Modern cork processing follows traditional methods, albeit helped by increasingly sophisticated technology nowadays. The strips of cork bark yielded by the annual stripping are stacked and left outside for seasoning (just as other woods are in barrel making) for at least six months. Good drainage and airflow are important at this stage to minimize the risk of tca development. Still in large strips, the cork is then boiled for about 90 minutes, both to make it more flexible and in an effort to kill off any moulds and other contaminants. The planks are then left to rest in the warehouse of the cork processing plant for three weeks before being sorted by hand and cut into strips as wide as the length of the final cork stopper. Corks are then punched out of these strips, usually by hand-operated punches but using an increasing degree of mechanization (machines are more efficient, but are so far unable to scrutinize each strip for impurities). Maximizing yield is a significant factor since only about 40% of all the commercially viable cork harvested is suitable for stoppers.