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Kind of a strange question, but say something has spoiled, i.e. smells bad, tastes bad, etc.

Will it actually always make you sick? Like for instance spoiled dressing, let's say it tastes sour, smells nasty and you eat it, will it make you sick?

If so what actually makes you sick? The toxins/bacteria? or it just being unappetizing?

I ask because I tried a TINY bit of ranch and while it tasted mostly fine (it was a bit tangy) it smelled quite tangy but looked fine. I also saw this article about people eating bad food and not getting sick.

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    Ranch dressing is made from buttermilk and so is supposed to have a sour smell and taste. Does the dressing smell worse than when you bought it or is the the first time you've sampled it? – Ross Ridge Mar 25 '15 at 18:59
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    Also would like to add, that things that smell bad might also be considered "good". i.e. en.wikipedia.org/wiki/Fermented_fish – Viktor Mellgren Mar 26 '15 at 9:30
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What most people don't get when it comes to food safety: Spoiled food has a chance of making you sick.

When food is visibly spoiled, it has large bacterial colonies growing in it. This means that it has been exposed to conditions which were promoting bacterial growth. Anything which was present on your food will have grown, unless outcompeted by something else.

The bacteria which make food gross are rarely the ones which make you sick. But if they had the chance to grow, the real pathogens had a chance to grow too. If they were present, then they grew, and you will consume them, together with whatever waste products their colony produced.

Assuming that there were infectious bacteria on your food, you will ingest them. From there on, it depends on a ton of factors whether you will get sick or not - the amount you ate, the acidity of your stomach contents, the state of your immune system. The last does not mean "you'll only get sick if you're immunocompromised" - it is like being around a person with the flu, even if your immune system is intact, you can still catch it.

There are pathogens which cannot infect you, but simply produce toxins. Botulinum is the poster child for these, but I think B. cereus also works through a toxin. In this case, eating the food poisons you. Depending on the type and amount of toxin ingested, as well as the current state of your body again (mostly liver function), the symptoms can run the entire spectrum from "so weak you don't notice them" to "you die despite being correctly diagnosed and treated".

Then there are a few more exotic possibilities, for example Hep C cannot cause liver cancer alone, but it does so (sometimes) in the presence of a type of toxin produced by moulds. But this is not actually considered foodborne illness any more, even though it's possible that eating mouldy food was the trigger. It falls outside of the scope of food safety, as the fault cannot be pinned on a specific dish you ate. Nevertheless, it's one more reason to not eat spoiled food.

There is also the possibility that your spoiled food has no pathogens at all, and the spoilage microbes outcompeted all the bad guys, and then of course nothing happens.

So, to sum it up: if you eat spoiled food, there are four possible outcomes:

  • You get sick (right away or with an incubation period of 2-3 days) and notice it
  • You get sick (right away or with an incubation period of 2-3 days) but the symptoms are so weak you don't even notice it
  • You get a different health problem, soon after eating or even years later, without knowing that it was the spoiled food which caused it
  • Nothing happens.

There is no way to predict which one will occur. Actually, these outcomes are possible with any food you eat, including safe food, but eating unsafe food increases the probability of the first outcomes a lot, and eating spoiled food increases it even more.

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    Most bacterial pathogens require an incubation period after being ingested before symptoms show up. You might not get sick until the next day or even days later. This is why outbreaks can be hard to trace back to their source. People tend to blame what they just ate but really it was something they ate a couple of days ago. – Ross Ridge Mar 25 '15 at 19:06
  • @RossRidge good point, I'll update it. Although the incubation period is rarely too long - my memory says 48 to 72 hours, do you have better data? – rumtscho♦ Mar 25 '15 at 19:09
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    @rumtscho Compared to what people think the incubation period is ("I ate at this restaurant and then I got really sick!"), two days is quite a long time. – Cascabel♦ Mar 25 '15 at 19:22
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    According to the CDA it seems to vary a lot. For symptoms to appear: 12-72 hours for salmonella, 6-24 hours for clostridium perfringens, 2-5 days for campylobacter, 1-10 days for E. coli O157:H7. cdc.gov/salmonella/general/index.html cdc.gov/foodsafety/clostridium-perfingens.html cdc.gov/nczved/divisions/dfbmd/diseases/campylobacter cdc.gov/ecoli/general/index.html – Ross Ridge Mar 25 '15 at 19:23
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    The moral of the story is: Don't ever assume that what you just ate was what caused you to get sick, and as a corollary, don't assume that you're "in the clear" if you manage to get through the night after eating something unsafe. Food poisoning isn't just hard to trace, it's basically impossible, and most people have dangerously wrong ideas about how accurately they can identify the cause. – Aaronut Mar 26 '15 at 5:00
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I like rumtscho's answer, but feel the need to add to it.

Amount ingested is a main factor in if you get sick, so to answer your main question - a TINY bit of bad Ranch Dressing is unlikely to make you sick.

A lot of foods are intentionally spoiled - yogurt, buttermilk, cottage cheese, cheese (especially Blue Cheese and the other moldy ones), meat is aged so bacteria can break down the tissues. Fermenting also includes growing bacteria so technically sourdough, wine, and beer are also made with spoiled food. Penicillin comes from mold. So, no... not all spoiled food will make you sick, just like not all germs are bad.

I think a lot of whether you get sick depends on your immune system, and you can actually build an immunity to certain germs (like a vaccination of sorts).

Case 1: Americans get sick when drinking the water in Mexico - most Mexicans don't, probably because they are exposed to it from an early age.

Case 2: My ex had a bad habit of only rinsing our daughter's bottles which left a coating of milk in them, and I'm pretty sure I didn't catch him every time. The small amount of germs were never enough to make her sick (thankfully). One day when she was 1 1/2 he gave her a bottle in the crib and it disappeared. I still have no clue where she hid it, but a couple days later I went to wake her from her nap and she was drinking it (YUCK!!! Daddy DID get yelled at for all of this if you were wondering). She didn't get sick, and I have to wonder if it has anything to do with the fact that Dad kept exposing her to small amounts of the germs with those rinsed bottles.

Case 3: Even in the middle of the woods you aren't supposed to drink from streams because animals poop near them and that can make you sick (I think it's Giardia or something like that). Are you going to tell me animals didn't poop near streams back when Native Americans and the Pioneers were alive? And you don't hear of most of them always being sick or dying from drinking out of streams.

Case 4: My boyfriend's sister gave us some stew once... after I'd had about half a bowl I found a piece of moldy food in the bowl (YUCK!!!). I never got sick from it (except for the nausea when thinking about eating mold - YUCK!!!).

But to answer your main question... No, spoiled food doesn't always make you sick. No, you aren't likely to get sick from a TINY taste of spoiled ranch dressing, unless it's horribly spoiled (and I'm not even sure if that's possible) or your immune system is compromised.

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    regarding Case 3, this reminds me of the Oregon Trail outcome: "You have died of dysentery" – Michael Mar 25 '15 at 22:30
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    Giardia may not kill you, but if not treated will effect your ability to properly digest food, potentially for the rest of your life. Hence historic populations with Giardia generally catch other diseases more easily, and generally do not live very long – TFD Mar 25 '15 at 23:23
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    Re case 2: there are strong indications that small amounts of bacteria are indeed beneficial. We're not born with working immune systems. As children grow, the immune system learns to recognize bacteria and develops immune reactions to them. The rise in allergies might be caused by immune systems failing to learn from normal, common bacteria and instead targeting the body itself. – MSalters Mar 26 '15 at 11:10
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    @MSalters You're mixing up auto-immune diseases and allergies, the two are very different. Allergies do not turn "against the body", they just mean an overreaction to some foreign object. They still attack the foreign object, not your body. Contrast gluten intolerance or wheat allergy and coeliac disease (auto-immune - presence of gluten causes your body to attack your own intestines). – Luaan Mar 27 '15 at 8:49
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    Ugh, more anecdotal evidence on food safety. This really is complete nonsense. Perhaps the biggest problem with anecdotes like this is that when people say "I/he/she didn't get sick", they invariably mean that they didn't appear to get sick immediately afterward. The problem is that sickness can develop hours or even days later, and by that time, most people have forgotten about the original incident. Anyway, you can tell that this is unscientific because it refers to fermentation culturing, and dry-aging as "intentionally spoiled". – Aaronut Mar 28 '15 at 17:27
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Physician opinion: Foods that use fermentation, chemicals (vinegar, salt, sugar), bacteria, fungi, yeast, etc., in their production are not "spoiled", they are "processed", and include many regional specialties found less palatable by many in the U.S., except alcohols.

Spoilage refers more to when foods become inedible due to either excessive growth of added processing organisms (think three week-old Camembert), the accumulation of pathogenic populations, or their toxic by-products, or dehydration due to poor storage. Generally, the by-products of intentionally added organisms are not harmful, else we wouldn't use them, but the ammonia in the cheese is too much (and sets off explosive detectors in airports !, as I found out).Pathogens are organisms, or their by-products, harmful to humans.

There are several routes to illness from food: 1) Food can be contaminated in its production with highly virulent organisms which grow well even in small amounts (hepatitis A, typhoid, cholera) and point out the need to avoid employing carriers of these, and to practice good hand washing. No amount of proper storage prevents these.

2)Food can be contaminated in its production cycle with less virulent organisms which can then survive stomach acid and grow in the intestines and produce illness. Washing of foodstuffs (E. coli) and full cooking to bactericidal temperatures (Salmonella) reduces the chance of illness, as does cold storage to decrease the numbers of these bacteria to which a consumer is exposed. The immune system can deal with smaller numbers of these contaminants. Immune-compromised or immune-immature persons (children) do less well here, hence the instructions to fully cook eggs for children.

3) Food can be stored poorly such that less virulent pathogens grow to a population size that makes illness highly likely. Where food is already inhabited by an added organism, such as yoghurt, the waste products of the organism tend to suppress other organisms, and thus make spoilage less of an issue, hence the use of the method to prolong storage, historically. But, in other foods, if bacteria are present (and they are in the air constantly), warm storage will cause overgrowth. If poor preparation methods are used (cutting vegetables on meat boards, etc.), then the chance of Salmonella overgrowth is high. Keep surfaces and tools clean and separated! Richer foods, larger surface area (i.e. chopped meat), exposure to air-borne organisms, moisture and warmth combine to support bacterial growth, so closed, cool, dry, unchopped storage is good practice.

4) When storage conditions allow bacterial growth, two methods of illness occur. First, a direct pathogen grows to numbers too high for the body to cope with, and ingestion leads to growth in the intestines. This method usually appears as an illness after a couple of days, when the pathogen, say Salmonella, either invades the tissues or produces a toxin responsible for a symptom (diarrhea, etc.). Illness is more rapid dependent on the amount of ingestion.

5) Much food poisoning, however, is due to an indirect method: the organism produces a toxin as it grows in the "stored" food, and ingestion results in a rapid illness as the toxin is absorbed, often in the stomach. This type accounts for the '30-90 minutes after eating' illnesses, frequently caused by Staphylococci found on human skin, and not deactivated by heating. Again, prep methods are usually at fault, worsened by warm storage.

6) It is not often possible to determine whether food is contaminated as above, because some pathogenic organisms, and toxins, produce no obvious markers detectable by humans. But, if there is spoilage due to a non-harmful organism, this is obviously a sign that harmful things could also be present, and thus the food is better avoided, no?

7) Hepatitis B and C are communicated by blood-to-blood contact, not oral ingestion. And, hepC causes cancer in the liver through prolonged infection leading to scar tissue which transforms into cancer after 25 years or so, with some help from host-viral interaction, as far as we know.

8) Most molds are harmless, but high concentrations taste bad, even on cheese. Peanut mold is not, as it produces aflatoxin. There are plenty of other things that grow on food, such as slime molds, which are disgusting to look at, but not harmful. Even so, they are a marker for spoilage.

I cut mold off of cheese because I can't bear to waste it. I prefer fresher stuff, tho. I store everything I can fit into the fridge as this retards oxidation, which tastes bad, but is not harmful per se. Think olive oil.

The long and short of it is, know your food cycle.

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    "Physician opinion", does this mean you are a physician, or documenting an opinion that a physician gave? – rackandboneman Jan 20 '17 at 9:14
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It depends what's in there; in most cases it's the toxins that make you sick, rather than the organisms themselves, but that's a generalization. Moldy cheese is bad - unless, of course, it's supposed to be moldy, in which case it's good (well, not to me, but to folks that like blue cheese it is...)

Bacteria are bad - unless, of course, they happen to be "friendlies" as in sour cream or yogurt...possibly the source of your "tang" - but possibly not, or not the only thing...

Some of the worst critters don't actually make the food smell or taste bad at all (our good friend botulism, re-hashed last night a bit.) You could have some spoiled food that smells and tastes horrid and does you no harm, but if a similar-smelling batch has botulism too, that one might kill you - or you might have some that smells fine, but still kills you.

It's a tricky subject.

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  • The "tang" in dairy is almost certainly lactic acid. This is a trick by Lactobacillus to keep its foodsource to itself. It's far more acid-resistant than most competing bacteria, so by excreting acid it keeps out other species. – MSalters Mar 26 '15 at 11:26
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    I wouldn't equate mold with bacteria. Most mold is bad, with only a small variety of strains being used in cheese-making; conversely, most bacteria are benign, and only a handful of strains cause serious problems. – Aaronut Mar 28 '15 at 17:29
  • I disagree that bacteria are bad. Right now, each one of us has trillions of bacteria in and on our bodies. The vast majority of these are beneficial in a symbiotic way: without them, we would die. While it is gross to think of these tiny, single-celled organisms crawling on us... pooping on us... reproducing on us... we need them and they need us. There is a very small minority of bacteria are bad for us. – user21524 Mar 29 '15 at 3:12
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There are also things which can happen to your food which won't cause any immediate symptoms, but which are quite bad for you in the long run. For instance, some molds produce mycotoxins (the most famous being aflatoxins) which can increase the risk of cancer.

Apart from this, I believe rumtscho covered the reasons you shouldn't eat spoiled food pretty well in her post.

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There's a lot of good answers here, so let me focus on the part that's mostly omitted.

A huge chunk of the food we eat is spoiled. Intentionally. The reasoning for each is very wide, from preventing harmful spoilage (food preservation), to improving taste, texture etc.

The most obvious of those foods are of course cheeses and yoghurts. Even the simplest cheeses can be considered "spoiled" - they were made by exposing milk to stomach enzymes. "Cultured" cheese include either molds or bacteria on top of that. Yoghurt depends completely on introducing bacteria to the milk. Knowing this, claims of "antibiotics in milk" are pretty absurd, heh? :) Note that before you get used to eating (a new kind of) cheese or yoghurt, a lot of your safety systems are screeming "don't eat that smelly, ugly, bad-tasting stuff!". In fact, smelliness is a great indicator of spoilage (it's one of the principal uses of smell in humans, unsurprisingly). What it can't detect is whether the spoilage is actually harmful.

Hams are an often overlooked example. They usually don't have any bacterial cultures added during production, because they're quite fine with what they already have. The key part is adding salt. Unlike what you might have heard, salt doesn't kill bacteria. In fact, there's a nice little bunch of bacteria that quite like salty meat, you might have heard of those - lactobacilli. The reason salt can be used to preserve meat is that it causes the bacterial growth in the meat to balance heavily in favour of those lactobacilli, which are harmless (and in fact, beneficial) to us. Other cultures, some of which may be harmful, find themselves unable to compete, so their numbers stay relatively low.

If you go further, even bread can be considered "spoiled" to some extent. Its production depends heavily on enzymes and micro-organisms that break down the starches in wheat and rye. Apart from yeasts, you will also find our good old friendly lactobacilli responsible - in "natural" sourdough bread, the lactobacilli predigest the dough for the yeasts, or (especially in rye bread), they are responsible for pretty much the whole leavening.

Alcoholic beverages can also be considered a spoiled food. Like bread, they are made through fermentation of fresh (or even pre-spoiled) food, changing the sugars in the original fruits (or e.g. grains) into ethanol and various other byproducts. Not to mention that it also loses a lot of its micro-nutrients, a common consideration in harmful spoilage.

Many local delicacies tend to include spoilage - basically, any time you eat some delicacy that smells awful, it's probably (controlled) spoilage. For example, Olomoucké tvarůžky, a very bad smelling kind of Moravian cheese, actually use the same bacteria that causes smelly feet. You will find the same in many traditional wines as well (remember the part where people crush the grapevines with their bare feet? Yum, yum, right? :P).

So, what kind of spoilage you should always be critical of?

  • Molds are generally dangerous, and may cause chronic, incurable diseases; unless it's a cultured mold (e.g. moldy cheese), stay away. Cooking the food will not help, since the harmful part usually aren't the molds themselves, but rather the toxins they produce - those will not disappear through cooking. Cutting out the moldy part will usually not help either - by the time you can see the mold, it's usually grown all the way through the whole food.
  • Anything with eggs or poultry. You really don't want Salmonella, it's really ugly. This of course includes stuff like dressings, mayonaisse and similar. Just stay away. If in doubt (as in, I've had those eggs for a while, but they don't show any visible or smellable sign of spoilage), make sure you cook the food thoroughly. Using such eggs in a baked meal that's been in the oven for hours on 70-90°C is probably fine. Using them in e.g. scrambled eggs is a bad idea. Using them in mayonaisse is just asking for trouble.
  • Food left in contact with the ground, especially when left out in the sun.

Humans are actually pretty tolerant to lots of different kinds of spoilage, but unless you're actually used to it, it's pretty risky.

But even for other spoilage, always treat food that doesn't smell or look good with suspicion. If you insist on eating it, try a very small portion first - amount is a huge factor in a lot of the bad stuff that can happen to you. However, as @rumntscho noted, this is far from fire-proof - a lot of infections and poisonings can take time to manifest, and you might not even notice trouble in a small dosage.

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Food rarely spoils on its own. The spoilage is usually a result of many different bacteria and fungi living on the foodstuff, consuming it, and depositing waste or metabolic by products. These by products may be toxic to you. The organisms themselves may be poisonous, or may simply invoke a strong immune reaction (which can also be uncomfortable).

However, even without the bacteria, food can undergo chemical reactions that degrade it. The degraded food can still be bad. Arguably, microorganisms are the most dangerous part of spoiled food, but not the only one: You could easily sterilize a piece of rotten food, and chances it would still be not good to eat.

Firstly, it may have a strong odor or taste, and may actually have developed noxious substances, which could make you vomit. Besides that, the chemicals in spoiled food may poison you.

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As an addition to rumtscho's answer:

You can mitigate the risk of bacterial infection by cooking it (except for the exceptional bacteria that can survive cooking, but those are rather rare and rarely dangerous).

You cannot mitigate the toxins by cooking (in general, they might break down due to the heat, but most don't).

Other factors to take into account are the way in which the food went bad. Raw milk spoils very differently from pasteurized milk (raw milk usually is still kind of okay, but if you want to be safe don't bank on it, might contain e-coli).

We've developed fine senses of smell and taste to deal with this. If you're worried about something being bad, smell and/or taste it. If you don't like it, don't eat it. Otherwise, you could take the risk. If you're not used to this, get other people's opinion as well (some people seem to not smell or taste bad food that well, you'd want to find out you're one of those without getting food poisoning).

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