Microbial safety and protein composition of birch sap

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.