New Food Spoilage Bacteria in Refrigerated Foods ◾ 279
liquid showed the presence of large numbers of motile rods, many with terminal oval spores. The
strain was isolated in pure form and found to have some biochemical and physiological characteristics different from those of Clo. laramie NK 1 because it could be grown in agar medium under
anaerobic conditions. However, it is psychrophilic and does not grow at 25°C or above.
Spoilage of Unripened Soft Cheese by Leuconostoc Species
Spanish-style soft cheese, made by coagulating pasteurized milk with lactic acid and rennet and
collecting the curd and vacuum packaging, showed gas formation and liquid accumulation in
the package within 30 days at refrigerated temperature. (The expected shelf life is 50+ days.) 8 The
curd texture was soft and gassy. Examination of the liquid under a phase-contrast microscope
revealed short chains of leuconostoc-like cells. Enumeration in pH 5.0 plating media revealed
10 8–9 CFUs/g product. Purified cells from the colonies were Gram-positive, heterofermentative,
and biochemically identified as Leuconostoc spp. The bacteria possibly got in the product during
extensive handling of the product following pasteurization of milk. Even when they are initially
present at a low level, during long storage they can multiply, form gas, and produce the defect. Any
temperature abuse accelerates the spoilage.
Spoilage of Low-Heat-Processed Meat Products by Leuconostoc Species
Many types of commercially processed, vacuum-packaged refrigerated meat products, following spoilage, were collected from local supermarkets. The products were restructured ham steak,
chopped and formed ham, chopped sliced ham, chunked and formed turkey ham, cooked luncheon meat (sliced), several brands of frankfurters (beef, chicken, and turkey), wieners, cooked
and smoked Polish sausage, summer sausage, and hot-link sausage. The common symptoms were
accumulation of large quantities of gas and cloudy purge (Figure 21.1c,d). 5–8 The products did not
have any disagreeable odor. The pH ranged from 5.0 to 6.0. Some were low-fat products. Phasecontrast microscopy of the purge revealed predominantly the presence of leuconostoc-like cells.
Colony enumeration of the purge revealed lactic acid bacteria counts of approximately 10 8–9 / mL
but lower APCs. Bro. thermosphacta, when present, was <10 3 /mL. Biochemical tests revealed that
the predominant lactic acid bacteria were Leu. carnosum and Leu. mesenteroides, and many could
produce bacteriocins. It was suspected that bacteriocin-producing leuconostocs, by inhibiting the
growth of other Gram-positive bacteriocin-sensitive contaminants, grew preferentially and produced CO 2 and acid and were responsible for gross distension (or bloating) of the packs and release
of liquid (purge). Most likely, the leuconostocs contaminated the products as post-heat contaminants from the equipment and other sources during handling before vacuum packaging. 11
Ammonia Odor in Turkey Roll
Low-fat (2%), high-pH (6.5) vacuum-packaged turkey breast rolls showed gas and liquid accumulation with a strong ammonia odor (diaper smell) when opened. 8 Examination of the liquid
under a phase-contrast microscope showed predominantly leuconostoc-like cells as well as motile
single or small-chain rods and some lactobacilli-like cells. Purification and Gram staining revealed
the leuconostoc- and lactobacilli-like cells to be Gram-positive and the motile rods to be Gramnegative. APCs and lactic acid bacteria counts of liquid were approximately 10 8 /mL. Biochemical
analysis revealed the Gram-negative species to be Ser. liquifaciens, and the Gram-positive bacteria
were Leu. mesenteroides and Lab. sake. It was suspected that these predominant bacterial types
liquid showed the presence of large numbers of motile rods, many with terminal oval spores. The
strain was isolated in pure form and found to have some biochemical and physiological characteristics different from those of Clo. laramie NK 1 because it could be grown in agar medium under
anaerobic conditions. However, it is psychrophilic and does not grow at 25°C or above.
Spoilage of Unripened Soft Cheese by Leuconostoc Species
Spanish-style soft cheese, made by coagulating pasteurized milk with lactic acid and rennet and
collecting the curd and vacuum packaging, showed gas formation and liquid accumulation in
the package within 30 days at refrigerated temperature. (The expected shelf life is 50+ days.) 8 The
curd texture was soft and gassy. Examination of the liquid under a phase-contrast microscope
revealed short chains of leuconostoc-like cells. Enumeration in pH 5.0 plating media revealed
10 8–9 CFUs/g product. Purified cells from the colonies were Gram-positive, heterofermentative,
and biochemically identified as Leuconostoc spp. The bacteria possibly got in the product during
extensive handling of the product following pasteurization of milk. Even when they are initially
present at a low level, during long storage they can multiply, form gas, and produce the defect. Any
temperature abuse accelerates the spoilage.
Spoilage of Low-Heat-Processed Meat Products by Leuconostoc Species
Many types of commercially processed, vacuum-packaged refrigerated meat products, following spoilage, were collected from local supermarkets. The products were restructured ham steak,
chopped and formed ham, chopped sliced ham, chunked and formed turkey ham, cooked luncheon meat (sliced), several brands of frankfurters (beef, chicken, and turkey), wieners, cooked
and smoked Polish sausage, summer sausage, and hot-link sausage. The common symptoms were
accumulation of large quantities of gas and cloudy purge (Figure 21.1c,d). 5–8 The products did not
have any disagreeable odor. The pH ranged from 5.0 to 6.0. Some were low-fat products. Phasecontrast microscopy of the purge revealed predominantly the presence of leuconostoc-like cells.
Colony enumeration of the purge revealed lactic acid bacteria counts of approximately 10 8–9 / mL
but lower APCs. Bro. thermosphacta, when present, was <10 3 /mL. Biochemical tests revealed that
the predominant lactic acid bacteria were Leu. carnosum and Leu. mesenteroides, and many could
produce bacteriocins. It was suspected that bacteriocin-producing leuconostocs, by inhibiting the
growth of other Gram-positive bacteriocin-sensitive contaminants, grew preferentially and produced CO 2 and acid and were responsible for gross distension (or bloating) of the packs and release
of liquid (purge). Most likely, the leuconostocs contaminated the products as post-heat contaminants from the equipment and other sources during handling before vacuum packaging. 11
Ammonia Odor in Turkey Roll
Low-fat (2%), high-pH (6.5) vacuum-packaged turkey breast rolls showed gas and liquid accumulation with a strong ammonia odor (diaper smell) when opened. 8 Examination of the liquid
under a phase-contrast microscope showed predominantly leuconostoc-like cells as well as motile
single or small-chain rods and some lactobacilli-like cells. Purification and Gram staining revealed
the leuconostoc- and lactobacilli-like cells to be Gram-positive and the motile rods to be Gramnegative. APCs and lactic acid bacteria counts of liquid were approximately 10 8 /mL. Biochemical
analysis revealed the Gram-negative species to be Ser. liquifaciens, and the Gram-positive bacteria
were Leu. mesenteroides and Lab. sake. It was suspected that these predominant bacterial types