Onion Quality

The principal aims of bulb-onion storage are to maintain the 'quality capital' present at harvest (Guerber-Cahuzac, 1996) and to satisfy consumer demand for extended availability of onions of satisfactory quality. The bulbs of edible alliums are naturally dormant organs adapted to maintaining plant viability during periods unfavourable to growth (Brewster, 1994). Following dormancy breaking, they normally resume growth and progress towards flowering and seed production. Appropriate pre- and post-harvest treatments can slow down or arrest this process.

A wide choice of cultivars is available on the world market: their storage potential varies from short to very long. Onions grown specifically for storage are cured, dried and held in long-term stores before being cleaned, trimmed, graded and bagged for marketing (Timm et al., 1991). Sweeter and softer onions, historically grown for the fresh bulb market, need special treatment to keep them dormant if they are to be sold later. Recent advances in the science and technology of onion storage have extended the potential life of onion bulbs of both types.

Criteria for onion quality differ between countries. In the UK (Love, 1995) and Australia (Jackson et al., 1989), size and skin finish are paramount. Skin colour is important: a range from pale straw through to a deep copper colour is acceptable for most European markets (Gorini and Testoni, 1990) and other temperate countries. For the UK market, bulb shape should be the globe, with only moderate variations: completely oval or very flat bulbs are not acceptable. Thick and badly trimmed necks are also rejected. In France, lack of internal bulb defects, homogeneity of size, acceptable trimming and firmness are the main marketing criteria (Guerber-Cahuzac, 1996). A

neck length up to 4 cm is allowed under European Community (EC) quality standards. Firmness and at least one complete skin are required, and skin cracking should not be evident. Sprouting is not allowed in Class I, but early signs of sprouting are allowed in Class II, provided that the shoots would not become visible for at least 10 days after purchase. Bacterial rots, watery scale and fungal storage rots make the bulbs unsaleable.

In the USA, No. 1 grade onions ('Bermuda', 'Granex' and 'Grano' types) should have typical cultivar characteristics, be mature, fairly firm and well shaped and free from decay, stains or sunscald damage, doubles (more than one distinct bulb joined only at the base) and bottlenecks (elongated bulbs with abnormally thick necks) (USDA, 1997). The onions should be free from seed-stems, splits (bulbs with more than one obvious neck), dry sunken areas, sunburn, sprouting, staining, dirt or foreign material, tops and roots, translucent or watery scales, moisture, disease and insects.

Quality factors can be affected by mineral nutrition, timing of irrigation or rainfall (Chung, 1989), cultivar differences and the use of MH (Love, 1995). Onion flavour, defined by pungency and sweetness, varies with cultivar and growing conditions: there is an increasing demand in the USA and the UK for sweeter onions with low pungency.

Postharvest chemical application is best avoided, as it is too close to the consumer; controlled-atmosphere (CA) storage is therefore of increasing interest, since it can extend storage life beyond that achievable with cold storage alone. It also influences sweetness and pungency (H.S. MacTavish, in preparation). The shelf-life of onions after consumer purchase can be affected by the conditions of warming to ambient temperature after cold storage, conditions throughout the marketing chain and the packaging used.

Maintenance of skin integrity and the firmness, colour and flavour of onions is of paramount importance during curing and in the choice of storage regime. Respiration, resumption of growth and pathological breakdown are the biological factors involved in the deterioration of onions.

Bulbs also lose water by evaporation or may be physically damaged. Careful handling and the choice of a suitable storage method for the cultivar type in question are vital to ensure that the product retains its quality until it reaches the consumer. 'Cosmetic quality', i.e. retaining an attractive appearance, is of increasing importance in competitive retail markets.

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