The researchers studied how different types of corn kernels affect the thickness of the outer layer. They found that certain combinations of kernel types can lead to thicker or thinner outer layers. The sugary-sugary2 combination had the thickest outer layer, which could make the corn less tender. However, the shrunken2 type, a popular new type, had a similar outer layer thickness to the standard sugary type, so it wouldn't affect tenderness as much.