Very important point @doitvoluntarily - the consumer is much better informed nowadays thanks to the Internet. Thus, apps which aim to make this ultra-conscious consumer EVEN MORE informed will easily beat out the competition.
It would be especially interesting to build a machine learning app that takes into account user preferences and choices to recommend not only related food products (like the Instacart mobile app) but also the most nutritious choices available. A sort of "healthier food recommendation engine"! Let's see Costco and Whole Foods compete with that! ;)