Dominant microflora of picante cheese: Independent role upon proteolysis and lipolysis in model systems

Four species of bacteria (two species of enterococci, Enterococcus faecium and Enterococcus faecalis, and two species of lactobacilli, Lactobacillus plantarum and Lactobacillus paracasei) and three species of yeasts (Debaryomyces hansenii, Yarrowia lipolytica and Cryptococcus laurentii), previously isolated from Picante cheese were assayed for biochemical performance in proteolysis and lipolysis. In addition to the difference of the microbiological strains, the milk type (caprine or ovine), the ripening time (0 to 65 days) and the concentration of NaCl (0 to 14%(w/v)) have been deliberatly fixed in vitro curdled milk (previously prepared from heat-sterilized milk, coagulated with animal rennet and inoculated with each strain) and subject to 12 ºC. High proteolytic activity was demonstrated by Y. lipolytica and by all the other strains to a lesser extent; Y. lipolytica produced ca. 85% of WSN by 65 days of ripening whereas E. faecium, D. hansenii and C. laurentii produced levels of WSN ranging in 40-50%, and E. faecalis, L. plantarum and L. paracasei in 30- 40%. In terms of peptidolytic activity, measured by NPN contents and by release of free amino acids, once again Y. lipolytica presented the highest activity, followed by L. plantarum, L. paracasei, E. faecium and E. faecalis. Milk type, ripening time, and content of NaCl revealed to be statistically significant processing factors in terms of proteolysis; caprine milk, 65 days of ripening and lower contents of NaCl led to the highest values. The lipolytic activity, assessed by the release of butyric acid from tributyrin, was strong for Y. lipolytica and C. laurentii, whereas release of free fatty acids was observed at different rates for all strains under study. Ripening time proved to be a statistically significant factor for lipolysis, whereas milk type was not; lipolytic activities, measured as fat acidity index, were strongly affected by NaCl content and, as happened with release of free amino acids, the extent of fat hydrolysis was much more affected by the increase of NaCl from 0 to 7% than by its increase from 7% to 14%. Although it is not possible to directly compare results obtained in vitro using pure, single cultures with those obtained in loco using actual cheese, our results suggest that a mixed-strain starter for Picante cheese including L. plantarum, E. faecium (or E. faecalis) and D. hansenii (and/or Y. lipolytica) would be of potential interest.