Abstract
Consumers’ demand for birch sap, a nutritional juice tapped directly from birch trees, for human consumption is growing. This study aimed to investigate the time- and weather-related variability of the microbiota and protein content in birch sap throughout a complete tapping season, and the effect of birch sap freezing on its shelf life. Birch sap was collected daily during the 2018 season and harvested once a week during three consecutive weeks in 2019. Microbiota and protein content was 0.6–5.7 log(CFU/mL) and 3–60 μg/mL, respectively, with the highest content of both being in the end of the season. Daily temperatures correlated statistically with microbiota counts throughout the tapping season but not with protein concentration. The most prevalent bacteria was the genus Pseudomonas. Freezing birch sap for two weeks reduced the microbiota counts ∼1 log unit but did not affect the shelf life and type of bacteria. Twenty proteins related to plant defence against pathogens and abiotic stress were identified. In conclusion, birch sap harvested in the beginning of the tapping season had a longer shelf life and contained less protein than at the end of the season, which is of importance when developing procedures for microbial safe collection of birch sap and for the collection of sap containing bioactive substances.
Highlights
IntroductionBirch sap is a colourless liquid from birch trees that has been used for centuries all around the world, as a beverage or syrup (Maher et al, 2005; Salminen et al, 2005; Zhang and Shi, 2005; Svanberg et al, 2012), as a food ingredient or as a probiotic after fermentation (Sem jonovs et al, 2014)
The first 13 days of the season the aerobic bacteria counts were below 2 log(CFU/mL), after which they increased to a maximum of approximately 5.5 log(CFU/mL) in the end of the season (Fig. 1)
Our results indicated that the initial level of bacteria, yeast, fungi, and proteins was lower in the beginning of the tapping season compared to the end of the season
Summary
Birch sap is a colourless liquid from birch trees that has been used for centuries all around the world, as a beverage or syrup (Maher et al, 2005; Salminen et al, 2005; Zhang and Shi, 2005; Svanberg et al, 2012), as a food ingredient or as a probiotic after fermentation (Sem jonovs et al, 2014). The xylem of birch trees, is harvested from different species of Betula in cooler regions of the Northern Hemisphere in a period of two to five weeks in early spring, from a drilled hole in the tree, a process named tapping (Drozdova et al, 1995; Jiang et al, 2001; Maher et al, 2005; Ozolincius et al, 2016; Shaoquan et al, 1995; Zhang and Shi, 2005; Zyryanova et al, 2005). Trees grown along the edge of the forests exude more sap than trees from the interior (Zajączkowska et al, 2019)
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