In Turpan, Xinjiang, and other regions of China, the summer and autumn seasons are characterized by high temperatures, low humidity, minimal rainfall, and significant temperature differences between day and night. These conditions make it ideal for natural air-drying of grapes, which is commonly used to produce high-quality green raisins. The process involves several key steps:
**1. Natural Air-Drying Method:**
This method relies on the region’s favorable climate. When the grapes reach full maturity, they are carefully harvested and processed using traditional techniques.
A. **Raw Material Selection:** Grapes are picked at the right time, ensuring that they are intact, with thin skins and plump flesh. Any damaged, overripe, or pest-infested grapes are removed. They are then placed in shaded areas to allow some moisture loss and softening, making them easier to handle during drying.
B. **Pylons (Drying Stands):** The selected grape clusters are sent to a dedicated drying room. They are hung layer by layer, from the bottom up, ensuring proper airflow. Fallen grapes are promptly removed to prevent spoilage.
C. **Management:** A dedicated person oversees the drying process to protect the grapes from birds, animals, and dust. The racks must remain stable to avoid fruit falling off.
D. **Harvesting and Sorting:** When the grapes become slightly shriveled and their surfaces turn white, they are collected. They are transported to the drying field, where stems, leaves, and debris are removed. Only the best quality grapes are selected, sorted by size and color, and finally packaged as finished raisins.
**2. Traditional Drying Method:**
This technique also begins with selecting high-quality grapes.
A. **Raw Material Selection:** Grapes with high sugar content, thin skins, and firm flesh are chosen. They must be fully ripe but not over-ripened.
B. **Cutting and Arrangement:** Large clusters are cut into smaller bunches, and individual grapes are spread out in a single layer on drying trays.
C. **Alkali Treatment:** Selected grapes are briefly immersed in a sodium hydroxide solution (1.5% to 4%) for 1–5 seconds. After rinsing, they are left to dry for 3–5 hours to enhance the texture and color.
D. **Sun Exposure:** The treated grapes are laid out in a single layer on drying trays. After about 10 days, when the outer layers begin to dry, the trays are flipped to expose the other side. This process is repeated until the grapes no longer release juice. After one week of further drying, the moisture content should be between 15% and 17%. The clusters are then removed, stems trimmed, and stored for 10–20 days before packaging and storage.
**3. Artificial Drying Method:**
This method uses controlled environments to speed up the drying process while preserving quality.
A. **Raw Material Selection:** Grapes with moderate ripeness, thin skin, and high sugar content are selected.
B. **String Cutting:** Larger clusters are divided into smaller strings and arranged on drying trays.
C. **Alkali Soaking:** The grape clusters are soaked in a sodium hydroxide solution (1–3%) for 10–30 seconds. For thinner-skinned grapes, a mixture of sodium carbonate or bicarbonate with sodium hydroxide can be used for 3–6 seconds. This helps break down the outer layer, promoting even drying.
D. **Rinsing:** After soaking, the grapes are rinsed 3–4 times with clean water until there is no alkaline residue. Excess water is drained off.
E. **Sulfur Fumigation:** In a sealed chamber, 1.5–2.0 kg of sulfur per ton of grapes is used to treat the fruit. This helps deactivate enzymes that cause browning and preserves vitamin content.
F. **Drying Process:** The sulfur-treated grapes are moved to a drying room. The temperature is gradually increased: first to 40–50°C for 1 hour, then to 60–70°C for another 2 hours, and finally to 70–75°C. The relative humidity is reduced to around 25%. The entire process takes 15–20 hours, resulting in well-dried, high-quality raisins ready for packaging and storage.
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