It's scary when stores have to pull unsafe food. It can take days to pinpoint the batch or supplier that caused the problem, which could lead to people getting sick... or at best, force the store to yank more products than necessary. Walmart believes technology could offer a better way. It's trying out blockchain's distributed ledger as a means of identifying the sources of poisonous food. Digital receipts for the food identify everyone in the supply chain, ranging from the original location through to the inspectors and shipping companies. If everything passing through a specific warehouse is making people ill, Walmart can identify that troublesome food and yank it within minutes -- and there's no easy way for unscrupulous suppliers to alter the info.