It doesn’t matter if you prefer whole milk, 2%, or skim, when it comes to the plastic containers that your milk comes in all share one thing in common: they’ve got an inverted circle built into one side of the jug. If you’re a milk drinker, you’ve probably seen this circle…but do you know the ingenious reason for it?
That concave circle on the side of your jug of milk actually provides structural integrity to the jug, itself. A full gallon of milk with rigid flat sides would be fine simply sitting on a shelf in your fridge; but if it were to drop onto the floor, it would most likely rupture. That concave circle is what helps prevent that from happening. When a jug hits the ground, the circle flips outward & gives the milk a place to go when it expands on impact. By working in some literal wiggle room into the design actually makes the container more flexible & durable.
The same feature comes in handy as the milk approaches its expiration date. Milk contains microbes that will expel gases over time. As those gases accumulate, pressure within the jug builds up, and it’s that flexible dimple that stops the jug from exploding. If you want to see a more extreme example of this process, just put a jug of milk in the freezer. See, liquids expand when frozen, which is why freezing a bottle or can of soda ultimately leaves you with a shattered container & a mess in your freezer. The milk jug’s inverted circle facilitates that expansion, though, so you could stick your milk directly in the freezer without it becoming a wet bomb.
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