Food Preservation Storage

Lesson 08: Preservation by Use of HeatPreservation by Use of Heat


Pasteurization is a mild heat treatment for relatively brief duration to kill part of the microorganisms and to eliminate human pathogens present in food. It is used specially when the aim is to kill pathogenic microorganisms and where the spoilage organisms are not very heat-resistant and the product cannot stand high-temperatures or frozen.

The main purpose of pasteurization in low acid foods is destruction of pathogenic microorganisms whereas in acid foods it aims at killing spoilage microorganisms along with enzyme inactivation. For example pasteurization is used to kill pathogenic microorganisms Brucella abortis, Mycobacterium tuberculosis and Coxiella burnetti in case of milk (63°C for 30 minutes; 71.5°C for 15 seconds) and spoilage microorganisms in beers (lactic acid bacteria and yeasts at 65°C–68°C for 20 minutes in bottle), fruit juices (yeast and fungi along with pectinesterase and polygalacturonase inactivation at 65°C for 30 minutes; 77°C for 1 minute; 88°C for 15 seconds) etc. In addition to destroying some microorganisms, pasteurization also inactivates some enzymes. Pasteurization does not change the colour and flavour to any significant level.

Since pasteurization does not kill all the microorganisms, this process is usually combined with another preservation method like refrigeration, freezing etc. Typical other preservation methods used in combination with pasteurization include refrigeration as in the case of milk; chemical additives- pickles, fruit juices; fermentation (additives)- sauerkraut, cheeses; and packaging (anaerobic conditions)- beers, fruit juices.

The index microorganism for pasteurization is Mycobacterium tuberculosis. If this microorganism is killed by pasteurization it is assumed that all other pathogens are also destroyed.

Methods of pasteurization
Three methods of pasteurization are used viz. low temperature long time (LTLT), high temperature short time (HTST) and ultra high temperature (UHT) method.

Low temperature long time (LTLT) method:
In LTLT pasteurization, the pasteurization time is in the order of minutes and related to the temperature used; two typical temperature/time combinations are used: 63 to 65°C over 30 minutes or 75°C over 8 to 10 minutes. The minimal heat treatment for market milk is 62.8°C for 30 minutes in (LTLT) holding method and for grape juice is 76.7°C for 30 minutes.

High temperature short time (HTST) method:
Rapid pasteurization involves temperatures of about 85° to 90°C or more and time in the order of seconds. HTST method involves temperature of 71.7°C for about 15 seconds in case of milk pasteurization and grape wines are generally pasteurized for one minute at 81 to 85°C.

Ultra high temperature (UHT) method:
Rapid, high or flash pasteurization involves temperatures of 85-90°C or more and time in order of seconds. These are also known as ultra high temperature (UHT) treatments. Typical temperature/time combinations may be: 88°C for 1 minute; 100°C for 12 seconds; 121°C for 2 seconds. This treatment will destroy all but the most heat resistant spores resulting in commercially sterile product.

Bacterial destruction is very nearly equivalent in all temperature-time combinations; however, the 121°C for 2 seconds treatment gives the best quality products in respect of flavour and vitamin retention. Very short holding times to the tune of seconds, however, require special equipment which is more difficult to design and generally is more expensive than the LTLT/ HTST type of processing equipment.

Last modified: Saturday, 10 March 2012, 5:19 AM