A study of the antimicrobial activity of combined black pepper and cinnamon essential oils against Escherichia fergusonii in traditional African yoghurt

Abstract

The antimicrobial activity of the essential oils of black pepper (BPE) and cinnamon bark (CE) extracts against E. fergusonii was assessed in pasteurized full cream milk during and post-fermentation. The milk was fermented with 1% (v/v) of Lactobacillus delbrueckii subspecies bulgaricus (NCIMB 11778) and Streptococcus thermophilus (NCIMB 10387) (approx. 106 cfu/mL each) and incubated and stored at 25 °C for 5 days (144 h) or at 43 °C for 24 h and then stored at 25 °C for 120 h. The milk was spiked with E. fergusonii at the start of fermentation by the lactic acid bacteria (pre-fermentation contamination) for after fermentation (post fermentation contamination). BPE and CE were applied at concentrations based on their minimum inhibitory concentration of 0.5% and 0.25% respectively as follows: 0.5% BPE alone; 0.125% BPE with 0.1875% CE; 0.25% BPE with 0.125% CE; 0.375% BPE with 0.0625% CE; 0.25% CE alone. Results showed that during fermentation at 25 °C, E. fergusonii grew to a similar level (approx. 109 CFU/mL) in control samples and 108 CFU/mL when BPE or CE were added alone. Whereas, in the samples with the combined essential oils, the bacterium grew to 106–107 CFU/mL only. During the milk fermentation at 43 °C, E. fergusonii grew to approx. 109 CFU/mL in samples without treatment. However, it was not detected in samples containing mixed BPE with CE after 8, 10 and 12 h of fermentation. Subsequent storage at 25 °C resulted in undetectable levels of the bacterium in all the samples treated with BPE or CE after 24 h of storage. These results indicated that BPE in combination with CE reduced growth during fermentation and was bactericidal during storage.