Freshly harvested walnuts are wrapped in the thick fleshy green skin that needs to be removed, but after removing the green skin, the skin of the walnut is darkened or stained by the green juice exuded from the green shell. So how does it turn black? Because the green peel of walnut contains a large number of tannin juice, which will oxygenize, tannic acid and Gallic acid and other substances under the oxidation reaction will make walnut’s surface turn black. Walnut dried through natural air with skin unpeeled is inclined to turn black for the green peel stains the walnut shell deeply. For this reason, fruit farmers had better choose green walnut peeling machine to ensure the pleasing appearance of walnut and to avoid moldy shell.
How to choose walnuts? 
steps1. Check the pattern
Look at the pattern on the walnut shell, if the pattern is relatively intensive and light, it must be good. Because during the walnut growth process, those patterns are conveyers for nutrient transport. Thus, the more patterns, the more nutrients walnut has absorbed.
Steps2. Smell walnuts
Many walnuts sold in the market have been bleached, which makes them look perfect, but it is harmful to the human body. Because many walnut shells are not completely crack free, therefore, when they are soaked into the bleach agent for a long time, it can easily lead to bleach water entering the walnut and contaminate the walnut kernel. So it is necessary to smell walnuts to see if there is a fragrance or odor, then this kind of walnuts can be bought, if there is bleach or moldy smell, sour smell and other odors, it is best not to buy.
Steps3. Weigh it in your hands
Hold a walnut in your hand and weigh it. If it is relatively heavy, it is a good walnut. Don’t just look at the size, pay more attention to the weight of the walnut. I have seen a lot of big hollow walnuts, while a lot of small walnuts contains full kernels.
Steps 4. Observe walnuts
Pick a walnut, crack open it to observe the inside of the walnut, if its membrane is full and yellow and is of healthy luster, then it must be a quality walnut. And if the walnuts are black or brown and dotted by black spots, they are moldy.