Research on Storage and Fresh-keeping Techniques for Jujube Fruit
Jujube is one of the fruit trees unique to China, with a cultivation history of over 2,000 years. The jujube tree is cold-resistant, drought-resistant and tolerant of poor soil, with strong adaptability, and is known as the "ironclad crop." Jujube fruit is rich in nutrients, containing more than twice the sugar of ordinary fruits as well as relatively large amounts of protein, fat, iron, phosphorus, calcium and other substances, and has high nutritional value and dietary therapy functions.
Domestic research on fresh jujube storage and its postharvest physiology began in 1980, and studies and explorations have been carried out on the postharvest physiology and storage of jujube, with a great deal of work done on physiological and biochemical mechanisms, but no new breakthrough has been made in storage technology, that is, in extending the storage period of fresh jujube.
Progress in Storage Conditions and Techniques
There is a folk custom in China of eating fresh jujubes, and some preservation methods have also been accumulated. Records in the Qing palace archives mention storing fresh jujubes in ice cellars, with a storage period of less than one month. For more than ten years, many scientific research and teaching institutions have conducted extensive research and exploration on the storage conditions and techniques of fresh jujube, and the results show that fresh jujube storage is affected by many factors.
1. Variety
The storage tolerance of fresh jujube varies greatly among varieties. According to our storage tolerance tests on 30 varieties collected from Taigu, Shanxi and the Shanxi Institute of Pomology, Xiangfen Yuanzao, Linfen Tuanzao and Yongji Hamazao have the best storage tolerance. Taigu Hulutao, Tuntunzao and Xiangzao are intermediate, while Langzao and Junzao are the worst.
2. Harvest Maturity
Harvest maturity has a clear effect on the storage life of fresh jujube; the lower the maturity, the more storage-tolerant it is, but the fruit loses water faster than fruit with higher maturity.
3. Environmental Conditions
Temperature
Temperature is the most important environmental factor affecting the storage life of fresh jujube. Within a certain range, the lower the temperature, the better the storage effect, but below the freezing point frost damage will occur. According to measurements, the freezing point of half-red-stage Langzao is -2.4 to -3.8°C.
Humidity
Fresh jujube is a fruit that easily loses water. Under conditions of a storage temperature of 6 to 10°C and a relative humidity of 70% to 80%, Langzao, Red Star apple and hawthorn stored for 14 days had water loss rates of 22.2%, 1.2% and 11.2% respectively, showing the importance of maintaining relatively high humidity for fresh jujube storage. Storing fresh jujube in 0.03 to 0.06 mm perforated polyvinyl chloride bags has a good moisture-retaining effect.
Gas Composition
Tests with different gas compositions show that fresh jujube is not tolerant of carbon dioxide, and carbon dioxide above 5% accelerates the softening and browning of fresh jujube flesh. Under essentially no carbon dioxide, 1.5% low oxygen did not cause varieties such as Xiangfen Yuanzao and Jinsi Xiaozao to soften and brown earlier than the air control, indicating that fresh jujube is more sensitive to carbon dioxide than to low oxygen.
4. Other Physical and Chemical Methods
Calcium is an important component of the middle lamella in fruit cells, and many studies have shown that calcium treatment can increase fruit firmness and storage life. Soaking Langzao in 2% calcium chloride immediately after harvest resulted in a good-fruit rate 20% higher than the control at 2 months of storage. Treating Junzao with 5% calcium chloride for 5 minutes before storage and storing it in perforated bags at 0°C for 70 days gave a firm-fruit rate of 15.6%, while the control had a firm-fruit rate of 6.2%. However, other studies have shown that soaking fruit in calcium chloride has no obvious effect compared with the control.
In recent years, research on fresh jujube storage has continued. Many physical and chemical methods, such as coating, preharvest and postharvest hormone treatment, cobalt-60 radiation treatment, vacuum preservation, electromagnetic radiation, rapid cooling and heat treatment, have been explored. Although some methods have a certain effect on delaying jujube fruit softening, no relatively significant effect has yet been seen. New methods and techniques are still being explored.