Unveiling Metschnikowia spp.: mechanisms and impacts of bioprotection in winemaking

Abstract

Bioprotection, leveraging beneficial microorganisms, has emerged as a sustainable approach to modern winemaking, minimizing reliance on chemical preservatives like as sulfur dioxide (SO₂). Among these microorganisms, non-Saccharomyces yeasts, particularly Metschnikowia pulcherrima (MP), have garnered significant attention. By establishing a favorable microbial environment during the early stages of fermentation, MP reduces the risk of undesirable fermentations and spoilage caused by lactic acid bacteria, Brettanomyces bruxellensis, or acetic acid bacteria (AAB). Pulcherriminic acid, an iron-chelating compound responsible for the red pigment, pulcherrimin, has long been considered the key mediator of MP bioprotection through iron deprivation.  This yeast species also plays a dual role in shaping microbial dynamics and enhancing wine quality.

This study explored the broader bioprotective potential of Metschnikowia species, including those other than MP, against a model AAB. The strains demonstrating the most effective bioprotection (BP) were further assessed for their oenological suitability and the underlying mechanisms supporting their BP activity were investigated.

In a first step, forty-six strains of Metschnikowia spp. were co-inoculated with the AAB Gluconobacter oxydans (Go), responsible for acetic souring in commercial grape juice. Monitoring bacterial growth over time revealed three distinct yeast BP profiles. Only one strain (M. reukaufii) exhibited no BP capacity. The majority of Metschnikowia strains (43) significantly delayed Go growth, but resulted in only a modest reduction in bacterial population after 7 days of incubation. Two strains (MP) demonstrated highly effective BP capacity, with minimal bacterial growth observed even after 7 days.

The four MP strains with the highest bioprotective effect were then evaluated for their on Saccharomyces cerevisiae fermentation and their bioprotection in Chardonnay must. Their presence had no significantly effect on fermentation duration, though slightly reducing S. cerevisiae fermentation rate. Analysis of central carbon metabolism and volatile organic compounds revealed that MP positively interacted with S. cerevisiae in sequential inoculation, with an increase in glycerol and acetate ester content.

In the last phase of the study, we quantified pulcherrimin in the medium by its absorbance (550 nm) and of pulcherriminic acid precursor by LC-MS/MS, and no significant correlation between the presence of these compounds and the BP capacity of Metschnikowia strains against Go was detected. To further evaluate alternative mechanisms, we assessed the influence of nutritional limitation (nitrogen sources, sugar, oxygen and iron), inhibitory compounds production and cell-cell contact, using differential bacterial growth media. Interestingly, Go growth was inhibited only in the presence of MP cells, regardless the medium composition, suggesting a key role of cell contact in the BP capacity of MP. These findings provide new insights into the efficient use of Metschnikowia as a natural alternative to sulfite addition for controlling specific spoilage microorganisms in winemaking.

References

Aragno J, Fernandez-Valle P, Thiriet A, Grondin C, Legras JL, Camarasa C, Bloem A. Two-Stage Screening of Metschnikowia spp. Bioprotective Properties: From Grape Juice to Fermented Must by Saccharomyces cerevisiae. Microorganisms. 2024 Aug 13;12(8):1659. doi: 10.3390/microorganisms12081659. PMID: 39203501; PMCID: PMC11356803.