What exactly are these unpleasant “strange substances”? Are they mold, as many rumors suggest? If they really are mold, what should we do to prevent household spices from becoming moldy in this hot and humid weather?

Professor Cao Xianle, from the School of Biotechnology and Food Science at Jiangsu University (China), conducted several small experiments to uncover the real reason behind mold in soy sauce. He also shared some scientific and effective tips for preserving spices.

What are the foreign substances that appear in soy sauce?

Mainly mold.

Many people may have guessed that the foreign “film” resembling patches is actually mold. Laboratory tests confirmed that the white, gray, yellow, brown, or moss green fuzzy substances are indeed mold species, which thrive and grow rapidly after prolonged exposure to soy sauce.

Note: Some people tend to discard the white layer and continue using the rest. This is strongly discouraged. Just like with other moldy foods, once soy sauce shows signs of mold, it should not be consumed—regardless of whether it’s still within its shelf life.

Why does soy sauce go moldy just a few days after opening?

The main reason is improper storage after opening, which allows airborne mold to invade.

Soy sauce becomes moldy in much the same way as bread or fruit left out too long. Once opened, prolonged exposure to air gives mold in the environment the opportunity to use the food's nutrients to multiply—leading to spoilage, off smells, and quality degradation.

Soy sauce is a nutrient-rich environment, which mold "loves." Even if it looks fine to the naked eye, an unsealed cap or sauce residue around the bottle mouth allows mold to grow once exposed to air.

So after each use, make sure to dry the bottle mouth and tightly seal the cap immediately to minimize air contact.

Does humid, rainy weather increase the risk of mold in soy sauce?

Yes.

Mold thrives in warm and humid conditions—typical of the rainy season. To prove this, the research team used four incubators to simulate different seasonal conditions with varying temperature and humidity levels.

The results showed that mold grows rapidly when the temperature exceeds 25°C and humidity is above 85%—ideal conditions for the rainy season.

High humidity also dilutes soy sauce, making it more susceptible to mold contamination.

Therefore, during the hot and humid rainy season, keep your kitchen clean and dry, pay close attention to cleaning the bottle mouth, and wipe it dry immediately if it gets wet.

Could soy sauce be moldy right from the factory?

No.

Some people worry, “Could soy sauce already be moldy when it leaves the factory?” – This is not possible with reputable manufacturers, because:

• Before bottling, soy sauce is typically pasteurized at over 90°C. Mold dies at temperatures above 50°C.

• Once sealed, with no exposure to air, mold cannot survive inside the bottle.

The golden formula: High-temperature sterilization + airtight sealing = no mold growth.

Additionally, in a verification experiment involving 9 common soy sauce brands (each tested with one opened and one unopened bottle in a mold-prone environment), the opened bottles developed mold while the sealed ones remained mold-free—proving that contamination happens only after opening.

Is soy sauce without preservatives or with low salt content more likely to get moldy?

Yes.

Many people prefer “green” products without preservatives or with low salt, but these are more vulnerable to mold and require stricter storage—ideally in the fridge.

Preservatives and salt help extend shelf life. Without them, mold can grow easily unless the product is kept cold and sealed.

Is it only soy sauce that gets moldy?

No! Other grain-fermented seasonings can also get moldy if not stored properly.

Products like soy sauce, fermented bean paste, oyster sauce (especially those made from real oysters), ketchup, etc., are all prone to mold. These products are rich in protein, amino acids, and minerals—perfect “food” for mold.

In a simulated “open environment” experiment, soy sauce, oyster sauce, bean paste, and ketchup all developed mold. Only vinegar and cooking wine (due to their high acidity and alcohol content) did not show visible mold.

Still, vinegar and cooking wine should be stored properly—away from sunlight and heat, in a cool and dry place.

How to store spices at home to avoid mold?

The most important rule: Keep bottles tightly closed, and bags securely tied!

Three key steps for proper storage:

1. After use, clean the bottle or bag mouth.

2. Seal the cap tightly or tie the bag securely—any exposure can lead to mold.

3. Carefully read and follow the storage instructions on the label—legitimate products always provide clear guidance.