There is no evidence that any u-boat ever had to head home due to lack of food. I'm not sure if that's because they just didn't mention it, but the fact is they didn't mention it.
http://www.uboat.net/forums/read.php?20,73516,73533
http://www.uboat.net/men/foodstuffs.htm
A ton-and-a-half of potatoes.
A ton-and-a-half-of-veggies.
A quarter-ton of eggs.
A ton of bread.
A quarter-ton of butter.
Three-quarter-ton of milk.
A ton of fruit.
And that's just the perishables, which they say were consumed in the first two weeks.
Two-and-a-half tons of meat.
Note a Type IX carried food for 12 weeks at sea. Not likely to have problems.