TA Documents

Department of Nutrition Science

Purdue University

Purdue griffin

Post Lab Discussion

Lab Preparation


End of Lab Discussion and Questions

Experiment 1 - Histochemical Localization of Pectic Substances


1. Where were the pectic substances observed? What role do they play in the intact plant?

Pectic substances are found in the middle lamella (i.e. between the cell walls in plants). Pectic substances act as intracellular "glue".
2. What changes do pectic substances undergo as fruits ripen? At what stage are they useful to the food industry?

pectic substance changes
Molecular weight decreases as you go from protopectin to pectic acid (chain length decreases as well as demethylation).

Methylation decreases as you go from protopectin to pectic acid.

*Pectin methylesterase -- cleaves off methyl groups.

Polygalacturonase -- cleaves polymer chain between sugar units.

Commercial Pectin - range of molecular weight available

range of methoxyl content

heterogeneous mixture of pectinic acids

From: lemon and lime peels and apple pomace

Experiment 2 - Pectin gels

Pectin in water solution is stabilized by:

a) shell of water

b) negative charges on COO- due to pectin methylesterase activity COOH => Increasing pH => COO- (like charges repel)

Pectin methylesterase action increases with ripening.

Pectin forms a colloidal dispersion. To make a gel you want to disturb this by neutralizing COOHs and strip the shell of water by adding sugar that competes for the water.

Therefore, to form a gel: need to strip shell of water, control pH.
Optimum for jelly:

Pectin: 0.5 - 1.0% pectin

pH: 2.8-3.5

sugar: 65% w/w

endpoint temperature: 104.5o C -- which gives you 65% sugar (this is based in the colligative point property of boiling point elevation, i.e., the relationship between the concentration of a solution and its boiling point)

A. Vary Sugar

1. half the sugar but still 104.5o C endpoint so still 65% sugar. More water is evaporated to get to the endpoint. -Results: 1/2 yield

2 times the pectin (1-2 percent)

Tougher gel

Decreased sag

Decreased tenderness

Increased force to penetrate by TA

2. Control

3. 1 1/2 sugar but still 104.5o C endpoint so still 65% sugar.

Less water is evaporated to get to the endpoint. -Results: 1 1/2 times the yield

Decreased pectin

Weaker gel

Increased sag

Increased tenderness

Decreased force to penetrate by TA

B. Vary pH

4. Control

5. pH = 2.4 -- too low and all COO- are protonated

The COO-'s are fully protonated Results: Tougher gel

Syneresis results

C. Vary Endpoint Temperature

6. 101.5o C -- less than 65% sugar Did not evaporate enough water off to increase sugar percentage Results: Higher yield

Decreased pectin concentration

Weaker gel

7. Control

8. 107.5o C -- greater than 65% sugar

Evaporated more water off when heated to high temperature Results: Lower yield

Higher pectin concentration

Tougher gel

D. Vary Amount of 150 Grade Pectin

150 grade means 150 lb. of sugar is required per 1 lb. of pectin to form a gel. All variations have the same endpoint temperature, so all will have 65% sugar and all yields are approximately the same.

 Grade is determined by gel forming potential => high grade has low molecular weight pectin (need more of a higher grade to form a gel)

9. 1/2 pectin -- weak gel

10. control

11. 2 times the pectin -- tough gel

E. Type of Pectin

12. Low methoxyl pectin + 10 g sugar + CaCl2 -- ~30% methylated Free COO- groups form salt bridges with Ca and forms chemically set gel. Since sugar concentration is decreased, this is a dietetic gel, exhibiting a more brittle nature, a higher water activity (not bound to sugar), and shorter shelf life due to microbial growth. Sugar--COO-(Ca+2 )-OOC--Sugar

13. Fast setting. High degree of methylation ~ 70%.

Gel forms easier due to hydrophobic interaction, sets at 85o C.

Fruit stays dispersed throughout, not enough time to float to top.

14. Slow setting. Low degree of methylation ~50-60% More charges COO-

Harder to form a gel, sets at 55o C

Fruit has time to float to top.


1. What physical property of a juice does the jelmeter measure?

Jelmeter measures viscosity that is given by the concentration of pectin.
2. Why does alcohol precipitate the pectin?

Alcohol precipitates the pectin because pectin is insoluble in alcohol.

The alcohol test is quick and easy way to see if there is enough pectin to make a gel in the fruit itself.
3. What stabilizes pectin in a colloidal dispersion?

The shell of water and the like charges on the carboxyl groups stabilize it.
4. What factors in jelly making destabilize the colloidal dispersion?

pH -- lowering the pH protonates carboxyls and destroys repulsion stabilization achieved by like negative charges.

sugar -- competes for the shell of water.

heating -- strips the shell of water.
5. What function does cooking jelly to 104.5o C?

It insures 65% w/w sugar in the product.
6-8. See Experiment 2 results.
9. If the jelly were made from pectin that was 30% versus 70% methylated what would be the expected results? How might the formulas differ? Which of these would be fast setting and which would be slow setting?

70% methylated -- more hydrophobic interaction possible. Gel sets faster and at a higher temperature.

30% methylated -- more negative charges on COO-. Harder to form gel, sets slower at a lower temperature.

10. See Exp. 2 results.



Preparations for Lab


Experiment 1 - Histochemical Localization of Pectic Substances

Get out razor blades - make sure they are sharp and wrapped for safety. They should be located in the top drawer of the front left bench. Prepare ruthenium red dye - there is some already made in the solution cabinet. This needs to be prepared fresh every semester.

Get out microscopes, slides and cover slips. Microscopes are in the back of the room by the left window in a low cabinet. The slides and cover slips should be in the top drawer of the front left bench.

Experiment 2 - Pectin gels

Jan in the storeroom will make the juice. Check early in the week to see if there is enough pectin in the juice. Some might need to be added. Test with the jelmeter.

Get out the 150 grade pectin, low methoxyl pectin, fast setting and slow setting pectin. All should be located on the middle shelf behind the first blackboard.

Get out the CaCl2 which is located in the chemical cabinet.

Prepare the citric acid solution - 286.02 g hydrous citric acid (make sure that this is citric acid monohydrate) dissolved in 300 ml H2O The citric acid is located in the chemical cabinet.

Obtain a balance that will be able to weigh the jelly jars 

Get out refractometer, Kimwipes, disposable pipettes and squirt bottle of water.

Prepare texture analyzer with the cone probe

Lab 8 Experiment 3 - Starch Gels

Get the sample cups of starch gels out of the freezer and refrigerator and allow them to come to room temperature.

Prepare texture analyzer with the cone probe


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