Those new to the gluten-free diet are often surprised to learn that soy sauce and vinegar can contain gluten, and can raise questions of safety for people with a gluten-related illness. Both of these pantry staple items have in common that they require fermentation reactions in their production to achieve their final products.

Fermentation basically involves the addition of a bacterial culture or yeast to the raw ingredients, which feed on the carbohydrates in the ingredients, converting them into alcohol or acid. Different bacterial microbes and yeasts perform slightly different fermentation reactions, which is why even though beer and wine, yoghurt, and vinegar all undergo fermentation, they have vastly varying tastes and qualities.

Soy sauce and some varieties of vinegar are made of gluten-containing raw ingredients. As such, scientists recently looked into whether the fermentation reaction might have some impact on the amount of gluten present in the finished product, with interesting results.

Soy sauce is composed of two main ingredients: soybeans and wheat. The traditional Japanese method of producing soy sauces involves these two ingredients, with the addition of brine (salt water), undergoing two fermentation processes2. The second of this fermentation process can take as long as 6 months to complete2. After this time, filtration and refinement occurs, resulting in the sauce that we know so well.

Like soy sauce, vinegar typically undergoes two fermentation processes, firstly involving alcohol (ethanol) production, and then further digestion of the alcohol to produce acetic acid4, giving the condiment its sour taste. Plain white distilled, balsamic, rice, and apple cider vinegar varieties are all typically gluten-free and safe3. However, malt vinegar is the problematic one to watch out for, due to malt being derived from barley, a known gluten-containing grain.

Could fermentation reduce the gluten content of these products?

An Australian study published in 2018 examined how fermentation in the making of these foods might influence gluten content. Researchers used protein detection methods at various stages during the manufacture of GF labelled vinegar and soy sauces, and regular malt vinegar and soy sauces that had barley and/or wheat as labelled ingredients1.

Expectedly, the gluten-free labelled soy sauces and distilled vinegar contained no detectable gluten (below 1mg/L). Malt vinegar was confirmed as unsafe with significant gluten proteins detected in the sample, so you can definitely rule it out from your gluten-free pantry.

The twist in the results came in with the testing of regular soy sauces. After the second fermentation reaction, gluten proteins were only detectable in very low concentrations, below the threshold of the testing methods. So despite wheat being listed as a primary ingredient, the presence of the gliadin protein was surprisingly low.

Long fermentation processes, along with distillation in the case of white vinegar, may be a reason for low detectable gluten content in these samples. However, it is nearly impossible to know how long a product has undergone fermentation for, and as such gluten content will likely depend on individual manufacturing. So for GF consumers, it is certainly safest to stick to specifically gluten-free products.

Luckily, there are GF alternatives available…

You might have come across tamari, a sauce that is becoming more widely available in supermarkets and grocers. Tamari sauce is also produced by fermenting soybeans, either with less than 10% wheat added, or no wheat for gluten-free varieties. Perfect for dipping sushi in, or creating a delicious stir-fry sauce, it is an ideal alternative to soy sauce if you are on the spectrum of gluten-sensitivities. And as mentioned previously, most varieties of vinegar are considered gluten free with the exception of malt vinegar, so it is generally pretty safe to include in your gluten-free cooking.

As always, make sure to double-check labels before purchasing and consuming any product, and when dining out always be conscious of dishes where malt vinegar or soy sauce might be hiding in sauces and marinades. Remember to keep a bottle of GluteGuard handy when heading out for a meal to help reduce the occurrence of symptoms of medically diagnosed gluten sensitivity caused by inadvertent gluten ingestion.


  1. Li, H., Byrne, K., Galiamov, R., Mendoza-Porras, O., Bose, U., Howitt, C.A., & Colgrave, M.L. (2018). Using LC-MS to examine the fermented food products vinegar and soy sauce for the presence of gluten. Food Chemistry, 254, 302-308. doi: 10.1016/j.foodchem.2018.02.023
  2. Cao, W., Watson, D., Bakke, M., Panda, R., Bedford, B., Kande, P.S., Jackson, L.S., & Garber, E.A.E. (2017). Detection of Gluten during the Fermentation Process To Produce Soy Sauce. Journal of Food Protection, 80(5), pp. 799-808. doi: 4315/0362-028X.JFP-16-483
  3. Baker Heart & Diabetes Institute (2019). Gluten Free Supermarket Shopping Guide. Retrieved from
  4. Lynch, K.M., Zannini, E., Wilkinson, S., Daenen, L., & Arendt, E.K. (2019). Physiology of Acetic Acid Bacteria and Their Role in Vinegar and Fermented Beverages. Comprehensive Reviews in Food Science and Food Safety, 18, 598-601. doi: 10.1111/1541-4337.12440

Author: Cass, Glutagen