By the end of this lab, students should be able to:
- Explain the science behind gel formation in jams and jellies
- Identify the basic ingredients in jams and jellies and their functions
- Prepare a strawberry jam
Jams and jellies are sweet, semi-solid spreads. Jellies are made from the strained fruit juice, and jams made from the fruit pulp. In order to form a gel, they must have the right balance of water, sugar, pectin, and food acid. Pectin can form a network in water, trapping the rest of the food material to create a gel. However, it does not do this independently. Sugar and pH of the food also play a key role. Sugar absorbs and traps water, increasing the thickness of the gel. Sugar competes with pectin for water. As the sugar pulls water away from the pectin, the pectin molecules are drawn closer together like a belt, trapping the rest of the material in a net-like gel structure. Without enough sugar, the structure of the gel will be too watery, while too much sugar causes a leathery, tough and grainy gel. The graininess is due to recrystallization of the sugar after cooling. For good results, the final sugar concentration of jams and jellies should be between 55 – 65%.
As I mentioned, the gel consistency is also dependent on the pH of the fruit juice. The juice should be acidic enough with a low pH of between 3.0 – 3.5. A low pH causes the pectin molecules to lose the natural negative charge it has when it is dissolved in water. This negative charge typically causes pectin molecules to repel (push away) from each other, making gel formation impossible. However, when pectin is neutralized, they associate to form the gel network.
If you are working with a juice that naturally has a high pH, you can lower the pH by adding citric acid, lemon, or lime juice. An interesting point to note here is that your jam or jelly does not have to be fruit-based. You can also use a vegetable. The important thing is to have the right balance of pectin, sugar and acid.
Materials and Equipment
- Stainless steel pot
- Mixing spoon
- Gram scale
- Jam jars (8 oz) and lids
- pH meter
|Fresh chopped strawberries||450|
- Weigh 450 g of fresh wholesome strawberries
- Remove sepals (green leaf-life structures) and wash strawberries
- Use a knife to chop strawberries into small pieces
- Add strawberries to pot and then add 60 ml (60 g) of water
- Cook on stove to soften strawberries (this should take about 5 minutes)
- Add 0.5 g citric acid. (Note: This should bring the pH to 3.5 of below which is desirable for jam setting. You can verify the pH using a pH meter)
- Add pectin gradually while mixing, in order to prevent clumping
- Bring to a rolling boil and then add sugar while mixing
- Return to a rolling boil and cook for about 3 – 5 minutes while continuing to mix. This will avoid sugar caramelization and off-taste
- Check that the jam is set by placing a drop of the jam in a cup of cold water. If the drop remains lumpy, the jam is set. If it disintegrates, the jam needs more time to cook.
- Once set, pour jams in 8-ounce jam jars. It should yield approximately three jars. (Note: Careful! The jam is very hot and can cause serious burns)
- Seal jars, label, and leave to cool until the next lab session
- When you return, conduct the following tests on your jam
- Brix (This is the term used for percentage sugar. You need a refractometer for this)
- Taste testing with crackers or bread
- Report the final pH of the jam: _____________ (2 points)
- Report the final brix of the jam: ____________ (2 points)
- Write a description of the sensory attributes of the jam. You may include texture, spreadability, taste (sweetness), flavor, aroma, appearance and your overall impression (5 points)
- Discuss what could cause the following problems in a jam or jelly and explain how it could be corrected.
- Gel is too soft (2 points)
- Gel is too gummy (2 points)
- Product contains sugar crystals (2 points)
- Product has darker than normal color (2 points)