SOME FACTORS AFFECTING ARTIFICIAL CURING OF ONION BULBS AND ITS EFFECTS ON THE STORABILITY

Abstract

This research aimed to study some factors affecting the artificial curing of onion bulbs and its effects on the storability of bulbs in terms of (total soluble solids TSS and onion storage losses) comparing with the conventional curing method in Egypt. The study also aimed to determine the influence of different levels of maturity stages (harvesting time), about of 20, 30, 40% and 50 % tops down and bulb sizes (large, medium and small) on the change in the bulbs moisture content and the time required for complete curing. The obtained results showed that, during field curing, the bulbs moisture content decreased gradually from initial moisture of 87-90 % ±0.5% (w.b) to a final level of 81.5 % ±0.5% (w.b) after 16 day (384 h). At maturity stages of 20, 30, 40, and 50 % of tops down, the results revealed that, curing of onion bulbs artificially can reduce the required time of curing by about 68.8, 71.9, 75 and 81.3 % for large size of bulbs, and by 71.9, 75, 78 and 81.3 for medium and small size of bulbs, compared with traditional curing method of bulbs (control treatment). The results also showed that artificial curing of bulbs at 20, 30, 40, and 50% of tops down, may save the time required for curing bulbs comparing to traditional curing method of onion bulbs, that in turn increase the available time for preparing the land of the next crop (maize, rice and cotton crops; and other summer vegetable crops) by about 13, 13.5, 14 and 15 day when curing large size of bulbs, while it save about 13.5, 14, 14.5 and 15 day when curing medium and small size of bulbs. The storability of onion bulbs improved with about of 12.73 % for bulbs harvested at 40 and 50 % tops down, while it was 12.44 % for bulbs harvested at 30 % tops down comparing with traditional curing. * Senior Researcher, Agric. Eng. Res. Inst. (AEnRI), Giza. Misr J. Ag. Eng., 26(2): 905-921 PROCESS ENGINEERING Misr J. Ag. Eng., April 2009 906 INTRODUCTION ostharvest losses of food after, especially for perishable crops, and in developing countries are very high. The three main objectives of applying postharvest technology to harvested fruits and vegetables are to maintain quality (appearance, texture, flavor and nutritive value), to protect food safety, and to reduce losses between harvest and consumption. Effective management during the postharvest period, rather than the level of sophistication of any given technology, is the key in reaching the desired objectives. Onion (Allium cepa L.) is one of the most important field crops widely grown in Egypt, and it has a very high potential for exportation. In Egypt, the cultivated area of onion crop is 18061 feddans, which annually produces over 252166 tones yearly (Ministry of Agriculture and Land Reclamation, 2008). Onion bulbs are always harvested at complete mature stage with green topping leaves and high moisture content. Increasing productivity of onion with high quality is an important target by the onion growers. One of the major problems facing exportation of Egyptian onion is the lower storability characteristics when shipping to the European countries that exposing the bulbs to decay and hence facing rejection from the exporter. Sanguansri and Gould (1992) showed that a pilot onion curing plant using forced heated air was assembled and tested as an alternative to field curing. The bulbs from the curing plant were placed in storage and tested for weight loss, firmness and color. Results showed that cured onions had lower weight loss and more enhanced color. Kader (1992) showed that some products are cured (onions, garlic's, sweet potatoes, and new crop potatoes) after harvesting and before storage or marketing. Onions and garlic's are cured to dry the necks and outer scales. Curing helps heal harvesting injuries, reduces water loss, and prevents entry of decay causing organisms during storage. Curing may be done in the field, in curing rooms, or during transit. The treatment was most effective if the crop was removed from the field for drying within 48 h of topping, thus avoiding severe infection of the damaged green neck tissue. Pandey et al. (1993) subjected dark red onions for various combinations