Does Fermentation Reduce Phytic Acid in Soy?

Spend any time reading about plant-based eating and you will bump into the word "antinutrient," usually attached to phytic acid. Soybeans contain a fair amount of it, so one of the most common questions we hear is whether fermenting soy into tempeh, natto or miso actually gets rid of it. The honest answer is more interesting than yes or no.

Last reviewed: October 2026 by the Haelan Publishing Team

Phytic acid (called phytate when bound to minerals, or inositol hexaphosphate) is the form in which plants store phosphorus in seeds, including soybeans, grains, nuts and other legumes.

What is phytic acid, and why is it called an antinutrient?

Phytic acid earns the "antinutrient" label because its structure carries several negative charges that bind positively charged minerals such as iron, zinc, calcium and magnesium into complexes the gut cannot easily absorb. A 2023 review in Frontiers in Chemistry describes phytate as polyanionic at intestinal pH, which lets it form insoluble salts with these minerals and lower their bioavailability, an effect most pronounced when a diet is both very high in phytate and low in trace minerals [1].

I like to think of phytate as a parking garage the seed builds for its own minerals: phosphorus, zinc and iron are locked inside until the seed sprouts. The attendant with the keys is an enzyme called phytase, which snips phosphate groups off the molecule and lets the minerals out. Humans make very little phytase, which is why a 2015 review in the Journal of Food Science and Technology points to processing steps that bring phytase to the food first, including soaking, germination and fermentation [2].

Soybeans sit at the higher end among legumes at roughly 1 to 2.2 percent phytate by weight, alongside kidney beans and above chickpeas and lentils [1]; a 2025 laboratory study measured 1.61 grams per 100 grams of dry matter in one raw cultivar [3]. For the other compounds fermentation changes, see our explainer on fermented vs. unfermented soy.

Does fermentation reduce phytic acid in soy?

Yes, fermentation can reduce phytic acid in soy, but the size of the reduction varies enormously between studies, and some report no meaningful change. The deciding factor is whether the fermenting microbe produces enough phytase under the conditions used.

Tempeh, made by growing the mold Rhizopus oligosporus through cooked soybeans, is the most studied case. A 2012 study in Food Science and Technology that made tempeh from four soybean cultivars found significantly lower phytic acid in only two of them and described overall content as "similar" between raw grains and tempeh. The authors cited a 2003 study reporting a roughly 30 percent reduction after 36 hours, and literature ranges of 1.0 to 2.3 grams per 100 grams in raw soybeans versus 0.69 to 0.96 in tempeh [4].

A 2025 study in Food and Bioprocess Technology is a useful reality check. Researchers cooked soybeans for 30 minutes, then fermented them with one R. oligosporus strain for 48 hours, and phytic acid did not change significantly. They contrasted this with published reductions of 10 to 66 percent and suggested their strain produced little phytase, that 48 hours was too short, or that the pH was wrong for the enzyme [3]. "Fermented" on a label does not guarantee a particular phytate level.

The same principle applies to natto, fermented with the bacterium Bacillus subtilis, and to miso and doenjang, which rely on koji, a culture of the mold Aspergillus oryzae: each organism can produce phytase, but activity depends on strain, time and temperature. A 2020 study in Plant Foods for Human Nutrition took a different route, making natto from a low-phytate soybean line, so the lower phytate came from the bean rather than the fermentation [7]. Our guide to tempeh vs. tofu covers what else fermentation changes.

How do soaking, cooking, sprouting and fermenting compare?

No single kitchen step removes most of the phytate in soybeans, but each contributes, and traditional preparation stacks several of them.

Method	How it works	What studies report
Soaking	Activates the bean's own phytase; some phytate leaches out	Modest reduction on its own [2][6]
Sprouting	The seedling switches on phytase to free its phosphorus	Consistently lowers phytate in grains and legumes [2][6]
Cooking	Heat degrades some phytate; a little diffuses into the water	About 20% after 60 minutes at 95°C (2025); little change before 40 minutes [3]
Fermentation	Molds, bacteria or yeasts secrete phytase	0% to roughly 66% in soybean studies; over 50% in sourdough and yeast breads [3][6]
Added phytase	Purified enzyme mixed in during processing	Can bring phytate close to zero; used in research and industry [5]

Traditional tempeh making touches most of these levers in sequence:

1. Soak the soybeans, often overnight, which begins to activate the bean's own phytase.
2. Dehull and cook the beans, which degrades a portion of the phytate.
3. Cool, inoculate with Rhizopus spores and incubate warm for 24 to 48 hours, when the mold may secrete more phytase.

Wondering whether heating a finished fermented food undoes its benefits? We looked at that in does cooking destroy probiotics in fermented foods.

Does phytic acid in soy block iron and zinc absorption?

Phytate does reduce how much iron and zinc your body absorbs from the same meal, and this has been measured directly in people. In a 1992 study in the American Journal of Clinical Nutrition, 32 adults ate test meals containing soy protein isolates with different phytate levels. Removing phytic acid (from about 4.9 to 8.4 milligrams per gram down to under 0.01) raised iron absorption four- to five-fold, yet even tiny residual amounts still inhibited absorption; a clear improvement appeared only below about 0.3 milligrams per gram, or roughly 10 milligrams per meal [5].

That study has an honest twist: with nearly all phytate gone, iron absorption from the soy meal was still about half that of the egg-white control, so the authors concluded that soy protein itself also lowers iron bioavailability [5]. Phytate is a big part of the story, not the whole story.

For zinc, scientists use the phytate-to-zinc molar ratio of a diet. A 2013 review in the Medical Journal of Australia reports that ratios above 15 have been associated with suboptimal zinc status, while leavening and fermentation typically bring the ratio below 12. It adds an easily missed point: wholemeal bread had half the zinc bioavailability of white bread yet delivered more absorbed zinc overall, because it contained more zinc [6]. Soy foods likewise supply iron and zinc, and soy is a complete protein, which is part of why they remain staples in so many traditional diets.

Is phytic acid actually bad for you?

For most people eating a varied diet, phytic acid is a minor factor rather than a problem, and researchers are increasingly curious about what else it does. The 2023 Frontiers in Chemistry review surveys small clinical trials exploring phytate's other properties, including antioxidant activity through metal chelation, and is candid that these trials are few, small (many with 8 to 35 participants) and short [1]. The practical question is less "how do I eliminate phytate" and more "does my diet supply enough iron and zinc, and am I preparing legumes in ways that help."

What the evidence does and does not show. The fermentation studies are laboratory experiments on specific cultivars, strains and protocols; results from one setup do not transfer automatically to the tempeh, natto or miso on a store shelf. Several measured phytate but not mineral absorption in people, so a drop on paper is not a measured improvement in iron or zinc status. The human absorption data come from single-meal isotope studies in small groups, which tend to exaggerate dietary effects compared with how people eat over months. The honest position is that fermentation often lowers phytate, the amount varies, and for most people it is one modest factor among many.

A quick note: this article is for educational purposes only and is not a substitute for individual advice. If you have questions about your iron or zinc status, or whether fermented soy fits your situation, talk with your healthcare professional.

Who should talk to a clinician before adding fermented soy?

Fermented soy foods are ordinary foods for most people, but a few groups have specific reasons to check in first. None of these relate to phytate itself.

Situation	Why it matters	Source
Taking thyroid medication (levothyroxine)	Soy can reduce absorption of the medication; Mayo Clinic advises an empty stomach and at least an hour before eating soy foods	[8]
Taking an MAO inhibitor	Fermented soy foods such as miso, soy sauce and soybean paste can contain tyramine, which people on MAOIs are advised to avoid	[9]
Soy allergy	Fermentation does not remove the allergen; the ACAAI lists miso, tempeh, tofu and soy sauce among foods to avoid	[10]
Managing low iron or zinc	Phytate and soy protein lower iron absorption from the same meal; timing and overall intake are worth discussing	[5][6]
Pregnancy, or a hormone-sensitive condition or related medication	Soy isoflavones are a separate topic from phytate; review dietary changes with your clinician	See does soy raise estrogen

Common questions

Does tempeh have less phytic acid than soybeans?
Often, but not always. A 2012 study cited literature ranges of 1.0 to 2.3 grams per 100 grams for raw soybeans and 0.69 to 0.96 for tempeh, yet found a significant drop in only two of four cultivars, and a 2025 study found no change with one Rhizopus strain. Strain, time and pH decide the outcome [4][3].

Does cooking soybeans remove phytic acid?
Partly. In a 2025 laboratory study, boiling soybeans at 95°C produced no significant change until 40 minutes and about a 20 percent reduction at 60 minutes, with only about 4 percent of the original phytate found in the cooking water. Cooking helps most when combined with soaking and, where applicable, fermentation [3].

What is phytase, and do humans make it?
Phytase is the enzyme that breaks phytic acid apart by removing its phosphate groups, releasing bound minerals. Plants switch it on during sprouting, and many molds, bacteria and yeasts secrete it during fermentation. Humans produce very little, which is why processing that brings phytase to the food before eating is the main way to lower phytate intake [2].

Is phytic acid in soy something to worry about?
For most people eating a varied diet, no. Reviews describe the mineral-binding effect as most significant when a diet is very high in phytate and already low in trace minerals. Soy foods also supply iron and zinc, and total mineral content in a meal often matters more than its phytate level [1][6].

Which fermented soy food is lowest in phytic acid?
There is no reliable ranking. Published values for tempeh, natto and miso vary widely because the microbes, soybean cultivar and conditions differ between studies. A 2020 study made lower-phytate natto simply by starting with a low-phytate soybean line, showing the bean can matter as much as the fermentation [7].

The bottom line

Fermentation can reduce phytic acid in soy, sometimes substantially, but it is not a guarantee: outcomes depend on the microbe, strain, time and conditions, and some careful studies found no change. Soaking, sprouting and cooking contribute too, and traditional recipes combine them. Phytate lowers iron and zinc absorption from a meal, which matters most in diets dominated by unrefined grains and legumes. The reasonable takeaway is to enjoy well-prepared soy foods as part of a diet that supplies minerals from several sources.

That is the spirit in which we approach fermented soy at Haelan. Haelan 951 is a concentrated organic, non-GMO fermented soy beverage made with a proprietary nitrogen fermentation process, classified by the FDA as a food rather than a supplement. It contains the naturally occurring plant compounds found in soy, including isoflavones, saponins and phytosterols. Serving suggestions are on the product label, and your healthcare professional is the right person to ask if any of the considerations above apply to you.

Discover Haelan 951 →

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References: [1] Pires SMG et al. Phytates as a natural source for health promotion: A critical evaluation of clinical trials. Frontiers in Chemistry, 2023.  [2] Gupta RK, Gangoliya SS, Singh NK. Reduction of phytic acid and enhancement of bioavailable micronutrients in food grains. Journal of Food Science and Technology, 2015.  [3] Gbedo C et al. Impact of Cooking and Rhizopus oligosporus Fermentation on Antinutritional Factors and Isoflavones in Soybeans. Food and Bioprocess Technology, 2025.  [4] Bavia ACF et al. Chemical composition of tempeh from soybean cultivars specially developed for human consumption. Food Science and Technology, 2012.  [5] Hurrell RF et al. Soy protein, phytate, and iron absorption in humans. American Journal of Clinical Nutrition, 1992.  [6] Saunders AV, Craig WJ, Baines SK. Zinc and vegetarian diets. Medical Journal of Australia, 2013.  [7] Qin D, Hara Y, Raboy V et al. Characteristics and Quality of Japanese Traditional Fermented Soybean (Natto) from a Low-phytate Line. Plant Foods for Human Nutrition, 2020.  [8] Mayo Clinic Staff. Expert answer on soy and levothyroxine absorption. Mayo Clinic, 2023.  [9] Mayo Clinic Staff. MAOIs and diet: Is it necessary to restrict tyramine? Mayo Clinic.  [10] American College of Allergy, Asthma & Immunology. Soy Allergy.

This content is for educational purposes only and is not intended to diagnose, treat, cure, or prevent any disease. Always consult your healthcare professional before making changes to your diet or health routine.