Taste threshold, fatty acid

Determining the threshold value is difficult because subthreshold levels of one compound may affect the threshold levels of another. Also, the flavor quality of a compound may be different at threshold level and at suprathreshold levels. The total range of perception can be divided into units that represent the smallest additional amount that can be perceived. This amount is called just noticeable difference (JND). The whole intensity scale of odor perception covers about 25 JNDs this is similar to the number of JNDs that comprise the scale of taste intensity. Flavor thresholds for some compounds depend on the medium in which the compound is dispersed or dissolved. Patton (1964) found large differences in the threshold values of saturated fatty acids dissolved in water and in oil. 

Although dietary lipids are mainly constituted of triglycerides (TG), long-chain fatty acids (LCFA more than 16 carbons) seem to be responsible for oral lipid perception. In a free-choice situation, rats have a weaker preference for TG and medium-chain fatty acids (8 to 14 carbons) than for LCFA (Tsuruta et al. 1999 Fukuwatari et al. 2003). This chemical selectivity is very tight, as LCFA derivatives, such as methyl LCFA, are not recognized (Tsuruta et al. 1999). The ability of rodents to detect LCFA specifically has also been confirmed with the conditioned taste aversion paradigm. It is noteworthy that both rats and mice can be conditioned to avoid specific LCFA (McCormack et al. 2006 Gaillard et al. 2008), with a submicromolar detection threshold (McCormack et al. 2006 Yoneda et al. 2007). 

The studies cited in this paper show that for n-aliphatic alcohols (C1-C12) and -acids (C2 Cg), olfaction and taste act in similar ways as chemotaxis and anesthesia. Jain et al. ( ) came to a similar conclusion for many other membrane systems. Alcohols and fatty acids were used in the present study since olfactory and gustatory data on these compounds could be compared with those on many other systems. It should be kept in mind that threshold... 

A major roadblock in identifying gustatory properties of stearic acid is the absence of a delivery system that can present high concentrations or amoimts of hydrophobic stimuli in the absence of tactile (and olfactory) cues. For example, Chale-Rush et al. [4, 22] and Mattes [2] foimd that the detection of smell and taste thresholds of fatty acid stimuli differed with presentation conditions. This absence of a suitable delivery system ciurently limits suprathreshold studies on fatty acid chemosensation in the oral cavity. 

Odor and taste threshold values of fatty acids are compiled in Table 3.3 for cream, butter and cocoa fat. The data for cream and coconut fat indicate lower odor than taste threshold values of C4- and C6-fatty acids, while it is the reverse for Cg- up to C 14-fatty acids. 

Table 3.3. Aroma threshold values (odor and/or taste) of free fatty acids in different food items...

Unsaturated fatty acids emulsified in water taste bitter with a relatively low threshold value for a-linolenic acid (Table 3.9). Thus an off-taste can be present due to fatty acids liberated, as indicated in Table 3.9, by the enzymatic hydrolysis of unsaturated triacyl glycerides which are tasteless in an aqueous emulsion. 

Lipases in stored butter gradually release fatty acids from tri-acylglycerols, and their presence can be detected as a rancid and soapy flavour when they reach 30-40% of their threshold concentrations, which is a result of their synergism. The reaction is called hydrolytic rancidity. Responsibe for the rancid flavour is mainly butyric acid, followed by caproic acid. Caprylic acid has a rancid soap-like flavour, capric and lauric acids only have soapy flavours. Odour (and taste) threshold concentrations in butter made from sweet cream are 50 (60) mg/kg for butyric acid, 85 (105) mg/kg for caproic acid, 200 (120) mg/kg for caprylic acid, >400 (90) mg/kgfor caprinic (capric) acid and >400 (130) mg/kg for lauric acid, respectively. In long term stored butter, active oxidative rancidity products are (E)-non-2-enal, (Z)-non-2-enal in particular, while less active products are (Z)-hept-4-enal, oct-l-en-3-one and others. The rancid and soapy odour in butter can also be caused by contamination with anion active detergents, such as natriumdodecyl sulfate. 

During lipid oxidation, the primary oxidation products that are formed by the autoxidation of unsaturated lipids are hydroperoxides, which have little or no direct impact on the sensory properties of foods. However, hydroperoxides are degraded to produce additional radicals which further accelerates the oxidation process and produce secondary oxidation products such as aldehydes, ketones, acids and alcohols, of which some are volatiles with very low sensory thresholds and have potentially significant impact on the sensory properties namely odor and flavor [2, 3]. Sensory analysis of food samples are performed by a panel of semi to highly trained personnel under specific quarantined conditions. Any chemical method used to determine lipid oxidation in food must be closely correlated with a sensory panel because the human nose is the most appropriate detector to monitor the odorants resulting from oxidative and non-oxidative degradation processes. The results obtained from sensory analyses provide the closest approximation to the consumers approach. Sensory analyses of smell and taste has been developed in many studies of edible fats and oils and for fatty food quality estimation [1, 4, 5]. 

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