treated UHT processed milk; but the combination of SO + AT (1%) completely inhibited lipolysis.
