Thermal processing of foods involves applying heat for a specified time to reduce or eliminate microorganisms. There are three main categories: blanching, pasteurization, and sterilization. Blanching uses mild heat (up to 100°C) to inactivate enzymes as a pretreatment for other processes. Pasteurization uses heat below 100°C to destroy heat-sensitive microbes and extend shelf life. Sterilization employs high heat to eliminate all microbes, including spores, through methods like canning. The time and temperature used depends on the thermal resistance of the target organisms.

1. OBT554
Unit 3
Dr K.Geetha
Associate Professor, Dept of
Biotechnology, Kamaraj College of Engg
& Tech, Madurai

2. Definition: the combination of temperature and time
required to eliminate a desired number of
microorganisms from a food product.

3.  The basic purpose for the thermal processing of
foods is,
 to reduce or destroy microbial activity,
 reduce or destroy enzyme activity,
 to produce physical or chemical changes,
 to make the food meet a certain quality standard.

4.  There are three main temperature categories employed
in thermal processing:
 Blanching - Mild processes
 Pasteurization - Mild processes
 Sterilization - More severe processes like Canning,
Baking, Roasting, Frying

5.  The primary purpose of blanching is to destroy
enzyme activity in fruit and vegetables.
 It is not intended as a sole method of preservation,
 But as a pre-treatment prior to freezing, drying and
canning.

6.  Blanching is carried out at up to 100°C using hot water
 Some use of fluidised bed blanchers, utilising a
mixture of air and steam.
 There is also some use of microwaves for blanching.

7.  Steam Blanchers –
 preferred method for foods with large cut surface areas as
lower leaching losses.
 food material carried on a mesh belt or rotatory cylinder
through a steam atmosphere, residence time controlled
by speed of the conveyor or rotation.
 Hot Water Blanchers –
 Includes various designs which hold the food in hot water
(70 to 100°C) for a specified time, then moves it to a
dewatering/cooling section.
 In blanchers of this type the food enters a slowly rotating

8.  faster,
 more uniform heating,
 good mixing of the product,
 reduction in effluent,
 shorter processing time and hence reduced loss of
soluble and heat sensitive components

9.  Pasteurization is a relatively
mild heat treatment in which
food is heated to <100°c.
 It is widely used throughout
the food industry.
 It can be used to destroy
enzymes.
 It can be used to destroy
relatively heat sensitive
Micro-organisms
 It is used to extend shelf life.

11.  Thermoduric: organisms that can survive exposure to
relatively high temperatures but do not necessarily
grow at these temperatures e.g. Streptococcus and
Lactobacillus.
 Thermophilic: organisms that not only survive
relatively high temperatures but require high
temperatures for their growth.

12.  Pasteurizers may be batch or continuous.
 A simple batch type may be a water bath in which
crates of the food are heated to a pre-set temperature,
and then cooled by draining and adding cold water.
 A continuous version may convey containers through
a hot water batch followed by a cold water bath.

13. 1. Batch (holding) Method
In this
method every
particle (e.g.
milk) must be
heated to at
least 63°C and
held for at
least 30
minutes.

14.  In this method the heating of every particle of milk to
at least 72°C and holding for at least 15 seconds.
 Carried out as a Continuous process.
 Ultra Heat Treatment (UHT) a sterilisation treatment,
can also be performed using higher temperatures and
shorter times e.g. 1 s at 135°C
 Eg: Tunnel Pasteurizer

15.  Sterilization is a controlled heating process used
to completely eliminate all living micro-organisms,
including thermo resistant spores in milk or other
food.
 It can be achieved by,
 Moist heat,
 Dry heat,
 Filtration,
 Irradiation,
 Chemical methods

16.  Phase 1 = (heating phase) heating medium (water or
steam) the product temperature is increased from ambient
to the required sterilization temperature
 Phase 2 = (holding phase) temperature is maintained for a
defined time.
 Phase 3 = (cooling phase) the temperature in the can is
decreased by introduction of cold water into the autoclave.

17.  Autoclaves or retorts
 Simple small autoclaves
 Larger autoclaves
 Rotary Autoclaves

18.  If a population of microbes is held at a constant high
temperature, the number of surviving spores or cells
plotted against time (on a logarithmic scale) will look
like the following graph – which is referred to as the
‘death rate curve’.

19.  If D-value versus time is plotted – again on a
logarithmic scale, the graph looks very similar to the
one previously. This one is called the Thermal death
time (TDT) curve.

20.  Factors affecting heat resistance of microorganism
 Age of cell: younger cell are less heat resistance
 Initial concentration of spore or cells: more the number
greater the heat treatment
 The medium in which growth has occurred: a more
nutritious medium increases heat resistance
 Moisture content: dry food tend to require more serve heat
treatment during sterilization
 pH of medium: cell or spore have great heat resistance at or
natural pH values

21.  Two strategies:
 A) High Temparature-Short Time (HTST) processing,
with associated aseptic packaging
 B) By delivering heat in new ways e.g. by Microwaves or
by Electric resistance (“OHMIC”) heating of food
 Examples:
 Retorts
 Using plastic packaging in retort operations
 Ohmic heating
 Microwaves sterilization

22.  The Campden and Chorleywood Food Research
Association is a centre of technical excellence for the
food industry.