Proteins hydrolysates, bitter peptides from

Bitter peptides from peptic soya protein hydrolysates... 

Methods for eliminating bitter peptides in partial protein hydrolysates are known, but they cause a significant loss of essential amino acids. These procedures usually include additional enzymatic hydrolysis under controlled conditions (a shorter time for the hydrolysis leads to higher peptides that are not bitter) and a selection of suitable proteases, such as aminopeptidases, carboxypeptidases and some other proteases. Enzymes of plant and microbial origin have been successfully used for this purpose. For example, the intracellular peptidases from Lactococcus lactis ssp. cremoris and Brevibacterium linens, which have high proteolytic activity, successfully hydrolyse bitter peptides in cheeses. 

Cho, M.J., Unklesbay, N., Hsieh, F.H. and Clarke, A.D. (2004). Hydrophobicity of bitter peptides from soy protein hydrolysates. Journal of Agricultural and Food Chemistry, 52,5895-5901. 

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. 

It seems that the negative charges can also be on a peptide chain. Fujimaki describes the bitter masking action of peptides rich in glutamyl residues (29 ) and the isolation and identification of acidic oligopeptides from a flavour-intensifying fraction from fish protein hydrolysate (30). 

Series of bitter peptides have been isolated from enzymatic hydrolysates of proteins, esp. casein and soybean protein. 

From soybean protein hydrolysates several series of bitter peptides have been isolated. As an example Table XIV shows bitter peptides isolated by Fujimaki (69., 70) As before the high Q-values are evident. 

It is interesting to see that proteins with high Q-values above 1400 as e.g. soybean protein, casein wheat gluten, potato protein, Zein are the "parents" of bitter peptides, whereas no bitter peptides have been isolated from hydrolysates prepared from collagen or gelatin, proteins with Q-values below 1300. 

The bitter peptide BPI a 114, isolated by Okai et al.239 > from casein hydrolysates, and delicious tasting peptides from fish proteins, will undoubtedly achieve practical importance in the food industry. 

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-... 

Table U- Bitter Peptides Isolated from Proteinase Hydrolysates of Proteins...

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. 

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. 

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