Pathogen growth slowed by polylysine and MAP

Polylysine by itself was effective but combination with the CO₂​ packaging method at a higher temperature delivered better results, according to researchers.

The specimens were treated with 2000 ppm and 8000 ppm of polylysine for E. coli and Salmonella.

At an incubation temperature of 4 °C, 4.3 log and 2.4 log reduction in bacterial numbers were observed after 7-day incubation for E. coli O157:H7 and Salmonella, respectively, in polylysine-added beef.

Polylysine was designated generally recognized as safe by the US Food and Drug Administration in 2004 and has shown antimicrobial activity over a wide pH range, said the researchers.

Temperature difference​

When effectiveness of the CO₂​-packaging combined with polylysine was investigated, CO₂​ did not have additional inhibiting effect on bacterial growth compared to only polylysine-treated samples when incubated at 4 °C.

However, effectiveness of CO₂​ was observed when incubated at 10 °C where 2.9 log and 4.4 log reduction in E. coli​ cell numbers were observed in only polylysine-treated samples and polylysine- and CO₂​-treated group.

The temperature of 10 °C was selected as 8 °C is the lowest temperature at which E.coli can grow, according to previous research.   

“With an incubation temperature of 4 °C, the action of polylysine with CO₂​ treatment was not found to be significantly different from that of polylysine treatment alone against ​E. coli​ O157:H7 and ​Salmonella​. ​

“However, in a simulated case of temperature control deficiency at 10 °C, both species of bacteria were found to have proliferated after 5 and 7 days when treated with polylysine alone. When polylysine was used along with CO₂​ exchange, this proliferation was inhibited.”​

A 1.7 log reduction in Salmonella cell numbers were observed in polylysine-treated samples and 3.5 log  decrease in the polylysine and CO₂​-treated group, respectively.

However the researchers warned that the product tested differs from actual marketed beef in the amount of bacteria normally present in public facilities because of sterilization by trimming.

Polylysine resistance​

For the measurement of the minimum inhibitory concentration (MIC) for enterohemorrhagic E. coli, three strains of O157, and five strains of non-O157 were used.

Seven strains of Salmonella, (S.​ Typhimurium ATCC 13311, S. Braenderup T11, S.​ Infantis T12, S.​ Thompson T18, S.​ Montevideo T20, S.​ Hadar T24, and S.​ Anatum T31) were analysed.

Three strains of E. coli O157:H7, and of Salmonella whose MIC measurement results showed resistance to polylysine (S.​ Typhimurium ATCC 13311, S.​ Montevideo T20, and S.​ Anatum T31) were used for the analysis.

On the surface of the 25-g blocks of beef, 10⁹​ CFU/ml preparations of E. coli O157:H7 and Salmonella bacterial suspensions were added in 10-μL aliquots at two locations.

Three strains were dripped onto the blocks, which were left in a safety cabinet for 15 minutes and then immersed in a filter-sterilized polylysine solution (3000 ppm or 5000 ppm for E. coli​, and 8000 ppm for Salmonella​) at 4 °C for 40 minutes, and stored for up to seven days.

Following polylysine treatment, the numbers of E. coli and Salmonella were determined after one, three, five and seven days, with CHROM agar O157 agar and MLCB agar.

Taste barrier​

To investigate the influence of polylysine on the food taste, the 5000 ppm and 8000 ppm polylysine solutions were added to Japanese amberjack sashimi by immersion at 4 °C for 40 minutes, followed by sensory evaluation.

Although 5000 ppm of polylysine solution has a detectable bitterness, there was no substantial change in the taste of the immersed sashimi, so the use of polylysine in raw foods would not be problematic.

At 8000 ppm, a slight bitterness could be sensed in the immersed sashimi but for meats relatively strongly flavored as per yukhoe, it is unlikely to be a problem, said the researchers

Source: Food Control, volume 37, March 2014, pages 62-67

Online ahead of print, DOI: 10.1016/j.foodcont.2013.09.028​

“Development of a controlling method for ​Escherichia coli​ O157:H7 and ​Salmonella​ spp. in fresh market beef by using polylysine and modified atmosphere packaging”​

Authors: Satoko Miya^, ​Hajime Takahashi^, ​Miho Hashimoto^, ​Moemi Nakazawa, Takashi Kuda, Hiroaki Koiso and Bon Kimura