Science Fair: Do Plant-Based Cups Really Break Down?

The Cup Question

1The cafeteria at Brookside Middle School hands out about 400 single-use cold cups every school day. This year, the supplier offered a new cup labeled “plant-based and compostable.” Seventh graders Hana Kobayashi and Mateo Rinaldi wondered whether the new cups were really better, so they made it their science fair project.

2The old cups are made of polystyrene (PS), a plastic that starts as crude oil or natural gas. At a chemical plant, small molecules taken from these resources react and link into very long chains called polymers. The new cups are made of PLA. PLA usually starts as corn. The starch in the corn is broken down into sugar, and microorganisms in large tanks ferment the sugar into a substance called lactic acid. At a factory, lactic acid molecules react and link into long polymer chains. The pellets of PLA are then melted and molded into cups.

3For their test, Hana and Mateo cut 5.0 g strips from each kind of cup and from an uncoated paper towel, which is made of wood fibers and is known to rot. They sealed each strip in a mesh bag. One set of bags went into the compost bin in Hana's backyard. With permission, a worker at the city composting facility buried a second set in a large pile. Workers turn these piles often, which keeps them hot, usually about 55 to 60 °C. The backyard bin stayed near the outdoor temperature.

4After 12 weeks, the students opened the bags while wearing gloves. They rinsed the strips, let them dry for two days, and weighed them. They tested three strips of each material in each place and recorded the average.

5Both kinds of cups are light, cheap compared with glass and hard to break, which helps a busy cafeteria. Making PS uses resources that cannot be replaced once they are used up. Making PLA uses a crop that farmers can grow again each year, but growing corn takes farmland, water and fertilizer. PLA cups can also cause problems if people put them in plastic recycling bins, because they do not mix well with other plastics.