Threshold values aldehydes

Strecker aldehydes, the main volatiles of Group 2, tend to have low threshold values (phenylethanal, 4 /j.g L 1 in water - thresholds will be given in these units unless stated otherwise and are taken from Rychlik et al.104 and Maarse276 3-methylbutanal, 0.35 methional, 0.2). Some Group 1 volatiles have comparable olfactory power (HDMF, 0.6 butanedione, 4), but others do not (norfuraneol, 2100 maltol, 9000 acetic acid, 22000). The threshold value of acetaldehyde (15), although a Strecker aldehyde, is only moderately low. 

The above discussion focused mainly on defining the major volatile constituents of the cilantro herbs. Due to their high abundance and overall low threshold values, the n-aldehydes and 2-alkenals are hypothesized to play predominant roles in the characteristic aromas of these herbs, especially nos. 2, 3, 8 and 11 in C sativum, and ( )-2-dodecenal (no. 8) in E. foetidum. Meanwhile, the volatile composition of P. odoratum differs markedly from C. sativum and E. foetidum in that P. odoratum does not contain ( )-2-alkenals. Instead, this herb contains mostly n-aldehydes, in particular decanal (no. 2) and dodecanal (no. 6), which are most likely the principal contributors to this herb s characteristic aroma. However, it is possible that additional minor constituents are important in the overall aromas of these herbs. In addition, it is not at all apparent by the above results which compounds actually contribute the most to the characteristic aroma of each herb. Therefore, aroma extract dilution analysis (AEDA) was applied to the volatile extracts prepared from C. sativum, E. foetidum, and P. odoratum. 

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat." The off-flavor properties of these compounds depend on the structure, concentrations, threshold values, and food systems. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. 

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat. The off-flavor properties of these compounds depend on the structure, concentration, threshold values, and the tested system. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. The peroxidation pathway from linoleic acid to various volatiles is determined in several researchs, - by using various techniques (Gas chromatography mass spectrometry, GC-MS, and electron spin resonance spectroscopy, ESR), identified the volatile aldehydes that are produced during the oxidation of sunflower oil. In both cases, hexanal was the major aldehyde product of hydroperoxide decomposition, whereas pentanal, 2-heptenal, 2-octenal, 2-nonenal, 2,4-nonadienal, and 2,4-decadienal were also identified. 

Classes of chemical compounds having different function groups and odor descriptors, some of which are useful to the flavor or perfume industries were selected for this initial study. For example, alcohols, aldehydes, pyrazines and various benzenoid compounds which have been isolated in the volatiles of cooked meat as reviewed by Hornstein (22) were studied. For each class of chemical compounds literature threshold values obtained only from one laboratory were used in order to prevent errors associated with technique or methodology between laboratories that occur for threshold determinations as discussed by Guadagni al. ) and Powers and Ware (23). 

Another area of further study is the reproducibility and accuracy of derived predictive equations. Two different data sets of aliphatic aldehyde threshold values in water were subjected to QSAR techniques to determine whether log P can be used to accurately reproduce predictive equations for odor intensity data of a compound in a given medium determined by two different laboratories. Results in Table II indicate that equations 10 and 13 have slopes, intercepts, correlation coefficients and standard deviations which are not statistically different at the 95% level of confidence. Both data sets also produced equations giving poor correlations of E and log (1/c) which were not statistically significant. 

Ketones form a minor fraction in alcoholic beverages. Ketones can be said to be potential aroma compounds. In particular, diketones such as 2,3-butanodione and 2,3-pentanodione are of great importance to the aroma of alcoholic beverages because of their low sensory threshold values. Those aldehydes with 8-10 carbon atoms, such as (E)-2-nonenal, octanal, nonanal, decanal, or (E,Z)-nonadienal, are also strong odorants, related with important off-flavors. 

The volatiles derived from oils containing hnolenic acid (soybean and canola oils) have significant sensory impact and lower threshold values than the volatiles derived from oils containing linoleic acid (cottonseed, com and sunflower oils) (Table 5.1). The most sensory-significant linolenate-derived aldehydes (with lower threshold values) were characteristic in having n-3 unsaturation. These trends explain why linolenic acid oils develop undesirable odors and flavors at much lower levels of oxidation (peroxide value of less than 1) than linoleic acid oils (peroxide value of 10). Similarly, potent volatile aldehydes have been identified in fish oil oxidized at very low levels of oxidation by static and dynamic headspace GC (see F.2) and detected by GC-MS at parts per billion levels, including cw-4-heptenal (1250 ppb), fran, cw-2,6-nonadienal (1231 ppb)andCiXcw-3,6-nonadienal(627 ppb). Cis-4-heptenal is produced by decomposition of fran, cw-2,6-nonadienal, which can be produced in turn by the decomposition of n-7 and n-9 hydroperoxides derived from the oxidation of 20 4, 20 5 and 22 6 n-3 PUFA (Chapter 4, D4). 

Not all of the potent volatile compounds are derived from lipid oxidation, including a number of lactones that come from naturally occurring hydroxy fatty acids, diacetyl and vanillin in butter oil (from melted butter). The concentrations of the mixtures of carbonyl compounds exceed the flavor threshold values for individual aldehydes, and the oxidized flavor results from a combination of volatile compounds. 

During soya processing, volatile degradation compounds (hexanal, etc.) with a bean-like aroma defect are formed because of the enzymatic oxidation of unsaturated fatty acids. These defects can be eliminated by the enzymatic oxidation of the resultant aldehydes to carboxylic acids. Since the flavor threshold values of these acids are high, the acids generated do not interfere with the aroma improvement process. 

The rapid deterioration of food containing linolenic acid should not be ascribed solely to the preferential oxidation of this acid but also to the low odor threshold values of the carbonyl compounds formed, such as (Z)-3-hexenal, (E,Z)-2,6-nonadienal and (Z)-l,5-octadien-3-one (Table 3.32). Aldehydes with exceptionally strong aromas can be released in food by the autoxidation of some fatty acids, even if they are present in low amounts. An example is octadeca-(Z,Z)-11, 15-dienoic acid (the precursor for... 

The odor threshold values for methyl ketones are substantially higher than those for aldehydes (cf. Tables 3.32 and 3.47). Nevertheless, they act as aroma constituents, particularly in flavors of mold-ripened cheese (cf. 10.2.8.3). However, methyl ketones in coconut or palm oil or in milk fat provide an undesirable, unpleasant odor denoted as perfume rancidity . 

Amino acid precursor Srecfer-aldehyde Odor threshold value... 

The Amadori compounds as such do not contribute to sensory changes however, they generally can be used as chemical markers for an early detection of the Maillard reaction in foods (5, 6, 9). By decomposition of Amadori compounds melanoidins and volatile Strecker aldehydes having a very low sensory threshold value are formed. The ratio between the concentration of Amadori compounds and the intensity of sensory changes depends on the reaction conditions. The CB- and HB-tomato flakes under investigation show only a slight burnt and adstringent off-flavor. 

Table III shows that the concentrations of all listed volatile compounds are far above the sensory threshold values (determined in water) thus strongly contributing to flav(M they may contribute to off-flavor, if th exceed certain concentrations. Therefore the Strecker aldehydes and dimethyl sulfide listed in Table III can be used as very sensitive marker substances for sensory changes during heat processing of tomatoes, espedally 2- and 3-methylbutanal, since their concentrations are more than hundredfold higher in tomato flakes than in tomato paste.

The characteristic compound for raw carrot is 2-xec-butyl-3-methoxypyra-zine, which has an extremely low (2 ppt) threshold value. Its sniffing port aroma in gas chromatography-olfactometry has been described as raw carroty (34). Unsaturated aldehydes contribute to the flavor of cooked carrot, the most significant being ( )-2-nonenal ( fatty-waxy ) (34). 

The volatiles of fresh pineapple (Ananas comosus [L] Merr.) crown, pulp and intact fmit were studied by capillary gas chromatography and capillary gas chromatography-mass spectrometry. The fnjit was sampled using dynamic headspace sampling and vacuum steam distillation-extraction. Analyses showed that the crown contains Cg aldehydes and alcohols while the pulp and intact fruit are characterized by a diverse assortment of esters, h rocarbons, alcohols and carbonyl compounds. Odor unit values, calculated from odor threshold and concentration data, indicate that the following compounds are important contributors to fresh pineapple aroma 2,5-dimethyl-4-hydroxy-3(2H)-furanone, methyl 2-methybutanoate, ethyl 2-methylbutanoate, ethyl acetate, ethyl hexanoate, ethyl butanoate, ethyl 2-methylpropanoate, methyl hexanoate and methyl butanoate. 

Environmental criteria have been established for many of these, but the utility and applicability of such criteria for indoor environments is controversial for at least four reasons. Eor example, the goals of the threshold limit values often do not include preventing irritation, a primary concern in indoor environments with requirements for close eye work at video display terminals. For most of the pollutant categories, the problem of interactions, commonly termed the multiple contaminants problem , remains inadequately defined. Even for agents that are thought to affect the same receptor, such as aldehydes, alcohols, and ketones, no prediction models are well established. Finally, the definition of representative compounds for measurement is unclear. That is, pollutants must be measurable, but complex mixtures vary in their composition. It is unclear whether the chronic residual odor annoyance from environmental tobacco smoke correlates better with nicotine, particulates, carbon monoxide, or other pollutants. The measure total volatile organic compounds is meanwhile... 

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