Fruit extracts reduce bacterial levels.

A variety of antimicrobial compounds are naturally present in plant sources. Among these are fruit extracts that reportedly possess antimicrobial properties.

With this in mind, researchers at Kansas State University determined the antimicrobial impact of seven fruit extracts on four foodborne pathogens: E. coli O157:H7 ATCC 35150, S. enteritidis USDA-FSCS 15060, L. monocytogenes 19115 and S. aureus ATCC 25178. The testing was done in a nutrient broth (NB) medium.

The investigators dissolved each commercial fruit extract--blueberry, cranberry, elderberry, concord grape, red raspberry, strawberry and red sour cherry--in powder form in 10-mL NB tubes. They made a fruit solution of 5% w per v. A cocktail of pathogens was inoculated into fruit solutions at 5-log cfu per mL. After incubation at 35 C for 0, 3, 6, 12 and 24 hours, samples were serially 10-fold diluted.

The proper diluent was spread-plated on PCA and pathogen-specific agar in duplicate. After incubation at 35 C for 24 hours to 36 hours, researchers obtained viable cell counts. The experiment was replicated three times. For total pathogenic bacteria on PCA, cranberry and strawberry yielded a greater than 5-log reduction of bacterial levels, and blueberry caused a 4.5-log reduction, compared to the control at 24 hours. Strawberry showed the highest antimicrobial activity against E. coli O157:H7 on MSA by achieving a 6.7-log reduction of bacterial levels. Cranberry caused a 5-log reduction, and blueberry, a 4.3-log reduction at 24 hours.

Both cranberry and strawberry showed the highest microbial reduction capability against S. enteritidis on XLD by yielding about a 6.4-log reduction. Blueberry had a 4.6-log reduction at 24 hours. Against both L. monocytogenes on MOX and S. aureus on BP, blueberry, elderberry, concord grape, red raspberry and red sour cherry caused a 3-log to 4-log reduction in microbial levels. Cranberry and strawberry achieved about a 5-log reduction at 24 hours.

Although research is still ongoing, it appears that extracts of cranberry and strawberry at 5% levels had the highest activity against all test cultures in BHI medium.

Further information. Daniel Fung, Department of Animal Sciences and Industry, Kansas State University, 225 Call Hall, Manhattan, KS 66506; phone: [phone]; fax: [phone]; email: [e-mail].