This is a really good idea, one suggestions include :
1) Developing a piece of hardware which can be stored in the fridge. Essentially what this would do is it would monitor the air samples exhaled by certain foods and based off that sample, link up to the app which would notify the user that these foods and quantity in your fridge is about to go off. Ideally Developing such hardware would require a lot of testing of a wide spectrum of common foods , however you could set up a system which would let everyday users to record or submit a sample of a food product which could be added to a database and reconfirmed by a test team. As an incentive for everyday users providing such data, you could offer discounted coupons to various grocery chains or points which could lead to a gift card etc.