Effects of Citric Acid and Lactic Acid on the Survival of Escherichia Coli O157:H7 Grown On Meat and Carrot

Abstract

Outbreaks of Escherichia coli O157:H7 infections associated with meat, vegetables, fruits and root crops have occurred with increasing frequency in the recent years. This study was carried out to investigate the effects of citric acid and lactic acid on the survival of E. coli O157:H7 grown on meat and carrot. Five hundred grams each of meat and carrot samples obtained from the market were washed with sterile distilled water and then cut into small pieces using a sterile knife. The substrates were divided into 10g and packed in polythene bags and kept inside freezer for 7days after which they were thawed at 4OC for 24 hours before use. The samples were inoculated with E. coli O157:H7 cells at concentration of 105cfu/g. The survival of E. coli O157:H7 cells were determined by dipping the meat and carrot samples separately in solutions containing citric acid and lactic acid at concentrations of 0, 3.0, 3.8, 4.6, 5.4 and 6.2 for the period of 0, 10, 20, 30, 40, 50 and 60 min. The effects of the combination of the two acids were also determined by dipping the two substrates in solution containing both acids at the same concentrations for 0, 10, 20 and 30 min. After acid treatments the numbers of E. coli O157:H7 cells that survived were determined by plate count method. The results show that at pH levels of 6.2 and 5.4, citric acid was not effective (p>0.05) in reducing numbers of E. coli O157:H7 attached to meat and carrot surfaces at dipping times of up to 60 min. However, at pH levels of 4.6, 3.8 and 3.0, citric acid reduced counts of the organism (p>0.05) with increase in dipping time for up to 40 min. The results show that at pH values of 6.2 and 5.4, lactic acid was also ineffective (p>0.05) in reducing numbers of E. coli O157: H7 attached to the meat and carrot surfaces at dipping times of 60 min, while at pH value of 5.4, reduction started occurring at dipping time of 50 min. There was reduced effectiveness of the acids in decontaminating E. coli O157:H7 from the surface of meat compared to that of carrot. Additive interactive rather than synergistic effect were seen when citric acid – lactic acid treatment was applied on both meat and carrot. It is clear from this study that both acids could not completely eliminate E. coli O157:H7 from food samples however citric acid and not lactic acid may have more value as surface decontaminant of food than lactic acid