Detailed Explanation of Key Techniques for Jujube Storage and Fresh-keeping

Jujube is one of the fruits unique to China. It is rich in nutrients, containing relatively large amounts of sugar, protein, fat, iron, calcium, phosphorus and other substances. At the same time, its content of vitamin C and cyclic adenosine monophosphate (CAMP) is also very high. In clinical medicine, it has preventive and therapeutic effects on the human body in terms of antioxidation and increasing myocardial nutrition, and is therefore favored by consumers. However, jujube fruit is a fruit that does not store well. At room temperature, it loses its original quality in about 20 days, and at low temperature it can only be stored for 40-60 days. Therefore, the storage and fresh-keeping technology for jujube fruit is extremely important.

(1) Storage characteristics and varieties of jujube fruit. During storage, fresh jujubes mainly develop symptoms such as alcoholic softening, flesh browning and rot. Generally, as the storage period of jujube fruit is prolonged, the flesh gradually softens, and the nutritional components of the fruit undergo great changes. The content of vitamin C and total sugar decreases significantly, while the ethanol content increases significantly, so a distinct alcoholic taste can be clearly perceived when eaten. The appearance of the jujube fruit shows dehydration and shriveling, the peel color deepens, and the flesh begins to turn yellow and brown from near the pit and may even become moldy and rotten.

There are many varieties of jujube fruit in China, and the storability of each variety differs greatly. Late-maturing varieties store better than early-maturing varieties, varieties used both for drying and fresh eating store better than fresh-eating varieties, crack-resistant varieties store better, and large-fruited varieties are usually not suitable for storage.

(2) Harvesting of jujube fruit. Harvest maturity has a significant effect on the storage life of fresh jujubes. Generally, the lower the maturity, the better the storability. At the same time, for fully red (100% colored), half-red (about 50% colored) and initially red (less than 25% colored) jujube fruits picked from the same tree, when stored at 0°C, initially red fruits store best, while fully red fruits have the poorest storability. Therefore, initially red fruits should be selected for long-term storage of jujube fruit.

The harvesting methods for jujube fruit mostly involve knocking them down with poles and picking up fallen jujubes. These methods require a great deal of labor and are highly labor-intensive, and the jujube fruits are easily damaged during harvest, which affects the storage effect. Therefore, for fruit intended for long-term storage, hand picking is still the better harvesting method.

(3) Storage of jujube fruit

① Simple storage method. This method is suitable for northern regions where the temperature is relatively low during the jujube ripening season. Choose fully ripe varieties that store well, and the best maturity for the fruit is when the peel is half-red. During storage, in order to reduce water evaporation, the fruit should first be put into perforated polyethylene film bags 0.04-0.07 mm thick, and the bag openings should be tied closed, then stored on layered storage racks in a cool shed (or cave dwelling).

② Cold storage method. After jujube fruit is harvested, it should be carefully selected as soon as possible, bagged and then placed in a cold storage warehouse. The methods for selecting and bagging the fruit are the same as for the simple storage method. During the storage period of jujube fruit, the temperature in the warehouse must be stably maintained at (0 +/- 1)°C, the relative humidity must be maintained at 90%-95%, and timely ventilation must be carried out. This prevents the carbon dioxide concentration in the air from becoming too high and affecting the quality of the jujube fruit.

③ Controlled atmosphere method. Jujube fruit cannot tolerate carbon dioxide above 5% in the environment, so fresh jujubes are not suitable for spontaneous modified atmosphere storage under sealed plastic bag conditions. However, some recent studies have shown that controlled atmosphere storage is more effective than cold storage for some varieties. For example, research by Liang Xiao'e and others on Jun jujube showed that controlled atmosphere storage with 5% oxygen and 0% carbon dioxide was significantly better than perforated bag storage. In addition, Yuanling large jujube also stores fairly well under a controlled atmosphere with 3%-5% oxygen and 2% carbon dioxide. Therefore, whether the controlled atmosphere method is suitable for jujube fruits of different varieties still depends on the variety.

In recent years, some physical and chemical methods such as film coating, pre-harvest hormone treatment, 60Co radiation treatment, vacuum fresh-keeping and heat treatment have all been explored. Some methods have shown certain effects on the storage of jujube fruit, but no breakthrough progress has yet been made, and new methods and technologies are still being explored.