Electrical properties

Question 1

applications

Answer 1

Food Quality: post-harvest / microbial growth /colour

Question 2

Electrical properties of food

Answer 2

- 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

Question 3

Potential difference

Answer 3

electric potential energy per unit charge
the driving force that makes electrons flow

Volts (V)

Question 4

Electric current

Answer 4

Ampere (A)

1 A = flow of electrons of 6.26 x 1018 s-1

Question 5

quantity of electricity or charge Q

Answer 5

coulombs (C)

Charge Q (C)= current I (A) x time t (s)

Question 6

Electrical Units

Answer 6

• Direct current

- Alternating current

Question 7

Electrical Resistance

Answer 7

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

Question 8

Ohm’s law

Answer 8

Ratio of voltage to current = constant

Voltage V / current I = resistance R

Question 9

Resistance R & unit

Answer 9

(Ω)

Conductors, insulators, semiconductors

Question 10

Resistivity ρ

Answer 10

Resistivity values are used to compare the conducting properties of different materials. (Ω m) [units]

R = (ρ x Length) / Area

Question 11

Inverse of resistance = conduction

Answer 11

V/I = R
I = V x 1/R
I = V x G

Question 12

Electrical conduction G

Answer 12

G = I / V = Ω-1

siemens (S)

Question 13

Electrical conductivity σ k

Answer 13

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]

Question 14

Conductivity

Answer 14

-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

Question 15

Factors influencing electrical conductivity of food

Answer 15

• 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

Question 16

Conductivity liquid foods

Answer 16

Temperature dependence
- Viscosity

Not all liquid foods follow a linear function

Question 17

Bovine milk

Answer 17

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

Question 18

Conductivity solid foods (Plant origin)

Answer 18

• 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

Question 19

Expect electrical conductivity to increase in fruit and vegetable products during

Answer 19

processing operations as milling squeezing, crushing and mashing

Question 20

how to increase conductivity solid foods (Plant origin)

Answer 20

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

Question 21

Conductivity solid foods (Animal origin)

Answer 21

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

Question 22

Anisotropic electric conductivity

Answer 22

• laminar structure
• layers going in one direction
• So conductive properties depend upon direction relative to tissue layers

Question 23

Electrical Conductivity Measurement

Answer 23

Temperature Compensations

- automatic, linear temperature compensation method

Question 24

ohmic heating

Answer 24

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