Chapter 3

Question 1

At what temperature can crystals form?

Answer 1

Low temperature

Question 2

What does polymorphism mean?

Answer 2

Molecule can crystallize in different crystals.

Question 3

How is it called a network of crystals?

Answer 3

Percolation

Question 4

What is one assumption that is made in the Hildebrand equation? (Hint: equation for crystallization)

Answer 4

That enthalpy of fusion is constant which is not as it is temperature dependent. Another assumption is that the solution is ideal so that aw=xw

Question 5

Why should we remove part of the freezing line and crystallization line which goes under the eutectic point?

Answer 5

Because under the eutectic point there is equilibrium between ice and e.g sucrose crystals. Therefore a horizontal dashed line should be drawn which passes through the eutectic point.

Question 6

What is a requirement to get crystal nuclei to start?

Answer 6

Supersaturation. This can be achieved by evaporating water.

Question 7

What is the relation between the melting and glass transition temperature. And what does the nature of the molecule influence?

Answer 7

If a molecule is very mobile and can rotate easily then it cannot get trapped in a network and form a glass. As a consequence Tg is low. However, the more rigid a molecule is the higher the Tg. Therefore, after cooling down under the melting temperature, molecules will start to organize. If this molecules are very rigid then they will not have enough time to form crystals as they already enter the glassy state.

Question 8

Is nucleation in pure systems feasible?

Answer 8

Yes but it is very difficult. Example is pure water. Only when concentration increases in localized point can crystallization occur.

Question 9

What are the three ways of starting a nuclei for crystallization?

Answer 9

1. Seeding (planting manually crystals in solution)
2. Heterogeneous nucleation (in a glass bottle small irregularities can lead to formation of crystals and from there crystallization occurs)
3. Homogeneous nucleation ( Example is pure water. Only when concentration increases in localized point can crystallization occur.)

Question 10

What is homogeneous and heterogeneous nucleation dependent on?

Answer 10

Supersaturation. Nuclei increase non -linear with increasing supersaturation.

Question 11

What factors determine the size of crystals?

Answer 11

The rate of crystal growth is dependent on diffusion rate. DIffusion rate decrease at lower temperatures and higher viscosities meaning the crystal growth has a max. The more nuclei, the smaller crystals, the less nuclei the larger crystals.

Question 12

Are large crystal more stable than small crystals?

Answer 12

Yes, small crystals will try to become large and can do so by disproportionation by Ostwald ripening. This phenomenon is stronger at higher temp.

Question 13

What is sintering or accretion?

Answer 13

It is the phenomena were crystals start touching each other forming a neck where migration can occur. This will result in a solid like structure.

Question 14

How is sintering facilitated?

Answer 14

High temp, presence of plasticizer, small powder particles as more surface area present so more ‘necks’ can be created.

Question 15

How can destruction of cell wall be prevented when ice crystals form? (in storing like vegetable product in freezer)

Answer 15

By fast cooling, smaller crystals will be formed.

Question 16

Looking at planks equation, when L»L^2 and when L«L^2 what does this mean

Answer 16

If L»L^2 then the heat transfer from the air towards the product is limiting. While if L«L^2 then the heat conduction in the product is the limiting factor.

Question 17

If the thickness of my product doubles what happens to the time it requires to freeze?

Answer 17

It become four times larger.

Question 18

How can a more healthy ice cream be prepared? What machine do i need?

Answer 18

Using a single screw extruder allows to break large crystal into small. In addition also air bubbles are kept small which give overall a fine structure which is creamy. Consequently less fat has to be added.

Question 19

What are the consequence of keeping a product above Tg, so still spoonable e.g. ice creame, to the structure?

Answer 19

Collapse of structure can occur through Ostwald ripening or sintering, but also degradation of enzymes or other components due to oxidation or browning.

Question 20

Which are the solution to cope with having a frozen product which can be still spoonable but not collapse?

Answer 20

Using cryoprotection -> low molecular components are added, e.g. sugar, to penetrate matrix and remove water. This will make the main component sugar and not water and so crystal cannot form. However not good for nutritional health.
Using cryostabilization -> using large molecular weight components, e.g starch, to increase Tg and avoid formation of crystals.

Question 21

What is the Freeze Flow tm?

Answer 21

Is a cryoprotectant and uses fructose syrup which Tg -Tm is little meaning that crystallization cannot occur and allows product to be spoonable from freezer.

Question 21

What other methods apart from cryoprotectant and cryostabilizers can we use?

Answer 21

Active proteins-> adhere at water ice interface and inhibits growth of ice crystals. However, this does not mean that collapsing phenomena cannot occur.

Question 21

Why do we lower temperature under Tg line in freeze drying?

Answer 21

To get supercooling effect and initiation of crystals.

Question 21

What is better for instant soups, freeze drying or air drying

Answer 21

Freeze drying

Question 21

How can you create fibrillar products?

Answer 21

Application of freeze allignment which consist of 2 plates pressing and causing vertical crystal formation.

Question 22

What is tempering?

Answer 22

Tempering is the formation of specific crystals in fats. Different shapes can be made which all have different melting points. Therefore, by keeping solution at certain temperature one can decide which crystals to form.

Question 23

What is a spherulite?

Answer 23

A spherulite is many needle shaped crystals which have formed due to a defect during crystallization of fat.

Question 24

To avoid blooming of fat in chocolate what measures can you undertake?

Answer 24

1. Reduce storage temperaute
2. Use high melting fat (V and VI)