Variations of Quality and Nutrients of Soybeans when Boiled

Soybeans are a commonly eaten in salads and as snacks, and are sold in their pods or shelled both fresh and frozen. This study was designed to understand what happens to the nutrients of soybeans when they are boiled at varying amounts of time. Changes in the content of enzyme levels, color, texture, sugar and mineral content due to steaming or blanching in an electric stove or kettle, were estimated. Observations showed that blanching sufficiently inactivated enzyme levels, or trypsin inhibitor activity (TIA), and increased color, texture, sucrose and mineral levels.

Raw soybeans contain high amounts of inhibitors of an important digestive enzyme — trypsin inhibitors. These lead to difficulty in digestion if consumed uncooked. Processing soybeans by soaking and then blanching or cooking, has been found to decrease the TIA by 80 percent. Also, cooking has been proven to enhance protein efficiency and reduce unwanted flavors. The changes occurring to the nutritional content of soybeans by cooking is influenced by the duration and temperature of cooking. This is due to the differential heat capabilities of steam, gas flame, or electric stove. This study attempted to understand the ideal time-temperature combination for effectively cooking soybeans while maintaining maximum nutrition.

* Of the two different soybean samples (comprised of whole pods as well as seeds alone), one (2006 crop) was subject to cooking in boiling water on an electric stove; while the second (2007 crop) was blanched using a steam-jacketed kettle.
* Cooking and blanching times in boiling water were noted. Samples for nutrition analysis were taken at raw, five, 10, 15 and 20 minutes of cooking.
* Color and texture of both samples were measured.
* The seeds were dried, ground and assessed in laboratory for changes in TIA, contents of soluble sugars (such as glucose, fructose, sucrose, raffinose and stachyose), iron, nitrogen and calcium. The values were statistically evaluated.

* Only raw samples varied with genotype and environment of cultivation.
* When cooked on the electric-resistance stove, the TIA reduction was 88 percent at five minutes and 92 percent at 10 minutes. There was no further change thereafter. At five minutes, no change was noted in texture, glucose-fructose and iron. Color decreased, sucrose increased by 1.4 to 1.8 percent, calcium and nitrogen levels were enhanced. Only nitrogen and iron levels were unaffected by cooking condition; all others changed. At 20 minutes, texture and color reduced while sucrose increased substantially.
* When cooked in a steam-jacketed kettle, two minutes of blanching eliminated 80 percent of TIA, unaltered by further blanching; texture and color improved slightly. At one minute of blanching, shelled samples recorded insignificant decrease in sucrose, no variation in glucose-fructose, insignificant increase in calcium and nitrogen in seeds and iron in the pods. At a later time point, texture was unaffected, color changed, sucrose was similar to raw sample.

Next steps/Shortcomings
Several initial hypotheses were defeated in this study because of a varied assumption employed by the researchers. These hypotheses cannot be confirmed in this study because the criteria measured were ill designed. Only nitrogen was measured and not protein. Also, the mineral and nitrogen content in the cooking liquid was not estimated for validation of loss during cooking.

The difference in heating capacity between the electric stove (representing home cooking) and the kettle (representing industrial preparation), was observed to have a differential effect on the cooking process and duration. Blanching for just two minutes was sufficient to lower TIA, while retaining original texture, color and high sugar levels. It is recommended to use soybeans as pods while blanching to ensure retention of sucrose. However, the simulation of home cooking on the electric stove indicated compromise on texture, color and sucrose. With increase in cooking time, the pivotal nutrients were seen to decrease. This could be directly impacted by the end user’s taste requirements.

For More Information:
Quality Attributes of Vegetable Soybean as a Function of Boiling Time and Condition
Publication Journal: International Journal of Food Science and Technology, 2009
By Leandro A. Mozzoni; Pengyin Chen; University of Arkansas, Fayetteville, Arkansas

*FYI Living Lab Reports Are Summaries of the Original Research.
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