Subject: Re: onions flavor
From: firstname.lastname@example.org (Jay Mann)
Date: Feb 13 1996
Frank Bridges (email@example.com) wrote:
: why do onions acquire a sweet taste from cooking?
: does the heat convert starch to sugar?
Onions contain fructans, not starch. Fructans are polymers of fructose,
whereas starches are glucose polymers. Prolonged moist heat hydrolyses
the fructans, producing oligofructosides and some free fructose, which of
course is sweet. I believe that fructans are not hydrolysed in the human
small intestine, however, thereby providing a meal for bacteria in the
large intestine. Jerusalem artichoke is another plant with fructan
instead of starch in the root.
Almost all of the dry weight of onions is from the fructan content, with
cell walls quite minor. Dry weights can be, say, 6% in the case of
fast-growing poor-keeping Japanese or (I think) Mexican cultivars, about
12-14% in your average yellow onion, and 20-25% in onion cultivars bred
for production of onion flakes. Garlic has something like 60% dry matter.
The percentage dry matter is inversely linked to the average chain length
of the fructans. The low-DM onions have short fructans, the high
DM-onions and especially garlic have longer larger fructans. I once
measured freezing point depression (that is, water potential) in juice
squeezed from different sorts of onions; they all corresponded to the
equivalent of 0.3 Molar (from memory). This suggests some sort of
regulatory mechanism, the mechanism of which is unclear. It could either
be variety differences in the fructan-synthesizing enzymes, or in the
relative amounts and substrate specificities of endogenous fructans.
Fructan is synthesized by an interesting method, in which two sucroses
are put together to form a triose plus free glucose. Then the
triose (F-G-F) donates a fructose to a pre-existing oligosaccharide.
I tried mightily to get financial support to study whether the synthetic
enzymes in high- and low-DM onions differed, but no $$.
I'll bet that's a lot more than you ever wanted to know about onions.
Jay D Mann <firstname.lastname@example.org>
Christchurch, New Zealand