Post-Harvest De-Greening and Rinsing Methods for Walnuts

  1. De-greening treatment. According to measurements, freshly harvested walnut green husks have a moisture content of 40%-45%, and the kernels have a moisture content of 20%-25%. Such high moisture content makes walnuts prone to rotting and deterioration after harvest. Therefore, after harvesting, walnuts should promptly undergo de-greening treatment. Common methods for removing walnut green husks include the heap-retting peeling method, the chemical peeling method, and the mechanical peeling method.

  (1) Heap-retting peeling method. This is a traditional walnut peeling method in China. Its key technical points are: after harvesting, walnuts should be promptly transported to a shaded outdoor area or indoors, and must never be exposed to direct sunlight; then pile them to a thickness of about 50 cm. Piling too thick easily causes rotting. If a layer of dry grass or dry leaves about 10 cm thick is added on top of the fruit pile, it can raise the temperature inside the pile, promote nut ripening, and speed up the de-greening process. Generally, after heap-retting for 3-5 days, when the green husks have separated from the shells or cracked by more than 50%, they can be knocked off with a wooden stick. Walnuts whose green husks have not yet been removed can be heap-retted for several more days until all husks are removed. During heap-retting, never let the green husks turn black or even rot, so as to prevent contaminated liquid from seeping into the shells and polluting the kernels, thereby reducing the quality and commercial value of the walnut nuts.

  (2) Chemical peeling method. Because the heap-retting peeling method has disadvantages such as long peeling time, low work efficiency, and a high fruit contamination rate, since the 1970s people began researching ethephon ripening and peeling technology and achieved success. The specific procedure is: after harvesting, dip the walnuts in a 3,000-5,000 mg/kg ethephon solution for about half a minute, then pile them to a thickness of about 50 cm in a shaded area or indoors. At a temperature of 30°C and a relative humidity of 80%-95%, after about 5 days, the peeling rate can reach as high as over 95%. If a layer of dry grass about 10 cm thick is placed over the fruit pile, the husks can be separated in about 2 days. According to measurements, the first-grade fruit rate of this method is 52% higher than that of the heap-retting method, the kernel deterioration rate drops to 1.3%, and the fruit surface is clean and attractive. The length of ethephon ripening time is related to the concentration used and the maturity of the fruit. The higher the fruit maturity, the lower the concentration used and the shorter the ripening time.

  (3) Mechanical peeling method. Based on the principle of rubbing, walnuts with green husks are placed between a rotating grinding disc and a hard steel wire brush for abrasion and rubbing, so that the green husks are separated from the nuts. If the green husks have low moisture content and the kernels are shriveled, and the rubbing force is too great, the kernels are very easily damaged during de-greening. Therefore, when using the mechanical peeling method to remove walnut green husks, removal must be carried out within 1-2 days after harvest.

  2. Rinsing treatment. After the green husks are removed, washing can remove soil, residual rotten husks, and twigs and leaves from the nuts. Washing methods include manual washing and mechanical washing. The manual washing method is to put the peeled nuts into baskets, place the baskets in a water pool or flowing water, and stir-wash them with a bamboo broom. When washing in a pool, the clean water should be replaced promptly, and each wash should last about 5 minutes. The washing time should not be too long, so as to prevent dirty water from seeping into the shells and contaminating the kernels. If bleaching is not needed, the washed nuts can be spread on mats for drying in the sun. The work efficiency of mechanical washing is 3-4 times that of manual washing, and the finished product rate also increases by about 10%.

  In order to make finished walnuts smooth, clean, and attractive in appearance, fruit growers often bleach walnuts after washing. The specific procedure is: in a ceramic vat (iron and wooden containers are prohibited), first dissolve 0.5 kg of bleaching powder (containing 80% sodium hypochlorite) in 3-4 kg of warm water, filter out the residue, then pour it into a ceramic vat with 30-40 L of clean water to make a bleaching solution, then put the washed nuts into the bleaching solution, stir with a wooden tool for 8-10 minutes, and when the walnut shell surface turns white, immediately scoop them out and rinse twice with clean water, then dry in the sun. As long as the bleaching solution is not turbid, it can be used continuously. After use, add 0.25 kg of bleaching powder to the used bleaching solution to continue rinsing. Each time, 40 kg of walnut nuts can be rinsed.