Can I tell if a food is safe or unsafe to eat by the way it looks and smells?

By Susan Brewer, Ph.D., University of Illinois,
Department of Food Science and Human Nutrition

 
In general, bacteria that ruin the taste of food do so by breaking down the components of the food into compounds they can use for growth.  Often these compounds, because they are smaller, contribute off-odors and off-flavors and cause texture changes which make foods watery, slimy, etc. Those that cause illness do so by growing in the body or by producing very specific toxins (which are relatively large molecules) in the food, or, in some cases, inside the body.  These toxins have very specific targets.  Large molecules are unlikely to cause flavor or odor changes.  So the short answer is, yes, generally they are different.  Spoilage organisms usually grow more rapidly than do disease-causing (pathogenic) organisms, producing signs of spoilage that are a warning sign that the product may be close to being unsafe.  However, foods containing pathogens may exhibit no signs of them.
 
Source(s):
Kendall, P. 2008.  “Bacterial food-borne illness.”  Cooperative Extension Service. Colorado State University, Fort Collins, CO.  Fact sheet No. 9.300.
U.S. D.A Food Safety and Inspection Service. 2006. “Basics for Handling Food Safely.”