Application of Yingtai Vacuum Freeze-Drying Technology in Soybean Extraction
The protein content of soybeans ranges from 38% to 46%, and they contain a comprehensive variety of amino acids, making them one of the most important plant-based protein sources. Soybean peptides are small protein fragments or amino acid chains with a molecular weight below 6,000 Da and a length of 2–20 amino acids, obtained through hydrolysis, separation, and purification of soybean protein. Due to their low viscosity, high solubility, good water absorption, and low osmotic pressure, small-molecule soybean peptides are more easily digested, absorbed, and utilized by the body compared to soybean protein. They exhibit superior and broader biological activities, making soybean peptides a research hotspot in functional foods, pharmaceuticals, and feed industries.
Introduction to Soybean Protein
Soybeans are a crucial food crop, with seeds rich in oil and protein, widely used in medicine and food. On average, soybeans contain about 40% protein and 20% oil on a dry matter basis. In Asian countries, soybean protein is a major protein source and can be processed in various ways, such as tofu, miso (fermented soybean paste), natto (fermented soybeans with a sticky coating), soy milk, and fried tofu (deep-fried tofu slices). The introduction of fibrous textures in tofu-based products can mimic the texture of meat. Additionally, researchers have gained new insights into the formation of “yuba”, a film-like soybean food made from heated soy milk, which contains oil bodies, granular proteins, soluble proteins, and carbohydrates.
Soybean protein is a mixture of multiple proteins, classified into four categories based on sedimentation coefficients: 2S, 7S, 11S, and 15S. It provides an excellent amino acid profile, including all essential amino acids, and contains physiologically beneficial components that can reduce cholesterol levels and lower the risk of hyperlipidemia and cardiovascular diseases. Moreover, it exhibits superior processing capabilities, such as gelation, emulsification, water retention, and oil retention.
Besides protein and oil, the byproduct remaining after oil extraction (soybean meal) serves as a high-quality protein feed for poultry, pigs, dairy cows, and beef cattle. Additionally, soybean isoflavones and their physiological benefits have attracted significant attention. Soluble soybean polysaccharides extracted from okara (the residue from tofu production) have been proven to be excellent emulsifiers and are widely used in the food industry.
Preparation of Soybean Peptides
The primary raw materials for preparing soybean peptides include soybean protein isolate (SPI), soybean meal, and soybean protein concentrate. Using SPI as a raw material offers advantages such as a protein content (dry basis) exceeding 90% and easier purification of the resulting peptides. However, the high cost of commercially available SPI increases production expenses. Therefore, enzymatic hydrolysis for soybean peptide production typically starts with low-temperature defatted soybean meal, which undergoes alkali extraction, acid precipitation, and spray drying to obtain SPI before further hydrolysis and purification.
Compared to enzymatic hydrolysis, fermentation methods for producing soybean peptides tend to directly use low-temperature defatted soybean meal as the raw material, with pre-treatments such as heating and grinding based on fermentation strains and process conditions.
1. Chemical Method
The chemical method emerged early and is relatively well-studied. It involves hydrolyzing soybean protein using acids or alkalis. However, the reaction conditions are harsh: alkaline hydrolysis is difficult to control, often leading to excessive hydrolysis into free amino acids, many of which are racemized (D-amino acids cannot be utilized by the human body). Acid hydrolysis, on the other hand, corrodes equipment, yields low levels of small peptides, and completely destroys tryptophan under high temperatures. This method is now nearly obsolete.
2. Enzymatic Hydrolysis
Enzymatic hydrolysis is currently the primary industrial method for producing soybean peptides. It operates under mild conditions, ensuring high safety of the hydrolysates. Different proteases have varying cleavage sites and mechanisms, leading to different hydrolysis effects on the same substrate. This enables industrial production of soybean peptides with tailored functional properties. Therefore, the selection of proteases and optimization of hydrolysis conditions are critical. Enzyme modification and genetic engineering of enzyme-producing strains remain key research areas worldwide.
Enzymatic hydrolysis can be classified into single-enzyme hydrolysis and composite-enzyme hydrolysis. In practice, composite-enzyme hydrolysis is more commonly used, combining different enzymes to enhance efficiency and improve the flavor of soybean peptides. Based on their optimal pH ranges, proteases are categorized into acidic, alkaline, and neutral proteases. Based on cleavage mechanisms, they are divided into endopeptidases and exopeptidases.
- Endopeptidases cleave peptide bonds within protein molecules, breaking them into smaller peptide fragments and exposing hydrophobic groups.
- Exopeptidases act on the terminal ends of proteins or peptides, releasing free amino acids. They are mainly used to remove hydrophobic amino acids from peptide terminals, reducing the formation of bitter peptides.
Yingtai Freeze-Drying Experiment
Sample: Soybean phospholipid hydrolysate
Objective: Complete removal of moisture via freeze-drying
Freeze Dryer Model: MP2 Pilot Scale
Procedure:
1. Pre-freeze the sample in a -80°C ultra-low temperature freezer for 4 hours.
2. Pre-cool the freeze-dryer, place the sample on the shelf, and start the vacuum.
3. Freeze-drying program:
- Sublimation temperature: 25°C–35°C
- Vacuum level: 1 Pa
- Duration: 48 hours (ensure complete sublimation)
4. After the program ends, release the pressure and retrieve the freeze-dried sample. Check if the moisture content meets the standard. If the experimental goal is achieved, store the sample or proceed with further experiments.
