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 sometimes, after a few months, allow electrolysis between the steel can itself and the tin-plating. (Two metals in an electrolytic (acid) solution). When this occurs, the metals corrode slowly, with hydrogen gas being released. This can be detected as a slight outward pressure at 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. Stored cans of acidic canned foods should ideally be consumed within 6 months to avoid this effect.  This applies to tomato products, canned fruits, canned sauerkraut, and some vegetables packed in a salt solution.  Non-acid, non-salty, 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 the "swollen can" resembles what happens when the deadly C. botulinum bacteria have been allowed to grow.

 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 be 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 "flat-pack" 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. Both are completely safe of course. 

  

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Q: Can eating tuna straight from the package give you food poisoning?

​A: Commercially-canned foods have been heated to 250°F(121°C), meaning they are now sterile (and obviously cooked, ready to eat).

      But there is more to the story specifically with tuna.  Tuna belong to the group of Scombroidiae fish in which excessive histamine can develop after death due to bacterial growth within the fish due to poor temperature control on the fishing boat or the wharf.     The high histamine survives processing and heating, meaning it can be in the canned tuna. It comes from poor processing, and there is no way to detect it as a consumer.

Scombroid poisoning is not usually serious; flushing, headache, sometimes vomiting, sensation of tingling around head and neck. Reduced by taking antihistamines. Should the person also be taking certain drugs which are histaminase-inhibitors (such as Isoniazid), the effect of the histamines is amplified. 

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Q: Why does canned food have an expiry date?

To begin with, they don't.  Only baby foods have "expiry dates" All of the rest are "best before dates, completely without any enforceable merit or obligation.  They are simply an advisory notice for you to ignore or be informed by at your pleasure.   

      Microbial decomposition is NOT the purpose here. The manufacturer is exercising caution that OTHER forms of deterioration may have taken place, and besides, it’s a good opportunity to frighten you enough to dump perfectly good food and buy more (“25% off if you buy the case!”) 

     Properly-retorted (pressure-processed at 121C for specific time) canned foods are among the safest foods ever produced. They have indefinite shelf life and some have been found in food caches more than a century old, still in edible condition. It goes without saying of course that, damaged, leaking, perforated, punctured, or externally-corroded cans are not to be used, and, apart from carbonated drinks, ANY can that appears swollen or spurts contents outward when opening.

So what are these “other” forms of decomposition that could be reason not to consume the contents?

      Corrosion of the can structure from INTERNAL causes. This can sometimes be seen in long-storage acid foods (e.g. tomato products, fruits, juices, sauerkraut), where the acid has reacted with the inner tin lining and has begun to attack the steel. Electrolysis will eventually cause perforation, but in the early stages, the food can contain excessive tin and iron compounds and have undergone colour and taste changes. Don’t forget to examine the inner lining and discard if the lining is severely mottled or corroded.

     Physical changes can occur after long storage, such as separation, discoloration, and other cosmetic defects. An example here could be canned evaporated milk, or canned meat products containing gelatin that has liquefied. None of these are harmful.

Finally, declared nutrients may have diminished, such as vitamin C. It is unlikely you were depending upon the minuscule quantities of this vitamin from canned food, but it’s important to point out that it may be less than you expected over long storage.

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