Botulism is a serious form of food poisoning caused by toxins produced by Clostridium botulinum bacteria.
Know Your Enemy
Botulism is a rather serious form of food poisoning caused by toxins produced by Clostridium botulinum bacteria. Botulism is very rare (about 20-30 adults in the US get it from food each year) but is fatal in about 5% of cases, so it should be taken very seriously . Strangely, the toxin is used as a beauty treatment but is also feared as a possible biological weapon. Symptoms of botulism generally present themselves in 18 to 36 hours.
Clostridium botulinum has a couple of important properties. It’s an anaerobic bacterium, which has both an active and a spore state. In order to grow in the active state, it prefers temperatures above 38 degrees Fahrenheit (2.5 degrees Celsius), needs water, and requires a pH above 4.6. Unfortunately, in the spore form it can survive just about any environment, except temperatures well in excess of boiling . Botulinum bacteria are present in many common places, most notably in soil, and, thus, on most vegetables. While there are some signs to watch for, it is possible for foods to be contaminated with botulinum toxin and show no detectable sign (like a change in appearance or taste) .
There’s Something About Oil
The problem with many flavored oils is that they give the perfect environment for botulinum to grow in : Vegetable matter like garlic and herbs will often [†] have botulinum spores on it waiting to find a nice place to grow and provides ready supplies of food and water. By submerging them in oil, you eliminate any surrounding air thereby providing an anaerobic environment, and if you keep the oil at room temperature (as one normally does with oil) then you’ve given the bacterium the right growing temperature to multiply. Most vegetable (including Wasabia japonica) do not generally have a pH less than 4.6.
Documented cases of infused oils causing botulism are difficult to find, but it is the common wisdom that putting herbs into oils has good potential to produce significant number of this bacterium  . There are several documented cases of people getting botulism in the USA and Canada from chopped garlic stored in oil in the mid to late 80’s . Today people still sell infused oils and garlic stored in oil. The development of botulinum can be stopped by adding acids to lower the pH or other preservatives, which is how many of those products are made safe. After the outbreaks in the 80's, stricter guidelines were put in place to ensure the safety of those type of products .
So how do you make it safe?
This is a difficult question to answer. Research appears to indicate that the majority of homemakers that produce flavoured oils agree the FDA recommendation that homemade flavored oils should be kept refrigerated and be discarded in a week  to 10 days .
A different view has been taken by food scientists Drs. Shirley VanGarde and Margy Woodburn  who claim that infusing under the right conditions and straining all the food materials out of the oil after infusion is also safe. In fact, this was the method that we used commercially for many years with a problem. See this article for more details for making Wasabi Oil.
Killing the Spores
The spores are on most vegetables (and many other foodstuffs), so generally you’re going to have to kill them if you want your product to be spore free. This is what is done when canning low-acidity foods. According to information I found from New Zealand Food Safety Authority, botulinum spores can be killed by exposure to temperatures in excess of 121 C (250F) for more than 3 minutes . However, in their guide to home canning, the USDA is quite a bit more conservative, and recommends holding the material at 240-250 F for 20 to 100 minutes. I can’t account for this discrepancy, except that this might be to ensure that the correct temperature is reached throughout pieces of vegetables in the canned goods, and it may be because it is difficult to reach the correct temperature in the watery environment of canned goods (usually it requires a pressure cooker, because liquid water can’t go above 212 F at 1 atm of pressure). Thankfully, oil can easily be brought up to 250 F, but you should probably use a thermometer to make sure. It should also be noted that freezing does not kill the spores.
If you want to make a spore free product, it's probably best to follow guidelines similar to those used in canning low-acidity foods. That means you should heat the product to 250 F, with the times as discussed above. It would probably be wise to consult the USDA guide on canning safety, which can be found at http://nchfp.uga.edu/publications/publications_usda.html for more information and more specific guidelines on times for heating. You should also use a clean, smooth, sterilized container, such as a glass container that has been submerged in boiling water for at least 10 minutes . The containers should be drained and turned upside-down to dry for a moment then filled while still warm. Also remember that times for boiling in water will be different at higher altitudes.
Canned goods are sealed in air-tight containers and spoil from other factors fairly quickly when opened, so it’s not entirely clear what the risk of re-contamination is if the oil is kept in an unsealed container (as would be normal). If nothing else is put into the oil and it is only in contact with air, it would seem the danger is small. This is consistent with statements by Shirley VanGarde and Margy Woodburn . You could also follow the procedures of the next section as well to further reduce the risks.
Preventing Toxin Production
The second strategy is to allow for the possibility that there are botulinum spores in your product and try to keep them from living in the active state and producing toxins. After all, we just said there are probably botulinum spores on a lot of the vegetables you buy (and they’re also in your honey), but you haven’t dropped dead (though the botulinum in honey can be dangerous to babies). They’re only harmful if allowed to go active and produce the toxins. I outlined the conditions necessary for botulinum to live in the active state, so the idea is to deprive them of at least one, and preferably as many of those conditions as possible.
In commercial products they use acids or preservatives to keep the bacteria inactive. As long as the pH is below 4.6, the food should be safe. This is also why infused vinegars are safe, since it’s basically a solution of acetic acid. There do not appear to be any guidelines on how to do this safely at home. If you just add vinegar to your oil you’ll just basically have salad dressing, but you might be able to add more concentrated acid (without the water or other flavoring agents) to do the job. You’re going to need some litmus strips handy, though, if you want to make sure you’ve done it right. They seem to often use citric acid in commercial chopped garlic packed in oil. Salinity (salt or brine) and other preservatives can also be used to inhibit the growth of botulinum .
Temperature is the next factor in reducing the possibility of producing toxins. If you can keep the product continuously above 122 F (50 C)or below 38 F (3 C) then you’ll prevent the toxin from being produced. Keeping it hot probably isn’t a possibility, and, unfortunately, refrigerators often average a temperature at or slightly above 40 F (5 C). Doesn’t sound like much, but the FDA apparently thinks it’s enough that they recommend not keeping home made oil infusions for more than about a week in the refrigerator. You can, however, keep it in your freezer for long term storage , since that should be below 32 F (0 C). Just remember that freezing does not kill the spore.
Food scientists Drs. Shirley VanGarde and Margy Woodburn suggest depriving the bacteria of food and water . One way to do this is to strain out the vegetable matter after infusion (with cheese cloth and/or a wire strainer). Of course, this won’t do you any good if the toxins are already there, so you have to do the infusion quickly (probably with hot oil) or do it under conditions where the toxin won’t be formed quickly (like in the refrigerator). Another way to deprive the bacteria of water is to use only dried ingredients (dried chilies or peppercorns, for example). This means no fresh garlic, though. Remember that the effectiveness of these methods will depends on how dry your ingredients are or how well you strain it, so it may be difficult to tell if you’ve done it well enough. Also, I could not find independent verification that these techniques are regarded as safe, though they do make sense.
Destroying the Toxins
If your product does become contaminated with botulinum toxin, then it can be detoxified. The toxin can be broken down by heat above 176 F (80 C) applied for at least 6 minutes or heat above 185 F (85 C) applied for more than 1 minute . The USDA recommends boiling in water for 10 minutes to destroy the toxins . So you could, in principle, not worry about preventing botulinum toxin in your oil and just make sure to cook it appropriately each time before use. But you’d better make sure you do that and that no one else uses the oil without heating it first. This is very likely not a good plan. On the other hand, if you have oil you think might have become contaminated, you can detoxify it in this way and then store it safely in one of the other ways mentioned.
The safest recommendation is probably to follow the FDA suggestions. In that case you should either infuse your oil in a relatively short period of time (probably an hour or less) and then keep it refrigerated or infuse it while in the refrigerator. All together, you shouldn’t keep it around much longer than a week (including infusion time), according to them. You can also put it in the freezer if you want to keep it for longer, just remember that as soon as all the time out of the freezer totals more than about one week total you should discard it.
It is possible that the oil would turn viscous at refrigerator or freezer temperatures. This depends on how the oil was manufactured. “Good virgin olive oil (extracted mechanically only) will stay perfectly liquid at fridge temperature. Cheaper oils, using heat or chemical extraction however, often do solidify somewhat in the fridge.”  In any case, oils don’t have a very high specific heat, so it should be fairly easy to warm them to the desired temperature. Just remember that if you leave the whole bottle out to warm up, all that time at room temperature adds up.
Based on my research there are some other methods that seem like a good bet. If you use dry ingredients then it should be pretty safe (according to the food scientists I mentioned above ). If you infuse with fresh ingredients but you either do it in a short period of time (an hour or so) or in the fridge over a few days and then strain out all the particles (using a strainer and/or cheese cloth as necessary to get everything out) you should also be in good shape. In those cases you can keep the resulting oil at room temperature.
Finally you could try kill all the botulinum spores. However, this requires heating to at least 250 F (122 C), as mentioned above. The problem is that if you do this with fresh ingredients, they’ll probably be deep fried by the time you’re done. The other issue is that if you use a delicate, flavorful oil like, say, extra virgin olive oil, heating to this temperature will probably destroy much of it’s flavour.
If you think a batch of oil may be contaminated, and you heat it to try to detoxify it, you must heat it to 176 F (80 C), which may also effect the flavor somewhat , and may cook some of the ingredients.
Botulism is a very rare form of food poisoning, with only 20-30 people in the US each year, that works out to something in the neighborhood of 1 in 10 million people. That certainly seems pretty remote, but remember those would be the odds of picking a person in the US at random and getting someone who had had botulism from food in the past year. What we’re really interested in is if one repeatedly makes flavored oil in one of the “unsafe” ways discussed above, what are the odds that that person will eventually get botulism. A complete answer would require more statistical information, but we can make a sort of back of the envelope calculation and get an upper bound (a worst case figure). Suppose all those cases of food botulism are from flavored oils. Now we must guess how many people make flavored oils in a given year in a way that could cause botulism. That’s very hard to estimate, but let’s guess it’s at least 1 in 1000 (because not everyone does these things, and some people are children, etc.). We would then say that the odds of getting botulism from improperly made flavored oil in a given year must be less than 1 in 10,000. That is, surely, still pretty remote.
So, is it worth the risk to throw caution to the wind? Well, clearly that’s a subjective judgment. As I tried to impress upon you at the beginning, we’re talking about something much more serious than most forms of food poisoning. The effects can last years and even be fatal. On the other hand, most of us probably do a lot of things that carry a greater risk of death than 1 in 10,000 per year. Personally, I look at the choice in terms of cost versus benefit. Driving a car is likely quite a bit more dangerous; however, it is nearly a necessity in the US today and it affords one great freedom, so it’s worth the risk, in my reckoning. In the case of the flavored oil, the danger is small, but it seems relatively simple to follow the safety guidelines above and reduce the risk by many orders of magnitude. So, to me, it’s very much worthwhile to take these precautions. If I had to stop making flavored oils entirely, that would be a different choice, and might just take my chances, but here it seems easy to mitigate the risk and still get the tasty result. The other issue is that if you’re going to serve this stuff to other people, then unless you lay this all out for them, you are making that choice for them too. I’d be very careful about making decisions that risk other peoples’ lives, even if the risks are small.
In terms of how stringently the guidelines must be adhered to, I’m not sure. It’s safe to assume that the FDA is playing it safe by a wide margin, and some sources say the oil can be refrigerated up to 3 weeks , though I don’t know the reason for that discrepency. I’m not sure how far one can bend the rules, however, especially considering that bacteria growth is exponential (so it’s constantly speeding up). I’d say try to stay somewhere close to the rules above if you want to play it safe, though a few extra days probably won’t hurt.
This article is supplied purely as information for those who want to make Wasabi Oil using the method described here. We used that method for 4 years with some of the oil being stored for all that time at room temperature with no botulism growing in the oil. Were we lucky? I don't know, but we know Wasabia japonica kills most bacteria, so maybe the method we used was self sterilising.
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