Effect of acids on the survival of Salmonella attached to chicken meat
thesisposted on 02.03.2017 by Tan, Sin Mei
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Salmonella is one of the most common pathogens of concern on poultry meat and products. Research on effective interventions to reduce Salmonella on these products is of significant current research interest. Marination is one such intervention which has been suggested to enhance the safety of meat by inhibiting the growth of microorganisms. The buffering effect of poultry meat may, however, neutralize the acidic pH of marinades and nullify their antimicrobial effects against Salmonella. There is limited literature on the influence of buffering effect on the survival of Salmonella attached to marinated chicken skin and meat. The first part of this project was an investigation into the buffering effect of chicken skin and meat with particular reference to the role it plays in protecting Salmonella against acidic pH induced by HCl. The results indicated that chicken meat buffered better than chicken skin and that the buffering effect of chicken skin and meat protected four strains of Salmonella against pH stress. Since the presence of organic acids specifically (in addition to pH alone) play an important role in the antimicrobial activity of marinades on meat, the effect of organic acids on the survival of Salmonella on chicken skin and meat was also examined. Of four acids examined, acetic acids resulted in the highest reduction of viable count of Salmonella on chicken (in a range of 5.79 - 10.63 log CFU/ml(or g)/pH-unit; p < 0.001), followed by citric acid (4.08 - 7.80 log CFU/ml(or g)/pH-unit, p < 0.05), lactic acid (4.92 - 8.79 log CFU/ml(or g)/pH-unit, p < 0.05) and hydrochloric acid (HCl; 2.92 - 6.52 log CFU/ml(or g)/pH-unit, p < 0.001). Results also showed that the buffering effect of chicken protecting Salmonella against HCl did not provide sufficient protection in the case of organic acids. This indicated that marinades with organic acids should be effective in reducing Salmonella on chicken despite the buffering effect of chicken skin and meat. In the second part of this study the role played by fat in chicken skin and meat play on their buffering capacity was investigated. In addition, the survival of Salmonella attached to chicken skin and meat with or without fat and treated with acetic acid was determined. Results showed that chicken skin has a higher fat content as compared to chicken meat. The extracted fat and skin remnants (without fat) did not have a strong buffering capacity (7.0 mmol H+/ (pH*kg) and 6.9 mmol H+/ (pH*kg) respectively; p > 0.05). When the components occurred together, however, a strong buffering capacity (13 mmol H+/ (pH*kg); p < 0.05) was observed. This indicates that fat contributes to some extent to the buffering capacity of chicken skin and meat. However, Salmonella Typhimurium ATCC 33062 was better protected when attached to skin remnants without fat (~3.5 log CFU/g; p < 0.05) from acetic acid treatment than skin (no viable count), extracted fat (no viable count), meat remnants (~1.5 log CFU/g; p < 0.05) and meat (~2.5 log CFU/g; p < 0.05). Salmonella cells attached to chicken skin (with fat) were more susceptible to acetic acid than those attached to skin remnants (without fat). It is suggested that fat in chicken skin may enhance the vulnerability of attached cells to acetic acid treatments. The third part of this project was carried out with the objective of investigating the antimicrobial mode of action of acetic acid against Salmonella with respect to cellular membrane disruption and the role of undissociated acid molecules in cellular energy depletion. It was established using a nucleic acid/protein assay and a SEM study that acetic acid did not cause leakage of intracellular components from the Salmonella strains. Some elongated cells observed in the micrographs indicated a possibility of acetic acid may inhibit DNA synthesis in Salmonella cells. Using an ATP assay it was found that at a neutral pH acetic acid caused energy depletion with ADP/ATP ratio in the range between 0.48 to 2.63 (p < 0.05) for four strains of Salmonella, probably due to the action of undissociated acid molecules. The antimicrobial effect of acetic acid was better under acidic conditions (ratio of 5.56 ± 1.27; p < 0.05) where the role of both acidic pH and undissociated acid molecules may act together. It was concluded that the inhibitory effect of acetic acid is not solely attributable to acidic pH but also the undissociated acid molecules. Taken together the results of this project suggested that marinades or other acetic acid treatments should be effective against Salmonella on chicken because of the acetic acid induced-energy depletion and inhibition of DNA synthesis in the bacterial cells. This should happen regardless of the buffering effect of skin and meat on the acidic pH. The fat in chicken skin and meat was suggested to support the antimicrobial activity of acetic acid against Salmonella attached to them. The presence of fat together with the application of organic acids may contribute to an effective intervention in reducing Salmonella on poultry meat.