Freeze-dried preparations of bacteriocins inhibit L. monocytogenes, S. aureus.

There has been a continuing movement to produce and consume ready-to-eat and more natural foods through the use of fewer chemicals. This shift to more natural foods has generated a great interest in using bacteriocins from lactic acid bacteria as biopreservatives.

Scientists at the University of Idaho found that inexpensive dairy-based media can be used to produce bacteriocins. In addition, freeze-drying is a viable option for concentrating bacteriocin preparations for use in dairy products stored at refrigeration temperatures.

The researchers identified certain bacteriocins that can be produced in inexpensive dairy-based media and then concentrated them using a freeze-drying process. These bacteriocins can be used in dairy products to inhibit L. monocytogenes and S. aureus. The scientists used L. monocytogenes V7 1/2a, S. aureus ATCC 25923, Lactobacillus sake 15521, and Lactobacillus plantarum NCDO 995 as indicators. The agar-well-diffusion method was used to detect bacteriocins.

Another technique used by researchers to detect bacteriocin activity involved the growth inhibition of indicators inoculated into cell-free culture supernatants of bacteriocin producers. Freeze-dried bacteriocin powders were prepared by growing bacteriocin producers in dairy-based media, freezing the cell-free culture supernatants at -70 C, -85 C or -196 C, and then freeze-drying the samples.

The researchers inoculated cheddar cheese samples with a known number of indicator cells. The inoculated samples were placed into a buffered suspension of freeze-dried bacteriocin powders, incubated at 4 C for 24 hours to 72 hours, and plated onto appropriate selective media.

Cheddar cheese whey supported bacteriocin production better than did demineralized whey powder. When grown in cheddar cheese whey, the incubation time for maximum bacteriocin production for Lactococcus lactis subsp. lactis ATCC 11454, Lactobacillus curvatus LTH 1174, Lactococcus lactis GI3 and Lactococcus lactis BFE 920 reached 24, 24 to 30, 16 to 18, and 16 to 24 hours, respectively.

All freeze-dried bacteriocin preparations were active against L. monocytogenes and S. aureus indicator strains. Bacteriocin preparations that were frozen at -196 C before freeze-drying appeared to be the most active.

Further information. Gulhan Yuksel, Department of Food Science and Toxicology, University of Idaho, Agricultural Biotechnology Laboratory, Room 205, Moscow, ID 83844; phone: [phone]; fax: [phone]; email: [e-mail].