Influence of Harvesting Month and Year on the Select Nutrients and Chemical Properties of Maple Sap from the Upper Peninsula of Michigan

Abstract

Objectives: Maple sap is a clear, watery liquid extracted directly from maple trees, consisting of 95–99% water. Historically used as a tonic, it is now consumed as a functional beverage. The primary objective of this study was to determine the influence of harvesting month and year on select nutrients and chemical properties of maple sap collected from the Upper Peninsula (UP) of Michigan. Methods: A total of 18 maple sap samples (8 in 2023 and 10 in 2024) were collected during the months of March and April in each year. The pH, titratable acidity (TA, %), and total soluble solids (TSS, °Brix) of the samples were measured. Organic acids were quantified using ultra high-performance liquid chromatography with diode array detection (UHPLC-DAD), while sugars were measured using HPLC with refractive index detection (HPLC-RI). Mineral content was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES), and total phenolic content (TPC) was determined using the Folin–Ciocalteu method. Phenolic compounds were identified and quantified using UHPLC-QTOF-MS and UHPLC-QQQ-MS, respectively. Non-targeted UHPLC-QTOF-MS analysis was used to identify a wide range of polar metabolites in maple sap, including amino acids. Statistical analysis was conducted using a univariate general linear model (GLM) in SPSS (version 29), and significance was determined at p < 0.05. Results: The pH, TA, and TSS of maple sap samples collected across two seasons ranged from 5.99 to 7.87, 0.01% to 0.03%, and 0.73 to 2.73 °Brix, respectively, with all three parameters significantly influenced by the harvesting year (p < 0.05). Malic acid was the predominant organic acid, present at concentrations ranging from 11.45 to 284.65 mg/L across all sap samples, but its concentrations did not differ significantly between months or years (p > 0.05). Among the six sugars analyzed (galactose, fructose, glucose, sucrose, maltose, and lactose) in selected samples collected in March and April of 2023 and 2024, only sucrose was quantifiable ranging from 1.37 to 2.65 g/100 mL, with no significant differences by month or year. In all sap samples harvested during March and April over the two years, TPC ranged from 21.6 to 89.6 mg GAE/L and was significantly affected by year (p = 0.045) and year × month interaction (p = 0.019). Calcium (3.09–46.70 mg/L), potassium (13.76–95.23 mg/L), and manganese (0.42–12.03 mg/L) were the most abundant minerals in maple sap samples, with stable concentrations across both years and months (p > 0.05), except for magnesium, which showed a significant year × month interaction (p = 0.043). A total of 15 compounds were identified in collected maples sap samples. Only vanillin was present in quantifiable amonts with concentration ranging from 0.40 to 2.14 mg/L, with no significant differences observed based on month or year in collected samples. Non-targeted analysis enabled the comprehensive profiling of polar compounds, including amino acids, and other metabolites in maple sap. Conclusion: This is the first study to report on the select nutrients in maple sap collected from the UP of Michigan, highlighting the influence of harvesting year and month on select sap properties. Malic acid, sucrose, calcium, potassium, and manganese were consistently detected as key components. It also contained phenolic compounds, with vanillin present in quantifiable amounts in all the collected samples. Overall, these results indicate that maple sap from this region has a stable composition across harvesting years and has the potential to be used as a low calorie, naturally sweetened, and nutritious functional beverage.