Peptide bitter taste

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). 

Some attempts to reduce sodium chloride intake have been carried out. The first is to use some socUum chloride substitutes. Pottasium chloride is widely used for this purpose. However, pottasium chloride is not thought as perfect sodium chloride substitute because it contains bitter taste. Okai and his associates have synthesized several salty peptides (5). These peptides are expected to be good for hypertension, gestosis, diabetes mellitus and other deseases because they contain no sodium ions. These peptides, however, are not expected to be used as sodium chloride substitutes immediately because of the difficulty of in their synthesis and their cost. Although they are struggling to establish a new synthetic method of peptides in a mass production system with reasonable costs and to improve the salty potency of peptides, they have not dissolved this problem. Since the threshold value of ionic taste is around 1 mM regardless their kinds, it seems to be very difficult to prepare an artificial sodium... 

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. 

Some peptides enjoy the property of masking the bitter taste of foods. Ohyama et al. (24) conducted sensory analyses using synthetic peptides and found that neutralized peptides consisting of aspartic acid and glutamic acid had a taste similar to that of monosodium glutamate. They termed this umami taste or relish. ... 

Enzymatic hydrolysates of various proteins have a bitter taste, which may be one of the main drawbacks to their use in food. Arai el al. [90] showed that the bitterness of peptides from soybean protein hydrolysates was reduced by treatment of Aspergillus acid carboxypeptidase from A. saitoi. Significant amounts of free leucine and phenylalanine were liberated by Aspergillus carboxypeptidase from the tetracosapeptide of the peptic hydrolysate of soybean as a compound having a bitter taste. Furthermore, the bitter peptide fractions obtained from peptic hydrolysates of casein, fish protein, and soybean protein were treated with wheat carboxypeptidase W [91], The bitterness of the peptides lessened with an increase in free amino acids. Carboxypeptidase W can eliminate bitter tastes in enzymatic proteins and is commercially available for food processing. 

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). 

The sweet dipeptide esters of the L-aspartic acid and the L-amino malonic acid (15-21) are interesting exceptions to the bitter taste shared by all other members of the peptide series. Fig. 

The studies on peptides began with a correlation between sweet amino acids and peptides. Since the projection formula of L-Asp-Gly-OMe (4) is similar in size and shape to that of e-Ac-D-Lys (3) which is sweet, we predicted that L-Asp-Gly-OMe would taste sweet in spite of the bitter taste in the literature. Therefore, we synthesized the peptide and tasted it. As expected, it was sweet and its sweetness potency was almost equal to that of e-Ac-D-Lys. Thus, the dipeptide could be correlated to the amino acid. Lengthening (5) or enlargement (6) of the alkyl group of the ester did not affect its sweetness potency (Table 1). 

L-Asp-L-Cap-OMe (19) was sweet, whereas the ethyl ester (20) was not sweet but bitter, though we could draw the sweet formula (A) to it. This may show that the increased hydrophobicity in the molecule changed the property of the sweet peptide to a bitter property, because it has been known that bitter-tasting compounds are composed of charge and hydrophobic properties. 

Figure 3 gives the sequence of p-casein - which represents JO % of casein - and the bitter peptides derived from it and isolated by the groups of Clegg (49), Kloster-meyer (46), Gordon (6k). Here also the Q-values of the bitter peptides are above 1400. Please note, that no special single amino acid or sequence is needed to impart the bitter taste. 

Above this molecular weight, also peptides with a Q-value above 1400 will no longer exhibit bitter taste. It is clear therefore, that 2 ways exist to come to non-bitter protein hydrolysates. As demonstrated in Figure 4... 

Figure 4. Molecular weight, average hydrophobicity Q, and bitter taste of peptides...

Stevenson, D.E., Ofman, D.J., Morgan, K.R., and Stanley, R.A. 1998. Protease-catalyzed condensation of peptides as a potential means to reduce the bitter taste of hydrophobic peptides found in protein hydrolysates. Enzyme Microb. Technol. 22, 100-110. 

Bitter-Tasting Amino Acids and Peptides 4.16.3.4.1 Amino acids... 

They also observed that the bitter taste of the peptides depended on many parameters DH being one of them. It seemed clear from their work that non-bitter hydrolysates could be obtained at high DH-values (above 20%) an observation which is in accordance with the results of Clegg and McMillan (4) who removed the bitter taste of casein hydrolysates by applying an exopeptidase. 

Small peptides in solution are generally random coils however, above a certain critical length, the peptides will be able to have secondary structures distinctly different from the random coil. Thus, the critical length for a-helix formation is 7-9 amino acid residues (22). It is not generally possible to predict at what chain length two hydrophobic side-chains are able to interact, whereby the peptide becomes U-shaped, because this must depend on the actual position and nature of the side-chains. This hydrophobic interaction masks the side-chains, resulting in a reduction of the bitter taste. 

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. 

Many bioactives have an undesirable taste and odour. Peptides are known for their bitter taste, mineral salts for their metaUic tastes and marine oils rich in omega-3 fatty acids for fishy taste and odour. Further, the addition of soluble iron salts to foods catalyses the oxidation of fats and amino acids and imparts undesirable metallic tastes to foods (Zimmermann 2004 Yang and Lawless 2006). A variety of added ingredients (e.g., sugar, flavours) have been used to mask these tastes, but with limited success. 

Bitter taste is elicited by structurally diverse compounds, including phenols, ions, amino acids and peptides, alkaloids, acylated sugars, glycosides, nitrogenous compounds, and thiocarbamates. Taste receptor cells are primarily associated with papillae on the tongue. The signal transduction mechanisms by which taste perception occurs are well not understood, but are the focus of intensive research as reviewed recently (6). 

Studies on the taste of peptides have been done only recently. The bitter taste produced during the storage of cheese and in the fermentation of the traditional Japanese food "miso" and "soy sauce" has been shown to be caused by the peptides in the hydrolysate of proteins. Since then, a number of studies on bitter peptides have... 

Almost all peptides of hydrophobic L-amino acids elicit a bitter taste, which indicates that the bitterness of peptides is caused by the hydrophobic property of the amino acid side chain. Ney (12) has reported that whether a peptide has a bitter taste or not depends on its hydrophobic value Q. The value Q is obtained by adding the Af-values (Table 3) of each constituent amino acid residue of a peptide and dividing the sum by the number of amino acid residues (n). 

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. 

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