Historically all cider was left in its natural state, unprocessed. In time, airborne yeasts present on apple skins or cider making machinery would start fermentation in the finished cider. Left on its own, alcohol would develop and forestall growth of harmful bacteria. When modern refrigeration emerged, cider and other fruit juices could be kept cold for long periods of time, retarding fermentation. Any interruption of the refrigeration, however, could allow bacterial contamination to grow. Outbreaks of illness resulted in government regulation requiring virtually all commercially produced cider to be treated either with heat or radiation.