What are the conditions and methods for storing walnuts?
Compared with fruits, the physiological metabolism and compositional changes of walnut kernels are relatively stable, which gives walnuts a relatively long storage life. However, when walnuts are not sold or processed immediately, suitable storage conditions must be provided for them, and reasonable storage methods must be adopted to ensure the quality of the walnut kernels.
1. Storage conditions The length of the walnut storage period and the quality of the storage result are determined by two aspects: the walnuts' own conditions and external conditions. Their own conditions include the characteristics of the walnut variety, the degree of damage to the nuts, and the moisture content of the kernels. Generally, paper-shell walnuts store the least well, while thick-shell walnuts store the best; the higher the degree of damage, the shorter the storage period, and intact, unblemished nuts have the longest storage period. By comparison, the moisture content of the kernels is the more important determining factor. Generally, walnuts intended for long-term storage are required to have a moisture content of no more than 7%.
(1) Temperature of the storage environment A low-temperature environment is the first choice for long-term storage of walnuts. Research shows that the storage time of walnut nuts and kernels extends as the temperature decreases. Some researchers have confirmed that at 10°C, walnut kernels can have a shelf life of up to 1 year, with their physical, chemical, sensory and other quality indicators all within the specified range. Other researchers recommend 0-2°C as the optimal storage temperature for walnuts. Research shows that when walnut nuts are sealed in polyethylene bags, they can maintain good quality for more than 2 years under frozen conditions.
(2) Relative humidity of the storage environment Controlling the relative humidity of the environment is very important for whether walnut kernels can maintain their color, aroma and texture. Generally, a relative humidity of 50%-60% is appropriate. If the relative humidity is less than 40%, the kernels will lose water and shrivel, reducing nut quality; if the relative humidity of the storage environment reaches 70% or more, a large amount of mold will develop, and the moisture content of the kernels will also increase, which will inevitably affect the storage result.
(3) Gas composition of the storage environment Similar to the storage of other fruits, walnut storage also requires an environment low in oxygen and high in carbon dioxide. Walnuts have a very high fat content, so special attention must be paid to the oxygen content. Generally speaking, the oxygen content of the storage environment should be below 1.5%, and the carbon dioxide or nitrogen concentration should reach 70% or more. In such an environment, respiration can be inhibited, consumption reduced, oxidation prevented, and mold activity can also be inhibited to prevent mildew and rot, while effectively reducing insect damage and rodent damage.
(4) Management during storage During the walnut storage period, regular observation and monitoring should be carried out, and spoiled and rotten nuts should be removed promptly. During storage, the problems most likely to occur are mildew and rot, insect damage, and the development of a rancid flavor. The main cause of mildew and rot during walnut storage is infection by microorganisms. Some of these microorganisms infect the walnut fruit in the orchard, then lie dormant and cause damage, while most are infected during post-harvest handling. Walnut kernels are fragrant and delicious and rich in protein and fat, making them easy to attract insect damage. The cause of rancidity is that walnut kernels contain a relatively large amount of fat, and when the temperature is high, there is much moisture, and oxygen is abundant, the fat is prone to oxidative deterioration, producing a rancid flavor. During storage, attention should be paid to preventing and addressing the above phenomena.
2. Storage methods
(1) Ordinary storage This is divided into two methods: dry storage and wet storage. Walnuts for eating are generally stored by the dry method, while those used for sowing can be stored by the wet method. Before storage, it should be confirmed that the walnut nuts have been completely sun-dried. For dry storage, put the walnuts into cloth bags, burlap sacks, or baskets and place them in a ventilated, cool, dry place. To be safe, hanging them up for storage is also acceptable. During storage, regular inspection and turning should be carried out to prevent rodent damage, mildew and rot, and heating.
(2) Low-temperature storage Walnuts that need to be kept for a long time must have a low-temperature storage environment. For small storage quantities, the nuts can be sealed in polyethylene bags and then kept in a refrigerator at 0-5°C. Where conditions permit, large quantities can be packed in burlap sacks and stored in a low-temperature controlled-atmosphere cold storage facility (temperature -1°C, relative humidity 50%-60%, oxygen concentration below 1%) for a better result.
(3) Plastic film tent storage Its principle is similar to controlled-atmosphere warehouse storage. Walnuts harvested at the proper time and processed are bagged and stacked into piles, stored in a low-temperature location, covered with a large plastic film tent, and carbon dioxide gas is introduced into the tent (nitrogen can also be used) to reduce the oxygen concentration. In the early stage of storage, the carbon dioxide content can reach more than 50%, and afterward is maintained at around 20%, with oxygen at around 1.5%. Sealed storage using plastic tents should be carried out in a low-temperature, dry season so as to maintain low humidity inside the tent. Research has confirmed that after 4 weeks of storage under carbon dioxide enrichment at 24°C, the color and flavor are clearly different from storage in air, and after 25 weeks the quality is still relatively good, whereas storage in air shows oil exudation and deterioration.