Electrical properties Flashcards
applications
Food Quality: post-harvest / microbial growth /colour
Electrical properties of food
- electrical conductivity
- Impedance - the measure of the opposition that a circuit presents to a current when a voltage is applied
- capacitance -the ability of a body to store an electrical charge
Potential difference
electric potential energy per unit charge
the driving force that makes electrons flow
Volts (V)
Electric current
Ampere (A)
1 A = flow of electrons of 6.26 x 1018 s-1
quantity of electricity or charge Q
coulombs (C)
Charge Q (C)= current I (A) x time t (s)
Electrical Units
- Direct current
- Alternating current
Electrical Resistance
When a current flows through a conductor there is a direct relationship between the potential difference across the conductor and the size of the current
Ohm’s law
Ratio of voltage to current = constant
Voltage V / current I = resistance R
Resistance R & unit
(Ω)
Conductors, insulators, semiconductors
Resistivity ρ
Resistivity values are used to compare the conducting properties of different materials. (Ω m) [units]
R = (ρ x Length) / Area
Inverse of resistance = conduction
V/I = R I = V x 1/R I = V x G
Electrical conduction G
G = I / V = Ω-1
siemens (S)
Electrical conductivity σ k
the amount of electric charge transferred per unit time across unit area under the action of a unit potential gradient
σ = 1 /ρ = Ω^-1 m^-1 = [S m^-1]
Conductivity
-How easily an electric current transmitted through material
- capable of transmitting an electric current
= positively or negatively charged electrolytes
= charged molecules or maromolecules
- transmit electric current
= carriers for the charged ions
= these carriers are mobile
Factors influencing electrical conductivity of food
- Concentration of charge carriers = salinity, formulation
- Charge & number of charge carriers = single chared or twice charged ions
- mobility of charged carriers = aggregate state, molar mass, type of bonding
Conductivity liquid foods
Temperature dependence
- Viscosity
Not all liquid foods follow a linear function
Bovine milk
0.4 to 0.55 S m^-1
- potassium and chloride ions are major contributors
- Increasing total solids concentration?
= Increase to 0.78 Sm-1 at 28% total solids
= extremely complicated salt balance between the colloidal and soluble phases
- fat tends to decrease the specific conductance
10^-14 S m^-1
Conductivity solid foods (Plant origin)
- Most fresh fruits and veg high in water content
- Normally very poor electrical conductivity
- Water held immobile within cells and intercellular spaces within the tissue structure of the plant materials
Expect electrical conductivity to increase in fruit and vegetable products during
processing operations as milling squeezing, crushing and mashing
how to increase conductivity solid foods (Plant origin)
Thermal heating, mechanical crushing enzymatic activity
> Breaks down cell structure
Water containing charged ions free to flow about
- mobile carrier for charged ions
- electrical conductivity can increase significantly
Conductivity solid foods (Animal origin)
Same situation as in fresh fruit and veg - High water content but much is immobile
Cellular structure broken down
> water containing charged ions free to flow
> mobile carrier for charged ions
> electrical conductivity significantly increase
Cellular structure breakdown in nature.
Electrical conductivity temperature dependence
- linear range of 20-60°C
Anisotropic electric conductivity
Anisotropic electric conductivity
- laminar structure
- layers going in one direction
- So conductive properties depend upon direction relative to tissue layers
Electrical Conductivity Measurement
Temperature Compensations
- automatic, linear temperature compensation method
ohmic heating
alternating electrical current passes through a food sample, resulting in internal energy generation in the food. This produces an inside- out heating pattern.
In food, excited cells vibrate, causing friction and energy dissipation in the form of heat
