Plant L-ascorbic acid: chemistry, function, metabolism, bioavailability and effects of processing

Humans are unable to synthesise L-ascorbic acid (L-AA, ascorbate, vitamin C ), and are thus entirely dependent upon dietary sources to meet needs. In b oth plant and animal metabolism, the biological functions of L-ascorbic aci d are centred around the antioxidant properties of this molecule. Considera ble evidence has been accruing in the last two decades of the importance of L-AA in protecting not only the plant from oxidative stress, but also mamm als from various chronic diseases that have their origins in oxidative stre ss. Evidence suggests that the plasma levels of L-AA in large sections of t he population are sub-optimal for the health protective effects of this vit amin. Until quite recently, little focus has been given to improving the L-AA con tent of plant foods, either in terms of the amounts present in commercial c rop varieties, or in minimising losses prior to ingestion. Further, while L -AA biosynthesis in animals was elucidated in the 1960s,(l) it is only very recently that a distinct biosynthetic route for plants has been proposed,( 2) The characterisation of this new pathway will undoubtedly provide the ne cessary focus and impetus to enable fundamental questions on plant L-AA met abolism to be resolved. This review focuses on the role of L-AA in metabolism and the latest studie s regarding its biosynthesis, tissue compartmentalisation, turnover and cat abolism. These inter-relationships are considered in relation to the potent ial to improve the L-AA content of crops. Methodology for the reliable anal ysis of L-AA in plant foods is briefly reviewed. The concentrations found i n common food sources and the effects of processing, or storage prior to co nsumption are discussed, Finally the factors that determine the bioavailabi lity of L-AA and how it may be improved are considered, as well as the most important future research needs. (C) 2000 Society of Chemical Industry.