Odour units

Van Harreveld measured the odour emission of a composting plant. As it was a composting plant with a surface of 4 ha it was impossible to measure the flowrate. He solved this problem in measuring the ratio mg NH3/m3 odour units/m3 in the air leewards of the plant. 

As it was known from other experiments that the NH3 losses per ton of compost produced were 2, 1 kg, he could calculate the emission in odour unit/s from this composting plant. 

Condensation can be avoided by the predilution of the sample by diy, odour-free air. It is important to know in which ratio the sample is diluted for odour unit or odour threshold determination. 

In the following some aspects of the guidelines are given in detail. Guideline VDI 3881 consists of four parts. The drafts of parts 1, 2, and 3 were published in the VDI handbook. Part 4 is in preparation. The draft of part 1 was already revised. The new version will be published in a few months. The most important result of the revision is the definition of odour concentration expressed as odour units per cubic meter (GE/m 3). According to this definition one odour unit is the amount of odorants in one cubic meter of air at odour threshold level. The new definition is a real concentration and gives a better form of input parameter for dispersion models. 

They should also make it possible to use forced choice methods by providing at least two sniffing ports which each may carry the odourous stimulus or pure air. Panels used to obtain valid odour threshold data should at least have 20 members. For practical work using relative measurements only panels of at least 6 trained subjects are recommended. Finally, it is pointed out that odour units/m3 are no direct indication of the perceived intensity of an odour. 

Since it is the object of olfactometry to give an indication of the perceived intensity of the odours in the environment, a general warning should be given as to the use of the concept of odour units/m. Even if the number of odour units/m is determined correctly, it does not give a direct indication of the perceived intensity as was already pointed out by Frijters (4). The slope of the curve which relates perceived odour intensity to the odour concentration may vary considerably from odour to odour. A schematic example is given in figure 2 for two substances A and B. 

According to Stevens law the logarithm of the perceived intensity is linearly related to the logarithm of the odour intensity. In the figure this relationship is given for two substances, one with a slope of 1.00 and one with a slope of. 67. As can be seen from the figure, this means that an odour concentration of 100 odour units/m3 is related to very different perceived odour intensities for the two substances. This means that odour concentrations computed in odour units/m3 should not be used as an indication of perceived odour intensity, but can only be used in relative measurements where the effects of measures taken to reduce odour pollution are compared, or in studies where dispersion models are used to find the distance to the source at which threshold is reached. 

As part of a study to test the use of population panels as a method for assessing odour annoyance in a direct way, a comparison of the odour annoyance experienced by such population panels and the odour immission concentrations expressed in odour units/m3 of air samples taken simultaneously, was made. It was found that in a city where odour annoyance occurs regularly, no relationship could be found between the amount of odour annoyance experienced and the immission concentrations. Furthermore, it was shown that the odour concentrations of pleasant smells (meadows forest) in an unpolluted area may be as high as 24 odour units. It is concluded that the amount of annoyance caused by odours can not be deduced from concentration measurements, but should be assessed in a direct way. Population panels provide a good means of obtaining such data. They are reliable and can give indications about the important sources of annoying odours in complex industrial areas. 

As can be seen from this table, odour concentrations of 20 odour units or more are quite common in a rural area without industry. Forest odour, the odour of a meadow or ordinary street odours have already to be diluted twenty times before they become indiscriminable from pure air in the laboratory. 

Concerning the rel. odour concentration in the cleaned air, a large difference is evident between the presented results and the assertion that a limit value of 100 odour units can be achieved. Two interpretations can be offered ... 

Odour concentration expressed in odour units per cubic meter (O.U.m-3) is the number of dilutions to the detection threshold. (The odour concentration of the undiluted sample being at threshold level is 1 O.U.nT3)... 

Description Colourless to pale yellow liquid with pungent odour (United States National Library of Medicine, 1997)... 

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. 

In the classical methods the threshold level is defined as the level at which 50% of a given population will detect the odour. One of these methods is the threshold method, which is standardised in many countries for the evaluation of outdoor air [23]. In this threshold method an air sample is diluted stepwise (for each step, by a factor of 2) with clean (odour-free) air to determine the dilution at which 50% of a panel of eight persons can no longer distinguish the diluted air from odour-free air. This number of dilutions, expressed in odour units per cubic metre of air of 20 °C, is the numerical value for the odour concentration of the original air sample. Some measurements using the classical threshold level method, have been made on indoor air, ventilation systems and building materials [22,24]. 

Frijters J.E.R. (1979) Some psychophysical notes on the use of the odour unit number. In Progress in Flavour Research (2nd Weurman Flav. Res. Symp., Norwich, UK, 2-6.4.1978) Land D.G. and Nursten H.E., Eds, Applied Science Publishers pp. 47-51. 

Peak no. (a,b) Fuggle Hallertau Idaho Cluster Brewer s Gold Brewer s Gold Threshold cone. (PPb) Odour units (X 10- ) % Total odour units... 

The results of a more detailed survey of the odour intensities of individual components of hop oil [54] are incorporated into Tables 13.3 and 13.4. By means of a panel the threshold concentration (T ) in parts per 10 (ppb) at which a component in distilled water could be detected by smell was measured. This was arbitrarily defined as one odour unit. If the fraction or component concentration (Z. ) in the whole oil is also expressed in ppb then the ratio FJT = the number of odour units attributable to any fraction or component. If the sum of the odour units from individual fractions equals that for the whole oil, the relative contributions by each fraction may be estimated. As shown in Table 13.8 the sum of the odour units found in the hydrocarbon and oxygenated fractions of a sample of oil from Brewer s Gold was equal to that of the whole oil. 

The threshold values of twenty-five of the components found in the sample of oil from Brewer s Gold were estimated and are included in Tables 13.3 and 13.4. In this sample, the hydrocarbons accounted for 87 % of the oil but only 61% of the odour units. Myrcene, which comprised 63% of the oil, was responsible for 58 % of the odour units. The twenty oxygenated components studied accounted for 6-6% of the oil and provided 14% of the odour units... 

The power factor n is assumed to be 1 00 and the constant is often omitted so that R is measured as SjT. This ratio has been given various names including odour units (see p. 451) and flavour units [15]. The power function n in the equation ... 

Frijters, J.E.R. A critical analysis of the odour unit number and its use. Chem. Senses Flavour 3, 227 (1978)... 

The ability to perceive the odour of a particular substance also depends on its odour threshold (aroma, fiavour) value, which is the concentration of a substance detectable by the sense of smeU. The odour detection threshold value is the lowest concentration of a stimulus (odoriferous substance), which can be detected in comparison with an environment that does not contain this substance. The odour recognition threshold value is the concentration at which a substance can not only be detected, but also recognised. It corresponds to a concentration that allows identification of the odour quality of a substance, which is usually higher than the odour detection threshold. Both values are measures of the odour intensity, but depend considerably on the environment, solubility, partition coefficients between air and water (oil) and some other factors. For example, values measured in air are typically several orders of magnitude lower than the values measured above aqueous solutions. The substance with a high odour threshold value must be present in foods in higher concentrations than substance with a low odour threshold value, otherwise its smell is imperceptible. The measure of whether the substance acts as an odour-active substance is its odour unit, its actual concentration divided by odour threshold concentration. 

F. Drawer and N. Christoph, Significance of the sniffing-technique for the determination of odor thresholds and detection of aroma impacts of trace volatiles. Analysis of Volatiles Methods and Apphcations (P. Schreier, ed.), De Gruyter, Berlin, 1984. J. E. R. Frijters, Some psychophysical notes on the use of the odour unit number. Progress in Flavour Research (D. G. Land and H. E. Nursten, eds.). Applied Science, London, 1979. 

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