What happens when you add fresh pineapple juice to jello?

Adding fresh pineapple juice to jello will prevent it from setting correctly. This is because pineapple contains an enzyme called bromelain, which breaks down the gelatin in jello. Bromelain is a proteolytic enzyme, which means that it breaks down proteins. Gelatin is a protein, so bromelain will break it down and prevent it from forming a gel.