Hydrophobic amino acids, bitterness

The enzymatic hydrolysates of milk casein and soy protein sometimes have a strong bitter taste. The bitter taste is frequently developed by pepsin [9001 -75-6] chymotrypsin [9004-07-3] and some neutral proteases and accounted for by the existence of peptides that have a hydrophobic amino acid in the carboxyhc terminal (226). The relation between bitter taste and amino acid constitution has been discussed (227). 

Bitterness of 0-Aminoacyl Sugars Containing Hydrophobic Amino Acids. Hydrophobic amino acids are known to produce a bitterness. Therefore, we... 

Bitter peptides have been identified in hydrolyzates of casein (12,13), cheese (13a,b), and soy bean (14,15,15a). The bitter taste has been related to the hydrophobic amino acid content (16-20) and to chain length. Ney and Retzlaff (21) established a formula relating the bitterness of peptides to their amino acid composition and chain length. Too large a proportion of hydro-phobic amino acids gives rise to bitterness yet above a certain molecular weight, bitterness is not perceptible even when there are hydrophobic amino acids (21). Peptides that were responsible for bitterness in Cheddar cheese were rich in Pro, which occurred predominantly in the penultimate position (21a). 

Otagiri et al. (22) used model peptides composed of arginine, proline, and phenylalanine to ascertain the relationship between bitter flavor and chemical structure. They reported that the presence of the hydrophobic amino acid at the C terminus and the basic amino acid at the N terminus brought about an increase in the bitterness of di- and tripeptides. They further noted a strong bitter taste when arginine was located next to proline and a synergistic effect in the peptides (Arg)r(Pro) ,-(Phe) (/ = 1,2 m, n = 1, 3) as the number of amino acids increased. Birch and Kemp (23) related the apparent specific volume of amino acids to taste. 

The extent of hydrolysis of protein hydrolysates is measured by the ratio of the amount of amino nitrogen to the total amount of nitrogen present in the raw material (AN/ TN ratio). Highly hydrolyzed materials have AN/TN ratios of 0.50 to 0.60. To obtain the desired level of hydrolysis in a protein, a combination of proteases is selected. Serine protease prepared from Bacillus lichenifor-mis has broad specificity and some preference for terminal hydrophobic amino acids. Peptides containing terminal hydrophobic amino acids cause bitterness. Usually a mixture of different proteases is employed. The hydrolysis reaction is terminated by adjust-... 

To summarize the model above At low DH-values, the majority of the hydrophobic side-chains are still masked and bitterness is low. At increasing DH-values more and more peptides will be too small to form proper secondary structures, the hydrophobic side-chains become exposed, and bitterness increases. At still higher DH-values, however, the peptides are so small that a significant fraction of hydrophobic amino acid will be either free or in terminal position and this will tend to reduce the bitter taste. 

Hydrolysis of proteins without taste by proteases often produces bitter peptides. Hydrophobic amino acid residues located in the interior of protein molecules in aqueous solution are exposed by fragmentation of the protein molecules treated with proteases, and the peptides containing a number of hydrophobic amino acid residues occur in the solution (13). Many bitter peptides as shovm in Table 4-have been isolated from protein digests with proteinases (14-22). 

The comparison of the amino acid sequence of the above-mentioned bitter peptides shows a large proportion of hydrophobic amino acids in each peptide. And the amino acid sequence of peptides also plays an important role in the intensity of the bitter taste. For example, the bitterness of Phe-Pro is more intense than that of Pro-Phe, and the bitterness of Gly-Phe-Pro is more intense than that of Phe-Pro-Gly (23). C-terminal groups of all bitter peptides in pepsin hydrolysates of the above-mentioned soy protein were characterized by the location of the Leu residue (14-17). The research on the relationship between the structure and bitter taste intensity of Arg-Gly-Pro-Pro-Phe-Ile-Val (BP-Ia) showed that Pro and Arg located on center and the N-terminal site, respectively, played an important role in the increment of bitter taste intensity besides the hydro-phobic amino acids located on C-terminal site (24-26). This may indicate that the peptide molecular structure formed by the arrangement of Arg, Pro and hydrophobic amino acid residues contributes to the bitter taste intensity of the peptide. 

Molecular and enzymatic properties of serine carboxypeptidase (EC.3.4.1.6.1, CPase Top), isolated and refined from the common squid (Todarodes pacificus) liver, were studied. It was found that this enzyme reacts well at the C-terminal position of peptides having hydrophobic amino acids. Because of this property, it was anticipated that this enzyme would have the effect of eliminating bitterness of some peptides. This enzyme was used on bitter peptides prepared by hydrolysis of proteins with pepsin and trypsin. It was found that this CPase Top can eliminate bitter peptides prepared from soy protein and com gluten. 

It has been demonstrated that CPase Top enzyme is useful for eliminating bitterness originating from soy protein and com gluten. With respect to the bitterness from casein, although the bitterness was not completely eliminated, it was effective in reducing most of the bitterness. It is postulated that the bitterness of peptides is due to the hydrophobic amino acids in the C-terminus position (4). This assumption is necessary for the elimination of bitterness by liberating a hydrophobic amino acid at the C-terminus by the substrate specificity of CPase Top. 

In the food industry, peptidases have several applications (Olempska-Beer et al. 2006). The bitterness found in protein hydrolysates has been classically associated with the release of peptides containing hydrophobic amino acid residues... 

Ney 1979). Significant reductions in bitterness have been observed after peptidase treatment of food. The peptidases used are exopeptidases including amino- and carboxypepti-dases, which cleave hydrophobic amino acid residues (Saha and Hayashi 2001 Raksakulthai and Haard 2003 FitzGerald and O Cuinn 2006). Thermolysin is used as a peptide and ester synthetase in the production of the artificial sweetener aspartame (Ager et al. 1998). O Table 7.1 summarizes some commercial peptidases and their applications. 

Bitterness of 0-Aminoacyl Sugars Containing Peptides Composed of Valine or Phenylalanine. Since hydrophobicity of the amino acid moiety seemed to be an important key for bitterness of 0-aminoacyl sugars, we then prepared O-... 

Thus it must follow that, in the case of peptides, irrespective of the c onfigurat.ion of the amino acids involved, only bitter taste can be expected if the other preconditions (hydrophobic side chains) are satisfied. The examples investigated confirm this assumption quality and intensity of taste do not depend on the configuration (Table VII). Intensity also seems to be independent of the sequence (Table VIII). 

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. 

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