"The apple is food."
Translation:Eplet er mat.
Føde means nourishment rather than food. Rarely used nowadays as far as I know, unless as in phrases like fast føde which you'd say about babies when they start eating "solid food" (as opposed to being breastfed). Or flytende føde if you can't chew and need to live off soups and "liquid foods".
I think I would use plural to say that apples was føde. Epler er føde. I tried to google it and it seems it is often used figuratively now (as in "food for thought"), or "CO2 is the nourishment for all life on Earth" kind of statements.