The milk is a little past its expiration date, but we need some for our coffee or cereal, so we sniff the gallon to decide whether it’s still “good”. If it smells sour, we make a face and toss it down the drain, preferably with water running to wash it away as quickly as possible. We dodged a bullet there, right?
The problem is that the sniff test tells us nothing about whether there are pathogens (disease-causing microbes) in the milk which will make us sick. All the sniff test tells us is whether or not putting the milk in our coffee or on our cereal will make us gag. Why is that? Well, for milk purchased from a store, it’s because the milk was pasteurized to kill any dangerous microbes, but for most other foods it’s even more basic: most pathogens do not actually cause food spoilage, therefore spoilage ≠ pathogens present***.
This reality is highly counterintuitive, since the chemicals spoiling food gives off tell our brains that we definitely should not eat that food. My best guess for why we have that intuitive reaction is that in the days before refrigeration and an understanding of germs, spoiled food might very well contain both pathogens and spoilage microorganisms. Therefore, being averse to eating spoiled food could save you from a very nasty bout of foodborne illness. (Interesting side note: highly spiced foods may have developed in part to disguise “off” flavors in the meats used).
So what is spoilage, really, and what kinds of microbes are responsible for it? Put simply, spoilage is microorganisms digesting a food product before we have a chance to eat it ourselves. Just as humans produce waste from food, microbes also produce waste from digesting the nutrients in foods; these wastes are the smells, textures, and tastes we associate with spoilage. Below are details on some of the more common food spoilage microorganisms (pictures following of both the microbes and the spoilage they cause).
- Pseudomonas spp. Pseudomonas frequently causes spoilage in red meats, fish, and poultry as well as vegetables (1,2,4). In addition to causing an “off” smell, it also can cause a slimy layer to form on top of the meat (1). Pseudomonas grows well at refrigeration temperatures (1).
- Shewanella putrefaciens: Shewanella causes spoilage of fish products (3), sometimes in combination with Pseudomonas (2). Again, think nasty smells (as implied by the “putre” in putrefaciens)
- Clostridium species: these are the exception to the rule that spoilage microorganisms don’t cause foodborne illness and foodborne pathogens don’t cause spoilage (3). These bacteria can cause food spoilage, but they won’t grow if oxygen is present, so they’re only a problem in improperly canned or vacuum-packed foods. Don’t eat spoiled canned goods; they could literally kill you.
- Penicillium expansum and Botrytis cinerea: these are both fungi which cause spoilage in fruits, primarily through infecting cuts and bruises on the fruit’s surface (4). They are not dangerous for humans.
- Erwinia carotovora is a bacterium which causes soft rot in fruits and vegetables. If you’ve ever seen vegetables slowly collapse into ooze in your crisper drawer, Erwinia is likely responsible (4).
In summary, most “spoiled” food is actually perfectly safe to use, and finding creative ways to use it reduces food waste. For example, if you can’t stomach the taste for drinking slightly sour milk, or feel better about using it for a cooked product, you could use it for yogurt or soft cheese. Wilted or soft-rotting vegetables and fruits are also OK., food safety-wise. If you don’t want the taste, cut out the spoiled bit and use the rest. If the texture is off, it could go in a cooked dish or be mashed. A slightly malodorous bit of meat, poultry, or fish could be used in a dish where its flavor will be disguised by heavy spicing and where the texture is less important. How much slightly spoiled food you use vs throw out food is up to you and how picky you are about the flavor and texture of your food. Just be sure to cook your food properly, as always.
And now, some images of these lovely pathogens and the damage they can cause:
***Caveat: this assumes the food has been stored properly and doesn’t already contain pathogens (think produce or raw milk). If either of the assumptions are false, the food may in fact contain pathogens which could make you ill; they just aren’t the ones causing the food to look or smell weird. Using slightly spoiled foods in cooked dishes is probably the better way to go on spoiled food.
Sources and Additional Information:
(1) Spoilage Microorganisms, Chapter 1.2 (pp 4-7) : https://books.google.com/books?hl=en&lr=&id=trtQAwAAQBAJ&oi=fnd&pg=PP1&dq=food+spoilage+microorganisms&ots=97GmrvBOZN&sig=5C2XGSZssyrFxE-HAmD_RHHzZp4#v=onepage&q=food%20spoilage%20microorganisms&f=false
(2) A research paper, so a bit more technical. Gram et al., 2002. Int. J. Food Microbiol. https://s3.amazonaws.com/academia.edu.documents/34831759/Food_spoilage-interactions_between_food_spoilage_bacteria.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1516152983&Signature=BJ5nQB9BAI%2BsAWaz7IDorF%2FoWQI%3D&response-content-disposition=inline%3B%20filename%3DFood_spoilage_interactions_between_food.pdf
(3) MicrobeWiki. A good quick source for images and basic information on many different microbes:
(4) On fruit and vegetable spoilage–another academic treatment. Barth et al. . http://seward.co.uk/resources/wp-content/uploads/sites/3/2013/03/food-microbiology-fruit-vegetable-TVC-stomacher-400.pdf