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MK Seeds Home Page
MAGNUS KAHL SEEDS P/L.
3422 Melbourne Rd.
Lancefield
Victoria 3435
Australia
Office Tel Australia
+ 613 54292455
Office Fax Australia
+ 613 54292488
Email:
dan@mkseeds.com.au
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Sweet
/ Pungency Analysis for MKS Varieties |
Below is a chart prepared to give
information re: Sweetness in Various Types of Onions.
| Variety |
Type |
Pyruvic
Acid |
Sugar
(gm /100 gm dryweight) |
| umole/gm |
Fructose |
Glucose |
Sucrose |
| Sweets |
|
Domenica |
Intermediate Day Sweet |
5.23 |
24.1 |
26.3 |
5.00 |
| Sweet Golden
Brown |
Short Day Sweet |
4.79 |
23.41 |
28.35 |
9.10 |
| F1 Sweet Sensation |
Intermediate Day Sweet |
4.46 |
22.7 |
26.37 |
7.53 |
Non Sweets |
|
| Belvedere |
Early Brown |
8.10 |
16.5 |
16.3 |
8.30 |
| Vecon Regular |
Creamgold |
7.11 |
11.90 |
14.40 |
13.50 |
| Patrick |
Brown Storage |
11.04 |
18.10 |
20.40 |
17.53 |
| Red Marksman |
Red Storage |
9.82 |
19.03 |
22.3 |
14.54 |
| Pasquale |
Yellow Storage |
11.34 |
17.94 |
21.04 |
15.03 |
| Notes - These
figures were prepared by University of Queensland for MKS. |
 | A significant variation in results was apparent for different
bulbs in each variety. The above figures are for tests carried out on samples of
groups of 6 bulbs. |
| Generally, sweet onions are characterised by high fructose,
glucose and sucrose levels. |
| Re measurements by brix alone - In general, it appears that
low brix, say 6%, means there are sugars, water and low dry matter. High brix, say 18%
would mean low sugars, high fructan polymers and a lot of dry matter. The difficulty is
with say 10% brix, where you are going from one sugar / fructan / dry matter accumulation
system to another. This means that measuring fructose / glucose / sucrose is much more
interpretable than brix alone. |
| As can be seen from the chart, sweet onions are characterised
by low pungency, high fructose, gluctose and low sucrose.
I refer you also, to a very good scientific paper on this subject. |
Title : “Genetic and Environmental Effects Influencing
Flavour in Onion”
By : William Randle, University of Georgia |
Sweet
Onion Notes |
Definition of Pungency:
Pungency in onions is caused by volatile sulfur compounds.
Some of these compounds affect the eyes when onions are first cut and produce tears (often
called lachrymatory effect).
Other sulfur compounds cause a pungent, burning sensation in
the back of the mouth and the throat. Others produce the milder, more pleasant and typical
onion flavour.
All of these flavours are produced when the onion is cut and
an enzyme called alliinase is released and instantly breaks down the flavour precursors.
There is no chemical method which can measure all of these
flavours from many samples. Instead, we measure pyruvic acid which is a byproduct of the
flavour reaction and is relatively quickly and easily measured.
Pyruvic acid has been shown to correlate well with flavour
perception.
Pyruvic acid levels vary between about 1 and 18 micromoles
for different onion cultivars. We use the following guide to pungency strength which is
also used by the Sweet Onion industry in Georgia, USA.
| Low Pungency = 0-3 micromoles pyruvic acid per gram fresh
onion. |
| Med Pungency = 3-7 micromoles pyruvic acid per gram fresh
onion. |
| High pungency = above 7 micromoles pyruvic acid per gram fresh
onion. |
|
As a rule of thumb, Vidalia onions grown under
low sulfur are in the low pungency category, Pukekohe Longkeeper (Creamgold) onions are
usually about 7 and above and white dehydration onions are usually in the 15 range.
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Definition of Sweetness:
Sweetness in onions is caused by the sugars glucose, fructose
and sucrose. These three sugars vary in their sweetness. If sucrose has a sweetness of
100% then glucose has a sweetness of 70% and fructose has a sweetness of 170%. Onions also
contain polymers of fructose called fructans, although these are not sweet.
Onion cultivars differ quite markedly in the relative amounts
of sucrose, glucose, fructose and fructans which they contain. They also differ in sugars
according to length of storage and location in the bulb. Short day cultivars, which are
poor storers, tend to have higher levels of sucrose, fructose and glucose, but hardly any
of the fructans. In contrast, long day cultivars and cultivars such as Pukekohe Longkeeper
have less sucrose, glucose and fructose and higher amounts of fructans.
The fructans do not play a role in sweetness. No one knows
for sure why such plants as onions have fructans and not starch. One possibility is that
they play a role in osmotic regulation, so that they control how much water is taken up by
the bulb and thus what the dry matter of the bulb is.
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Relationship between Pungency and
Sweetness:
The balance between levels of pungency and levels of sugars
determines the perception of sweetness in an onion. High levels of pungency can mask high
levels of sugars so that the onion is not perceived as sweet. Also, onions with low levels
of pungency but low levels of sugars can be perceived as bland. Ideally a sweet onion
would have high levels of sugars and lower levels of pungency.
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Producing Sweet Onions:
In order to produce a truly sweet onion, a number of factors
must be considered. The process begins with the selection of a variety which possesses
suitable genetic characteristics. Variety is the main factor in determining pungency in
onion cultivars.
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