Precipitated protein isolate

In some protein sources storage protein may be isolated from non-storage protein, or acid-precipitated proteins isolated from whey proteins. In the preparation of classical protein isolates by solubilization and acid precipitation, the amino acid make-up of the isolate differs from that of the extract. Consequently, consideration must be given to the amino acids chemically present in a refined protein. 

FIGURE 4.2 Effect of pH on nitrogen solubility index of rapeseed protein isolates. PPI—precipitated protein isolate SPI—soluble protein isolate. (From Xu, L. and Diosady, L. L. 1994. J. Amer. Oil Chem. Soc. 71 935-939. With permission.)... 

So far, two processes involving membrane separation for canola protein isolation are considered promising, and both are currently under commercial development Researchers at the University of Toronto developed a membrane-based process for canola protein isolation from defatted meal [29,42], in which, after precipitation, the soluble proteins were ultraflltered to be concentrated and diaflltered for purification. Two protein isolates were produced precipitated and soluble, with a combined protein recovery of more than 70% of total meal protein. Both products were high in protein (>85%), low in phytates (<1%), essentially free of glucosinolates (<2 (xmol/g) and had desirable functional properties comparable to those of soy protein. While the methionine content of both protein isolates was similar to the reported values, the soluble product was found to have a higher level of lysine than any canola proteins obtained before, and the precipitated protein isolate was, on the other hand, farther enriched with leucine [43]. This amino acid composition makes them suitable for nse in infant formulae (Table 4.6). Despite their excellent nutritive quality. 

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 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. 

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. 

The preliminary precipitation of proteins from milk is realized through the addition of solutions of acetic acid (1,7 mol/1) and sodium acetate (lmol/1) at t = 40-45°C before chromatographic isolation of OxTC. The precipitated proteins are separated by filtration. OxTC is detenuined in filtrate after its isolation on chromatographic column. Contents of OxTC was determined on calibration curve which is linear within concentration range 0,01-1,0 p.g/ml. 

The preparation of antibodies specific for the individual plasma proteins has greatly facilitated their smdy, allowing the precipitation and isolation of pure proteins from the complex mixmre present in tissues or plasma. In addition, the use of isotopes has made possible the determination of their pathways of biosynthesis and of their turnover rates in plasma. 

Protein isolation with affinity precipitation has been discussed in detail by Mattiasson and co-workers (see, e.g. Galaev and Mattiassion, 1997) and they have provided an exhaustive tabulation. Polymers varied from alginate.s/chitosan to dextran to NIPAM. Examples of the used proteins are from trypsin, p-glucosidase, xylanase, alkaline protease, etc. It is remarkable that affinity precipitation can sometimes give results comparable to affinity chromatography. 

The denatured protein isolate (DPI) was prepared as follows. Acid precipitated curd is washed, resuspended at pH 7.0, heated at temperatures above 90 C, and spray dried. DSC measurements indicate this isolate to be totally denatured. 

Concanavalin A is an antibody-like protein isolated from the jack bean. 82 It was shown to form a precipitate with glycogen and with yeast mannan, and was later also used to differentiate between glycogen-like polysaccharides from various sources.283 284 It has now been shown that concanavalin A forms a precipitate only with branched polysaccharides which contain terminal, nonreducing a-D-glucopyranosyl or a-D-manno-pyranosyl groups.18 The combining sites of the protein appear to be directed against the 2-deoxy-a-D-aratano-hexopyranosyl system.2 ... 

Fig. 8. A typical commercial soy protein isolate process. The soy proteins are extracted with alkali, precipitated with acid, washed with water, redispersed and dried. (Reprinted with permission from [14], 1989 Institute of Food Technologists)...

Our alternative soy protein isolate process utilizing the swellable gel technology is shown in Fig. 9. Instead of precipitating the protein, we subject the mother liquor to a series of five poly(N-isopropylacrylamide) gel treatments to concentrate and purify the protein stream. In the first gel stage, the mother liquor is contacted with collapsed gel at 5 °C. Approximately 40% of the water in the mother liquor is absorbed by the gel as it swells. The swollen gel is then separated from the retentate in a centrifuge. This retentate is diluted with water... 

Thongboonkerd V, McLeish KR, Arthur JM, Klein JB (2002) Proteomic analysis of normal human urinary proteins isolated by acetone precipitation or ultracentrifugation. Kidney Int 62 1461-1469... 

Whey Protein Isolate occurs either as a liquid or as a dry product. It is the substance obtained by the removal of sufficient nonprotein constituents from whey so that the finished dry product contains not less than 90% protein. Removal of nonprotein constituents is accomplished by separation techniques such as precipitation, membrane filtration, and/or ion exchange. The acidity of Whey Protein Isolate maybe adjusted by the addition of safe and suitable pH-adjusting ingredients. The final product is pasteurized. 

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