According to the regulations regarding acidified foods, an acidified food is a low acid food to which acids or acid foods are added so that the final product has 1) a pH less than 4.6 and 2) a water activity above 0.85 (more on water activity later). This is stated a lot more formally in the Code of Federal Regulations, Title 21 Part 114.

Let’s take a straightforward example: vinegar pickles. Cucumbers, which are low acid (high pH), are packed into a jar with spices and then a brine of vinegar and water is poured over the solids. The vinegar, which is very acidic (low pH), will cause the cucumbers to become acidic too. The pH of the whole cucumber/brine system will eventually reach equilibrium to a pH value that is higher than that of the vinegar, but lower than the pH of the cucumbers. The resulting pickles are a textbook example of an acidified food – a low acid food (cucumbers) to which acid is added in order to drop the pH of the final product below 4.6.

What is so special about pH 4.6? The biggest concern when heat is used to preserve food is the potential for growth of pathogenic microorganisms that are resistant to heat. Among these, Clostridium botulinum is of special concern. C. botulinum can produce a deadly toxin, botulin (even though it is the main component of botox), and, because it can produce spores, this bacterium can survive even in boiling water for a long time! Luckily, however, C. botulinum growth (and the ability to produce its toxin) can be inhibited if pH is 4.6 or lower. So, adding enough acid to a low acid food, to drop the pH below 4.6, makes the low acid food safer.

The concern with acidified foods is that the amount of acid or acid ingredients added to the low acid base must drop the pH enough, and keep it low. This is a particular concern for shelf stable foods because, along with a pH above 4.6, C. botulinum will only grow in places which do not have oxygen. The typical way to make something shelf stable is to can it, which creates a vacuum and removes oxygen. If the pH of the final product rises above 4.6, and there is no oxygen, C. botulinum can thrive. Why can’t we just kill C. botulinum with heat? You can, but you need the equivalent of 250ºF for 18 minutes to do it, which would turn your pickles into cucumber soup. Also, you would need some expensive equipment (retorts) to do that.

So okay, C. botulinum is a big deal, and I can see why vinegar pickles are acidified foods, but what about my corn & bean salsa or barbecue sauce? Why are they acidified?

As process authorities, we look at the formulation of a food together with its pH and ask questions such as: what role do the acids play in this food? What is the final pH and how close is it to the danger value of 4.6? How much will the low acid ingredients affect the pH? All of these questions aim towards the same fundamental concern: how reliant is the food on the acid or acid ingredients to get that final pH below 4.6, and how likely is it that the food’s pH will rise before it reaches equilibrium?

In our example, Corn & Bean Salsa is acidified because the corn and beans act as “buffers”, which means that the absorb or neutralize a lot of acid. As a result, it takes a lot of acid to acidify corn and beans, and it takes that acid a long time (three days sometimes) to penetrate the corn and beans fully. Barbecue Sauce is acidified depending on the ingredients and the amounts used. If the sauce has a lot of brown sugar, molasses, and/or soy sauce, for example, the sauce may be acidified.