chapter 9

Question 1

when is a foam considered to be stable

Answer 1

when the size of its air bubbles does not change

Question 2

explain disproportionation

Answer 2

higher pressure in small bubbles –> higher soluability in the air continious phase –> dissolves –> migrates to bigger bubbles –> smaller bubbles disappear

Question 3

what are casein micelles and name some properties

Answer 3

they are aggregates of individual proteins
- smaller than fat
- stable aggregates
- hydrophobic
- coated with k casein

Question 4

Name two essential colloidal structures in milk and their typical dimensions

Answer 4

fat globules (10 micron) and casein micelles, 0.1 micron

Question 5

Which type of milk shows more creaming, homogenised or natural milk, and
why?

Answer 5

Natural, since the fat globules are much larger while homogenised milk has
many of the globules being much smaller by the homogenisation, and therefore are
exhibiting Brownian motion and therefore do not show creaming at all

Question 6

Explain why milk is white, in terms of what happens to light as it travels through
milk.

Answer 6

There is multiple scattering of light within the sample, and that light stems from
being reflected by the large fat globules and the casein particles (both large
relative to light wavelength).

Question 7

Which two colloidal structures are mainly responsible for the white appearance of
milk?

Answer 7

fat globules and the casein particles

Question 8

What occurs with stability of casein micelles if the pH becomes lower than the natural pH of 6.6? Does it become more or less stable?

Answer 8

stability decreases –> the casein will start to interact with each other leading to aggregates

Question 9

Explain skim formation on top of milk when heated in a pan. Suggest two ways to counteract this skim formation when milk is being heated.

Answer 9

Water evaporates, thereby increasing the concentration of caseins and Calcium,
thereby inducing aggregations and thus skin formation. One way out would be
to put a lid on the pan in order to prevent water evaporation. Another way
would be to foam the milk so that water evaporation is retarded.

Question 10

What happens in milk when it is heated and at the same time acidified? Choose one colloidal structure that you want to consider in your reasoning.

Answer 10

The casein micelles coagulate

Question 11

Do fat particles in homogenised milk have the tendency to aggregate more or less
than fat particles in natural milk, and why?

Answer 11

More than in natural milk, since the membranes separating the globules are not all in tact any more due to the homogenisation process