There are three reasons:

1. The appearance of aged walnuts may be small or large, but if the inside changes from white to yellow, it is a sign of corruption. Walnut kernels will gradually lose their original color and fragrance, and become darker and darker in color; they will have a "harsh" taste. This kind of walnut kernels have lost their use value. In severe cases, they may even have carcinogenic effects from Aspergillus aflatoxin, so do not eat them;

2. Walnut kernels turn black due to damage by diseases and insect pests during the growth of walnuts on the tree, such as "walnut black" (walnut limb-lifting moth) damage, sunburn, severe black spot, ring spot, etc. The damage caused by this kind of walnuts naturally has no edible value;

3. This is the key point to say. Due to the continuous rain during the maturity season of walnuts, the inner seed coat of the walnut kernel turns black on the tree. This kind of The main symptoms are that the walnut shell is dark in color; the inner seed coat of the walnut kernel is dark and brown or even close to black, or there are molds of varying severity; but the walnut kernel is normal white. This kind of walnut is generally edible and does not even affect the flavor and nutrition. Extended information

The walnut kernels have also changed color and appear black or yellow. It is best not to eat them, as they may be spoiled. Also pay attention to the taste of walnuts, and do not eat them if they taste bitter or rancid.

Eating walnuts regularly has many benefits. Walnuts contain a large amount of unsaturated fat, which is beneficial fat to the human body. Regular consumption can make blood lipid levels more reasonable and prevent cardiovascular disease. The protein content is not low either, with 14.9 grams of protein per 100 grams of dried walnuts, making them a very good source of plant protein.

People often say that eating walnuts can nourish the brain, because the shape of walnut kernels is somewhat similar to that of the brain, and it is rich in nutrients. It has been popular among the people that walnuts can nourish the brain. Walnuts do contain ingredients that are beneficial to the brain and can delay brain aging, but it does not directly prove that eating walnuts can immediately improve IQ, it is just good for the brain.