The Common Onion, Allium cepa

Onion oil is obtained by steam distillation of the crushed bulbs of the common onion. Allium cepa L. It is an amber-yellow to amber liquid with a strongly pungent, lasting, characteristic onion odor. 

FIGURE L Chromosome complements in root tips A) the horse bean, Vida faba B) the tree onion, Allium proliferum (C) the common onion, Allium cepa. Acetic alcohol, Feulgen squash. 

The effect of applied plant residues can be positively influenced by an increase of temperature. Mallek et al. (2007) established that dried and milled crop residues of onion (Allium cepa L.) or garlic (A. sativum L.) were able to reduce seed germination of barnyard grass (E. crus-galli (L.) R Beauv.), common purslane (Portulaca oleracea L.), london rocket (Sisymbrium irio L.) during their decomposition in soil, but only at the elevated (39°C) soil temperature. It can support combination of methods for weed control, in this case allelopathy with soil heating treatments (e.g., solarization). 

Onion Allium Cepa The outer skin of this common vegetable is one of the most useful and readily available dyestuffs. It is ideal for the novice dyer s first experiments since it reliable produced rich vibrant shade of yellow, orange, rust and brown on all fibres and does not impart any odour to the dyed material. 

The onion, like the other related vegetables (Allium genus), has long been classified as a member of the Lily family (Liliaceae), but now some botanists place the onion and its close relatives in a new family called the Alliaceae. The common onion is classified as the variety cepa to distinguish it from other varieties of onions such as the shallot, which is the aggregatum variety. Fig. 0-23 shows typical onion bulbs. 

Alkyl sulfides and thiols. Some alkyl thiols and sulfides, notably those from commonly ingested Allium sativum (garlic) and Allium cepa (onion) (Alliaceae), are variously bioactive as odorants and antimicrobials. Propanethial S-oxide (CH3-CH2-CH=S=0) is a lachrymatory irritant principle of onion. Allicin (S-oxodiallydisulfide CH2=CH—CH2-SO-S-CH2— CH=CH2), diallyldisulfide (CH2=CH-CH2-S-S-CH2-CH=CH2) and diallylsulfide (CH2=CH—CH2—S—CH2-CH=CH2) are major odorants of garlic that are reactive and irritant because of the allyl groups. Dimethyl disulfide (CH3—S—S-CH3), dipropyl disulfide (CH3-CH2-CH2-S-S-CH2-CH2-CH3), methyl allyl disulfide (CH3-S-S-CH2-CH=CH2) and propane-1-thiol (CH3-CH2—CH2—SH) are further Allium odorants. Methane thiol (methyl mercaptan CH3—SH) is a widespread plant volatile and notably derives from anaerobic bacterial degradation of cysteine as in human flatus and bad mouth odour. The aliphatic disulfides allicin and ajoene inhibit proinflammatory expression of iNOS. 

The natural flavors from garlic (Allium sarivum), onion (A. cepa), and other Allium species, like those from many other common vegetables and fruits, are not present as such in the intact plants but are formed by cnaymatic processes when the plants are chewed or cut (49). Additional flavors, also considered natural, arc formed during cooking as a result of the thermal breakdown of the initial enzymatically produced flavorants in either an aqueous on nonaqueous (c.g., cooking oil) medium. If the breakdown products are unstable, other compounds can be formed, which can contribute to the aroma and taste of the food. 

---