Agriculture Reference
In-Depth Information
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 t/ha respectively in 2007, compared to an average
world yield of 18.7 t/ha; 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.
