Applications of FTIR microspectroscopy in oenology: shedding light on Saccharomyces cerevisiae cell wall composition and autolytic capacity

Abstract

Many microbial starters for the alcoholic and malolactic fermentation processes are commercially available, indicated for diverse wine styles and quality goals. The screening protocols cover a wide range of oenologically relevant features, although some characteristics could also be studied using underexplored powerful techniques. In this study, we applied Fourier Transform Infrared (FTIR) microspectroscopy [1,2] to compare the cell wall biochemical composition and monitor the autolytic process in several wine strains of Saccharomyces cerevisiae. After cell death, autolysis trigger the release of mannoproteins and β-glucans, major components of yeast cell walls, influencing color, aroma, body, foaming properties, and stability of wine. Moreover, they can stimulate the metabolism of malolactic bacteria, while some fatty acids, also released during autolysis, act as inhibitors. Analysis of the cell wall structure was carried out both on cells grown in liquid medium and on cell walls previously separated from the other components. The autolytic capacity was assayed by sampling the cells at different times during induced autolysis in clarified and pasteurized must. From five to ten infrared spectra were acquired on each sample in the 4,000-700 cm-1 range in Attenuated Total Reflection on a 50×50 μm2 area. Spectra were analyzed after pretreatment through Hierarchical Cluster Analysis and Principal Component Analysis. Preliminary results were evaluated in relation to conventional spectrophotometric methods to quantify mannoproteins and β-glucans. The thickness of the cell walls was determined by means of scanning (SEM) and transmission electron microscopy (TEM). From the point of view of cell wall composition two groups of yeasts were distinguished by multivariate statistical analysis on the FTIR spectra since the strains EC1118, MY11 and PEDRO2000E showed higher absorption bands of mannoproteins and β-glucans. With conventional methods, the cell walls of the first two strains, alongside K1 and MY8, displayed a higher content of parietal polysaccharides, while the latter had the thickest wall among all the tested yeasts. The strains BM45 and D47 have a thinner surface structure. Regarding the autolytic process, again two different clusters were found distinguishing the behavior of the strains EC1118 and FRB with a similar timing of autolysis on one side from CH and Q20 on the other side. Furthermore, the latter strain presented a higher absorption in the spectral zone related to lipids, which can be correlated with a greater release of fatty acids in the medium. In conclusion, FTIR microspectroscopy proved to be an accurate and informative technique, suitable to highlight profound differences among S. cerevisiae strains as concerns both the content of parietal polysaccharides and the evolution of autolysis. Thus, this technique may become an option for the selection of starter cultures with properties fo great interest for the wine sector.

References

[1] Burattini, E., Cavagna, M., Dell’Anna, R., Malvezzi Campeggi, F., Monti, F., Rossi, F., & Torriani, S. (2008). A FTIR microspectroscopy study of autolysis in cells of the wine yeast Saccharomyces cerevisiae. Vibrational Spectroscopy, 47(2), 139-147. https://doi.org/10.1016/j.vibspec.2008.04.007.
[2] Cavagna, M., Dell’Anna, R., Monti, F., Rossi, F., & Torriani, S. (2010). Use of ATR-FTIR microspectroscopy to monitor autolysis of Saccharomyces cerevisiae cells in a base wine. Journal of Agricultural and Food Chemistry, 58(1), 39–45. https://doi.org/10.1021/jf902369s.