6.2 Thermal Processing (Sterilization, Microwave Heating, Food Irradiation)

It is a thermal process designed to kill all microorganisms in food.

Sterilization

Why is absolute sterility impossible to achieve in food processing?

Because microbial death follows a logarithmic (inverse exponential) trend, meaning microbes are reduced but never fully reach zero.

The destruction of all viable microorganisms of public health significance and those capable of growing under normal, non-refrigerated conditions of storage and distribution.

commercial sterility

Can dormant spores remain in commercially sterile foods?

Yes, dormant spores may remain but cannot grow or reproduce under normal conditions.

Because complete microbial elimination is impossible; instead, survival is expressed as a __.

probability of spoilage

What is the industry standard probability of spoilage for sterilized canned foods?

1 in 1 billion cans (N = 1 × 10⁻⁹ surviving cells/spores per can)

What is the assumed initial spore load per can when calculating probability of spoilage?

10³ spores per can

Who conducted the original study that led to the development of the 12D process?

Esty and Meyer (1922), using 60 billion spores of Clostridium botulinum (types A and B)

Who corrected the TDT value for heating lag, and what was the corrected value?

Townsend et al. (1938) corrected it to 2.45 minutes (Fo = 2.45 min)

What is the industry standard minimum botulinum cook?

3 minutes (F250 = 3 min)

What is the D-value (D250) for Clostridium botulinum according to Schmidt (1964)?

0.20 mins

What does the term “12D process” mean?

It represents a 12 log cycle (10¹²-fold) reduction of C. botulinum spores.

How is the 12D reduction calculated?

2.45 minutes ÷ 0.20 minutes = 12 → reduces 60 billion spores by 10¹² times.

What theoretical survival probability does the 12D process ensure?

One spore surviving in 10¹² cans.

How may the processing level vary depending on the product type?

dog food requires a 20D process

What does “D-value” stand for in thermal processing?

Decimal Reduction Time

The time (in minutes) required at a specific temperature to reduce a microbial population by 90% (or 1 log cycle)—that is, to kill 90% of the organisms or spores present.

D-value

What does a higher D-value mean? What does a lower D-value mean?

The organism is more heat-resistant; The organism is more easily destroyed by heat.

What is the main microorganism of concern in thermal processing?

Clostridium botulinum

the most potent natural toxin, lethal when ingested

botulinum toxin (BoTox)

What microorganisms are used as a laboratory surrogate for C. botulinum?

• Clostridium sporogenes (P.A. 3679)

• Bacillus stearothermophilus

Clostridium sporogenes is also known as __.

PA 3679

A putrefactive anaerobe with D₂₅₀°F = 1.5 minutes.

Clostridium sporogenes (P.A. 3679)

What type of microorganism is C. sporogenes and what is its D-value?

A putrefactive anaerobe with D₂₅₀°F = 1.5 minutes

What is the D-value of Bacillus stearothermophilus?

D₂₅₀°F = 2.5 minutes

general procedure for canning

1. raw mats

2. preparation

3. filling into cans

4. exhausting/sealing

5. heat processing

6. cooling

7. storage

What is the ideal headspace during filling in canning, and why is it important?

~1/8 inch — allows food expansion during heating and creates a partial vacuum upon cooling

This is done before sealing to remove trapped gases to prevent pressure buildup and eliminate oxygen to avoid oxidation and can corrosion.

exhausting

What is the ideal headspace for cans and jars?

Cans: ~1/8 inch
Jars: <6% of content volume

What is the target temperature for cooling canned products?

40ºC

This is used in the cooling of a canned food product to prevent contamination.

chlorinated cooling water

What are the recommended storage conditions for canned goods?

cool, dry area (<35°C)

A method of heating food using electromagnetic radiation in the microwave range that produces dielectric heating through oscillating electric fields.

microwave heating

Microwave heating is a method of heating food using electromagnetic radiation in the microwave range that produces __ through oscillating electric fields.

dielectric heating

Microwave heating is a method of heating food using __ in the microwave range that produces dielectric heating through oscillating electric fields.

electromagnetic radiation

What is the frequency of home microwave ovens?

2450 MHz (2.45 billion cycles per second)

What are the advantages of microwave heating?

• Uniform and rapid heating throughout the product.

• Energy-efficient – heat is generated within the food, minimizing heat loss.

• No browning or surface crusting, unlike frying, grilling, or baking.

What is the typical frequency used in commercial microwave operations?

915 MHz

A non-thermal process that uses ionizing radiation to inactivate microorganisms without significantly raising the product’s temperature

Food irradiation

How do microwaves generate heat in food?

Microwaves create rapidly alternating electric fields that cause polar molecules (like water) to oscillate or rotate rapidly, producing frictional heat inside the food.

How does ionizing radiation inactivate microorganisms?

By dislodging electrons from molecules, forming unstable ions that cause lethal mutations through DNA and RNA destruction.

Food irradiation is also called __.

cold sterilization

Electromagnetic radiation ranging from infrared to ultraviolet wavelengths.

radiation

The total radiation striking a surface per unit area.

irradiation

The total radiation leaving a surface.

radiosity

What are the two main sources of ionizing radiation used in food irradiation?

• Cobalt-60 (Co-60)

• Cesium-137 (Cs-137)

What are the disadvantages of food irradiation?

• Formation of off-flavors due to free radical reactions, especially in fats (autooxidation).

• Generation of free radicals can alter lipid components, leading to potential quality deterioration.