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Q & A: Extra space at the top of vegetable oil bottles

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Most recent answer: 10/22/2007
Q:
why is there extra space at the top of a vegetable oil bottle?
- Sam (age 14)
Madison HS, Vienna, VA
A:
I can think of two reasons, one probably much more important than the other.

One only makes sense if the bottle cannot change shape when the pressure increases. If you drop the bottle of oil (say, a plastic one, which is usually how vegetable oil is sold), the bottle will dent in where it hits the ground. The volume of oil remains constant, and something else in the bottle has to reduce in volume, or the bottle has to bulge out somewhere else. Air compresses much more easily than the oil, and this may provide some protection against breaking the bottle when it hits the ground. Many vegetable oil bottles have complicated shapes (mostly so you can still hold onto them even if they are oily and slippery) and bulging out in some places is probably not going to break them. Gallon jugs of milk often have circular indentations which will bulge out when the jug is dropped, which is preferable to breaking the jug, spilling milk everywhere. You could also protect the milk bottle by only filling it up partway, allowing the air to compress when the jug is dropped, but this would cause the milk to spoil faster and people will feel cheated if they think they're buying a lot of air.

This probably isn't the real reason, however, as there is usually rather little air at the top and the oil bottles can deform. What I suspect is the real reason is that if the bottling company tries to fill up each bottle all the way to the tippy-top, then on some fraction of the bottles, oil will spill over the sides, and this would have to be cleaned off the bottle before sale (and the oil would stain the labels). Furthermore, it's probably harder to get that little freshness seal glued on to the lip of the bottle if the lip is oily.

Tom

(published on 10/22/2007)

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