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Study on the Effect of Isoelectric Point Shift of Soybean Protein in Acid Precipitation Process Based on Transglutaminase

Yuqiang Shi*, Rucui Liu, Jun Ma, Xuejiao Cheng, Peng Chen, Weigang Yuan

Review Transglutaminase (TG) is mainly used in food modification to adjust food texture and improve protein
nutritional value. It works by catalyzing the amide transfer reaction between the γ-formamide group in glutamine
and the primary amine group in lysine, thereby crosslinking the two amino acids. Soy protein contains 18 amino
acids, and cross-links are formed between soy proteins through TG, thereby forming new protein structures that can
cause isoelectric point shifts. In order to investigate the change rule of TG changing the isoelectric point of soybean
protein and its influence on the production of soybean protein isolate, the following studies were carried out:
The low-temperature skimmed soybean meal was dissolved under alkaline conditions, and soy milk was obtained
by centrifugation; TG was added to the soy milk, and then protein isoelectric point adjustment was performed;
materials that reach the isoelectric point were centrifuged to obtain soy protein and soy whey; the change trend of soy
whey and the shift direction of protein isoelectric points after adding enzyme were analyzed. When different ratios
of TG enzyme were added, the shift trend of the isoelectric point of soybean protein was studied. At these isoelectric
points, the recovery rate of soybean protein was studied. The results showed that: When the TG was added at a ratio
of 1.0% of the dry matter content of the soymilk, the Crude Protein (CP) of the soybean whey was 1.2 percentage
points lower than that without TG at the normal protein isoelectric point (pH 4.5). And the pH value of isoelectric
point in soymilk with TG was shifted from 4.5 to 3.7, then the CP of the soybean whey was reduced from 23.8%
to 20.0% at the new isoelectric point. Under different TG addition, the degree of deviation of the isoelectric point
of soybean protein was different, and the pH value of isoelectric point decreased first and then increased with the
addition increase. When the addition amount was 1.0%, the pH value of isoelectric point reached the low of 3.7.
The recovery of soybean protein increased with the TG addition, and when the addition amount was 1.5%, the
recovery was 91.1%. By adding TG to soybean milk, soybean protein could be effectively cross-linked, the pH value of
isoelectric point of soybean protein changed, the recovery rate of soybean protein improved, the content of soybean
protein in soybean whey reduced, and the utilization of soybean could be improved. It has positive significance for
protecting the environment and promoting the development of the soybean protein industry.