2009 Annual Report
1a.Objectives (from AD-416)
The overall objective of this research is to develop methods to prevent the growth of pathogenic and spoilage microorganisms in minimally preserved, brined, and fresh-cut foods and optimizing safety, quality, and sensory attributes specifically through:.
1)development of fundamental knowledge of the biochemistry of bacterial adaptation to acidified environments;.
2)determination, through analysis of gene/protein expression profiles, the responses to intrinsic and extrinsic stressors and, in particular, the effect of oxygen imposed on pathogenic bacteria during production, processing, and storage of acid and acidified foods;.
3)development of data and its use for the development of mechanistic models for growth, survival and inactivation of pathogens.
1b.Approach (from AD-416)
The effect of common food acids and acid preservatives will be evaluated for their relative ability to enhance killing of acid-tolerant food pathogens, particularly Escherichia coli O157:H7, in the absence of oxygen and independent of pH. Work will be carried out at biosafety level 2 (BSL-2) due to the organisms under investigation. Selected acid/pathogen strain combinations will be analyzed using genetic and biochemical analyses to determine the mechanisms by which acids are responsible for killing E. coli and other pathogens. This information will be utilized to identify common metabolic targets for the killing effects of acids and acid preservatives, as well metabolic targets unique to particular acids. Since we have found that oxygen increases the killing rates of acid-tolerant pathogens at low pH, similar investigations will be done to determine the genetic and metabolic responses of acid-tolerant pathogens to acids in the presence of molecular oxygen and oxygen radicals. These results will be used to determine the mechanisms by which oxygen species enhance killing of pathogens in acid and acidified foods. Data from genetic and metabolic experiments will be used to develop mechanistic mathematical models and validate the models that are developed in order to test hypotheses developed from genetic and metabolic investigations of acid-killing and acid resistance of food pathogens.
The safety of acidified foods is of concern to FDA and producers of these products. Some disease causing bacteria are very acid resistant and can survive for up to several months in some acidified foods such as cucumber pickle products, if these foods are not properly treated. Bacterial strains of a common acid resistant organism Escherichia coli O157:H7 were analyzed to determine how they survive under conditions of acidified foods. It was found that survival and acid resistance varied depending on source of isolation of the strain. Strains isolated directly from animal sources were more acid resistant than strains isolated from foods or human disease.
The interaction of oxygen and components of brine for acidified vegetable products was found to be important in the survival of acid resistant disease causing bacteria under conditions similar to acidified food products. It was found that salt concentration and dissolved oxygen can have synergistic effects and significantly affect survival in acid environments for E. coli O157:H7.
Use of linear models for thermal processing of acidified foods. Process filings for acidified foods have previously used tables of time-temperature values for determining the safety of process conditions. FDA has recently required that linear killing kinetic models be used for process filing forms. The linear models needed were similar to the ones used for canned foods, but had to be adapted for use with pickled vegetables. The model parameters for acidified foods were determined from existing processing data, and a model was developed. The technology was made available to industry through trade association meetings, and university extension specialists. The model parameters are now commonly used by industry for acidified food process filings.
Oh, D., Pan, Y., Berry, E.D., Cooley, M.B., Mandrell, R.E., Breidt, F. 2009. Escherichia coli O157:H7 strains isolated from environmental sources differ significantly in acid resistance compared to human outbreak strains. Journal of Food Protection. 72(3):503-509.
Kreske, A.C., Bjornsdottir, K., Breidt, F., Hassan, H. 2008. Effects of pH, dissolved oxygen, and ionic strength on the survival of Escherichia coli O157:H7 in organic acid solutions. Journal of Food Protection. 71(12):2404-2409.