Pesto is one of life's delights. The aroma of fresh basil and garlic mixed with the satiating cheese, oil, and pine nuts makes pesto difficult to pass up. And with just a few blended ingredients, it's really easy to make.
Wait- did I say garlic & oil? These two are as much a recipe for disaster as they are for pesto! The FDA has declared garlic and oil infusions a food safety hazard. The combination of garlic and oil create the perfect environment for botulism spores to grow and multiply to a level considered fatal.
C. botulinum bacteria thrive in anaerobic (no oxygen), moist, alkaline environments where food is present (1). Garlic and other vegetables from the ground are potential carriers of C. botulinum. In fact, C. botulinum is all around us. However, it won't harm humans as long as oxygen is present. But when you add garlic (C. botulinum carrier) to an oil mixture like pesto (food with moisture and no air), the risk of botulism increases exponentially.
Why haven't I ever heard of this before?
There actually have been many cases of botulism poisoning due to homemade garlic and oil infusions. These cases have included individuals who had a homemade or unpreserved oil and garlic mixture and suffered anything from dizziness, to double vision, to difficulty breathing to progressive paralysis. And sadly, about one-third of the cases have died. Naive home cooks have killed enough people to get the FDA to issue warnings about garlic and oil infusions. And getting the FDA to take action is no easy task!
What about those garlic infused oils they sell at stores and markets?
As far as garlic in olive oil infusions- whether dressings or pesto- at stores and markets, they may or may not be safe. Commercial sellers use a preservative that either inhibits microbial growth or acts as an acidifying agent to eliminate that alkaline factor needed for botulism to grow. These usually come in the form of citric acid or phosphoric acid. Don't fear the preservatives; some preservatives like these can save your life!
However, not all sellers of garlic and oil infusions use preservatives and buyer beware. Especially smaller vendors that you may see at markets may not be regulated or even know about the potential for bacteria growth between these two products. Always check the label for citric or phosphoric acid, or some other preservative specific to inhibiting bacterial growth before you purchase pesto or garlic-infused oil.
How do I make and serve pesto without killing anyone?
To ensure you don't inadvertently grow botulism in your pesto, you can choose a number of different options:
Make your pesto and serve it immediately. The longer it sits, the better the chance of botulism poisoning.
Store your pesto in a clean, sanitized jar in the refrigerator for up to 3 days. After 3 days, despite the cool temperatures, the risk of botulism increases. It may take longer to grow, but it can still manifest itself as a toxin.
Freeze your pesto immediately. When you're ready to serve it DO NOT THAW AT ROOM TEMPERATURE. Of course, if you do, you invite that time and temperature factor back in to play. Thaw the pesto in the refrigerator overnight and serve the next day.
Add an acid. Although pesto typically doesn't contain acid, by including lemon juice, tomato, or vinegar, the pH may be lowered enough to prevent botulism growth.
Finally, my recommendation is to make the pesto initially without garlic and store it in the fridge. When you're ready to serve it, mince the garlic and add it to the pesto for immediate consumption.
In addition to preventing botulism growth, this also limits the time garlic is exposed to oxygen. (Although oxygen prevents botulism growth, it also removes garlic's antioxidant properties- a double-edged sword). The best way to get the most out of garlic's health benefits is by chewing a raw piece of garlic or chopping it and eating the minced raw garlic within minutes. Once it's cut or cooked, time starts ticking on its health benefits. Garlic loses its antioxidant power within hours of being cut and minutes after being exposed to heat (2). (Note: Swallowing whole garlic appears to be ineffective at improving health because the enzymes aren't released before the garlic comes into stomach acid. Therefore, it's important to chop or chew garlic for any nutrition benefit.)
Here is a recipe for Botulism-Free Pesto.
1-2 handfuls of fresh basil (about 2 cups)
1/3 cup pine nuts
1/2 cup Parmesan cheese
1/2 cup extra virgin olive oil
pinch of salt & pepper
3 cloves garlic, minced (to be added upon serving)
In a food processor or blender, add basil and pine nuts.
Once nuts are blended into the basil, add cheese and process.
3. Slowly pour in the olive oil while the blender is going. Add a pinch of ground pepper and salt if desired.
4. Scrape the pesto into a clean, sanitized Mason jar and tighten the lid.
5. Refrigerate up to a week.
6. When ready to serve, mince the garlic and stir into the sauce. Add to your favorite pasta, toast or raw veggies to enjoy immediately.
(1) Kendall, P. & Rausch, J. (2012). Flavored vinegars and oils. Colorado State Extension, Fact Sheet 9.340. https://extension.colostate.edu/docs/pubs/foodnut/09340.pdf
(2) Linus Pauling Institute. (2019). Garlic. Oregon State University. https://lpi.oregonstate.edu/mic/food-beverages/garlic#organosulfur-compounds