R. Pla
E. Beltran
J. Yuste
CER Planta de Tecnologia dels Aliments (CeRTA, XIT),
Departament de Ciència Animal i dels Aliments, Facultat de Veterinària,
Universitat Autònoma de Barcelona.
Bellaterra, Barcelona, Spain
High pressure processing is applied to several poultry products for safety enhancement and shelf-life extension. Pressurization induced large decreases in counts of psychrotrophic, lactic acid bacteria, and especially enterobacteria and Listeria spp. Reductions were similar to or even higher than those caused by heat treatment. High pressure and nisin, pressurization at 50 ºC, and cycle pressure treatments effectively inactivated indigenous and inoculated microbiotas (e.g., Escherichia coli and Salmonella Enteritidis). High pressure processing greatly improves the microbiological quality of poultry products.
Introduction
General aspects
High pressure processing is the technology by which a product is treated at or above 100 MPa. Pressure is transmitted uniformly and almost instantaneously throughout the food, regardless of its size and shape. Furthermore, once the pressure value is reached, there is no need for extra energy. Pressurization can be performed at temperatures lower than those used with heat treatment. Thus, the nutrient content and some organoleptic characteristics, such as odour and taste, do not markedly change.
Microbial sensitivity
Indigenous microbiota grows more readily than inoculated laboratory collection strains. Gram-positive bacteria are more resistant than Gram-negative bacteria, and cocci are more resistant than rods.
Bacterial spores are the most pressure-resistant microorganisms. Cells in the logarithmic growth phase are more sensitive than those in the other phases.
Treatment effectiveness
Increasing pressure and exposure time lead to increased lethality. Treatments consisting of several cycles are generally more effective than continuous treatments, especially against bacterial spores. The effectiveness of pressurization is greater in inoculated suspensions and model systems than in foods. The lower the pH and the higher the water activity, the more lethal the treatment.
Objective
To investigate pressure inactivation of pathogenic and spoilage bacteria in poultry products for safety enhancement and shelf-life extension, with particular emphasis on techniques to detect sublethally injured cells.
Materials and methods
Raw materials
Mechanically recovered poultry meat, cooked poultry sausages, whole and minced chicken (breast and thigh), liquid whole egg, egg yolk and white.
High pressure processing
Combinations of several pressures (100 to 900 MPa), times (1 to 30 min, continuous and cycle pressurizations) and temperatures (−20 to 75 ºC). With two discontinuous isostatic presses: one, with a ca. 2-L volume pressure vessel, from ALSTOM (Nantes, France) for 100- to 500-MPa treatments; and the other one, with a ca. 275-mL volume pressure vessel, from Standsted Fluid Power (Standsted, UK) for 600- to 900-MPa treatments.
Treatment temperature monitored with a thermocouple. Samples allowed reaching the treatment temperature inside the pressure vessel before pressurization. The effect of pressure treatment usually compared with that of conventional heat treatment.
Microbiological analysis
Counts of indigenous and inoculated pathogenic (Clostridium perfringens, Escherichia coli, Listeria monocytogenes, Salmonella Enteritidis, Salmonella Typhimurium, Staphylococcus aureus, Yersinia enterocolitica) and spoilage bacteria (lactic acid bacteria, enterobacteria, total/faecal coliforms, sulfite-reducing clostridia), on nonselective and selective media. Techniques to detect and recover sublethally injured cells.
Results
The following are some particular results.
Mechanically recovered poultry meat (Table 1)
Mechanically recovered poultry meat had high initial counts (7 log CFU/g) and reached 9 log CFU/g at 15 days. Increased inactivation occurred with increasing magnitude of pressure. Psychrotrophs were more pressure-sensitive than mesophiles.
Combination of high pressure and 200 ppm nisin plus 1 % GdL was very effective: psychrotroph counts of mechanically recovered poultry meat treated at 450 MPa were less than 1 log CFU/g throughout the 30-day experiment.
Cooked poultry sausages (Table 2)
Cooked poultry sausages had high initial counts and, at 6 weeks, reached ca. 8 log CFU/g of lactic acid bacteria, 4 log CFU/g of Listeria spp. and 5.5 log CFU/g of enterobacteria. High pressure processing was more effective against lactic acid bacteria and Listeria spp. than heat processing. Both treatments were very effective against enterobacteria. No Listeria spp. and enterobacteria growth occurred in pressurized sausages throughout the 18-week experiment.
Liquid whole egg (Tables 3 and 4)
Listeria innocua was more pressure-resistant than E. coli. Furthermore, since Listeria is a psychrotrophic organism, surviving (injured) L. innocua cells recovered and developed and counts increased during the refrigeration storage. In contrast, surviving (injured) E. coli cells died and counts decreased, because E. coli is not psychrotrophic.
Listeria innocua was more sensitive to high pressure-nisin combination than E. coli: L. innocua was not detected throughout the 30-day experiment. This is because nisin is more effective against Gram-positive bacteria.
Salmonella Enteritidis was more pressure-sensitive than E. coli. Treatments at 50ºC were the most effective and those at 2ºC were the least effective. Cycle pressurization was more effective than continuous pressurization, which was very clear in liquid whole egg treated at 20ºC.
References
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Ponce, E., R. Pla, M. Capellas, B. Guamis, and M. Mor-Mur, 1998. Inactivation of Escherichia coli inoculated in liquid whole egg by high hydrostatic pressure. Food Microbiol. 15:265-272.
Ponce, E., R. Pla, E. Sendra, B. Guamis, and M. Mor-Mur, 1998. Combined effect of nisin and high hydrostatic pressure on destruction of Listeria innocua and Escherichia coli in liquid whole egg. Int. J. Food Microbiol. 43:15-19.
Ponce, E., R. Pla, E. Sendra, B. Guamis, and M. Mor-Mur, 1999. Destruction of Salmonella enteritidis inoculated in liquid whole egg by high hydrostatic pressure: comparative study in selective and non-selective media. Food Microbiol. 16:357-365.
Yuste, J., M. Capellas, R. Pla, D. Y. C. Fung, and M. Mor-Mur, 2001. High pressure processing for food safety and preservation: a review. J. Rapid Methods Automation Microbiol. 9:1-10.
Yuste, J., M. Mor-Mur, M. Capellas, B. Guamis, and R. Pla, 1998. Microbiological quality of mechanically recovered poultry meat treated with high hydrostatic pressure and nisin. Food Microbiol. 15:407-414.
Yuste, J., R. Pla, M. Capellas, E. Ponce, and M. Mor-Mur, 2000. High-pressure processing applied to cooked sausages: bacterial populations during chilled storage. J. Food Protect. 63:1093-1099
From Proceedings of the "XVI European Symposium on the Quality of Poultry Meat" and the "X European Symposium on the Quality of Eggs and Egg Products", Saint-Brieuc Ploufragan, France.
