Allium cepa L. (Onion)

2016

The Allium species have been a source of food flavors and medicinal compounds in many areas of the world for several thousand years. The attraction of the alliums as a flavor source is primarily the pungent volatile constituents which are released when the fresh tissue is cut or chewed, and also the presence of milder odors in the cooked vegetables. The major alliums used as food in Western Europe include the onion (Allium cepa L.), garlic (A. sativum L.), chives (A. schoenoprasum L.) and leek (A. porrum L.), but Allium fistulasum L. and Allium tuberosum are grown on a large scale and eaten raw or cooked in China, Japan, and South East Asia. All the alliums referred to may be eaten raw, or as a cooked vegetable, or used as a flavor additive to fresh or cooked foods (). On a commercial scale, the flavor may be added as a powder, an oil, or as dried shredded bulb tissue. The importance of Allium is indicated by the fact that flavor derived from this source (usually garlic or onions) is the major flavor additive to convenience foods.

The therapeutic value of fresh and extracted Allium has always been recognized, as can be judged by the list of ailments that are reported to be cured by garlic and onion. These are hemorrhoids, rheumatism, dermatitis, abdominal pain, coughs, loss of apetite, stomach disorders, loss of weight, skin rashes, parasitic infections, leprosy, snake bites and the bite of mad dogs (). Today there has been a revival in the use of Allium as a cure for more specific diseases such as dysentery, typhoid, and cholera, and modern diseases such as arterio sclerosis and hypertension. In the face of such worldwide and popular use of these plants as medicinal agents, it is not surprising that there have been many attempts to isolate the active principle and to test it in vitro and in vivo. The initial investigations were carried out on garlic (). Steam distillation produced a distillate which contained no antibacterial activity. However organic extraction of garlic at room temperature yielded a compound, allicin, which did have pronounced antifungal and bacteriostatic (rather than bacteriocidal) activity against a range of Gram-positive bacteria, including those bacteria (Staphylococcus, Streptococcus, Vibrio cholerae, Bacillus typhorum, B. dysenteriae, and B. enteridius) that are involved in intestinal diseases.

More recent evidence suggests that garlic may also contain anti-thrombotic compounds. It was found that the breakdown of the unstable allicin leads to the production of a large number of sulfide compounds, amongst which was detected a compound, ajoene, which was effective in preventing the aggregation of blood platelets (). The evidence from such in vitro studies provided a more factual basis to the claims of Allium as a source of useful medicinal compounds. The inclusion of garlic extract into present day “health” pills is an indication of the public’s continuing faith in the curative powers of Allium. Clearly there was and is a major commercial interest in the flavoring and medicinal properties of these species.

Considerable effort is now being devoted to flavor production in tissue cultures (). Most cell cultures, however, still show a lower level and a more restricted range of flavors, or other secondary products, than the intact plant. In the early experiments the approach was essentially empirical and did not attempt to understand or examine the basis for the low yields in tissue cultures. Tissue cultures of onion, as summarized by Collin and Musker (), were no exception to this general rule, since the undifferentiated callus on a maintenance medium did not accumulate any S-alkyl cysteine sulfoxides except for a small amount of Me Cys SO. If secondary product and, specifically, flavor production is going to be enhanced in culture, then the metabolism of the cultured cells must be fully understood. In order to stimulate the secondary metabolism in the onion, it is important to understand the control of secondary pathways in the tissue culture, and the intact plant. The following approach should be made to establish the control mechanisms in onion flavor biosynthesis:

  1. Identify the control enzyme(s) on the path of synthesis of trans-Pren Cys SO.
  2. Locate the intracellular site of synthesis of the trans-Pren Cys SO.
  3. Confirm the secondary pathways for synthesis of methyl, propyl, and allyl cysteine sulfoxide, since these also contribute to the flavor, then repeat steps 1-2 for these compounds.
  4. Examine the mechanism of action of picloram as an inductive agent in the path of trans-Pren Cys SO synthesis.
  5. In the long term, attempt to characterize the control enzymes(s) in trans-Pren Cys SO synthesis in order to manipulate its activity by genetic engineering.

 

Selections from the book: “Medicinal and Aromatic Plants V”, 1993.