A series of winter storms and widespread power outages gripping Texas and other states not used to such extreme low temperatures are creating big challenges in the nation’s food supply networks. Grocery chains like Walmart and Publix have been forced to close some stores either because of lack of power or they couldn’t get workers. And at retailers that have remained open, customers complain of long lines outside and facing empty shelves of water, bread and milk when they get in. Meanwhile, retailers and food suppliers are having to throw out spoiled food, creating devastating consequences for food banks that are a lifeline to the poor.