As consumers, we inevitably fall prey to the ambiguousness of the food labels placed on packaging by food manufacturers. The most commonly misunderstood labeling involves the terms, "Use by", "Best if used by" and "Sell by." Each term is sound, but what should be more defined is, whether the term relates to economics or food safety.
Grocery products cannot last forever. Grocery manufacturers want you to buy their products, and they spend considerable capital on designing a product that is palatable and presentable. It's about quality!
Food quality terms
The manufacturers use labeling terms such as, Use by, Best if used by, Best by, and Best before, to ensure that their product is purchased while the quality of the product is optimal. For example, if a consumer purchases a jar of mayonnaise and realizes at home the Best by date is a week away, there need not be a food safety concern, but quality will begin to degrade. As the ingredients start to degrade--emulsifiers and inhibitors lose effectiveness and colors fade--the product slowly loses the best quality characteristics. A mayonnaise product that is considerably past a Best by date may begin to separate and darken in color, but still be safe to eat. Manufacturers profit by turn-over of product on grocery store shelves--these dates assist in moving product and ensuring quality for a reasonable amount of time. As well, these terms are commonly placed on shelf-stable products (semi-perishable).
Food safety terms
Shelf-life terms that relate to food safety are Sell by and Expiration date. Manufacturers of perishable food products, such as fresh meat, pre-packaged salad greens and dairy foods use the term Sell by to ensure perishable foods maintain a status of "safe-to-consume" within a reasonable time-frame. For example, pre-packaged salad greens typically have a 10--14 day shelf-life under refrigerated conditions of 40°F. If a consumer stores the packaged greens appropriately, the product could still be safe to consume several days after the Sell by date. Any fluctuation of storage conditions or temperature abuse will dramatically decrease the ability of a product to last much longer than the Sell by date.
The term Expiration date will most likely be seen on products like baby food, infant formula and nutraceuticals (food products that provide health benefits in addition to their basic nutritional value). Food products with an Expiration date should be used by the date indicated and discarded after that date.
The bottom line--if a food product does not smell, taste or look right, do not eat it! Consumers can find various "home" food safety tips at www.fda.gov.