Electromagnetic radiation

applications in Food Processing

Alistair Grandison

Modules FB2EFP, FBMFP1,

FBMFP2

• Some refs for Modules FB2 EFP and FBM FP1 – EM processing and novel processes

• Food Processing Handbook (2011) J.G.Brennan & A.S.Grandison(ed.), Wiley-VCH (on line

access available)

•

• Ramaswamy, H. and Marcotte, M. (2006) Food Processing : Principles and applications.

Taylor & Francis, London

•

• Barbosa-Canovas, G.V. et al. (1998) Nonthermal preservation of food, Marcel Dekker, New

York.

•

• Fellows, P.J., Food Processing Technology: principles and practice, 3rd Ed., Woodhead

Publishing Ltd., Cambridge, 2009

•

• Brennan J.G., Butters, J.R., Cowell, N.D and Lilly, A.E.V., Food Engineering Operations, 3rd

edition, Elsevier Applied Science, London,1990 – out of print now unfortunately

•

• “Electromagnetic Radiation Properties of Foods and Agricultural Products” N N

MOHSENIN; 1984; Gordon & Breach

What is EM radiation?

Is it a wave form?

OR

Does it consist of particles?

Answer - Well yes and no really to both questions!!

Speed of light (3 x 108 ms-1) c = F λ

(F=frequency, Hz; λ=wavelength, m)

Consider EM radiation to be stream of photons – a photon is a

“quantum” of energy which possesses no resting mass, but

contains energy and momentum.

Energy of photon increases with F:

Energy of photon (J) Ep = hF

(where h=Plank’s constant, 6.63 x 10-34 Js).

Listed below are the approximate wavelength, frequency, and energy limits

of the various regions of the electromagnetic spectrum.

Wavelength (m) Frequency (Hz) Energy (J)

Radio > 1 x 10-1 < 3 x 109 < 2 x 10-24

Microwave 1 x 10-3 - 1 x 10-1 3 x 109 - 3 x 1011 2 x 10-24- 2 x 10-22

Infrared 7 x 10-7 - 1 x 10-3 3 x 1011 - 4 x 1014 2 x 10-22 - 3 x 10-19

Optical 4 x 10-7 - 7 x 10-7 4 x 1014 - 7.5 x 1014 3 x 10-19 - 5 x 10-19

UV 1 x 10-8 - 4 x 10-7 7.5 x 1014 - 3 x 1016 5 x 10-19 - 2 x 10-17

X-ray 1 x 10-11 - 1 x 10-8 3 x 1016 - 3 x 1019 2 x 10-17 - 2 x 10-14

Gamma-ray < 1 x 10-11 > 3 x 1019 > 2 x 10-14

Chemical analysis of foods by EM

• UV – e.g. proteins absorb at 280 nm

• Visible range – many colorimetric assays

• IR – e.g. Dairylab/Lactoscope

• Much research into non-invasive analysis

of foods – both chemical analysis and

texture

Sorting & Grading etc. – usually

based on reflectance

• Ripeness – red/green tomatoes

• Removal of e.g. blackened peas and

beans or blemished fruit and vegetables

• Detection of fruit pips – e.g. cherries

Wavelengths used for processing

• Solar drying

• UV sterilisation – e.g. packaging materials

Wavelengths used for heat

processing

• Infra red

• Dielectric principle • - Microwaves

• - Radiofrequency

Stefan’s law Rate of energy emission from a radiating body:

Q = σ ε A T4

(Js-1

) (m2)(K

4)

σ = Stefan’s constant = 5.7 x 10-8

Js-1

m-2

K-4

ε = Emissivity (1 for black body ; 0 for perfectly reflecting or

transmitting material)

A = surface area; T = Absolute temp.

The net rate of heat transfer between two bodies:

Q = ε σ A (T 4

1 - T 4

2 )

Where T1 (K) is the temperature of the emitter and T2 (K) is the

temperature of the absorber

INFRA RED HEATING

IR energy produced by radiant heaters:

Electrical – Ni/Cr/Fe alloy filaments in metal or ceramic

sheath (500-10000 C surface temp); or W sheathed in quartz (up

to 30000 C)

Gas (up to 9000 C).

Radiant energy converted to heat directly on absorption by

directly increasing molecular motion of molecules (not dielectric

effects).

Depth of penetration D

Approximate depth of penetration of IR

radiation

(λ approx. 1μm)

Material Depth (mm)

Ice 30

Bread 7-12

Dough 4-6

Raw potato 6

Apple 4

Tomato paste 1

To avoid interference with radio and telecommunications only

certain permitted frequencies – major ones are:

Microwave – 2450 and 915 (896 in Europe) MHz

Dielectric – 27.12 MHz

Rate of heating derived from power developed in

the “dielectric” (P0)

P0 = 55.61 x 10-14 E2 . F . εr . tan δ

E = electrical field strength

F = frequency

εr = relative dielectric constant

tan δ = loss tangent

The term εr . tan δ is known as the “loss factor” (referred to as ε″r

in some publications) depends on composition of food, and varies

with temperature and frequency.

E and F are properties of machinery.

e1

21

)tan.(2

r

D = Depth of penetration (distance at which power

falls to

th incident power):

D ≈

e.g. for water at 950C : D=29.5cm at 915 MHz; D=4.8

cm at 2450 MHz).

Characteristics of radiofrequency

• Very fast heating (one tenth conventional)

• Penetrative

• Local overheating minimised

• Working space reduced

• Clean, continuous, automatic

• No surface browning

• Directional

Microwaves

• Rapid, in-depth heating

• Compact, flexible processing lines

• Material heated inside insulating

packaging

• Disadvantage – expensive in terms of

equipment and energy