Optimization of process conditions of dry longan pulp fermented by complex lactic acid bacteria

Abstract: Longan (Dimocarpus longan Lour.) is a subtropical fruit, and it contains many nutritional and functional components such as carbohydrates, fiber, and phenolics. However, the fresh longan fruit is susceptible to spoilage and has a short storage life, so it is necessary to develop a new product of longan by fermentation. The purpose of this study was to obtain the complex lactic acid bacteria being suitable for fermentation of dry longan pulp, and the optimizaiton of process conditions of fermentation. The effect of seven lactic acid bacteria (Lactobacillus acidophilus, Leuconostoc mesenteroides, Lactobacillus casei, Pediococcus pentosaceus, Lactobacillus plantarum subsp·Plantarum, Lactobacillus delbrueckii subsp. lactis ) fermentations on the Lactobacillus colonies, the pH value, the acidity and the content of reducing sugar and free phenolic was analyzed. The result showed that the content of reducing sugar was consumed and the content of free phenolic was increased in seven lactic acid bacteria fermentations in dry longan pulp. Leuconostoc mesenteroides and Lactobacillus plantarumsubsp·Plantarum were selected by comparing the effect of seven lactic acid bacteria fermention. The conditions of longan pulp fermented by complex lactic acid bacteria were optimized through the single factor experiment, and factors included solid-liquid ratio, fermentation temperature, fermentation time, complex lactobacillus inoculation, complex lactobacillus matching shift. The result showed that solid-liquid ratio, fermentation temperature, fermentation time, and complex lactobacillus inoculation were significant influence factors in dry longan pulp fermented by complex lactic acid bacteria. And the suitable conditions in dry longan pulp fermented by complex lactic acid bacteria were solid-liquid ratio1:7 (g/mL), fermentation temperature 35 ℃, fermentation time 48 h, complex lactobacillus inoculation 1%. Complex lactobacillus matching shift could not significantly effect on the fuzzy evaluation value in dry longan pulp fermented by complex lactic acid bacteria, which synthetically expressed the changing situation of the content of free phenolics and sugar in dry longan pulp. The conditions of longan pulp fermented by complex lactic acid bacteria were optimized through Box-Benhnken central composite experimental design based on the indicators of the fuzzy evaluation value. A regression model of four factors including the fermentation time, the mixed lactobacillus inoculation, solid-liquid ratio and the fermentation temperature was established. Through regression analysis and validation test, the optimum conditions were solid-liquid ratio1:7 (g/mL), fermentation temperature 37 ℃, fermentation time 53 h, complex lactobacillus inoculation 1.4%, complex lactobacillus matching shift 1:1. These four factors which impacted on the free phenolics and reducing sugar content in descending order were fermentation time, fermentation temperature, complex lactobacillus inoculation and solid-liquid ratio. Fermentation time and fermentation temperature were significant interaction terms. Complex lactobacillus inoculation and fermentation time were significant interaction terms. The fuzzy evaluation value prepared under the optimum conditions was 89.32, and the relative error was 2.70% between predictive fuzzy evaluation value (91.80) and true fuzzy evaluation value. The free phenolics and reducing sugar content prepared under the optimum conditions respectively were (161.77±3.93) mg/(100 g), (122.80±3.64) mg/g. This research provides a theoretical guidance for the development of longan beverage fermented by lactic acid bacteria, which contains the lower sugar content and is suitable for some patients with diseases such as diabetes and obesity.