Volatile isothiocyanates

BARTLET, E BLIGHT, M.M., HICK, A.J., WILLIAMS, I.H., The responses of the cabbage seed weevil (Ceutorhynchus assimilis) to the odour of oilseed rape (Brassica napus) and to some volatile isothiocyanates., Entomol. Exp. Appl., 1993,68,295-302. 

Enzymic hydrolysates of glucosinolates from seeds of nine species of Tropaeolum contain volatile isothiocyanates. ... 

Gentle drying at 40°C of wet raw material stored for 1 h was not suitable to restore the typical attributes. By drying, it was possible only to reduce the intensity of the negative attributes musty fusty and to remove the volatile isothiocyanates responsible for the smell and taste for horseradish, but the oil was still not suitable for human consumption. This experiment shows that already very short storage times at higher moisture contents result drastically in the loss of quality. That has to be avoided absolutely after the harvest of canola seeds. 

Figure 8 A comparison of the changes observed for the volatile isothiocyanates of horseradish isolated from (a) a sample of cocktail sauce maintained at 4°C and (b) a subsample of the same cocktail sauce held at 30°C for 5 days. The volatiles were isolated from the cocktail sauce samples by headspace SPME extraction using a 100- rm polydi-methylsiloxane fiber after spiking the samples with phenyl isothiocyanate (peak 3, internal standard) at a concentration of 10 ppm. Analysis of the components was by GC/MS, and quantitation was performed using selected ion extraction chromatograms of the respective molecular ions of the isothiocyanates. The other numbered peaks are aUyl thiocyanate (1), allyl isothiocyanate (2), and phenylethyl isothiocyanate (4).

Paralleling the various phosphorus trihalides are numerous pseudohalides and mixed pseudohalide-halides of which the various isocyanates and isothiocyanates are perhaps the best known. Most are volatile liquids, e.g. 

Volatile Inhibitors. Of the volatile components that influence plant growth and development, ethylene has received the most attention. Literature concerned with the variety of effects produced by ethylene, factors which influence its production, and the mechanisms through which responses are expressed has been reviewed by Evenari (57). Other gaseous excretions with inhibitory effects considered by Evenari include hydrogen cyanide, ammonia, essential oils, and mustard oils (probably allyl isothiocyanate and /3-phenethyI isothiocyanate). 

Nonvolatile Inhibitors. Glycosides A number of toxic constituents are known to be released by the enzymatic degradation of various glycosides. Some of the volatile components have been mentioned previously—i.e., isothiocyanates from mustard oil glycosides and hydrogen cyanide from cyanogenic glycosides. 

An example of a larval parasitoid that responds to the host sex pheromone is seen with Cotesiaplutellae (Braconidae), also a parasitoid of the diamondback moth. These insects were attracted equally to the pheromone blend (31,32,33, see above), the acetate 32, or aldehyde 31 components [80]. This larval parasitoid, however, was also strongly attracted to host frass volatiles, in particular, dipropyl disulfide 34, dimethyl disulfide 35, allyl isothiocyanate 36, and dimethyl trisulfide 37. In contrast, the egg parasitoid Trichogramma chilonis was only weakly attracted to 36. In both, T. chilonis and C. plutellae, plant volatiles, in particular (3Z)-hex-3-en-l-yl acetate 38, significantly enhanced attraction by the pheromone [80]. 

Host frass volatiles [attraction] Dipropyl disulfide 34, dimethyl disulfide 35, allyl isothiocyanate 36 dimethyl trisulfide 37 [80]... 

Exchange reactions in volatile isocyanates and isothiocyanates of silicon,... 

Scheme 7.4 Products of thioglucosidase (myrosinase) hydrolysis of glucosinolates. Volatile iso-thiocyantes and their corresponding nitriles are important flavoirr compounds, in particular in vegetables of the cabbage family. At low pH the formation of nitrile is favoured, whereas neutral or high pH favours the formation of the isothiocyanate...

The edible portion of broccoli Brassica oleracea var. italica) is the inflorescence, and it is normally eaten cooked, with the main meal. Over 40 volatile compounds have been identified from raw or cooked broccoli. The most influential aroma compounds found in broccoli are sulfides, isothiocyanates, aliphatic aldehydes, alcohols and aromatic compounds [35, 166-169]. Broccoli is mainly characterised by sulfurous aroma compounds, which are formed from gluco-sinolates and amino acid precursors (Sects. 7.2.2, 7.2.3) [170-173]. The strong off-odours produced by broccoli have mainly been associated with volatile sulfur compounds, such as methanethiol, hydrogen sulfide, dimethyl disulfide and trimethyl disulfide [169,171, 174, 175]. Other volatile compounds that also have been reported as important to broccoli aroma and odour are dimethyl sulfide, hexanal, (Z)-3-hexen-l-ol, nonanal, ethanol, methyl thiocyanate, butyl isothiocyanate, 2-methylbutyl isothiocyanate and 3-isopropyl-2-methoxypyrazine... 

The buds and the leaves (less often) of the Brussels sprout plant (Brasska olera-cea var. gemmifera) are eaten cooked with the main meal. In Brussels sprouts, breakdown products from glucosinolates are dominant and represent about 80-90% of the volatiles in headspace samples [176]. The residual volatiles are mostly sulfur compounds [176]. Compounds likely to be associated with the aroma of Brussels sprouts are 2-propenyl isothiocyanate, dimethyl sulfide, dimethyl disulfide and dimethyl trisulfide (Table 7.6) [35,176]. 

The roundish flower head, the curd, of the cauliflower plant (Brasska oleracea var. botrytis) is the edible portion of this vegetable. It can be eaten raw in salads or as a pickled condiment in vinegar. More often it is boiled and eaten with the main meal or is converted into sauces and soups. Over 80 volatile compounds have been identified in raw and cooked cauliflower. Among the compounds potentially active in cooked cauliflower, certain sulfides such as methanethiol, dimethyl sulfide and dimethyl trisulflde have often been incriminated in objectionable sulfurous aromas and overcooked off-flavours [169, 177, 178, 181-183]. Additional aldehydes have been found to be the most abundant cauliflower volatiles, with nonanal as a major component [175,177]. A recent study showed that volatiles such as 2-propenyl isothiocyanate, dimethyl trisulflde, di-... 

In another investigation, linalool (Fig. 8.1) was detected in relatively low concentration in the solvent-extracted volatiles of fresh papaya pulp from Sri Lanka [41]. The authors attributed the characteristic sweaty note of this papaya fruit mainly to methyl butanoate. Phenylacetonitrile was also found in high amounts (17.7%), which, according to the authors, combined with lesser concentrations of benzyl isothiocyanate (1.5%) can play a role in the aroma of papaya. 

Treat the solution (R i) remaining in the distilling flask after the volatile acidic and neutral compounds have been removed with 10-20 per cent sodium hydroxide solution until distinctly alkaline. If a solid separates, filter it off and identify it. Distil the alkaline solution until no more volatile bases pass over (distillate no longer turbid, or not basic to litmus water-soluble bases also possess characteristic odours) add more water to the contents of the flask if the solution becomes too concentrated during this distillation. [Set aside the solution in the distilling flask (S2) for Step 3.] If the volatile basic compounds are insoluble in water, remove them by extraction with two 20 ml portions of ether, and identify the bases (compare Sections 9.6.21, p. 1273 and 9.6.22, p. 1278) after evaporation of the ether. The water-soluble amines may be identified with phenyl isothiocyanate it is best to concentrate the bases by redistilling and collecting the first half of the distillate separately. 

Black mustard is a light olive-brown powder obtained by grinding the dried ripe seeds of several varieties of plants of the Brassica genus moisture yields from this, through enzymatic hydrolysis of a contained glycoside, are about 0.6% of the volatile oil of mustard, which has as its main ingredient the irritant compound allyl isothiocyanate. Mustard is available in bulk and as the official plaster, the latter used only for the local irritant (counterirritant) properties it develops when applied to the skin after moistening with tepid water. 

Other spices - e.g. mustard, cloves - are widely used as food additives to enhance palatability. Both are characterized by volatile components which produce two responses pungency and characteristic aroma. The simple phenol, eugenol, is the active component in cloves various isothiocyanates, allyl- in black, allyl-and 3-butenyl- in brown, characterize the mustards. The non-... 

---