The prototype plastic bottle offered ketchup users several benefits: it was lightweight, shatterproof, squeezable for better access and portion control, and had a convenient, nonremovable flip-top cap. Unlike glass bottles, however, the plastic bottle had to be refrigerated after opening. The plastic flip-top cap allowed greater air inflow than the lug-style metal closure used on the glass bottle which provided a tighter seal. Refrigeration was necessary to retard the ketchup's oxidation, the chemical reaction of the ketchup with oxygen which resulted in the dark residue that formed around the seal. Although shatterproof, the plastic bottle was breakable—another potential drawback since consumers might mistakenly assume that all plastic containers were unbreakable. Refrigeration added to the problem since the plastic (polypropylene) became brittle with cooling