Most sweet corn is ready 15 to 22 days after silking, and is hand harvested by grasping the ear and pulling downward while twisting the wrist to snap the ear off the stalk. Sweet corn may also be harvested using machines, which are becoming more common. As the kernels mature, they pass through growth stages termed pre-milk, milk, early dough, and dough. At the dough stage, sugars in the kernels’ pericarp change to starch and the kernels become tough. The time to harvest is when kernels just reach the milk stage. Look for the following:
- Kernels will be nearly full-size, but still soft and tender and filled with clear to milky juice when punctured with the thumbnail.
- The tip of the ear will be filled out.
- Silks will be dried and brown beyond the end of the husk.
Harvest when 70% of the ears in the patch are in this condition. Sweet corn may only stay in prime condition for 1 to 2 days if daytime temperatures are consistently above 86°F, so harvest timing is critical for optimal flavor and quality.
Postharvest temperature management is imperative to maintain ear quality. Ears will be 15 to 30°F cooler in the morning than at midday, so pick ears early, if possible. Chill corn as soon as possible after picking. Standard sugary (su) types lose sweetness in just a few days. In fact, half of the sugar in su kernels is lost within 24 hours when stored above 86°F. When kept near freezing, only 8% of the sugar is lost each day. Extra sweet and super sweet types remain sweet for a longer period, but cooling the harvested ears is still critical to maintain quality.
When sweet corn is shipped or stored for more than 2 to 3 days, maintain the corn at 32°F. Cooling with air is common but not as effective as other options because it often takes 24 to 48 hours for ears to become sufficiently chilled.
In larger operations, hydro-cooling (soaking or sprinkling ears in chilled water) and then top-icing is commonly used to reduce ear temperatures quickly. Temperatures can be reduced by 20°F in as little as 20 minutes if ears are immersed. When using sprinklers, one gallon of water is needed for every 4 lbs of corn.
Hydro-cooling and ice making systems are expensive, costing several thousand dollars for refrigeration units and assembly. Refrigeration systems that produce crushed ice will dissipate heat more quickly than those producing cubed ice. For shipping, add one lb of ice for every five lbs of pre-chilled corn in the shipping containers. Contact the local health department or government food safety administrator for potential restrictions, building codes, and other safety issues.
If using an air refrigeration system, immerse sweet corn ears in tanks of pre-chilled water to lower their temperature quickly. After the initial bath, store the ears in the cooler at approximately 98% humidity to maintain ear quality.