Microbial Spoilage

a. General. Milk obtained from the cow may not be sterile. During and after milking, the milk is subjected to organisms from various sources. However, contaminated equipment used to handle, transport, store, and process the milk seems to be the main source of organisms.

b. Examination of Product. You must examine the product for abnormal color(s), abnormal odor(s), abnormal flavor(s), and abnormal texture or consistency which may indicate microbial deterioration. Microbial spoilage may be caused by bacteria, mold, and/or yeasts. The particular color, odor, flavor, or texture/consistency change(s) involved will depend upon the type or species of microorganisms involved. If microbial spoilage is suspected, DO NOT TASTE THE PRODUCT. You should contact the veterinary officer for further examination of the product.

c. Color Changes. The following color changes may indicate microbial spoilage: blue, bluish gray, pink, yellow, red, and brown.

d. Odor and Flavors. The following odors and flavors may be associated with microbial deterioration of dairy products: sour or acid, bitter, burnt or caramel, soapy, malty, rancid, fishy, fruity, oxidized, alcoholic, and putrid. Although bacteria may be responsible for a number of different flavor defects in both raw and pasteurized milk, only those defects described as acid, malty, and fruity can be recognized as being of microbial origin by sensory perception alone. The flavors described as stale, barny, unclean, bitter, foreign, rancid, and feedy (like animal feed) can be caused by bacteria, but determination of the actual cause is often difficult without bacteriological analyses because of the similarity of these flavors to flavors due to other causes.

(1) Acid flavor. Because of the universal distribution of Streptococcus lactis in the environment of milk production, most milk is unintentionally inoculated with this organism immediately after milking. If the milk is not cooled immediately to 4.4°C (40°F) or below, it eventually will develop an acid taste due to proliferation of the organism and its conversion of lactose to lactic acid. The development of lactic acid in milk is accompanied by an odor usually described as sour. Since S. lactis is destroyed by proper pasteurization, acid development in milk subsequent to pasteurization is not likely. However, pasteurization will not improve the flavor of raw milk if acid already has developed.

(2) Malty flavor. A flavor and aroma which in the past has been described as cooked, burnt, caramel, or malty, may develop in raw milk as a result of the metabolism of S. lactis subspecies maltigenes. This organism enters milk through contact with improperly sanitized equipment during production.

(3) Fruity flavor. The aroma, which may develop in pasteurized milk and other processed dairy products as a result of Pseudomonas fragi, has been described as strawberry-like or fruity. P. fragi, a psychrotrophic water and soil organism, is distributed widely in dairy environments. The organism is very heat sensitive, and its presence in pasteurized products is due to post-pasteurization contamination. Strains of Bacillus have also been isolated from milk with a fruity flavor. These spore-forming organisms may be the cause of flavor defects in aseptically packaged "sterilized" milk and fluid milk products.

(4) Unclean, bitter, and putrid flavors. Unclean and bitter flavors may be due to other causes. However, unclean, bitter, and putrid flavors are often caused by growth of psychrophilic organisms in pasteurized milk. The resulting flavor defects usually become evident upon extended storage of milk.

e. Defects of Butter. One of the main defects of butter is rancidity. This can be due to oxidation as well as to microbial growth. Molds are able to grow on the surface of butter. Putrid, proteolytic, and fruity flavors in butters are caused by psychrophilic bacteria. Surface taint and yeasty butter are other defects of this product caused by microorganisms.

f. Defects of Cottage Cheese. In cottage cheese, the flavor defects of milk may be accompanied by a gelatinous or tapioca curd formation. Pseudomonas fragi and Alteromonas putrefaciens have been associated with a slimy curd defect on the surface of cottage cheese. A fruity, putrid, or rancid odor and a fruity or bitter flavor may accompany this defect. Surface discoloration of cottage cheese may occur due to the growth of the pigment Flavobacterium. E. coN can cause barny or unclean flavors and, if the cottage cheese is held at room temperature, the organism can cause a gassy defect. The yeast Rhodotorula produces pink spots which may become a pink slime. Torulopsis also produces a slime, but it is yellow. Geotrichum produces off-white, tan, or yellow surface discolorations.

g. Defects of Italian Cheese. A pink discoloration of Romano and other Italian cheese varieties occurs as a uniform band of color near the cheese surface or as discoloration throughout the entire cheese. This discoloration is due to Lactobacillus helveticus and L. bulgaricus.

h. Summary. A summary of microbial conditions of dairy products may be seen in figure 4-1.



Pasteurized milk

Rancidity, Ropy or Slimy, Sour, Bitter, Fruity

Canned milk

Swelling, Gas


Surface Taint



Cottage cheese

Slimy Curd, Putrid Odor, Gelatinous, Fruity



Figure 4-1. Microbial conditions of dairy products.

Figure 4-1. Microbial conditions of dairy products.

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