Methods and Technical Key Points for the Fresh-keeping and Storage of Jun Jujube
Red beauty, nourishing life. For thousands of years, red jujube has been regarded as an excellent product for beautifying the complexion, resisting aging, and promoting longevity, and it is also one of the important food sources for the survival of people in jujube-growing areas and for getting through famines. Red jujube is different from ordinary dried and fresh fruits; it combines medicinal, health-care, and edible functions in one. As a rare fruit and the "king of vitamin C," Jun Jujube, as one of the four famous jujubes of Shanxi, is favored by consumers for its unique shape, deep red color, thin skin and thick flesh, small pit and large fruit, and crisp, sweet, fragrant taste. According to measurements, every 100 g of fresh Jun Jujube contains 28.7 g of sugar, 0.45 g of acid, and 432 mg of vitamin C, with an edible rate of 96.3%; while every 100 g of dried jujube contains 75.6 g of sugar, 1.58 g of acid, and only 16 mg of vitamin C.
Under natural conditions, Jun Jujube will quickly lose water after harvest at room temperature without any treatment. Due to excessive water loss and abnormal metabolism, the flesh tissue becomes saccharified, vitamin C is severely lost, and nutritional value declines. Only through effective storage and fresh-keeping can the nutrients of fresh jujube be preserved to the greatest extent and market competitiveness be improved.
1. Storage Characteristics
The storage characteristics of Jun Jujube mainly include the following aspects: ① High respiration intensity. Jun Jujube is a climacteric fruit, and its postharvest respiration intensity is very high, about 2 to 3 times that of Red Fuji apple, which is the fundamental reason why Jun Jujube is not storage-tolerant after harvest. ② Rapid water loss. Jun Jujube has thin skin and poorly developed epidermal tissue, so water evaporates easily. After harvest, fresh jujube loses its fresh crispness within 2 to 3 days under natural conditions. Jun Jujube with low maturity loses water faster than that with high maturity. ③ Sensitive to carbon dioxide. When the volume fraction of carbon dioxide in the storage environment is higher than 2%, it will aggravate the softening and browning of Jun Jujube; however, it is less sensitive to low oxygen and can tolerate a low-oxygen state with a volume fraction of 1% at 0°C. ④ Maturity has a great influence on storage tolerance. Initially red Jun Jujube is the most storage-tolerant, followed by half-red fruit, and fully red fruit is the least storage-tolerant.
2. Preharvest Management
Preharvest management of Jun Jujube mainly includes 3 aspects:
① Reduce pesticide pollution. Strictly control the amount of pesticides applied. Within the effective concentration range, use low concentrations as much as possible for pest and disease control. Generally, pesticides with restricted use can only be used once a year, and their use should be stopped 20 days before fruit harvest to ensure that there is no pesticide residue in Jun Jujube. ② Inhibit ethylene metabolism. As the fruit matures, both the ethylene concentration in the fruit and the respiration intensity show peak periods. Jun Jujube is relatively sensitive to ethylene at the white-ripe stage and insensitive to ethylene after coloring and maturing. Therefore, timely use of ethylene biosynthesis inhibitors such as aminooxyacetic acid and aminooxyvinylglycine is beneficial for enhancing the storage tolerance of Jun Jujube. ③ Increase fruit firmness. Calcium can form calcium pectate with pectic acid in the middle lamella of jujube fruit somatic cells, and plays an important role in maintaining fruit firmness, regulating tissue respiration, and delaying senescence. Therefore, spraying the tree canopy and jujube fruit with a 0.2% mass fraction calcium chloride solution half a month before harvest can increase the calcium content of Jun Jujube and improve its storage tolerance.
3. Warehouse Disinfection
Half a month before harvesting Jun Jujube, the warehouse should be cleaned and disinfected, and facilities such as refrigeration equipment and electrical devices should be maintained and inspected. When disinfecting the warehouse, every 1 cubic meter of space can be fumigated with 15 g to 20 g of sulfur for 24 h to 48 h, or the walls and floor can be sprayed with formalin or 84 disinfectant at a mass fraction of 1% to 2%, or bleaching powder at a mass fraction of 0.5%. For the disinfection of tools and containers used in the warehouse, a 0.25% mass fraction calcium hypochlorite solution can be used for soaking or whitewashing, or they can be placed under the sun for exposure disinfection.
4. Harvesting Methods
During harvest, beating or shaking the tree is strictly prohibited; fruits should be picked by hand, with light picking and light placing. Practice has proved that harvesting while retaining the fruit stalk is an important measure to improve the storage tolerance of Jun Jujube. After the jujube fruits are harvested, they should first be graded according to the size of Jun Jujube, and those that are diseased, insect-damaged, deformed, or injured should be removed.
5. Precooling Treatment
Precooling treatment is a necessary step to improve the storage and fresh-keeping effect of Jun Jujube. Its purpose is to quickly dissipate a large amount of field heat and reduce the cooling load upon storage. In addition, precooling treatment can also avoid the chilling injury that Jun Jujube is prone to when it immediately enters a cold storage state. After picking, Jun Jujube is best cooled to about 0°C within 24 h. Precooling of Jun Jujube can be carried out in a precooling room in a cold storage or through forced ventilation cooling, or the fruit can be spread in a ventilated and cool place for 2 to 3 days so that its temperature drops to close to the storage room temperature.
6. Fresh-keeping Storage
At present, widely used storage and fresh-keeping methods include fresh-keeping agent preservation, mechanical refrigeration preservation, controlled atmosphere preservation, and other methods. In addition, some efficient and energy-saving new storage and fresh-keeping methods using natural cold sources, high-voltage electrostatic fields, and electrically generated functional water are also constantly emerging.
(1) Fresh-keeping bag cold storage method. According to the actual conditions of storage and fresh-keeping at the present stage, the storage and fresh-keeping of Jun Jujube should preferably adopt the fresh-keeping bag cold storage method, and the relative air humidity inside the fresh-keeping bag should be controlled within the range of 90% to 95%. The fresh-keeping bag can promptly remove harmful gases produced during the storage of Jun Jujube. Place this fresh-keeping bag into a plastic box, put 10 kg of red jujube in each box, tie the bag mouth tightly, and then stack it for storage. The temperature in the storage warehouse should be controlled within the range of -1°C to 0°C, and the volume fraction of carbon dioxide should be lower than 2%. A stable low-temperature state should be maintained. The fluctuation of warehouse temperature should not exceed 1°C, and it is most ideal if the product temperature fluctuation of Jun Jujube does not exceed 0.5°C. Under such an environment, Jun Jujube can be stored for 60 to 100 days, and the good fruit rate reaches 50% to 90%.
(2) Controlled atmosphere storage. Among many storage methods, controlled atmosphere storage has the best effect.
Temperature and humidity in controlled atmosphere storage are the main factors affecting the storage effect of jujube fruit. It is advisable to use a temperature of -1°C to 0°C and a relative humidity of 90% to 95%. After storage, the fruit tastes good and its flavor is maintained.
A high volume fraction of carbon dioxide can cause fruit poisoning or anaerobic respiration, accelerating the softening and browning of Jun Jujube. The suitable gas indicators should be a carbon dioxide volume fraction of 2% to 4% and an oxygen volume fraction of 3% to 6%.