Agriculture Reference
In-Depth Information
most others (Hertog
et al.
, 1992). The quercetin content of onion remains high
even after chopping and boiling (Hirota
et al.
, 1998; Makris and Rossiter,
2001).
In living onion cells, nearly all quercetin exists combined with either one
or two glucose units, as quercetin 4-glucoside or quercetin 3, 4-diglucoside
(see Fig. 8.9) (Hirota
et al.
, 1998). Concentrations reported in the edible rings
of red onion cultivars range from about 0.3 (Price and Rhodes, 1997) to 1.4%
(Patil and Pike, 1995); weights were expressed as quercetin free of any
attached glucose. Quercetin concentrations are higher in the outer scales of
the bulbs, towards the top of each scale and in the abaxial (outward facing)
epidermis of each scale (Patil and Pike, 1995; Hirota
et al.
, 1998).
Concentrations are highest in red cultivars, about 50% lower in the yellow
cultivars and about 99.7% lower in white cultivars. The dry skin of a red
cultivar can have 3% of its dry weight as quercetin, of which 2% can be free
(non-glucosides) (Patil and Pike, 1995).
During the senescence and drying of onion outer scales as they are curing
to form the dry outer tunic of the bulb, flavonols are converted to various
phenolic compounds, some of which condense to form brown pigments (see
Chapter 7, 'The Curing of Onion Skins').
Other chemical constituents
Alliums contain the nutritive substances normally associated with vegetables.
They also contain a number of other complex biochemicals in addition to their
prominent flavour compounds and carbohydrates. The chemical composition
of alliums was reviewed by Fenwick and Hanley (1985a, 1990a), and the
following remarks are based on those reports.
The dry matter content of allium vegetables is usually in the range 7-15%,
with leafy shoots usually a percentage point or two lower in dry matter than
bulbs. Garlic bulbs are the exception, ranging in dry matter content from 30 to
56%, depending on cultivar. On a fresh-weight basis these vegetables typically
contain 1-2% protein, around 0.2% fats and 5-12% carbohydrates, the latter
depending very much on dry matter content which, with onion bulbs and
garlic bulbs, can vary greatly with cultivar, as discussed previously. Protein
contents of garlic bulbs of 4-6% are common, in accord with their high total
dry matter content. The ash content of vegetable alliums is typically 0.6-1.0%,
the higher values again being associated with higher dry matter percentages.
Typically, their calorific value is around 35 calories/100 g, but is around 140
for garlic bulbs.
A number of phenolic substances have been isolated from alliums, in addition
to protocatechuic acid from the outer skins of onion bulbs, as discussed in Chapter
7, 'The Curing of Onion Skins'. Most of the amino acids commonly found in
proteins have been detected in onions and Japanese bunching onion. They contain
