Department of Nutrition Science

Purdue University

Purdue griffin


Group Assignments and Tables


The forms and tables included in these pages are for the purpose of illustrating how you should prepare tables in your notebook before coming to lab. They are not for the purpose of direct entry of data during the lab. Please use them in the way intended. The grade sheets indicate the materials that should be included in the formal write and the point value assigned to each element.

Download your gelatin handout here!!




animated collagen

Gelatin is derived from the triple helix protein collagen, shown in this figure.

(Image from 

Effect of gelatin concentration on gels

 As gelatin concentration increases:

1. gel strength increases and penetration force increases

2. viscosity of sol increases and outflow time increases

3. setting time decreases and liquifying time increases- 1.5% gelatin is the optimal concentration for gel formation

- Rapidly cooling gels (like we did in lab) results in the formation of an unstable gel which liquifies easily (melts). Stable hydrogen bonds (junction zones) are not allowed to form, or at least a sufficient number over a considerable length of the gelatin polymer..

Effect of pH on gels

At the isoelectric point (pI) proteins have no net charge, and therefore do not repel each other. When proteins are at their pI, this results in the formation of STRONG GELS. (pH of 4.7-5.2 is the pI of gelatin proteins).

Proteins are also more globular (if they tend to a globular form) when at the pI.

When the pH is away from the pI, (pH below or above 5), proteins are charged. They are negatively charged at basic pHs and positively charged at acidic pHs. Proteins are more rodlike due to electrostatic repulsions when charged and bond with water easily. Therefore, acidic and basic conditions will cause gels to become tenderized due to increased bonding to water.

As the pH moves further from the pI:

1. setting time increases

2. gel strength decreases and penetration force decreases

3. liquifying time decreases

4. viscosity increases because fibrous, rodlike proteins are more viscous than globular proteins. (At their pI, proteins are more globular)

Effect of sucrose concentration on gels:

At high concentrations sugar tenderizes gels by competing for water which is needed for gelatin swelling, unfolding and maximum hydrogen bonding. Also, there is adirect offect of sucrose acting as a plasticizing agent between the gelatin molecules.  Therefore, viscosity will decrease at high sugar concentrations. At lower sugar concentrations, viscosity of gelatin sols will increase because it does not compete with the gelatin excessively for water at lower concentrations. 

As sucrose concentration increases:

1. gel strength decreases and penetration force decreases

2. setting time increases

3. liquifying time decreases

4. viscosity increases at low sugar concentrations, viscosity decreases at high sugar concentrations

Effect of proteolytic enzymes on gels:

Proteolytic enzymes cleave the protein chains and prevent gel formation.

e.g. ficin - figs

bromelain - pineapple

papain - papaya

- heat treatment of enzymes in canned fruits destroys the proteolytic enzymes.



Experiment 1 is the only experiment you'll execute during lab!


Gels are related to the number of junction zones (arrows) and the size of the junctions.  The bigger the junctions are, the stronger the gel.  Also important is the speed at which the gel occurs.  With a slower rate of gelling, a more stable gel will be formed.


6% - 500ml of stock solution

3% - 250ml of stock solution + 250ml water

1.5% - 125ml of stock solution + 375ml water

0.5% - 42ml of stock solution + 458ml water


Measured with 5ml pipette

Let 5ml of gelatin dispersioni flow and time it

Make sure gelatin is at 60 degrees C (Warm, not hot!)


Pour into test tube

Place in ice bath

Time it


Use gel in test tube from “setting” experiment

Make sure to label well enough to see while in 40C incubator

Place tube upside-down in test tube rack, on tray with paper towel, in incubator

Mark time when gel has liquefied

Gel Strength

Pour into plastic cup

Place in fridge

Once solidified, use Texture Analyzer with the cone probe to test gel strength


When you increase the concentration, you increase the number of molecules and the number of junctions.  The gel will be much stronger with this increase in concentration.


When you adjust the pH, you are changing the number of positive and negative charges on the protein.  When are at the isoelectric point (IEP), there is an equal number of positive and negative.  At this point, your protein is globular.  When you have a globular protein, you have a smaller hydrodynamic radius and low viscosity.  When you get further away from the IEP, the protein has either more positive or negative charges, resulting in a more linear protein, with a larger hydrodynamic radius and a high viscosity.  When are at or near the IEP, outflow time is the quickest, the setting time is the shortest, and the liquefying time the longest.


Sucrose will fill the gaps where the junction zones would be, creating a weaker association than with protein-protein.



The sugar strips the water molecules surrounding the protein, so the gel is not as strong as without sugar.  The gel will become very tender and has trouble solidifying and liquefying.


In this case, papain (found in the desiccator in the freezer) will eliminate certain bonds in the protein, decreasing the overall size of the original protein.  Gels rely on big proteins, so the decrease in size will have an effect on the gelling process.  You may not be able to get these solutions with papain to solidify for use in the experiments, so be aware.  You will also need to allow the solution to cool before adding papain.


You will need to work as a team in your group, especially with setting and outflow times.  It is critical that your gelatin solutions for these specific exercises are at 60C. Any major variance in temperature could result in skewed data.


Group Assignments, Further Instructions and Tables

Group Assignments


Group 1-----------------effect of sucrose

Group 2 ----------------effect of pH

Group 3 ----------------effect of concentration

Group 4-----------------effect of enzyme

Group 5-----------------effect of enzyme



1. Use ice bath for cooling gels

2. Do objective measurements in triplicate

Disperse gelatin in a small amount of cold water first, then add hot water. Then blend in Sorvall or blender.



Effect of concentration on gelatin


% gelatin

seconds of


setting time

liquifying time

TA (g)











































Effect of pH



seconds of


setting time

liquifying time

TA (g)






















































Effect of sucrose concentration


sucrose conc.


seconds of outflow

setting time

liquifying time

TA (g)












































Effect of proteolytic enzymes



seconds of outflow

setting time

liquifying time

TA (g)























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Web Author: James R. Daniel, Ph. D.

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