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The Battle Of The Milk Bags [Blog]

Updated: Jul 30, 2021

Author: Parmin Sedigh

[Image courtesy of Food Network Canada; modified by Parmin Sedigh.]


Did you just say milk bags?


Yep. In Eastern Canada, that includes Ontario, Quebec, and the Maritime Provinces (New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland and Labrador), most large quantities of milk comes in bags. For example, going into any family home, you’ll probably find a large bag, like the one below, that has 3 smaller bags full of milk, amounting to 4L of milk in total. Each small bag is then put into a jug and a corner (or two) is snipped so the milk can come out.


Milk Bags Found in an Ontarian Grocery Store (Courtesy of CBC)


If you’ve never heard of bagged milk, you’re not alone. Not even all of Canada uses milk bags. But why? Why are Eastern Canadians the only ones having milk in bags? According to the CBC, it goes back to the 1960s. That’s when dairy producers were experimenting with milk bags as cheap alternatives to glass bottles. Bags still weren’t very popular until some new regulations regarding measurements came into effect. In the 1970s the Canadian government took a strong position against the imperial system and enforced lots of new rules to get Canada shifted over to the metric system. This included getting the milk industry to move away from quarts and towards liters. Changing from one-quart glass bottles to 1.3 L ones was expensive since so many were already in circulation and changing machinery was also costly. But these experimental milk bags could very easily be changed to 1.3 L instead. So that’s what happened. That doesn’t mean that milk bags were consumer’s favorite. When milk jugs became cheaper than plastic bags, much of Canada moved over to them. To add on to that, the Progressive Conservative government of Brian Mulroney relaxed some of the metric regulations in the 1980s, further allowing the milk industry to say bye-bye to milk bags. Nowadays, you can find milk jugs in Quebec and the Maritimes. But Ontario just can’t seem to let go of the milk bag. It’s because of even more regulations. Up until 2018, if retailers or producers want to sell or produce 4L milk jugs, they must either pay a deposit or have a recycling program in place. Even now, Ontarians are just too used to those milk bags, even though it perplexes most others south of the border or in the west.


How do milk bags work?


Glad you asked; let’s get down to some physics. Here’s the part that everyone agrees on. You grab your bag of milk, pop it into a reusable plastic jug with no lid, and then you cut it open. But that’s where the science comes in.

According to a Huffington Post poll of 500 people, 70% always snip off just one corner of the milk bag, 20% always snip off two corners, and the rest are somewhere in the middle. As you can see below, whenever the milk bag is full, it’s very likely to get messy. I conducted experiments, and both methods (one and two snips) resulted in a messy first pour. However, with a half full bag, the pour was quite simple and clean, again regardless of the different number of cuts.


Milk Pouring Video (Full Bag)


Milk Pouring video (Half Full)


So two questions pop up!

  1. Why are people so set on either method when they perform nearly the exact same? (By the way, don’t take my word, watch this video, about someone super confident in the two-snip method who ended up spilling the full bag anyways!)

  2. Why is the pouring of the full bag so much messier? (And this involves a little more physics than just the fact that there’s more milk in one of them!)


Let's answer the two questions!


Does the number of cuts matter?


Short answer: not really in this case. But that’s not what you’re here for! The reasoning that most of those in favor of the two snips go to is that the second hole relieves air pressure and allows better air flow which prevents lots of spillage. They aren’t exactly wrong, but they aren’t right either. This is absolutely true for containers like cans where having just one hole will lead to the liquid not coming out. This is because air really wants to get into that can and when the hole only has enough room for the milk, the air tries something different. The air pushes the milk back in. When there are two holes, air can go into one and the liquid can come out the other. Having a larger hole also works. The air can get in while the liquid comes out simultaneously. Going back to our milk bag, you don’t want to cut a huge hole that’s going to allow for little control of the pour but you also want the milk to come out easily and without struggle. So you cut another small hole on the other side. This would be true except for the fact that nearly always, the hole cut for milk to come out is big enough for air to come out of too! So team two-snip was really close and had some solid physics behind them, but it just fell apart at the end.


What supporters of two cuts believe happens during a pour (Image courtesy of Food Network Canada; modified by Parmin Sedigh).


Why's pouring a full bag so messy?


You might be really confused about this question; “well, because it’s full of milk so it can easily spill out!” That is partially true but there’s something else at play here. Going back to our air flow above, when the bag is half full, the other half is full of something else, air! Since there’s already some air in there, the air pressure isn’t as high when pouring milk out. However, when the bag is nearly full of milk and has very little air, the air from outside the bag is desperately trying to get in. And a three-way fight pursues! It’s between you and gravity against the air. And the only thing left at the end is a milky mess.


Who thought there’d be physics at work during such a simple (but bizarre to some) task like pouring milk out of a milk bag? Now you know about the history of milk bags and the science behind them so go out and inform the world!



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