Heat treatments given canned foods are generally thought of as sterilization processes, though those given high-acid canned foods and even those given some acid canned foods are commonly referred to as pasteurization processes. Whether the term sterilization or pasteurization is used to label a heat treatment designed to reduce the microbial population of a food, the basic purpose the heat treatment is the same-that is, to free the food of microorganisms that may endanger the health of food consumers or cause economically important spoilage of the food in storage and distribution. In other words, the purpose is to sterilize the food with respect certain types of microorganisms, regardless of how the heat treatment applied is labeled. Therefore, the chief difference between pasteurization and sterilization is in concept and common usage. Pasteurization is the term that is most often applied to relatively mild heat treatments given foods, that because of their mature will not support growth of the more heat-resistant organisms or that are refrigerated, frozen, concentrated or dehydrated to prevent significant growth of the more heat-resistant organisms. Sterilization is the term that is most often applied to more severe heat treatments, e.g., those given most low-acid canned food items that are designed to free foods of virtually all microorganisms regardless of their heat resistance. Obviously, there can be no sharp line of demarcation between the two terms in either concept or usage.
Pathogenic Microorganisms
Consideration here will be confined to a number of microorganisms that may, not uncommonly, foods that when ingested with foods have the ability to pass trough the intact mucosa of the alimentary tract, grow in the body tissues and produce damage therein causing disease syndrome. Infections diseases so produced are often referred to as food infections in contrast to afflictions caused by microorganisms, e.g., Clostridium botulinum, and most staphylococci, that normally are not pathogenic when ingested. When growing in foods before ingestion, however, they produce toxins that are absorbed from the alimentary tract when the food is ingested and thereby cause intoxication syndrome. The latter are usually referred to as food poisonings or food intoxications. Often infectious and intoxication disease syndromes are very similar, e.g., in the case of Salmonella infections and Staphylococcus food poisonings.
Mycobacterium tuberculosis
This is a non-spore-forming, gram-positive, acid-fast, rod-shaped bacterium that causes tuberculosis, a disease may appear in a diversity of forms, such as the more common pulmonary tuberculosis or the less common tubercular meningitis and tuberculosis of the bone. Tubercle bacilli readily pass through the intact mucosa of the alimentary tract and from there may spread to involve practically every organ of' the body. Historically, tuberculosis stands out as one of the next important scourges of civilized community.
From the standpoint of food pasteurization, M. tuberculosis is of chief concern in milk because it is infectious to cattle and frequently occurs ill the milk of infected animals; in fact, milk in times past was the most common vehicle of infection, although other foods, particularly meat from infected animals, and sputum may be source of infection also. The first real defense against transmission of tuberculosis through milk from cattle to humans was pasteurization, a heat treatment, based on the early work of Louis Pasteur mentioned above. Pasteurization of milk as practiced, to ensure inactivation of M. tuberculosis, was clearly defined in the Milk Ordinance and Code, recommended by the United States Health Service (1953) as follows: “The terms '’pasteurization,’ ‘pasteurized,’ and similar terms shall be taken to refer to the process of heating every particle of milk or milk products to at least 143 degrees F., and holding at such temperature continuously for at least 30 minutes, or to at least 161 degrees F., and holding at such temperature continuously for at least 15 seconds, in approved and properly operated equipment: provided that nothing contained in this definition shall by constructed as barring any other process which has been demonstrated to be equally efficient and which is approved by the State health laboratory.” “Boiled down" this code "specifies, for milk pasteurization, any heat treatment that is equivalent in lethality, with respect to the destruction of M. tuberculosis, to the time-temperature relationship of 30 minutes at 143 degrees F. and 15 second at 161 degrees Fahrenheit.
Brucellae
The brucellae are non-spore-forming, gram-negative, minute rod-shaped bacteria that cause brucellosis in animals and man. This disease is very infectious and causes contagious abortion (Bang’s disease) in cattle and undulant fever in humans. Though the disease may be acute in man, it is more generally chronic and characterized by recurring undulating fever. There are three recognized species of importance: Brucella abortus, Brucella melitensis, and Brucella suis, identified with brucellosis in cattle, goats, and swine, respectively. All three species may cause brucellosis in humans, although B. suis is most probably most infectious for man. Brucella suis is also highly infectious for cattle and may be transmitted to man through the milk from cows infected with this species.
The chief source of brucellosis infection in man is milk from infected cows. However, the disease can be transmitted in meat from infected animals. Like M. tuberculosis, Brucellae organisms pass readily through the intact mucosa of the alimentary tract. As with tuberculosis, milk pasteurization is also the oldest and most effective defense against transmission of brucellosis from cattle to humans. It should be noted that, in regard to both tuberculosis and brucellosis, eradication programs have markedly reduced the number of infected cattle and consequently have greatly, reduced the magnitude of the transmission problem; however they have not been and most likely never will be effective enough to eliminate the necessity of milk pasteurization.
The brucellae are appreciably less resistant to heat than are the most resistant strains of M. tuberculosis. It becomes obvious that pasteurization processes designed to sterilize with respect to M. tuberculosis are ample for sterilizing with respect to Brucella spp.
Coxiella burnetti
This organism causes a febrile disease in man known as Q fever. Epidemiological studies have implicated cows, sheep, and goats as sources of this infectious organism with regard to humans. When these animals are infected, their milk contains the organism, which probably explains the mode of transmission from animal to man. The infection in humans is characterized by a febrile condition that is often accompanied by pneumonitis. The mortality rate is low.
Prior to the studies on thermal resistance of Coxiella burnetti, minimum standards for milk pasteurization were based on the resistance of M. tuberculosis. As indicated in the above of M. tuberculosis, the milk pasteurization standards were equivalent in lethality to 30 minutes heating at 193 degrees Fahrenheit.
Salmonellae
The salmonellae are non-spore-forming gram-negative, rod-shaped bacteria that cause salmonellosis in animals and man. The three species recognized are Salmonella cholerae-suis, Salmonella typhii, and Salmonella enteridis.
As a group the salmonellae, compared to M. tuberculosis, C. burnetti, and many non pathogenic vegetative forms, have been found have appreciably lower resistance to heat, making their removal from certain foods (e.g., liquid eggs) possible by the application of relatively mild pasteurization processes. The average resistance reported for most of the strains is equivalent to a D150 of about 0.014 minutes, that of the least resistant strain to a D150 of about 0.007 minutes, and that of the most resistant strain to a D150 of about 0.070 minutes.
Shigella dysenteryae
This species is closely related to salmonellae and is often included with them in discussion. It causes the enteric disease, shigellosis, which symptomatically is so similar to salmonellosis that differential diagnosis usually depends on isolation of the causative organism and species identification. Its thermal resistance has been found to be intermediate compared with the wide range of resistance reported for various strains and serotypes of salmonellae.
Toxin-Producing Microorganisms
Micrococcus pyogenes var. aureus – Coagulases-Positive Staphylococci
This is a gram-positive and micrococcus the cells of which often appear in culture. It is widespread in nature and is well known as a pathogen. It causes a number of infections in man and animals, notably, pimples, boils, carbuncles, internal abscesses, osteomyelitis, and septicemia. Though highly infectious under certain circumstances, it commonly resides on the skin of the healthiest individuals. It commonly causes mastitis in cows and contaminates the milk of infected cows. Because of its prevalence, in minor and major infections and in residence on the skin of healthy individuals, and because it readily multiplies outside the animal body placed in a suitable environment, its chance of contaminating a wide variety of foods is indeed great.
Though many strains of this species are highly pathogenic under certain circumstances, this characteristic of the organism must be considered of minor importance from the standpoint of food contamination. Normally, they are noninfectious when ingested by healthy individuals. They are of chief concern as food contaminants because of their ability to grow in a wide variety of foods and produce an exotoxin that induces acute gastroenteritis when ingested. As a group they are mesophilic and facultatively anaerobic. They can grow and produce toxin in foods having pH values slightly above 4.00 and upward. Virtually all strains of Micrococcus pyogenes var. aureus are coagulase positive, that is, they produce the enzyme that coagulase that coagulates human (and rabbit) blood plasma. It is a distinguishing characteristic of the group. In fact, in discussion of food poisoning, these micrococci are generally referred to as simply coagulase-positive staphylococci.
Clostridium botulinum Type E
This is a gram-positive, sporulating, anaerobic, motile bacillus that produces a powerful neuroparalytic exotoxin. It is not pathogenic in the sense of causing infection in man. However, its toxin when ingested causes an often-fatal intoxication known as botulism.
The incubation period in botulism is usually 8 to 24 hours. The course of events somewhat as follows: abdominal symptoms of nausea, vomiting, pain, and distension may develop within a few hours after ingestion of toxin. These are generally followed by persistent constipation and the onset of a peculiar paralytic syndrome. When sufficient toxin has been absorbed from the alimentary tract to reach sites adjacent to cranial and peripheral motor nerve endings, it exerts an inhibitory effect upon the mechanism of acetylcholine synthesis, or release, thereby interfering with the transmission of neural impulses to muscles and glands. Frequently, the cranial nerves supplying the muscles of the eyes and throat are affected first, resulting in dim, fuzzy, or double vision, and difficulty in swallowing. This is usually accompanied by respiratory and cardiac failure. In fatal cases, death usually occurs between the second and fifth day but may occur as early as 20 to 24 hours, or as late as 10 to 14 days after ingestion of affected food (with minor exception, this quite accurately describes the events in botulism caused by other types as well).
There are six known Types of C. botulinum, designated alphabetically from A to F. Human botulism is nearly always caused by Types A, B, and E. The spores of Types A and B are the most heat resistant, their resistance being characterized by D250 values in the range of 0.1 to 0,2 minutes. They are of great concern in the sterilization of canned low-acid foods.
Spoilage Microorganisms
Nonsporulating Bacteria
Reference here is to saprophytic bacteria that normally are neither pathogenic nor toxin producing. Virtually all such organisms, and there is a multitude of different species and strains, may if given the opportunity grow and cause spoilage of foods of one sort or another. However, among the more important genera from the standpoint of food spoilage are Pseudomonas, Achromobacter, Lactobacillus, Leuconostoc, Proteus, Micrococcus, and Aerobacter.
Unfortunately, there is a grave paucity of definitive heat resistance kinetic data available for most of these organisms. However, judging from the effectiveness of pasteurization processes employed for treating a wide variety of foods for extending their keeping quality (under refrigeration) it may be deduced that the average maximum heat resistance of most of these organisms may be characterized by D150 values in the range of 1.00 to 3.00 minutes.
