Impact of broken cells of lactococci or propionibacteria on the ripening of Saint-Paulin UF-cheeses: extent of proteolysis and GC-MS profiles

UF-cheeses have been successfully developed in many countries. However, proteolysis and the general extent of ripening were shown to be far slower compared to traditional varieties. The absence of starter lysis has been cited as a plausible explanation. In this work, propionibacteria and lactococci cells were disrupted using a new pilot homogenizer. Crude broken suspension (CBS), or cell-free extract (CFE), obtained after centrifugation, were added to UF-St Paulin retentate (concen- tration factor 6) made with a usual lactic starter. RO water was added to the control. Proteolysis was estimated by NCN, NPN and free amino acids. Neutral volatile compounds were determined by GC- MS. A low extent of ripening was noted in the control and the absence of starter lysis was effectively proved using immunodetection of lactococci cytoplasmic proteins. The addition of lactococci CBS or CFE increased free amino acid content (1.5 to 3 times) whereas propionibacteria CBS or CFE exhibited no significant increase, even when cheeses were ripened at 20 o C instead of 12 o C. By con- trast, addition of propionibacteria CBS generated a significant increase in several volatile compounds like alcohols and ketones, whereas CFE did not, showing that the presence of live cells was required to form these compounds. CBS or CFE of lactococci did not significantly change the volatile com- pound profile. In conclusion, it was possible to influence the ripening of UF-cheese by the addition of crude broken bacterial cell suspensions. Other strains and species should now be investigated. ultrafiltration / UF-cheese / GC-MS profile / broken cells / ripening / proteolysis / Lactococcus / propionibacteria