personally, i think that a food budget of 20€/day for a single person is quite large.

i live in frankfurt and i'd eat cold the whole way...

i break it down like this (at Frankfurt inner city prices/countryside would be much less)...

3 fresh baguettes @ 0.99€
1 loaf fresh bread @ 1.99€
150g pre-sliced pepper cover salami @ 1.29€
150g sliced and smoked ham @ 1.29€
100g spreadable cheese w/herbs @ 0.99€
250g Emmentaler cheese (Swiss mountain cheese) @ 2.49€
200g Saint Agur or Gorgonzola @ 1.99€
6 large tomatoes @ 0.99€
fresh rosemary @ 0.30€
balsamic vinegar @ 0.50€
1 package field salad @ 0.99€
0.7L bottle Crémant @ 3.50€
a metal tube of mustard @ 1.49€ and some spiciness would be nice for the salad as well.

that doesn't include any water, but you could easily substitute the Crémant for water.

to be honest, most students I know who don't get paid tend to live on between 5-10€/day for food. 5€ is they only eat cold and 10€ if they want one warm decent meal per day.

also, I'm not sure where you're headed in France but in central Germany, you'll pass through tiny towns on the bike paths/tours and can easily pick up stuff on a daily basis (if you so desire.) even the bakeries are open on Sunday if you want bread/pastries (however, nothing else will be open on Sundays.)