Some researchers of University of Foggia (Italy) published in 2009 a review on chemical, physical, and biological stress factors that can inhibit the microbial growth, especially bacterial pathogens survival, on minimally processed food.Among the bacterial pathogens, the most common on fresh produce are Escherichia coli O157:H7, Salmonella spp., and Listeria monocytogenes. These pathogens can contaminate produce whenever the GAP (Good Agriculture Practices), GHP (Good Hygiene Practices), and GPP (Good Processing Practices) are not applied and respected.
Pathogen survival and growth on food-produce are influenced by a number of interdependent factors, such as storage temperature, product type, minimal processing operations (washing, sanitizing systems, cutting, etc.), new mild technologies, package atmosphere and the competition with the natural microflora present on food-produce.
In the paper, the authors described the different stress factors, such as treatment with cold (<10°C), heat (>40-50 °C), acids, solvents, salts, oxidant compounds, irradiation, biological compounds (essential oils, and GRAS compounds), and biological agents (lactic acid bacteria and Enterobacteriaceae), and the stress response of bacterial.
Cold stress causes in bacterial cell several physiological changes, such as decrease in membrane fluidity, stabilization of secondary structures of nucleic acids reducing the efficiency of mRNA translation and transcription, inefficient folding of some proteins, and hampered ribosome function. Heat stress by using hot water or steam has been widely applied in food industry to eliminate foodborne pathogens. The high temperatures generally damage the nucleic acids, cause the protein denaturation leading to destruction of enzyme activity and enzyme-controlled metabolism in microorganisms.
Low pH causes the denaturation of intracellular proteins, bacterial pathogens do not develop at pH<4, generally pH is lowered by using organic acids. Oxidative stress is a key stress in bacteria, treatments with oxidant compounds (hydrogen peroxide, nitric oxide, ozone, etc.) destroy microorganisms by the progressive oxidation of vital cell components, such as lipids, proteins and DNA repair enzymes. Osmotic stress by using potassium ions (K+) consists of increasing the solute concentration of the surrounding medium where bacterial cell grows. In this way the stress acts on the inner osmotic pressure of bacterial cell, which will die of dehydration.
The treatment with UV-C radiations can inhibit microbial growth by inducing the formation of pyrimidine dimers, which alter the DNA helix and block microbial cell replication.
Hurdle technologies
The pool of these stress factors has allowed to develop the hurdle technologies and to implement the cross protection in the food industry. These technologies involve the use of a number of synergic mild preservation techniques known as hurdles, instead of using one robust method that can change the organoleptic quality of food or induce stress resistances in the pathogens.
Several stresses have been shown to induce cross protection, including heat stress, cold stress, acid stress, and osmotic stress. Many studies are assessing the synergic or antagonistic action of different stress factors combinations to develop hurdle technologies that are effective against microbial growth and do not induce stress response in the microorganisms.
These technologies allow to inhibit microbial growth without changing organoleptic characteristic of food, thus they can be used during fresh-cut processing to ensure the fresh-cut product safety.
Original research. Capozzi V., Fiocco D., Amodio M.L., Gallone A., Spano G., ‘Bacterial Stressors in Minimally Processed Food’, 2009, International Journal of Molecular Sciences, Issue No. 10, pagg. 3076-3105. For more details: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2738913/pdf/ijms-10-03076.pdf