Soybean proteins alkali

It may also be surprising how easily this racemization may occur. Friedman and Liardon (126) studied the racemization kinetics for various amino acid residues in alkali-treated soybean proteins. They report that the racemization of serine, when exposed to 0.1M NaOH at 75°C, is nearly complete after just 60 minutes. However, caution must be used when examining apparent racemization rates for protein-bound amino acids. Liardon et al. (127) have also reported that the classic acid hydrolysis, employed to liberate constituent amino acids, causes amino acids to racemize to various degrees. This will necessarily result in D-isomer determinations that are biased high. Widely applicable correction factors are not possible since the racemization behavior of free amino acids is different from that of amino acid residues in proteins (which can be further affected by sequence). Of course, this is not a problem for free amino acid isomer determinations since the acid hydrolysis is unnecessary. Liardon et al. also describe an isotopic labeling/mass spectrometric method for determining true racemization rates unbiased by the acid hydrolysis. For an extensive and excellent review of the nutritional implications of the racemization of amino acids in foods, the reader is directed to a review article written by Man and Bada (128). 

M Friedman, R Liardon. Racemization kinetics of amino acid residues in alkali-treated soybean proteins. J Agric Food Chem 33 666-672, 1985. 

Severely alkali-treated herring meals did not support normal growth in chicks in fact some toxic effects were observed (13). Dispersing soybean protein concentrates with sodium hydroxide resulted in decreased growth in lambs (13). 

Alkalies and Acids. The older literature dealing with the treatment of soybean proteins with alkaline substances is quite extensive since these agents have often been used for protein extraction, solubilization, and property modification, including improved solubility, increased adhesive properties, and lower viscosity in dispersion and fiber formation (12, 21, 23, 24). The alkali treatment of soy protein for industrial use is done under conditions which are more severe (higher temperature and higher pH, such as possibly 50 °C at pH >13) than those intended for food usage. 

Hayashi and Kameda have reported 40% to 70% decreases in pepsin-catalyzed hydrolysis of lysozyme, soybean protein, casein and rlbonuclease A due to alkali-treatment under slightly milder conditions than ours (16, 29). Friedman, Zahnley and Masters reported an 80% decrease in digestibility of sodium hydroxide-treated casein measured as hydrolysis by trypsin (17). However, trypsin is specific for lysyl residues and lysine levels decreased to about half control values during the alkali-treatment, with a concomitant Increase in LAL formation. The somewhat lower digestibilities reported by these laboratories compared to our observations may be due to LAL formation in the proteins other than zein. 

Tables 6 and 1 compare the effects of several organic and inorganic compounds on the lysinoalanine and lysine contents of alkali-treated wheat gluten and soybean protein. These results show that all these compounds partly inhibit lysinoalanine formation. The extent of inhibition may vary from protein to protein and should be related to both the content and reducibility of the disulfide bonds (Friedman, 1978a Finley et al., 1978 a,b Masri and Friedman, 1982).

Table II shows the effect of alkali treatment on some other amino acid digestibility. The hydrolytic release rate for individual amino acids was different from the mean rate of release for protein nitrogen and varied according to the protein. For example, lysine from rapeseed protein was liberated slowly. Lysine was more available from soybean protein and its release rate was equivalent to that of serine. The latter amino acid is equally available from both protein sources.

Ishino, K. and Okamoto, S. (1975), Molecnlar interaction in alkali denatnred soybean proteins . Cereal Chemistry, 52 (1), 9-20. 

Modifying SPI with alkoxy silanes is a useful treatment prior to paper coating application. Silanated SPI is produced by mixing alkali extracts of soybean flakes with alkoxy silanes at pH 11 and 50°C (122 F) for 1 h. Modified protein is then separated by isoelectric precipitation using H SO (Krinski Steinmetz, 1987). Silanated SPI... 

In 1969, deGroot and Slump also demonstrated decreases for in vitro digestibility of alkali-treated soy protein Isolate and decreases In absorption of some amino acids by everted Intestinal sacs (14). These workers also observed decreases In net protein utilization for treated soybean meal, treated soy protein Isolate and treated casein. These decreases In net protein utilization were correlated with Increases In LAL formation. In these experiments, therefore. It was possible that protein utilization was hindered by the presence of LAL. 

In contrast to their widespread use in furniture and paper products, animal glues have not proved useful as struetural adhesives for wood. When used as the principal protein constituent, their water sensitivity is excessive compared with other available proteins. When eombined with soybean, blood, or casein, animal gelatin glues are completely hydrolyzed and destroyed by the strong alkalies required to disperse these proteins. In addition, they soften when severely heated, which, by law, prohibits their use in structural wood products [5]. 

The key to the problem seems then to be the measurement of the enzymatic releasability of protein-bound LAL, as mentioned by Slump (1978). Little on this topic can be found in the literature. Using a closed system (pepsin-pancreatin and pepsin-pronase-prolidase-aminopeptidase), Slump (1978) found an LAL release with treated soybean of 3% (1.3 g LAL/16 g N), and of 2% with treated casein (5.5 g LAL/16 g N content) or 0.5% (1.0 g LAL/16 g N content). Using a different vitro digestion system, Finot et al. (1978) reported a 27% release of LAL from alkali-treated lactalbumine. 

Although these amino acids act as precursors of lysinoalanine, their enzymatic release was not uniformly decreased after treatment. In fact, 4- or 8-hour alkali treatment markedly increased the release of lysine in both proteins, especially rapeseed. Serine liberation was not affected in rapeseed, but significantly decreased in soybean. These variations in amino acid availability were more marked in the first stages of digestion. They tended to disappear with time in treated and untreated proteins. 

Two processes are currently used to prepare protein-based films the wet method ( casting ), which involves the solubilization of protein and a plasticizer in a solvent followed by the formation of a protein network on evaporation of the solvent and the dry method, which is based on thermoplastic characteristics of proteins and combines the use of pressure and heat to plasticize protein chains [25, 32]. Dehulled soybean, after solvent defatting and meal grinding, becomes a fat-free, low fibre soy flour (48.5% protein). The soy flour, after leaching out of the water/alcohol soluble sugars, is termed soy protein concentrate (above 65% protein). The soy protein concentrate, if it is further extracted by alkali and reprecipitated by acidification, becomes the purest commercially available soy protein isolate (above 90% protein). 

The term "vegetable protein source or concentrate" applies to oilseeds and legumes, the oil-expressed/extracted meals and protein Isolates recovered by acid or alkali digestion and precipation. These materials normally originate from vegetable protein sources such as soybean meal, cotton seed meal, peanuts, peanut meal, etc. 

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