Use of Fourier transform infrared spectroscopy to evaluate the proteolytic activity of Yarrowia lipolytica and its contribution to cheese ripening.

The purpose of this study was to investigate the suitability of Fourier transform infrared (FTIR) spectroscopy as rapid technique to be used to investigate the secondary structure of proteins in aqueous solution and its changes as a consequence of microbial proteolytic activity as well as to assess or identify the contribution of different strains of Yarrowia lipolytica to cheese ripening. The comparison of spectroscopic data with SDS-polyacrylamide gel electrophoresis (PAGE) electrophoretograms outlined the consistency between the two instrumental approaches and showed a remarkable biodiversity between strains of Y. lipolytica. The strains tested were shown to modify the secondary structure of the caseins, inducing transitions between alpha helix and beta sheets, thus confirming the ability of this species to hydrolyse alphaS1 casein "in vitro" and to induce protein breakdown in skim milk, although strong heterogeneity in the hydrolytic activity of the strains was observed. Significant differences in the Amide I and Amide II bands of protein of curds and cheeses obtained from milk inoculated with Lactococcus lactis subsp. lactis and different strains of Y. lipolytica occurred during ripening. Y. lipolytica Y3 presented the most relevant enzymatic activity inducing transitions between alpha helix and beta sheets of proteins, as evidenced by the strong absorbance reduction of the Amide I band and the increase of the Amide II band. In the control and in the samples inoculated with the Y. lipolytica strains PO1 and RO13 changes resulted in a strengthening of the interactions with solvent molecules and of the hydration state of the caseins.