Given soy’s ubiquity in our lives and diets, I thought it would be instructive to explore how those fields of beans become nondescript additives creeping into so much of our Western diet. “Processed foods” is a bit of a black box but by looking at this one food source I can show you that it actually supports massive swaths of our agricultural and food systems, and a bit of what this means for our diet.
Soybeans are legumes native to East Asia but the United States has long embraced this staple crop and is, along with Brazil, the leading global producer of soybeans. USDA research projects that more than 84 million acres will be harvested annually through 2026, surpassing corn in harvested acreage. This is primarily due to demand increases domestically and globally, especially in China, which can’t meet its consumption with domestic supply. Most production is genetically modified (GM) soybeans engineered to be insensitive to glyphosate, allowing increased efficiency and yield by reducing nutrient depletion of the soil by weeds. Other traits are selected in both GM and conventional crops that increase the value of the agricultural product. Most soy (85%) cultivars are oilseed, which is ultimately processed into meal for animal feed and oil. Whereas the vast majority of soybean meal (98%) is used as animal feed, the vast majority of soybean oil (95%) is used in food. Other cultivars used for food, meal, or oil vary by their macronutrient content, but typically contain more protein and less fat than oilseed.
Domestic US soy is planted between May and July, as it grows well in warm temperatures and is tolerant of a wide range of soil conditions. Seeds are planted in rows and begin sprouting within a week. In late summer the plants bloom into white or purple flowers, which eventually produce the seedpod. During growth, soybeans (and other legumes) depend on a proteobacteria called Rhizobia that fix nitrogen inside the root nodules. Many farmers specifically inoculate their crop with these diazotrophic bacteria to increase crop productivity. As the end of the year approaches, the leaves yellow and die leaving only the mature seedpod. Seedpods are harvested using a combine, and at this point the mature crop can be dried and stored or processed in a variety of ways.
The fate of an oil seed crop is as follows. Once grown, seeds must be cleaned, optionally cooked, and separated from their skins, which are called hulls. The hull is about 12% protein and is considered a by-product of the oil extraction process. Hulls are separated by physical aspiration and separation, then roasted and milled for use as a feeding supplement for livestock. Dehulled soybeans are heat-dried, rolled into flakes, and then enter a solvent mill where they are mixed with an organic solvent (typically hexane) to extract the oil for use in other products. The solvated soybean oil is then distilled to get crude soybean oil. Crude soybean oil is degummed of its lecithin, the phospholipid-rich amphipathic fraction of the crude, which is then used as an emulsifier in processed foods. Recently some other derived emulsifiers were determined to be causal agents in low-grade gut inflammation, and administration of carrageenan, carboxymethylcellulose (two other gums derived natural sources), or polysorbate-80 is used to establish colitis animal models of irritable bowel disease1,2. However, the aforementioned emulsifiers are large, polymeric molecules quite different in structure (if not function) from soy lecithin, and the levels of emulsifiers found to cause inflammation may be well below their percent abundance in food products. Soy lecithin is generally recognized as safe and permitted for food use in both the US and EU.
Degummed raw soybean oil can then be used in other processes, but for food use the final steps are neutralization, bleaching, and deodorizing. Refined as such, soybean oil is then processed further for its various destinations, such as blending for use as vegetable oil. One controversial processing for vegetable oil is hydrogenation, which involves the addition of hydrogen atoms to the unsaturated content of the oil, converting it to saturated fat. Fats with a higher saturated content, such as butter, margarine, or shortening are more desirable for certain cooking and baking applications. A major problem with hydrogenation is it produces trans-fatty acids byproducts, which are associated with increased cardiovascular mortality. Hydrogenation also stabilizes the bulk product against rancidification by converting oxidation-prone omega-3 linolenic acid (ALA). However, ALA is also an essential fatty acid and one of the healthiest components of the oil. Apart from the proven risks associated with trans fats, the conversion of an essential fatty acid into a saturated fatty acid is yet another detriment to their nutritional value. One solution to avoid hydrogenation has been the development of soybean cultivars with low ALA and other omega-3 content. DuPont accomplished this in a genetically-modified strain DP305423 with a silenced desaturase gene, halting the production of ALA in the plants; likewise, a low-ALA strain called Vistive Gold will be available from Monsanto in 2018. My solution is to simply avoid most processed foods, which are the predominant destination of hydrogenated oils.
Soybean meal is the non-oil component left after solvent extraction. The vast majority, in various formulations (with/without hulls, fat content, etc.) is used as animal feed for poultry, swine, cattle (beef and dairy), and fish. However, soybean meal can be milled finer for use in other soy-based products such as soy flour. Milled soy is emulsified into water and refined to a desired consistency and flavor to produce soymilk. Soy protein concentrate and isolate involves additional washing of milled soy with alcohol to remove the carbohydrate portion, leaving only the protein.
Textured vegetable protein (TVP) has been a ubiquitous form of soy in processed foods dating back to the 1960s, especially meat substitutes. Soy flour or concentrate is extruded into small different shapes depending on the application, in a similar fashion to how pasta is made. As the condensed, heated proteins exit the extruder they puff up into less dense flakes, which store well and can be rehydrated later. In addition to being used as a meat substitute, TVP is also used as an extender to increase the protein content of meat products or dishes.
Not all soy foods are come from processed intermediates. Another preparation of soymilk is to soak, puree, and finally heat whole soybeans to a desired consistency. From soymilk, tofu can be produced by ionic or acidic coagulation followed by compression of the curds. Conversely, tempeh is made by acidic fermentation of precooked soybeans with a fungus of the Rhizopus genus. Likewise, soy sauce is made from fermented boiled soybeans with Aspergillus fungus. Therefore, most of these products are minimally processed foods incorporating the bulk of the soy content.
Black soybeans are heavily pigmented and used extensively in Eastern cuisine. In China, fermented black soybeans are called douche, used as the base for many sauces. Interestingly, one study comparing 5 Korean cultivars of soybeans found that the one with a black seedpod, Cheongjakong 3, contained the only detectable anthocyanin content of the cultivars, but also the lowest isoflavone content3. Take that for what you will, I don’t know that it’s meaningful.
Soybeans flow into multiple aspects of our food economy. They are the feedstock for commercial meat, dairy and fish production. Soy’s rotation with cereal grains enriches the nitrogen content of the soil and increases the yield of cereal crop. Prepared foods like soymilk, tofu, and tempeh can substitute dairy and meat products, but TVP and soy protein are probably in more products than you’d assume. Finally, soybean oil is one of the most widely-consumed cooking oils and is used to produce not just margarine and shortening, but also biodiesel and many commercial products that are outside the scope of this post.
My takeaway is that processed food is not so much a problem of it’s composed of, but what it’s lacking. Each step of soybean processing, such as the removal or hydrogenation ALA, or separation of the fibrous hull, renders a product which is more desired for shelf life or physical properties than nutrition. But I wouldn’t call any of those products “food” on their own. Personally, this is one of my main motivations to consume a diet that is more focused on whole foods, so I don’t miss out on nutrients that are removed for aesthetic or textural concerns in granola bars or microwave burritos.
Finally, note that I touched on some bacterial and fungi actors in the soy story here – the diazotroph that fixes nitrogen for the plant’s roots, as well as the molds used to produce fermented soy products. It’s about time I mentioned a bit about the microbiome on this blog, and that’s what my next post will be about. Stay tuned!
Web resources that were helpful in composing this post:
- Chassaing B, Koren O, Goodrich JK, et al. Dietary emulsifiers impact the mouse gut microbiota promoting colitis and metabolic syndrome. Nature. 2015;519(7541):92-96. doi:10.1038/nature14232.
- Martino JV, Van Limbergen J, Cahill LE. The Role of Carrageenan and Carboxymethylcellulose in the Development of Intestinal Inflammation. Front Pediatr. 2017;5:510. doi:10.3389/fped.2017.00096.
- Kim EH, Kim SL, Kim SH, Chung IM. Comparison of isoflavones and anthocyanins in soybean [Glycine max (L.) Merrill] seeds of different planting dates. J Agric Food Chem. 2012;60(41):10196-10202. doi:10.1021/jf3031259.