In Turpan, Xinjiang, and other regions of China, the summer and autumn seasons are characterized by high temperatures, low humidity, minimal rainfall, significant temperature differences between day and night, and long hours of sunlight. These conditions make it ideal for natural air drying of grapes, allowing the production of high-quality green raisins. The traditional method involves several key steps:
**1. Natural Air Drying Method:**
A. **Raw Material Selection:** Grapes are harvested at the right time, ensuring they are intact, with thin skins and full flesh. Any damaged, overripe, or pest-infested fruits are removed. They are then placed in shaded areas to allow gradual water loss, making them easier to handle during the drying process.
B. **Pylon Preparation:** The selected grape clusters are transported to a dedicated drying room. They are hung layer by layer, from bottom to top, and any fallen grapes are promptly removed.
C. **Management:** A dedicated person is assigned to monitor the drying process, protecting the grapes from birds, animals, and dust. The racks must also be kept stable to prevent fruit from falling off.
D. **Harvesting and Sorting:** When the raisins become slightly soft and show visible wrinkles and white spots, they are collected. They are then moved to the drying field, where stems, leaves, and debris are removed. Only clean, uniform grains are selected as finished raisins.
**2. Traditional Drying Method:**
A. **Raw Material Selection:** High-quality grapes with high sugar content, firm flesh, and thin skin are chosen. They must be fully ripe but not over-ripe.
B. **Cutting and Arrangement:** Small grapes are cut, and large clusters are separated into smaller strings. These are arranged in a single layer on drying trays.
C. **Alkali Treatment:** The grapes are briefly soaked in a 1.5% to 4% sodium hydroxide solution for 1 to 5 seconds, then rinsed thoroughly with clean water. For white raisins, this treatment lasts 3 to 5 hours.
D. **Sun Exposure:** After the initial drying, the grapes are turned over multiple times under the sun to ensure even drying. Once the moisture is reduced to between 15% and 17%, the clusters are collected, stems are removed, and they are left to rest for 10 to 20 days before packaging and storage.
**3. Artificial Drying Method:**
A. **Raw Material Selection:** Grapes with moderate ripeness, intact structure, thin skin, and high sugar content are selected.
B. **String Cutting:** Large clusters are divided into smaller strings and spread evenly on drying trays.
C. **Alkali Soaking:** The grapes are soaked in a 1% to 3% sodium hydroxide solution for 10 to 30 seconds. For thinner-skinned varieties, a mixture of 0.5% sodium carbonate or bicarbonate with sodium hydroxide can be used for 3 to 6 seconds. This helps break down the outer skin, promoting faster and more even drying.
D. **Rinsing:** After soaking, the grapes are rinsed 3 to 4 times with clean water until no alkaline residue remains. Excess water is drained.
E. **Sulfur Fumigation:** In a sealed chamber, 1.5 to 2.0 kilograms of sulfur per ton of grapes is burned to treat the fruit. This helps deactivate enzymes like polyphenol oxidase, preventing browning and preserving vitamin content.
F. **Drying Process:** The sulfur-treated grapes are placed in a drying room. The temperature starts at 40–50°C for one hour, then gradually increases to 60–70°C for two hours, and finally reaches 70–75°C. The relative humidity is maintained at around 25%. After 15 to 20 hours, the raisins are ready for packaging and storage.
This comprehensive process ensures that the final product is not only visually appealing but also rich in flavor and nutrients, making it a popular choice in both local and international markets.
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