Organisms of Greatest Importance in The Spoilage of Canned Foods

Microbial spoilage of heat-processed canned foods is caused by microorganisms, which survive the heat processes or that gain entrance, through container leakage, subsequent to the heat processing. It is virtually impossible to predict what types of microorgan­isms may gain entrance through container leakage. However, there are important measures for minimizing this type of spoilage. These will be discussed in some detail later. From the standpoint of establish­ing sterilization processes, spore-bearing bacteria except in highly acid products are the organisms of chief concern. Because the type of microbial spoilage, as well as the heat resistance of bacteria, in closely related to the acidity of foods, for convenience of discussion the different foods are often grouped in classes with respect to pH.

Classification of Food with Respect to Acidity

Various classifications have been proposed. Cameron and Esty (1940) suggested the following:

1. Low-acid foods: pH 5.0 and higher.

2. Medium- (or semi-) acid foods: pH 5.0 to 4.5

3. Acid foods: pH 4.6 to 8.7.

4. High-acid foods: pH 3.7 and lower.

Except for the dividing line between acid and high-acid foods, this classification serves as well perhaps as any. In practice, it has been found that pH 4.00 is a more realistic dividing line between acid and high-acid foods. Seldom will spore-bearing bacteria grow in heat-processed foods with pH values of 4.0 or lower. Some of the butyric anaerobes and Bacillus coagulans (B.thermoacidurans) will grow in laboratory culture or even in foods at pH values as low as 3.7; however, this generally occurs only with very heavy inocula. Some workers recognize only three classes of foods, namely:

1. Low-acid foods: pH above 9.5.

2. Acid foods: pH 9.0 to 9.5.

3. High-acid food: pH below 4.0.

The dividing line between low-acid and acid foods is taken as 4.5 because some strains of Clostridium botulinum will grow and produce toxin at pH values as low as about 4.6. Some of the highly heat­-resistant saccharolytic anaerobes-for example, Clostridium thermo­saccharolyticum – grow in and cause spoilage of foods in this semi­-acid range. Therefore, until more is known of bacterial heat resistance and bacterial growth in the 'semi-acid foods, they perhaps should remain in the low-acid grouping.

Classification of Spore-Bearing Bacteria with Reference to Oxygen Requirements

In low-acid and acid foods, the spore-bearing bacteria are of greatest concern from the standpoint of sterilization. With respect to oxygen requirements they may be classified as follows: (1) obligate aerobes-Bacillus spp.; (2) facultative anaerobes-Bacillus spp.; (3) obligate anaerobes-Clostridium spp.

Obligate Aerobes

This group includes those species that require molecular oxygen for growth. From the standpoint of food sterilization, it is the least important of the three groups. Under present-day methods of canning, most foods contain very low levels of molecular oxygen, insufficient to support appreciable growth. Beyond this, spores of most obligate aerobes are low in heat resistance compared with spores of a number of organisms in the other two groups. Heat proc­esses designed to free foods of the more-resistant organisms in groups 2 and 3 are generally more than adequate to free foods of the aerobic bacilli. However, as with most things, there are exceptions. In canned cured meat products containing nitrate, the Bacillus subtilis - Bacillus mycoides group of organisms may at times be of greater economic importance than some other groups of bacteria.

Some canned cured meat products are given relatively mild heat processes, inhibitory action of the curing agents, and in some cases refrigeration, being depended on to prevent spoilage by organisms in groups 2 and 3. It is usual for spores of aerobic bacilli to survive in some of these products. A number of strains of these bacilli are capable of reducing nitrate to obtain oxygen for metabolic activity. If not destroyed by the heat process, and if subjected to favorable temperatures, many of them will grow in the cured meat products and cause gaseous spoilage. It should be noted that the dry spices used in seasoning some of these products are sometimes heavily contaminated with spores of the aerobic bacilli.

Facultative Anaerobes

Spore-bearing bacteria of the facultative anaerobic group are among the more important from the standpoint of food sterilization. Of particular importance in low-acid and acid foods are thermophilic spore-bearing bacilli. Some of these produce spores that are more resistant to heat than are spores produced by most obligate anaerobes. They generally cause what is known as "flat­-sour" spoilage; that is, they produce acid but little or no gas. Though the most troublesome ones are classified as thermophiles, many of them have the faculty to grow, though more slowly, at or near ambient food-handling temperatures.

Of greatest importance in low-acid foods is Bacillus stearothermophilus and related species. These flat-sour-producing organisms are often not identified with respect to species by the food bacteriologist butt simply labeled as belonging to the B.stearothermophilus group. A number of these strains and species produce spores characterized by D₂₅₀ values in excess of 4.00 minutes, D₂₅₀ being the time, in minutes, at 250⁰F required to destroy 90% of the spores in a given population. This is in contrast to a small fraction of a minute at 250⁰F required to destroy 90% of the spores of most obligate aerobic bacilli. The optimum growth temperature for these flat-sour thermophiles varies considerably among strains and species from about 40⁰ to 55⁰C (120⁰ to 131⁰F.). Most of them seldom grow at temperature below about 38⁰C. (100⁰F.)

Of greatest importance in acid foods are three species of facultative anaerobes-namely, Bacillus coagulans (B. thermoacidurans), Bacillus macerans, and Bacillus polymyxa. Of these, Bacillus coagulans is the more important, particularly in spoilage of tomatoes and tomato products. Bacillus macerans, and Bacillus polymyxa have been isolated as causative organisms in spoilage of some fruits and fruit products. There is some doubt whether spores of these latter organisms resisted the heat processes given or whether the organisms gained entrance through container leakage subsequent to the heat processing.

Bacillus coagulans has been the cause of economically importance spoilage of acid foods, particularly tomato product. It is quite acid-tolerant, growing at pH values of 4.0 or slightly lower. It grows well in tomatoes products and some other semi-acid foods in the pH range 4.0 to 4.6. It is of little concern in low-acid foods, however, because heat processes for these foods are generally more than adequate to destroy its spores. The D₂₅₀ values of B. coagulans spores are of the order of 0,1 and less. Bacillus coagulans is generally classified as a thermophile; however, it has the faculty to grow quite well at ambient food-handling temperatures.

Obligate Anaerobes

Some species of spore-bearing anaerobes produce spores those are relatively quite heat-resistant. With reference to canned food spoilage these may be classified in two groups, mesophilic and thermophilic. Of the thermophiles, the most important are the saccharolytic organisms, which do not produce hydrogen sulfide. Clostridium thermosaccharoliticum is generally considered the type species of this group. These organisms are very saccharolytic, producing large quantities of gas, chiefly carbon dioxide and hydrogen, from a wide variety of carbohydrates. Consequently, they cause spoilage of the “swell” or gaseous type. They often produce a butyric or “cheesy” odor on foods. They generally are of greater importance in the spoilage of semi-acid products (pH 4.5 to 5.0) than are flat-sour types. Their optimum growth temperature is about 32⁰C. (131⁰F). They seldom grow at temperature below about 32⁰C. (90⁰F). They can be a source of real trouble at temperatures of 35⁰C. (95⁰F.) and above. Some strains produce spores of extremely high heat resistance, making severe processing necessary if their spores are present in appreciable numbers.

Thermophilic spore-bearing anaerobes that produce hydrogen sulfide are responsible for the so-called “sulfur stinker” spoilage of canned foods. Clostridium nigrificans is considered the type species of this group. These organisms are proteolytic, and hydrogen sulfide is the only gas they produce in great quantity. They are only weakly, if at all saccharolytic, because the hydrogen sulfide is soluble in the product, spoiled cans usually remain flat. Many products spoiled by these organisms become black, owing to the interaction of the hydrogen sulfide and iron. Spoilage by these organisms is comparatively rare for two reasons: Incident of their spores in most products is generally low, and the spores of most strains are relatively low in heat resistance compared with spores of the saccharolytic thermophilic anaerobes and the flat-sour thermophilic facultative anaerobes. Neither of the above groups is of importance in foods having pH values below about 4.5.

Next in importance, in low-acid foods, to the groups discussed above are spore-bearing mesophilic anaerobes. Because of its public health significance, the toxin-producing organism, Clostridium botulinum, should perhaps be considered of greatest importance among organisms of this group. Of the different botulinum types, A, B, and E are of greatest significance. Spores of Types A and B are considerably more heat-resistant than spores of Type E and therefore of greatest concern in the sterilization of canned foods. Even the spores of A and B are not nearly as heat-resistant as the spores of a closely related nontoxic organism identified only as P.A.3679. Whereas the most-resistant spores of Types A and B botulinum are characterized by D₂₅₀ values of the order of 0.10 to 0.20, the most-resistant spores of P.A.3679 are characterized by D₂₅₀ values of the order of 0.50 to 1.50.

Fortunately, the incidence of the more-resistant strains of P.A.3679 is very low in most foods. For this reason, canning processes designed to assure a high degree of safety with regard to C. botulinum are often adequate to prevent economically important spoilage by even the most resistant of these nontoxic putrefactive anaerobes. Perhaps the greatest importance of P.A.3679 is as a test organism to check the adequacy of canning processes. It is almost ideal for this purpose, not only because of its heat resistance, but because it is nontoxic, it is easy to cultivate, and its spoilage of foods is accompanied by voluminous production of gas.

Other proteolytic or putrefactive organisms most frequently causing spoilage of low-acid and semi-acid foods are C. putrificum, C. Histalyticum, C. bifermentans, C. sporogenes, and related species. Heat processes designed to assure a high degree of safety with regard to C. botulinum are often adequate to prevent economically important spoilage by these organisms; however, not infrequently, more severe processes than considered adequate to control C. botulinum are necessary to prevent spoilage of a number of food items by the more heat-resistant members of this group, including P.A.3679.

Important in the spoilage of some foods are saccharolytic spore-bearing anaerobes of lower heat resistance than the more-resistant putrefactive anaerobes. Of greatest importance in this group are C. pasteurianum, C. butyricum, and related butyric anaerobes. Some strains of these grow well in foods with pH values in the range of 4.0 to 4.5. However, their spores are usually less resistant to heat than are the spores of the flat-sour organism B. coagulans. Therefore, heat processes designed to free acid foods of B. coagulans are generally adequate to free them of the butyric anaerobes.

In summary, of the mesophilic obligate anaerobes, in spoilage of low-acid and semi-acid foods (pH 4.5 and above) the putrefactive anaerobes, C. sporogenes and related species, are of greatest import­ance. In spoilage of acid foods (pH 4.0 to 4.5) the butyric anaerobes, C. pasteurianum and related species, are of greatest importance. The temperature range of optimum growth of these organisms is 25⁰ to 35⁰C. (77⁰ to 95⁰F.).

Non-Spore-Bearing Bacteria, Yeasts, and Molds

Except when they gain entrance through container leakage, these organisms are of greatest importance in spoilage of high-acid canned foods (pH below 4.0) given relatively mild heat processes-pickles, grapefruit, citric juices, rhubarb, cranberries, etc. They are of some importance also in spoilage of concentrated and sweetened products given mild heat processes, the high soluble solids content being depended on to inhibit growth of spore-bearing organisms-for ex­ample, fruit and vegetable concentrates, jellies, preserves, sweetened condensed milk.

Of the non-spore-bearing bacteria, Lactobacillus and Leuconostoc spp. are most important. A wide variety of yeasts have been found as spoilage agents of number of high-acid foods given mild heat processes. The more resistant of these yeasts and bacteria are charac­terized by D₁₅₀ values of the order of 1.00. Pasteurization processed based on this resistance value usually prove adequate. Molds are generally considered significant as spoilage agents in canned foods.

The temperature range of optimum growth for most of these non­-spore-bearing bacteria, yeasts, and molds is from 20⁰ to 35⁰C. (68⁰ to 95⁰F.).

Organisms Entering via Container Leakage

If a container "breathes" even shortly, bacteria may be sucked in with contaminated air or water. They are far more likely to enter with contaminated water than with air. In most cases, the openings causing leakage are small enough to filter out bacteria-laden dust from the air. This is not the case for bacteria suspended in water. Because of thin and because of stream on can seams and jar closures during the cooling operation, bacteria are most likely to gain entrance during cooling or shortly thereafter, while the containers are still wet. With cans, rough handling on contaminated conveyors, and elevators, while the cans are wet is a major contributing factor to can leakage and bacterial contamination.

The beat defenses against spoil­age because of container leakage are:

1. Good can seams and jar closures.

2. Careful handling to avoid container damage, such as dents close to can seams.

3. Chlorinated cooling water (3 to 5 p.p.m. active chlorine).

4. Rapid drying of containers after cooling.

5. Dry and sanitary post-cooling can-handling equipment.

Because of varied types of bacteria in contaminated cooling water and on can-handling equipment, many different species of bacteria have been associated with leakage spoilage. Usually, though not always, a mixed flora causes leakage spoilage of a container. Commonly found are micrococci, gram-negative rods of the Pseudomonas-Achromobacter group, and yeasts. It should be recognized that very minute amounts of heavily contaminated water could cause spoilage. For example, if the water were contaminated to the extent of 1 million bacterial cells per milliliter, only one-millionth milliliter would have to enter the container to cause spoilage.