Free and glycosidically bound aroma compounds in lychee (Litchi chinensis Sonn.)

Free and glycosidically-bound volatile compounds were isolated and separated from fresh clear lychee juice using an Amberlite XAD-2 column. The volatile compounds from the bound fraction were released by hydrolysis with almond β-glucosidase. Volatile components of both free and bound fractions were then determined by GC and GC–MS, and they showed similar volatile profiles. Totally, 25 compounds were identified in both fractions, including one ester, 14 alcohols, two aldehydes, four acids, two ketones and two terpenes. In the free fraction (2907 mg kg−1), the major volatile compounds found were acetoin (30.1%), geraniol (15.6%), 3-methyl-2-buten-1-ol (15.3%), octanoic acid (7.28%), 2-phenylethanol (4.91%), cis-ocimene (4.32%), and butyric acid (3.40%). In the bound fraction (1576 mg kg−1), the latent major volatile compounds found were geraniol (73.7%) and geranial (7.95%). In aroma evaluation, the free volatile fraction showed a fresh-fruity, lychee-like aroma whereas the bound fraction was odourless. The characteristic lychee-like aroma was noted in the bound fraction after enzymatic hydrolysis. On combination of the free and hydrolysed bound fractions, a strongly fruity, lychee-like aroma was perceived.