Abstract
Maple sap is a complex nutrient matrix collected during spring to produce maple syrup. The characteristics of sap change over the production period and its composition directly impacts syrup quality. This variability could in part be attributed to changes in tree metabolism following dormancy release, but little is known about these changes in deciduous trees. Therefore, understanding the variation in sap composition associated with dormancy release could help pinpoint the causes of some defects in maple syrup. In particular, a defect known as “buddy”, is an increasing concern for the industry. This off-flavor appears around the time of bud break, hence its name. To investigate sap variation related to bud break and the buddy defect, we monitored sap variation with respect to a dormancy release index (Sbb) and syrup quality. First, we looked at variation in amino acid content during this period. We observed a shift in amino acid relative proportions associated with dormancy release and found that most of them increase rapidly near the point of bud break, correlating with changes in syrup quality. Second, we identified biological processes that respond to variation in maple sap by performing a competition assay using the barcoded Saccharomyces cerevisiae prototroph deletion collection. This untargeted approach revealed that the organic sulfur content may be responsible for the development of the buddy off-flavor, and that dormancy release is necessary for the appearance of the defect, but other factors such as microbial activity may also be contributing.
Highlights
Maple syrup is a natural sweetener obtained from the concentration of sap collected in the spring from sugar maple trees (Acer saccharum Marsh.) and related species (Acer rubrum, Acer nigrum)
We based our analysis on an index representing the remaining Sum of cumulative temperature necessary to reach the stage of Bud Break (Sbb), which we formulated using the model of Raulier and Bernier[28], in which Sbb = 0 corresponds to bud break
Sbb was highly correlated with the Julian date (JD) (Spearman ρ = −0.99, P-value = 2.2e−16) but for different years, the predicted bud break occurs at different dates (Supplementary Figs 1, 2a)
Summary
Maple syrup is a natural sweetener obtained from the concentration of sap collected in the spring from sugar maple trees (Acer saccharum Marsh.) and related species (Acer rubrum, Acer nigrum). A model was proposed by Raulier and Bernier[28] based on winter chilling and spring warming records to predict the date of bud break, that is, the stage at which leaves are visible but not yet unfolded[31] This particular decoupling between sap flow and dormancy release is a conundrum for maple syrup production, because some years, climatic conditions are such that sap flow is still heavy, but the tree physiology has reached a point where important quality defects appear in the syrup produced. We sought to identify the changes in maple sap with respect to dormancy release, associated with the appearance of quality defects, first with amino acid content profiling, with a systems biology approach using a prototrophic yeast deletion collection as a tool for monitoring sap composition variation. We found that changes in syrup quality coincide with the conversion of sulfate to organic sulfur in maple sap in the late stage of dormancy release
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