Compounds odors, discussion

Abstract A problem-solving approach to retrosynthesis is introduced. Basic principles for good disconnections are postulated. Examples of interconversion and disconnection of carbinols, alkenes, ketones and nitro compounds are discussed. Concepts of retro-Diels-Alder and re/ro-Wittig disconnections are presented and the mechanisms of reactions explained. Application of the Wittig reaction on the industrial scale is exemphfied by the synthesis of the analog of bombykol, the principal of pear odor and anti-appelizer chlorphentermine. 

Enantiomers of a chiral compound show many different physiological responses, including those of odor and taste2 and it has long been known that enantiomers of some sulfur-containing compounds may have different odors. The examples discussed here are for sulfur-containing compounds where the chirality is based on carbon. While certain compounds can show sulfur-based chirality, there are apparently no known cases where enantiomers dependent on sulfur chirality exhibit different odors. 

The joint session of both groups discussed the use of instrumental methods for evaluating odours, it was agreed that these methods could have advantages in terms of cost and labour input. In some cases identification of the odorous compounds was necessary to ensure that the correct treatment system is used. Cross reference to olfactometric measurements is necessary to ensure a good correlationship between the instrumental results, offensiveness ratings and odour concentration by panels. 

Pheromone propagation by wind depends on the release rate of the pheromone (or any other odor) and air movements (turbulent dispersion). In wind, the turbulent diffusivity overwhelms the diffusion properties of a volatile compound or mixture itself. Diffusion properties are now properties of wind structure and boundary surfaces, and preferably termed dispersion coefficients. Two models have dominated the discussion of insect pheromone propagation. These are the time-average model (Sutton, 1953) and the Gaussian plume model. 

Removal of the objectionable odors due to the presence of H2S and mercaptans is the objective of the fuel sweetening process. Several methods can be utilized to remove these undesirable compounds including caustic washing, copper chloride sweetening, sulfuric acid treating, Merox processing, and hydrotreating. These methods will be discussed below. 

Alkyl-2-(2-furyl)-l-vinylpyrroles are high boiling liquids with a slight characteristic odor, soluble in diethyl ether, acetone, ethanole, chloroform, DMSO, and insoluble in water. Typical IR and UV spectra of these compounds are presented in Table XVIII. The H-NMR spectra have been discussed in detail (78ZOR2628 79IZV2372). 

This important flavor compound was identified in the head-space volatiles of beef broth by Brinkman, et al. (43) and although it has the odor of fresh onions, it is believed to contribute to the flavor of meat. This compound can be formed quite easily from Strecker degradation products. Schutte and Koenders (49) concluded that the most probable precursors for its formation were etha-nal, methanethiol and hydrogen sulfide. As shown in Figure 5, these immediate precursors are generated from alanine, methionine and cysteine in the presence of a Strecker degradation dicarbonyl compound such as pyruvaldehyde. These same precursors could also interact under similar conditions to give dimethyl disulfide and 3,5-dimethyl-l,2,4-trithiolane previously discussed. 

The chlorinated propenes are obnoxious compounds. Unlike other compounds discussed so far in this section, their pungent odors and irritating effects lead to an avoidance response in exposed subjects. They are irritants to the eyes, skin, and respiratory tract. Contact with the skin can result in rashes, blisters, and bums. Chronic exposure to allyl chloride is manifested by aching muscles and bones it damages the liver, lungs, and kidney and causes pulmonary edema. 

The composition of the volatile fraction of bread depends on the bread ingredients, the conditions of dough fermentation and the baking process. This fraction contributes significantly to the desirable flavors of the crust and the crumb. For this reason, the volatile fraction of different bread types has been studied by several authors. Within the more than 280 compounds that have been identified in the volatile fraction of wheat bread, only a relative small number are responsible for the different notes in the aroma profiles of the crust and the crumb. These compounds can be considered as character impact compounds. Approaches to find out the relevant aroma compounds in bread flavors using model systems and the odor unit concept are emphasized in this review. A new technique denominated "aroma extract dilution analysis" was developed based on the odor unit concept and GC-effluent sniffing. It allows the assessment of the relative importance of the aroma compounds of an extract. The application of this technique to extracts of the crust of both wheat and rye breads and to the crumb of wheat bread is discussed. 

GC-effluent sniffing of wheat bread aroma concentrates has shown the presence of low level volatiles that smell like the fresh bread crust. As discussed in the preceeding sections, these compounds (3-7 in Figure 1) were proposed to be responsible for this odor note. 

Crust volatiles were isolated immediately after baking by extraction with dichloromethane and sublimation in vacuo ( ). Application of aroma extract dilution analysis 6) to the acid-free crust extract led to the detection of 31 odorants. After separation and enrichment, these compounds were identified by comparison of the MS/EI, MS/Cl and retention data on two columns of different polarity to reference compounds. Aroma quality was also assessed. The results of the identification experiments (Table I) revealed that 2(E)-none-nal (No. 1), followed by 2(E),4(E)-decadienal (No. 2) and 3-methyl-butanal (No. 3) showed the highest FD-factors in the crust of the chemically leavened bread. Additionally l-octen-3-one, 2(Z)-nonenal, 2(E),4(E)-nonadienal and an unknown compound with a metallic odor contributed high FD-factors to the overall flavor (For a discussion of FD-factors, see Chapter by Schieberle and Grosch, this book). 

Hodge et al. (45) discussed mechanisms for formation of methyl furanones and related substances from Amadori compounds. They have been produced by heating D-ribose and D-ribose phosphate with ammonia (46 47). Hicks and Feather (48) demonstrated that the Amadori compound 1-benzylamino-l-deoxy-D-threo-pentulose dehydrates to 4-hy-droxy-5-methyl-3(2H)-furanone and it has also been identified as a degradation product of L-ascorbic acid. This compound is believed to be formed from ribose-5-phosphate, and gained prominence when it was isolated from beef by Tonsbeck et al. (49). It became more apparent as a precursor of meat flavor when Van den Ouweland and Peer (50) reacted it and its thio analog with HaS to produce a number of sulfur compounds, some of which had meaty odors. 

The precursors used for process meat flavors are reviewed and also discussed will be non-Maillard interactions of ribose-5-phosphate and lipid degradation products with sulfur giving a real meaty odor and meat specie specific odor compounds, respectively. 

In the previous discussion of the limit value concentration, the influence of solubility of the odor compound in the packaging material on the limit value has been ignored. When one takes into consideration the KP/p value than in equilibrium one gets instead of Eq. (13-4) ... 

At this point in the chapter it is convenient to introduce a discussion about the role that the different wine odorants can play in the aroma of wine. This discussion is based on the experience gained in the last few years, which have shown that there are different and clearly identifiable roles in the way in which aroma compounds contribute to the formation of the different aroma nuances of wine. 

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