Amino fish protein concentrate

The functional properties of food proteins is a subject of considerable interest and importance. For example, in the last two decades much effort has gone into the development of various nonconventional sources of protein. The vast majority of this effort, however, has been concentrated on various aspects of production of economical protein resources and not on the actual utilization of these resources. The result is that many nonconventional protein resources, while they can now be produced fairly easily, can in many cases be produced only in a form lacking desirable functional properties. The point is simply that a protein, even though it may have excellent amino acid balance and all other prerequisites for a nutritionally superior protein, will have no impact on human nutrition unless it has the functional properties necessary for its incorporation into food systems. A specific example of this problem is the case of fish protein concentrate, the utilization of which is severely restricted by its very limited solubility in water. There are, of course, many other examples, but for present purposes it is sufficient to emphasize that there is a real need for the development of chemical, physical, or enzymatic methods that increase the usefulness of proteins in food systems. 

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. 

While several laboratories have shown that severe racemiza-tion of proteins can occur during treatment with sodium hydroxide (6,18,22-24,61,62) the effects of other alkalis used in food processing are documented less well. Jenkins, et al. (70) have observed substantial differences in the degree of racemization caused by lime or caustic soda treatment of zein. Lime causes only 50% to 90% of the racemization observed for several amino acyl residues compared to when caustic soda is used. Because a substantial amount of calcium ion remained bound to the protein (approx. 10,000 ppm) compared to l/20th that amount of sodium ion for the caustic soda-treated zein, it is possible that divalent calcium may stabilize the protein making it less susceptible to racemization. Tovar (14) observed increases of 40% to 50% in serine and phenylalanine racemization and a decrease of 30% aspartate racemization for caustic soda-treated fish protein concentrate compared to lime-treated protein (see Table II). These studies indicate that different alkalis have different effects on racemization of proteins specifically, lime may cause less racemization than caustic soda at a similar pH. 

Table 1.44. Amino acid composition (weight-%) of plasteins with high tyrosine and low phenylalanine contents from fish protein concentrate (FPC) and soya protein isolate (SPI)...

The most important role of animal protein is to correct the amino acid deficiencies of the cereal proteins, which supply about two-thirds of the total protein intake, and which are notably deficient in the amino acid, lysine. The latter deficiency can also be filled by soyhtean meal, fish, protein concentrates and isolates, synthetic lysine, or high-lysine com. But such products have neither the natural balance in amino acids nor the appetite appeal of animal protein. 

Enzymes have also been used in the preparation of protein ingredients from animal wastes. Mohr (1980) described the use of proteolytic enzymes in the preparation of protein concentrates from fish offal. Fish protein waste was ground with protease and incubated to yield a concentrated fish protein, which was then dried. Generally, the contractile and connective tissue proteins are hydrolysed most efficiently. Sarcoplasmic proteins tend to aggregate and resist enzyme attack. The proteins are broken down into peptides and individual amino acids, and the longer the hydrolysis continues the higher the yield of hydrolysates but the greater the peptide breakdown. For many food applications peptide breakdown needs to be carefully controlled. 

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Nowadays, ACE inhibitory peptides have been isolated from meat, remaining muscle proteins, skin collagen and gelatin, bone, and internal organs of fishes such as Alaska pollack, bonito, tuna, salmon, shark, and sardine. Table 16.1 provides a partial summary of ACE inhibitory peptides derived from marine fish sources, their amino acid sequence, the enzyme used for hydrolysis, and IC50 values. The IC50 value is the concentration of peptide that inhibits 50% of ACE activity. 

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