6533b835fe1ef96bd129e6c3

RESEARCH PRODUCT

Monitoring of mixed culture fermentations

subject

description

Current and future constraints linked to climate change and evolution of wine consumer demand are prompting the winemaking industry to consider adopting new practices to address the technical challenges resulting from this context. These challenges include the need to maintain a constant alcohol level despite increased sugar contents in the must, and to seek a wider diversity of aromatic profiles, while maintaining acceptable reproducibility.Fermentations with addition of non-Saccharomyces yeasts to the Saccharomyces cerevisiae species traditionally used to conduct alcoholic fermentation seem to be an interesting alternative to achieve these objectives. However, the numerous interactions between these different yeasts are still poorly understood, which complicates the control of these cocultures.A state-of-the-art review of the interactions occurring between Saccharomyces and non-Saccharomyces yeasts and their consequences on the characteristics of the resulting wine was conducted, with a specific focus on the methodologies used to study these interactions. This review pointed out the existence of a great diversity of results depending on the studies considered, and thus to a possible impact of fermentation conditions on the interactions between yeasts.This study therefore focused on studying the impact of selected fermentation conditions on interactions between Saccharomyces cerevisiae and Lachancea thermotolerans, a yeast with technological interests (acidification, aroma compounds, bioprotection). This study was carried out using a multi-criteria approach (population dynamics, fermentation progress, production of aroma compounds) based on an experimental design.The results show multiple impacts of the fermentation parameters, which differ depending on the variables studied. Yeast growth rates were affected by temperature, inoculation ratio and by a probable competition for nitrogen. The persistence of Lachancea thermotolerans was modulated by three interacting factors: temperature, inoculation ratio and oxygenation. The duration of fermentation is strongly affected by temperature. Ethanol production is influenced by both inoculation ratio and oxygenation, whereas lactic acid production is only affected by the ratio. To modify the production of aroma compounds, temperature, inoculation ratio and in some cases nitrogen concentration constitute significant levers. These results illustrate the complexity of controlling fermentation in mixed cultures, and the necessity to find the best parametric compromise to achieve the targeted technical objectives.In order to control these cocultures, the control of population dynamics is therefore a key point, as they impact both the fermentation process, microbial interactions and wine quality. A second line of work therefore aimed to evaluate the possibility of using a method of continuous monitoring of microbial populations, giving precise results with the least possible delay. The feasibility of automating flow cytometry in the oenological field was validated: this technology makes it possible to continuously monitor bacterial or yeast populations in various situations, particularly in the context of mixed culture fermentations.The two parts of the thesis (impact of fermentation conditions on interactions and automation of flow cytometry) are complementary, and allow to envision real-time control of fermentation conditions in mixed cultures, with future applications in research, quality control and even in the cellar.