Food Microbiology

Maple syrup is a natural sweetener produced from the concentration of maple sap during the North-American spring season. Over the last decades, maple syrup production transitioned from artisanal to industrial, leading to annual productions of nearly ten million gallons in Canada. The increased volumes now justify the need to address quality variation in the product. A range of color grades, but also flavors and defects are encountered in maple syrup. We previously showed that maple sap microbial contamination and predominant populations are correlated with syrup quality. However, the causality underlying these correlations remains to be explored. Are microorganisms directly responsible for changes in quality, or are they only reflecting changes in maple sap chemical composition driven by the tree metabolism? To improve our understanding of maple sap composition variation, we applied a systems biology approach using a collection of barcoded Saccharomyces cerevisiae deletion strains as a biological reporter. By competing these genetically modified yeasts in various maple sap samples, we identified metabolic pathways linked to nutrient utilization that vary over the collecting season, one of which is associated with a major flavor defect. We also identified allantoic acid as the principal nitrogen source in map sap, which may play a major role in modulating microbial contamination. The next step is to identify how these molecules directly contribute to maple syrup quality and how maple sap microorganisms respond to variations in these limiting nutrient sources. Such knowledge may ultimately help devise microbial-based strategies to improve maple syrup quality.