Plastein taste

Pish protein concentrate and soy protein concentrate have been used to prepare a low phenylalanine, high tyrosine peptide for use with phenylketonuria patients (150). The process includes pepsin hydrolysis at pH 1.5 ptonase hydrolysis at pH 6.5 to Hberate aromatic amino acids gel filtration on Sephadex G-15 to remove aromatic amino acids incubation with papain and ethyl esters of L-tyrosine and L-tryptophan, ie, plastein synthesis and ultrafiltration (qv). The plastein has a bland taste and odor and does not contain free amino acids. Yields of 69.3 and 60.9% from PPG and soy protein concentrate, respectively, have been attained. 

More details of the plastein reaction and its application to remove pigments such as chlorophyll, or to remove off-flavor components such as the beany taste of soybeans, are shown in Figure 2. The protein of the food system is solubilized and denatured (in order to achieve proteolysis), a protease is added, and the hydrolytic reaction is allowed to proceed. On partial hydrolysis of the protein the pigments and flavor constituents are released from the protein they are removed, the hydrolyzate is concentrated, and resynthesis and/or rearrangement of the amino acid sequence of the polypeptides is catalyzed by the same or a different protease. Resynthesis also can be carried out in the presence of added amino acid esters in order to improve the nutritional/functional properties of the protein. 

Enrichment of proteins in specific a.a. can be achieved in the plastein reaction, i.e., protease-catalyzed transpeptidation in concentrated solutions of a.a. ethyl esters and protein hydrolyzates (Figure 7.8). Incubation of a protein hydrolyzate, concentrated to 30-40%, with ethyl esters of Lys, Met, or Trp, e.g., with an appropriate endopeptidase at pH 4-7 at about 37°C, leads after a few days to accumulation of peptides of 2-3 kDa enriched in the respective a.a. residues. Plasteins free of Phe residues can also be obtained for phenylketonuric patients. The rate of incorporation of a.a. into the plastein increases with the hydrophobicity of the a.a. Thus selective removal of hydrophobic a.a. from the hydrolyzate and decrease of its bitter taste are possible. 

Solubilization of Protein. Fish protein concentrate has high nutritional quality as determined both from its essential amino acid composition and from animal feeding experiments. Unfortunately, the concentrate is quite insoluble in water because of its denaturation by the solvent extraction method used in processing thus it contributes no functional properties to a food and must be used in bakery products primarily. A potentially useful method of solubilizing the protein is by proteolysis (9-12). As is the case with protein hydrolysates of casein and soybean protein, bitter peptides are formed during the hydrolysis. Papain and ficin produce more of these bitter peptides than does Pronase, for example (12). Pronase was found to produce a more brothy taste (13). A possible method of removing the bitter peptides is to convert the concentrated protein hydrolysate to plastein by further proteolytic enzyme action (14) to remove the bitter peptides. 

Fish protein concentrate, although a good protein source nutritionally, is very insoluble this limits its use in foods. The protein can be solubilized by proteolytic enzymes, but the hydrolysate is bitter (12). It would be useful to investigate the plastein reaction for removal of the bitter taste as well as for changing the solubility properties of the digest (14). The plastein reaction is described more fully in Chapter 6 of this monograph. 

Proteins with an increased content of glutamic acid show an interesting sensory effect partial hydrolysis of modified plastein does not result in a bitter taste, rather it generates a pronounced meat broth flavor (Table 1.43). 

Table 1.43. Taste of glutamic acid enriched plasteins...

Elimination of the bitter taste from a protein hydrolysate is also possible without incorporation of hydrophihc amino acids. Bitter-tasting peptides, such as Leu-Phe, which are released by partial hydrolysis of protein, react preferentially in the subsequent plastein reaction and are incorporated into higher molecular weight peptides with a neutral taste. 

In certain foods a bitter taste is definitely not desirable, therefore different debittering methods have been developed. Methods for removing the bitter taste of enzymatic protein hydrolysates (such as casein hydrolysates) are described in Section 2.3.2.2. These methods are mainly based on controlled proteolysis, plastein reaction, extraction with azeotropic mixtures of alcohols and masking of bitter substances. 

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