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Lecture 13

Control of Microbes

There are two main methods of controlling microbes one is to kill them and the other is to inactivate (stop them growing) the microbes. Both methods have their advantages and disadvantages.

Killing microbes can make a product safe in the short term but it is only remains "safe" as long as it prevented form of re-contamination. i.e. it must be sealed in some way to prevent new organisms entering and growing in the product. A disadvantage of killing bacteria is that act if all the microbes are killed a product can be more dangerous then before it was processed if not stored correctly. This is because before processing the products have had many different bacteria present. Then if it was kept under the wrong conditions this mixture of y bacteria soon show obvious deteriorate (food spoilage) thus preventing consumers from eating a potentially dangerous product. In fact the food spoilage organisms will grow faster and more obviously than the food poisoning bacteria and in some cases spoilage organisms will actually inhibit the growth of dangerous microbes. Hence unprocessed food is less likely to cause food borne disease than cooked foods.

The spoilage bacteria act as an indicator of poor handling. So food spoilage can protect the consumer from disease.

If all the microbes are killed this protection no longer exists. Once a sterilized product becomes contaminated with a "food poisoning microbe" it is possible that this microbe can grow to a dangerous level with out any visible sign. This is why most food poisoning in western communities come from cooked food not uncooked food.

The option of inactivating microbes can retain the protection provided by spoilage organisms as well as the controlling the dangerous bacteria by stopping them growing. It is possible to combine the two methods (killing and inactivating) by using a minimal processing method (pasteurizing). In pasteurization the pathogenic bacteria that don't have to grow to become dangerous are killed then the spoilage bacteria survive to remain an indicator of poor handling.

The food is however only safe as long as the inactivating action remains in place. I.e. a died product must remain dry, a salted product must remain salted and a frozen product must remain frozen. Once the product is reconstituted the microbes can grow just like they had never been inhibited.

Killing Methods

Microbes can be killed by heat, radiation, poisonous gases, chemicals, or ultra-sound.

In every method of killing there are two things must be taken account of every time: Time and Intensity of treatment

E.g.. The temperature / time combination needed to kill TB bacteria in milk is 63 C for 30 min. If the temperature is increased by only 10 C the exposure time is reduced by 99.2% i.e. 73C requires only 15 sec. To have the same killing effect as 63 C for 30 min.

In every case the exposure time is inversely related to the Intensity.

Organisms vary in relation to their resistance to killing agents and the Intensity/Time combination required is can be greater for resistant organisms.

The Exposure time required is also Greater for larger populations than smaller populations. Once a killing process has been chosen the initial population must be kept below expected initial numbers to make sure not too many organism survive the treatment

Killing with High Temperature

The exposure time to kill organisms depends on the:
  • Processing temperature ,
  • The type of microorganisms(s) that are in the food
  • Physical and chemical properties of the food.
  • The number of organisms


Killing bacteria using a temperature less than 100 C

Pasteurization of Milk

LTLT: The temperature / time combination needed to kill TB bacteria in milk is 63 C for 30 min.

This requires holding large volumes of milk in vats for a long time. The process is referred to as a batch process and is expensive. The low temperature has less effect on flavour than pasteurization at higher temperatures

HTST 73C requires only 15 sec. To have the same killing effect as LTLT but the very short time allow the heat treatment to be applied while the milk passes between heated plates

The process can be run as a continuous method allowing large volumes to be pasteurized quickly and economically but with a slightly inferior taste.

Pasteurization of Other Foods The composition of the food may limit temperature used.


Pasteurization temperature of eggs is limited so that the egg does not get "hard Boiled"

Egg pulp maximum 600 C 3.5 min

Egg yolk maximum 600 C 6.2 min

Fruit Juice The temperature needed for pasteurization of fruit juice is higher than needed to kill pathogenic bacteria because it must also is inactivate endogenous enzymes that would cause spoilage. Temperature may be up to 850C
Boiling at 1000C

            A few seconds at 1000C is all that is need to kill most vegetative cells however boiling for 30 min. may be needed to kill more thermoduric organisms. Boiling for 5 min. is often recommended to purify water. However some cysts of Giardia  may survive 5 min. Endo spores can survive boiling for many hours. Sterilization is the only practical way to control spores.

Intermittent Boiling (Tyndallization) or "Fractional sterilization" This involves boiling the media at  1000C for 30 min. on three successive days. and leaving the material at 370C during the intervening times so that surviving spores can germinate. Each boil kills of the newly germinated vegetative cells.
After three incubations there should be no more ungerminated spores remaining. The initial cooking period acts as "heat activation" to initiate sporulation. This was once a popular method for home preserving and is better than nothing if a pressurized vessel is not available. However not all spores are obliging enough to sporulate when expected. So this method is not as reliable as sterilization.


Killing bacteria using a temperature over 1000C At temperatures over 1000C most bacteria found in food are killed instantaneously

However a small number produce heat resistant endospores that will withstand boiling for many hours.

Temperature greater than 100 C must be used to reduce the exposure time to a practical

    1. Commercial sterilization
    2. 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
      The problem with spore is that they are not all killed by the same amount of heat.
      Types of commercially sterile processes include canning, bottling, and aseptic processing
      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 the start to grow. If the damage is in regard to food spoilage and not a heath risk the acceptable number will depend on what the company accepts as an acceptable number of consumer complaints. A decision has to be made on the commercial damage caused by too many consumer complaints compared to the commercial damage done by heating the product longer and causing an inferior product in relation to taste and appearance.
      There are a few spores that do represent a health risk. The most significant is from Clostridium botulinum. If botulinum spores germinate the bacteria can produce a lethal toxin. The number of acceptable spores for Cl. botulinum is 1 in 1,000,000 containers. Food that can support the growth of Cl. botulinum must be given a 12D cook.
      12D cook
      If it is assumed that a container had one million spores per can the heat treatment needed to reduce the number to one in one million i.e. from 106 to 10-12 involves a reduction of twelve decimal places
      i.e. from 1,000,000 to 0.0000001
      This is called a 12D cook
      A "d" value is the time needed to reduce the population of spores by one decimal reduction
      I.e. from 100 to 10 or 100,000 to 10,000
      F value
      The killing effect of a time / temperature combination is referred to as the F value
      F = 1 is heat killing effect equivalent to 1 min at 121 C
      The F value required to achieve a 12D cook depends on the resistance of the particular type of bacteria. One of the most resistant species is Bacillus stearothermophilus which is 5 or 6 time more resistant than botulinum.
      A 12-D cook for Cl. botulinum may require an F value of 2.52
      A 12 D cook for B. stearothermophilus may require F = 18
      The lethality of a thermal treatment will also be influenced by the composition of the food
      Moist Heat vs Dry Heat
      Moist Heat kills bacteria by coagulating proteins whereas dry heat kills by oxidation of cell contents.
      Moist heat in requires less heat (temperature or time) than dry heat.
      121 C for 10 min of moist heat is equivalent to about 30 min at 200 c dry heat.
      For this reason a lot of sterilization procedures use super heated steam that provides moist heat.
      Temperature over 100 C requires cooking under elevated pressure, (like in a pressure cooker) 121 C require 100 kpa extra pressure.
      It is important that no air pockets are allowed to develop when a product being sterilized with steam. In air pockets food is exposed to dry heat and not the time /temperature is not enough.
      Killing with Irradiation

      Three forms of radiation are common

      1 UV light

      2 Atomic radiation

      3 Microwaves

      How does irradiation work?
      UV light of 260 nm wavelength is absorbed by DNA causing lethal mutations. Microorganisms are killed or inactivated.
      However UV light does not penetrate far though food or packaging and has little use in the food industry
      It can be used to control contamination of surfaces.
      Atomic Radiation
      When food in containers is passed through a shielded area, radiation passes through the food and breaks molecular bonds in the water producing ions and free radicals. These Free radicals react with DNA and kill bacteria, parasites, viruses, fungi, and insects
      The food must be exposed to an irradiation source for an amount of time that depends on the food and the type of radiation.
      Atomic radiation includes alpha, beta and gamma rays. However only gamma rays have a useable penetrating ability and can pass through most food packaging.
    3. Irradiation can also delay ripening and prevent sprouting in fresh fruits and vegetables.
    4. It is thought by some that the food is left virtually unchanged. However it is possible that changes can occur to the flavour and level of oxidation and may leave residual radiation. Radiation is not allowed in Australia but it is permitted in USA.
      1. Radiation is a cheap and effective method of treating large volumes of grain and in areas of the world where food is in short supply it is an attractive alternative.

 Prepared by Barry Brazier