Growth of specific lactic acid bacteria (PCR-DGGE) in relation to volatile compounds (SMart Nose and GC/MS) in biofilm inoculated milk under Ragusano cheese-making conditions.

Ragusano cheese is a brine-salted pasta filata raw-milk cheese. Starters are not added, but the biofilm of the wooden vat (tina) and the natural milk flora are responsible for the milk acidification. The aim of the present study was to investigate the growth of specific LAB species in relation to the development of volatile compounds in milk which has been inoculated with biofilm from different tinas and which has undergone the usual cheese-making procedure except for the rennet addition step. Biofilm samples were obtained from the inner surface of 11 wooden tinas (A-K) with sterile swabs suspended in peptone water. Aliquots of a UHT milk sample were inoculated with the biofilm samples. Tina surface area per volume milk was 1.5 times the usual exposure conditions. Incubated samples were analyzed by PCR-DGGE to determine LAB species, with Smart Nose® and by GC/MS. The following LAB species were identified: L. plantarum and L. paracasei (mesophilic lactobacilli), Lc. Lactis and L. mesenteroides (mesophilic lactococci), L. delbrueckii and L. helveticus (thermophilic lactobacilli), S. termophilus (thermophilic lactococcus) and E. faecalis (enterococcus). Data were evaluated with principal component analysis (PC1: 23%; PC2: 14%). Four (B, E, F, J) represented the greatest volatiles’ variation. Samples E and F were similar in profile, but different from B and J. Profiles of B and J differed also. L. paracasei was associated to hexane 2,3,4-trimethyl and heptane-2,2,4,6,6-pentamethyl, E. faecalis and L. delbrueckii to tetradecanal, L. plantarum to nonanoic acid and benzeneacetaldehyde, Lc. Lactis to octanal. Differences in volatile compounds were most related to L. mesenteroides and L. paracasei which separated B from E and F samples, but not to L. plantarum, S. termophilus and L. delbrueckii . Sample J was distinguished from the others by 2-dodecanone. Further studies are still needed to better understand the relationship between individual LAB species and flavour production in cheese.