NOVEL THERMAL TECHNOLOGIES IN FOOD PROCESSING

KAUTKAR S.S.Ph. D. (Process & Food Engg.)

G.B.P.U.A.&T. Pantnagar

Email: sskautkar15@gmail.com

Objectives of thermal processing

ⱴ Ensure safety (kill microorganisms)

ⱴ Increase digestibility

ⱴ Increase shelf life (destruction of enzymes, toxins)

ⱴ Add value (texture, flavour, colour)

ⱴ Make varieties of new products

ⱴ Meet the needs of specific section of population

Preservation Processes

ThermalNon thermal

Drying

Frying

Freezing

Radio frequency heating

Infrared heating

Microwave heating

Ohmic heating

Chilling

Blanching

ExtrusionOscillating

Magnetic Field

Electron Beam

Cold Plasma

Ozone

Ultrasound

Irradiation

Pulsed Light

Pulsed Electric Field

High Pressure

MICROWAVE HEATING

The father of microwave: Percy L. Spencer (1946).

Microwaves are electromagnetic waves in frequencies between 300 MHz and 300 GHz

It consists of both electric and magnetic fields

It is part of electromagnetic spectrum with wavelengths from 1 mm to 10 cm

Mechanism：the principle of frictional heat production using microwaves.

Microwaves are also used in telecommunications, e.g., radars, wireless computer networks and mobile phones.

Working principle of microwaves

Food containing water is a good absorber of microwave energy.

Water is a dipolar in nature contains positive and negative charge

Microwaves excites and polarized the water molecules in food

water molecules get pulled back and forth at the rate of about 2.5 billiontimes per second by the electric fields.

This rapid back-and-forth motion between the water molecules createsfriction, and hence heat.

Microwaves components

How microwave can heat the product?

1. Magnetron :

The microwave generator

2. Waveguide :The

path of microwave

3. Cavity: Where

the food is heated

Microwave properties of food

I. Dielectric loss factor: Amount of heat absorbed by food.

II. Penetration depth: Distance at which heat generation drops to 37%.

III. Dielectric constant: Ability of microwave to store energy.

Applications of MW heating

• Microwave Drying

• Thawing

• Pasteurization and sterilization

• Baking and cooking

• Blanching

• Waste treatment under microwave irradiation

• Puffing

Rapid heating

Reduced loss of nutrients than conventional heating

No contamination of foods by products of combustion

Equipment is small, compact, clean in operation

Surface of the food does not overheat

Automatic process control

Low penetration depth

Product may hazardous to health

High initial cost

Non uniform heating

Less energy efficient than ohmic heating

AdvantagesLimitations

Hazardous of MW heating

• Hidden hazardous of MW cooking (Dr. Joseph Mercola , 2005)

I. Cancer causing defects

II. Nutritive destruction of foods

III. Biological defects of exposure

• May cause stomach and intestinal cancerous growth

• Degradation cellular tissue

• Disorder of digestive system

INFRARED HEATING

• Discovered by William Herschel

• Infrared: below red (infra: below)

• Red is the color of the longest wavelengths of visible light

• That is wavelength between 0.7 and 1000 µm.

• “Wavelength longer than visible light but shorter than those of radio waves are the infrared

waves.”

Types of Infrared waves

1. Short waves: 0.76-2 µm

(Near IR waves)

2. Medium waves: 2-4 µm

(Medium IR waves)

3. Long waves: 4-1000 µm

(Far IR waves)

Working principle of Infrared waves

IR energy is electromagnetic radiation emitted by hot objects

When it is absorbed, the radiation gives up its energy to heat materials.

Rate of heat transfer depends:

1

• Surface temperatures of heating & receiving materials

2• Surface properties of the two materials

3

• Shapes of the emitting & receiving bodies. Principle of heating

Types of IR equipment (Radiators)

1 . Gas heated radiators (efficiency 30-40%)

-uses natural gas or propane to heat the radiators

2 . Electrically heated radiators (efficiency 40-70%)

– tubular/flat metallic heaters (long waves)

– ceramic heaters (long waves)

– quartz tube heaters (medium, short waves)

– halogen tube heaters (ultra short waves).

Short wavelength, medium wavelength and long wavelength

Applications of Infrared heating

• Drying and dehydration

• Enzyme inactivation

• Pathogen inactivation

• Baking and roasting

• Pasteurization and sterilization

• Blanching

Alternate source of energy

Fast heating rate

Shorter response time

Uniform drying temperature

High degree of process control

Cleaner working environment

High thermal efficiency

Low penetration power

Prolonged exposure may cause fracturing

Not sensitive to reflective properties of coatings

AdvantagesLimitations

OHMIC HEATING

• Advanced thermal processing method

• Developed by United Kingdom Electricity Research and Development Center

• Licensed to APV Baker Ltd for commercial exploitation

• Also called electrical resistance heating, Joule heating, or electro-heating

• Food material is heated by passing electric current through it

• Electrical resistance of the food causes the power to be translated directly into heat

• Electrical energy is dissipated into heat, which results in rapid and uniform heating

Working principle of ohmic heating

Food material containing liquid and electrolyte is placed directly between two electrodes

AC is passed to the food

EC helps in passing electric current through the product

Heat is rapidly generated directly within the product

Ohmic heater design

A. Static system : number of electrode inserted within container

B. Continuous flow throw system:

i. Parallel plates: For low conductivity fluids (<5S/m)

ii. Parallel rods: Used where cost considerations are paramount

iii. Collinear system: For high conductivity fluids (wider electrode spacing)

iv. Staggered rod: low cost option

Characteristics of electrodes

i. High electrical conductivity

ii. High resistance to corrosion

iii. High resistance to acid foods

iv. High mechanical strength

v. Easy for fabrication and repair

vi. Low cost, easily available

For high product quality: Stainless steel is preferred

Where product quality is not essential: Low cost carbon electrode

Types: Carbon, Graphite, Aluminum, Stainless steel, Platinum, Titanium

Factors affecting ohmic heating

Electrical Conductivity

4

4.5

5

5.5

6

6.5

7

7.5

8

3 5 7 9 11 13 15

Poin

t Ele

ctr

ical C

onducti

vit

y,

mS/c

m

Voltage Gradient, v/cm

0% Salt Level

1% Salt Level

2% Salt Level

4

4.5

5

5.5

6

6.5

7

7.5

8

3 5 7 9 11 13 15

Bulk

Ele

ctr

ical

Conducti

vit

y, m

S/c

m

Voltage Gradient, v/cm

0% Salt Level

1% Salt Level

2% Salt Level

Point Electrical Conductivity Bulk Electrical Conductivity

Voltage Gradient

0

0.1

0.2

0.3

4 5 6 7 8 9 10 11 12 13 14

Heati

ng R

ate

, ˚C

/s

Voltage Gradient, V/cm

Heating rate

Applications of ohmic heating

PASTURIZATION

CONTINEOUS HTST PROCESSING

Various Parts

1. Launching tank

2. Pump

3. Ohmic chamber

4. Holding tubes

5. Cooling tubes

6. Aseptic tank

Working of continuous HTST process

Advantages & Limitations

Suitable for continuous processing

Temperature required for HTST processes can be

achieved very quickly.

Uniform heating (product does not experience large

temperature gradient)

Reduced problems of surface fouling or over heating

No residual heat transfer after the current is shut off

high energy conversion efficiencies (95% of the

electrical energy is converted into heat)

Environmentally-friendly system

Ease of process control with instant switch-on and

shut-down.

Reducing maintenance cost (no moving parts).

lower capital cost than microwave heating

Require liquid or liquid with

particulates food

Require great attention

Sometime dangerous

Microwaves

forms of electromagnetic

energy

direct methods

Heat is generated by

molecular friction in water

molecules

Depends on moisture content

of the food

Mainly used to preserve foods

Limited penetration depth

thermal conductivity is not a

limiting factor

Foods having moisture can

support efficiently

Infrared waves

forms of electromagnetic

energy

indirect method

infrared energy is simply

absorbed and converted to heat

Depends on surface

characteristics and colour of

the food

mostly used to alter the eating

qualities

Limited penetration depth

thermal conductivity is a

limiting factor

May be used for liquid or solid

foods

Ohmic heating

uses the electrical resistance of

foods

direct methods

Heat is generated due to

electrical current

Depends on depends on the

electrical resistance of the food

Mainly used to preserve foods

penetrates throughout the food

thermal conductivity is not a

limiting factor

Require liquid or particulates

food

Ohmic heating Vs Pulsed electric field