Microbial Spoilage

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a. Characteristics of Deterioration in Food. The microbial deterioration of a food is usually manifested by alterations in the appearance, texture, color, odor, or flavor or by slime formation. Alterations in appearance include color changes, formation of pockets of gas or swelling, and microbial growth (especially molds). As some meat products deteriorate, they tend to become soft or mushy. Degradation of food results in the formation of compounds which have odors and flavors different from those of the fresh food.

b. Microbial Spoilage. Examine the product for abnormal color(s), odor(s), and texture(s) which may indicate microbial deterioration. Microbial spoilage may be caused by yeast, mold, and/or bacteria. The particular color, odor, or texture change will depend upon the type or species of microorganisms involved.

c. Color Changes. The following color changes may indicate microbial spoilage: red spots; blue, green, purple, and yellowish discoloration; green core (sausage); greenish-blue to brownish-black spots; abnormal pink, cream color, or white coloration.

d. Particular Odors. The following odors have been associated with microbial deterioration: tainted, gassy, sour, cold storage flavor, musty or earthy, putrid, and acid.

Poultry products may also exhibit a dishrag odor, similar to the smell of a dirty dishrag, usually a strong, sour odor.

e. Texture Changes. The following texture changes are associated with microbial spoilage: surface slime, stickiness, whiskers (fuzzy growth), spongy texture, surface growth, swelling (of vacuum-packed meats), and cloudiness.

f. Tasting the Product. If microbial spoilage is suspected, DO NOT TASTE THE PRODUCT. The veterinary officer should be contacted for further examination of the product.

g. Growth of Microorganisms. Since fresh animal products are perishable, they are chilled and stored in ice or a refrigerator at 0° to 4°C (32° to 39°F). This means that psychrophilic organisms (organisms capable of growing at refrigeration temperatures) become dominant. Microorganisms are the primary cause of spoilage in meat and meat products. Spoilage of meat is due to the growth and metabolism of large numbers of microorganisms on the surface or the interior. Most spoilage is on the surface. The number of organisms that are present when spoilage is evident varies from 106 to 108 per cm2 of meat surface. This variation apparently is due to the activity of the organisms present, as well as the criteria used by the investigators to determine spoilage.

h. Spoilage Organisms. The temperature of the meat is perhaps the most important factor that determines the predominant microflora and the resultant spoilage. At temperatures of 50°C (122°F) or higher, thermophilic bacteria can grow. Usually these bacteria cause spoilage of heat-processed meats that have been improperly handled during processing. Intermediate temperatures (15° to 30°C) (59° to 86°F) may occur when the chilling of meat is slow, delayed, or the meat is held in this temperature range. In these cases, the main spoilage organisms are the mesophilic clostridia. They cause internal spoilage or bone taint in beef. At low temperatures (0°C, 32°F), spoilage is evidenced on the surface. Under storage conditions which foster a moist meat surface, spoilage is due to gram-negative bacteria, especially Pseudomonas species.

i. Water as a Factor. The water activity of the product can play a role in microbial spoilage. Water activity (aw) is defined as the ratio of the water vapor pressure of the food substance to the vapor pressure of pure water at the same temperature. This is an index of the availability of water for chemical reactions and microbial growth. At an aw of 0.96 or less, most of the usual microorganisms causing spoilage of fresh meat are inhibited. When the surface of meat has an aw lower than 0.96, the slower-growing fungi become evident. The conditions seen will be "whiskers" caused by Thamnidium, black spot caused by Cladosporium, and white spot caused by Sporotrichum. Generally, the activity of molds is limited to the outer surface of meat where aerobic conditions prevail.

j. Odor and Slime. The main conditions of microbial spoilage in poultry are off-odor (which appears at a bacterial load between 106 to 108 per cm2) and slime formation (which occurs soon after off-odor is noted). Slime is the descriptive term which describes the massive accumulation of microbial cells on the product surface as the result of extensive growth. Species of Pseudomonas are the principal spoilage organisms of poultry. Some of the chemical compounds found on spoiled chicken included H2S (hydrogen sulfide), acetone, methanol, and many other odoriferous compounds.

k. Gas Formation. The development of gas in cured meats is occasionally encountered, especially in the manufacture of fermented sausages. Lactic acid bacteria (lactobacilli and leuconostocs) and yeasts produce copious quantities of carbon dioxide from the fermentation of the added sugar. This gas produces large numbers of pinholes in sausage and a constant swelling of the product. Burst casings are often encountered. Gassiness in hams and frankfurters is also observed. Among frankfurters, the effects become dramatic when vacuum packages are used. The packages retain the gas as it diffuses from the product and may become inflated.

l. Summary of Microbial Conditions. The various microbial conditions seen in meat and meat products are summarized in figure 2-1.

PRODUCT

CONDITION(S)

Fresh red meats

Off-odor, Sliminess (stickiness), Discoloration, Moldiness, Whiskers, White spot, Black spot, Bone taint, Gassiness, Souring

Vacuum packaged (fresh)

Acidity, Sweetness, Rancidity

Bacon

Cheesy texture, Souring, Rancidity, Discoloration, Slime formation

Vacuum packaged (cured)

Cabbage odor Taint

Ham

Surface slime, Gassiness or puffiness, Green discoloration Bone and meat sours

Sausages

Slime on surface, Gas production, Greenish discoloration

Fermented sausage

Slime Spots (discoloration)

Canned meats

Gas, Putrefaction, Souring, Discoloration

Poultry

Off-odor Slime

Vinegar-pickled meats

Cloudy or ropy brine

Figure 2-1. Microbial conditions of meat and meat products.

Figure 2-1. Microbial conditions of meat and meat products.

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