Aroma value

As already indicated, compounds with high aroma values may contribute to the aroma of foods. The aroma value A of a compound is calculated according to the definition  

The examples presented in Fig. 5.1 show that the exponent n and, therefore, the dependency of the odor intensity on the concentration can vary substantially. Within a class of compounds, the range of variations is not very large, e. g., n = 0.50—0.63 for the alkanals C4-C9. 

In addition, additive effects that are difficult to assess must also be considered. Examinations of mixtures have provided preliminary information. They show that although the intensities of compounds with a similar aroma note add up, the intensity of the mixture is usually lower than the sum of the individual intensities (cf. 3.2.1.1). For substances which clearly differ in their aroma note, however, the odor profile of a mixture is composed of the odor profiles of the components added together, only when the odor intensities are approximately equal. If the concentration ratio is such that the odor intensity of one component predominates, this component then largely or completely determines the odor profile. 

Examples are (E)-2-hexenal and (E)-2-decenal which have clearly different odor profiles (cf. Fig. 5.2 a and 5.2 f). If the ratio of the odor intensities is approximately one, the odor notes of both aldehydes can be recognized in the odor profile of the mixture (Fig. 5.2 d). But if the dominating odor intensity is that of the decenal (Fig. 5.2 b), or of the hexenal (Fig. 5.2 e), that particular note determines the odor profile of the mixture. 

The evaluation of volatile compounds on the basis of the aroma value provides only a rough pattern at first. The dependence of the odor intensity on the concentration must also be taken into account. In accordance with the universally valid law of Stevens for physiological stimuli, it is formulated as follows  

---

For estimating the contribution of volatile compounds to bread aroma Rothe and coworkers (S) defined "aroma value" as the ratio of the concentration of some volatile compounds to the taste threshold value of the aroma. This concept was further developed by Weurman and coworkers (9) by introducing "odor value", in which aroma solutions were replaced by synthetic mixtures of volatile compounds in water. These mixtures showed the complexity of the volatile fractions of wheat bread, because none of them resembled the aroma of bread. Recently two variations of GC-sniffing were presented (10-11), in which the aroma extract is stepwise diluted with a solvent until no odor is perceived for each volatile compound separately in the GC effluent. The dilution factors obtained indicate the potency of a compound as a contributor to the total aroma. 

Aroma value, definition, 193 Ascoibic acid, natural antioxidant, 58 Ascoibyl palmitate, natural antioxidant, 58... 

One way to quantify the odor impact of a compound is to determine the aroma value or odor activity value (OAV). This is calculated by dividing the concentration of the compound by its perception threshold. Therefore, the odor impact of a compound increases in proportion to its OAV when this value is >1. Thus, compounds exhibiting higher OAV values are more likely to contribute to the aroma of wine and have an important influence on its sensory characteristics. 

A more general approach to estimate the importance of a flavor compound in a particular food is the calculation of the ratio of its concentration to its flavor (odor and taste) threshold (9) or to its odor threshold (1 , 11) The result is denoted "aroma value" 19), "odor unit" (10) or "odor value" (11) the higher the value or unit, the more intensely this component contributes to the flavor or odor of the food. 

Rothe (5, , 1 5) calculated the aroma values of some volatiles identified in the crumb of wheat bread and the crust of rye bread. The data listed in Table I indicate that ethanol, isobutanal, iso-pentanal, diacetyl and isopentanol contribute with high aroma values to the aroma of the wheat bread crumb. During baking of rye bread, the two Strecker aldehydes, isobutanal and isopentanal, increased so much in the crust that they showed the highest aroma values of the volatiles investigated. 

Table I. Aroma Values of Some Volatiles Identified in the Crumb of Wheat Bread and in the Crust of Rye Bread 15, 9, 15)...

On applying the concept of odor units with our current knowledge we can designate 15 to 20 compounds with high aroma values which only amount to 10 - 20 ppm. 

Gasser, U., Grosch, W. (1988). Identification of volatile flavour compounds with hight aroma values from cooked beef. Z Lebensm. Unters. Forsch., 186, 489-494. 

Among all the volatile compounds, only a limited number are important for aroma. According to a proposal by Rothe and Thomas [2] only those compounds actually contribute to aroma whose concentration in food exceeds their odour thresholds. To estimate the importance of a volatile compound for the aroma of a particular food, the ratio of concentration to its odour threshold was calculated. This value was denoted aroma value [2], odour unit [3] or odour activity value (OAV) [4], In the following the latter term is used. 

The volatile components of both wasabi and horseradish formed by the hydrolysis of thioglucosides with myrosinase are known to possess a strong pungency. The difference between wasabi and horseradish is a green odor. The ratio of the concentration to the odor threshold, C/T, is called the aroma value. Sensory response is logarithmically proportional to the amount of stimulus. In this study, the log of the aroma values, log(C/T), for 14 isothiocyanates have been used instead of their conventional aroma values. co-Alkenyl isothiocyanates, which possessed higher log(C/T) values than those of horseradish, contributed to the green odor of wasabi. Furthermore, the yields and the values of log(C/T) of the isothiocyanates catalyzed by myrosinase were affected by pH and temperature. 

The concentration divided by the odor threshold value, C/T is called the aroma value (22). Odorants with high aroma values are important contributors to the characteristic flavors. In general, the estimated feel of the stimulus is logarithmically proportional to the objectively measured strength of the stimulus... 

The yields and the log of the aroma values of the isothiocyanates hydrolyzed by wasabi myrosinase are significantly affected by pH and temperature. 

ODOR ACTIVITY VALUE (OAV) OR ODOR UNIT OR AROMA VALUE OR FLAVOR UNIT... 

Rothe M. (1976) Aroma values. A useful concept Nahrung 20, 259-66. 

Cherry flavor Seven compounds with very high aroma values have been found in the juice of sour cherries benzaldehyde, linalool, hexanal (see alkanals), ( )-2- hexenal, phenylacetaldehyde, ( ,Z)-2,6- nonadienal, and eugenol. Benzaldehyde is reported to be the impact compound of cherry flavor. 

According to their relative threshold values, volatile compounds are also referred to as aroma active . The aroma value (also referred to as odor unit ) of a compound x is defined as ... 

Table 11.18. Volatile flavor compounds with high aroma values from cooked beef ...

Compound Cone. (%) Description Odor threshold value (/ig/kg)" Aroma value FD- factor ... 

From Belitz and Grosch (1986). Aroma value = concentration/odor threshold concentration 2-Et-3,5-diMepyrazine was listed as 2-Et-3,6-diMepyrazine. 

The nor-isoprenoids most frequently found in flavors, a- and P-ionone (244) and (245), have a particularly high aroma value due to their characteristic odor qualities and their low threshold concentration (Table 3). Though both ketones had, for a long time, been... 

Rothe, M. Aroma Values — a Useful Concept Proc. int. Symp. Aroma Research, Zeist, p. 111. Wageningen Pudoc. 1975. 

Gasser, U. and W. Grosch, Identification of volatile flavor compounds with high aroma values from cooked beef, Zeitschrift Lebens. Untersuch. Forsch., 186, 6, p. 489,1988. Gasser, U., W. Grosch, Primary odorants of chicken broth a comparative study with meat broths from cow and ox, Zeitschrift Lebens. Untersuch. Forsch, 190, 3, p. 3, 1990. 

II, odor threshold of the compound in an aqueous HD3F solution having a concentration (6.75 mg/1, aroma value A = 115) as high as in a coffee drink. 

---