Leucine Catabolism Affects Acetate Ester Production in Bananas (cv. Senorita)

The aroma of fresh fruits is an important factor in determining quality and consumer satisfaction. Esters are the major compounds responsible for aroma in most ripened fruits. Isobutyl acetate and isoamyl acetate are the predominant esters, which are bio-synthesized by enzymatic combination of alcohols, derived from amino acids and acyl CoAs, catalyzed by alcohol acyltransferase in bananas. However, ripened 'Senorita' banana has a very poor banana-like aroma production compared to other banana types. In order to elucidate the low level of isoamyl acetate production in Senorita bananas, effect of leucine catabolism on the ester production was investigated in 'Senorita' bananas. Concentration of isoamyl alcohol in 'Senorita' and 'Cavendish' bananas was determined by gas chromatography (GC). Senorita banana pulp was incubated with different precursor compounds; isoamyl alcohol (4 mM), isovaleraldehyde (5 mM), alpha-keto isocaproic acid (10 mM) or leucine (7, 14, 50 mM) separately. After incubation, formation of isoamyl acetate was determined by GC. The levels of amino acids (leucine, isoleucine, and valine) present in 'Senorita' and Cavendish bananas were also identified and quantified by capillary electrophoresis.  The results indicated that isoamyl alcohol concentration of 'Senorita' was lower than that of 'Cavendish', which has a rich banana-like aroma. Addition of any of the precursor compounds to the pulp, increased the production of isoamyl acetate. However, only a slight increase in ester production was detected by the addition of 50 mM leucine to the pulp, which was incubated for  5 h. When the pulp was incubated with lower concentration of leucine (7 mM) for 19h, isoamyl acetate production was higher than that of incubated for 5 h with 50 mM leucine. In both banana types, approximately similar amounts of amino acids were detected. In spite of the presence of adequate amount of amino acids in the pulp, the ester forming capacity was rather low in 'Senorita', most probably due to lack of isoamyl alcohol. The data obtained from this study indicate that the production of isoamyl acetate ester is considerably low in ripened 'Senorita', possibly due to limitations in the conversion of leucine to alpha-keto isocaproic acid by amino acid transaminase and then to isoamyl alcohol.