Users who want to refrigerate and preserve grapes can refer to the following information on what precautions to take in actual grape cold storage.
1. Temperature Management
The pre-cooling time for grapes usually refers to the time it takes for their temperature to reach or approach freezing point (0 to -1°C). However, in reality, without a dedicated pre-cooling warehouse or with a small cold storage room, reaching 0°C in open pre-cooling for 1–2 days is difficult. Therefore, the initial cold storage temperature can be adjusted to be 0.5°C lower than required. This accelerates the pre-cooling process, which is crucial for effective grapes cold storage.
For cold storage facilities holding Kyoho grapes, lower the temperature to -1.5 to -1°C for the first week. Once the fruit temperature reaches approximately 0°C, adjust the storage temperature accordingly. The specific temperature control varies depending on the grape variety and its ripeness. Grapes with high lignification of the stem and higher sugar content are more resistant to low temperatures. Temperatures vary in different parts of the cold storage; the area near the fans is the coldest, while the area near the entrance without fans is slightly warmer. When stacking grape boxes, select appropriate locations based on the variety and quality.
The grapes in the cold storage's fan vents and the top layer of each stack can experience fluctuating temperatures (during operation and shutdown). Experienced grape growers typically cover the grape boxes near the fan with plastic film or burlap sacks. If using crates without lids, cover the top layer of grape boxes in each stack with two layers of newspaper. To save energy, when the outside temperature drops to 0°C, turn on the cold storage's ventilation fans. This accelerates cooling and reduces humidity. When outside temperatures fall below -6°C, avoid using natural cooling sources.
Secondly, early spring is a critical period for managing cold storage temperatures. By this time, we have shipped out and sold most of the grapes, leaving only a few behind. Consequently, warehouse owners often neglect to start the machine in time, a situation very common in large ammonia-refrigerated cold stores. Therefore, install mercury thermometers in various locations, regardless of whether the facility uses automatic temperature control or ammonia refrigeration. These should have an accuracy of 0.1°C. The temperature inside the cold storage should be based on the internal thermometer. Monitor and adjust the difference between the automatic system setting and the internal temperature. It's also important to prevent the automatic system from malfunctioning. Ensure timely maintenance of the temperature control and refrigeration system, which plays a vital role in efficient grapes cold storage.
2. Humidity management of Grapes Cold Storage
Currently, most grape storage in China involves lining grape boxes with plastic wrap. Therefore, humidity control in cold storage is closely related to the selection of plastic wrap. All types of plastic wrap have a degree of moisture permeability; PVC plastic wrap is particularly permeable. In northern regions, late autumn and early winter see relatively dry air. Condensation easily occurs to varying degrees inside grape boxes at this time. Therefore, the humidity in early-stage cold storage should be minimized with careful management of grapes cold storage. However, when storing moisture-resistant varieties, like Kyoho grapes, consider later humidification of the cold storage. Additionally, consider the storage facility's humidity level. For example, facilities completed during the rainy season in the first year will be too humid. In rainy southern regions, storage facility humidity is generally high; strengthen ventilation early on to reduce this.
3. Gas Flow
After grapes are placed in storage, their respiration rate is high. Some varieties may release harmful gases such as ethylene. Therefore, cold storage should use low nighttime temperatures and ensure ventilation beforehand. Regular ventilation is essential during management to maintain fresh and clean air, which is vital for effective grapes cold storage.
Investigation and treatment of fruit storage conditions in cold storage
The grapes in each storage room should be stacked separately by variety and quality grade. This facilitates monitoring of changes during storage. Choose representative grape boxes for monitoring each type of fruit, including those harvested from different vineyards. Different parts of the cold storage have variations in temperature and humidity. Check these different monitoring boxes regularly: weekly during the early and late stages of storage, and every two weeks during the middle stage. Inspection typically involves checking for mold, dry stems, or signs of chemical bleaching through the plastic wrap. Sometimes, samples must be taken and inspected, or plastic bags removed from boxes to check for changes in the bottom bunches of grapes. Address any issues promptly to ensure successful grapes cold storage.
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