A crusher is a mechanical device consisting of paddles and rollers that break the grape berries and extract the juice. Crushing must be delicate enough so that the grape seeds are not broken, which would release their bitterness into the wine. With a crusher, a screen is necessary to separate the juice, skins, and seeds from the stems and leaves. With a crusher-stemmer, however, the stems and leaves are automatically expelled. Large commercial wineries have continuous-feed crusher-stemmers that can process up to 150 tons of grapes an hour.