Sorbic acid resistance and metabolism of Brettanomyces bruxellensis in the spoilage of low sugar soft drinks

Brettanomyces bruxellensis is an emerging spoilage yeast of low-sugar ethanol fermentation processes and alcoholic beverages. As soft (non-alcoholic) drinks manufacturers transition towards low sugar formulations, this study investigated the ability of B. bruxellensis to grow in different soft-drink and preservative conditions. Multiple B. bruxellensis isolates grew comparably to the common spoilage yeast Z. bailii in a variety of soft drink formulations, including zero sugar lemonades, low-sugar fruit juices, and carbonated beverages. Growth assays with B. bruxellensis in laboratory minimal-medium supplemented with low (0.1 %) glucose were characterised by turbid biomass accumulation (a spoilage indicator) and resistance to the major food preservative sorbic acid (SA), known to cause oxidative stress and to inhibit respiration. Analysis of respiro-fermentative metabolism revealed that B. bruxellensis favoured respiration over fermentation regardless of glucose concentration, with oxygen limitation significantly reducing its growth. Cell-to-cell heterogeneity was used as a tool to test whether cellular levels of respiratory reactive oxygen species (ROS) influence the organism's SA resistance phenotype. At low glucose, sorted cell-subpopulations with high background ROS were more SA resistant than low ROS cells. Furthermore, the antioxidant N-acetyl cysteine (NAC) hyper-sensitized these cell subpopulations to SA. Therefore, one explanation for SA resistance despite the organism's primarily respiratory metabolism could be that respiratory ROS builds cells' resilience to (subsequent) SA-induced oxidative stress. The work shows that B. bruxellensis is capable of growth in zero- or low-sugar media and drinks formulations, and in the presence of relatively high sorbic acid levels.