Newer methods of thermal processing

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.