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)
