Understanding the Zinc Content in Corn
The zinc content in corn is not a single, fixed number but rather a range influenced by several factors, including the type of corn, growing conditions, and agricultural practices. For common sweetcorn varieties, the zinc content can range from 1.7 to 3.5 mg per 100g, depending on cultivation and processing. In contrast, the baseline concentration for mature maize grain is typically lower, averaging around 14.7–24.0 mg kg⁻¹ (or about 1.5–2.4 mg per 100g). Biofortified varieties developed through specific breeding or fertilization techniques can significantly increase this concentration, sometimes reaching over 30 mg kg⁻¹.
The Critical Role of Bioavailability
While raw zinc numbers are important, the mineral's bioavailability—the proportion that the human body can absorb and utilize—is even more critical. Much of the zinc in mature maize kernels is bound to phytic acid (or phytate), an antinutrient compound that inhibits mineral absorption. The concentration of phytate is particularly high in the germ of the maize kernel. Research shows that while zinc in the endosperm of immature sweetcorn is more bioavailable, the zinc in mature maize endosperms becomes increasingly bound to phytate over time, reducing its nutritional value. This issue is particularly relevant in developing countries where maize is a dietary staple and processing methods don't sufficiently reduce phytate content.
Factors Influencing Corn's Zinc Levels
Soil pH and Composition
Soil pH is one of the most critical factors affecting zinc availability to plants. Zinc is most soluble and available in slightly acidic to neutral soils (pH 5.0–7.0). In alkaline soils (pH above 7.0), zinc's solubility dramatically decreases, leading to deficiencies. Soils with low organic matter or sandy texture also tend to be more prone to zinc deficiency.
Nutrient Interactions
High levels of phosphorus can induce zinc deficiency in corn, especially in soils that already have marginal zinc levels. This P-induced deficiency results from an imbalanced nutrient ratio, where high phosphorus interferes with the plant's metabolism and uptake of zinc. However, high phosphorus from sources like manure may not cause the same issue, as manure often contains significant amounts of zinc as well.
Weather Conditions
Cool, wet soil conditions, particularly early in the growing season, can intensify zinc deficiency in corn seedlings. These conditions limit root growth and slow the release of zinc from organic matter, restricting the plant's ability to take up the nutrient. This is often a temporary issue that resolves as the weather warms and soil conditions improve.
Strategies to Increase Bioavailable Zinc in Corn
Agronomic Biofortification
Applying zinc fertilizers to the soil or foliage is a rapid and effective way to increase the zinc concentration in the maize grain.
- Soil Application: This involves applying zinc sources, such as zinc sulfate, either by broadcasting before planting or banding with starter fertilizers during planting. This method can build up soil zinc levels for several years.
- Foliar Application: Spraying zinc directly onto the leaves is another method, often used to correct deficiencies during the growing season. This provides rapid uptake and bypasses issues with soil availability.
Genetic Biofortification
Plant breeders are developing and releasing new maize varieties with enhanced zinc content through genetic selection. These biofortified varieties, such as BIO-MZN01, have been successfully introduced in regions like Colombia to combat malnutrition.
Post-Harvest Processing
Even for non-biofortified corn, certain processing techniques can improve zinc bioavailability for human consumption. Soaking, sprouting, and fermentation can help break down phytates, increasing mineral absorption. For example, bread made with a sourdough starter has more bioavailable zinc than unleavened flatbreads.
Comparison of Zinc Bioavailability in Foods
| Food Type | Example | Typical Zinc Content | Bioavailability Consideration |
|---|---|---|---|
| Animal Products | Oysters | Very High | Excellent bioavailability, especially in meat and seafood, due to absence of phytates. |
| Legumes | Lentils, Peas | High | Lower bioavailability than animal products due to phytates. Soaking and cooking can improve absorption. |
| Whole Grains | Corn, Wheat, Rice | Variable | Generally lower bioavailability due to phytates, especially in mature grains. Biofortified varieties are designed to counter this. |
| Nuts & Seeds | Cashews, Hemp Seeds | High | Phytates can reduce absorption, but certain processing can help. Still a good source in a balanced diet. |
Conclusion
While corn does contain zinc, its total quantity is highly dependent on factors like the specific variety, soil pH, and nutrient management. More importantly, the high concentration of phytates in mature maize kernels significantly reduces the amount of zinc the human body can absorb. Efforts in biofortification, through both agronomic practices and genetic breeding, offer a pathway to develop corn varieties with higher and more bioavailable zinc content, addressing nutritional deficiencies in vulnerable populations. For consumers, choosing biofortified products and using traditional processing methods like fermentation can help maximize the health benefits of this essential trace mineral from corn.
For more detailed agricultural information on managing zinc levels in crops, consult resources like the Purdue University Extension.
How to Enhance Zinc Absorption from Corn
- Choose biofortified varieties: These are specifically bred for higher zinc content and bioavailability.
- Proper preparation: Soaking, sprouting, and fermenting corn products like flour can significantly reduce phytate levels, improving zinc uptake.
- Add protein: Consuming corn with high-protein foods, especially animal-based products, can positively affect zinc absorption.
- Select sweetcorn: When possible, consuming sweetcorn that is harvested immaturely can offer more bioavailable zinc in its endosperm.
- Combine with other nutrient-rich foods: Pair corn dishes with other dietary zinc sources like meat, beans, or nuts to balance absorption inhibitors.
