Can blackened lotus root still be eaten?
Core tips: Why does lotus root turn black? What are some tips to prevent it from turning black? Can it still be eaten after it turns black?
You might have had this experience: the lotus root and pork rib soup you stewed in an iron pot accidentally turned into a black ink; the lotus root, which was as white as jade when you bought it yesterday, has turned as dark as Bao Gong's face just after a day in the kitchen. It's said that lotus root emerges from the mud unstained, but why is it so prone to turning black in the air?
I. Why does lotus root turn black?
There are many explanations circulating online. The most "scientific" one sounds like this: "Lotus root turns black after heating because it is rich in iron, which oxidizes when heated, causing the color to darken." If this were true, then lotus root cooked in a clay pot should also turn black, but that is not the case. Besides, the iron content of lotus root is far less than that of wood ear mushrooms or even pork liver, and you've never heard of these things turning soup black just by being cooked in an iron pot.
In fact, the reason lotus root turns black in an iron pot is related to its polyphenolic chemical compounds. These substances have a common property: they can bind with iron ions to form colored complexes that are purple or blue. One of the abundant polyphenols in lotus root—gallic acid—combines with iron ions to form a blue-black substance, which is a main component of blue-black ink. No wonder it can turn the pork rib soup into ink soup.
The mystery of the ink-like soup from stewed lotus root in an iron pot is solved, but why does lotus root that hasn't touched any ironware and is just placed in a vegetable basket also turn black? This is because the polyphenolic substances mentioned above are oxidized into chemicals called "quinones" under the action of polyphenol oxidase (PPO). These quinones then polymerize to form melanin. In fact, a cut apple turning brown or a banana in the refrigerator turning black is for the same reason.
II. Tips to Prevent Blackening
Actually, keeping lotus root white is not difficult. Blanching it in boiling water at 100°C for 70 seconds can inactivate all polyphenol oxidase, thus stopping the conversion of polyphenols to quinones. However, this method is probably only suitable for dishes like stewed pork ribs with lotus root that require a soft and tender texture. If you use this method for cold lotus root slices, they won't turn black, but they will also lose their crisp texture.
Don't worry. Besides high-temperature treatment, adding acidic substances can also "tie up" polyphenol oxidase. When making cold lotus root slices, immediately adding vinegar to the blanched slices can largely prevent the catalytic action of polyphenol oxidase, keeping the lotus root white. In fact, some ready-to-eat packaged lotus root slices contain citric acid and other substances that inhibit polyphenol oxidase activity.
For fresh lotus root that you can't finish eating right away, how can you keep it white? This is also not difficult. Although polyphenol oxidase has a strong catalytic effect, it requires oxygen for the phenolic substances in the lotus root to oxidize into quinones. Therefore, isolating it from oxygen is another method. Soaking the lotus root in water can slow down the blackening process.
III. Can blackened lotus root still be eaten?
Based on the reasons why lotus root turns black, blackened lotus root is edible. However, if it's blackened because it has been stored for too long, it is definitely not fresh anymore and should not be eaten.