Summary Food Process
Engineering
1. Food material properties
Food: heterogenous system
- Homogenous parts: phases (separated by clear boundaries)
o Properties are considered uniform
State of phase: dependent on
- Pressure
- Temperature
- Composition
After critical point: No distinguishment between gas and liquid
Gases
Mixture of gases Gases have partial pressures
- Sum of all pressures is overall pressure
Vapours = Gases which tend to condense under conditions close to room temperature and pressure
- Saturated water vapour pressure is dependent on the temperature
o Air that is at equilibrium with water (saturated air)
Boiling point elevation: Product becomes more concentrated because of evaporation, results in a
higher boiling point
Liquids
Liquids have a viscosity
- Newtonian = Viscosity is constant
- At a certain concentration, dissolved components start to interact with each other
o Yield stress will form, liquid seems a solid (but can be poured)
- No long-range ordering of molecules: amorphous
Gels and rubbers: Much stronger solid behaviour, dominate above liquid properties
- Cannot be deformed without limit like liquids (breakage)
Glasses: viscosity is so high that for all practical purposes the material becomes solid
- Flows immeasurably slowly
- All fluids and solutions can become glassy if cooled down quick enough
Solids
Crystalline state
- Very ordered
1
,Freezing point depression: More concentrated solution Lower freezing point
State diagram = Modified phase diagram that also shows kinetic
limitations (glass transition)
Phase diagram >
< State diagram
Lever rule
Chemical potential μ [J/mol]J/mol]]
- Depends on pressure, volume, temperature and composition
o Standard chemical potential is reference state
- Equilibrium, when chemical potentials are equal
- Mixture of gases: xi is partial pressure divided by total pressure p
RH = pw / pw.sat
- Aw is eual to RH of vapour that is at equilibrium with the solution
Ideal solutions = Activity is equal to the molar fraction
- γ activity coefficient is 1 for ideal solutions
o γ < 1: strong interaction, good solubility γ > 1: poor interaction
- Low concentration of component γ is constant Henry’s law applies
o Kh is Henry’s coefficient
Volume fraction of water φw
Parameter that expresses the interaction between water and dissolved components χ
- χ = 0, Interaction between matrix and water is the same
2
, - χ<0 Solutes have great affinity for water χ>0 Solutes do not like moisture
Sorption isotherm = Relation between RH and amount of moisture in solid
- Moisture content is not the same as the affinity
Affinity: activity, depends on:
- Temperature
- Composition
Glasses -> Not in equilibrium with its surroundings
Water vapour sorption isotherm = Relation between the amount of moisture in the product and its
water activity (S-shaped relation)
Δhem should be in J/mol, when put in equations
2. Mass transfer
Glassy state: crispy behaviour
Hysteresis = Difference in adsorption and desorption
- Caused by the different layers in a food product (in mesostructure)
Requirements mass transfer
- Driving force
o Difference in concentration, T, p
- Molecules should be mobile: Brownian motion
Diffusion = Motion of individual molecules through a stagnant layer of other molecules
- Because of gradient in concentration
- Motion is caused by thermal motion of every molecule
o The higher the temperature, the larger the motion
o Result of random motion of the diffusing component
- Rate of diffusion in gas is 3 orders of magnitude smaller than in liquid (less collisions)
o In solid: only little vibration
- Steady state: Driving force of diffusion is equal to friction
Friction
- Dependent on difference in velocity between diffusing molecule u1 and surroundings u2
o u2 is often 0 , because in most systems the environment does not move
- x2 is mole fraction of the surroundings
- ζ12 friction coefficient
o The larger the coefficient, the larger the friction, the more difficult to move through
the matrix
D12 = RT / ζ12 = Maxwell-Stefan diffusion coefficient [m2/s]
Ṅ1 Molar flux [mol/m2s]
V̇1 Volume flux [m3/m2s] = Same equation as molar flux but without C1
3
Engineering
1. Food material properties
Food: heterogenous system
- Homogenous parts: phases (separated by clear boundaries)
o Properties are considered uniform
State of phase: dependent on
- Pressure
- Temperature
- Composition
After critical point: No distinguishment between gas and liquid
Gases
Mixture of gases Gases have partial pressures
- Sum of all pressures is overall pressure
Vapours = Gases which tend to condense under conditions close to room temperature and pressure
- Saturated water vapour pressure is dependent on the temperature
o Air that is at equilibrium with water (saturated air)
Boiling point elevation: Product becomes more concentrated because of evaporation, results in a
higher boiling point
Liquids
Liquids have a viscosity
- Newtonian = Viscosity is constant
- At a certain concentration, dissolved components start to interact with each other
o Yield stress will form, liquid seems a solid (but can be poured)
- No long-range ordering of molecules: amorphous
Gels and rubbers: Much stronger solid behaviour, dominate above liquid properties
- Cannot be deformed without limit like liquids (breakage)
Glasses: viscosity is so high that for all practical purposes the material becomes solid
- Flows immeasurably slowly
- All fluids and solutions can become glassy if cooled down quick enough
Solids
Crystalline state
- Very ordered
1
,Freezing point depression: More concentrated solution Lower freezing point
State diagram = Modified phase diagram that also shows kinetic
limitations (glass transition)
Phase diagram >
< State diagram
Lever rule
Chemical potential μ [J/mol]J/mol]]
- Depends on pressure, volume, temperature and composition
o Standard chemical potential is reference state
- Equilibrium, when chemical potentials are equal
- Mixture of gases: xi is partial pressure divided by total pressure p
RH = pw / pw.sat
- Aw is eual to RH of vapour that is at equilibrium with the solution
Ideal solutions = Activity is equal to the molar fraction
- γ activity coefficient is 1 for ideal solutions
o γ < 1: strong interaction, good solubility γ > 1: poor interaction
- Low concentration of component γ is constant Henry’s law applies
o Kh is Henry’s coefficient
Volume fraction of water φw
Parameter that expresses the interaction between water and dissolved components χ
- χ = 0, Interaction between matrix and water is the same
2
, - χ<0 Solutes have great affinity for water χ>0 Solutes do not like moisture
Sorption isotherm = Relation between RH and amount of moisture in solid
- Moisture content is not the same as the affinity
Affinity: activity, depends on:
- Temperature
- Composition
Glasses -> Not in equilibrium with its surroundings
Water vapour sorption isotherm = Relation between the amount of moisture in the product and its
water activity (S-shaped relation)
Δhem should be in J/mol, when put in equations
2. Mass transfer
Glassy state: crispy behaviour
Hysteresis = Difference in adsorption and desorption
- Caused by the different layers in a food product (in mesostructure)
Requirements mass transfer
- Driving force
o Difference in concentration, T, p
- Molecules should be mobile: Brownian motion
Diffusion = Motion of individual molecules through a stagnant layer of other molecules
- Because of gradient in concentration
- Motion is caused by thermal motion of every molecule
o The higher the temperature, the larger the motion
o Result of random motion of the diffusing component
- Rate of diffusion in gas is 3 orders of magnitude smaller than in liquid (less collisions)
o In solid: only little vibration
- Steady state: Driving force of diffusion is equal to friction
Friction
- Dependent on difference in velocity between diffusing molecule u1 and surroundings u2
o u2 is often 0 , because in most systems the environment does not move
- x2 is mole fraction of the surroundings
- ζ12 friction coefficient
o The larger the coefficient, the larger the friction, the more difficult to move through
the matrix
D12 = RT / ζ12 = Maxwell-Stefan diffusion coefficient [m2/s]
Ṅ1 Molar flux [mol/m2s]
V̇1 Volume flux [m3/m2s] = Same equation as molar flux but without C1
3
