Physical and Chemical properties 2 Flashcards
what can freezing curves be used for in regard to ice cream
hardness at a given temperature
what is hardness of ice at a given temperature determined by
amount of water/ % of solutes
Describe the region of A-S on a freezing curve (refer to slides if needed)
cooling of liquid water
removal of sensible heat
nucleation (initiation of phase change)
What happens at point S
super cooling
-Cooling below the initial freezing point without ice formation
Describe the region of S-B on a freezing curve (refer to slides if needed)
Ice crystals form at nucleation points
-latent heat of crystallisation is released increasing temperature.
Describe the region of B-C on a freezing curve (refer to slides if needed)
-Ice crystals grow
-removal of latent heat, temp remains relatively constant (FP plateau)
-FP depression as solute become concentrated
Describe the region of C-D on a freezing curve (refer to slides if needed)
-Supersaturated solutes may precipitate (crystalise) releasing heat of fusion
Describe the region of D-E on a freezing curve (refer to slides if needed)
-crystlisation of ice and solutes continues
-a portion remains unfrozen water
Describe the region of E-F on a freezing curve (refer to slides if needed)
-ice crystal form until theres no more water (E)
-temp falls to that of the freezer through removal of sensible heat
-glass transition can be reached
at what stage of the feezing process do commercial freezer stop at
D-E
draw a temp vs heat removal graph of pure water vs food system
look at notes
-water: linear decline (senisble heat removal)-> super cooling-> slight increase-> plateu (latent heat removal)-> linear decline (removal of sensible heat)
-Food: linear decline-> slight increase-> curved decline
when does glass transition occur
when critial temp has been reached
what is the glassy state?
when a viscoelastic liquid changes to a brittle amorphous solid
describe the viscosity during the glass phase
viscosity is so high it resists shear stress (for long time)
what is the critical temperature called ?
glass transition (Tg)
