Abstract:

【Objective】The optical fermentation conditions of longan pulp by lactic acid bacteria were established, and the change of volatile flavor compounds of longan pulp before and after fermentation was clarified. This research can guide the development of functional longan beverage. 【Method】Lactic acid bacteria (Lactobacillus bulgavicus﹕Streptococus thermophilus=1﹕1) was domesticate in the medium containing 60%, 70%, 80% and 90% longan pulp and 10% skim milk. The acidity and pH of longan pulp were analyzed during the domestication. The conditions of fermented longan pulp by lactic acid bacteria were optimized through Box-Benhnken central composite experimental design based on the indicators of total acid. A regression model of four factors was established including the fermentation time, fermentation temperature, the content of skim milk powder and inoculation amount of lactic acid bacteria. Separation and analysis of the main volatile flavor compounds in longan pulp before and after fermentation were performed by headspace solid phase micro extraction and gas chromatography - mass spectrometry. 【Result】The lactic acid bacteria with good fermentation ability was obtained in longan pulp with high concentration. After 12 h of fermentation, the acidity of 90% longan pulp increased from 9.5ºT to 104.4ºT, and pH decreased from 7.12 to 4.44. The acidity of 90% longan pulp was significantly higher than that of 60% and 70% longan pulp (P<0.05), there was no significant difference of pH between longan pulp with four concentrations. The results showed that the optimum conditions are as follows: fermentation time 12h, fermentation temperature 45℃, 5% the content of skim milk powder and 3% inoculation amount of lactic acid bacteria. Under this condition, the acidity of fermented fruit pulp increased from 9.5ºT to 105.1ºT. The relative error to the value predicted by the model was 1.2%. It indicated that this model can be used in practical production. The volatile flavor compounds in longan pulp significantly changed after the fermentation. 53 volatile flavor compounds were detected in fermented longan pulp, which contained 15 terpenes, 6 alcohols, 18 esters, 7 ketones, 3 aldehyde, 2 acids and 2 others. 18 volatile flavor compounds were newly produced by fermentation. After the fermentation, the contents of alcohols, aldehydes, esters, ketones and acids increased, and the contents of terpenes decreased. The main volatile flavor compounds in fermented longan pulp were ethanol 25.68 μg·g^-1, 4-(1-methylethyl)- benzenemethanol 2.42 μg·g^-1, acetaldehyde 1.95 μg·g^-1, benzaldehyde 1.25 μg·g^-1, ethyl acetate 2.19 μg·g^-1, methyl benzoate 1.05 μg·g^-1, methyl salicylate 1.93 μg·g^-1, 3-hydroxy-2-butanone 1.085 μg·g^-1, (E)-β-ocimene 97.81 μg·g^-1, allo-ocimene 1.923 μg·g^-1, and acetic acid 1.84 μg·g^-1, respectively. Benzaldehyde and ethyl acetate were newly generated. The contents of ethanol, acetaldehyde, allo-ocimene and acetic acid increased after the fermentation. 【Conclusion】 Lactic acid bacteria fermentation significantly increased total acid and volatile flavor compounds of longan pulp. Longan pulp beverage fermented by lactic acid bacteria with a unique flavor can be developed by controlling fermentation conditions.

Key words: longan pulp, lactic acid bacteria, fermentation, volatile flavor compounds, total acid