Soy protein hydrolysates

Plasteins ate formed from soy protein hydrolysates with a variety of microbial proteases (149). Preferred conditions for hydrolysis and synthesis ate obtained with an enzyme-to-substrate ratio of 1 100, and a temperature of 37°C for 24—72 h. A substrate concentration of 30 wt %, 80% hydrolyzed, gives an 80% net yield of plastein from the synthesis reaction. However, these results ate based on a 1% protein solution used in the hydrolysis step this would be too low for an economical process (see Microbial transformations). 

The sulfur amino acid content of soy protein can be enhanced by preparing plasteins from soy protein hydrolysate and sources of methionine or cystine, such as ovalbumin hydrolysate (plastein AB), wool keratin hydrolysate (plastein AC), or L-methionine ethyl ester [3082-77-7] (alkaU saponified plastein) (153). Typical PER values for a 1 2 mixture of plastein AC and soybean, and a 1 3 mixture of alkah-saponified plastein and soybean protein, were 2.86 and 3.38, respectively, as compared with 1.28 for the soy protein hydrolysate and 2.40 for casein. 

The present work was carried out with the purpose of relating taste, solubility, emulsifying capacity, foaming capacity, and viscosity of soy protein hydrolysates to the DH of these hydrolysates. This tentative approach to the manufacture of functional soy protein hydrolysates was chosen, because DH is easily controlled during hydrolysis by means of the pH-stat technique (5), and because the properties of the hydrolysate are presumed to be related to the DH-value rather than to hydrolysis parameters such as temperature, substrate concentration, and enzyme-substrate ratio (6). 

In addition to the 9 hydrolysates mentioned above, 4 soy protein hydrolysates (plus 1 control) were produced on a pilot plant scale, using Alcalase (J3, 6). All processing conditions were identical to those mentioned previously except that the enzyme inactivation was performed by pasteurisation at 90°C for 30 sec. in an Alfa-Laval small-scale plate pasteuriser P-20 with a flow of 2.6 1/min. Ten litres of hydrolysate were pasteurised, freeze-dried, and the DH determined as described above. 

Taste Evaluation. The four hydrolysates produced in pilot plant were evaluated for bitter taste by the laboratory s taste panel. Tasting took place in the taste panel room which is equipped with separate booths, and the panel has been selected and trained specially for discrimination of bitterness. The panel was instructed to rank two samples and four bitter-tasting standards, containing 20, 40, 80, and 160 ppm quinine hydrochloride dissolved in non-bitter iso-electric soluble soy protein hydrolysate (5, 6). 20 ppm quinine hydrochloride in this solvent had in previous experiments been established as the panel s threshold value. The protein (N x 6.25) concentration in the samples and standards was 4.0% and pH was adjusted to 6.5 with 4 N NaOH or 6 N HC1. 

Viscosity. Soy protein hydrolysate dispersion containing 10% protein (N x 6.25) and 0.5 M NaCl were prepared using a blender during repeated periods of 30 sec. until the slurry was homogenous. The slurry was centrifuged at 500 rpm for 3 min. in order to remove dispersed air and finally gently stirred with a spatula. 

Table I. Results from the Organoleptic Evaluation of the Bitterness of Four Soy Protein Hydrolysates...

Figure 4. Nitrogen solubility tw. pH for soy protein hydrolysates. The hydrolyses were carried out using alcalase(B> at pH 8.0 as described in the experimental...

Similar results as ours have recently been published by Zakar-ia and McFeeters (24). They studied the emulsifying activity of soy protein hydrolysates made by peptic hydrolysis and observed that with an increasing concentration of free amino groups in the hydrolysate the emulsifying activity exhibited an increase followed by a decrease. However, because of the differences in experimental conditions between their work and ours, a quantitative comparison would not be justified. 

The picture is somewhat different if we consider an inhomoge-nous system such as soy protein hydrolysate. Figure 10 shows that the viscosity as a function of DH is characterized by a sudden, initial large drop between DH = 1% (control sample) and the hydrolysates with the lowest DH-values. No further drop in viscosity is observed when DH is 3% or above. The figure also shows that the final level of viscosity seems to be dependent on the enzyme rather than on DH which is in marked contrast to the results obtained with gelatin hydrolysates (Figure 9). 

The heat treatment of the Alcalase-treated hydrolysates does not cause any changes in the viscosity, in contrast to the Neu-trase-treated samples which show an irreversible increase in viscosity, in particular at DH = 1.5%. No gel formation was observed in these experiments, whereas Pour-el and Swenson (26) were able to introduce gel formation capacity in soy protein hydrolysates. This indicates that the removal of the iso-electric soluble phase as carried out in their work (26), is necessary for obtaining gelation ability. Pour-el also proposed that the presence of small peptides would hinder the three-dimensional cross-linking necessary for gel formation (27). 

An industrial process has been developed for production of isoelectric soluble soy protein hydrolysate with no bitterness and a bland taste (13). The raw material may be acid washed soy white flakes, soy protein concentrate or soy protein isolate. The raw material is hydrolyzed by the alkaline protease ALCALASE to a specified degree of hydrolysis using the pH-stat at pH 8.0... 

Figure 8. Flow sheet production of a nonbitter, soluble soy protein hydrolysate suitable for incorporation into soft drinks and other low pH foods.

Hyperfiltration serves the purpose of concentration of both plasma and hydrolysate separately. Flux data are very similar to those obtained on soy protein hydrolysates, and also the total economy of such process seems attractive. The main reason is that slaughterhouse blood in most cases is regarded as a waste product having no value, or even a negative value. 

As described previously in the present publication, we have developed a batch process for producing isoelectric soluble soy protein hydrolysate (ISSPH) with a bland taste. From studies of the kinetics of the hydrolysis reaction, which takes place in this process, we have come to the conclusion that the reaction is adequately controlled by keeping pH constant and monitoring DH. Termination of the reaction at a preset value of DH ensures a reproducible, optimal organoleptic quality of the product. 

Wang, W E. de Mejia. Anticancer potential and mechanisms of lunasin and soy protein hydrolysates. Abstracts, 233rd ACS National Meeting, Chicago, IL, March 25-29, 2007. 

In the early 1970s, Puski carried out a systematic investigation on functional soy protein hydrolysates. Puski hydrolyzed soy protein isolate with Aspergillus oryzae protease at pH 7 for 3 hours with different levels of enzyme/... 

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. 

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