Today’s post is inspired by a rash of questions I’ve gotten recently in regard to my recipe for Honey-Sweetened Peach Vanilla Jam. A number of you are concerned because while that recipe contents lemon zest, it doesn’t contain any lemon juice. That jam is safe as written, but we need to dig a little deeper into canning science to understand why. Read on!
If you’ve been canning for any length of time, you’ve probably heard mention of acid levels in relation to safe boiling water bath canning. Anything that is preserved in a boiling water bath must have a high acid content. The reason that high acid levels are important is that the presence of acid inhibits the germination of botulism spores into the botulism toxin. Botulism spores can only develop into the botulism toxin in low acid, oxygen-free environments.
When you preserve something in a boiling water bath canner, you heat the jars and their contents to the boiling point (that temperature varies depending on your elevation, but at sea level the boiling point is 212 degrees F). That heat is enough to kill off the micro-organisms that can cause spoilage, mold, or fermentation, but it’s not enough to kill botulism spores (they require far higher temperatures). The process of boiling the jars also helps to drive the oxygen out of the jars, creating a vacuum seal. For jars that have sufficient acid content, the result is a jar of food that is safely preserved and shelf stable.
The way food scientists (and home canners) determine whether something is high or low in acid is by pH. If something has a pH of 4.6 or below, it is deemed high in acid and is safe for boiling water bath canning. If the pH is 4.7 or above, it is considered low in acid. We’ll talk more about how to preserve those foods that are low in acid and have a pH of 4.7 or above another day, but to give you just a hint, that’s often where a pressure canner comes in.
If a food is close to the 4.6 pH point, you can often add enough acid to bring that product into the necessary safe zone. Fruits like tomatoes, figs, asian pears, melons, persimmons, papaya, white peaches and white nectarines, and bananas are often just a bit too low in acid in their natural state for safe canning. So in order to lower the pH to a safe level, we add either bottled lemon or lime juice, or powdered citric acid to products featuring those ingredients. Once the acid levels are high enough to inhibit the botulism spore’s ability to germinate into a deadly toxin, that product is safe for boiling water bath canning.
However, there are a world of foods out that naturally have a pH that is well within the zone for safe preservation in a boiling water bath canner. Here’s where we come around to the peach jam I mentioned in the introduction to this post. That recipe specifically calls for yellow peaches, which typically have a pH of 3.4 to 3.6. I know the general pH range for yellow peaches because the FDA provides a handy reference page on their website that lists the general pH range of most common fruits and vegetables.
You could certainly add lemon juice to my jam in order to balance the flavor and add a little extra pectin (citrus fruit is naturally high in pectin), but it’s not necessary for safety.
Updated to add: One last thing! It’s important to remember the pH of the entire jar counts here. This is why it’s so vital to follow tested, reliable recipes for things like tomato sauce or salsa. Sure, you can add bottled lemon juice to your tomatoes to lower the pH, but if you’ve also added onions, garlic, and basil to your sauce, you’re not just balancing the acid of the tomatoes, you’re also taking the rest of the ingredients into account. That’s why salsa recipes designed for canning contain so much bottled lemon or lime juice, or vinegar.