Researchers at UC Berkley and National Chiao Tung University have discovered that bacteria in milk can change the resonant frequency of a circuit in contact with milk. They’ve devised a way to monitor this frequency and alert the consumer when this happens.
There are other technologies for sensors to detect bad food, but this one looks to be very cheap. In the prototype, specially equipped bottle caps are 3d printed with electrodes and resonant circuits that can be read using technology similar to card reader.
Here’s an article from UC Berkley that has more details: http://news.berkeley.edu/2015/07/20/3d-printed-electronic-smart-cap/
As the sensors get cheaper we’ll see them embedded in more and more consumer products. Could cheap sensors be included in our products to aid the consumer?