Sunday, August 26, 2012

Jam Science

First, let me be clear: I am the worst jam maker I know.  My excuse is that both my mom and mom-in-law made awesome preserves so I never needed to learn. I tried a few times over the years with hilarious and awful results but now I have three reasons to master this finicky art. First, my mom has been gone for years and  my mom-in-law is now in a nursing home, so the torch must pass to someone. Second, I love to garden so I have a lot of jammable fruits to try out. Third, and I think most people would agree with me, homemade jams and jellies outclass any purchased product. Home jam makers can achieve a flavor intensity and freshness no one else can.

Jam-making is an art, a science, a little alchemy and maybe even sorcery. I've tried several cookbook recipes that never set or don't taste like I hoped for. Over the years I've developed "canner syndrome" (every time I even think of making jam my blood pressure goes up). To my shock, a few days ago, I finally made good sour cherry (Evan's cherry) jam.

Having failed so many times before, I started to wonder: how does jam-making work? In this article I hope to answer some of my own, and maybe your, nagging questions.

Jam making is really a lesson, well, many lessons, in chemistry and microbiology. I pored over dozens of recipes online and was shocked at the variance between them - some required the addition of pectin, some didn't. The ratio of sugar to fruit was all over the map. Anti-spoilage procedures ranged from simply closing the jar's lid to sealing with wax to processing in boiling water for several minutes.

Jam History

Jam, jelly, preserves - it's all about preparing delicate and perishable fruits for long-term storage, and it all started several centuries ago. Although the exact origin of jam (I'll refer to all these fruit preserves simply as jam from now on) is a matter of debate, it likely started in the Middle East where a ready supply of cane sugar could grow. Jam was a very valuable delicacy because sugar, even in the 16th and 17th centuries, was extremely expensive and hard to come by. It was so rare it was considered to be a spice. Arabs grew and refined cane sugar, making way for a lucrative export product to late Middle Age Europe. It didn't take long for Europeans to experiment with preserving fruit with sugar. Only royalty could afford such extravagance - historians describe how marmalades finished off the feasts of Louis XIV, for example. The first jam cookbooks came out in the late 1600's, when some of the wealthy merchant class could afford to buy sugar. Here's an Elizabethan recipe I came across:

"Marmalade of Damsons
Take two Pounds of Damsons, and one Pound of Pippins, pared and cut in pieces, bake them in an Oven with a little sliced Ginger, when they are tender, poure them into a Cullender, and let the Syrup drop from them, then strain them, and take as much sugar as the Pulp doth weigh, boil it to a Candy height with a little water, then put in your Pulp, and boil it till it will come from the bottom of the Skillet, and so put it up"

 - a recipe from Hannah Woolley, in a cookbook entitled:

The Queen-like Chest
Scored with all manner of
Perserving, Candying and Cookery

printed in 1672.

Eventually, early European settlers brought the art of jam to the New World, using maple sugar, honey and molasses instead of cane sugar to make jams from wild berries and tree fruits. They also discovered along the way that apple peelings could be used to thicken their jams.

Jam is Simple (ish)

Just two or three ingredients - fruit, sugar and maybe a few chopped up apple peelings - were all one needed to make jam. That basic ingredient list hasn't changed much since. What makes jam-making challenging is not how complicated the ingredients are, but how the quality, ripeness and type of fruit used, how much sugar is used, and how preparation is carried out that significantly affect the finished product.


All fruit has some pectin in it. Apples have a lot of it. Pectin makes jams gel. So what is pectin? Scientists call it a heteropolysaccharide, a long carbohydrate chain made up of sugar acid units called galacturonic acid units which are branched with other sugars, esters and/or carboxyl groups all linked together. Glycogen (an energy storage molecule in your body), starches, cellulose and chitin (this is what the shells of crabs and insects are made of) are also polysaccharides. Different characteristics of different polysaccharides come from how long the chains are, what kinds of sugars make them up and how branched they are. The cell walls in every land plant on Earth have pectin in them. Although the pectin naturally found in fruit has jelled jams for centuries, it wasn't isolated and described until 1825 by Henri Braconnot. This is what pectin looks like in powder form:
Plant cells use cellulose to maintain cell rigidity. The non-woody parts of plants also utilize pectin to maintain cell shape and hold cells together. When fruit ripens it softens as pectin breaks down through the action of two enzymes. A very similar process happens to the ends of deciduous leaf stems, allowing leaves to fall in autumn.

Firm fruits like apples, gooseberries, plums and oranges have lots of pectin, whereas soft fruits like sweet cherries and strawberries only contain a small amount.

Here's a brief list of low and high pectin jam fruits:

Apricots Apples and crab apples
Blueberries and saskatoons Citrus rinds
Cherries (sweet and sour) Cranberries
Elderberries Currants
Peaches Gooseberries
Pears Sour Plums
Raspberries and Blackberries  Grapes
Sweet plums

Today most commercial pectin is extracted from dried citrus peel (30% pectin fresh weight) and apple pomace (1.5% pfw), both byproducts of juice production.

How Pectin Works - A Chemistry Lesson

Pectin is a long branched molecule:
Each of the six-sided rings is called galacturonic acid, and a pectin molecule may have between a few hundred and a thousand of these rings. On this particular pectin molecule, each COOCH3 branch is a methyl ester group (there are 3). Each COOH branch is a carboxyl group (there are 2). It is very difficult to generalize about the structure of pectin because its length as well the amount and types of branching vary widely, even within individual plant samples. It also changes during isolation from plant material, during processing and during storage (that is why pectin products have a best-before date).

The structure of a pectin polymer can be very complicated. This three dimensional complexity enables plant cells to have firm shapes. It provides a supportive mesh. Inside plant cells, pectin molecules contain some regions dominated by galacturonic acid (called smooth regions; like the diagram above) and other regions (called hairy regions) are rich with various sugar molecules in a highly branched arrangement.

Sugar molecules look very much like galacturonic acid does. They all have a very similar ring structure. For example, here's what ordinary granulated sugar looks like:
Sugar (sucrose to chemists) is a disaccharide of two sugars - glucose (left ring) and fructose (right ring). Imagine the long pectin molecule above with several of these sugar rings bound to it. Galacturonic acid looks so much like these molecules because it is an oxidized form of a sugar called galactose. Oxidization makes it a sugar acid. Below, galactose is on the left and galacturonic acid is on the right (look at the top functional groups):
When pectin is extracted, many of the hairy regions are destroyed, leaving a smoother more linear chain of mostly galacturonic acid. However, processed pectin molecules are still long and very complex. This makes them very useful to jam makers.

Because pectin molecules are long and highly branched, they can tangle up with each other and that is what causes jams to gel. Here is what happens in your jam pot: fruit has a lot of water in it and pectin dissolves in water. The pectin molecules don't interact much with each other. When pectin is heated along with sugar during jam making, less water is available to dissolve pectin because it is dissolving the sugar molecules. As the cooked jam cools, the pectin molecules are free to bind to each other through a combination of hydrogen bonds and hydrophobic interactions, forming a gel network. This chemical reaction, like many, works best at a particular pH. The optimum pH for most pectin is around 3.8, which is quite acidic, as a pH of 7 is neutral. High pectin fruits are also high in fruit acids (our previous galacturonic acid discussion tells us why). Most recipes add lemon juice to lower acid fruits such as sweet cherries, strawberries and peaches. Some jam makers suggest adding a few tablespoons of lemon juice to any jam to help ensure a low pH. I added 2 tablespoons fresh lemon juice to my successful sour cherry jam (anything to help).

Different Pectin Products

As if all this isn't complicated enough, there are two different kinds of pectin commercially available to jam-makers. The above process describes what happens when high-ester pectin is used. Esters are created when carboxyl groups (-COOH) lined up along galacturonic acid molecules bond with an alcohol, usually methanol (-OCH3). In order for high-ester pectin to work, your fruit mixture must contain at least 60% sugar. Most "regular" granular and liquid pectin is high-ester pectin, which is why jams will not set when we try to make a less sweet jam, a mistake I've made a few times.

Lots of us want a less sweet, more fruity jam. The product manufacturers have been listening. Now we can buy low-ester pectin for jam making. Less sugar is needed in order for these jams to set. In nature, about 80% of the carboxyl groups in pectin are esterified. High-ester pectin products have more than 50% carboxyl groups esterified whereas low-ester pectin has less than 50% esterified. If we look again at the pectin molecule diagram (below), you'll notice that 60% of its functional groups are esters (-COOCH3) and 40% are carboxyl groups.
This particular molecule is 60% esterified so it's a high-ester pectin. Low-ester pectin requires calcium as well as some sugar to form a gel. In this case, ionic bridges are formed between calcium ions (Ca2+) and ionized carboxyl groups, like the one shown below (an oxygen, O, has a little negative charge sign) on the galacturonic polymer chain.
The addition of a calcium salt such as dicalcium phosphate (added to or sold along with the pectin) is required.

This is the reaction that tangles up the low-ester pectin molecules into a gel. This process works best at a very low pH. Pectin molecules normally repel each other but as pH is lowered, they can associate and form a gel, so products such as Certo Light contain low-ester powdered pectin (already acidic) with acid. I haven't tried any light pectin products but I plan to.

There are several other ways in which pectin can be customized for various applications for use in products like yogurt, candies and cake fillings. Products now on the market for jam-makers include powdered pectin, liquid pectin, no-cook freezer jam pectin, low-sugar pectin, no sugar pectin, etc. helps you browse the types available and explains pectin in very user-friendly way. This might help you avoid standing in the grocery aisle looking stupefied like I did. But a word of caution as you choose - my research (and experience) tells me it is crucial to add the correct amount of sugar called for in the recipe (and to start with tested recipes included with the product). Different pectin products require their own exact sugar amounts. For example, I noticed that when using powdered pectin, my cherry jam requires 41/2 cups sugar to 4 cups prepared fruit. If I use liquid pectin, I will need 7 cups sugar to 4 cups fruit.

I hope this chemistry lesson helps us understand why we have to do things the way the box tells us to. Still, we are utilizing a variable and complex molecule designed by nature to do our chemical bidding. In spite of all the chemistry we learned here, pectin has built into it a little tweak of natural sorcery (and there is no reason why we can't use this excuse when things go south, which we'll explore next).

Reasons Why Jam Won't Set

When my jam doesn't set, I usually end up cussing around my kitchen for a while, but eventually I ask myself why it didn't work. Some makers swear that humidity in your kitchen can affect jam gelling. I can't tell you how this might work. Others insist that jams set much more reliably with cane sugar than with beet sugar. Wikipedia tells us that sugar beet pectin contains acetylated galacturonic acid esters in addition to methyl esters. Acetylation inhibits gel formation but increases the stabilizing and emulsifying effects of pectin, sort of a trade-off it seems to me. But the catch is how much pectin is in refined beet sugar. There shouldn't be any. For the record, Certo (the only brand I've tried so far) simply specifies granulated white sugar, and I used (Roger's) beet sugar in my successful sour cherry jam.

By far the most common reasons for jams not setting are:

1) not enough sugar
2) too little pectin (if you add extra fruit, especially low-pectin fruit, you risk this)
3) not reaching a high enough temperature (recipes tell you to reach a full rolling boil - on the highest heat possible. Your jam needs to reach around 218°F in order for the pectin and sugar to react with each other - that's a full rolling boil that foams up and you can't stir down)
4) cooking too long (the highly branched pectin molecules will eventually break down with long high-heat exposure)
5) doubling the recipe (a brimming jam pot will take much longer for the jam to heat up to 218°F and some pectin can break down in the meantime - and you are bound to boil over)

You can fix situations 1, 2 and 3 above by correcting the sugar level and remaking your jam (adding new additional pectin). I successfully corrected one of my too-low-sugar jams. Or, you can tell your family you intended to make pancake syrup and/or ice cream sauce all along. What they don't know won't hurt them. Be sure to label "sauce" not "jam" for authenticity.

To further ensure your success: Unripe fruit has much more pectin in it than fully ripe fruit. Many jam makers suggest picking about 1/4 under-ripe fruit, if you can, to boost your natural pectin content. Again, I did just this with my (successful!) sour cherry jam.

A final note: Fruit acidity and pectin will vary year to year even from the same tree depending on the growing season and how ripe most of it is. For this reason alone, jams can differ slightly year-to-year, or even mysteriously fail to set some years. Again, I file this under natural sorcery.

How the Homesteaders Made Jam

Pectin wasn't available here in western Canada until just a few decades ago, so my grandmother and others made jam the old way, which means cooking it a long time to boil away moisture, release natural pectin and thicken the added sugar into a syrup.

Most people shy away from the traditional method for several reasons: 1) it's even more tricky - adding pectin makes it more likely to set properly, 2) you cook away some vitamins as well as fresh fruit flavour, 3) it takes longer, 4) you can't skimp on sugar and 5) some low pectin fruits are difficult if not impossible to set. Frankly I don't know how these brave ladies (and the odd man) did it.

Some jam lovers, however, will tell you that old-fashioned jam has a rich caramelized undertone that you can only get from long cooking. You simply mix fruit, sugar and (often) a little lemon juice to help with the chemistry, and then cook until you reach a ketchup-like consistency (trusting that the natural pectin has been released from the fruit but not yet broken down) and place in jars. Exact sugar measurements are not necessary either. I've got enough on my plate with Certo jams before taking on this challenge, but foodies call this artisanal jam, and there is indeed a whole artisanal jam movement out there.

Making Jam Safely

There is no point in making jam if you're going to kill yourself. First, some rules of thumb: sealed processed jam can be stored safely for at least 1 year, and is considered safe as long as the seal is intact and there are no visible signs of spoilage from molds or yeasts. Some makers claim that non-processed jam will tend to darken faster in storage and may succumb to spoilage sooner. However, even the Certo website says this step is unnecessary. Below I will argue why I think it is better to process your jam just to be safe. Once processed, home food preservation guides suggest storing your jam in a cool dry dark place.

Microbial Threats to Jam

Bacteria, molds and yeasts are the three biggest threats to all home-canned foods. These microorganisms are in the air, on surfaces and on and in us. There is no way to eliminate them short of a laboratory clean room.

Jams, however, are an acidic high-sugar environment, which means that most microorganisms cannot live in them. High-sugar jams tend to last longer than those with reduced sugar because sugar is a natural preservative.


Many people worry about botulism in home-canned foods, for good reason. The botulinum bacterium thrives in an oxygen-free environment and it is very difficult to kill. A sustained temperature of 250°F is required. Even most home pressure canners can't reliably reach this temperature. Luckily, the risk of contamination with botulism is very low with jams because they are much too acidic for this bacterium to live. Home-canned low acid foods (meats, fish, green beans, carrots, asparagus, etc.) can be very dangerous however. Other common bacterial food contaminants - Salmonella, Staphylococcus aureus, Listeria and E. Coli - likewise do not thrive in an acidic sugary environment. But some bacteria can be present in your jam, as inactive endospores.


Aside from bacteria, jams can and do succumb to molds and yeasts. Molds can tolerate sugary low pH environments like jams, but these organisms can't live in a sealed air-free environment. However, when a jar's seal is broken, mold and yeasts become a real threat to jams. This is the main reason why sealing jam with wax isn't recommended. A microscopically perfect seal does not always occur and wax can crack over time as well, creating tiny spaces where jam can be exposed to airborne molds and yeasts. Although some molds are not dangerous, others can release toxins that can make us very sick. When a jar is contaminated, it is not easy to tell whether the mold or yeast is harmless or not. Mold and yeast growth can also increase the pH of the jam by consuming the natural acids and sugars in it, making bacterial contamination a possibility. All sources I researched tell us to throw out jars with any signs of mold or yeast, even if they appear to be sealed. Once jam is unsealed and in the fridge it is safest to discard any jam that begins to grow mold on it, rather than spoon off the mold. Most sources recommend that opened jam be kept refrigerated and consumed within a month or two.

Maintaining a Clean Workplace

Although jam resists microbial growth, minimizing contamination is highly recommended. Here is the method I use, adapted from the National Center for Home Food Preservation site:

Have 6 (or how many will fit in your canner) clean jars with screw caps and new snap-on lids on hand.

Wash jars, tongs, metal ladle, large flat metal spoon for skimming, jam funnel, metal cake rack and heatproof stirring spatula with hot soapy water. Leave in clean sink to air-dry. Place empty jars (minus lids and screw caps) in the canner and fill with hot tap water to just over 1 inch above to top of the jars. Put tongs (to take out jars and place lids later on) up to its handles in hot water as well. Place over high heat and bring to a boil. Boil with the lid on for 10 minutes.

Turn off heat, keeping lid on. Meanwhile, wipe down countertop with hot soapy water.

Prepare lids according to the box instructions to soften the sealing strip.

Prepare fruit, adding pectin, lemon juice and sugar and boiling according to the recipe. Once done boiling, turn off heat and skim off foam as jam cools for 5 minutes. Remove hot jars as needed with sterile tongs and fill each jar to 1/4 inch from the top using a jam funnel (these are easily available in stores and eliminate the need to wipe down jars - a possible contamination source).

Using sterile tongs place a lid on top of jar and screw on band until it is finger-tight. Place on wire rack. Repeat to get 6 filled jars. Turn heat on high under canner, put lid back on and bring to a boil. Place filled jars (keep them upright at all times - you don't want jam to spill into the sealing ring area) in canner and boil gently with lid on for 10 minutes.

Raise jars up with the canner rack and gently remove to the wire cake rack (I use oven mitts but I'd like to find a set of jar tongs). Once on the rack, do not touch jars! Don't tighten the screw caps. Soon you will hear popping sounds as the jams begin to cool and contract, creating a vacuum under the seal. Popping and an indented lid mean your jam is properly sealed. Any jars that don't seal should be put in the fridge and used within a month. Let jams cool completely, overnight is often recommended. Label and store. Rejoice in your jam-making prowess and share jams with friends and family. Jam can take up to a week to set. If it still isn't set, refer to my "reason why jam won't' set" paragraph.

Jam naturally resists microbial growth but it does not kill any vegetative (endospore) bacteria, molds or yeasts that might be present. Though the Certo site and Certo pectin instructions don't mention processing your jam, a boiling water canner is now recommended by the USDA for all jams. This process will kill all the bacteria I mentioned with the exception of botulinum, and it encourages a strong seal.

Jars and Lids

One more safety note: If you look at the first picture in this article (of my cherry jam), you'll see the decorative jar has a rubber ring seal. You can seal with rubber ring seals but they must be brand new (this one isn't so it's my fridge jar). Rubber seals quickly develop minute cracks that can inhibit a good seal and eventually let air in so only new perfectly smooth ones are recommended. I prefer the snap lids because you can easily see if they are sealed or not. For the same reason, avoid banging jars into each other and always check each jar's sealing edge to make sure it isn't chipped.

Try Making Jam!

There is a whole jam world to explore. The insert that comes with pectin products includes recipes for many different fruits to try. Pectin, jars and jam-making equipment all show up in stores (here) in August and September. Later this fall I have a plan to try making cranberry claret jelly, a grown-up recipe in the liquid Certo insert. It sounds perfect for soft cheese and crackers at Christmas.

Yesterday I made my first successful crabapple jelly too! Here is my batch just out of the canner:

If you look really closely you can tell that about half the lids have already snapped inward (the center outfacing dimple is gone). The crabapples I used are Rosybrook, green/yellow with a pink blush, so they made this light gold jelly. It gelled just right! The jam gods are pleased. For now.


A year later (August 2013) I made my sour cherry jam using two thirds of the original amount of sugar and low-ester pectin (Certo Light brand) crystals. Perfect jam! Less sweetness brought out the natural tang of the cherries and it jelled quickly and well. Next year I plan to try these crystals for apple jelly!


  1. Pectin in jam has a great effect on its thickness. There are numerous health benefits to it as well.

  2. interesting. thanks for sharing. love learning about the history of food. And interesting to learn about the chemistry of it.

  3. This is great one to see your blog last time I see same design to my friend constraction building in chennai. Majorly used Aluminium scaffolding equipment used for glass room design. Really very gud looking.


  4. This sweet, Spices infused Pineapple Homemade Jam is made of Ripe, juicy pineapples which are handpicked from our pesticides free Home grown Garden.