Effect of a Methanol-Acetone Extract of Skim Milk Fermented by Streptococcus thermophilus on Color, Microbial Counts, and pH of Fresh Beef

Abstract

Fresh beef steaks were treated with two concentrations of a methanol-acetone (MA) extract ( Streptococcus thermophilus -fermented skim milk) for color stability, pH change, and microbial growth at storage temperatures of 0 and 10°C. Beef color was measured by a color difference meter (Gardner) and a sensory panel. The Gardner "a/b" values for the untreated samples (control) were significantly higher (P<0.05) than values for treated samples (500 and 1000 ppm MA) through 9 d of storage (0°C). At 15 d of storage (0°C) there were no significant differences between treatments (P>0.05). The "a/b" values at 10°C indicated a significant difference (P<0.05) after 3 d of storage between untreated and treated samples. After 6 d of storage (10°C), no significant difference (P<0.05) was found between untreated and treated meat. Sensory panel scores indicated that both untreated and treated samples were of an acceptable quality through 9 d of storage (0°C); however, the untreated samples were of inferior color quality compared to treated (P< 0.05). All samples stored at 10°C deteriorated to an unacceptable color quality between 3 and 6 d. The growth of Pseudomonas fluorescens on beef steaks was significantly higher (P<0.05) in the untreated samples (by 1 to 2 log₁₀ cycles) than the treated (0°C, 15-d storage). The same pattern was observed at 10°C; however, spoilage was apparent after 6 d of storage while spoilage was apparent after 9 d at 0°C. The pH change, between 0 and 3 d of storage, was not significant (P>0.05) at either storage temperature (0°C and 10°C). From 3 through 15 d, the pH increased from 5.3 to 6.1, with no significant difference (P>0.05) between treatments (after 15 d of storage, 0°C). At 10°C, no significant difference (P>0.05) was observed between treatments (after 6-d storage). Results from this experiment indicate that the MA extract may have some practical use in extending the shelf life of fresh beef by reducing factors that are responsible for color deterioration, e.g., microbial activity. However, it seems likely that higher concentrations of the MA extract would be necessary to control microbial activity at higher storage temperatures.