FOOD SAFETY Q&A

Q:  How long does canned food last past the expiry date?     

A:  Acidic foods (e.g., tomato products, fruits, fruit juice, sauerkraut) can, after a few months, allow some electrolysis in the can between the steel and the tin-plating. This will slowly release hydrogen that can be detected as a slight outward pressure on the ends of the can instead of concave ends due to the vacuum. This is termed "hydrogen swell", and is one of the causes of the contents spurting outwards upon opening, instead of normally, air rushing into the can when opened. Hydrogen swell with acidic foods is not dangerous (although if you consumed the contents you may be ingesting some salts of tin (e.g., stannous citrate), which is better avoided. Acidic canned foods should ideally be consumed within 6 months to avoid this effect.  Non-acid foods (e.g., milk, meats, fish, beans, chowder, potatoes) can remain unchanged for years if the can is undamaged and unperforated, regardless of any date shown.    [The real reason you should avoid ALL swollen cans is because of the very small but deadly possibility it contains botulism toxin.  More info here]   

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Q:  How can I tell if canned evaporated milk is bad?

A:  Evaporated milk is normal milk that has had approximately 50 percent of the water removed. Milk is an amphoteric fluid, meaning that it is neither acidic nor basic, and it will remain unchanged in the can for years, unless the can itself has been compromised in some way.  A can of evaporated milk that has been on a shelf for months or a year or two may need a good shake before opening (otherwise it can begin to separate slightly), but you should find perfectly good evaporated milk inside. As with all canned food, watch for cans stored in damp or wet locations, where rust has been allowed to develop, which can lead to perforation from the outside. Also be aware of any physical damage such as from a box-cutter or staple which may have cut the metal. 

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Q:  The supermarket is selling expired canned food at a reduced price. Is this legal?

A: To begin with, the only foods with a true "expiry" date are baby/infant foods.  The others are actually best-before or use-before or sell-before dates. These are, at best, advisory in nature and assist in stock rotation both for the retailer and the consumer. As with all "dated" foods, there is no guarantee that a food will be perfectly edible or safe even before the date, especially if it has been damaged, or abused, while in most cases the item will be edible, safe, and perfectly acceptable long after the date has passed. Learn to use your senses to understand when the food item is suspicious. 

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Q:   I have heard that dented cans cause botulism; is this true?

A:   Completely untrue. The serious food-borne illness called botulism is caused by ingestion of powerful neurotoxin that can formed in the food before it is consumed. It certainly has a connection with canned foods, in that the bacteria are strict anaerobes (will only grow in complete absence of oxygen), and the environment inside a sealed can of non-acid food will allow the bacteria to multiply and produce one of nine neurotoxins, which can be fatal. How does it get into the can?   These bacteria can be found almost everywhere, in dust, on surfaces, or blowing in the wind.  The food itself almost certainly has some spores of C. botulinum on it when it was placed in the can or jar, and we rely on proper heating after canning to kill it off.  Boiling water at 100C (212F) will NOT be enough.  Non-acid foods need to be processed in a pressure-cooker at 15 psi (reaching 212C or 250F) for 20 minutes to be sure of killing off this extremely heat-resistant bacterium. 

It's the inadequate heat-treatment that allows botulism to grow in canned food, not a dent.  [More information about botulism here]       

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Q:  How can I inspect my canned foods at home?

A:  Food inspectors use the following techniques:

• Perception: does the can appear to be distorted, or are the ends convex instead of concave? 

• Has the can been damaged by a nail, box-cutter, staple, causing a leak?

• Are dark stains visible on the outer wall, or showing through the label, that suggests a very slow leak (a puncture)?

• Is there evidence of rust on the top, bottom or sides of the can?I 

• Press each end of the can in turn; does the opposite end 'flip' out?

• Percussion: Using a finger, tap the end of the can. There should be a dull thud, not a sound like a drum.

• When shaken (liquid contents) is there a sound of foamy liquid sloshing around, or does it sound more like liquid in a vacuum (almost silent)?

• When opening a can, does the air rush into the can or do the contents foam or  spurt out?

• Examine the contents. They should resemble (sight, smell) the advertised content on the label?

• Tip out the contents and examine the inner surface of the can: is it clean and bright (can be tinplate, or amber resin, or a white polymer lining), or are there signs of internal corrosion (such as with acid foods causing electrolysis). 

• Note that some cans are naturally "flat", such as sardines. The lack of a firm concave top is perfectly acceptable in this case because of the shape of the can. 

• Note also that aerated soda/pop drinks are naturally "bulged", and freshly roasted coffee beans sometimes give off gas causing pressure in the can.  

  

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