We mostly avoid eating spoiled food by detecting its peculiar, but still all-too-familiar smell and colour. However, bacteria such as salmonella and E.coli contaminate food without displaying any signs that are easily detectable by the senses of sight and smell. These bacteria are a major cause of food poisoning.
Temperatures hotter than 75°C kill most bacteria within a few seconds. But to do the job, it's the food's internal temperature that has to reach 75°C, not just the outside. And with the exception of poultry, cooking meats or vegetables to that degree renders them all but inedible. Fortunately, we have size on our side. Bacteria are really small, and it takes quite a lot of them to make us sick. So rather than killing them, and ruining our food in the process, we merely have to stop them from multiplying — or at least slow them down — so that there's never enough of them to do us any harm. We do that by controlling the food's temperature during every stage of storage and preparation.
Bacteria won't multiply in the colder temperatures of a refrigerator or freezer, or at temperatures hotter than 60°C. Where they thrive is between 5°Cand 60°C, a region known as the "Food Temperature Danger Zone." To substantially reduce your chances of contracting, or passing along, a food-borne illness, make sure that your perishable foods spend more than an hour in the Food Temperature Danger Zone.
Armed with the knowledge that temperature plays a critical role in controlling the effect of bacteria on foods, we can effectively minimize the chances of food-borne illness. Other factors we need to keep in mind include time, moisture, and acidity level.
Ref :
http://culinaryarts.about.com/od/safetysanitation/a/bacteria_2.html
http://www.cooksrecipes.com/tips/meat-cooking-temperature-chart.html