Is Pesto Acidic?

Short answer
There are no ingredients in freshly made, classic basil pesto that are acidic. However, for ambient pesto sauces to be safe for long periods of time, producers have no choice but to ensure they contain a certain level of acidity.

Long answer
Let's start with a quick science lesson. Any food that has a pH of 4.7 or below is deemed acidic, while those over 4.7 are deemed alkaline. The reason that is so critical to pesto producers is because all pesto recipes contain garlic and oil, which, if handled incorrectly, can lead to the production of harmful toxins that can make people extremely sick.

The toxins in question are those produced by Clostridium botulinum spores, which are present on the surface of garlic but are not in themselves harmful. However, when these spores are placed in an anaerobic (i.e. no oxygen) alkaline environment, they can start to create harmful toxins.

If you make a fresh pesto using the official recipe, you'll end up with an alkaline sauce with a pH of around 4.8 - 5.2. Providing it is refrigerated, that sauce will remain safe for several days.

To counteract any risk, producers of long-life, ambient pesto not only have to heat-treat the sauce (via pasteurisation or sterilisation) but also drop the pH to below 4.7 to make it acidic and therefore impossible for the harmful toxins to be produced. Common acids used vary from lemon or lime juice (which is high in citric acid) to powdered acids like lactic, malic, and tartaric. Some producers may also use vinegar.

Because different batches of ingredients can vary in their acidity and because pH can alter over time, pesto manufacturers take a 'belt-and-braces' approach and drop the acidity to well below 4.7. We took pH meter readings from a random selection of shop-bought pestos, and on average, their acidity levels were 4.2. The most acidic pesto we found had a pH of 3.8, but it was so sour that we found it unpalatable. That level of acidity can be especially bad for anyone who suffers from acid reflux or heartburn.

Luckily for us, Mediterranean cuisine makes ample use of their world-class lemons, so adding a generous amount of lemon juice to our sauces not only improves the flavour but makes them suitably acidic in the process. Win-win!