There are such things as "Food Grade" polyurethane.
You can read more abut them by a google search for "food grade polyurethane".
Just the name "polyurethane" doesn't tell you about the chemical additives in it that may cause you problems. It doesn't necessarily mean that the harmful additives are present either.
You could consider use of a paper bag or bags inside of the PU sack to contain your bulk foods, or the paper or plastic bags that are furnished at the point of purchase of bulk foods. Those bags are likely to be food grade.
Our local news show had a feature article on the contamination that your food can get from the reuse of those "environmentally friendly" grocery bags. One solution was -- use the "food grade" bags in the produce aisles to prevent contact between the "environmentally friendly" reusable grocery bags and your food.
What is your proposed hygiene control solution for any polyurethane bag you would use for this purpose?