If you’ve ever tried your hand at jam-making, you’ll know that it’s something of a tricky process. A number of factors need to be just right to achieve a perfectly set jam – and chemistry can help explain why. There are three key chemical entities that go into jam-making: sugar, pectin, and acids. Here, we’ll look at each in turn, and how they help jam achieve its eventual consistency.
Pectins
Pectins are long, linked chains of sugar molecules, which are found naturally in plant cell walls. Although we refer to them in general as ‘pectin’, their structures are variable, as well as hard to determine; a rough general structure is given in the graphic, but in reality the overall structure can be much more complicated.Pectins arefound in fruits, particularly in the peels and cores. When jam sets, pectin plays a vital role.
Boiling the jam releases the pectins from the fruit used; with the correct amount of sugar and acidity, which we’ll discuss in due course, the long pectin chains can bind to each other via intermolecular interactions, forming a gel network. This network generally forms at the ‘setting point’ of jam, which is approximately 104˚C. Once it has formed, the jam can be allowed to cool, and the gel network ‘traps’ the water content of the jam, leading to setting.
The pectin content of different fruits varies: fruits such as apples and blackcurrants have higher levels of pectin than those such as strawberries and raspberries. In cases where a jam is being made from a low pectin fruit, either a higher pectin fruit must also be included, or commercial pectin must be added. Commercial pectin is obtained from the peel of citrus fruits, which have a naturally high pectin content.
Sugar
An important part of jam is, of course, the sugar content, which is vital for the flavour and also plays a role in helping jam set. Many jam recipes recommend the use of a 1:1 ratio of fruit to sugar in jam-making. As well as sweetening the jam, the sugar also helps the pectin set – it enhances the pectin’s gel-forming capability by drawing water to itself, decreasing the ability of the pectin to remain in separate chains. Additionally, sugar imparts a preservative effect. By binding water molecules to itself, it reduces the amount of water available in the jam, to the point at which it is too low for microbial growth, helping to ensure that the jam doesn’t go off too rapidly after it’s been made! The final sugar content of jam should be between 65-69%.
Acids
Acids are also important in helping the pectin to set. The COOH groups in the pectin are usually ionised, and thenegative charges on the molecules this ionisation causes can cause repulsion, and prevent the pectin chains from being able to form the gel network. To avoid this, we need the pH of the mixture to be roughly in the range of 2.8-3.3. At these more acidic pHs, the COOH groups aren’t ionised, lowering the magnitude of the repulsive forces.
Fruits naturally contain acids – the most well known is citric acid, but malic acid and tartaric acid are also found in a number of fruits. Whilst some acid will be contributed by the fruit from which the jam is made, often this won’t be enough to reach the desired pH, and for this reason more must be added. This is commonly in the form of lemon juice, which contains citric acid,though powdered forms of acids can also be used.
In summation, then, the three factors of pectin, sugar and acid have to be in perfect balance for jam to set. If it doesn’t, you can often point to one of those three factors being somehow amiss – and understanding the chemistry behind why jam sets in the first place can often help you identify how to fix it!
A modified version of this graphic will be included in the upcoming book “Why Does Asparagus Make Your Wee Smell?”, available to pre-order now!
The graphic in this article is licensed under aCreative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. See the site’scontent usage guidelines.
References & Further Reading
- The Science & Magic of Jam-Making – Guardian Science
- Jam Science – Scientific Explorer
- Chemistry of Pectin – P Sriamornsak
- Science of Jams & Jellies– Dr M Bourne
Tags: cooking fruit jam
14 CommentsClose Comments
14Comments
-
Ana
Posted September 27, 2014 at 7:12 pm 0Likes
Great article!
-
Compound Interest
Posted September 28, 2014 at 11:19 pm 0Likes
Thanks Ana, glad you enjoyed it!
-
- Trackback: What Makes Jam Set? - The Chemistry of Jam-Maki...
- Trackback: The Chemistry of the Real World | Pearltrees
- Trackback: The Chemistry of Jam/Jelly Making :) | A Food Journey To Go
- Trackback: Caramelized Vanilla Pear Jam |
- Trackback: The Chemistry of Making Jam Set Up | The Homestead Survival
-
satish
Posted July 26, 2016 at 11:41 pm 0Likes
what about the pectins that allow for low or no sugar? strange they have added SODIUM CITRATE and FUMARIC ACID, how does this pectin get thick when there is no sugar? And why cant they add citric acid?
-
satish
Posted July 26, 2016 at 11:49 pm 0Likes
is is there any heat needed. i seen the videos of the them cooking the jam?
- Trackback: The Berry Trifecta – U pick, U learn
-
clothespin
Posted November 23, 2016 at 6:12 pm 0Likes
Thank you so much for this! As a botanist (well, now a mommy) and a home canner, this has come in handy. I’m working on a recipe for hibiscus tea jelly… and thanks to a ph test in the lab and your information, I at least have a starting point on where to go to make the recipe work. (The tea is super acidic so am going to try adding another juice to raise it up.) As I tell my girls… cooking is just science that you can eat!
-
Compound Interest
Posted November 23, 2016 at 6:55 pm 0Likes
You’re welcome! If you’re into canning, this graphic might also be of interest: http://cen.acs.org/articles/94/i36/Periodic-graphics-chemistry-canning.html
-
- Trackback: 10 Traditional Jam Making Troubleshooting Tips Part 2 – Melissa K. Norris
- Trackback: Hóa học đằng sau mứt trái cây - Hóa Học Là Chia Sẻ
Comments are closed.
As an enthusiast and expert in the field of food science and chemistry, particularly regarding the principles behind jam-making, I can provide comprehensive insights into the chemical components and processes involved. My expertise stems from extensive study, practical experience, and ongoing engagement with scientific literature, recipes, and experimentation in this area.
The article discusses the chemistry behind jam-making, highlighting three essential elements: pectin, sugar, and acids.
-
Pectins:
- Pectins are complex polysaccharides found in plant cell walls, especially in the peels and cores of fruits.
- They play a crucial role in jam setting. When heated, pectins are released from the fruit and form a gel network, provided the correct balance of sugar and acidity is present.
- Different fruits contain varying levels of pectin. Fruits like apples and blackcurrants have higher levels, while strawberries and raspberries have lower amounts. For fruits with low pectin content, additional pectin (often from citrus fruits) or commercial pectin is added.
-
Sugar:
- Sugar contributes to both the taste and setting of jam. It aids in the formation of the gel network by enhancing pectin's ability to bind together.
- Additionally, sugar acts as a preservative by reducing water activity, making it less conducive to microbial growth. The recommended final sugar content in jam ranges between 65-69%.
-
Acids:
- Acids are crucial in jam-making as they influence the pH level necessary for proper pectin setting.
- The COOH groups in pectin molecules need a specific pH range (approximately 2.8-3.3) to prevent repulsion between molecules and facilitate gel formation.
- While fruits naturally contain acids, additional acid (commonly from lemon juice) is often required to attain the desired pH level for effective jam setting.
Regarding additional queries posed in the comments section of the article:
- Some pectins allow for low or no sugar content by incorporating substances like sodium citrate and fumaric acid, which aid in thickening without relying solely on sugar.
- Heat is indeed necessary in jam-making. Boiling the jam helps release pectins and activates the gel-forming process.
- Adjusting pH levels by adding specific juices or acids is a strategy employed by home canners and recipe developers to ensure proper jam consistency.
In essence, achieving the perfect jam consistency requires a delicate balance of pectin, sugar, and acids. Understanding the chemistry behind these elements is pivotal in troubleshooting and mastering the art of successful jam-making.