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How to Make Safe (and Perfect) Jam with Boiling Water Canning
With in-season produce flooding the market, it’s time to think long term.
Sarah Wilson
Sarah Wilson

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Here in the northeast, we’re just starting to see strawberries and rhubarb dominate the market produce aisles, and it has us thinking about making some jam. But first, we are sharing a few key reminders on safely doing so. For starters, there are two methods of home canning approved by the USDA: boiling water canning used for high-acidic foods like fruits and pickled vegetables, and pressure canning for foods that have very little natural acid, like non-pickled vegetables, meats, poultry, seafood, and dairy. Our Online Cooking School course, Preserving 101, goes into detail about the topic of canning. Our focus here is on boiling water canning, and putting out a great batch of fruit jam, whether it’s a large batch for long-term storage, or just a couple of pints for the fridge. Our first priority is safety!

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Tip 1: Understand boiling water canning and when to use it.

Proper canning (or “processing”) prevents the growth of undesirable microorganisms, destroys enzymes, and removes oxygen from the jars so that the food inside can be stored for long periods of time. The USDA has established processing times through laboratory testing. They range depending on the ingredients and altitude.

So when is this process not necessary? When you want to put up a small batch of jam that you plan to eat in the next eight weeks. Make the jam according to the recipe, pour it into a couple of clean jars, and store it in the fridge for immediate consumption (or sharing!). Check out this recipe for Strawberry Refrigerator Jam for a great example of an easy, small-batch jam.

Tip 2: When it comes to acid, consistency is more important than fresh-squeezed.

Achieving the proper acidity (pH) level is key for preservation. Without the right pH, boiling water canning is not considered safe. Likewise, without enough acid, unprocessed foods will only have a short lifespan, even in the fridge. Acidity also plays a key role in gelling abilities of pectin; without a consistent pH it can be difficult to predict how a jam or jelly will set. Fresh lemon juice has too much variation from lemon to lemon to consistently predict how much of it will increase the acidity of a given preserve; bottled lemon juice, however, has a tightly controlled pH that is always consistent no matter the time of year or variety used. Note: Citric acid can also be used to ensure acidity, but we found bottled lemon juice easier to find and more likely to be already on hand.

Tip 3: Temperature is the key to proper thickening.

All fresh fruit contains natural pectin, but some fruits contain more than others. While it would seem obvious that we should add commercial pectin to low-pectin fruit jams, we actually found this was not necessary in most cases (it is, however, necessary with most jellies). No matter if you’re using commercial pectin or relying on the natural pectin from the fruit, what is important is having the three key factors—water, sugar, and pH—at the right temperature and in the right proportions before the pectin will do its thickening job. Otherwise, the pectin molecules will simply float around in the jam mixture and do nothing. When these factors fall into place, the pectin molecules bond to each other to form a web-like structure that holds the fruit and sugar in place, making the jam set. You’ll need an instant-read thermometer to know when the jam has hit the ideal temperature. See our favorites (for every price range) below.

The Best Thermometers

Best Overall Thermoworks Thermapen Mk4

Our longtime winner is still the best instant-read thermometer on the market.

 

Best Midprice Option Lavatools Javelin PRO Duo

This thermometer gave more expensive models a real run for their money. It’s fast and accurate and has a large handle that keeps you safely away from the heat.

 

Best Inexpensive Option Thermoworks ThermoPop

Our former inexpensive winner had another strong showing. It was dead accurate and extremely fast.

 

Tip 4: Long-term storage requires boiling water canning.

As we’ve stated, not all jams require boiling water canning. We love when we find a perfect fruit at the market and can make a quick batch of jam, fill a couple of clean jars, and pop them in the fridge. But when you want to make a large batch—which implies long-term storage—boiling water canning is non-negotiable. See this article for more in-depth explanation of these steps, but for a quick rundown, here are the eight essential steps for canning a batch of jam for long-term storage:

  1. Heat the jars
  2. Fill the jars
  3. Measure the headspace
  4. Release the Air bubbles
  5. Add the lids and rings
  6. Process the jars
  7. Let the jars seal themselves
  8. Store the jars

Tip 5: Know what botulism is so you can avoid it.

Food-born botulism is a rare but serious illness, and most cases are due to improperly home-canned foods. Luckily, it is very easy to avoid botulism as long as you follow these rules.

  • Follow our recipes exactly. The processing times given in our recipes are derived from the USDA Complete Guide to Home Canning; these times are long enough to neutralize any dangerous toxins inside the jars.
  • Ensure the pH of your jam is well below the minimum level (pH of 4.6) for food safety. You don’t need to measure the pH, you just need to follow the recipe and be sure to use bottled lemon or lime juice and do not substitute other types of vinegar.
  • Work cleanly. Thoroughly wash the food, jars, rings, and lids, counter, sink and your hands before starting a home-canning project to reduce the possibility of contamination.
  • When in doubt, throw it out! Always discard food from jars that have lost their seal during storage, as it is a sign that bacteria, mold, or other toxins might be growing inside the jar.

For more tips on making your own jam and lots of other dishes, check out our Cooking School classes.


What is your favorite fruit to use in jam recipes? Let us know in the comments!

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