I’m learning how to make red bean (hong dou, or azuki) soup today, and it’s a rather simple recipe that follows the main principles of bean-cooking. What struck me first was advice from my girlfriend’s mom to soak the beans for 2-3 hours in water prior to cooking. Then I read online to soak the beans overnight. I talked to my dad this morning about it, and he said, “[We] never soak the beans, but we boil them and let them cook for a long, long time. [. . .] Do NOT add the sugar to the soup until the very end, or they will stop cooking.” At this point, I suddenly remembered the lab I had just done on Friday, where we pushed cell solution at 1x PBS into a microfluidic PDMS trap and replaced the liquid with 0.1x PBS. The cells bloated and bordered on exploding, with a 1.5-fold increase in diameter (coinciding with approximately a 3-fold increase in volume). Yet another recipe I just came across warns you not to add any salt whatsoever to the soaking water – hard evidence that maintaining a strong concentration gradient is the key to cooking red beans.
Just as making crunchy Chinese pickles must be done by patting the cucumbers with loads of salt in order to coax the water out, to bloat and explode the beans so that they form that characteristic grainy red soup, one must only use pure, solute-free water. Because the sugar (carbohydrate) concentration of in beans in extraordinarily high, adding sugar too early to the soup would be deadly – something that seeing the ingredients list off-hand would not convey.
Wouldn’t it be fun to compare the osmotically-driven expansion curve of the white blood cells in dilute saline solution (or even ddH2O) to the expansion curve of red beans in soup solution?
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