PTR-ToF-MS for the Online Monitoring of Alcoholic Fermentation in Wine: Assessment of VOCs Variability Associated with Different Combinations of Saccharomyces/Non-Saccharomyces as a Case-Study

Abstract

The management of the alcoholic fermentation (AF) in wine is crucial to shaping product quality. Numerous variables (e.g., grape varieties, yeast species/strains, technological parameters) can affect the performances of this fermentative bioprocess. The fact that these variables are often interdependent, with a high degree of interaction, leads to a huge ‘oenological space’ associated with AF that scientists and professionals have explored to obtain the desired quality standards in wine and to promote innovation. This challenge explains the high interest in approaches tested to monitor this bioprocess including those using volatile organic compounds (VOCs) as target molecules. Among direct injection mass spectrometry approaches, no study has proposed an untargeted online investigation of the diversity of volatiles associated with the wine headspace. This communication proposed the first application of proton-transfer reaction-mass spectrometry coupled to a time-of-flight mass analyzer (PTR-ToF-MS) to follow the progress of AF and evaluate the impact of the different variables of wine quality. As a case study, the assessment of VOC variability associated with different combinations of Saccharomyces/non-Saccharomyces was selected. The different combinations of microbial resources in wine are among the main factors susceptible to influencing the content of VOCs associated with the wine headspaces. In particular, this investigation explored the effect of multiple combinations of two Saccharomyces strains and two non-Saccharomyces strains (belonging to the species Metschnikowia pulcherrima and Torulaspora delbrueckii) on the content of VOCs in wine, inoculated both in commercial grape juice and fresh grape must. The results demonstrated the possible exploitation of non-invasive PTR-ToF-MS monitoring to explore, using VOCs as biomarkers, (i) the huge number of variables influencing AF in wine, and (ii) applications of single/mixed starter cultures in wine. Reported preliminary findings underlined the presence of different behaviors on grape juice and on must, respectively, and confirmed differences among the single yeast strains ‘volatomes’. It was one of the first studies to include the simultaneous inoculation on two non-Saccharomyces species together with a S. cerevisiae strain in terms of VOC contribution. Among the other outcomes, evidence suggests that the addition of M. pulcherrima to the coupled S. cerevisiae/T. delbrueckii can modify the global release of volatiles as a function of the characteristics of the fermented matrix.

Influence of Non-Saccharomyces on Wine Chemistry: A Focus on Aroma-Related Compounds

“…All these results are likely to be the evidence of the changes of in the set of all volatile compounds produced by the yeast (volatome) due to the different combinations of the strains, endorsing the concept that the complexity of the wine can reflect the complexity of the starter cultures [ 9 , 53 , 158 ].…”

“…In the past, non- Saccharomyces species have been considered ‘bad fermenters’ because of their general low fermentative efficiency, low tolerance to enological additives such as sulfur dioxide and their production of acetic acid. However, in recent years several scientific evidences have encouraged researchers and producers to reconsider the action and contribution of the non-Saccharomyces species to the improvement of wine processing and product quality, especially in synergy with S. cerevisiae [ 2 , 3 , 7 , 8 , 9 ]. However, in the past, non- Saccharomyces fermentations have been generally associated with high volatile acidity, ethyl acetate production, off -flavours and wine spoilage.…”

“…However, in the past, non- Saccharomyces fermentations have been generally associated with high volatile acidity, ethyl acetate production, off -flavours and wine spoilage. Emerging evidence has demonstrated that the exclusive use of S. cerevisiae could result in a sensorial flattening of wines when compared with distinctive aromatic features of wines produced by spontaneous fermentation driven by the above-described yeast consortium [ 9 , 10 ]. Although the oenological potential of Saccharomyces strains has been deeply investigated, there is scarce information related to the impact of different non- Saccharomyces genera/species on the aromatic profile of wines [ 3 , 4 , 10 , 11 , 12 , 13 , 14 , 15 , 16 ].…”

“…Recent studies have shown that non- Saccharomyces yeasts have different oenological properties compared to those of S. cerevisiae, which can be used to solve specific technological issues (e.g., reduce volatile acidity, biocontrol strategies to prevent spoilage) in order to modulate and enrich sensory properties of wines by playing a role in releasing volatile compounds from non-volatile precursors [ 2 , 11 , 17 , 18 , 19 ]. The use of non- Saccharomyces species as part of mixed starters together with S. cerevisiae , and eventually also with malolactic bacteria, has been suggested as a way to mimic spontaneous fermentations and enhance wine complexity [ 8 , 9 , 13 , 17 , 20 ]. Additionally, the use of these mixed yeast starters can avoid the risks of fermentative stuck [ 6 , 8 , 21 , 22 ].…”

Saccharomyces bayanus Enhances Volatile Profile of Apple Brandies

“…However, to obtain characteristic flavours, increased concentration of desirable esters, and added aroma complexity, winemakers turn to non-Saccharomyces species [4] or strains that belong to the Saccharomyces genus other than S. cerevisiae [5]. It must be highlighted that there are several commercially available strains that belong to both these groups [6,7]. In many cases, indigenous microbiota provides wines with desirable flavour, intense aroma persistency, distinction, and vintage variation [8].…”