Heat kills microorganisms by changing the physical and chemical properties of their proteins. When heat is used to preserve foods, the number of microorganisms present, the microbial load , is an important consideration. Various types of microorganisms must also be considered because different levels of resistance exist. For example, bacterial spores are much more difficult to kill than vegetative bacilli. In addition, increasing acidity enhances the killing process in food preservation.

1. BY
ASSIST. PROF. DR.BERCIYAL GOLDA. P
Principles of Heat
Transfer

2. CONTENTS
Introduction
Principles of Heat Transfer
Conduction
 Convection
Radiation

3. Introduction
• Heat kills microorganisms by changing the physical and
chemical properties of their proteins.
• When heat is used to preserve foods, the number of
microorganisms present, the microbial load , is an important
consideration.
• Various types of microorganisms must also be considered
because different levels of resistance exist.
For example, bacterial spores are much more difficult to kill than
vegetative bacilli.
• In addition, increasing acidity enhances the killing process
in food preservation.

4. • Heat transfer is important in food processing. In the unit
operations several issues of steady and unsteady heat
transfer occur.
• So for instance heat transfer processes are important for
the hygienic, nutritional and sensory quality of the food product.

5. Principlesof Heat Transfer
Heat is transferred via three processes:
1. Conduction (solid material )
2. Convection (liquids and gases )
3. Radiation (electromagnetical waves )
1. Conduction
• Conduction is heat traveling through a solid material. On
hot days, heat is conducted into your home through the roof,
walls, and windows.
• Heat-reflecting roofs, insulation, and energy efficient
windows will help to reduce that heat conduction.

6. • Heat transfer occurs because of the transfer of vibration
energy from one molecule to the adjoining molecules closely
associated in a solid mass.
• Free electrons in molecular level might carry the thermal
energy and electrical energy with in a system.
• Physical movement of body does not take place in
conduction.
Example:- heating of metal body

10. 2. Convection
• Convection is another means for the heat from your walls
and ceiling to reach you.
• Hot air naturally rises, carrying heat away from your
walls and causing it to circulate throughout your home.
• As the hot air circulates past your skin (and you breathe it
in), it warms you.
• Convection is associated with fluids since heat transfer in
a system occurs due to the mass movement within the system.
•Example:- boiling of water

14. 3. Radiation
• Radiation is heat traveling in the form of visible and non-
visible light. Sunlight is an obvious source of heat for homes.
• In addition, low-wavelength, non-visible infrared radiation
can carry heat directly from warm objects to cooler objects.
• Infrared radiation is why you can feel the heat of a hot
burner element on a stovetop, even from across the room.
• Older windows will allow infrared radiation coming from
warm objects outside to radiate into your home; shades can help to
block this radiation.
• Newer windows have low-e coatings that block infrared
radiation. Infrared radiation will also carry the heat of your walls
and ceiling directly to your body.

15. •Radiation is the transfer of thermal energy in the form of
electromagnetic spectrum in vacuum.
• Generation of electromagnetic spectrum occurs at high
temperature range.
Example:- Microwave heating, heating of bread crust inside
baking oven