Thermal Control Of Microorganisms

 The minute living organisms, not visible to the naked eye and classified as
microorganisms, are virtually everywhere. Those of primary medical interest
are bacteria, viruses, spirochetes, rickettsia, molds, and yeasts.

Thermal Control Of Microorganisms

They flourish in
the soil of the farms that grow our grains, fruits and vegetables, on the hides
and feathers of our meat animals and on the fins and organs of the seafood we
eat. Though there are innumerable genera and species of each class of
microorganisms, not all are of medical significance or involved in disease

Many of these organisms can be beneficial. In fact the
predominance are composed of those that are necessary to food production,
friendly environments, and metabolic processes, examples being cheese/wine production, decomposition of organic matter, and digestion of food. 

Lactic acid
bacteria in the dairy industry, yeasts in the baking and brewing industries,
molds for specialty cheeses are examples of “domesticated” microorganisms.
But in a many cases these microscopic flora create serious problems in our
food supply. 

These problems fit into two categories. Food spoilage occurs
when the food becomes unpalatable as the result of microbial growth. Products
develop undesirable flavors, odors, appearances or textures via microbial
action. The other, more dangerous problem is food poisoning, which occurs
when the organisms present in food cause human illness or death. 

microorganisms either produce a toxin or cause an infection, generally
intestinal, when consumed. Those organisms that spoil product are typically
called spoilage organisms, while those that can make people sick are referred
to as pathogens. Therefore, to avoid both of these problems we need to
understand the techniques which prevent their growth.

Food preservation has been around for a long time. The technique of food
preservation may vary but the goal of food preservation has been the same i.e.
to keep the food in a stable condition over a period of time so that it will
not spoil or make people sick. 

There are various ways of food preservation,
including chemical preservation, modified atmospheres, irradiation, low
temperature preservation, preservation by drying and high temperature


The most common method of killing microorganisms is to subject them to a
heat treatment. High temperatures act by killing vegetative cells and also
spores and denaturing the food enzymes. It may also act to destroy toxins
produced by certain microorganisms. 
The heat treatment used depends on the following factors. In order to safely
preserve foods using heat treatment, the following must be known:
• What time-temperature combination is required to inactivate the most heat
resistant pathogens and spoilage organisms in one particular food? The
higher the temperature, the less time needed and vice versa. 
destruction of microorganisms is a gradual phenomenon the longer is the
treatment time at lethal temperatures, the larger is the number of
microorganisms killed. As higher is treatment temperature the shorter is the
time required to kill microorganisms and lower is heat induced damage to
food products. 
• What are the heat penetration characteristics in one particular food,
including the can or container of choice if it is packaged? 
• What are the types of micro-organisms present in the food material? The
thermal death time of different microorganisms vary widely with the
species. Different foods will support growth of different pathogens and
different spoilage organisms so the target will vary depending upon the
food to be heated. 
• What is the concentration of the microorganisms? The higher the
concentration, the more time is needed
• What is the state of the microorganism? Spores are more resistant than
vegetative cells. Organisms that have been stressed are more susceptible to
• What is effect of heat on the product? Obviously, the temperatures required
to kill microorganisms affect most food products. 
• The degree of heat penetration also must be considered. Preservation
processes must provide the heat treatment which will ensure that the
remotest particle of food in a batch or within a container will reach a
sufficient temperature, for a sufficient time, to inactivate both the most
resistant pathogen and the most resistant spoilage organisms if it is to
achieve sterility or “commercial sterility”, and to inactivate the most heat
resistant pathogen if pasteurization for public health purposes is the goal 
• What is the effect of various environmental factors, such as pH and salts or
solutes. Food acidity / pH value has a tremendous impact on the target in
heat preservation/ processing.


The most common type of food preservation by high temperatures is cooking.
However, there are many more processes that involve the use of temperature
above that of ambient air.


By sterilisation we mean complete destruction of micro-organisms. Because of
the resistance of certain bacterial spores to heat, this frequently means a
treatment of at least 121° C (250° F) of wet heat for 15 minutes or its
It also means that every particle of the food must receive this heat
treatment. If a can of food is to be sterilized, then keeping it at 121° C or retort
for the 15 minutes will not be sufficient because of relatively slow rate of heat
transfer through the food in the can to the most distant point. In such cases
time needs to be increased.

Commercially Sterile Food Products

Sterile means free of life of every kind and is actually achieved under very
limited conditions. The control of microorganisms in medicine, industry,
sanitation, food, and feed service involves the acceptance that sterilization is
most often not achievable without destroying or severely damaging the
Only Low Acid Foods, having pH higher than 4.6, must be
sterilized, because all microorganisms are able to grow in LAF. More acid
products [pH equal/lower than 4.6] do not allow the growth of pathogenic
spore forming bacteria. Then Sterilization is not required. 
Hence Commercial
Sterility is a term commonly used in the canning industry meaning the
condition achieved by the application of heat sufficient to render the processed
product free from viable microorganisms (including those of known public
health significance), capable of growing in the food under normal nonrefrigerated temperatures at which the food is likely to be held during
distribution and storage.
The process was developed by Nicolas Appert and published in 1810. All
vegetative organisms that could grow in the food and cause spoilage under
normal handling and storage conditions are destroyed. 
However commercial
sterile foods may contain a small number of heat resistant bacterial spores, but
they will not multiply under normal handling and storage conditions. Types of
commercially sterile processes include canning, bottling, and aseptic
Commercial sterilization must make sure the numbers of surviving
spores are at an acceptable level. The acceptable number of spores will depend
on what type of damage they are capable of causing if they start to grow.


In the previous section you have read about strerilzation and commercially
sterile foods. Now we will discuss milder heat treatment i.e. pasteurization. It
is one type of preservation by heat that most people are familiar with. It is
process of heating a liquid, particularly milk, to a temperature between 55 and
70 degrees C (131 and 158 degrees F), to destroy harmful bacteria. 
process is named after the French chemist Louis Pasteur, who devised it in
1865 to inhibit fermentation of wine. Pasteur’s aim was to destroy bacteria,
molds, spores etc. He discovered that the destruction of bacteria can be
performed by exposing them to certain minimum temperature for certain
minimum time and the higher the temperature the shorter the exposure time
Through this process, all of the bacteria (such as E.coli, Lysteria, and
Salmonella) are not destroyed, it still exists in pasteurized products, but in very
low concentrates. Refrigeration keeps the bacteria from further growth, very
low. There are other bacteria that aren’t harmful to humans, but they produce
acids that turn the milk sour. 
They are called lactophilic because they consume
the lactose in milk and produce acids. The extent of the pasteurization
treatment required is determined by the heat resistance of the most heatresistant enzyme or microorganism in the food. For example, milk
pasteurization is based on Mycobacterium tuberculosis and Coxiella burnetii. 
These two organisms are the most heat resistant of pathogens that are not spore
forming. Milk is a product that most people know is pasteurized. It is
pasteurized by heating at a temperature of 63 degrees C (145 degrees F) for 30
minutes, rapidly cooling it, and then storing it at a temperature below 10
degrees C (50 degrees F).
Pasteurization is a comparatively low order of heat treatment, generally at a
temperature below the boiling point of water. The more general objective of
pasteurization is to extend product shelf-life from a microbial and enzymatic
point of view. 
Pasteurization is frequently combined with another means of
preservation – concentration, chemical, acidification, etc. Blanching is a type of
pasteurization usually applied to vegetables mainly to inactivate natural food
enzymes. Depending on its severity, blanching will also destroy some
Depending upon time and temperature treatment there are three kinds of
pasteurization processes.

Low Temperature Long Time (LTLT)

Where pasteurization time is in the order of minutes and related to the
temperature used; two typical temperature/time combinations are as following:
63°C to 65°C for 30 minutes or 75° C over 8 to 10 minutes. Pasteurization
temperature and time will vary according to:
• nature of product; initial degree of contamination; 
• pasteurized product storage conditions and shelf life required.
In LTLT pasteurization it is possible to define three phases:
• heating to a fixed temperature; 
• maintaining this temperature over the established time period
(= pasteurization time); 
• cooling the pasteurized products: natural (slow) or forced cooling.
This is a typical batch method where a quantity of milk is placed in an open vat
and heated to 63°C and held at that temperature for 30 min. Sometimes filled
and sealed bottles of milk are heat-treated in shallow vats by that method and
subsequently cooled by running water.

High Temperature Short Time (HTST)

HTST pasteurization is characterized by a pasteurization time in the order of
seconds and temperatures of about 85° to 90° C or more, depending on holding
time. Typical temperature/time combinations are as follows:
• 88° C for 1 minute 
• 100° C for 12 seconds 
• 121°C for 2 seconds.
While bacterial destruction is very nearly equivalent in low and in high
pasteurization processes, the 121°C/2 seconds treatment give the best quality
products in respect of flavour and vitamin retention. This is the most widely
used process. The “hold time” is typically 125°C to pasteurize milk. This
process is a continuous method and a “hold tube” is used. 
The “hold tube” is
the tubing in the system that transports the milk after the point where the
product is heated. The tubing is sized so that it takes 15-20 seconds for the
product to travel all the way through it. When it reaches the end, if the
temperature is at 125°C or hotter, it is considered pasteurized. 
It is then cooled
and put in storage. The warm milk passes through the cooling section where it
is cooled to 4° C or below by coolant on the opposite sides of the thin, stainless
steel plates. The cold, pasteurized milk passes on to a storage tank filler for

Ultra High Temperature (UHT) Processing Treatments

In this method, milk is exposed to a brief, intense heating, normally to
temperatures in the range 135-140 °C but for a very short time, a second or
less. The treatment kills all microorganisms that would otherwise spoil the
product. The process depends upon a fairly complicated sterilizer/aseptic
filling design. 
The two stages of effective heat sterilization followed by aseptic
filling represent an integral system. Frequently the packaging material for UHT
milk is cardboard which must be chemically sterilized prior to the filling
This method is used mainly for coffee creamers and boxed juices with the
exception of Europe. They pasteurize milk in this way. 
After this is done,
there is no need to refrigerate, because it sterilizes the product. Sometimes the
products can have a “cooked” taste that can be detected after being brought to
such a high temperature.
Industrial applications of pasteurization process are mainly used as a means of
preservation for milk and fruits and vegetable juices and specially for tomato

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