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Milk bags are plastic bags that contain milk. They are usually stored in a pitcher or jug with one of the corners cut off to allow for pouring. A typical milk bag contains approximately 1 1⁄3 litres (2.3 imp pt) of milk in Canada and 1 L (1.8 imp pt) of milk in South America, Eastern Europe and the Baltics.
Milk bags in Canada
Bagged milk is common in many parts of Canada. The innovation was introduced in 1967 by DuPont using European equipment. The new packaging quickly found favour with the domestic dairy industry, being lighter and less fragile than glass bottles. However, the consumer public preferred plastic jugs for years, but largely accepted the new containers in certain regions in the 1970s. A major reason for that shift was with the national conversion to the metric system, which was easier to adjust to on production with bags while doing the same for jugs required entire systems to be wholly redesigned.
They are sold in eastern Canada, but not widely in western Canada (Manitoba, Saskatchewan, Alberta and British Columbia). Three bags are sold together in a larger bag containing a total of 4 L (7 imp pt) of milk. The bags are not sold individually, and are either not labelled at all or labelled with only the expiry date, the lot number and sometimes the type of milk contained in the bag. The three-bag 4 L (7 imp pt) package is the largest normally sold at retail, with the lowest unit price. Some convenience store chains offer 4 L (7 imp pt) plastic jugs instead of milk bags, even in eastern Canada.
Two accessories are commonly associated with Canadian milk bags: pitchers and bag openers. The key-shaped bag opener with a clip and a magnet was invented in Toronto in 1979. These bag openers are a common type of refrigerator magnet, although the bags can be opened with scissors or knives.
In other countries
Milk bags were also common in the former Soviet Union[dubious ], and other Eastern bloc countries, such as Poland, East Germany, Hungary, Romania, Yugoslavia, and Bulgaria. They remain popular in the Baltics (e.g. Estonia) and some Eastern European countries, where they may also be seen used for packaging yogurt or kefir. A resurgence of milk bags is beginning in Britain amid concerns that plastic bottles are not being recycled. Milk bags were used in Australia (Greater Shepparton, Victoria), in the late 1990s, distributed by Shepparton-based dairy company Ducats. They were also used in Gympie, Queensland, in the 1970's and early 1980's, and also in Caboolture, Queensland around the same time. These were 1 pint in size.
Dairies in the United States used the bags in the 1980s, but today are confined mainly to regional convenience store chains with in-house dairies such as Kwik Trip in the Upper Midwest and other boutique dairies.
In Russia, milk bags are often made not from the single-layer LDPE, but from sturdy laminated paper that uses at least two paper and three LDPE layers, similar to what is used for the common milk cartons, and may include aluminum foil for better protection of the bag's insides. Such bags are much more difficult to pierce, which greatly simplifies handling and reduces losses in transportation. It also makes a bag more rigid, so it is able to sit on the counter upright by itself, and can be poured without help of a holding pitcher, though it still remains difficult to reseal. It is possible to seal it again with the hot clamps that make the polyethylene coatings stick to each other, but barely anyone bothers.
In the United Kingdom, Sainsburys began a pilot experiment on distributing milk in bags in 2008 in conjunction with Dairy Crest. It was originally targeted at 35 stores at the same price as a regular 2-imperial-pint (1.1 l) plastic bottle of milk, the product was expanded through the North of England nationwide in 2010 at which point the bags retailed at a discounted price compared to traditional containers.
In the UK, the bags are usually used in conjunction with a specialized plastic jug. The bag fits snugly inside the jug, one corner of the bag is secured under a bar at the front of the jug, and as the lid is closed the bag is pierced and a spout slides into the hole, allowing the milk to be easily poured and maintaining freshness. Doorstep deliveries in the United Kingdom are normally associated with traditional glass milk bottles, but the Dairy Crest/Milk and More service also deliver milk bags, and sell Jug-It brand plastic jugs specially designed to hold the milk bags.
There are no real benefits to buying bagged milk over cartons, but rather a cultural preference. 
When pouring, the top of the bag can tump over, causing the milk to spill. Spillage can be avoided by cutting a secondary hole at the other side of the bag for air intake, by pinching the top of the bag while pouring, or by using a pitcher with a lid to keep the milk bag in place. Milk bags cannot easily be sealed once open, although some consumers fold over the spout and use clips to help maintain freshness. Also, a common single-ply LDPE bag is easy to pierce and tear, and must be handled and transported with care to avoid product losses and messes.
While milk bags use less plastic than the traditional jug, there is no incentive on consumers to recycle the bags, such as the refunds provided when jugs are recycled at a bottle depot. In Canada, where recycling services are municipally or regionally managed, milk bags are not always able to be recycled. In some municipalities milk bags are required to be thrown away  and in others they are recyclable.
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Media related to Milk bags at Wikimedia Commons
- Milk Bags Demystified A woman demonstrates the nature of the packaging and how it is readied for use.