Apilactobacillus kunkeei as a promising fermentation candidate for reducing free sugars in fruit juices

BACKGROUND: Fruit juice is a rich source of vitamins, dietary fiber, and polyphenols; however, it also contains high concentrations of free sugars, such as glucose, fructose, and sucrose. Excessive consumption of these sugars negatively affects human health. For example, high fructose consumption is regarded as a contributing factor to metabolic syndrome. Therefore, various methods have been explored to reduce free sugar content in fruit juice, among which microbial fermentation is a promising approach due to its simplicity and cost-effectiveness. In this study, we investigated the reduction of free sugars in apple and orange juices through fermentation using fructophilic lactic acid bacteria (FLAB) which possess a unique ability to preferentially utilize fructose over glucose as a carbon source. METHODS: Ten FLAB strains and five lactic acid bacteria (LAB) strains were used. Apple and orange juices were fermented by each strain at 37 °C for 18 h. The concentrations of glucose, fructose, sucrose, and mannitol in the fermented juices were measured using high-performance liquid chromatography. Based on the sugar reduction performance, Apilactobacillus kunkeei was selected for further evaluation under high osmotic pressure and low pH conditions. RESULTS: FLAB strains reduced glucose and fructose levels more effectively than the other LAB strains. Notably, A. kunkeei and Apilactobacillus zhangqiuensis markedly reduced not only glucose and fructose but also sucrose levels compared with other Apilactobacillus species. The total free sugar reduction rate by these two strains exceeded 40% in apple juice and 50% in orange juice. Furthermore, A. kunkeei retained its sugar-reducing ability under low pH (pH 4.0) and high osmotic pressure (40° Brix) conditions. CONCLUSION: This study demonstrates that FLAB, especially A. kunkeei, are capable of efficiently reducing free sugars, i.e. glucose, fructose, and sucrose, in apple and orange juices. The sugar-reducing activity of A. kunkeei remains effective even under acidic conditions and high osmotic pressure. These characteristics suggest that fermentation with A. kunkeei is a promising approach for reducing the free sugar content in fruit juices.