Agriculture Reference
In-Depth Information
during long-term storage (Hansen
et al.
, 1999; Benkeblia
et al.
, 2004) as
fructans in bulb-scales are converted to fructose and sucrose and these are
translocated to the bulb base plate to drive sprout growth (Pak
et al.
, 1995; see
Chapter 7). Fructans accumulate transiently during seed development in
onions, but do not ultimately form a significant seed reserve (Pollock and
Lloyd, 1994).
The enzymes for fructan synthesis can be rapidly induced in onions; for
example, when onion leaves were given 5% sucrose in the light, they started to
accumulate the m-RNA to synthesize 1-SST after 4 h and the m-RNA for 6 G-
FFT after 12 h, and 1-kestose and neokestose were also detectable after 4 and
12 h, respectively, indicating activity of 1-SST and 6 G-FFT enzymes at these
time points (Vijn
et al.
, 1998). Transfer of onion plants from short to long
photoperiods or from a non-inductive to inductive light qualities (730:660 nm
ratio, see Chapter 4) induces a rapid increase in fructans in leaves and leaf bases
up to 10 days before increases in bulbing ratio are visible (Darbyshire and Steer,
1990; Kahane
et al.
, 2001). The hydrolysis of some of the accumulated fructans
may subsequently drive the expansion of outer leaf bases to give visible bulbing
(Darbyshire and Steer, 1990).
The utility of fructans
The soluble carbohydrates in edible alliums are an important determinant of
their qualities as foodstuffs. For the production of dried onion products - for
example, onion powder or flakes, which are widely used in cooking and food
manufacture - a high dry matter content of the bulb is required, as this reduces
the energy required for evaporating water per unit of dried material. Also, bulbs
for dehydration should have low quantities of reducing sugars (free glucose and
fructose), since these can cause non-enzymatic browning reactions, resulting in
an undesirable darkening of the dried product.
Fructan oligosaccharides have some sweetness that decreases with DP,
and also water-binding and gelling properties that increase with DP. The
latter properties make them useful as a fat substitute and as an ingredient
in manufactured soups or sauces. Shallots, which typically contain 20%
dry matter, have large amounts of fructans of DP 8-14 (Ernst
et al.
, 1998)
and are regarded as ideal for making savoury sauces in French cuisine.
Fructans are largely resistant to hydrolytic enzymes in the human
intestinal tract.
1-2 fructan links are completely resistant to digestive
enzymes, but
2-6-linked fructans may be partially hydrolysed (Delzenne,
2003). Therefore, they can impart many desirable qualities to food yet
have a low nutritional energy content, and hence they are of great interest
to food manufacturers.
