Factors affecting apple aroma/flavour volatile concentration: a review

Typical apple (Malus domestica Borkh.) favour develops during ripening. Maximum endogenous volatile concentration occurs at the climacteric peak but it is not known whether the volatile biosynthetic enzymes are constitutive or induced during the climacteric. Exposing apples to hypoxic conditions induces accumulation of high concentrations of acetaldehyde and ethanol; after return to air ethyl esters are enhanced and non-ethyl esters decrease. There are differences in degree of ethyl ester enhancement among cultivars. These may be because of: differential activity or synthesis of alcohol acyl CoA transferase (AAT) or alcohol dehydrogenase (ADH), separate iso-forms of AAT and ADH each with their own substrate specificity; variation in alcohol precursors in different cultivars; or a combination of all three. Volatile production is greater at higher temperatures in the range from 0 to 30 degrees C but exposure to low temperatures (<3 degrees C) for more than 3 months decreases production. Temperature influences the production of specific volatiles with some compounds only being produced at certain temperatures. It is not known how temperature will affect volatile production after exposure to hypoxia. It is suggested that the enhanced volatile production that occurs in apples following an hypoxic treatment might overcome or reverse the decreases that are induced by low temperatures and controlled atmosphere (CA) storage. The use of hypoxia to enhance volatile concentrations may be a beneficial side effect when such treatments are used for disinfestation purposes. It is possible that given equal efficacy, hypoxia could be either preferred or used as an adjunct to heat treatments to eradicate insects. In addition hypoxic treatment of fresh fruit could induce significant increases in volatile concentrations that could be used in production of high quality essences from apple juice.