Spoilage organisms usually make food smell (and look) off, whereas harmful organisms rarely do. Manufacturers are super-cautious about use-by dates, and they'll tell you that they have your safety at heart. What they really want is for you to throw perfectly good food away and buy more. It's fine to use your nose and eyes but you need to use your intelligence as well. Dairy products and leafy salads are among the worst offenders when it comes to giving you a dodgy tum. It's also worth remembering that products such as soured cream, yoghurt and creme fraiche are produced using carefully-nurtured cultures. They're not just fresh stuff that's gone a bit off. and that pasteurising milk is a compromise to avoid changing the taste of the milk by too much.