Key points on the operation of quick-frozen sweet corn kernels

Raw material harvesting

The best harvesting period for sweet corn is the milky stage, which is 20 to 22 days after pollination. Generally, the pollination time starts when the corn ear spins out about 1 inch (3.3 cm) of filaments.The moisture content of corn kernels when harvested should be 70% to 73%. At this time, the taste and flavor of sweet corn are very good.After harvesting, corn should not be left in the field for a long time or exposed to the sun. Do not over-squeeze during transportation and handling to avoid damage to the corn kernels.

2. Raw material acceptance

The leaves of sweet corn should be green when harvested.The seeds are plump, yellow or light yellow in color, uniform in color, without variegated grains, the grain size and grain arrangement are uniform and neat, and there are no serious bald tips, missing grains or insect infestations.

3. Peel and remove filaments

After entering the factory, sweet corn should be placed in a cool place and peeled and processed immediately.The time from corn harvesting to processing cannot exceed 6 hours. If the corn cannot be processed within 6 hours, it must be stored in a fresh-keeping warehouse at around 0°C for a short period of time.Manually peel off corn bracts and remove corn silk.Corn ears with bracts removed should be handled with care and put into special baskets.

4. Trimming, screening and grading

First, remove the ears of sweet corn that are too old, too tender, too infested, or have extremely untidy kernels.Use a knife to dig out the ears of sweet corn that have some insect-eaten and variegated kernels.Then according to the corn diameter classification, 2 to 3 grades can be set according to different corn varieties, and the diameter difference between grades is set at about 5 mm.

5. Cleaning and threshing

Since quick-frozen corn kernels are no longer thawed and washed before consumption, they should be cleaned with running water.Threshing is carried out on a special corn thresher. The depth interval of the thresher's knives is adjusted according to the grade to avoid cutting the corn kernels too deeply or too shallowly.If the corn is cut too deep, the cob may be cut off, affecting the appearance and taste of the product; if it is cut too shallow, fewer kernels will be cut, the yield will be low, and raw materials will be wasted.

6. Blanching

Blanching is a very important step in the processing of young corn, and the blanching temperature and time must be strictly controlled.The purpose of blanching is as follows: it can inactivate the enzymes in the corn tissue. Low-temperature refrigeration will not deactivate the enzymes, but still undergoes slow physiological and chemical feedback, destroying the nutrients of the kernels; blanching can also kill some microorganisms and insect eggs, ensuring the hygiene of the product; the air in the blanched corn kernels is squeezed out, reducing the stretching pressure when ice crystals are formed during condensation, increasing the resistance to stretching pressure, and correspondingly reducing the oxidation degree of the raw materials, maintaining the color and nutrition of the product.

Blanching can use boiling water or steam, the temperature is 93 ~ 100 ℃, the blanching time is controlled according to the water temperature, usually 3 ~ 8 minutes.If the blanching time is too long, nutrients will be seriously lost, and quality indicators such as color and taste of the finished product will also be greatly reduced.After threshing, the corn kernels should be blanched in time.Blanching can be done in a jacketed kettle or a continuous blanching machine.The blanching technique is to boil the water first and then add sweet corn kernels. The ratio of water to corn kernels is 4:1.

7. Cool down

The blanched corn kernels should be cooled in time, otherwise the remaining heat will seriously affect the quality, such as the color will darken, the drying loss will increase, the quick freezing time will be lengthened and energy will be wasted, and it will also provide conditions for the growth of microorganisms. Therefore, cooling must be completed in real time to ensure the color and quality of the product.

In order to save water, segmented cooling technology can be used. Start using spraying techniques to lower the temperature of the corn kernels around 90°C to 25-30°C; then soak and cool them in ice water at 0-5°C to reduce the temperature in the middle of the corn kernels to below 5°C.

8. Screening

Pick out cob scraps, filaments, discolored grains and other foreign impurities. In order to ensure product quality, improve manufacturing rate, and reduce the pressure of screening before packaging, manual screening is performed on the conveyor belt after cooling to remove overripe, under-blanched, and broken corn kernels.

9. Drain

Draining can remove the moisture on the surface of the corn kernels, preventing excessive surface moisture from condensing and causing adhesion between ice cubes and corn kernels, affecting the surface and weight, and also reducing the cost of electricity. Draining can be done with shaking, but it is best to blow dry with cold air. First, it can be drained completely, and second, it can be further cooled.

10.Quick freezing

Quick freezing is a decisive factor in ensuring product quality. The faster the coagulation rate, the better the quality of the product; conversely, the longer the coagulation time, the worse the quality. Quick-frozen corn kernels use a fluidized bed quick-freezing tunnel. The corn kernels are laid flat on the conveyor belt, and multiple fans under the conveyor belt blow cold air from bottom to top at a speed of 6 to 8 meters/second, so that the corn kernels are suspended, and the temperature in the middle of the corn kernels reaches -18°C. The quick-frozen corn kernels should not stick to each other and should be frost-free on the surface. The evaporation temperature of the machine is -34~-40℃, the cold air temperature is -26~-30℃, the thickness of the corn kernels is 30~38 mm, and it takes 3 to 5 minutes to condense to meet the requirements.

11. Filter

The coagulated corn kernels are further screened to remove defective kernels and broken kernels, and can be sieved if necessary.

12.Packaging

Quick-frozen sweet corn kernels should be packaged at -6°C. Generally packed in polyethylene plastic bags, 250g/bag or 500g/bag according to needs. After packaging, seal and stamp the manufacturing date on the seal. After packing, send to the cold storage for refrigeration.

13. Refrigeration

The temperature of the cold storage should be below -18°C and the relative humidity should be 95% to 98%. The temperature fluctuation limit in the cold storage should not exceed ±2°C. Otherwise, excessive temperature fluctuation will cause recrystallization and ice sublimation, which will increase the ice crystals in the corn tissue and destroy the cell tissue, affecting product quality. When stacking, leave enough space between stacks to facilitate air circulation.