Protein isolate

Commercial soy protein concentrates typically contain 70 to 72% cmde protein, ie, nitrogen x 6.25, dry wt basis. Soy protein isolates are prepared from desolventhed, defatted flakes. A three-stage aqueous countercurrent extraction at pH 8.5 is used to disperse proteins and dissolve water-soluble constituents. Centrifugation then removes the extracted flakes, and the protein is precipitated from the aqueous phase by acidifying with HCl at pH 4.5. 

The protein precipitate is washed with water, redispersed at pH 7, and then spray dried. Typical commercial soy protein isolates contain greater than 90% cmde protein, dry wt basis. 

A modification of the conventional soy protein isolate process has been investigated on a small pilot-plant scale. It is based on the absorption of water from the aqueous protein after extraction at pH 8.5 using temperature-sensitive polyisopyropylacrylamide gels, followed by spray drying to give a 96% protein isolate (111). 

Products prepared from soy protein products and resembling chicken, ham, frankfurters, and bacon are available commercially. Soy protein isolates are used in place of milk proteins or sodium caseinate in products such as coffee creamers, whipped toppings, yogurt, and infant formulas (see Dairy substitutes). Soy protein products also are used in snacks and in baked foods. 

A pepsin hydrolysate of flounder fish protein isolate has been used as the substrate (40% w/v) for plastein synthesis, using either pepsin at pH 5 or alpha chymotrypsin at pH 7, with an enzyme—substrate ratio of 1 100 w/v at 37°C for 24 h (151). The plastein yields for pepsin and alpha chymotrypsin after precipitation with ethanol were 46 and 40.5%, respectively. 

Protein-Based Substitutes. Several plant and animal-based proteins have been used in processed meat products to increase yields, reduce reformulation costs, enhance specific functional properties, and decrease fat content. Examples of these protein additives are wheat flour, wheat gluten, soy flour, soy protein concentrate, soy protein isolate, textured soy protein, cottonseed flour, oat flour, com germ meal, nonfat dry milk, caseinates, whey proteins, surimi, blood plasma, and egg proteins. Most of these protein ingredients can be included in cooked sausages with a maximum level allowed up to 3.5% of the formulation, except soy protein isolate and caseinates are restricted to 2% (44). 

SolubiHty is used in the refining of oilseed meals to give protein isolates and concentrates (18). Proteins are highly soluble in basic solutions and the... 

Soybean Protein Isolates. Soybean protein isolates, having a protein content of >90 wt%, are the only vegetable proteins that are widely used in imitation dairy products (1). Most isolates are derived from isoelectric precipitation, so that the soybean protein isolates have properties that are similar to those of casein. They are insoluble at thek isoelectric point, have a relatively high proportion of hydrophobic amino acid residues, and are calcium-sensitive. They differ from casein in that they are heat-denaturable and thus heat-labile. The proteins have relatively good nutritional properties and have been increasingly used as a principal source of protein. A main deterrent to use has been the beany flavor associated with the product. Use is expected to increase in part because of lower cost as compared to caseinates. There has been much research to develop improved soybean protein isolates. 

Soybean-based ice cream products, technologically feasible, are generally not in use because of flavor problems. An acceptable ice cream has been made by replacing 50% of the nonfat milk soHds with a dried soy protein isolate made up of cheese whey (21). Chocolate flavor has been widely used to mask the flavor of soybean proteins in ice cream (see Flavors and spices). 

Test preparation for both methods is similar. It comprises an extraction by a water-acetone mixture with its further degreasing protein isolation and redistribution of aflatoxin B into chloroform, concentrating by means of evaporation of the dissolvent on the rotary evaporator (t = 40°C). 

By measuring the L-isoleucine/o-alloisoleucine ratio in the protein isolated from the eggshells of an extinct Australian bird, a team of scientists recently determined that this bird lived approximately 50,000 years ago. Radiocarbon ( " C) dating is not accurate for samples older than about 35,000 years, so AAR is a useful addition to the tools available to paleontologists. 

FIGURE 4.4 The structures of several atniuo acids that are less cotntnou but nevertheless found in certain proteins. Hydroxylysine and hydroxyproline are found in connective-tissue proteins, pyroglutatnic acid is found in bacteriorhodopsin (a protein in Halohacterium halohium), and atninoadipic acid is found in proteins isolated from corn. 

The cytosolic dyneins bear many similarities to axonemal dynein. The protein isolated from C. elegans includes a heavy chain with a molecular mass of approximately 400 kD, as well as smaller peptides with molecular mass ranging from 53 kD to 74 kD. The protein possesses a microtubule-activated ATPase... 

Prendergast, F. G., and Mann, K. G. (1978). Chemical and physical properties of aequorin and the green fluorescent protein isolated from Aequorea forskalea. Biochemistry 17 3448-3453. 

KELL blood group antigen is a plasma membrane protein isolated from red cells homologous to zinc-binding glycoproteins with neutral endopeptidase activity. 

Enzyme-mediated chiral sulfoxidation has been reviewed comprehensively in historical context [188-191]. The biotransformation can be mediated by cytochrome P-450 and flavin-dependent MOs, peroxidases, and haloperoxidases. Owing to limited stability and troublesome protein isolation, a majority of biotransformations were reported using whole-cells or crude preparations. In particular, fungi have been identified as valuable sources of such biocatalysts and the catalytic entities have not been fully identified in all cases. 

This chapter will focus on simple and complex iron-sulfur-con-taining proteins isolated from sulfate reducing bacteria (SRB), in order to review the following topics types and distribution of proteins metal clusters involved and their association with other centers and... 

A preliminaiy characterization of a new iron—sulfur protein isolated from Desulfovibrio vulgaris Hildenborough was reported in 1989 124). The protein contained approximately 6 iron and 6 inorganic sulfur atoms per molecule. The FPR spectrum of the dithionite reduced protein exhibited an S = signal similar to what was found for synthetic compounds with a [6Fe-6S] core (prismane core). No other FPR signals were reported at this time, and based on the observed similarity it was suggested that this peculiar iron-sulfur protein contained a [6Fe-6S] cluster. Because it had no known function, the pro-... 

The actions of proteins isolated from sea anemones, or other coelenterates, involve mechanisms different from those described for saponins. Thus, hemolysins from sea anemone R macrodactylus are capable of forming ion channels directly in membranes (98). The basic protein from S. helianthus also forms channels in black-lipid membranes. These channels are permeable to cations and show rectification (99). This ability of S. helianthus toxin III to form channels depends upon the nature of the host lipid membrane (100). Cytolysin S. helianthus binds to sphingomyelin and this substance may well serve as the binding site in cell membranes (101-106). 

ALEKEL D L, ST GERMAIN A, PERESON C T, HANSON K B, STEWART J W, TODA T (2000) IsoflaVOne-rich soy protein isolate attenuates bone loss in the lumber spine of perimenopausal women. Am J Clin Nutr. 72 844-52. 

MARIOTTI F, MAKE S, BENAMOUZIG R, LUENGO 0, DARE S, GAUDICHON 0, TOME D (1999) Nutritional value of [15N]-soy protein isolate assessed from ileal digestibility and postprandial protein utilization in humans. /AmZ 129 1992-7. 

In order to produce soy protein, soybeans are first dehulled, flaked, and defatted to make white flakes . Soy protein concentrates are obtained by removing a portion of the carbohydrates from defatted and dehulled soybeans. Alcohol extraction is the method most commonly used to manufacture soy protein concentrates even though it results in the loss of isoflavones. Soy protein concentrates retain most of the fiber in the original soybean and must contain at least 65% protein on a moisture-free basis to meet quality standards. The most concentrated source of soy protein is soy protein isolates (or isolated soy protein, ISP), which is required to be at least 90% protein on a moisture-free basis. It is heat-treated during processing to insure inactivation of trypsin inhibitors. Most isolated soy protein is manufactured by water extraction from defatted and dehulled soybeans and it retains the natural isoflavones. 

SHIH F, DAIGLE K w (2000) Preparation and characterization of rice protein isolates. .ZIOCY, 77(8) 885-89. 

WANG M, HETTiARACHCHY N s, Qi M, BURKS w, siEBENMORGEN T (1999) Preparation and functional properties of rice bran protein isolate. J Agri, Food Chem, 47 411-16. 

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