Sweet Onion Pungency Analysis

Sweet / Pungency Analysis for MKS Varieties

Variety	Type	Pyruvic Acid	Sugar (gm /100 gm dryweight)
Variety	Type	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.

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.

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.

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.

	The concentration of pyruvic acid in an onion bulb is used to measure the pungency of an onion.
	Climate and soils also affect pyruvic acid concentrations.
	Sweet Onions should ideally be grown on soils which have a naturally low sulphur level.
	The observance of specific crop management strategies is critical to the final result. Sulphur based fertilisers should be applied only during the early stages of onion growth.
	Sweet Onions should not be subjected to water stress. Water stress causes the pungency level to increase.
	Sweet Onion bulbs should be provided with a constant supply of water during their growth period.
	High summer temperatures during the growth period are known to increase pyruvic acid concentrations. The timing of Sweet Onion sowing and harvesting should be organised to avoid the hottest periods in individual growing regions.
	Planting density also affects onion pungency. Low plant populations are recommended.
	Sweet Onions are at their sweetest when they are harvested. Onion pungency increases during storage while total bulb sugar content decreases.