Eiswein, designates German wines produced from grapes frozen on the vine, and pressed while still frozen. The deliberate picking of Eiswein with any significant frequency seems to have originated in the 1960s, and from the 1980s the practice became routine at the majority of top estates, excepting those that farm vineyards not prone to deep frost. Freezing concentrates not just the sugar in the grapes, but also acidity and extract, and Riesling Eiswein is routinely the highest in acid (as well as some of the highest-priced) of any German wines. For best results, a frost of at least −8 °C/18 °F is required, for which grapes are generally harvested between five and eight in the morning of the first sufficiently cold November or December days. Eiswein picked in January or even February is not unknown, but is seldom of as high a quality. Such a wine is labelled for the calendar year of the growing season. Before 1971, Eisweine were frequently labelled for the date of picking or nearest Saint’s Day (Nikolauswein, for example, designated a wine harvested December 6), but such information is currently not permitted on the label. Since 1982, Eiswein has been a separate prädikat with the minimum must weight of a beerenauslese, namely 110–128 oechsle depending on the region and variety in question. In austria too, the requisite must weight is the same as that of a Beerenauslese, 25 KMW (approximately 127 °Oechsle). The harvesting of Eiswein has become much more routine as a result of the widespread (if controversial) use of semi-permeable plastic sheeting that hugs the vines to protect grapes from birds and rain while waiting for a suitably deep frost. (Protection from wild boar is another matter, and more potential Eiswein is lost to these marauders than to any other cause.) While the classic concept of Eiswein for most growers is a wine from botrytis-free grapes, this is not a legal requirement, and the use of film in fact often promotes humidity and thus a low level of botrytis in the shrouded grapes. If the harvest does not achieve the requisite ripeness or the character deemed appropriate to Eiswein by the individual vintner, the wine usually ends up being bottled as an auslese or subsumed into another wine, even though this practice is technically legally questionable.