Growth of Indicator, Pathogenic and Psychrotrophic Bacteria in Mechanically Separated Beef, Lean Ground Beef and Beef Bone Marrow1

Abstract

Growth of Escherichia coli, Salmonella anatum, Staphylococcus aureus, Clostridium perfringens and naturally occurring psychrotrophs in mechanically separated beef (MSB), lean ground beef (LGB) and beef bone marrow (BBM) was studied. Six good grade steers were slaughtered and samples of MSB, LGB and BBM were prepared under proper sanitary care. Six hundred grams of each sample were collected; 100 g were used for chemical analysis. The remaining 500 g were divided into 10-g portions, each mixed with 10 ml of water and either frozen to −20°C or used immediately for bacteriological analysis. For growth studies, samples were inoculated with E. coli or one of the pathogens, incubated at 37°C up to 24 h and enumerated for colony forming units (CFU) on specific selective agar plates. During the first 8 h of incubation, E. coli and S. anatum multiplied rapidly in MSB and LGB but rather slowly in BBM. By 24 h, both species had multiplied to the same population level. Initial growth of S. aureus was rapid in MSB and LGB, but by 24 h its number was higher in LGB than in MSB or BBM. C. perfringens grew faster in LGB and slower in BBM during the 24-h period. Growth of psychrotrophs was determined by incubating the materials at 10, 7 and 3°C up to 10 d. The psychrotrophs grew fastest in MSB and slowest in LGB at all three temperatures, especially at 3°C. Rapid growth of various bacteria in MSB should be considered in its production, storage and subsequent use.