Some amino acids are known to be precursors of aroma-active compounds produced by yeast metabolism. This study examined the effect of the addition of L-isoleucine and L-phenylalanine on the volatile profiles of longan wine fermented by a co-culture of Saccharomyces cerevisiae var. bayanus EC-1118 and Williopsis saturnus var. saturnus CBS254 inoculated at a ratio of 1:1 000 cfu/mL with the aim of enhancing longan wine aroma. Significantly higher concentrations of active amyl alcohol (2-methyl-1-butanol), 2-phenylethyl alcohol and their corresponding acetate esters were found in the longan wine with added L-isoleucine and L-phenylalanine respectively, compared with the control (without added amino acids). The biosynthesis of other volatile compounds was either similar or minimally different, although the formation of some acetate esters, such as isoamyl acetate, hexyl acetate and ethyl acetate, was decreased. These results suggest that the added amino acids play an important role in enhancing the production of targeted aroma compounds in longan wine fermented with mixed yeasts, and that the addition of selected amino acids can be a valuable tool to modulate the formation of aroma compounds in longan wine.

Longan (Dimocarpus longan Lour.) is a type of tropical fruit. This study investigated the impact of L-leucine and L-phenylalanine on the volatile profiles of longan wine fermented with a non-Saccharomyces yeast Williopsis saturnus ssp. saturnus CBS254 with a view to enhancing longan wine aroma. The results revealed the ability of this yeast to enhance isoamyl alcohol and its ester isoamyl acetate (banana-like aroma), and 2-phenylethanol and its ester 2-phenylethyl acetate (rose-like aroma) with the addition of L-leucine and L-phenylalanine, respectively. The increased production of the targeted acetate esters appeared to be at the expense of other acetate esters except for methyl acetate, whereas the effects on the biotransformation of other volatiles were minimal. Therefore, these findings suggest that the combination(s) of an amino acid and yeast can be employed as a tool to manipulate the level of the targeted aroma compounds so as to impart a specific aroma or to add flavor complexity to longan wine. PRACTICAL APPLICATIONSTropical fruits such as longan are often in oversupply during the peak season and some are wasted because of lack of adequate processing technologies. Thus, there is a need to add value to longan fruit via processing. Although drying as a form of processing is commonly applied to longan fruit, fermentation is not widely practiced to process and preserve this fruit because of lack of research and information. Longan wine fermentation using the conventional grape wine fermentation technology results in longan wine with inferior flavor. The aim of this study is to investigate the fermentation of longan juice using a non-Saccharomyces yeast Williopsis saturnus var. saturnus with addition of L-leucine and L-phenylalanine so as to modulate the formation of aroma compounds.

Cofermentation of longan juice by mixed cultures of Saccharomyces cerevisiae var. bayanus EC-1118 and Williopsis saturnus var. saturnus CBS254 at two inoculation ratios (EC-1118:CBS254 = 1:100 and 1:1000 cfu mL )1 ) was performed to ascertain their impact on longan wine aroma compound formation. The results showed improved aroma compound profiles in the longan wine fermented with mixed yeasts in comparison with the longan wines fermented with single yeasts in terms of increased production of acetate esters, fatty acid ethyl esters, alcohols and carboxylic acids. The impact of cofermentation on longan wine aroma formation was affected by the ratio of S. cerevisiae EC-1118 to W. saturnus CBS254 with 1:100 cfu mL )1 being more effective. This research suggests that the inoculation ratio of mixed yeasts may be used as an effective means of manipulating longan wine aroma.

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