Exported walnuts often require bleaching, and the cost of this treatment has significantly increased compared to untreated walnuts. The bleaching process for walnuts is described below:
First, the green skin must be removed. After harvesting and transporting the walnuts, they are typically turned on bamboo rafts while peeling them. However, this traditional method can lead to serious surface contamination, which negatively affects the quality of the nuts. To improve efficiency and cleanliness, a more advanced method involves treating the walnuts with 5000 ppm ethylene. The nuts are fully immersed and placed in an environment with a temperature of 30°C and a relative humidity of 80%. After being left for five days, the peeling rate reaches over 95%, making the process much more efficient and reducing labor costs.
Next comes the bleaching and drying stage. Once the walnuts are peeled, they are washed thoroughly to remove dirt. For bleaching, a small amount of warm water is used to dissolve the bleaching powder, then mixed at a ratio of 80 kg of walnuts per 1 kg of bleaching powder. Alternatively, a sodium hypochlorite solution (containing 10–15% available chlorine) can be used, with 30–40 kg of water per 1 kg of the chemical. The washed walnuts are then placed into the solution and stirred continuously for about 10–15 minutes. When the shell turns from blue to white, the nuts are immediately removed and rinsed with fresh water. Each batch can treat up to 80 kg of walnuts.
After rinsing, the walnuts are spread out on foil or trays for drying. They should be turned regularly during the drying process. It usually takes 8–10 days for the kernel to change from milky white to golden yellow, and for the middle diaphragm to break, indicating that the drying is complete. This method ensures a clean, high-quality product suitable for export.
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