The Importance and Methods of Soybean Oil Extraction

Soybeans are a common oilseed crop in daily life and a major source of edible vegetable oil and plant protein. The oil content in soybeans ranges from 15% to 22%, which is much lower than that of peanuts and rapeseeds. However, the protein content in soybeans is between 35% and 50%. Soybean meal, which is left after extracting soybean oil, can be used to separate soybean protein, making soybeans highly valuable in the grain and oil industry.

Due to their low oil content, soybean oil can be efficiently extracted using a single solvent extraction process, with the residual oil in the extracted meal being less than 1%. The soybean meal can be used for protein extraction, animal feed, and other applications. Additionally, lecithin can be extracted from the refined oil foot for pharmaceutical use. These products have a higher value than soybean oil itself, which is considered a byproduct of the production process. This is one reason why soybean oil is priced lower in the market compared to other edible oils.

The consumption of soybean oil has been increasing annually, and there are many oil production manufacturers in the market. Research on soybean oil processing technology is extensive both domestically and internationally. The methods for oil extraction can be classified into physical methods, chemical methods, biological methods, and hybrid methods. However, the most commonly used methods in industrial production are physical and chemical methods.

The physical method refers to the pressing method, which has a long history in China. Its principle is to directly separate oil from the raw material by applying pressure. Historically, this process relied on human or animal power, resulting in large land occupation, low production efficiency, and high labor intensity. It has now been largely replaced by mechanical methods.

The mechanical pressing process for soybean oil includes cleaning, drying, crushing, sieving, pressing, and filtering to obtain crude soybean oil, retaining the original flavor of soybeans. Oil plants can choose between cold pressing and hot pressing methods depending on whether heating and cooking are applied before pressing. Generally, cold-pressed soybean oil has a relatively higher linoleic acid content, but hot-pressed oil has a higher oil yield.

The chemical method refers to the solvent extraction method, where an appropriate solvent is used to extract the oil. This mature oil extraction process can achieve large-scale production and is widely used in the industrial production of soybean oil. Mechanical pressing requires significant external force, making it difficult to break down the oilseeds and extract the oil. However, the solvent extraction method effectively addresses this issue, with good solubility between the solvent and oil, resulting in a residual oil rate in the extraction meal as low as 1%.

The widespread application of the solvent extraction method for producing soybean oil is due not only to its high oil yield but also to its ability to preserve soybean protein in the meal. Protein tends to denature at high temperatures, and the pressing method involves multiple pressing and higher production temperatures to increase oil yield, which adversely affects protein preservation and meal utilization. Therefore, most oil plants use the solvent extraction method to extract soybean oil, as it better preserves the protein in the meal.

Regardless of whether soybean oil is extracted by pressing or solvent extraction, consumers are primarily concerned with the oil quality. To ensure the quality of soybean oil, China has set clear standards for indicators such as acid value, color, and peroxide value. In actual sales, some consumers have observed that soybean oil purchased from supermarkets darkens in color during storage. This occurs due to factors such as light exposure, storage temperature, and the acid value of the oil increasing during logistics, storage, and usage, resulting in acidification and color change, which can impact the reputation of oil plants among consumers.