Onion Oil

Onion Oil
The distillation of so-called “onion oil” is very involved operation as yields are small and differ greatly depending on the variety of onion used and the distillation technique employed as the components are to some extent water soluble. The average yield is 0.015% of a dark brown oil which may crystallize on standing.

The comparative flavoring strength of the oil is in order of 4000 times that of fresh onion. In this form it is much too powerful to incorporate directly into foods and diluted versions, both liquid and dry, are available for this purpose. It is necessary to establish with the supplier the relative flavoring strength a compared with either fresh or dehydrated onion. It has to be appreciated that, though these products may have the flavoring character of fresh onion, they do not contribute any of the textural quality given by the raw vegetable.

In addition to their characteristics flavor notes, it is well recognized that onions when freshly cut have a marked pungency and lachrymatory property. This is an important factor in the flavor profile but these attributes are difficult to define, let alone assess accurately. The pungency level is somewhat related to flavor strength and aromatic character but its nature is not yet well understood.

Component of Onion oil
Oil of Onion obtained by steam distillation or by solvent extraction of the expressed juice differs in detail profile though each has been shown to contain a mixture of similar di – and trisulfides. The following components have been identified:
Ethanol
hydrogen sulfide
1-propanol
1-propanethiol
2-propanol
methyl disulfide
methanol
methyl 1-prophyl disulfide
propanal
1-prophyl disulfide
1-butanal
methyl trisulfide
acetone
methyl 1-prophyl trisulfide
methyl ethyl ketone
1-prophyl trisulfides

Sulfides are not the only odorous components but they are so powerful as to predominate.

The lachrymatory character of fresh onions (thiopropanal-S-oxide) does not exist in the essential oil and is not really part of the onion flavor. American wild onions are very flavorful but do not have this lachrymatory.
Onion Oil

The Flavor of Onion

The Flavor of Onion
In the natural products so far considered, the aromatic components exist per se in the plant material and can be recovered by simple distillation or extraction. The flavor of onions, on the other hand is an excellent example of the action of enzymes in flavor development as the characteristic notes are created spontaneously only when the onion is cut and the cells disrupted. The study has established that much of the odor and hence, flavor arise as a result of the conversion of odorless, nonvolatile, S-substituted cysteine sulfoxide derivatives, first to an unstable alkyl sulfenic acid intermediate by the action of an alliinase enzyme system.

The sulfenic acid moiety is very unstable and the reaction proceeds directly to a mixture of mercaptans, di-, tri-, and poly-sulfides as well as thiosulfonates. All possible combinations of methyl and prophyl derivatives have been isolated but little or no allyl compounds are present in onion. A similar system in garlic results in the formation of mainly allyl compounds, the difference being due to the nature of the precursor present.

Most onion flavor components are the result of the degradation of the thiosulfonates, resulting in a complex mixture of sulfides plus ammonia and pyruvic acid which is produced in the first stage of the reaction. A determination of pyruvic acid has been suggested as a measure of the flavoring strength of onion and onion products even though the acid itself does not contribute to the flavor profile.
The Flavor of Onion