Agriculture Reference
In-Depth Information
The non-structural carbohydrates of alliums make up much of the dry matter;
for example, 60-80% of the dry matter in onion bulbs (Rutherford and Whittle,
1982; Hansen, 1999). The biochemistry of these carbohydrates was reviewed
by Darbyshire and Steer (1990), but the subject has developed considerably
since then, accompanied by a growing recognition of its importance for human
nutrition and for the understanding of the physiology of allium plants. These
carbohydrates comprise glucose, fructose, sucrose and fructans. Starch is rare
in onions, but does occur in the stem (i.e. baseplate) around vascular bundles
and in the primary thickening meristem (see Chapter 2) during sprouting, and
also in root cap cells (Ernst and Bufler, 1994).
Fructans are linear and branched polymers of fructose that occur as
reserve carbohydrates in about 15% of flowering plant species (Vijn and
Smeekens, 1999). The fructans in alliums mostly contain a single sucrose unit
(glucose-1 linked to fructose-2), to which are attached chains of 1-2 linked
fructose units joined at either the 1-fructose of the original sucrose or the
6-glucose of the original sucrose, or branching from both these points (see Fig.
8.5; Shiomi
et al.
, 2005; Benkeblia and Shiomi, 2006).
The number of linked fructose units, plus the single glucose unit, gives the
degree of polymerisation (DP) of the fructan. Fructans of low DP are termed
fructo-oligosaccharides and are the type most common in allium vegetables.
Onion, shallots,
A. fistulosum
, most leek cultivars and chives contain fructo-
oligosaccharides of a DP up to 20, but the shorter the chain length the greater
their proportion of the total fructan. Garlic contains large amounts of high-DP
fructan (average DP 41) and little fructo-oligosaccharide. Chinese chives and a
winter leek cultivar contained both fructo-oligosaccharides and high-DP
fructans (Ernst
et al.
, 1998).
The dry matter and soluble carbohydrate content of onion bulbs can vary
by a factor of three or so, being only about 7% in some sweet types, but up to
about 22% in cultivars grown for dehydration (Sinclair
et al.
, 1995a). The
carbohydrate composition differs dramatically between these types. White
'Sweet Spanish', with 7.5% dry matter, contained about 75% of the bulb-
soluble carbohydrate as glucose plus fructose, about 18% as sucrose and 7% as
fructans (DP 3 or more). The cross cv. 'White Creole'
'Southport White
Globe' contained 17.2% dry matter and had approximately 3, 7 and 90% of the
carbohydrates in the above three categories, respectively. Also, the higher the
percentage dry matter content the higher the mean DP of the fructans in the
bulbs (see Fig. 8.6). The dry matter percentage of onion bulbs is highly
correlated with the soluble solids content of juice extracted from them, as
measured by refractometer (see Fig. 8.7). The high fructan DP of garlic bulbs
correlates with their high dry matter percentage.
Fructan biosynthesis in onions involves first the production of 1-kestose,
which is catalysed by sucrose:sucrose 1 - fructosyl transferase (SST). As its
