Pineapple contains bromelain, a group of digestive enzymes that break down the collagen fibres in chicken whilst marinating. It is a wonderful way to tenderise and make the chicken breast more succulent. However, if you over marinate the chicken, it will produce an unpleasant texture in the final product. Stick to 1-8 hours of marinating to be safe. These enzymes only work from fresh pineapple, not canned or pasteurised juice.