Does buying food in bulk cost less in the long run than buying it over time?

That depends. If you buy something that will spoil before you use it, it is a waste of money. If you buy something you don't like, it is a waste of money. If you don't have space to store it, it is a waste of money. If the time it takes to use it is an extended period of time such that the money you had to use could have made money in an interest-bearing account, it is a waste of money. Other than that, buying in bulk is generally a good idea.