The caps are made of sugarcane. It's a form of bioplastic, but it doesn't have many of the downsides of regular plastic. They are produced 100% CO2 neutral and they are 100% recyclable through regular plastic waste, but most importantly, they do not rely on the oil industry to be produced. The growing of the sugarcane filters the air for CO2 and it needs very little water and space. The stuff used to make the bioplastic from is actually a byproduct of the sugarcane, which would otherwise hardly be used.