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 grape jelly
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
- Jam jars (8 oz) and lids
|Ingredients||Weight (g)||Percentage (%)|
- Use a pH meter or pH strip to test the pH of grape juice and adjust to 3.0 – 3.5 with lemon juice or citric acid if necessary (Note: This is normally not necessary for commercial grape juice since its pH is usually within the acceptable range for gel formation)
- Measure and collect grape juice in stainless steel pot
- Add pectin and dissolve
- Apply heat and bring grape juice to a rolling boil
- Add sugar and return to a continuous rolling boil (temperature will be at about 104oC), and hold for at least 2-3 minutes
- Test for set point and bottle in sterilized jars. Note: Set point is achieved when jelly forms a single lump when dripped into a glass of cold water, or when it forms a firm clump when dropped on a cold glass plate).
- What is the difference between a jam and a jelly? (2 points)
- What are the key ingredients in jams and jellies that are essential for gel formation? (2 points)
- True or False: Jams and jellies can only be made using fruits (1 point)
- Discuss what could cause the following problems in a jam or jelly product, 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)