Food placed in a glass container at warm or room temperature will continue to evaporate moisture which will condense on the lid.
If the lid is plastic (as most are), the condensed water can then be contaminated by contact with the plastic.
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The food can then be contaminated when the condensed droplets of water drip onto the food. Or, if frozen, the condensed water — which turns to ice — will detach and drop on the frozen food when the plastic lid is deformed upon opening.
Covering the glass contained with aluminum foil before putting the lid on will prevent plastic contamination.
There may be some form of thin chemical coating on the aluminum foil which was used in the rolling process. We could find no information on this.