Lifestyle | Health & Wellness

How Fermentation Affects the FODMAP Content in Soy Foods and Cabbage

FODMAP and Food Processing Series

Article 2: Part 2

The topic of this installment of my FODMAP and Food Processing series is how and why fermentation changes the FODMAP profile of soy foods and cabbage. You can also check out my previous article on fermented milk products and sourdough bread, as well as the first article in this series on the effect dehydration, ripening, and storage conditions have on the FODMAP content in fruit

As mentioned in both of my previous pieces, it’s perplexing to discover that the same food can differ in FODMAP concentration depending on the environmental conditions it has been subjected to. That’s why the scientists at Monash University always test (and retest) all items included in the Monash University Low FODMAP Diet Smartphone App, and why my discussion will be limited to foods found in this essential resource.

Fermentation 101

Fermentation is an ancient form of food processing that utilizes microorganisms such as bacteria, yeast and mold to break down the carbohydrate components in food. Sauerkraut and certain soy products, as well as yogurt, kefir, cheese and sourdough bread are all examples of fermented foods.

Homemade Lemon Dill Sauerkraut - organically grown on my home farm!

When discussing fermentation, the issue of probiotics and the microbiome almost always comes up. This article will not make reference to either, because they are immaterial to the theme of this series, which is how various food processing techniques alter the FODMAP content in food. FODMAP Everyday® will tackle the complex topic of probiotics at some point, but my colleague, Jody Garlick, has already written an excellent review of the microbiome that I’m sure you’ll enjoy.

Soy’s Got Culture

Decades ago the humble soybean was not nearly as ubiquitous in the U.S. as it is now, but it’s been a staple in other countries for ages due to its low cost and high protein content. When most Americans hear the word “soy,” they probably think of tofu or soy milk, and then struggle to conjure up more products made from this legume. While these items may be the most well known soy foods, I’ll save them for a future article in this series because their FODMAP transformation has nothing to do with fermentation.

Soy beans and Soy Milk - How Fermentation Affects the FODMAP Content in Soy Foods and Cabbage- FODMAPS and FOOD PROCESSING SERIES-
Soy beans and soy milk.
Fermentation Changes Everything

In its natural state, the soybean is a high FODMAP food – ½ cup (85 g) of boiled beans rates a Red Light in the Monash Smartphone App due to excessive galacto-oligosaccharides (GOS) and fructans (both from the “O” FODMAP group). Tempeh and miso, on the other hand get a Green Light even though they are largely, if not wholly, made from soybeans. I don’t think it will come as a surprise to learn that fermentation is the reason these items get a FODMAP pass.

Miso Soup - How Fermentation Affects the FODMAP Content in Soy Foods and Cabbage- FODMAPS and FOOD PROCESSING SERIES-
Photo of Miso Soup by Satsuki.
Miso Is More Than Just Soup

Miso is the salty seasoning that gives the classic Japanese soup its rich flavor. Miso is typically made from soybeans that have been cooked and combined with salt and a type of mold called Aspergillus oryzae, or koji. This mushy mixture is then stashed away for as long as three years, allowing ample time for the fermentation process to yield a tasty low FODMAP product that has a multitude of culinary applications. Here are a few miso recipes that we’re particularly fond of here at FODMAP Everyday®:

How To Look For Low FODMAP Miso

Only one unbranded miso product is included in the Monash Smartphone App, so my advice is to look for products that have been aged for a long period of time and contain no other high FODMAP ingredients other than soybeans; we like this one from South River Miso Company. I also suggest that you start with the amount listed in the Monash Smartphone App, which translates to 12 g or 2 American teaspoons.

Tempeh: Meatless Protein

Tempeh is one of my hardest sells as a nutritionist. People tend to be turned off by its funky, moldy appearance, which is actually a by-product of the microorganisms that are essential to its preparation. They also don’t know what to do with this staple of Indonesian cuisine. That’s too bad, because tempeh is a nutritional powerhouse, providing 19 grams of high-quality protein in a Monash-approved 3.53 ounce (100 gram) serving.

Tempeh- How Fermentation Affects the FODMAP Content in Soy Foods and Cabbage- FODMAPS and FOOD PROCESSING SERIES-

Tempeh is produced by soaking and cooking whole soybeans, inoculating them with a special type of mold, and then letting this mixture ferment for a day or two. This process lowers the FODMAP content in soybeans significantly, creating a dense product that’s perfect for slicing and adding to a stir-fry or making into high-protein “croutons” by marinating chunks and baking until crispy.

You can find tempeh in the refrigerated section of any health food store, and in many supermarkets as well. So, the next time you’re considering a meatless meal and tofu is just not cutting it for you, give tempeh a try. Do make sure to buy “plain” tempeh as it can also come with additional ingredients that are high FODMAP.

Green Cabbage- How Fermentation Affects the FODMAP Content in Soy Foods and Cabbage- FODMAPS and FOOD PROCESSING SERIES-
Green cabbage.
Cabbage is Confusing

Finally, this exploration of fermentation would not be complete without mentioning cabbage and its processed counterpart, sauerkraut. In 2017, Monash University scientists tested fresh red and white cabbage, as well as the sauerkraut made from each. The results were…interesting!

Both red and white cabbage tested low in FODMAPs when limited to 1 cup (about 90 g) servings. Larger portions of white cabbage contained excess sorbitol (from the “P” FODMAP group) while larger servings of red cabbage contained excess GOS and fructans. So far, so good, but here’s the interesting part: the fermentation process actually increased the FODMAP content in these veggies instead of lowering it, as it did for all the other foods discussed previously.

Red Cabbage- How Fermentation Affects the FODMAP Content in Soy Foods and Cabbage- FODMAPS and FOOD PROCESSING SERIES-
Red cabbage.

One cup of sauerkraut made from white cabbage received a Red Light rating due to high levels of mannitol (from the “P” FODMAP group), while 1 cup of red cabbage sauerkraut got a Yellow Light because it contained moderate amounts of fructans. In other words, fermentation not only increased the total amount of FODMAPs in sauerkraut made from white cabbage, it also changed the type of polyol from sorbitol to mannitol. In the case of the red sauerkraut, the fructans present in larger portions of red cabbage appear to have become more concentrated, while the GOS disappeared!

What to make of this?

Hard to tell, especially since Monash has not offered an explanation on their website. I did, however, discover that mannitol is one of the products formed during sauerkraut fermentation, so that helps explain the white cabbage results. As for red cabbage, this is pure speculation on my part, but perhaps the microorganisms involved in creating sauerkraut are capable of digesting GOS but not fructans, and the latter increases as a defense mechanism.

If you read my first article in this series, you may remember that fructan levels rise when a plant is subjected to environmental stressors. Could it be that the acidic conditions inherent in the fermentation process are just such a stressor? If anyone has a different explanation, please leave a comment below!

Want to learn more about this subject? Read the other articles in this series: