Threshold odor concentration

The threshold odor concentration of amines in air is typically 10 to 40 ppb. These levels are considerable less than any regulatory limit but offer potential for triggering complaints in laundries, kitchens, and sterilizing equipment rooms. 

Aesthetic quality threshold odor concentration 1 - g/L WHO 1984a... 

Figure 17.2 shows the threshold odor as a function of pH and the concentration of chlorine dosage. Figure 17.2a uses a concentration of 0.2 mg/L and, at a pH of 9.0, the maximum threshold odor concentration is around 28 /tg/L. When the pH is reduced to 8.0 this threshold worsens to around 20 /tg/L, and when the pH is further reduced to 7.0, the threshold concentration becomes worst at around 13 /xg/L. Thus, chlorination at acidic conditions would produce very bad odors compared to chlorination at high pH values. This is very unfortunate, because HOCl predominates at the lower pH range, which is the effective range of disinfection. 

Figure 17.3 shows the reaction scheme for the breakdown of phenol to odorless low molecular weight decomposition products usiug HOCl. The threshold odor concentrations of the various chloride substituted pheuoUc compouuds are also indicated in brackets. Note that the worst offenders are 2-monochlorophenol and 2,4-dichlorophenol, which have an odor threshold of 2.0 /xg/L. In order to effect these breakdown reactions, superchlorination would be uecessary, which would also mean that the odor had increased before it disappeared. 

Threshold odor concentration Threshold taste concentration Health-based guideline... 

Odors are characterized by quaUty and intensity. Descriptive quaUties such as sour, sweet, pungent, fishy, and spicy are commonly used. Intensity is deterrnined by how much the concentration of the odoriferous substance exceeds its detection threshold (the concentration at which most people can detect an odor). Odor intensity is approximately proportional to the logarithm of the concentration. However, several factors affect the abiUty of an individual to detect an odor the sensitivity of a subject s olfactory system, the presence of other masking odors, and olfactory fatigue (ie, reduced olfactory sensitivity during continued exposure to the odorous substance). In addition, the average person s sensitivity to odor decreases with age. 

A laboratory where the measurement takes place must be free from odor and is typically air-conditioned with air filtration. The odor sample is placed in an olfactometer that basically is a device for dilution of the sample. Typically, the meter has two outlet ports diluted odorous air flows from one, and clean odor-free air flows from the other. In dynamic olfactometry, panel members assess the two ports of the olfactometer. The assessors indicate from which of the ports the diluted sample is flowing. The measurement starts with a dilution that is large enough to make the odor concentration beyond the panelists threshold. This concentration is normally increased by a factor of two in each successive presentation. Only when the correct port is chosen and the assessor is certain that the choice is correct and not just a guess, is the response considered a true value. 

In the CEN procedure, the European odor unit per cubic meter, ou m-3 is used. The odor concentration at the detection threshold is defined as equal to 1 ou m-3. 

One method of controlling response perseveration and other anticipation factors is to use a forced choice response indication based on two or more response categories. In the measurement of odors the panelist has to report the temporal position of positive stimuli in a series of random blanks. If the concentration is below the threshold, the test subjects will guess. As the odorant concentration will increase, the relative cumulative frequency for identification of the correct sample will be greater. In order to determine the relative odor recognition a correction must be made. 

The characteristic curve of the odor threshold is used. The relative cumulative frequency of positive answers is calculated for each odorant concentration and graphically plotted, while for odor concentration a logarithmic scale is used. The odor threshold can be obtained from the resulting curve as the 50-percentile and so can the associated 16- and 84-percentiles. 

Clear, colorless liquid with a rancid sweet odor similar to fusel oil. Experimentally determined detection and recognition odor threshold concentrations were 900 pg/m (300 ppbv) and 3.0 mg/m (1.0 ppmv), respectively (Heilman and Small, 1974). Odor threshold concentration in water is 500 ppb (Buttery et al, 1988). The least detectable odor threshold in concentration water at 60 °C was 0.2 mg/L (Alexander et al., 1982). Cometto-Muniz et al. (2000) reported nasal pungency threshold concentrations ranging from approximately 900 to 4,000 ppm. 

Pale amber liquid with a slight phenolic, floral, or musty-type odor. At 40 °C, the average and lowest odor concentrations detected were 0.36 and 0.088 pg/L, respectively. At 25 °C, the average taste threshold concentration and the lowest concentration at which a taste was detected were 0.97 and 0.94 pg/L, respectively (Young et al., 1996). 

The biologically significant active space where the odor concentration is above threshold is shaped like an overturned boat (Fig. 1.4). (If the molecules were able to spread in all directions, as from an elevated odor source, the active space would assume the shape of a cylinder with pointed ends.)... 

Odorant Concentration (pg/kg) Threshold (pg/kg water) Odour activity value ... 

ODOR. An important property of many substances, manifested by a physiological sensation caused by contact of their molecules with the olfactory nervous system. Odor and flavor are closely related, and both are profoundly affected by submicrogram amounts of volatile compounds. Attempts to correlate odor with chemical structure have produced no definitive results, Objective measurement techniques involving chromatography are under development. Even potent odors must be present in a concentration of 1,7 x I07 molecules/cc to be detected. It has been authentically stated that the nose is 100 times as sensitive in detection of threshold odor values as the best analytical apparatus. 

Methylamines are colorless liquids that are volatile at normal atmospheric conditions. They have threshold odor limits of less than 10 ppm, and at low concentrations they have a fishy smell. At high concentrations they smell like ammonia. The physical properties are given in Table 14.1 and Table 14.2. 

Table 13-3 Absolute odor threshold (OTa) concentrations of different chemical compounds.

The odorous air sample is diluted automatically with neutral air by use of an olfactometer. This procedure leads to the definition of an odor concentration according to Eq. 7-9 that is applied for the determination of mixing ratios of known flow rates V (dynamic dilution method). The abbreviation ou stands for odor unit (Treitinger and Meyer-Pittroff 1997). The odor concentration is inversely proportional to the dilution. Actually, odor concentrations are dimensionless (see Eq. 7-9) and they simply reflect a dilution to threshold ratio. However, it is common to express odor concentration in odor units per cubic meter ([odor]/ou m ). 

Therefore, 1 ou mT is defined to be equivalent to the odor concentration at which 50% of the test people can recognize the difference between neutral air and the odorous air sample. If a waste gas was determined to have 90 ou this waste gas must be diluted 90 times with neutral air to reach the odor threshold. In other words, the larger the value of the odor unit per the more intensive is the odor. However, an increase of odor concentration from 100 ou m to 1000 ou m increases the sensory impression of the human nose only by a factor of about two, in accord with a logarithmic dependence. 

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. 

In Figure 17.2b the concentration of chlorine has been increased to 1.0 mg/L. For the same adjustments of pH, the maximum threshold concentrations are about the same as in Figure 17.2a however, in the cases of pH s 7.0 and 8.0, the threshold odors practically vanish at approximately 3 to 5 h after contact as opposed to greater than 60 h when the dosage was only 0.2 mg/L. Thus, increasing the dosage produces the worst nightmare for odor production. 

Odor thresholds of some organic compounds in foods and beverages were reported by Guadagni et al. (78). The odor thresholds were measured in aqueous solutions. They applied this method for measuring odor value, which is expressed as the odor concentration divided by the odor threshold. 

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