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 100ยฐC 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 โ
๏ 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 โ
๏ Includes various designs which hold the food in hot water
(70 to 100ยฐC) 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 <100ยฐc.
๏ 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.
10.
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 63ยฐC and
held for at
least 30
minutes.
14. ๏ In this method the heating of every particle of milk to
at least 72ยฐC 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 135ยฐC
๏ 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 โ which is referred to as the
โdeath rate curveโ.
19. ๏ If D-value versus time is plotted โ 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 (โOHMICโ) 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.