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360 Crop Post-Harvest: Science and Technology , First Edition. Edited by Debbie Rees, Graham Farrell and John Orchard. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd. 17 Onions, Shallots and Garlic Lesley Currah, Katherine Cools and Leon A. Terry INTRODUCTION Onions Allium cepa L., shallots A. cepa group Aggregatum and garlic A. sativum are all plants of the genus Allium which produce edible bulbs and leaves. Botanically, the family Alliaceae is related to the Liliaceae and the Amaryllidaceae within the monocotyledons. The majority of species of the large Allium genus are distingui- shed by the presence of sulphur containing compounds which give the typical onion or garlic smell and flavour for which the allium crops are prized. Other important compounds include saponins and flavonols, which also contribute to the flavour and the health benefits from eating alliums. The main storage carbohydrates in onions are complex fructans, especially abundant in pungent onions, while in sweet onions there are higher levels of simple sugars Davis et al. 2007. Onions and other bulb alliums are important vegetables in world trade since they can be shipped when dormant. Annual figures published by the FAO Food and Agriculture Organization of the United Nations show that onion world production has risen steadily in recent years, reaching a total of 64.47 million tonnes in 2007 FAOSTAT 2008. Estimates of the proportion of onions traded internationally vary from 6 to 10: for example, the volume exported in 2005 Tables 17.1 and 17.2 was nearly 5 million tonnes, representing about 7.6 of reported production. However, we should be aware that a lot of ‘informal’ trade across frontiers is not reported in the official figures from Tanzania to Kenya for example. In addition to ‘dry bulb onions’, over 500 000 tonnes of ‘green onions plus shallots’ were also exported in 2005 Tables 17.3 and 17.4. For garlic, world annual production is about 15 million tonnes and of this, over two-thirds originate in China, which is also by far the most successful exporter Table 17.5, while many different countries are substantial importers Table 17.6 FAOSTAT 2008. Major onion producing countries extend from the tropics to temperate regions and onions are produced in countries ranging from the equator to Scandinavia. Since onions are sensitive to photoperiod, a very wide range of onion cultivars has been developed, from ‘short day’ to ‘very long day’ adapted, to cover all the latitudes where onions are grown. The highest national onion yields are recorded from temperate countries with long growing seasons e.g. Ireland and Korea, mean yields 58.0 and 57.0 tha respectively in 2007, compared to an average world yield of 18.7 tha; FAOSTAT 2008. Temperate climates allow the onion to develop a large vegetative plant before bulbing starts, thereby giving it the capacity to produce a large bulb. In many tropical countries with seasonal climates, onions grow and produce well in the cool season. However, they are more difficult to grow near to the equator in very humid climates and at low altitudes: here, tropical shallots are the traditional alliums produced e.g. in Indonesia. Traditional garlic production areas are mostly located in Mediterranean climates including California, Argentina and Chile and also in India and the East Asian countries.