Onion Market Requirements

The majority of onions are grown for the fresh market and are usually graded prior to marketing. A uniform size, shape and color, plus freedom from soil and disease are important characteristics desired by the retailer and consumer. Bulbs should be sufficiently dry for the intended purpose. Firmness is also critical, particularly in dry bulb onions to be stored for long periods. Most common bulb onions have 10-12% dry matter. A thin neck size is also desirable as these onions are less susceptible to disease problems during the curing process.

Even size is important in onion production. Generally, large size bulbs get the best prices, provided shape and skin quality etc., are suitable. Taste, texture, smell and pungency vary between types and cultivars. Onions for fresh consumption without cooking are usually sweeter and mild flavored. They are generally larger – with just one center, thereby suitable for slicing into onion rings. Pungency is largely due to the level of sulphur based non-protein amino acids they contain.

When cut, damaged or eaten, sulphenic acids, ammonia and pyruvate are released giving a range of strong smells. Measurement of this important characteristic is difficult. While levels of pyruvate can be assessed, this does not always provide the complete answer to an onion’s ‘taste’ or pungency. Sweetness as assessed by the levels of sugars in the bulb is useful, but to the consumer, the over-riding pungency due to pyruvate can ‘mask’ these levels, or is over-ridingly strong. It is only when there is a low level of sulphur in the bulb that the sweetness is ‘allowed’ to come through in the taste. 

Thus, onion quality is usually assessed by taste panels that place onions, according to a combination of traits, into a range of ‘sensory rating’ categories. These vary according to country, but in the US, onions are rated 1 to 18. An onion rated 1 has very little pungency when raw. Any onion above 6 can’t be comfortably eaten raw, while most Vidalia and Super Sweet onions score 2.5-4.