I'm not sure if this helps or not, but you can go from string to b, so I'm not sure you would need to go the other way, since you already have a string.
It's possible to go from b to string, but not trivial. At least I can't think of a quick way. But if you need to, it can be done.
using namespace std;
vector < bitset<8> > b;
for (int i=0; i<food.length(); i++)
for (int i=0; i<b.size(); i++)
cout << b[i] << endl;