Botulism thrives in high-moisture, low-acidity, low-salinity, anaerobic environments at between 50 and 130 degrees.
In your refridgerator pickles, you're:
- keeping cold
- adding acid
- adding salt
All of which should at least prevent the botulism from reproducing in great enough numbers to be toxic, if not outright killing it.
That's not to say nothing nasty can grow in refrigerator pickles — you're likely safe from botulism, however.
Virtually every case of botulism ever recorded in the past 50 years is due to improper home canning. The risk of botulism from a commercial product is so low that you literally have a better chance of being struck by lightning and almost as good a chance as being struck twice in the same year.
There are 145 cases reported in the U.S. each year and 65% of those are infant botulism, 20% from wounds. It's hard to get statistics on infant population, but there are 314 million people in the U.S., so I estimate the chance of getting botulism from food there to be 0.00000692675% in any given year.
You may not live in the U.S., but unless you live in a country with extremely poor or nonexistent safety standards (in which case you have more important things to worry about anyway), I'd advise you to stop worrying. You are about 20 times more likely to die in a fire and 200 times more likely to die in a car accident.
As far as the spores go, they generally aren't dangerous to healthy adults, they affect infants and those with depressed immune systems, hence the heavy proportion of infant botulism cases (often from honey). That's why the WHO warns people not to give honey to infants under 1 year old, but doesn't advise any similar precautions for adults (The reason for no advice for adults is that their stomach is acidic so the bacteria cannot grow and produce the toxin. But infants have lower acidity in their stomach and botulism bacteria from honey can grow there. Honey always has the bacteria. )
Botulism spores can only be killed at extreme temperature, i.e. above 120° C. Boiling water is 100° C, so don't even try to kill them this way. You would need a pressure cooker at very high pressure, and this is why low-acid foods must be pressure-canned; simply boiling is not enough. On the other hand, the botulism toxin is denatured at 80° C, so boiling anything for a reasonable time will make it safe from botulism, but not necessarily from the many other bacteria and/or toxins that can be in spoiled food, such as those produced by certain e.coli strains.
There are plenty of things to worry about with commercial food - salmonella in peanut butter, listeria in lettuce, listeria in spinach... the list goes on and on, but one thing that's generally not on it is botulism, and I think the constant attention here on it actually makes matters worse by taking attention away from other, more common and equally serious issues. Seriously, one person died from contaminated homemade ham and now you're afraid to eat commercial mozzarella cheese?
If you aren't an infant, don't do home canning or home food preservation, and refrigerate your food properly, you're not at risk for botulism. Period. You are, however, at risk of so many other things, and really should try to learn more about food safety in general - from which you'll definitely learn facts such as boiling for 20 minutes won't make spoiled food safe.
Best Answer
Anaerobic simply means "absence of air."
Any liquid food environment basically counts as "anaerobic." Yes, there may be some dissolved gases and exchange of air may happen near the surface of a liquid like water, but deeper in an undisturbed liquid, there's often not enough air to prevent botulism growth. Some basically "solid" foods count too, if they aren't porous or are highly viscous (but apparent solids) and contain a high enough moisture content for bacteria to grow and spread. Any solids that are relatively moist and close-packed are also a particular problem (think sausages).
Whether all of these anaerobic environments will grow botulism then depends on various other factors. Botulism bacteria need food. They dislike acidity or excess salt (think pickles) or excess sugar (think preserves) or alcohol (think fermentation), which are all traditional preservation methods. They need a certain level of water -- hence why drying was also a traditional preservation method and why solid hunks of stuff are less likely to cause problems (unless they have sufficient internal moisture). Also, oil-rich environments lack water and may not grow botulism (think mayonnaise) but water-containing food within oil can (think garlic or herbs in oil).
All of these may be "anaerobic" environments, but the other factors can prevent botulism.
Lastly, you have things like temperature -- chilling will slow botulism growth to a crawl and at low temperatures stop it completely. That's the last traditional preservation method: chilling or freezing.
Basically, anything under liquid left at room temperature for extended periods could conceivably grow botulism bacteria, assuming adequate "food" (for the bacteria) is present and none of the "preserving" environmental conditions mentioned above. Note the "extended periods" -- compared to other food poisoning bacteria, botulism grows more slowly and generally needs at least a couple days to grow to appreciable levels. And that's why any type of canning recipes, etc. from official organizations are rigorously tested with exact measurements for preservation agents (like acid, salt, etc.), unless they undergo a "pressure canning" step that heats the food hot enough to actually kill botulism spores (which will survive boiling). Similarly, preserved meats (especially processed ground ones) that can remain at room temperature need to be treated with excessive salt for long periods (sometimes acid as well), and often other agents (like nitrates or nitrites) to inhibit botulism growth.
So yes, as Harold McGee mentions, stock left at room temperature for a few days can definitely grow botulism bacteria. A casserole left at room temperature for a few days could likely grow them too. Meat left at room temperature can definitely grow botulism, which is actually one of the biggest problems for actual cases of botulism after canning -- improperly preserved meat not kept under refrigeration.
"Solid" meat and vegetables are less likely to have botulism bacteria far in their interior in the first place (and often aren't "wet" enough for the bacteria to spread far), but if they become contaminated, they can be a problem too. For example: baked potatoes stored in foil and kept at room temperature have been known to be a cause, as have pickled eggs that were pricked before pickling (thus inadvertently introducing botulism bacteria into the otherwise sterile yolk).
To quote the FDA: