Summary Food Engineering (FPE-20306). The summary is quite extensive but contains all information needed to successfully pass the exam. The summary contains figures and (parts of) the lecture slides for better understanding of certain topics and processes.
Food products are heterogeneous systems, consisting of
homogeneous parts (phases) with uniform chemical and physical
properties. These phases are separated by distinguishable boundaries.
One phase can have more than 1 components.
State of matter – form in which different phases can exist, dependent
on T, p and composition. Equilibrium lines in the state diagram of pure
substances show the phase transitions due to a change in T or p:
Dispersion – mixture of phases
Liquids have a wide range of viscosity:
! By slow cooling or drying of a concentrated solution, a highly ordered, suspension of sugar crystals is formed.
By fast cooling or drying of a concentrated solution, a disordered, glass is formed, because there is no time to
form crystalline structures.
For most substances, the S/L equilibrium curve has a positive slope. At the equilibrium S/L, an increase in
pressure will result in solidification, because it takes less space, and an increase of the melting T. For water,
the S/L equilibrium line has a negative slope, this is because the density of solid water is lower than the density
of liquid water. An increase in pressure will result in a decrease of the melting T, but an increase of the boiling
T. At boiling, the saturated vapour pressure is equal to the external pressure.
Critical point – at the CP, the densities for L and G are similar. The distinction between gas
and liquid disappears and there is no more phase separation. The CP depends on the
strength of intermolecular interactions.
Sublimation of CO2 only occurs at a very low p, below the triple point. The triple point is
above atmospheric pressure. Under atmospheric conditions (1 atm), CO2 is present as
solid or gas.
,State diagrams of mixtures are based on temperature and the amount of solids dissolved in the liquid. The
pressure is assumed constant. The diagram of a salt or sugar solution show regions with either 1 phase or 2
phases. Equilibria between phases:
- Boiling line: dissolved components elevate the boiling point
- Liquidus line: dissolved components suppress the freezing point
- Solubility line: gives the saturation concentration in relation to temperature
Eutectic composition – the composition of substances in which the mixture has a lower melting point than any
of the constituents
Lever rule – a rule used to determine the mole fraction or mass fraction of each phase of a 2 phase system.
! In the equilibrium situations of a system, the chemical potentials of two adjacent phases are equal.
The chemical potential of a component is dependent on pressure, temperature and composition. The
chemical driving force makes sure movement of molecules happens from a high to a low chemical potential.
Chemical potential in a gas or gas mixture: Chemical potential for ice:
L/G equilibrium for water (aw in solution = RH vapour = 1)
L/G equilibrium for water in mixtures (aw in solution = RH vapour = < 1)
When there is no equilibrium, first there is a transfer of components from high to low chemical potential
, Activity of a solute in a solvent is related to, but different from concentration:
! Mass fraction <-> molar fraction <-> activity
The activity coefficient y is a measure for the interaction between solute and solvent, the ‘escaping tendency’
of the solute from the solvent. When y < 1, there is a strong interaction between the solute and solvent.
Attractive forces dominate and the solute remains in the solvent. The Gibbs energy of mixing is negative,
which means the mixing is spontaneous. When y = 1, there is no specific interaction between the solute and
the solvent and the solution is an ‘ideal solution’. When y > 1, there is a poor interaction between the solute
and solvent. Repulsive forces dominate and the solvent destabilises the solute. Energy is required to achieve
mixing.
Henry’s law – a gas law that states that at a constant T, the amount (xi) of dissolved gas in a given type and
volume of liquid is proportional to the partial pressure of that gas (pi) in equilibrium with that liquid. The
solubility of a gas in a liquid depends on T and pi.
(kH is the proportionality constant and dependent on temperature and on
mass fraction or concentration, leading to the respective units bar or bar*L/mol)
Flory-huggins solution – mixture of water with biopolymers. The water activity depends on the volume
fraction of water and the affinity of solutes for moisture X. X < 0 for strong affinity and X > 0 for poor affinity:
Crystallization of water in polymer solution – equilibrium between water and ice crystal (µwater = µice):
Reaction rate changes in food in relation to water activity:
Moisture diffusion happens from high to low aw. Moisture
diffusion inhibition:
- Adapt formulation (match initial aw’s or add humectants,
which lower the aw)
- Use a moisture impermeable barrier layer, such as
chocolate
Sorption isotherm – the relation between aw and moisture%.
The sorption isotherm is important for product stability, drying/dehydration processes and designing
composed products. The isotherm can be described with a mathematical equation. The sorption isotherm is
temperature dependent. There is no fixed relation between moisture % and the state of a phase, but a low aw
corresponds to crispy (glassy), a medium aw to rubbery and a high aw to liquid.
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