New Food Spoilage Bacteria in Refrigerated Foods ◾ 281
The lactic acid bacteria as well as the Gram-negative species isolated from these samples were
heat-sensitive psychotrophic and facultative anaerobic. They definitely entered the product as postheat contaminants, indicating poor sanitation in the production facilities. The absence of large
amounts of gas in this product, as opposed to some products described before, could be a result of
the presence of very little metabolizable carbohydrate. The reason for the pH to remain unchanged
(~pH 6.0) during deamination of amino acids could be the use of high amounts of phosphate in
the formulation to bind water.
Gas Distension and Pink Discoloration of Sliced Turkey Rolls
Sliced low-fat turkey rolls (initial pH 6.5) were packaged by modified atmosphere (drawing vacuum and then flushing a mixture of 30% CO 2 and 70% N 2 in small amounts) so that the packages
remained slightly loose. The product was expected to have an eight-week shelf life at refrigerated
temperatures. By five weeks, many packages developed gross distension with gas, little to moderate
amounts of purge, and pinkish-red discoloration from the normal gray-white color of the breast
meat (Figure 21.1e). 8 The intensity of the color was more inside the slices than at the rim. The gas
had an off-odor (like wet socks), but no H 2 S was detected. Soon after opening a bag, the color of
the meat slices changed to normal. Vacuum packaging the meat slices again changed the color in
8–12 hours to pinkish red. Addition of ascorbic acid to the normal colored meat slices gave the
similar pinkish-red color quickly (in 1–2 hours). Examination of the liquid under a phase-contrast
microscope revealed leuconostoc- and lactobacilli-like cells as well as motile rods. Colony enumeration revealed the presence of lactic acid bacteria at approximately 10 7–8 /g and Enterobacteriaceae
at approximately 10 6 /g. While lactic acid bacteria were predominantly Leu. carnosum (heterofermentative) and Lab. curvatus (homofermentative), Enterobacteriaceae were Ser. liquifaciens.
Inoculation studies revealed that Ser. liquifaciens was important for the off-flavor. The pink color
was a result of formation of reduced metmyoglobin; however, it is not clear how it formed. It seems
that psychrotrophic Gram-positive and Gram-negative bacteria, entering the product as post-heat
contaminants, grew during storage and reduced the O-R potential of the products. This changed
the metmyoglobin from a reversible oxidized to a reduced state and the color from gray-white to
pink. In air (oxygen), metmyoglobin reversed from a reduced to an oxidized state, changing the
color from pink to gray-white. Absorption spectrum studies with a spectrophotometer of extracted
materials also supported the conclusion that the pink color was the result of a reduced form of
metmyoglobin. A large amount of gas was produced by Leu. carnosum from the metabolism of
glucose used in the formulation. Because the product had fairly large amounts of phosphate, pH
did not change, even from the growth of lactic acid bacteria; thus, there was only a small amount
of purge present in the bags.
Examination of the production facility revealed that the cooked products were contaminated
by these bacteria mainly from the slicer and the conveyors between the slicer and the packaging
machine. However, the initial contamination level was very low (<10/g). High numbers were reached
during storage. It was also revealed by the processor that during distribution from the warehouse to
stores in a van, the product temperature could be as high as 50°F–55°F (10°C–12°C) for 6–8 hours;
this could also result in rapid growth of psychrotrophs, causing reduction in shelf life.
Gas Distension (Bloating) of Ground Beef Chubs
Vacuum-packaged, low-fat ground beef chubs were found to accumulate large quantities of gas
within a few days of refrigerated storage (Figure 21.1f ). Initially (within one week of storage), there
The lactic acid bacteria as well as the Gram-negative species isolated from these samples were
heat-sensitive psychotrophic and facultative anaerobic. They definitely entered the product as postheat contaminants, indicating poor sanitation in the production facilities. The absence of large
amounts of gas in this product, as opposed to some products described before, could be a result of
the presence of very little metabolizable carbohydrate. The reason for the pH to remain unchanged
(~pH 6.0) during deamination of amino acids could be the use of high amounts of phosphate in
the formulation to bind water.
Gas Distension and Pink Discoloration of Sliced Turkey Rolls
Sliced low-fat turkey rolls (initial pH 6.5) were packaged by modified atmosphere (drawing vacuum and then flushing a mixture of 30% CO 2 and 70% N 2 in small amounts) so that the packages
remained slightly loose. The product was expected to have an eight-week shelf life at refrigerated
temperatures. By five weeks, many packages developed gross distension with gas, little to moderate
amounts of purge, and pinkish-red discoloration from the normal gray-white color of the breast
meat (Figure 21.1e). 8 The intensity of the color was more inside the slices than at the rim. The gas
had an off-odor (like wet socks), but no H 2 S was detected. Soon after opening a bag, the color of
the meat slices changed to normal. Vacuum packaging the meat slices again changed the color in
8–12 hours to pinkish red. Addition of ascorbic acid to the normal colored meat slices gave the
similar pinkish-red color quickly (in 1–2 hours). Examination of the liquid under a phase-contrast
microscope revealed leuconostoc- and lactobacilli-like cells as well as motile rods. Colony enumeration revealed the presence of lactic acid bacteria at approximately 10 7–8 /g and Enterobacteriaceae
at approximately 10 6 /g. While lactic acid bacteria were predominantly Leu. carnosum (heterofermentative) and Lab. curvatus (homofermentative), Enterobacteriaceae were Ser. liquifaciens.
Inoculation studies revealed that Ser. liquifaciens was important for the off-flavor. The pink color
was a result of formation of reduced metmyoglobin; however, it is not clear how it formed. It seems
that psychrotrophic Gram-positive and Gram-negative bacteria, entering the product as post-heat
contaminants, grew during storage and reduced the O-R potential of the products. This changed
the metmyoglobin from a reversible oxidized to a reduced state and the color from gray-white to
pink. In air (oxygen), metmyoglobin reversed from a reduced to an oxidized state, changing the
color from pink to gray-white. Absorption spectrum studies with a spectrophotometer of extracted
materials also supported the conclusion that the pink color was the result of a reduced form of
metmyoglobin. A large amount of gas was produced by Leu. carnosum from the metabolism of
glucose used in the formulation. Because the product had fairly large amounts of phosphate, pH
did not change, even from the growth of lactic acid bacteria; thus, there was only a small amount
of purge present in the bags.
Examination of the production facility revealed that the cooked products were contaminated
by these bacteria mainly from the slicer and the conveyors between the slicer and the packaging
machine. However, the initial contamination level was very low (<10/g). High numbers were reached
during storage. It was also revealed by the processor that during distribution from the warehouse to
stores in a van, the product temperature could be as high as 50°F–55°F (10°C–12°C) for 6–8 hours;
this could also result in rapid growth of psychrotrophs, causing reduction in shelf life.
Gas Distension (Bloating) of Ground Beef Chubs
Vacuum-packaged, low-fat ground beef chubs were found to accumulate large quantities of gas
within a few days of refrigerated storage (Figure 21.1f ). Initially (within one week of storage), there