A Detailed Guide to Walnut Drying and Dehydration Methods

  1. Natural Sun-Drying and Dehydration After the walnut nuts are cleaned, they must not be exposed to direct sunlight, to avoid the shells splitting and the kernels spoiling. The washed nuts should first be dried in the shade on bamboo mats or sorghum stalk mats for half a day, and once most of the moisture has evaporated, they can be spread out on reed mats or bamboo mats to dry in the sun. The spread layer should not exceed two layers of nuts; if too thick, they easily heat up, causing the kernels to spoil, and they also dry unevenly. During drying, they must be turned frequently, to prevent the shaded side of the shells from turning yellow, and care should be taken to avoid rain and nighttime dampness. Generally, they can be dried in 5-7 days. The criteria for judging whether they are dry are: the nuts make a crisp sound when knocked together, the inner membranes are easily crushed by rubbing with the fingers, and the kernel skin color changes from milky white to light yellowish-brown. If dried excessively, the kernels will release oil and the quality of the nuts will be reduced.

  2. Artificial Dehydration Processing Compared with natural sun-drying and dehydration, artificial dehydration has good heating devices and insulation equipment, ventilation equipment and better sanitary conditions. Although the cost of artificial dehydration is higher and the operating technique is relatively more complex, both the conditions for drying and dehydration and the quality of the dried nuts are superior to natural sun-drying and dehydration, and it represents the direction of food dehydration.

  At present, China's artificial drying equipment is generally divided, according to the mode of heat action during drying, into three types: convection drying equipment, thermal radiation drying equipment and induction drying equipment. In addition, there are batch drying rooms and continuous tunnel drying rooms, as well as low-temperature drying rooms and high-temperature drying rooms. The heat-carrying media used include hot water, steam, electric power, flue gas, etc. Batch drying rooms commonly use steam and electric power for heating, while continuous tunnel drying rooms mostly use infrared heating. Electromagnetic induction drying is not yet widely used. In production, the most commonly used are drying stoves and drying rooms, which use stove heating and rely on air convection to complete heat transfer.

  (1) Drying stove The drying stove is the simplest artificial dehydration equipment, and its forms are varied; some are built as stoves on the ground, while others are dug as pits in the ground. During dehydration, a fire is lit in the stove or at the bottom of the pit, wooden beams are set up above and mats are laid on them, and the raw materials are spread on the mats to dry; the required temperature is controlled by the size of the fire. This kind of dehydration equipment has a simple structure and low production cost, but its capacity is low, the drying speed is slow, and the labor intensity is high, making it suitable for individual farming households.

  (2) Drying room The drying rooms currently used by production units are mostly hot-air convection drying equipment heated by flue gas, generally relatively simple rectangular buildings of earth-and-wood construction. There are many forms of such buildings; according to the different heating methods, they can be divided into one-stove one-side straight-temperature-rise drying rooms, one-stove one-chimney return-flow temperature-rise drying rooms, one-stove two-chimney straight-temperature-rise drying rooms, one-stove two-side return-flow temperature-rise drying rooms, two-stove two-chimney straight-temperature-rise drying rooms, two-stove two-chimney return-flow temperature-rise drying rooms, two-stove one-chimney straight-temperature-rise drying rooms, two-stove one-chimney return-flow temperature-rise drying rooms, high-temperature drying rooms, etc. In addition, drying rooms can also be divided, according to the different roof forms, into ridge type, flat-roof type, cave-dwelling type, etc. According to the different ways of setting up the drying racks inside the drying room, they are divided into fixed-rack drying rooms and movable-rack drying rooms. The two-stove one-chimney return-flow temperature-rise drying room is currently a form commonly used in production; it is a rectangular earth-and-wood structure, generally 6-10 m long, 3-3.4 m wide and 2-2.2 m high, and its main equipment includes heating equipment (including 6 main parts: fire pit, ash door, furnace chamber, main flue, wall flue, chimney, etc.), ventilation and moisture-removal equipment (air inlet windows and exhaust pipes), loading equipment (drying racks and drying trays), doors, passageways, lighting equipment, temperature and humidity measuring equipment, etc.