First, the production process. Sweet corn is harvested at the optimal time—typically 20 days after pollination, during the milk stage. Once harvested, the ears are peeled and defibrated, then selected, trimmed, and graded. They are washed, shucked, blanched, cooled, and sorted again before being rapidly frozen, screened, packaged, and stored in refrigeration.
Second, common varieties used for processing frozen sweet corn include Super-sweet corn and enhanced sweet corn types. Popular choices are Green Superman, Xiangyu Chaotian No. 1, Yuetian No. 3, Jingketian No. 115, Sweet List No. 21, No. 3 Sweet Sweets, No. 6 Sweet Noodles, Super Sweet 2000, Huatianyu No. 1, Shentian No. 2, US No. 1, US No. 2, Japan Card 85, New Zealand’s 5015, and 5016. These varieties are known for their sweetness, texture, and suitability for freezing.
Third, the processing technology includes several key steps:
1. **Timely Harvesting**: The best time to harvest is when the leaves are still green, the silks have turned brown, and the kernels are plump and milky. Harvested corn must be processed within two hours to maintain quality.
2. **Peeling and Filament Removal**: After harvesting, the corn is placed in a shaded area and immediately stripped of its husks and silks. This should be done carefully to avoid damage, and if not processed right away, it should be stored in cold storage (around 0°C) for short-term preservation.
3. **Picking, Trimming, and Grading**: The corn is sorted to remove overripe, damaged, or deformed ears. Any with minor issues like insects or color variation are further cleaned. Ears are then graded by diameter, usually into 2–3 categories, with a difference of about 5 mm between each grade.
4. **Washing and Threshing**: The graded ears are washed with clean water and then threshed using a specialized machine. The blade setting is adjusted so that it just touches the cob without damaging the kernels.
5. **Blanching**: Immediately after threshing, the kernels are blanched in boiling water or steam at 95–100°C for about 5 minutes. This step helps preserve color, texture, and nutrients.
6. **Cooling**: After blanching, the kernels are cooled quickly. First, they are sprayed with cold water to reduce the temperature from around 90°C to 25–30°C, then submerged in ice water (0–5°C) to bring the core temperature below 5°C.
7. **Selection**: Any remaining impurities such as cobs, filaments, or discolored kernels are removed to ensure high-quality final product.
8. **Quick Freezing**: The kernels are frozen in a fluidized bed tunnel at -34 to -40°C. Cold air is blown upward at 6–8 m/s, suspending the kernels and ensuring even freezing. The process takes 3–5 minutes, and the center temperature reaches -18°C. The frozen kernels should remain separate and free of frost.
9. **Screening**: After freezing, the kernels are further inspected and sifted to remove any remaining impurities, broken grains, or crumbs. A 0.4 cm sieve may be used if needed.
10. **Packaging**: Frozen kernels are packaged at -6°C in polyethylene bags, typically 250 g or 500 g per bag. The date is printed on the seal, and the packages are immediately moved to refrigerated storage.
11. **Refrigeration**: Storage conditions require temperatures below -18°C and humidity between 95% and 98%. The freezer must maintain stable temperatures, with fluctuations no more than 2°C. Proper spacing between containers ensures good air circulation and uniform cooling.
Fourth, quality standards are crucial. Sensory characteristics include a light yellow or golden color, uniform kernel size, no broken or discolored pieces, and no foreign matter. The product should taste sweet and crisp when cooked. Health indicators include limits on heavy metals such as copper (≤5.0 mg/kg), arsenic (≤0.5 mg/kg), and lead (≤1.0 mg/kg), along with microbial safety meeting commercial aseptic standards.
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