Study of Lactococcus lactis during advanced ripening stages of model cheeses characterized by GC-MS.

Lactococcus lactis, known as dairy starter, has recently been detected, as metabolically active cells, in advanced ripening stages of some cheeses, by culture-independent methods. The main goal of the present study was to assess the vitality of L. lactis in model cheeses, up to 180 days of ripening, and to evaluate a possible correlation between the metabolism of this microorganism and volatile aromatic profiles of the cheeses. Three L. lactis commercial starters, constituted of subsp. lactis, were inoculated in pasteurized milk, checked for the absence of lactic acid bacteria populations, and model cheeses were manufactured and ripened for six months. Samples were analyzed, at different steps, in terms of vitality of L. lactis by both traditional plating and direct analysis of RNA by quantitative reverse transcription PCR and for the aromatic profile by Gas Chromatography-Mass Spectrometry. In addition, samples of curd and cheese were submitted to metatranscriptomic analysis. Based on RT-qPCR analysis, L. lactis was found viable throughout the whole process of cheesemaking and aging, with final average loads, at 180 days, of 103 CFU/g. On the contrary, the microbial colonies detected on M17 medium, did not always confirm the presence of the microorganism, probably in a state of physiological stress. The aroma profiles of the cheeses highlighted the presence of important contents of volatile compounds related to the metabolism of carbohydrates, amino acids and lipids. Metatranscriptomic data would allow to better understand the L. lactis gene expression shift between cheesemaking and ripening, adding new information about its metabolic activity.