Effect of gamma irradiation and temperature on fructans (fructo-oligosaccharides) of stored onion bulbs Allium cepa L
Introduction
Onions with other members of the Alliums family are generally consumed for their flavour, and their nutritive value has only been recently appreciated (Salunkhe & Wu, 1974). During their harvesting, handling, transportation, packaging and storage, onion bulbs are exposed to several treatments and environmental conditions which can affect their quality attributes and physiological characteristics. These effects could be responsible for several reactions and stresses, causing important biochemical changes to the bulb tissues (Benkeblia, 2003).
Bulb dry matter content is an important quality parameter of onion, and several investigations have attempted to relate bulb characteristics and storage life (Rutherford & Whittle, 1984). About 80% of bulb dry matter are non-structural carbohydrates (Darbyshire & Henry, 1981). The predominant members of these non-structural carbohydrates are glucose, fructose, sucrose and low-molecular-weight fructans, while starch and raffinose are absent (Benkeblia, Varoquaux, Shiomi, & Sakai, 2002; Darbyshire & Henry, 1981). The metabolism of sugars is closely linked to the dormancy and sprouting state (Kato, 1966), and the most important biochemical changes occurring during long term storage of bulbs, as of other vegetables, are the quantitative variations in the carbohydrate constituents. Variations of mono and disaccharides levels in onion bulbs during storage were previously reported (Benkeblia et al., 2002; Benkeblia & Varoquaux, 2003; Hurst, Shewfelt, & Schuller, 1985; Rutherford & Whittle, 1982); however, variation of fructo-oligosaccharides and the effect of gamma irradiation and long term storage on these constituents were not investigated.
Ionizing radiation is defined as a process in which food products are exposed to a controlled amount of radiant energy. Irradiation increases shelf life of fruits and vegetables, and reduces spoilage, and several investigations have been carried out throughout the world on the use of ionizing radiation to control sprouting in onions (Elias & Cohen, 1983; Matsuyama & Umeda, 1983). Irradiation doses ranging from 0.05 to 0.15 kGy inhibit bulb sprouting, and are more effective when applied during the dormancy period, specifically within 4–6 weeks following harvesting (Salunkhe & Wu, 1974). Ionized bulbs can be stored for several months without heavy spoilage, though ionization and storage can affect changes in the carbohydrate contents of onion tissues. However, despite the existence of numerous data on the commercial quality of irradiated onion bulbs, little information is available about the pattern of changes in the main chemical components, such as non-structural carbohydrates, during irradiation treatments and long-term storage.
The aim of this investigation was to assess the effect of irradiation and long-term storage on dry onion bulbs. Two irradiation doses were used (0.15 and 0.30 kGy) and treated bulbs were stored at three different temperatures (4, 10 and 20 °C) for 24 weeks. The concentrations of glucose, fructose, sucrose and other fructo-oligosaccharides were then determined.
Section snippets
Onions
Dry onion bulbs Allium cepa cv. Jaune d’Espagne (organic product, free of any preharvest chemical treatments), which had been freshly harvested and dried in the field for 2 weeks, were obtained from the local market. They were sorted for uniformity and absence of defects, packed in commercial plastic (PVC) trays and placed at 18 °C prior to treatments. Each tray contains 12 kg onions, and three trays were used for each irradiation dose and temperature.
Ionizing treatment
The irradiation treatment was applied two
Results
As shown in Fig. 1, glucose, fructose and sucrose constitute a major proportion of non-structural carbohydrates, while tri- and tetra-saccharides contents are lower. It was observed that concentration of high polymerized fructans decreased with their degree of polymerization. The distribution of the carbohydrate constituents is shown in Table 1. It was noted that glucose, fructose and sucrose constitute of a major part of the dry matter and non-structural carbohydrates, averaging 28%, 24% and
Discussion
Variation of mono and disaccharides in onions, particularly irradiated bulbs, was not extensively studied. Salama, Hicks, and Nock (1990) reported a decrease in total sugars and glucose in control onions stored for 5 months at 0, 15 and 30 °C, but fructose increased, particularly at 0 °C. Similar results on fructose were reported by Rutherford and Whittle (1982) at 0 °C. Hurst et al. (1985) noted a decrease in total sugars of onion kept during 6 months at 1 and 4 °C, but no variation was noted
Acknowledgments
Part of this work was conducted during a Postdoctoral Fellowship from the Japanese Society for the Promotion of Science (JSPS).
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