Full automation of oenological fermentations and its application to the processing of must containing high sugar or acetic acid concentrations

Abstract

Climate change and harvest date decisions have led to the evolution of must quality over the last decades. Increases in must sugar concentrations are among the most obvious consequences, quantitatively. Saccharomyces cerevisiae is a robust and acid tolerant organism. These properties, its sugar to ethanol conversion rate and ethanol tolerance make it the ideal production organism for wine fermentations. Unfortunately, high sugar concentrations may affect S. cerevisiae and lead to growth inhibition or yeast lysis, and cause sluggish or stuck fermentations. Even sublethal conditions cause a hyperosmotic stress response in S. cerevisiae which leads to increased formation of fermentation by-products, including acetic acid, which may exceed legal limits in some wines. Recently, an innovative fermentation system based on FT-NIR and modern process technology was developed by our group, allowing fully automated alcoholic fermentations. The system provides accurate real-time information about key-fermentation parameters including glucose, fructose, and ethanol concentrations throughout fermentations. This allows carrying out fed-batch fermentations at constant and low sugar concentrations thus reducing the hyperosmotic stress response of S. cerevisiae. In this research project, the automated fed-batch technique was compared to the traditional batch method and applied to the vinification of a white Chasselas (Gutedel) grape must under practical winery conditions. A research grade FT-NIR spectrophotometer with an InGaAs detector and an external transflectance probe was used providing non-destructive and non-diffusion limited in-line measurement of sugars. The population dynamics of Saccharomyces cerevisiae and apiculate yeast were followed throughout the fermentations, and samples were also analyzed for organic acids, glycerol, primary amino acids, ammonia, and aldehydes. The final wines were subjected to discrimination (2/5) and descriptive sensory (free sorting) analyses by a trained expert panel. The fed-batch technique allowed drastically reducing the titer (1-2 orders of magnitude) and impact of apiculate yeast. This lead to significantly different wines that were rated as being free of defects and fruitier by the trained panel. The kinetics of several key-wine compounds also differed considerably. Wine produced with the fed-batch technique contained no acetic acid and significantly reduced acetaldehyde levels. The research demonstrates the potential for the application of the fed-batch technique for high gravity musts, but also for musts with a high microbiological load. The drastic reduction of acetic acid concentrations offers a biological alternative to the membrane technology based reduction of acetic acid in musts and wines.