The Science of Jam Making

apricot jam

Yes, the last snows have just melted and any fresh fruit is still a while away, but being prepared never hurt nobody, right? Read on so you’ll be well-educated and ready when all those lovely local strawberries hit the stands of your farmers market!

Up until recently, I thought jam making was kind of an occult science. Weird, because I watched my grandmother and mother make batch after batch after batch as I was growing up, and it seemed to be the most straightforward thing in the world. Still, I was intimidated to try myself because I could only think of the ways it could go wrong – what if my jam spoils and I food-poison my family?

I don’t think either granny or my mom ever worried about their jar disinfecting practices or pectin percentages, but I am admittedly a bit of a nerd. So if you too are curious about how jam making works, geek out with me for a bit and learn the simple science behind this preserving method. It’s gonna be fun!

First of all, why bother with jam making at all? Two reasons. First, although the quality of commercial jams are improving, those are still just the shadows of a good homemade jam, often containing more coloring, preservatives and artificial flavors than actual fruit. And also because buying jam gives no way near the satisfaction of making it yourself!

Once I made my first batch, there was no stopping me. Pictured above is a vanilla-cardamom apricot preserve from last summer. I have dozens of flavor variations in a notebook that I want to try, and the list is growing.

So what is jam?

The wonderful jams of summers past are actually creative exercises in chemistry. Today, jam generally refers to a spreadable, chunky-textured mix made from the juice and flesh of fruit. A clever balance of pectins, sugar, and acid turns the cooked fruit into our favorite topping for toast.

The fruit

The choice of fruit for jam-making is simply endless. Always try to use seasonal fruit to get the best flavour. Remove leaves and twigs, wash if needed, remove cores and/or stones.

The equipment

You’ll need a wide-mouthed pan big enough that the fruit does not reach more than halfway up the side. Use one with a heavy bottom for even heat distribution – otherwise the jam will catch and burn. Also, a wooden preserving spoon (it has a flat head that helps keep the mixture moving and stops it sticking to the bottom of the pan), a ladle, a funnel, jars, and optionally a food thermometer.

The chemistry

To achieve a perfectly set jam, a number of factors need to be just right. The three key elements that go into jam making are sugar, pectin, and acids. Understanding the chemistry behind why jam sets can also help you identify and fix problems.

The sugar

Sugar imparts a preservative effect: binding water molecules to itself reduces the amount of water available in the jam, to the point at which it is too low for microbial growth. Binding the water molecules also frees up the pectin chains so they can form their network.

Finished jams have a sugar content of roughly 60%. To achieve this, recipes comprise mostly equal weights of fruit and sugar. While you can totally play with this 1:1 ratio, be aware that too much fruit and you may lose the preserving effect, while too much sugar and the jam will be the consistency of set honey, losing the color and aroma of the fruit.

The pectin

Pectins are long, chain-like sugar molecules found in the skins and cores of fruit. These compounds are the ones that turn the wet, sloppy fruit sauce into the semi-rigid, elastic substance of our dreams, a texture referred to as a gel.

Through boiling, pectin is released from the fruit and at one point forms a mesh structure that holds the liquid together. As the pectin content of different fruits varies, some may require an added dose in the form of commercially packed pectin or a pectin-rich other fruit. See this chart for specific pectin contents.

The acid

High acidity makes your jam an unpleasant place for micro organisms to breed, helping to extend the shelf life. Acids also help the pectin branches to bond. Fruits naturally contain acids (the most well-known is citric acid), but often this won’t be enough to reach the desired pH. For this reason, lemon juice or powdered forms of citric acid is added. As a rough guide, the juice of a whole lemon (30-40 ml) will be needed for low acid fruits, half a lemon will be enough for medium acid fruits, and you won’t need any for high acid fruits. In general, fruits with high pectin will also have high acidity.

The process

Heat the fruits slowly, without sugar, until a very gentle boil is reached, then cook until just tender. The heat will break down the structure of the fruit, release the moisture and cause the whole fruit to soften. Add the sugar (it may foam up, so be careful). Allow the sugar to dissolve over a low heat then bring to the boil. If you want larger fruit pieces, avoid stirring at this point. Not only does the texture change by boiling, the water content is also reduced, increasing the concentration of sugar and fruit components. Cook, stirring occasionally, until the setting point is reached.

How do I know it’s ready?

There are simple ways of telling when the pectin network has formed and you are ready to pour the jam out. Get the thermometer: if the temperature is around 104-105°C, the sugar content is high enough to allow the pectin branches to join. A direct measurement can also tell the doneness: pour a small blob of jam on a saucer, let cool in the fridge and then push with your finger. If the surface wrinkles, it means the pectin network has solidified and setting point has been reached.

You can now take the jam off the heat. Note that if you don’t boil it long enough the pectin network will not form properly and you’ll end up with a sloppy liquid. Boil it too long though and you risk having a too thick preserve, losing the fresh flavour and color.

Time to can

Allow the jam to cool and thicken for about 10 minutes before pouring into prepared jars (read how to disinfect jars and lids here). Don’t leave it any longer, as lukewarm jam is a breeding ground for mildew spores present in the air. Each jar should be topped up to just less than a centimetre below the surface. Adjust lids, process, than cool jars.

Don’t forget to test your seals before labeling and putting jars away in your pantry! Gently press the middle of the lids with a finger – if the lid springs up when you release your finger, it’s not sealed properly. No need to throw these out, just keep refrigerated and consume shortly.

Store jars in a cool, dry, dark place away from direct sunlight and accidental freezing. Once a bottle of jam is opened, micro organisms have been reintroduced. The same applies as with unsealed jars: keep in the fridge and consume shortly.

Hope all this makes sense and you’ll try your hand at jam making too! Let me know!

Love,

Fruzsi

 

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