How leftover tofu liquid can help make the next batch
做豆腐剩下的水，为什么还能帮助做豆腐？
做豆腐时，凝成的豆花和水分会逐渐分开。
During tofu making, the curds and liquid gradually separate.
这些分出来的水，并不一定到这里就完成了任务。
The separated liquid does not necessarily end its useful life there.
在中国的一种传统做法中，它经过发酵变成酸浆，又被用来让下一批豆浆凝固。
In one traditional Chinese method, it is fermented into a sour liquid and used to coagulate the next batch of soy milk.
这像一个小循环：上一批豆腐留下的液体，成了下一批的帮手。
It forms a small cycle: liquid left from one batch helps make the next.
不过，真正需要解释的是，酸浆为什么能把流动的豆浆变成凝块。
The question is why the sour liquid can turn flowing soy milk into curds.
2019年的一项研究分析了酸浆中的微生物，并分离出一种产酸能力强的乳酸菌。
A 2019 study examined the microbes in the sour liquid and isolated a lactic acid bacterium with strong acid production.
研究人员发现，酸是促进豆浆蛋白质凝固的主要因素，其他成分也有帮助。
The researchers found that acid was the main factor promoting soy protein coagulation, with other components also contributing.
豆浆加热后，蛋白质的结构发生变化。
Heating soy milk changes the structure of its proteins.
加入酸浆会改变酸碱条件，减弱蛋白质之间由电荷产生的相互排斥。
Adding the sour liquid changes the acidity and weakens electrical repulsion between proteins.
蛋白质因此更容易靠近、聚集，形成含有水分的凝胶。
The proteins can then approach and gather more readily, forming a gel that contains water.
这里的豆腐不是把豆浆里的水全部烧干后留下的固体。
This tofu is not the solid residue left after boiling away all the water in soy milk.
关键是让蛋白质改变组织方式，而酸浆把前一批的副产物带回了这个过程。
The key is changing how proteins organise themselves, while the sour liquid brings a by-product of the previous batch back into the process.
