8 - Production of off-flavours in fruit and vegetables under fermentative conditions

Abstract

A major problem in post-harvest storage and handling of fruit and vegetables is the development of off-flavors and loss of authenticity. The appearance and sensation of off-flavors are highly associated with the accumulation of the ethanol fermentation products acetaldehyde (AA) and ethanol. Ethanol fermentation is a two-step process in which pyruvate is first carboxylated to acetaldehyde (AA) by Pyruvate Decarboxylase (PDC) and AA is subsequently converted to ethanol by Alcohol Dehydrogenase (ADH). The off-flavor volatiles AA and ethanol normally accumulate at low levels during fruit maturation and ripening and, thereby play an important role in the biosynthesis of fruit aroma volatiles. However, following exposure of the fruit to anoxic conditions, the accumulation of the ethanol fermentation metabolites, AA and ethanol, may be enhanced to extreme levels. The occurrence of anaerobic conditions in the internal atmospheres of fruit and vegetables, and their tendency to develop off-flavors also depend on their anatomical structure and morphology. Fruit and vegetables may become exposed to anoxic conditions during their post-harvest handling and along the storage and marketing chain, and this may lead to induction of fermentative metabolism and accumulation of off-flavors. This chapter describes the effects of ripening and post-harvest management on ethanol fermentation metabolism and discusses the processes leading to the development of off-flavors and possible ways to manipulate and reduce them.