Sterilization Versus Pasteurization

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Thermal processing covers the broad area of food preservation technology in which heat treatments are used to inactivate microorganisms to accomplish either commercial sterilization or pasteurization. Sterilization processes are used with canning to preserve the safety and wholesomeness of ready-to-eat foods over long terms of extended storage at normal room temperature (nonrefrigerated) without additives or preservatives, and pasteurization processes are used to extend the refrigerated storage life of fresh foods. Although both processes make use of heat treatments for the purpose of inactivating microorganisms, they differ widely with respect to the classification or type of microorganisms targeted, and thus the range of temperatures that must be used and the type of equipment systems capable of achieving such temperatures.

Pasteurization

Pasteurization is a relatively mild heat treatment given to foods with the purpose of destroying selected vegetative microbial species (especially the pathogens) and inactivating the enzymes. Because the process does not eliminate all the vegetative microbial population and almost none of the spore formers, pasteurized foods must be contained and stored under conditions of refrigeration with chemical additives or modified atmosphere packaging, which minimize microbial growth. Depending on the type of product, the shelf life of pasteurized foods could range from several days (milk) to several months (fruit juices). Because only mild heat treatment is involved, the sensory characteristics and nutritive value of the food are minimally affected. The severity of the heat treatment and the length of storage depends on the nature of the product, pH conditions, the resistance of the target microorganism or enzyme, the sensitivity of the product, and the method of heating. Some of these are summarized in Table 1 (1).

Most pasteurization operations involving liquids (milk, milk products, beer, fruit juices, liquid egg, etc) are carried out in continuous heat exchangers. The product temperature is quickly raised to the pasteurization levels in the first heat exchanger, held for the required length of time in the holding tube, and quickly cooled in a second heat exchanger. For viscous fluids, a swept surface heat exchanger is often used to promote faster heat transfer and to prevent surface fouling problems. In-package pasteurization is similar to conventional thermal processing of foods except that it is carried out at lower temperatures. The thermal processing of high acid foods (natural or acidified) is also sometimes termed pasteurization to indicate that relatively milder heat treatment is involved (generally carried out at boiling water temperatures).

Sterilization

Sterilization implies the destruction of all viable microorganisms and is not the appropriate word to be used for thermal processing of foods, because these foods are far from being sterile in the medical sense of the word. The success of thermal processing does not lie in destroying all viable microorganisms but in the fact that together with the nature of the food (pH), environment (vacuum), hermetic packaging, and storage temperature, the given heat process prevents the growth of microorganisms of spoilage and public health concern. In essence, it represents a thermal process in which foods are exposed to a high-enough temperature for a sufficiently long time to render them commercially sterile. The process takes into account the heat resistance of the spore formers in addition to their growth sensitivity to oxygen, pH, and temperature. The presence of vacuum in cans prevents the growth of most aerobic microorganisms, and if the storage temperature is kept below 25°C, the heat-resistant thermophiles pose little or no problem. From the public health perspective, the most important microorganism in low-acid (pH > 4.5) foods is Clostridium botulinum, a heat-resistant, spore-forming, anaerobic pathogen that, if it survives processing, can potentially grow and produce the deadly botulism toxin in foods. Because C. botulinum and most spore formers do not grow at pH < 4.5 (acid and medium-acid foods), the thermal processing criterion for these foods is the destruction of heat-resistant yeasts and molds, vegetative microorganisms, or enzymes. The temperature and pH requirements of some common spoilage microorganisms are summarized in Table 2 (2). Because spore formers generally have high heat resistance, the low-acid foods that support their growth are processed at elevated temperatures (115-125°C), whereas acid foods need only to be brought to 80-90°C for adequate inactivation of enzymes or destruction of vegetative cells, yeasts, and molds.

STERILIZATION EQUIPMENT SYSTEMS Historical Perspectives

The practice of canning as a method of food preservation originated in the early 1800s in France when Emperor Napoleon Bonaparte offered a prize of 12,000 francs to anyone who could develop better and more diversified foods to feed his troops on their military campaigns. A man named Nicholas Appert won the prize for successfully preserving, for the first time, a variety of perishable food products by heat-processing them in glass jars and bottles. At the time of Appert's discovery, the reasons for food spoilage were not known; process times and temperatures were selected by trial and error. Pasteur discovered the existence of microscopic organisms many years later in 1860.

From Appert's work, plus the invention of the metal container and the pressure cooker or retort, evolved the present-day thermal processing technology. The development of metal and glass containers capable of withstanding more than 1 atm of added internal pressure was a ma-

Table 1. Pasteurization Objectives and Conditions for Selected Foods

Food

Purpose

Typical processing conditions

Fruit juice Inactivation of enzymes (pectinesterase and polygalacturonase)

Beer Destruction of spoilage microorganisms (wild yeasts, Lactobacillus sp.), and residual yeasts (Saccharomyces sp.) Milk Destruction of pathogens: Brucella aboritis, Mycobacterium tuberculosis, Coxiella burnettii Liquid egg Destruction of pathogens Salmonella seftenburg Ice cream Destruction of pathogens

1-4 min at 900-1,000 kPa 63°C for 30 min; 71.5°C for 15 s

64.4°C for 2.5 min; 60°C for 3.5 min 65°C for 30 min; 71°C for 10 min; 80°C for 15 s

Table 2. Spore-Forming Bacteria Important in Spoilage of Food

Approximate temperature (°C) range for vigorous growth

Acidity of food

Table 2. Spore-Forming Bacteria Important in Spoilage of Food

Approximate temperature (°C) range for vigorous growth

Acidity of food

Thermophilic (55-35°)

B. coagulons

C. thermosaccharolyticum C. nigrificans

Bacillus stearothermophilus

Mesophilic (40-10°)

C. butyricum

C. botulinum, A and B

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Responses

  • harding
    What is tge essence of strerilization ij fiid processing?
    3 years ago
  • teemu
    Why is pasteurization a better method of preserving milk than sterilization?
    6 months ago
  • elsie
    HOW IS PASTEURIZATION A BETTER METHOD OF MILK PRESERVATION THAN STERIZATION?
    2 months ago
  • marko
    What is commercial sterilization in food processing and preservation?
    1 month ago

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