Soy Protein Isolate (SPI for short) refers to powdered soybean products with a protein content of ≥90% on a dry basis. The production of soybean protein isolate by alkali dissolution and acid precipitation is currently the most commonly used process in the world. It uses low-temperature defatted and desolvated soybean meal as raw material, and removes soluble and insoluble carbohydrates in the defatted soybean meal through alkali dissolution and isoelectric precipitation.
Process flow: low-temperature soybean meal-alkaline extraction-centrifugal separation-acid precipitation-centrifugal separation-neutralization-sterilization-spray drying-soybean protein isolate.
Thermal denaturation of soybean protein in spray drying
Spray drying is a drying method for processing solution, emulsion, suspension or paste materials into powder and granular materials. It is the effect of the liquid passing through the atomizer, spraying into mist droplets, and relying on the drying medium (hot air) to mix evenly with the mist droplets for heat exchange and mass exchange, so as to vaporize the water and remove the physically bound water in the material The process of combining water with machinery has the advantages of stable product properties, low degree of denaturation, continuous, automatic, and high operational flexibility.
Heat treatment during spray drying greatly reduces the activity of trypsin inhibitors, thereby improving protein digestibility. Practice has proved that the use of a good spray drying system can help to better improve the functionality of soybean protein isolate, thereby improving the viscosity, density, color and flavor of the product.
In addition, after spray drying, the hydrophobic groups originally embedded in the soybean protein isolate were exposed, the surface hydrophobicity increased, a large number of thermal aggregates were formed, and the solubility and emulsification decreased. With the increase of protein concentration, the volume fraction of thermal aggregates in soybean protein isolate after spray drying increases, the particle size increases, and the solubility decreases. Lower air inlet temperature can reduce the degree of denaturation and help maintain the functional properties of the product.
Some scholars have studied that cyclodextrin is used as an additive to assist protein refolding. Therefore, while lowering the temperature, a certain amount of additive can be added to reduce the denaturation of soybean protein.
(1) Morphology of soy protein isolate cold gel
(2) Gel strength of soy protein isolate cold gel
Compared with traditional heating pretreatment, the two combined high-field-strength ultrasonic-heating pretreatments could significantly (P<0.05) enhance the water holding capacity and gel strength of soybean protein isolate cold gel. The gel strength of the cold gel prepared by process 1 (20 kHz, 400 W, ultrasonic for 0, 2, 4, 10 min and then heated for 20 min) gradually increased with the increase of ultrasonic time (gel strength by (5.83±0.31) g increased to (46.37±1.15) g, while the gel strength of cold gels prepared by process two (heating for 20 min and then ultrasonication for 0, 2, 4, and 10 min) rapidly increased within a short ultrasonic time (within 4 min). Increase (gel strength increased from (5.83±0.31) g to (37.57±2.57) g.