Checking the "best before" and "expiry date" labels on foods, from milk and cheese to bread and meats, is one of the first things consumers should do before throwing them in their grocery carts.

But what do these labels mean?

A Health Canada advisory issued earlier this week informs consumers about what they should know before stocking their fridges and cupboards.

For instance, in Canada, a best-before sticker must appear on almost all prepackaged foods that will keep fresh for 90 days or less, Kirsten Mattison of Health Canada's bureau of microbial hazards told CBC News on Friday.

Brenda Watson, executive director of the Canadian Partnership for Consumer Food Safety Education, adds: "Those best-before dates are the manufacturer's promise to the consumer that the packaging and the food within them will be of the highest quality standard and will contain the nutrients as outlined on the label.

"After that date, the promise no longer stands.…Definitely when a consumer purchases their food, they should be aware of the best-before date and if it has already passed, they should leave it on the shelf and look for something fresher," she says from Cambridge, Ont.

Some foods with a longer shelf life and that are critical to nutrition must carry expiry dates. These foods include meal replacements, nutritional supplements, infant formulas and formulated liquid diets, which should not be consumed after the date on the label has passed, Mattison says from Ottawa.

"That's absolutely critical for those products because they may be all the food people are ingesting."

Here are some other answers to common questions about the freshness of food products:

Does Opening A Product Affect The Best-Before Or Expiry Date?
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Some foods show a best-before date even if they are not required to do so, but these dates tell you about the freshness and shelf life of "unopened food," so once a product is opened, there's no guarantee it will have the same flavour, texture or nutritional value. "Consumers need to know once they've opened a carton of yogurt, and taken that seal off, that best-before or expiry date is no longer in effect," says Watson.

1. Does opening a product affect the best-before or expiry date?

Some foods show a best-before date even if they are not required to do so, but these dates tell you about the freshness and shelf life of "unopened food," so once a product is opened, there's no guarantee it will have the same flavour, texture or nutritional value. "Consumers need to know once they’ve opened a carton of yogurt, and taken that seal off, that best-before or expiry date is no longer in effect," says Watson.

2. What's the significance of an expiry date on food?

The expiration date is the date up to which the food maintains its microbiological and physical stability, and the nutrient content declared on the label. That means it's important to use that food before the expiry date to get the most nutritional value from it.

3. What if a food looks or smells OK, but has passed its best-before or expiry date?

Even after a best-before or expiry date has lapsed, a food may smell or taste fine, but that can be dangerous, warns Health Canada. When a best-before date has expired, "use your judgment. When in doubt, throw it out," says Health Canada. "When an expiration date has passed, there is no doubt, throw it out."

4. Does keeping food cold or freezing it extend its best-before date?

Foods should be refrigerated within two hours of purchase (sometimes sooner) for a best-before date to be valid. The Canadian Partnership for Consumer Food Safety says refrigeration slows down but does not stop bacterial growth, so food can still go bad. Some food can be frozen to keep beyond its best-before date, but how long it can be safely frozen depends on the type of food and its ingredients. Consumers can also contact manufacturers for information about freezing and storing their products.

5. Do foods in other countries have the same warnings about when they may go bad?

Standards about when foods expire or are best differ in various countries. For instance, the U.S. Food and Drug Administration does not require food firms to place "expired by," "use by" or "best before" dates on food products, although U.S. law states that foods sold in the country "must be wholesome and fit for consumption."

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