Will spoiled food make you sick?

I recently received a message from Jo, asking me to comment on this subject.  Jo actually raised a lot of questions and, of course, the answers are not simple.

First question:  If the food smells off, tastes bad, has a poor colour etc. is that a good indicator it will make you sick?

Humans have been on the Earth for a long time and one of the reasons they have survived for so long is that they have evolved self preservation mechanisms.  If the food smells off or doesn't look right, there is a chance that it is poisonous in some way and we tend to avoid it.  Sometimes I wonder what the first people to taste durian or blue cheese thought they were doing!  So, clearly, the answer has to be a qualified "Yes, it might make you sick, but some sort of tribal wisdom suggests that a few foods can look and smell awful but still be OK to eat".

Jo then went on to say that she thought that the food was unlikely to make you sick, as the changes were caused by spoilage bacteria and enzymatic reactions, which are not the same as pathogenic bacteria, which she thought do not alter the food in the same way.

This is where it gets complicated.  Spoilage bacteria do cause some changes, which, by definition, make the food unacceptable to most people.  Other bacteria, such as Salmonella, may grow alongside the spoilage bacteria.  When the food is eaten, the salmonellae  set up an infection in the gut and produce the familiar food poisoning symptoms.

Second question: Is it true that pathogenic bacteria don't alter the food?  Again, it depends on the bacteria.  Generally, clostridia alter the food a lot, producing many smelly compounds and gas.  This would put most people off eating the food, so they would be safe.  But some Clostridium botulinum strains are non-proteolytic.  That is, they don't break down proteins and they don't produce the foul smells that the proteolytic strains make.  So you could find improperly processed canned foods that appear quite normal, but could kill you.

Jo's third question was perhaps the most fascinating: she was most interested in how the food could potentially have both types of bacteria on it near to the time it was made and correctly identified contamination of the food as the cause, possibly by cross-contamination from some source.  A good example would be a careless food handler.

At this point, the food could cause food poisoning if consumed - Salmonella transferred from raw meat to a cream pastry would be a good example here.

Jo went on "However as time goes on and assuming the food gives them all the things they both need to grow, is it as simple as the pathogens might be around for a short time until the spoilage guys, who being better competitors for resources, take over; so by the time the food
is showing signs of being spoiled, the pathogens have been killed off?"

That's a really insightful comment.  Many fermented foods are actually safe because of this pattern.  Take raw cabbage and make sauerkraut:  we shred the cabbage and add about 2.5% salt to it and then press it into a container and seal it.  The salt draws out the tissue fluids from the shredded leaves and bacteria naturally on the leaves begin to produce lactic acid.  If we sample on the first day, we can find all sorts of bacteria, including Escherichia coli and possibly Salmonella.  However, as more acid is produced, the pH falls and the potential pathogens die off.  Finished sauerkraut has a pH around 3.1 to 3.7 and is perfectly safe to eat, though there has been a lot of bacterial growth in it.

So, to return to Jo's original question, "Is spoiled food potentially safe to eat, even though it looks and tastes awful?"    Sorry Jo, I can't answer that.   But if it tastes awful, why would you want to eat it?

One last point:  there is an anecdote that a lady opened a can of peas and thought that they looked a bit different.  She tasted one and cooked the rest.  She died of botulism.  Ironically, if she had cooked them and then tasted one, she would have lived, as the botulin toxin is heat labile.  I don't remember where I read this - it was somewhere around 1980, but it's a good example with which to finish this article.