Fundamental Food Microbiology, Third Edition - Fuad Fathir

NEW FOOD SPOILAGE BACTERIA IN REFRIGERATED FOODS 301
10 7 /g). However, levels of coliforms and Enterobacteriaceae decreased during storage.
Interestingly, fresh samples had ca. 6 ¥ 10 2 /g Proteus, but 24-d-old samples did
not have any in a 10 –1 dilution. It was suspected that cells of Enterobacteriaceae,
including Proteus sp., were killed during storage by the combined action of 1.8%
lactate, 1.2% salt, and 30�F (–1.1°C). Their proteolytic enzymes (exocellular and
endocellular) were able to break down proteins and sulfur-containing amino acids
to produce a small amount of H 2S associated with egg odor. To overcome such
problems, an important objective will be to keep the level of Enterobacteriaceae,
as well as the level of Gram-negative bacteria, very low.
N. Off-Odor in Frozen Chicken Meat Product
Skinless and boneless chicken thigh meat was diced and marinated (the mixture
contained different ingredients, including 3% sodium lactate and 1.8% salt). The
products (pH 6.3) were vacuum packaged and frozen to –20�C. The product,
expected to have a shelf life of 90 d or more, is thawed by customers immediately
before use. When thawed during storage, the products gave an off-odor, which
increased in intensity with storage period.
Vacuum-packaged meat samples consisting of frozen unmarinated and marinated
and frozen stored for 25 d and 100 d were enumerated for the levels of several
bacterial groups by the methods described in Section IV-M). The packages of the
marinated products were loose and the thawed meat had an off-flavor, which was
distinct in the 100-d stored products. The results (Table 20.2) revealed that unmarinated
product had low levels of four bacterial groups. In contrast, the 25-d stored
products had high levels of all four groups. In the 100-d storage products, the counts
of all four bacterial groups dropped by two to three log cycles.
It was suspected that the unmarinated product was heavily contaminated during
the process involving marination. Many of these were Gram-negatives (psychrotrophs,
coliforms, Enterobacteriaceae). During frozen storage, in the presence of high
lactate and salt, many of them were killed. Some of their enzymes were able to
catalyze reactions slowly at –20�C and caused slight proteolysis (and maybe lipolysis),
producing flavor compounds that could be identified after the meat was thawed
(see Chapter 21). To overcome the problem, it will be important to adopt good
sanitary practices and proper temperature control during processing and before
freezing the product.
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Table 20.2 Change in CFUs/g of Several Bacterial Groups in Frozen Chicken Meat
Product during Storage
Days at
CFUs/g
Meat Types –20����C
a
APC Psychrotrophs Coliforms Enterobacteriaceae
Unprocessed 7 1 ¥ 104 3 ¥ 104 5 ¥ 101 2.8 ¥ 103 Processed 25 1 ¥ 107 1.9 ¥ 107 1.1 ¥ 104 1 ¥ 105 Processed 100 4 ¥ 104 3.2 ¥ 104 1.4 ¥ 102 1.1 ¥ 102 a Average of two or more samples.