Sensory Threshold Values

One of the principal properties of an organic compound which is to be used in flavorings is its sensory threshold value(s). The American Society for Testing and Materials (ASTM) has provided the following definitions  

Detection threshold Minimum physical intensity detectable where the subject is not required to identify the stimulus 

Difference threshold Smallest change in concentration of a substance required to give a perceptible change 

FIGURE 10.4 Isomers and sterioisomers. (From Macleod, A.J., Proc. Symp. Zool. Soc., 45, p. 15, 1980. With permission.) 

Carboxylic acids -ic acid or -oic add -COOH R-COOH C Hs-COOH 

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Literature information about the sensory properties for nearly 450 Maillard reaction products has been compiled in a survey. It includes qualitative aroma and flavor descriptions as well as sensory threshold values in different media for the compounds, classified according to their chemical structure. 

The sensory properties of nearly 450 volatile Maillard reaction products and related compounds have been compiled (45). The review includes quantitative aroma and flavor descriptions, as veil as sensory threshold values for different media, classified according to chemical structure. 

It should be pointed out, that we were concerned with presence or absence of bitterness. Bitterness in terms of sensory threshold values or bitterness ratings was not assessed. 

Bitterness in terms of sensory threshold values or bitterness ratings was not assessed. 

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. 

Sensory threshold values for sorbic acid in wine range from as low as 135 mg/L (Ough and Ingraham, 1960) upward to levels that exceed legal maxima 300-400 mg/L (Tromp and Agenbach, 1981). Because the com-... 

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 efficacy of IDA will be demonstrated using the example of ethyl-2-methylbu-tyrate, a trace component of apple flavor—one which because of its low sensory threshold value, however, can persistently affect the flavor. 

A persistent idea is that there is a very small number of flavor quaUties or characteristics, called primaries, each detected by a different kind of receptor site in the sensory organ. It is thought that each of these primary sites can be excited independently but that some chemicals can react with more than one site producing the perception of several flavor quaUties simultaneously (12). Sweet, sour, salty, bitter, and umami quaUties are generally accepted as five of the primaries for taste sucrose, hydrochloric acid, sodium chloride, quinine, and glutamate, respectively, are compounds that have these primary tastes. Sucrose is only sweet, quinine is only bitter, etc saccharin, however, is slightly bitter as well as sweet and its Stevens law exponent is 0.8, between that for purely sweet (1.5) and purely bitter (0.6) compounds (34). There is evidence that all compounds with the same primary taste characteristic have the same psychophysical exponent even though they may have different threshold values (24). The flavor of a complex food can be described as a combination of a smaller number of flavor primaries, each with an associated intensity. A flavor may be described as a vector in which the primaries make up the coordinates of the flavor space. 

The following review focuses on the composition of flavour compounds in spirit drinks, their origin, and their sensory attributes like odour quality and threshold value. Important information on flavour-related aspects of technology, like distillation and ageing, as well as the main categories and brands of spirits to be found on the national and international markets are summarised. Finally, aspects of sustainability in the production of distilled spirits are discussed. 

Table 10.1 gives a summary of the main by-products of fermentation by yeasts and other microbiological activities which can be identified in distilled spirits from different raw materials, like fruits, wine, grain, sugar cane, or other carbohydrate-containing plants. Since the sensory relevance of a flavour compound is related to its odour thresholds and odour quality. Table 10.1 presents also odour qualities and a review of threshold values of the fermentation by-products in ethanol solutions [9-10] and/or water [11-14] (Christoph and Bauer-Christoph 2006, unpublished results). 

The influence of the sensitivity of the assessors on AEDA has been studied [11], with the result that the differences in the FD factors determined by a group of six panellists amount to not more than two dilution steps (e.g. 64 and 256), implying that the key odorants in a given extract will undoubtedly be detected. However, to avoid falsification of the result by anosmia, AEDA of a sample should be independently performed by at least two assessors. As detailed in [6], odour threshold values of odorants can be determined by AEDA using a sensory internal standard, e.g. ( )-2-decenal. However, as shown in Table 16.6 these odour threshold values may vary by several orders of magnitude [8] owing to different properties of the stationary phases. Consequently, such effects will also influence the results of dilution experiments. Indeed, different FD factors were determined for 2-methyl-3-furanthiol on the stationary phases SE-54 and FFAP 2 and 2 , respectively. In contrast, 5-ethyl-3-hydroxy-4-methyl-2(5H)-furanone showed higher FD factors on FFAP than on SE-54 2 and 2, respectively. Consequently, FD factors should be determined on suitable GC capillaries [8]. However, the best method to overcome the limitations of GC-O and the dilution experiment is a sensory study of aroma models (Sect. 16.6.3). 

Reliable quantitative data are a prerequisite for evaluating the contribution of a single odorant to a positive aroma or off-flavor. They are needed to calculate odor activity values (OAV) that are defined as the ratio of the concentration to the sensory threshold of a given compound in a matrix (Rothe and Thomas, 1963 Acree et al., 1984). These values give guidance in the evaluation of the impact of an odorant to the overall aroma profile. 

If qualitative data are required, and the enantiomers differ significantly in their odorqual-ity, then acceptable results may be obtained. If odor intensity measurements (OAVs or OSVs) or threshold values are required, then the conditions described above must be obtained if the data is to be of value. Bernreuther et al. (1997) and Koppenhoefer et al. (1994) have published the enantiospecific sensory data for a variety of chiral odorants. 

During AEDA, interactions between the odorants are not taken into consideration, since every odorant is evaluated individually. Therefore, it may be possible that odorants are recognized which are possibly masked in the food flavor by more potent odorants. Furthermore, the odor activity values only partially reflect the situation in the food, since OAVs are mostly calculated on the basis of odor thresholds of single odorants in pure solvents. However, in the food system, the threshold values may be influenced by nonvolatile components such as lipids, sugars or proteins. The following examples will indicate that systematic sensory model studies are important further steps in evaluating the contribution of single odorants to the overall food aroma. 

Sensory thresholds (in beer) and odor descriptors. a 5-Hydroxymethyl-2-furfuraI. h 5-Ethoxymethyl 2-furfural c Minimum. d Maximum. e Mean value. 

Detailed investigations of structure and taste require quantitative data on the sensory side. The taste recognition threshold value is especially well suited for this purpose as, according to BEIDLER (8), it is connected with the association constant of the stimulus-receptor-complex. 

From experience it has been established that the sensory threshold for coffee creamer and condensed milk products is on the order of 0.1 mg/kg (ppm) of styrene in the product. This observation is only partly supported by threshold values from the literature in Table 14-2 where values range from 0.2 ppm for 3 % yogurt, 1.2 ppm for 3.8 % fat milk and 2-5 ppm for condensed milk. This points out two problems with threshold concentration values caused by the way they are determined (e.g. experimental methods) and the definition of the threshold value being the value at which the substance is correctly identified by 50 % of the panelists (versus other possible ways of measur-ing/defining the taste threshold). 

Interpretation of result The calculated migration values here are realistic since results calculated using Eq. (14-4) cannot be larger than the mass balance result (Example 14-1). The calculated amount of styrene is still above the assumed sensory threshold limit of 0.1 mg/kg in the product for the worst case in step 4 but is equal to the estimation using the experimental diffusion coefficient in step 5. 

To estimate the sensory contribution of the 42 odorants to the overall flavor of the wine samples, their OAV s were calculated (Table II). To take into account the influence of ethanol, the odor threshold values of wine odorants were determined in a mixture of water/ethanol (9+1, w/w) and were used to calculate the OAV s for each compound. According to the results in Table A, 4-mercapto-4-methylpentan-2-one, ethyl octanoate, ethyl hexanoate, 3-methylbutyl acetate, ethyl isobutyrate, (E)-fi-damascenone, linalool, cis rose oxide and wine lactone showed the highest OAV s in the Scheurebe wine. With exception of 4-mercapto-4-methylpentan-2-one the above mentioned odorants also showed the highest OAV s in Gewurztraminer wine. Differences in the OAV s of ethyl octanoate, ethyl hexanoate, 3-methylbutyl acetate and ethyl isobutyrate between the two varieties are probably caused by differences in the maturity of the fruit at harvest and/or by the fermentation process. 

The era of publishing a large number of compounds identified as to chemical structures is slowly changing to an era in which constituents are identified as to which are the Important contributors to the characteristic odors. More and more sensory analyses are stating odor threshold values as well as odor quality. 

The flow of some materials may not commence until a threshold value of stress, the yield stress (ao) (see Figures 1-2 and 1-3), is exceeded. Although the concept of yield stress was questioned recently (Bames and Walters, 1985), within the time scales of most food processes the concept of yield stress is useful in food process design, sensory assessment, and modeling. Shear-thinning with yield stress behavior... 

Carbonyl compounds in oxidized fats and oils are the secondary oxidation products that originate from decomposition of hydroperoxides. They usually have low threshold values and hence are responsible for off-flavor development in oxidized oils. Therefore, content of carbonyl compounds corresponds with sensory data. 

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