Worldwide, the United States ranks third in the production of dry bulb onions after China and India (Food and Agriculture Organization of the United Nations, 2014). The Vidalia onion industry is an important component of Georgia’s agricultural economy, and is a significant portion of the U.S. onion market. Vidalia onions are a branded product protected by both state recognition and a federal marketing order. They are a short-day, yellow, high water content onion with a Granex shape. In 2010, almost 13,000 acres were harvested in Georgia with an estimated farm gate value of $139 million (Wolfe and Morgan, 2011). Onions ranked first among vegetables comprising ≈18.5% of total vegetable farm gate value in Georgia (Wolfe and Morgan, 2011), which makes onions the state’s most important vegetable crop.
Onions are divided into three categories based on the photoperiod length plants need to initiate bulb production, 1) short-day varieties that require 11–12 h of daylight, 2) intermediate-day varieties that will bulb when exposed to 13 h of daylight, and 3) long-day varieties that will bulb when exposed to photoperiods for 14–16 h (Brewster, 1990). Onions from different daylength classifications also have other significant differences, notably the degree of pungency and storability with short-day onions generally having the lowest pungency and the poorest storability. Due to the mild winter and early spring season in Georgia, short-day varieties are well suited for overwintering production in the state (Boyhan and Torrance, 2002).
Vidalia onions are relatively low in sulfur compounds, which makes them mild flavored (Boyhan and Torrance, 2002). However, having the characteristic of low sulfur content makes Vidalia onions more susceptible to infection from pathogens and diseases than high sulfur–containing onions (Maw et al., 1997b). Several fungal pathogens can attack sweet onions in Georgia. Botrytis neck rot (Botrytis alli) is one of the most common and damaging storage pathogens of short-day onions. In bad years, 70% of the total crop can be infected with botrytis neck rot rendering them unmarketable (Sanders et al., 2008). Under natural conditions, this pathogen infects the dead or dying tissue of the onion bulb, and then grows downward through the neck into the bulb proper (Pappelis et al., 1974).
The postharvest treatment required for long-term storage of onions is curing (Maw et al., 1997a). Curing is a drying process intended to dry out the necks (Bayat et al., 2010) and outer scales of the bulbs (Maw et al., 2004) to reduce loss of moisture and prevent decay during storage. From harvesting to storage, curing can occur at any stage, whenever the conditions around the bulb become favorable to remove moisture from the bulb (Maw et al., 2004). There are two ways of curing onion bulbs: artificial and natural. Natural curing can take place in the field under the sun and wind after harvest. It is the least expensive way of curing and can be helpful in enhancing onion quality by allowing downward movement of nutrients from tops into the bulb (Maw et al., 1997a). Drying onions by forcing heated air around them is another way of curing. Standard conditions for this type of curing are blowing hot, dry air around the onions with temperatures up to 38 °C (Maw et al., 1998). Duration of heat curing varies according to the harvest maturity of the bulbs. For immature onions, the duration of heat curing required is more than for onions harvested at optimal maturity. Onions of early maturity are benefited when the duration of heat was 72 h, while for onions of optimal maturity required only 48 h of heat curing (Maw et al., 1997b).
Based on the market window, short-day onions can be handled in different ways: 1) fresh market, where the onions are sold directly without storage, 2) early-season markets, where the onions are stored in dry, well-ventilated sheds, 3) midseason markets, where onions can be stored under refrigeration, and 4) late season markets, where onions can be stored under refrigeration and controlled-atmosphere storage (Maw et al., 1997b). For successful long-term storage of short-day onions, they must be kept in a dormant state, which can be achieved with controlled-atmosphere storage. Controlled-atmosphere storage has been used widely with various fruits and vegetables like apples (Malus ×domestica), pears (Pyrus sp.), kiwis (Actinidia deliciosa), blueberries (Vaccinium sp.), mango (Mangifera indica), bananas (Musa sp.), cabbage (Brassica oleracea var. capitata), chinese cabbage (Brassica rapa), etc. (Kader et al., 1989). In Georgia, Vidalia onions are stored in controlled-atmosphere storage (3% O2, 5% CO2) at 1–2 °C and RH of 75%, which helps in extending the market availability of Vidalia onions from May to September (Boyhan et al., 2008). Sumner (2000) reported good quality Vidalia onions after 7 months of storage under controlled-atmosphere storage.
Sulfur dioxide is not used in onion storage in Georgia, but may offer an alternative to current long-term storage methods. In California, SO2 has been used for the postharvest control of gray mold of table grapes (Vitis vinifera) caused by Botrytis cinerea, (Nelson, 1985). Marois et al. (1986) suggested that controlling gray mold on table grapes under commercial storage conditions was better when SO2 application was at a concentration of 200 mg·L−1 applied three times per week rather than the standard practice of 2500 mg·L−1 once per week. SO2 technology has been tested to control brown rot (Monilinia fructicola) in peaches [Prunus persica (Smith, 1930)], mold in raspberries [Rubus sp. (Spayd et al., 1984)], and postharvest decay and peel browning in longan fruit [Dimocarpus longan (Whangchai et al., 2005)]. SO2 does have some drawbacks; it can corrode metal surfaces of the storage chamber and can cause product damage (Rahman, 2007).
Ozone can be helpful in postharvest treatment of fruits and vegetables and is currently used on a limited basis in some onion operations in southern Georgia. It can be applied as a gas or ozonizated water either continuously or intermittently under controlled-atmosphere storage (Palou et al., 2001). Storage life of broccoli (B. oleracea var. italica) and cucumber (Cucumis sativa) can be extended with the help of O3 (Skog and Chu, 2001). Song et al. (2000) reported that onion stored at low temperature when exposed to O3 had half of the mold growth compared with the untreated onions.
Gubb and MacTavish (2002) observed that there are a number of factors that can affect the storage life of onions. This includes harvest time, temperature at the time of harvest, bulb composition, number of outer skin layers, and dry matter content. The objective of this study was to determine the influence of varieties, curing, storage atmosphere or fumigation, duration of storage, and poststorage time on marketability of Vidalia onions.
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