Optimised extraction and preliminary characterisation of mannoproteins from non-Saccharomyces wine yeasts

Abstract

The use of non-Saccharomyces yeast species for the improvement of wine technological and oenological properties is a topic that has gained much interest in recent years [1]. Their application as co-starter cultures sequential to the inoculation of Saccharomyces cerevisiae and in aging on the lees has been shown to improve aspects such as protein stability and mouthfeel [2]. These contributions have frequently been associated with higher levels of polysaccharides, specifically the cell wall-derived mannoprotein [3]. Furthermore, mannoprotein structure and composition has been shown to vary between yeast strains, which in turn may influence their behaviour in the wine matrix [4-6]. However, non-Saccharomyces yeasts are typically weak fermentors and are frequently out-competed in the fermentation medium. An alternative strategy to their use as co-starter cultures is the isolation of the compound of interest for exogenous application to wine [7]. Indeed, the addition of exogenous mannoprotein-containing products derived from the cell wall of the wine yeast S. cerevisiae is a fairly common winemaking practice [8]. Nevertheless, the extraction of mannoproteins from non-Saccharomyces yeasts has not yet been well described. AIM: This study aimed to optimise the extraction of mannoproteins from four non-Saccharomyces strains, and to perform a preliminary investigation into the compositional differences of the mannoproteins obtained from the different species.

METHODS: Four non-Saccharomyces wine strains, Saccharomyces cerevisiae, Saccharomyces boulardii, Metschnikowia fructicola and Torulaspora delbrueckii, were exposed to combined methods with varied parameters of ultrasound and enzymatic extraction with β-glucanase to optimise mannoprotein yield. Colorimetric assays were used to quantify protein and carbohydrate concentrations in the extracts.

RESULTS: Yeast cells subjected to 4 min of ultrasound treatment applied at 80% of the maximum amplitude with a 50% duty cycle, followed by an enzymatic treatment of 4000 U lyticase per g dry cells weight, showed the highest yield of mannoproteins from all species. Furthermore, preliminary evaluation of the obtained extracts revealed differences in carbohydrate/protein ratios between species and with increased enzyme incubation time, as demonstrated by the higher ratios obtained for T. Delbrueckii and S. cerevisiae after almost all treatments, in comparison to M. fructicola and S. boulardii.

CONCLUSIONS: The results obtained in this study form an important step towards further characterisation of extraction treatment impact and yeast species effect on the extracted mannoproteins. Their impact on the carbohydrate/protein ratio in particular is an important factor to consider for applications such as wine protein haze reduction and tartrate stabilisation, and requires more in-depth investigation of isolated mannoproteins.

Optimised extraction and preliminary characterisation of mannoproteins from non-Saccharomyces wine yeasts

Abstract

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