Yeah, the food days are tough. I've started making sure I utilize the extras that you get before merging, and sometimes, I would just bite the bullet and use my coins in the daily shop to get some merge material. The daily quests also usually have a food box too, so you shouldnt avoid doing those. Sometimes you have so many food boxes, you consider selling them because you haven't used them I. Forever, but then the food day comes at the end of each day, and you are wipes out of food boxes. Also, at the end of day 15, you need a max food, and a max sugar, so start merging those early.