When traveling, experiencing local snacks is a must. If you go to Beijing, you may go to Qianmen Dashilan, Huguo Temple or Longfu Temple to experience the flavor of the old imperial city. There you will see a snack called bean juice, served with burnt rings and pickles, which is a bit like "soy milk and fried dough sticks".

However, I would like to give you a friendly reminder that soybean juice is a dark dish for people who cannot accept its taste. It is not easy to drink a bowl of soybean juice calmly for the first time.

This is because Old Beijing bean juice has a sour smell similar to swill, and people who don’t like to drink it often describe it as “rotten soy milk.”

Will the soy milk at home turn into soy milk if it goes rancid? Not really. In fact, it is the leftovers for making mung bean starch, vermicelli, and vermicelli, and it is completely different from the soy milk you drink for breakfast.

It is said that bean juice has been popular among the people as early as the Liao and Song Dynasties, and even entered the imperial dining room during the Qianlong period of the Qing Dynasty. The general production process is that mung beans are first soaked, ground, and filtered to remove coarse bean dregs, then filtered and fermented to allow starch to precipitate to obtain raw bean juice, which is then boiled to become cooked bean juice.

Bean juice is a naturally fermented soy product. Scientists collected samples from Dongzhimen Doujiu Store, Longfusi Doujue Store, and Lao Ciqikou Bean Juice Store and identified the microbial composition through DNA sequencing.

It was found that the main fermenting microorganism is actually Lactococcus lactis. In addition, various microorganisms such as Lactobacillus, Streptococcus, and Acetobacter participate in the fermentation process of soybean juice.

Although the fermentation of soy milk is similar to that of yogurt, its flavor is far more complex than that of supermarket yogurt due to the presence of miscellaneous bacteria. After testing with modern scientific instruments, there are at least 40 kinds of flavor substances in soybean juice, mainly various alcohols, followed by organic acids, aldehydes, and ethers.

Some old Beijingers particularly like to drink soy juice, and they also like to go to specific bean juice shops to drink it. There is also a scientific reason for this. Different stores have different fermentation processes and fermentation environments, resulting in different fermentation microorganisms and different flavor substances produced by fermentation.

For example, the soybean juice from Huguosi has more alcoholic substances, while the soybean juice from Laociqikou is rich in aldehydes.

The sour smell in soybean juice is mainly caused by organic acids, sulfur-containing compounds, furans and other substances, especially methyl mercaptan, methyl sulfide, and furan. It only takes a very low concentration to make you smell it. A particularly "exciting" taste.

The good news is that after boiling the cooked bean juice, many of the sour and odorous components will evaporate, and the remaining flavor substances are mainly organic acids and aldehydes, and the taste will become relatively "softer".

If you still can't accept the taste, you can also try another traditional snack - "Ma Tofu". It is a by-product of boiling soybean juice and is fried in mutton oil. Its taste is much milder than that of soybean juice.

Although soybean juice smells like rancid water, it is actually quite safe because the pH value drops rapidly during fermentation by lactic acid bacteria, making it difficult for pathogenic bacteria to survive. As a fermented food, soy milk is also very nutritious and easy to digest, making it a healthy food. For example, its protein content can reach more than 4 grams per 100 ml, which is "higher protein" than most milk. Of course, if you're like me and can't tolerate soy juice because of the taste, you'd better just drink milk.

Text/Zhong Kai (Doctor of Food Safety)