Pectin: Gelling properties

Question 1

First to make a gel it is important to understand the charge of the pectin: explain how it works?

Answer 1

The galacturonic acids, have acid groups. This groups can have a negative charge depending of the pH.

• at neutral pH of 7: the carboxyl groups are negatively charged.
• acidic pH of 2-3: the carboxyl groups are neutral.

Question 2

There are two different types of gel that the pectin can make:

Answer 2

1. Calcium gel: LM-pectin forms a gel with Ca2+. Calcium ions have a positive charge they make a bridge, an ionic bond between two different pectin strands. Because there is a high relative pH, we have negative charges in the pectin, so everything is connected via the calcium. This bridge is called the `egg-box´model.
2. Sugar-acid gel: HM-pectin forms a gel with acid an sugar. The sugar is going to attract water, you are going to lower the water activity so there is less water available for the pectin molecules, so the pectin molecules are close to each other. And the pH also helps , by adding acids you take away the charge from the acids groups that are still present, so there is going to be no charge–> less repulsion between molecules.

Question 3

1. Why will a methylated group never become negatively charged?
2. Which type of pectin is used for jam? HM or LM?

Answer 3

1. Because there is no carboxyl group, the H cannot go anymore.
2. HM, which almost no acid groups are there, there is going to be a little bit of charge, and we want the least charge for this type of gel.