Current studies on onion and its pungency Part 2. Interactive effects of sulfur fertilization and drying under the aspect of solar energy utilization

Abstract

It is known that the consumer acceptance of fresh and dried onions is influenced mainyl by its flavour strength. Consequently, the interactive effects of eight treatments, which were combinations of four levels of elemental sulfur fertilizer (0, 357, 714 and 1071 kg/ha) and two drying methods i. e. solar energy and electrical vacuum drying on several characteristics associated with the nutritional potential, pungency, drying and rehydration efficiency of onion, were evaluated. Results revealed that sulfur fertilization, especially at 714 kg/ha, increased significantly (p ≤ 0.05) total solids, fructan, water soluble carbohydrates (WSC) and vitamin C; however slight decreases in individual sugars were recorded. Total-, background- and enzymatically formed pyruvic acid as pungency precursors, volatile sulfur content and sulfur containing amino acids (methionine, cystine and cysteine) increased significantly by sulfur application particularly at a rate of 714 kg/ha. The drying process induced significant losses in all nutritional and pungency indices, where the electrical vacuum drying was more deteriorative. Gas chromatograms of onion volatile oil indicated that sulfur fertilization resulted in a considerable increase in all volatile components particularly the major ones i. e. propanethiol (n-propyl mercaptan), propionaldehyde, di-1-propyl disulfide and methyl propenyl disulfide, which have the purest and strongest onion odour. The horizontal solar tunnel drier with a maximum temperature of 60°C proved to be favourable for drying of onion slices, as it not only led to maintenance of the volatile odour components and a good colour, but also to reduction of drying time (6 hrs) and promotion of the rehydration ratio of the dried onion slices.