The Farm Stand Mystery
1Every August, the garden club at Westbrook Middle School picks sweet corn from its raised beds and sells it at a farm stand on the first day of school. The club starts its corn seedlings in the school greenhouse each spring and moves them outside in May. Last year, several customers said that ears bought on the morning they were picked tasted much sweeter than ears left over from the day before. Seventh grader Tobias Okafor wanted to find out why, so he planned a test with the club's adviser, Ms. Ingrid Valdés.
2Ms. Valdés gave the club a model. While an ear is on the plant, the leaves make sugar and send it to the kernels. The kernel cells are alive, and they keep working for days after the ear is picked. They can do two things with sugar. They can link many sugar units together into starch, a much larger molecule that does not taste sweet. Building starch takes energy. The cells can also break sugar down in cellular respiration: sugar combines with oxygen to form carbon dioxide and water, and energy is released that the cells use for their work. In both processes, the atoms in the sugar are rearranged into new molecules.
3The club picked 18 ears of a standard sweet corn variety from one bed at 7 a.m. Kernels from 2 ears were tested right away. Eight ears were stored in a storeroom at 25 °C, and eight went into the cafeteria's walk-in refrigerator at 2 °C. All of the ears stayed in their husks inside loosely closed bags, so they did not dry out. After 12, 24 and 48 hours, kernels from 2 ears in each group went to the county extension lab, which measured the sugar and starch in them. The last 2 ears from each group were sealed in the lab's chambers to measure the carbon dioxide they gave off. No mold or rot was seen on any ear.
4The club also grew a supersweet variety in the next bed. Supersweet plants carry a gene change that blocks most of the step in which kernel cells link sugar into starch. Supersweet ears picked the same morning were stored at 25 °C and tested the same way.