Canning 101: Understanding Acid and pH in Boiling Water Bath Canning

pickles on a table

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.

 

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Links: Peach Ketchup and Baked Ricotta Cheese

Pie filling from Bucks County Preserves.

Sometime last week, I misplaced my blogging groove. Too many freelance deadlines combined with a need to tackle the mess that was our apartment before a canner’s potluck took me away from this space. But I’ll be around more this week, which at least one new recipe, some Canning 101 goodness, and a subversive tutorial. Here a few links to tide you over until them.

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Potluck with Preserving with Pomona’s Pectin Author Allison Carroll Duffy

potluck

Hey friends! Allison Carroll Duffy, author of the Preserving with Pomona’s Pectin is coming to Philadelphia and I’m throwing a little potluck to welcome her to town and give her a chance to meet some local preservers. We’ll be gathering at my place this Sunday, September 15 at 6 pm. If you’re in the area and you’d like to come, please enter your information in the form below. Once you’re signed up, I’ll send all the details along.

Hope to see some of you this Sunday!

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Links: Boozy Infusions, Blackberry Kvass, and a Winner

A late Monday morning breakfast.

It’s been a really long time since I’ve had a normal week. The kind where I’m home for seven days straight, don’t have any huge extra projects on the horizon, and actually get to cook my way through my CSA share. Happily, this week is shaping up to be wonderfully normal and I couldn’t be more pleased. Now, links!

assembled tomato strainer

roma by weston winner The winner of the Roma by Weston Electric Tomato Strainer is commenter #248/Kara B. She said, “My favorite canning helper is my Presto 23 qt. pressure canner. I even just started using it for water bath processing. I’ve been doing my tomato batches with a strainer and a rubber spatula. I need this little gadget so much!!!” Well Kara, looks like you’re in luck!

I’m going to take this week off from giveaways (it’s been quite the summer of great canning gear, hasn’t it!), but I’ll be back with something terrific for you guys next Monday.

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Upcoming Classes: Philly! Phoenixville! Greensgrow!

class image revised

Hello canners! Summer is coming to a close, but I’ve still got plenty of canning classes on the schedule. Check ‘em out!

September 14 – Tomato Jam at Indy Hall! In this class, we’ll make a batch of tomato jam and talk about all the ins and outs of canning tomatoes. Both boiling water bath and pressure canning will be discussed. Class is 11 am – 1 pm and costs $50. Leave a comment or email me to sign up.

September 18 – Tomato canning demonstration class at Henry Got Crops in Philadelphia’s Roxborough neighborhood. The class is from 6:30 – 8:30 pm and costs $10 for CSA members, $15 for non-members. Email henrygotcropsATweaversway.coop to sign up.

September 19 – Hands on tomato canning basics at Cooking Spotlight in Phoenixville, PA. Class runs 6:30 – 9 pm and costs $60. All participants will go home with a jar of tomatoes. Click here to sign up.

September 21 – Tomato Jam at Greensgrow. Class is from 12 noon to 2pm and costs $35. Click here to sign up.

October 12 – Spiced Apple Pie Filling at Indy Hall! This class will give students an opportunity to help peel, chop and process 10 pounds of apples down into a batch of fragrant, spicy apple pie filling. Class is 11 am – 1 pm and costs $50. Leave a comment or email me to sign up.

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A Food in Jars Workshop at the Rowe Center

six cases of jars

As you might have noticed, I teach a lot of canning classes and workshops. Most of the time, these events are just two or three hours long. It’s enough to hit the high points, work through a recipe, and get people comfortable with the basics of boiling water bath canning. But it’s not always enough, particularly for those of you who want to go a little deeper with your canning knowledge.

So this fall, I’m trying something new. I’ve teamed up with the Rowe Center (a camp and conference center based in Western Mass) to offer a weekend long canning workshop. The workshop will be a three-day, hands on canning extravaganza that will focus in on the many different ways to preserve autumn fruit.

We’ll make applesauce, apple butter, roasted quince chutney, pickled Asian pears, pear vanilla jam, apple mint jelly and apple rosemary jam and in doing so, make nearly every style of sweet and puckery preserve I know. Participants will go home with a jar of everything we make, along with a far deeper understanding of preserving.

This workshop will start on the evening of Friday, November 1 and will run through to the afternoon of Sunday, November 3. The workshop fee is based on a sliding scale and there are a number of different housing options and price-points.

I do hope some of you will join me for this week. I think it’s going to be fun!

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