Onion aroma

The effect of water content on the reaction yield is shown graphically in Figure 1. The sample prepared at 757. water content provided the highest yield. It also had the most balanced, meaty aroma. In contrast, the samples with 0%, 10% and 25% water had a biting aroma. The 50% water sample had a blended onion aroma at 75% water the aroma was pot-roasted, roasted, meaty and clean, while the 100% water sample was roasted and pot-roasted but also burnt. 

Normally consists exclusively of the 2a,4a,6a-form. Synthetic onion aroma. Colourless or yellowish oil. 

Thiophenes are found in many foods and often contribute significantly to their organoleptic properties. They are important as aromatic components of meat, roasted coffee, roasted nuts, onions and other foods. For example, while unsubstituted thiophene has an odour reminiscent of benzene, 2-methyl substituted thiophenes positively affect the flavour of canned meat. 2-Methylthiophene odour is described as green, heated onion, sidfurous and sweet dimethylthiophenes are important components of fried onion aroma, 2-acetyl-3-methylthiophene is reminiscent of honey and roasted nuts and 3-acetyl-2,5-dimethylthiophene has a sidfurous odour. 

The characteristic components of the fried onion aroma are 2,4-dimethylthiophene and 3,4-dimethylthiophene. They are produced from di(prop-l-en-l-yl)disulfide (Figure 8.74), which is formed from isoalliin via the corresponding thiosidfinate, in the same way as diallyl disulfide gives afliin via allicin. 

The flavor chemistry of sulfur compounds in onion is quite complex (19,20). Early reports of polysulfides and thiosulfinates were later demonstrated to be thermal artifacts from gas chromatographic analysis (20). Character impact sulfur compounds have been proposed for fresh, boiled, and fried onion. In raw, fresh onion, propyl propanethiosulfinate, propenyl propanethiosulfinate thiopro-panal S-oxide, and propyl methanethiosulfinate are impact contributors (18,19). A number of compounds contribute to the aroma character of cooked onion, of which dipropyl disulfide and allyl propyl disulfide provide key impact (18). Fried onion aroma is formed by heating the latter compound, and is characterized by... 

A number of volatile aliphatic compounds that contain nitrogen or sulfur atoms are important aroma constituents. Alkyl thiols, dialkyl sulfides and disulfides, and alkyl thiocyanates belong to this group. They occur widely in foods and spices and determine the odor of, for example, onions, garlic, and mustard. Because of their potent smell, they are used in high dilution and are often produced only in small quantities. The same is true for the following ... 

Allium oils are obtained from garlic and onion (Liliaceae). Their quality is assessed on the basis of their odor and aroma rather than their physical and chemical properties. The EOA specifications given below are, therefore, of limited value only. 

The most important flavour compound in raw onions is thiopropanal-S-ox-ide, the lachrymatory factor [145,146]. Other important flavour compounds are 3,4-dimethyl-2,5-dioxo-2,5-dihydrothiophene and alkyl alkane thiosulfonates such as propyl methanethiosulfonate and propyl propanethiosulfonate with a distinct odour of freshly cut onions [35, 36, 147]. Various thiosulfinates that have a sharp and pungent odour may also contribute to the flavour of onions. These compounds, however, are rapidly decomposed to a mixture of alkyl and alkenyl monosulfides, disulfides and trisulfides (Scheme 7.3) of which dipropyl disulfide, methyl ( )-propenyl disulfide, propyl ( )-propenyl disulfide, dipropyl trisulfide and methyl propyl trisulfide are the most important contributors to the aroma of raw and cooked onions (Table 7.5, Fig. 7.6) [148-150]. Recently, 3-mercapto-2-methylpentan-l-ol was identified in raw and cooked onions eliciting intense meat broth, sweaty, onion and leek-like odours [142, 151]. 

A very different source, vanillin, has also given accurate results. A saturated solution of vanillin is prepared by dissolving vanillin in ethanol and by adding a small portion of this solution to the basic analyte solution, NaOH. This type of olfactory titration is more appealing to students due to the pleasant aroma of the vanillin. Instructors might find this titration more attractive as well since the concentration of the NaOH is only 1M versus the 2 M required to quench the aromas of onions and garlic. A detailed account of the chemistry involved, procedures, and student handouts is available (Neppel et al. 2005). 

Yeoman et al. (35) demonstrated that supplying immediate precursors of capsaicin, the hot aroma compound of pepper, to Capsicum frutescens cultures increased the amount of capsaicin produced. Feeding more general precursors such as amino acids, also increased capsaicin production but to a lesser extent than when immediate precursors were applied. Feeding intermediate precursors to Allium cepa L. callus, enhanced levels of aroma components of onion flavor (36). 

Finally, aldehydes can react with nitrogen (31-32) and sulfur nucleophiles, including H2S, which may also be present in wines. These reactions may have dramatic effects on flavor and aroma (e.g., formation of ethyl mercaptan from acetaldehyde and H2S results in formation of a onion-like or burnt rubber aroma) and will decrease the levels of free aldehydes which can be readily quantitated (1). 

Thiiranes occur naturally in cruciferous vegetables. Thiirane and 2-methylthiirane have been detected by GC-MS in the aroma of canned beef. Beer with an onion-like off-flavor was found to contain (l-methylethyl)thiirane and 2-mercapto-3-methyl-l-butanol <88Mi 105-01). Thiirane is in the aroma of cooking mutton and is formed in the brewing of foods. 1 -Cyano-4,5-epithiopentane was the major component of rape seeds at 3 days of germination <85ABC217>. 

Maelkki, Y. Heinonen, S. Freeze-drying of high aroma onions. Journal of the Scientific Agricultural Society of Finland 50 (2), 125-136 (1978)... 

Model reaction trials and modem analytical methods (gas chromatography/mass spectrometry (GC/MS), gas chromatography/olfactometry (GC/0)) permitted the identification of key mechanisms responsible for flavour generation in process flavourings and some of the most important ones are detailed below. Often chemically complex precursor raw materials (vegetables such as onions, spices, yeast extracts, animal products) are used. Research work on these complex reactions is rare but necessary and allows the discovery of new key odorants and formation pathways. For example, Widder and co-workers [13] discovered a new powerful aroma compound, 3-mer-capto-2-methylpentan-l-ol in a complex process flavour based on onion. 

A powerful sulphury aroma compound, 3-mercapto-2-methylpentan-l-ol (51), has recently been identified [13] in a complex process flavouring. It has a low odour threshold of 0.15 mg/L of water. The formation could be traced back to the onions present in the process flavouring and its formation is explained from propanal present in onions via aldol condensation, addition of hydrogen sulphide and enzymatic reduction (Fig. 3.31). 

This article deals with the combinatorial synthesis of various sulfur-containing odorants. We used this approach for the identification of natural aroma components in different ingredients such as fiied onion or bell peppers. 

Sensory Properties. This mixture showed a very strong onion and leek-like aroma. All components can be sensorially characterized by GC-0. The sensory characteristics of the 1,3,5-dithiazines from isobutanal and ethanal used as starting aldehydes have already been reported earlier 17). 

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