The Metabolism of the Fructooligosaccharides in Onion Bulbs: A Comprehensive Review

Abstract

In the past two decades, there has been vast expansion in the research of fructooligosaccharides (FOS), including their chemistry, biochemistry and enzymology in higher plants. However, in spite of these considerable advances in fructan science, many other aspects of the mechanisms of fructan metabolism have not been fully understood. The bulbing and yield of dry onion cultivars vary, but they depend on the contents of the high dry matter and non-structural carbohydrates (fructooligosaccharides) which contribute to keepability. The knowledge of the mechanisms of the synthesis and the degradation of the FOS occurring during growth and storage are of great interest. Important progress has been made in the research area of onion FOS, and in addition to their role as accessible reserve carbohydrates in bulbs during sprouting, FOS participate in many physiological processes of production, protection and preservation of the bulbs. This review aims to contribute to a better understanding of the metabolism of FOS in onion bulbs, based on recent investigations. The activities of the main enzymes involved in the synthesis and the hydrolysis of the FOS during the pre-harvest (growth) and post-harvest (storage) life of the bulbs are reviewed, although present knowledge is too limited to explain clearly the mechanisms triggering the enzymes activities or the mechanisms by which FOS contribute to quality and long keepability of the bulbs.