As consumers seek convenience and healthier food, competition is rising among food retailers to meet demand. Long dominated by Whole Foods’ higher-end stores and Trader Joe’s smaller shops and limited assortment, retailers such as Sprouts and Natural Grocer are quickly expanding with messages of affordability. Amid recent failures, such as Tesco’s Fresh and Easy and Delhaize’s Bottom Dollar, retailers are trying smaller, affordable and convenient concepts. The latter group includes Kroger, Ahold and Whole Foods. …