Laboratory 9
Protein chemistry - foaming properties
Introduction
Proteins are used extensively in the food industry for their functional properties (i.e., gelation,
foaming, viscosity) as well as for their nutritional properties. Protein as a component in foods can also
influence their properties of texture, flavor, etc. Such an example is milk proteins and their contribution to
cheese, yogurt, ice cream, etc. and egg white proteins for foams. The ability of a protein suspension to
form a stable foam is dependent upon its physical form, the pH of the suspension and on the presence of
other constituents.
Proteins are surface active. This means they have the ability to interact both with air and water.
The hydrophobic parts of proteins interact better with air compared to the hydrophilic parts, which favor
interactions with polar solvents such as water. Since most of the hydrophobic groups of a protein are
buried in its interior it is important to properly unfold (open up) the protein to produce a good foam. This is
normally done by mechanical action where the protein solution is whipped, thus allowing for more contact
of air to the protein which makes it favorable for the protein to open up and expose its hydrophobic groups
to the air phase. During this process the protein will eventually form a strong cohesive network around air
pockets, with its hydrophobic groups sticking into the air phase and the hydrophilic into the water phase.
Different protein types can differ tremendously in their ability to form strong and stable foams. Also, by
changing the environment around the protein (pH, salt conc., temperature etc) the properties of the
protein can be greatly affected. The following lab will examine some of the variables that are important in
obtaining a stable foam with acceptable overrun (foam volume).
Materials
The instructor will assign to you one of the following protein sources:
1. Casein
2. Whey 1 from bovine milk
3. Soy Protein Isolate
4. Egg Albumin 1
5. Whey Protein Concentrate
6. Egg Albumin from fresh egg
7. Gelatin (Knox powder)
Procedures
Add Xg of protein in 50 mL of water, buffer or salt solution and blend for one minute at the highest speed
of the blender. Pour the foam into a graduated cylinder (l00 mL for whey protein, 250 mL for casein and
soy). The protein samples you will receive all have different protein concentrations. It is important to
compare them all at the same protein concentration. You will be provided with how many g of protein to
add.
Variables
1. Measure the foaming of protein suspensions as described above. Add X g protein to 50 mL of 0.067M
phosphate buffer adjusted to pH 5, 6, 7, 8 and .2M glycine/NaOH adjusted to pH 9.0. Which pH
gave the highest overrun at 0 and 15 minutes?
2. Select the pH that gave the best overrun at 15 minutes and prepare samples with 0, 5, and 10% sugar
added. Which sample gave the greatest overrun at 0 and15 minutes?
3. To the sample in 1 with the greatest overrun at 15 minutes, add 0, 1.5 or 2.0 % NaCl and evaluate.
Which sample gave the highest overrun at 0 and 15 minutes?
4. To the above sample selected as giving the highest overrun at 10 minutes add 0., 0.5 or 1.0% fat and
evaluate. Which sample gave the greatest overrun at 10 minutes?
