Perception thresholds

Diacetyl, acetoin, and diketones form during fermentation. Diacetyl has a pronounced effect on flavor, with a threshold of perception of 0.1—0.2 ppm at 0.45 ppm it produces a cheesy flavor. U.S. lager beer has a very mild flavor and generally has lower concentrations of diacetyl than ale. Diacetyl probably forms from the decarboxylation of a-ethyl acetolactate to acetoin and consequent oxidation of acetoin to diacetyl. The yeast enzyme diacetyl reductase can kreversibly reduce diacetyl to acetoin. Aldehyde concentrations are usually 10—20 ppm. Thek effects on flavor must be minor, since the perception threshold is about 25 ppm. 

Organic acids, including carbon dioxide, lower the wort pH during fermentation. The principal acids formed are lactic, pymvic citric, malic, and acetic acids, at concentrations ranging from 100—200 ppm. The main sulfur compounds formed during fermentation and thek perception thresholds are as follows H2S (5—10 ppb) ethanethiol (5—10 ppb) dimethyl sulfoxide (35—60 ppb) and diethyl sulfide (3—30 ppb). At low levels, these may have a deskable flavor effect at higher levels they are extremely undeskable. Sulfur dioxide also forms during fermentation, at concentrations of 5—50 ppm its presence can be tasted at levels above 50 ppm. 

The results clearly indicated that the ventilation noise was perceived as most acceptable when the tone was situated in the low er part of the frequency range. The experience of disturbance and the associated effects occur at exposure levels above the auditory perception threshold. Above this level, the risk of these effects increases as the perceived loudness increases, provided that the other conditions remain constant. Since the loudness can be predicted relatively accurately by means of technical measurements, any differences in the degree of disturbance can also be predicted by reference to these measurements, provided that they are dependent on differences in the loudness. 

The odor perception threshold for benzene in water is 2 ing/L. The benzene drinking water unit risk is 8.3 x lO L/pg. Calculate the potential benzene intake rate (mg benzene/kg-d) and the cumulative cancer risk from drinking water with benzene concentrations at half of its odor threshold for a 30 year exposure duration. 

This article contains a detailed summary of the experiments carried out by LCA Coates Lorilleux aimed at evaluating the sensorial perception thresholds of ethyl acetate (and other solvents) used in rotogravure printing of flexible packaging for food. The methodological approach applied in the present study can be deemed a valid tool of study for the organoleptic behaviour of solvents and/or volatile compounds other than ethyl acetate that may be present inflexible materials. 8 refs. 

For each of the identified osmogenes the relationship between the compounds actual abundance in the mixture and the concentration that represents the odour perception threshold must be evaluated. This is essential to give some indication of the osmogenes perceived intensity and assist in pinpointing which of the odour-intensive species are most significant. 

Similar to exogenous cannabinoids, anandamide has analgesic effects (Fride and Mechoulam 1993). Peripheral anandamide (10 mg/kg IM) produces antinociception in primates. However, centrally administered anandamide does not alter pain perceptive thresholds (Murillo-... 

Now we introduce the sensitivity (perception threshold) of the receiving animal. According to Bossert and Wilson (1963), the downwind maximum distance Xmax (in cm) at which the concentration of an odor remains above threshold is... 

It has been suggested that the disulfides may subsequently be reduced back to the volatile thiols (with lower perception thresholds) by SO2 present in the wine during bottle storage under low oxygen concentrations (Bobet et ah, 1990), or be released by hydrolysis from thioacetate esters (Rauhut et ah, 1996). 

The impact of MOX upon reductive aromas is being exanained in a further research project at the University of Auckland, using SPME GC-MS (Nguyen et ah, 2010). The wines subject to MOX, or stored in an 02-permeable Flextank, recorded lower concentrations of compoimds such as methanethiol (aroma of cooked cabbage, with a perception threshold of 0.3 pg/L Mestres et al., 2000), and 3-(methylthio)-l-propanol... 

One precondition of applying the scheme is that the toxicologically based guide values of individual substances are not thereby exceeded. A separate evaluation will however always be required when substances with low odor perception thresholds are involved which even at lower concentrations can be bothersome due to their odor activity or when noticeably high concentrations of individual substances occur. 

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. 

Some authors (Peinado et al. 2004a, 2004b Zea et al. 2007) have grouped aroma compounds in aromatic series based on their aroma descriptors. The overall value for each series is obtained by combining the odour activity value (OAV, which is defined as the ratio of the concentration of a compound to its perception thresholds) of the individual aroma compounds in the series. The combination of individual OAVs in order to calculate the overall value for a series cannot be interpreted as an arithmetic addition of odorant sensations. Some aspects of this classifying scheme can be subject to criticism, but in any case, the scheme is effective for comparing wines obtained with different aging methods inasmuch as the odour series always comprise the same compounds. 

SulfanyUiexan-l-ol (3SH) is always present in Sauvignon blanc, at concentrations of several hundred ng/L, ranging up to a few pg/L in some wines. As a result, this compound, which smells of grapefruit and has a perception threshold of 60 ng/L, also has considerable olfactory impact. Wines containing the most 3SH also have the highest concentrations of its acetate. The citrus zest aroma of... 

Entries in parentheses are the aromatic index of each compound concentration/perception threshold... 

Table 8B.8 Perception threshold and olfactory descriptors of the two enantiomers of 3SH and 3SHA (Tominaga et al. 2006)...

Empyreumatic odors, evocative of smoke, burnt wood, and cooked meat, are common descriptors used by wine tasters. Until now, a few sulfur compounds with these odors had been identified and assayed in wine at concentrations above their perception thresholds methylthiopropionic acid (Schreier and Drawer 1974), 2-sulfanylethyl acetate (V), and 3-sulfanylptopropyl acetate (X) (Lavigne et al. 1998). Two mercaptans, ethyl 3-sulfanylpropionate (VII) and ethyl 2-sulfanylpropionate (I), have been identified as constituents in the aroma of Vilis labrusca, Concord grapes (Kolor 1983) and both have also been found in Vitis vinifera wine (Tominaga et al. 2003a Blanchard 2000). 

The presence of 2-methy-3-furanthiol (II), an odoriferous compound evocative of cooked meat, has been reported in red Bordeaux (Bouchilloux et al. 1998b Kotseridis et al. 2000) and de Rioja wines (Aznar et al. 2001). Concentrations of this compound in wine vary from 25 to 140ng/L, with the highest values in Champagnes. As the perception threshold of 2-methyl-3-furanethiol in model dilute alcohol solution is 4 ng/L, this thiol certainly contributes to the toasty aroma in wines. 

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