Isoflavones are compounds found in soybeans that have powerful antioxidant characteristics and have been shown to prevent cancer and other diseases. Synthetic antioxidants are used as food preservatives to retard rancidity. Because of their superior health benefits, soy isoflavones have been identified as alternative, natural antioxidants to prevent rancidity in food fats.

View photos of the microwave extraction system and a schematic design.

Microwave-assisted extraction has proved to be a fast, cost-effective, energy-efficient and environmentally friendly process for extracting vegetable oils from various agricultural products. Continuous and batch microwave processing accelerate the extraction process by using heat and pressure to rupture cells. However, the potential of using a continuous, microwave-assisted extraction system for soybean oil on a commercial scale has not been studied in detail.

LSU Ag Center researchers are investigating the extraction of soy isoflavones from soy flour using microwave-assisted extraction and testing the ability of isoflavone extracts to prevent rancidity of fish oil.

To conduct these experiments, a continuous microwave extraction system and a separation system were built. Soy flour and a solvent were mixed together and pumped continuously through the microwave where the mixture was heated to temperatures necessary to extract the oil. Following heating and extracting soybean oil from the flour, the spent flour was separated from the solvent-oil mixture. Then the solvent was removed, and the oil was collected. Oils were extracted using different solvents and exposure times, and the three most prevalent isoflavones were measured.

Of the different solvents tested, ethanol was most effective in extracting total isoflavones. When ethanol was used as a solvent for longer extraction times at 153 degrees Fahrenheit, the yield increased by 330 percent, compared to conventional solvent extraction. Based on the results of the performance tests conducted using the laboratory-scale system, researchers developed and optimized a pilot-scale, microwave-based extraction process. With the pilot-scale system, about 98 percent of the total extractable oil in the soy flour could be recovered at extraction times shorter than six minutes.

The levels of total antioxidant phytochemicals and isoflavones in the defatted soy flour extract and in the soy oil also were compared. The defatted soy flour extract had a much higher level of phenolic compounds and isoflavones than the oil extracted with hexane by a conventional method. The defatted soy flour extract’s capabilities in preventing fish oil oxidation were significant. Hexanal, the major rancid substance produced from fat oxidation, was significantly reduced in the presence of the soy flour extract during four days of storage at room temperature. The soy isoflavone extract was capable of stabilizing menhaden oil and preventing oxidation during both heating and room-temperature storage. These results indicate that the extract could significantly retard oxidation of food fats during cooking or storage while also preventing the production of undesirable and toxic oxidation products.

This research shows that continuous microwave extraction is a viable method for efficiently extracting from soy flour and defatted soy flakes total isoflavones with antioxidant properties at higher yields than conventional, solvent-extraction methods. The process also used less solvent and decreased extraction times. The results will be useful to the soybean industry, which now has an alternative, more efficient method for extracting isoflavones from soy products and byproducts. The newly developed method has tremendous potential to revolutionize the way bioactive components are extracted from soybeans on an industrial scale, adding value to the Louisiana soybean industry.

Acknowledgment
The Louisiana Soybean and Grain Promotion Board for assistance with funding.

Cristina M. Sabliov, Associate Professor,Department of Biological & Agricultural Engineering, LSU AgCenter, Baton Rouge, La.; Zhimin Xu, Associate Professor, Department of Food Science, Baton Rouge, La.; Dorin Boldor, Associate Professor, and Marybeth Lima, Cliff & Nancy Spanier Alumni Professor, Department of Biological & Agricultural Engineering.

(This article was published in the spring 2011 issue of Louisiana Agriculture magazine.)