Chapter 3 Flashcards
At what temperature can crystals form?
Low temperature
What does polymorphism mean?
Molecule can crystallize in different crystals.
How is it called a network of crystals?
Percolation
What is one assumption that is made in the Hildebrand equation? (Hint: equation for crystallization)
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
Why should we remove part of the freezing line and crystallization line which goes under the eutectic point?
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.
What is a requirement to get crystal nuclei to start?
Supersaturation. This can be achieved by evaporating water.
What is the relation between the melting and glass transition temperature. And what does the nature of the molecule influence?
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.
Is nucleation in pure systems feasible?
Yes but it is very difficult. Example is pure water. Only when concentration increases in localized point can crystallization occur.
What are the three ways of starting a nuclei for crystallization?
- Seeding (planting manually crystals in solution)
- Heterogeneous nucleation (in a glass bottle small irregularities can lead to formation of crystals and from there crystallization occurs)
- Homogeneous nucleation ( Example is pure water. Only when concentration increases in localized point can crystallization occur.)
What is homogeneous and heterogeneous nucleation dependent on?
Supersaturation. Nuclei increase non -linear with increasing supersaturation.
What factors determine the size of crystals?
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.
Are large crystal more stable than small crystals?
Yes, small crystals will try to become large and can do so by disproportionation by Ostwald ripening. This phenomenon is stronger at higher temp.
What is sintering or accretion?
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.
How is sintering facilitated?
High temp, presence of plasticizer, small powder particles as more surface area present so more ‘necks’ can be created.
How can destruction of cell wall be prevented when ice crystals form? (in storing like vegetable product in freezer)
By fast cooling, smaller crystals will be formed.
Looking at planks equation, when L»L^2 and when L«L^2 what does this mean
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.
If the thickness of my product doubles what happens to the time it requires to freeze?
It become four times larger.
How can a more healthy ice cream be prepared? What machine do i need?
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.
What are the consequence of keeping a product above Tg, so still spoonable e.g. ice creame, to the structure?
Collapse of structure can occur through Ostwald ripening or sintering, but also degradation of enzymes or other components due to oxidation or browning.
Which are the solution to cope with having a frozen product which can be still spoonable but not collapse?
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.
What is the Freeze Flow tm?
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.
What other methods apart from cryoprotectant and cryostabilizers can we use?
Active proteins-> adhere at water ice interface and inhibits growth of ice crystals. However, this does not mean that collapsing phenomena cannot occur.
Why do we lower temperature under Tg line in freeze drying?
To get supercooling effect and initiation of crystals.
What is better for instant soups, freeze drying or air drying
Freeze drying
