Sweet Corn

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Key Factspending-external-review

  • Fresh-cut sweet corn kernels are extremely perishable, with very high respiration rates, making proper temperature control critical.
  • While there are concerns regarding various mycotoxins in field corn fed to animals, there is minimal risk associated with mycotoxins in sweet corn consumed by humans. According to the FDA, minimal traces of, fumonisins (a type mycotoxin), have been found in sweet corn (4-82 ppb).
  • It is not uncommon for fresh corn to be preserved via canning. However, improper home-canning practices can leave low-acid foods, such as corn, at risk of developing C. botulinum. Corn cannot be safely canned using a boiling water bath, but must be processed in a pressure canner.
  • Sweet corn contains a genetic mutation that causes kernels to store at least two times more sugar than field corn.
  • Sweet corn is the second largest crop in the U.S. (second only to tomatoes).
  • The U.S. exports of sweet corn are valued at $246 million per year, $46 million as fresh and $200 million processed and frozen (based on 2014 values).

Introduction

Sweet corn (Zea mays var. rugosa) comes in three colors: yellow, white and bicolored. Sweet corn contains a genetic mutation on the su locus that causes the endosperm to store at least twice as much sugar as field corn.  As soon as corn is harvested, the sugar begins converting to starch, reducing the sweetness. Therefore, sweet corn should be refrigerated or preserved within 2 to 6 hours of harvest to ensure best quality and retention of sweetness.

Bodacious sweet corn variety.
Bodacious sweet corn variety.

There are hundreds of different varieties, to include Bodacious, Incredible, Temptation, Delectable and Providence. Traditional varieties are su1 (sugary1) mutants and contain twice the sugar and eight to ten times more water-soluble polysaccharide as field corn. The increase in water-soluble polysaccharide is responsible for the creamy consistency but is also responsible for it’s short shelf life (maximum storage in refrigerator of 4 days). Supersweet varieties that contain the sh2 (shrunken2) mutation contain at least 4 times the amount of sugar as field corn but almost no water-soluble polysaccharide, making conversion of sugar to starch almost negligible. This allows Supersweet varieties to remain sweet at least 10 days after harvest if kept refrigerated, making these varieties ideal for sale at distant markets.

Top producers in the U.S. include Florida, California, Georgia and New York.  Sweet corn grown in the Midwest is generally available late summer through early fall while sweet corn grown in Florida is available December through May.

Foodborne Outbreaks

While there have not been many foodborne outbreaks related to sweet corn, there have been foodborne outbreaks related to frozen vegetables in which corn was included. In 2013, a multistate outbreak of Listeria monocytogenes was reported with a total of nine illnesses and three deaths. Epidemiological and laboratory evidence indicated that frozen vegetables from a frozen foods company in Washington State were the source. In late spring of 2016 there was a large recall of frozen vegetables with corn being one of the implicated vegetables. After the recall no further linked cases were reported. 

To contribute to the Broccoli-associated Foodborne Outbreak section, please follow this link: http://fsi.colostate.edu/suggest-a-topic/

Food Production

Pre-Harvest

Seeds of sweet corn should be plated about 1 inch deep in heavy soils and ½ to 2 inch in sandy soils. It is highly suggested that all seeds are treated with insecticide and fungicide, which can sometimes be applied before purchasing seed and should be verified with the supplier.  If Supersweet seeds are used, they must be planted about ½ inch shallower than other sweet corn seeds and planting should be delayed until warmer temperatures in the soil are reached. The corn earworm is the greatest insect concern with sweet corn. Commercial sweet corn operations require the use of an insecticide spraying program to manage this pest.

One sweet corn plant should be placed every 8 to 10 inches. Three-foot rows with this spacing will allow for about 17,000 plants per acre of cropland.  Fertilizer can be applied depending on the soil type. Consumers like to see sweet corn that has long, green flag leaves and dark green husks. To obtain this desirable appearance, a high level of nitrogen must be maintained in the soil. Moisture is crucial for sweet corn growth. It will contribute to crop uniformity, early enhancement, increased yields, and other desirable qualities. However, over-irrigation of sweet corn cropland can cause seeds to easily rot—especially in cold weather. Thus, irrigation must be closely monitored and seeds should be planted during the warm season. Diseases are rare with sweet corn. 

Harvest

Cornfields ready for harvest on Uesugi Farms in Gilroy, CA.
Cornfields ready for harvest on Uesugi Farms in Gilroy, CA.

The average yield per 100 feet of row is about 8 dozen ears of corn. Sweet corn is ready for harvest at approximately 15 to 23 days after the silks appear. If the weather is especially hot, the plant will tend to mature faster. When the kernels are fully developed, they are in the milk stage and will leak a milky liquid when punctured.  Sweet corn can be harvested by hand or machine. A large commercial operation will likely use machines for harvest. It is important that the ears be collected in the early morning or evening when the weather is cool.

Post-Harvest

It is extremely important to keep the corn cool following harvest. Sweet corn loses sweetness and freshness very quickly after harvest, especially when kept at increased temperatures. Because sweet corn has a high respiration rate, it produces heat which can build up in bulk loads after harvest. The longer it takes for the corn to be chilled and the longer it is able to stay at elevated temperatures, the more sugar is able to be converted to starch, thus greatly affecting quality of the product. Product must be moved quickly from the field to the packing shed where it should be sorted, packed, and cooled. There are several methods to chill sweet corn: hydrocooling using water, package icing (typically only used for local distribution), cold storage using refrigeration and relative humidity of at least 95%. It is ideal to store corn at 32°F but in commercial settings, 40°F is typically the lowest temperature reached. No matter how the corn is transported to market, it must be kept on ice to maintain quality.

Food Safety

Because of the conversion of sugar to starch, fresh sweet corn should be refrigerated or processed immediately after harvest. From a food quality aspect, sweet corn should be consumed within four to ten days of refrigeration, depending on the varietal. Fresh-cut sweet corn kernels are extremely perishable, with very high respiration rates, making proper temperature control critical.

It is not uncommon for fresh corn to be preserved via canning. However, improper home-canning practices can leave low-acid foods, such as corn, at risk of developing C. botulinum. Corn cannot be safely canned using a boiling water bath, but must be processed in a pressure canner. Home-canned corn should be heated to no less than 80 degrees Celsius for at least 10 minutes. Before consuming canned corn products, containers should be inspected for bulging, lid damage, and leakage, as these are signs of gas release produced by bacterial growth.

There have been no reported outbreaks of food-borne illness associated with sweet corn. While there are concerns regarding various mycotoxins in field corn fed to animals, there is minimal risk associated with mycotoxins in sweet corn consumed by humans. According to the FDA, minimal traces of, fumonisins (a type mycotoxin), have been found in sweet corn (4-82 ppb). This is 50 to 1,000 less than the maximum recommended levels in corn intended for human consumption (4 ppm). Additionally, according to the FDA, aflatoxin, another mycotoxin of concern in field corn, has not be found in fresh, frozen or canned sweet corn.

Consumption

Corn from harvester to trailer at John N. Mills & Sons farm.
Corn from harvester to trailer at John N. Mills & Sons farm.

Corn is one of the most versatile food products grown and consumed in the United States.  In addition to fresh sweet corn for human consumption, it can be further processed into other food products, sweeteners, starches, oils, alcohol, and ethanol based fuel. Corn is also the main feed grain in the United States, and is projected to account for 95% of the feed grain in 2015-2016. As of 2010, Americans were consuming an average of 26 pounds of sweet corn per person. Of this total amount, 9.6 pounds were from frozen sweet corn, 9.1 pounds were fresh sweet corn, the remaining 7.4 pounds were canned sweet corn products. In the 2006—2007 Foodnet Population Atlas of Exposures, 41.5% of the survey cohort reported eating fresh corn within the past 7 days.

Information on how to keep corn fresh and of safe quality please visit FoodKeeper App.

Nutrition

While sweet corn typically invokes images of summer barbeques, it also has many health benefits. Sweet corn is a great source of Vitamin C and Vitamin A, with one ear of corn containing over 10% of the Daily Value (DV) of Vitamin C and over 6% DV of Vitamin A. One large ear of boiled corn on the cob contains 113 calories, 4 grams of protein, 1.8 grams of fat, 5.4 grams of sugar and 2.8 grams of fiber (10% DV). The large amount of soluble fiber helps decrease cholesterol. Research has shown that cooking corn actually increases levels of beneficial antioxidants and phytochemicals.


 

References

Authors

Angelina Gerardo

Angelina Gerardo

Rachel Murphy

Rachel Murphy

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