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I got large container of vegetable shortening that I wasn't going to open right away, so I stuck it in bottom of the freezer (-14 F) to keep it from going rancid. I never really needed it, so I ignored it for the next four years. I pulled it out yesterday and let it thaw.

The first thing I noticed is that it's more translucent than fresh shortening. It looks just like paraffin wax, but it's still soft like shortening. The smell and flavor are identical to fresh shortening. The can still had the factory seal until I opened it yesterday, so it's unlikely that it got contaminated with anything. What caused the change in appearance?

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Fats can have a complex molecular structure:

Molecular diagram of fats

Not only do elements have different properties, resulting in different behavior when exposed to heat or cold, but there are bonds between atoms that may be affected by these different neighboring reactions. Think of this somewhat like the way a frying pan heats up very easily, but the plastic handle of the pan behaves a bit differently; or, the way an ice pop is very cold coming out of the freezer, but the wooden stick isn't nearly as cold and warms up quickly.

These differences in the way temperature effects substances could've acted on the molecular structure of the fats, damaging or altering them as a result. This can result in changes to consistency, color, etc. This is very likely similar to the way damage can happen to certain foods due to the fact that water expands when frozen, while other substances tend to contract (which can result in strange, spongey, and otherwise undesirable changes in texture and taste).

It's fortunate that it seems not to have effected the taste or smell!

Just keep in mind that there are certain more desirable temperatures for foods, and they may not maintain their structure internally, their taste, color, or other qualities, if kept outside of that desirable temperature range for too long. While freezing does help to prevent bacterial growth and decay, it doesn't protect food from the changes which result from temperature change itself.

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