Tyndallization and Pasteurization

Tyndallization and Pasteurization




Learning objectives

At the end of this lecture, student will be able to

      Discuss sterilization at temperature 100⁰C

      Discuss sterilization at temperature below 100

      Explain pasteurization and tyndallization

Thermal sterilization / Sterilization by heat

Moist heat sterilization / Steam sterilization

a)      Sterilization at temperature above 100⁰C (Saturated stream)

      Autoclaving / steam sterilization

b)      Sterilization at temperature of 100⁰C ( Boiling water)


      Sterilization by boiling water

c)       Sterilization at temperature below 100⁰C

                (Hot water below Boiling Point)


      Vaccine bath

e)      Heating with a bactericide

Sterilization at 100°C

  1. Boiling 
  2. Steaming
  3. Tyndallisation


       Boiling at 100°C for 30 minutes is done in a water bath

       Syringes, rubber goods and surgical instruments may be sterilized by this method

       After sterilization the materials should be removed with forceps already sterilised by dipping in suitable disinfectant or by heating in flame

       All bacteria and certain spores are killed


       Steam (100°C) is more effective than dry heat at the same temperature as:

      Bacteria are more susceptible to moist heat

      Steam has more penetrating power

      Steam has more sterilizing power as more heat is given up during condensation

Steam Sterilizer

       It works at 100°C under normal atmospheric pressure i.e. without extra pressure

       It is ideally suitable for sterilizing media which may be damaged at a temperature higher than 100°C

       It is a metallic vessel having 2 perforated diaphragms (Shelves), one above boiling water, and the other about 4" above the floor

       Water is boiled by electricity, gas or stove

       Steam passes up

       There is a small opening on the roof of the instrument for the escape of steam


       Fractional sterilization or tyndallization is a method used to destroy bacteria and endospores

       In this articles are  boiled or steamed at 212°F (100°C) for 30 min in a pot with lid, three days in a row

       Between the boiling steps the jars are kept warm, around 30°C to allow the remaining endospores to germinate

Principle: Any resistant endospores will germinate after the first heating and therefore be susceptible to killing during the second and third heating.

       Also called fractional sterilization – A fraction is sterilized on each day


       This method is used for sterilizing egg serum, sugar containing media and gelatin media which will be damaged at higher temperature


       This method is only applicable to sloutions which bacterial spores surviving the first heating have a chance to develop during the next 24 hours

Sterilization below 100⁰C – Pasteurization

       Louis Pasteur found a practical method of preventing the spoilage of beer and wine

       Pasteur used mild heating, which was sufficient to kill the  organisms that caused the particular spoilage problem without seriously damaging the taste of the product

       The same principle was later applied to milk to produce what we now call pasteurized milk

       Pasteurization of milk was to eliminate pathogenic microbes

       It lowers microbial numbers, which prolongs milk's good quality under refrigeration

Phosphatase test

       To determine whether the products have been pasteurised

       Phosphatase is an enzyme naturally present in milk

       If the product has been pasteurised, phosphatase will be inactivated


       Take 1 ml of milk sample

       Put 5 ml of nitrate Phosphate buffer solution

       Incubate it in water bath at temperature of about 37˚C

       Check for colour change after 15 minutes

       If colour changes then the milk is not pasteurized

Conclusion : In the image Test tube on left side with pale yellow colour is showing test of Raw milk confirms alkaline phosphate activity and test tube on right side is showing no colour change in case of packed milk confirming Pasteurization

Types of Pasteurization

  1. Classical method of pasteurization
  2. High Temperature Short-time (HTST) pasteurization / Flash method
  3.  Ultra-High-Temperature (UHT) pasteurization

1.    Classical method of pasteurization

·         The milk was exposed to a temperature of about 63⁰C for 30 minutes

       The milk proteins tend to protect the microorganisms but inspite of this mycobacterium tuberculosis is killed in 20mins but extra 10mins provides a very safe exposure which destroys the other pathogens found in milk and the lactic acid producing organisms responsible for souring

2.    High Temperature Short-time (HTST) pasteurization

       Uses temperatures of at least 72°C, but for only 15 seconds

       HTST pasteurization lowers total bacterial counts, so the milk keeps well under refrigeration

3. Ultra-High-Temperature (UHT) pasteurization

       Milk can be stored for several months without refrigeration

       The liquid milk (or juice) is sprayed through a nozzle into a chamber filled with high –temperature steam under pressure

       Sterilizing temperatures are reached almost instantaneously

       After reaching a temperature of 140°C for 4 seconds, the fluid is rapidly cooled in a vacuum chamber

Heating with a bactericide

       This method is based on the fact that bactericides are more effective at high temperature

       This method is used in a sterilization of aqueous solutions and suspensions that are destroyed by autoclaving

       Suitable proportion of a bactericide is added to the solution to be sterilized which is distributed in the final containers and sealed

       The sealed containers are then heated at 100⁰C in a steam sterilizer or water bath for 30 minutes.


       Pasteurization is a technique used to destroy pathogenic organisms helps to improve the keeping qualities of food products without affecting their taste

       Tyndallization is a process of fractional sterilization used to sterilize heat sensitive liquids like culture media

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