Pasteurization in food preservation

• It is heat treatment that kills part but not all of the microorganisms present.
• It usually involves the application of temperatures below 100⁰C.
• The heating may be by means of steam, hot water, dry heat, or electric currents and the products are cooled promptly after the heat treatment.

Pasteurization is used in following conditions:

1. When more rigorous heat treatment might harm the quality of the product, as with market milk.
2. When one aim is to kill pathogens, like in market milk.
3. When the main spoilage organisms are not very heat resistant, such as the yeasts in fruit juices.
4. When any surviving spoilage organisms will be taken care of by additional preservative methods to be employed, as in the chilling of market milk.
5. When competing organisms are to be killed, allowing a desired fermentation, usually by added starter organisms, as in cheese making.

Preservative methods used to supplement pasteurization include

1. Refrigeration, e.g., milk.
2. Keeping out microorganisms, usually by packaging the product in a sealed container.
3. Maintenance of anaerobic conditions, as in evacuated, sealed containers.
4. Addition of high concentrations of sugar, as in sweetened condensed milk.
5. Presence or addition of chemical preservatives, e.g., the organic acids on pickles.

Times and temperature in pasteurization process

• Time and temperature depend on the method employed and the product treated.
• The high temperature short time (HTST) method employs a comparatively high temperature for a short time, whereas the low temperature long time, or holding (LTH), method uses a lower temperature for a longer time.

Examples involving pasteurization process

• Some examples follow of pasteurizing treatments given various types of foods.
• The minimal heat treatment of market milk is 62.8⁰C for 30 min the holding method; at 71.7⁰C for at least 15 sec in the HTST method and at 137.8⁰C for at least 2 sec in the ultra-pasteurized method.
• One basis for the selection of this treatment is the thermal resistance of the rickettsia responsible for Q fever, Coxiella burnetti, an organism that may be transmitted by milk.
• The heat treatment is often greater when milk is to be used for other purposes, but it sometimes is slighted in cheese making, in which event the cheese should be aged, as is raw milk cheese.
• Ice-cream mix is pasteurized at various temperatures for different times, usually receiving a greater heat treatment than market milk.

• Grape wines may be pasteurized for 1 min at 82-85⁰C in bulk, whereas fruit wines sometimes are heated to 62.8⁰C or over and bottled hot.
• Beer may be pasteurized at 60⁰C or above, the time varying with the temperature.
• Dried fruits are usually pasteurized in the package at 65.6 to 85⁰C for 30 to 90 minute, the treatment varying with the type of fruit and the size of the package.
• The pasteurizing treatment given to fruit juices depends on their acidity and whether they are in bulk or in the bottle or can.
• Recommended for bottle grape juice is 76.7⁰C for 30 min or flash treatment in bulk at 80⁰C to 85⁰C.
• Similarly for apple juice it is 60⁰C if bottled and 85 to 87.8⁰C for 30 to 60 sec in bulk.
• The average heat treatment for carbonated juices would be 65.6⁰C for 30 min.
• When vinegar is pasteurized in the bottle in a water bath, all the vinegar is brought to at least 65.6⁰C.
• If flash pasteurized, the vinegar is heated so as to be at 65.6⁰ to 71.1⁰C when the bottle is closed.
• When pasteurized in bulk, the vinegar is held at 60 to 65.6⁰C for 30 min.

References: 

i) https://vtechworks.lib.vt.edu/bitstream/handle/10919/93378/FST-315.pdf

ii) http://ecoursesonline.iasri.res.in/mod/page/view.php?id=111460

Pasteurization in food preservation