C. botulinum is a spore-forming bacterium. It is common in soil and other everyday substances, including garlic. It is an obligate anaerobe, meaning that oxygen is poisonous to the cells. However, C. botulinum tolerates traces of oxygen due to the enzyme superoxide dismutase, which is an important antioxidant defense in nearly all cells exposed to oxygen.
C. botulinum is ONLY able to produce the neurotoxin during sporulation, which can ONLY happen in an anaerobic environment, such as being packed in oil. Sporulation is the formation of nearly dormant forms of bacteria. In a limited number of bacteria, spores can preserve the genetic material of the bacteria when conditions are inhospitable and lethal for the normal (vegetative) form of the bacteria. The commitment of a bacterium to the sporulation process sets in motion a series of events that transform the cell.
Some bacterial species, including C. botulinum, produce spores in an unfavorable growth environment to preserve the organism’s viability and permit survival in a dormant state until the spores are exposed to favorable conditions, at which time they “hatch” and create the toxin. This is why you don’t use fresh garlic in low temp (less than 140F) SV processing.
It’s also not the ONLY reason.
Even though salts, and sugar, to a much lesser extent, are the only flavorings that can penetrate the complex matrix of proteins that we call muscle, many people insist on putting other things in the bag. These other things are called “surface treatments,” for the apparent reason. Vegetables typically require higher temps, 183F+ to denature, so keep this in mind. They will not cook in the bag at lesser temps. Fresh garlic, can release botulinum toxins into the bag, because of the anaerobic environment, the lack of light, and the low temperature. This is why it is not safe to leave chopped garlic in oil on the shelf in the pantry. People have actually gotten sick from this scenario. So, instant garlic usually replaces fresh, if one is determined to add it to the bag.
PS – EVOO can introduce off flavors in the bag but that is for another post!
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