Science & Food

Jammin’ With Fruit

Now that summer is in full swing, what better way is there to use all the berries, apricots, plums, peaches, and nectarines in season than to make jam? Jam has a great texture that makes it the perfect spread for brioche toast and a sweet complement for porridge. The base ingredients of fruit, sugar, pectin, and acid are cooked until the jam reaches a spreadable consistency [1]. Each ingredient plays an important role in the texture of the final product.

Strawberry jam. Photocredit: Julia Khusainova (Jullclous/Flickr)

Strawberry jam. Photocredit: Julia Khusainova (Jullclous/Flickr)

Sugar Skills

Sugar has many roles besides adding sweetness to the jam. When sugar is mixed with mashed fruit, it begins to dissolve and draw water out of the fruits through osmosis [1]. This occurs because fruit has a lower concentration of sugars than the amount of sugar that is typically added. The hydrophilic groups on sugar make it miscible with other polar molecules like water. Sugar also acts as a preservative by forming bonds with water molecules, making fewer water molecules available to support the growth of various microorganisms that might cause spoilage, such as Aspergillus glaucus and Saccharomyces rouxii [2,3].

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The making of blackberry jam. Photocredit: (mrskupe/Flickr)

Gelation

Pectin is soluble dietary fiber that naturally occurs in certain fruits such as apples, plums, and quinces. When heated and mixed with acid, this carbohydrate creates a thick gel that contributes to the consistency of jam. Acid from citrus increases the hydrogen ion concentration in the solution, which results in the pectin molecules losing some charge. With less electrostatic repulsion, the molecules can now aggregate to form a physical gel at a higher temperature of around 220ºF [4], resulting in watery fruit liquid dispersing itself within a web of pectin molecules [5].

Typically, under-ripe fruits have more pectin because fruit enzymes convert it to pectic acid during the ripening process [5]. This means that high pectin content can often be a trade-off for lower flavor, so it is recommended to use two parts of ripe fruit for every part of under-ripe fruit for the best consistency and taste [1]. Alternatively you can buy powdered pectin from the store to use with fruits that naturally have low levels of this carbohydrate, such as apricots, peaches, and raspberries.

Sugar Inversion

Acid not only contributes to the texture of jam, but it also catalyzes the conversion of sucrose (from added sugar) into its constituent fructose and glucose molecules with the help of heat. This process is called sugar inversion, and it is necessary to prevent recrystallization during jam storage [6]. However, this rarely occurs because finishing a jar of jam usually does not take too long. As an added bonus, acid also contributes to the flavor balance of the jam, preventing it from being too sweet.

Different fruits vary in acid and pectin content, so adjustments may be necessary to obtain the right texture and taste. You can also use a Brix test to measure the endogenous levels of sugars and dissolved nutrients. Basically if your jam contains riper and more nutrient dense fruits, the test will give a higher reading. Develop the right recipe, and you will want to eat out of the jam jar with a spoon!

References cited:

  1. The Science of Jam and Jelly Making. University of Kentucky.
  2. Jam Making: Why all the sugar? Iufost.org
  3. Why does jam go mouldy, even in the fridge? University of Liverpool
  4. Fishman, M.L. & Jen, J.J. Chemistry and Function of Pectins. June 1986.
  5. Jam Making 101. Seriouseats.
  6. Inversion of Sucrose. Colby College.

 


Catherine HuAbout the author: Catherine Hu is pursuing her B.S. in Psychobiology at UCLA. When she is not writing about food science, she enjoys exploring the city and can often be found enduring long wait times to try new mouthwatering dishes.

Read more by Catherine Hu


 

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