Microbial Spoilage

a. Spoilage in Low-Acid Foods. The micrological groups associated with spoilage in low-acid foods (pH above 4.6) are as follows.

(1) Insufficient processing. Insufficient processing is indicated chiefly by the survival of bacterial spores, particularly those of the Clostridium species and those of the Bacillus species, which subsequently spoil the product. Generally, anaerobic mesophilic spoilage is associated with putrid odors.

(2) Storage at 110°F or above. Low-acid foods may be spoiled during storage above 43°C (109°F) by a variety of extremely heat-resistant, sporeforming, thermophilic microorganisms. These bacteria are not pathogenic. In low-acid foods, the most common forms of thermophilic spoilage and the causative microorganisms are categorized as follows.

(a) Flat sour spoilage is indicated when the container is not swollen, and when the pH of the product is significantly lowered. The causative microorganisms are sporeformers, such as Bacillus stearothermophilus, a facultative anaerobe.

(b) Thermophilic anaerobe (TA) spoilage is indicated by swelling and, commonly, by the bursting of the container. The condition is caused by obligately thermophilic, sporeforming anaerobes such as Clostridium thermosaccharolyticum. This organism produces large quantities of hydrogen and carbon dioxide. The product usually has a "cheesy" odor.

(c) "Sulfide stinker" spoilage is characterized by a flat container in which the contents are darkened and have the odor for rotten eggs. This type of spoilage is caused by the sporeforming, anaerobic obligately thermophilic microorganism, Clostridium nigrificans. No swelling of the container is produced because the hydrogen sulfide is very soluble in the food. However, it does react with any iron present to form black iron sulfide.

b. Spoilage in Acid Foods. Micro-biological spoilage in acid foods is caused by microorganisms capable of growing at pH 4.6 or lower. Clostridium botulinum does not grow in these products unless abnormal conditions first produce a pH in excess of 4.6. Groups of microorganisms that may be associated with spoilage in acid foods (pH 4.6 or lower) are described as follows.

(1) Sporeforming microorganisms. A variety of acid-tolerant, sporeforming microorganisms may survive processing. They are as follows.

(a) Butyric anaerobes, such as the mesophilic, sporeforming anaerobe Clostridium pasteurianum, which produces butyric acid as well as carbon dioxide and hydrogen.

(b) Aciduric flat sours, especially Bacillus coagulans, in tomato products.

(c) Heat-resistant molds, particularly in the case of the contamination of juice concentrates and fruits by these fungi prior to processing. The causative microorganisms are usually Byssochamys fulva and related or similar species which produce very heat-resistant spores. Spoilage is evidenced by a moldy taste and odor, color fading, the presence of mold mycelia in the product, and sometimes slight swelling of the container lid.

(d) Yeasts and bacteria that do not reproduce by spores (asporogenous) may cause spoilage in cases of grossly insufficient processing. This type of spoilage may be indistinguishable from leaker spoilage unless the containers are thoroughly examined for leakage and structural defects.

(2) Thermophilic microorganisms. Thermophilic (high-temperature) spoilage may occur in acid foods, especially tomato products.

(3) Bacteria, yeast, and/or mold contaminants. Pure or mixed cultures of acid-tolerant bacteria, yeast, and mold contaminants are commonly found in leaker spoilage of acid products. The containers may be swollen or flat. Gas and swelling of the can is commonly produced by bacteria or yeasts and sometimes by molds. Spoilage in flat cans is caused by bacteria (rods and/or cocci) which do not produce gas. A slight lowering of the pH usually occurs. Mold spoilage usually is evidenced by the presence of mycelia and fungal spores in flat containers having a leak large enough to permit entrance of oxygen.

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