Jam Making 101: The Secrets to Getting Jam to Set Like a Pro

It can seem like witchcraft: you start with a pile of fresh fruit, wave a wand (okay, actually a rubber spatula) over a bubbling, boiling cauldron and then everything comes together into a glistening, jewel-toned, spreadable jam.

There's chemistry behind the magic, though, and understanding it is the key to making great jam. Once I learned exactly how a jam's few ingredients interact, I was able to figure out what I was doing wrong when my first strawberry jam came out like a soda syrup and my second came out like the sliceable contents of the can of cranberry sauce my grandma used to open at Christmas.

The ultimate goal in jam-making is to form a molecular web that holds all of the fruit's juices together. And the molecule that forms the web is none other than pectin. Getting there, though, is a well-choreographed dance in which heat, sugar, and acid all pitch in to get the pectin to do its thing, pushing the mixture over the cusp from fruit syrup to jam—without going all the way to jello salad.

Let's start by taking a look at exactly what pectin is and why it's important.

Peter Preserver Picked a Peck of Polysaccharide Pectin

Pectin is a long chain of carbohydrates called a polysaccharide. It's found in the cell walls of all terrestrial plants, helping to give structure to stems, leaves, flowers, and fruit. It's most concentrated in the skins and cores of fruit, with some types, like apples and citrus, having particularly high levels.

As fruit ripens, enzymes in the fruit start to break pectin down into pectic acid, which is why very ripe fruit is both softer and more difficult to jam without adding extra pectin, a topic I covered in the first part of this series on how to select the best fruit for jam-making.

When heat is applied to fruit, its cells rupture and pectin-rich fluids leak out. Once that happens, the pectin is then capable of forming that web-like structure we've been talking about (technically called a colloidal system). On a microscopic level, a colloidal system is a mixture in which one substance (in this case, the fruit's watery juice) is dispersed in another (say, a web of pectin molecules), without the two substances combining to make something chemically new. But, without the right conditions, pectin dissolved in water is not a colloidal system—it's just free-floating pectin molecules in liquid.

The key, then, is getting those conditions just right for the pectin to form its web. To understand how this all works, it helps to take a slightly closer look at the pectin molecule (don't worry, not too close: no need to crack a chemistry textbook to follow along).

Why Pectin, We've Been Expectin' You

Pectin is an indigestible soluble fiber ("fiber" just refers to any parts of plant-based foods that your body can't digest or absorb, while "soluble" means it can dissolve in water). When pectin molecules are dissolved in water, they avoid each other for two reasons: First, they are hydrophilic (water-loving), meaning that they'd rather stick to water molecules than to each other. Second, they have a negative charge and repel each other, similar to how like poles of magnets refuse to join.

Water, I'd Like You to Meet Sugar—I Think You Two Have a Lot in Common

The goal here is to get pectin to stop bonding with water and start bonding with itself. Water is kind of a flirt, so the best thing to do is distract it: if we can get it to stop talking to the pectin and start talking to someone else...say, someone really, really instantly likeable, then the pectin will be left all by itself (poor pectin, being an indigestible soluble fiber, it's rarely the sexiest one in the room). So who's our star decoy, the one that will steal water's attention and cause it to abandon the pectin? Lovely, sweet sugar, of course (you gotta admit, she's pretty cute).

Incidentally, sugar is an essential part of what makes preserves, well, preserved. All of the microorganisms (yeasts, mold, bacteria) that cause spoilage need water to survive and proliferate. When the water molecules are all bound up with the sugar, though, the nasties can't survive.

It Just Takes a Little Acid to See the World...Differently

So once the water and sugar have gone off to neck in a corner, the pectin is left by itself, twiddling its fibrous thumbs. Sure, it can see another sad, lonely, dejected pectin across the room, but it's just not that interested. Its whole outlook is so negative, know what I mean? The answer here is to add something positive to get rid of that negativity, and one option is acid.

In water, acid increases the concentration of positive ions. Those positive ions swim off to the negative pectin (opposites attract after all), for an overall neutralizing effect. Once neutralized, pectin is suddenly willing to interact with other pectin, you know, chat, crack some jokes, maybe brush up against each other. Next thing you know pectin has its hands in all sorts of places and some major bonding is taking place. (Water and sugar should stop necking and take note because pectin is taking it to the next level...leave it to acid to make things interesting.)

Pectin aside, adding acid plays another crucial role in the jam-making process: It lowers the final jam’s pH level, which limits the growth of bad bacteria and extends the jam’s shelf life. Almost all fruit contains some acid, but the natural amount of acid can differ between fruit types, and even from batch to batch of the same type of fruit. To ensure that my jam has a proper acid level to gel properly and limit bacteria growth, I always add lemon juice to a jam mixture. Plus, I like the flavor a pop of lemon juice adds to my jam. It balances the sweetness of the fruit and brightens most fruit jams.

Water, We Actually Think It's Time For You to Go Home. Call You a Cab?

Once we have the sugar to tie up much of the water and leave the pectin free to mingle, and the acid to give the pectin enough of a positive outlook to actually have the motivation to do it, we're almost there. There's just one problem: There's still too much water, and it's just kinda gettin' in the way of things. This is where we enlist our third weapon: heat.

When a jam mixture is cooked, water is escorted out through evaporation. And when enough water has evaporated (with whatever's left still doing its thing with the sugar), the jam has finally arrived at its gel point. That means the pectin molecules are fully able to stick to each other and form a 3-dimensional web in which the remaining sugary juices are all held like water in a sponge.

The gel point usually corresponds to a specific temperature: the sweet spot tends to be 220°F (8 degrees higher than boiling water). The concentration of sugar at this point is about 65% and the pH is pretty acidic, somewhere between 3.1 and 3.6.

Can We Just Spoon Now?

After all this excitement, you'd think there'd be some elaborate process for determining the gel point. But there's no need to pull out the pH strips or the thermometer, because pinpointing the gel point requires one simple blunt instrument: a spoon.

See, the truth is that the pectin web doesn't really solidify until everything cools down. That means it's tricky to tell whether you've achieved the gel point while the action is still hot and heavy. Enter the spoon: Before you start your jam, set a plate with a few metal spoons in the freezer.

Then, when the foam has subsided and the bubbles have slowed, place a small dollop of jam onto one of the freezing-cold spoons and let it sit in the freezer for 5 minutes. When you pull it out, the jam should feel neither warm nor cold. If the jam has properly gelled, it will hold its shape pretty well when you tilt the spoon, neither running off too fast like a liquid, nor seizing up and not moving at all.

If the jam is still too runny, just keep cooking it and performing the frozen-spoon test every 5 minutes until you achieve the consistency you want (I used to keep 15 spoons in the freezer when I was first learning how to make jam—what can I say, I like to be prepared).

If, on the other hand, the jam is rock solid, that means you've gone too far and cooked it too long. You can try adding a little water to thin it out, but bear in mind that after overcooking a jam, you can't really get those fresh fruit flavors back.

An Improvised Jam Session

Let's say you have landed a gorgeous flat of apricots from a neighbor's tree and you want to turn them into jam without a recipe. How do you apply what we've just learned to making great jam?

First, Select the Right Pot

One thing to keep in mind right away is not to try to cram too much fruit into your jam pot. The mixture needs room to bubble up while it's heating in order for the water to evaporate and the pectin web to form. I try to fill the pot less than half full of the jam mixture—that's all the fruit and sugar and lemon juice mixed together (and you'll want to mix them together before dumping them into your jam pot if you're using copper. Just as an example, my pot is about 15-inches across, 4.5-inches deep, and has an 11-quart capacity, and I can usually fit between 5 and 6 pounds of fruit without overfilling it.

Next, Guesstimate Your Sugar

There is some pretty easy math that will help you figure out how much sugar to add. First, you'll need to weigh your fruit after it's been cleaned of cores, stems, pits, and anything else not going into the jam. I usually add somewhere between 40 and 70 percent of the weight of the fruit, depending on what kind of fruit it is.

If it's a low-acid, low-pectin fruit like ripe strawberries, then I'm going to have to add more sugar, closer to the 70-percent level. If the fruit is higher in pectin and has more natural acid—such as perfectly ripe blueberries with a handful of greener ones thrown in—then I'd add less sugar, closer to the 40-percent level.

Let's say our prepped apricots, now halved and pitted, weigh 6 pounds and are perfectly ripe. I'll often go for about 45 percent sugar (same for plums; whereas, with dark red cherries and tender peaches, I'd do a little more, about 60 percent). Since I have 6 pounds of prepped apricots, that means adding 2 2/3 pounds of sugar.

Lemon Juice and Fruit Jams: A Perfect Pairing

As I noted above, almost all fruit contains some acid, but I always add lemon juice to a jam mixture to ensure the acidity is high enough for gelling and food safety (and because I like the flavor). I generally add one ounce of lemon juice for every two pounds of fruit when jamming higher-acid fruit (like tart plums, cherries, or raspberries), and about two ounces for lower-acid fruit (like sweet strawberries). So for my six pounds of apricots, which are tart and acidic, I'd end up adding around three ounces of lemon juice. Remember: We can always add more acid but we can't take it away, so feel free to add more to taste.

One more note: If you add lemon juice towards the end of the process, you'll be introducing additional water, which can set you back a little in the pectin-web-making process. The solution then is to cook the jam for a few extra minutes to cook off the additional water that is added via lemon juice. In general, it’s important to remember that if you add water later in the cooking process, you will have to still cook down this added water to limit the final water activity level in your jam to achieve a proper gel set.

Feel free to experiment with acid sources: limes or sour oranges can add great flavor to some jams. You can also add neutral-flavored, water-free acids like powdered citric acid, or even malic or tartaric acid if you can find them, but those really pack a punch, so you have less margin of error. Of course, using another source of acid may involve tweaking the amount added per pound of fruit. Lime juice, for instance, has a lower average pH than lemon juice, thus you would need to add more to achieve the same pH in a jam. So, if you're going off-script with another acid source, I recommend checking the acidity level with test strips or a pH meter to make sure your finished jam has a pH of 4.6 or below before processing.

What About Just Adding Pectin That They Sell in Stores?

I think of adding commercial pectin to jam as an artist's choice. Some people use it to good effect, but I tend to avoid it because I have gotten much better results by just using pectin-rich fruit that is just ripe enough, and then dialing in on the sugar and acid and heat.

Jams that I've made with added pectin have often turned out too thick and rubbery, with a dull flavor. I do occasionally add a satchel of lemon wedges to a jam when I know there is likely to be lower amounts of pectin, like with strawberries. This helps add extra pectin that the berries don't have, but in lower amounts than if I were adding commercial pectin. (When the jam is finished, I open the satchel and squeeze any remaining jam back into the pot, then discard the lemon wedges.)

Still, I'm not saying that commercial pectin is never useful or that it's bad. Some jellies or jams made with low-pectin fruit like melons couldn't exist without it and are very much worth making. Whether to use a recipe that calls for added pectin is a personal choice, and I know that many, many successful jam makers have crafted delicious jams with it.

If you do decide to use added pectin, make sure that you use the specific type specified by the recipe—some types require higher levels of sugar, some require calcium to set, some are rapid-set, and some are slow-set. Recipes will call for different sugar amounts and cooking methods depending on the pectin type that's used.

Try It!

Now that you know all of the inner workings of jam-making, go ahead and start jamming. Just remember not to over-think it: Grandmas were making delicious jams a hundred years ago and probably didn't know about colloidal systems and negatively charged ions. Just enjoy experimenting, use some of the tips and techniques I've described to have a general sense of where to start, and remember: runny jams make delicious sauce for ice cream and too-firm jams can be melted down to glaze meats or galettes. There is almost never a failure when it comes to jam.