Threshold values, amino acids

L-Amino acid Threshold value, mg/dL Sweet Sour Bitter Salty Umami... 

Figure 9.1a reporting GC-MS results taken on laboratory blank in a GC-MS procedure [87] shows negligible amino acid contamination. Indeed, quantitating a protein as the sum of 14 amino acids, a value of about 200 ng is the minimum required threshold to positively consider the protein identification. Figure 9. lb reports the chromatogram of an amino acid standard solution acquired in the SIM mode and corresponds to the quantitation limit that is a content of about 700 ng in the sample at a confidence level of 95%. This seems amenable when working at the trace level. 

Many nitrogen- and sulfur-containing heterocycles have been identified in the aroma fractions of foods [214]. In roasted products (e.g., coffee) and heat-treated foods (e.g., baked bread or fried meat), these heterocycles are formed from reducing sugars and simple or sulfur-containing amino acids by means of Maillard reactions [215, 216]. Their odor threshold values are often extremely low and even minute amounts may significantly contribute to the aroma quality of many products [217, 218]. Therefore, N- and N,S-heterocyclic fragrance and flavor substances are produced in far smaller quantities than most of the products previously described. 

Notice that below the threshold, the glucose phosphorylation is not sufficient to sustain the beta cells energy expenditures. This is particularly seen in the vertical jumps for small values of Rox. However, in practice the threshold is smoother, because other nutrients like fatty acids and amino acids can take over [32]. 

With peptides, the threshold values depend on the amino acids involved. Tables IX and X demonstrate this on the basis of some of the dipeptides and tripeptides. 

Qualitative predictions concerning the degree of bitterness to be expected in peptides are possible with the aid of NEY s Q-value (12) (Table XI), which is based on TANFORD s studies (13). Using the Ft values of peptides, quantitative assessment of the expected range of threshold values is also possible on the basis of the amino acid composition (Table XI) (14). 

Fig. 3 demonstrates, on the basis of the sweet and bitter taste of the amino acids, that not only hydrophobicity, but also the shape of the side chains influences the threshold value. 

The ring size has a determining effect on the threshold values not only in the case of the sweet-bitter 1-amino-cycloalkane-1-carboxylic acids but also, as our investigations have shown, in the case of other cyclic compounds, as for example the bitter cyc-loalkanones, azacycloalkanes, lactams and lactones (Table XV), as well as the sweet cycloalkanesulfamates (Table XVI). From the tables it follows that the hydrophobic contact with the receptor apparently attains a maximum in general with ring sizes 6-8 on the part of sweet as well as bitter compounds. 

It was reported that q-glut amyl peptides, particularly peptides with hydrophilic amino acids, such as Glu—Asp, Glu-Thr, Glu-Ser, and Glu-Glu, elicited an umami flavor236 These peptides were isolated from the umami constituents in the enzymatically hydrolyzed products of soybean proteins. The same author also reported that tripeptides such as Glu-Gly-Ser also elicited the umami taste. The threshold value (0.15%) of these peptides is greater than that of MSG. The flavor of meat extract can be reproduced using these peptides with MSG and IMP. Recently,. Y-lacroyl glutamic acid, which is a condensation product of lactic acid with glutamic acid, was shown to elicit a weak umami taste, similar to MSG.237... 

Table 1. Taste, Threshold Value and Discovery of Amino Acids...

Amino acid Taste Threshold value(mg/dl) Where found... 

Though amino acids can elicit any one of the primary tastes, the threshold value of taste of each amino acid is high. As the levels of some free amino acids in natural foods are lower than their threshold values, it may be thought that they may not contribute directly to food taste. However, they may have an Important role in making the food savory because of the synergistic effect. 

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