Subject: Re: onions flavor
From: mannj@southern.co.nz (Jay Mann)
Date: Feb 13 1996
Newsgroups: sci.bio.food-Science

Frank Bridges (fabfab@atl.mindspring.com) wrote:
: query:
: 
: 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  <mannj@southern.co.nz>
Christchurch, New Zealand