How long is garlic butter safe, and why is it not a botulism risk like garlic in oil?

Unlike oil, butter isn't pure fat. Typically, it's around 80% fat in an emulsion with roughly 20% water and dissolved milk solids. So, no, it's not a truly anaerobic environment. If it were melted or clarified, it may be a different story. Furthermore, butter's crystaline structure -- at room temperature and below -- is bound to be aerated to some degree or another.

Also unlike oil, which is usually stored at room temperature, butter is usually stored at or below 40 degrees F, which retards bacterial reproduction of any kind. The botulism bacteria itself isn't inherently dangerous. It's pretty common in soil and also, therefore, in agricultural products, especially root crops like onions and garlic. When the bacteria reproduces under anaerobic conditions, however, there is a toxic chemical by-product of that specific process. Since low temperatures retard bacterial reproduction, they also reduce the risk that the toxin will be produced.

Realistically, the risks of garlic-in-oil preparations are probably mostly over-stated, but it can be a complex issue, and the consequences of botulism poisoning are too grave to take any chances.

That said, if you're making garlic butter with fresh garlic and storing it anywhere but the freezer, "months at a time" seems like too long, from a quality perspective if not a food-safety one.

3 days if not refrigerated. 14 days in a fridge. Freezing is not recommended.

Garlic butter should be safer because you make it by chopping up garlic and cooking it in butter. The cooking reduces the water content in the garlic to low enough levels that botulism bacteria should no longer an issue.

The garlic in oil issue is that at the water content and pH of garlic, oil blocks the oxygen, allowing the anaerobic bacteria to thrive. But if you change either the water content, by cooking, or the pH, by pickling, then the bacteria can't grow.

Of course, either cooking or pickling will change the taste of the garlic, but both are delicious, so it's fine. :)

There's no reason to believe it's safer.

Garlic in oil is "unsafe" by FDA standards. Which means that roughly one in 100,000 bottles of homemade garlic oil kills someone. Before reading about the botulism risk, my friends and I used to make garlic oil at home and hand it out; I'd say we distributed probably 100 bottles, some of which stayed on the shelf for years before being used. In that time, nobody got sick from it (most bottles went to friends, so we'd have heard).

So the fact that your garlic butter hasn't killed anyone yet just means that you're playing the odds. Chances are, unless you get really sloppy, you could go on making garlic butter for the rest of your life and never get botulism poisoning. But not everyone is comfortable with that risk.

EDITED PER BELOW: You can improve your odds of avoiding botulism by straining the oil/butter through cheesecloth (to eliminate solids which would hide spores), and heat-treating it to 160F or more for 45 minutes. This will not eliminate all risk of botulism, but will improve your odds.

Lactic acid has been shown to inhibit the growth of Clostridium botulinum. That is why it is safer to use butter than oil. link

I thought that our stomach acid kills botulism spores... Which is why we recommend that infants under 12 months do not eat honey. Honey can store botulism spores... But it's not dangerous for older kids and adults.

Clostridium botulinum grows best around body temperature.

Typically, butter — as opposed to oil — is stored in a cool place which significantly reduces bacteria growth in general, including the growth of Clostridium botulinum. The CDC in 2010 specifically recommended cooling as a means to prevent Botulism.

Botulism is rare to begin with (the CDC page gives a number of 110 annual cases in the U.S. of which only 25% are food-born.¹).

I'd be totally cool about food in the fridge ;-).

As an aside, the whole issue is moot (for adults) if the food is cooked for a few minutes before consumption. Botulism in adults is not an infection (our acidic stomach sees to that) but caused by the toxin already present in the food. The toxin is destroyed at boiling temperature. You can cook with your garlic oil no matter how old, just don't put it in the salad.

_{¹ It is perhaps worthwhile to put the numbers in perspective. In the US, about 5,000 people each year die from choking, as opposed to three or so from Botulism. The best advice is "we don't care how you store your garlic as long as you chew it properly" ;-).}