Know Your Onions

Subramaniam Sotheeswaran
University of the South Pacific

Onion is botanically described as Allium cepa and belongs to the plant family Lillaceae. It has been used as a spice for centuries. Onions are cooked, used raw in salads or used in flavouring foods. Three varieties of onions are known. They are the white, yellow or the red varieties and they differ in their tenderness and pungency. Approximate composition of the non-volatile matter in onions is given in Table 1. Onion is popular as a spice because of its volatile or flavour substances which are mainly organosulfur compounds. Some compounds have a remarkable effect assaulting the tear ducts or tantilising the tastebuds (1).

TABLE 1: Percentage Composition of the Non-Volatile Matter in Onions 

Investigations made by chemists over more than a century established that cutting an onion releases a number of low molecular weight organic molecules which are, in some instances, formed as the result of the action of some onion enzymes on naturally occurring sulfur compounds. These substances display a remarkable range of biological effects. Tearing brought on by slicing an onion is infact due to a chain of reactions set off that work on trans(+)-S-(1-propenyl)-L-cysteine sulphoxide, 1, (2), producing the lachrimatory factor, (3), which is a 19-1 mixture of (Z)- and (E)- propanethiol-S-oxide, 2a and 2b, (4). The onion enzyme, allinase which is capable of creating alkyl cysteine sulfoxides, catalyses this biochemical reacion as shown in figure 1. 

Frying onions transforms the pungent onion odour to a sweeter smelling one. The major component of the fried onion smell is the disulfide, 7,(1).

Biochemical experiments have shown that onion extracts have antibacterial, antifungal, antithrombotic and antiasthmatic properties, (1). The antithrombotic substances have been shown to be the organosulfur compounds named Cepaenes, 9, and Zwiebelanes, l0. The substance responsible for the antiasthmatic properties are the thiosulfinates, 5 and 6. The formation of the antithrombotic substance 9 from 2a is shown in figure 4. The biosynthesis of 10 probably uses dimethyl thiophenes as outlined in figure 5. 

Analysis of Headspace volatiles from onions showed, (5), the presence of fourteen compounds (see table 2). Some of the volatiles are not naturally occurring in onions but are formed during heating. For example the dimethyl thiophenes are reported, (5), to be formed from the disulfides as indicated in figure 5. 

 TABLE 2: Compounds Identified in the Headspace of Cut Onion

Steam distillation of onion gave an oil which was shown (5) to contain at least fifty compounds which were oxygen compounds which as aldehydes, monosulfides, disulfides, trisulfides, thiophene derivatives, tetrasulfides, thiols, and others. Most of them are not naturally occurring in the onion bulb but are formed during the chopping of the onion in the kitchen and on heating. Propanal (Table 2) is one of the most important flavour compounds in raw onions and is biochemically derived from the precursor 1 as shown in figure 1.

1. Emsley, J., Onions run rings around chemists, New Scientist, 30 September, 32, 1989.
2. Block, E., The chemistry of Garlic and Onions, Scientific American, 252(3), 94-99,1985.
3. Brodnitz, M.H., Pascale, J.V. and Derslice, L.V., Thiopropanal S-Oxide: A Lachrymator Factor in Onions, J. Agric. Food Chemistry, 19(2), 269-275, 1971.
4. Block, E., Revelle, L.K. and Bazzi, A.A., The Lachrymatory Factor of the Onion: An NMR Study, Tetrahedron Letters, 21 , 1227-1280, 1980.
5. Boelens, M., de Valois, P.J., Wobben, H.J. and Van der Gen, A., Volatile Flavour Compound from Onions, J. Agricultural Food chemistry, 19(5), 904-991 ,1971.