Ok I am not an expert, but a few thoughts
1. Birds for example, like Strawberries, so perhaps the ants want to protect them, covered in sand they are less visible, probably taste bad mixed with the sand too.
2. Perhaps the sand acts to protect from the sun (drying out)
3. Protect or even preserve them
4. Are ants able to secrete chemicals that will preserve or protect food
5. Does sand also contain salt?, esp if near the coast, salt is a preservative.