Influence of the product on microbial inactivation under pressure
Various types of meat products have been high-pressure-treated to extend their shelf-lives. Garriga and Aymerich (2009) distinguish between studies performed on raw meat products and those performed on cooked, cured, and fermented meat products. Table 1 highlights the variability of the high-pressure effect, which depends on both the bacterial species and the type of meat product. However, the results are difficult to compare because processing conditions and methods vary between studies.
Table 1. Table 1–Recent results obtained for microbial inactivation (given as a comparison between untreated control and pressure-treated samples at the same time after treatment) in different meat and meat products treated with high-pressure processing.
| ||Meat or meat product||Treatment||Reduction (log CFU/g)||Reference|
|Raw meat and products with high aw||Raw chicken meat||375 MPa, 18 °C, 15 min||Inoculated Listeria monocytogenes: 2 to 5 immediately after processing, depending on the strain||Simpson and Gilmour (1997)|
| ||Poultry sausages||500 MPa, 50 °C, 10 min 500 MPa, 60 °C, 10 min||Aerobic mesophiles: 3.28 the day after processing 5.18 the day after processing||Yuste and others (2000b)|
| ||Mechanically recovered poultry meat||450 MPa, 20 °C, 15 min||Aerobic mesophiles: 3.7 the day after processing||Yuste and others (2001)|
| ||Frankfurt sausages||500 MPa, 65 °C, 15 min||Aerobic mesophiles: 6.14 after 3 wk of chilled storage||Yuste and others (2000a)|
| ||Raw smoked pork loin||500 MPa, 30 min||Aerobic mesophiles: 1.36 just after processing 0.5 after 8 wk of chilled storage||Karlowski and others (2002)|
| ||Raw beef||560 MPa, 10 °C, 4 min||Aerobic mesophiles: 2.5 the day after processing||Jung and others (2003)|
| ||Marinated beef loin||600 MPa, 31 °C, 6 min||Aerobic mesophiles: 6.51 (of 6.51 initially present) immediately after processing||Garriga and others (2004)|
| ||Marinated beef loin||600 MPa, 31 °C, 6 min||Inoculated LAB: about 4 of 5 initially present, 2 d after processing||Jofré and others (2009b)|
|Dry cured products||Dry cured ham||600 MPa, 31 °C, 6 min||Aerobic mesophiles: 2.7 immediately after processing||Garriga and others (2004)|
| ||Dry cured ham||600 MPa, 31 °C, 6 min||Inoculated LAB: 1.6 (of 4 initially present), 2 d after processing||Jofré and others (2009b)|
| ||Dry fermented pork sausage||400 MPa, 17 °C, 10 min||Inoculated Listeria monocytogenes: 0.6 (of 6 initially present) the day after processing||Jofré and others (2009a)|
| ||Dry cured chorizo sausages||350 MPa, 20 °C, 15 min||Aerobic mesophiles: <1 immediately after processing||Ruiz-Capillas and others (2007b)|
| ||Dry cured beef “Cecina de Leon”||500 MPa, 18 °C, 5 min||Aerobic Mesophiles: 1.66 after processing 2.55 after 60 d of storage||Rubio and others (2007b)|
|Cooked meat and products (low acid,||Sliced cooked ham||400 MPa, 7 °C, 20 min||Aerobic mesophiles: 2.34 immediately after processing||Lopez-Caballero and others (1999)|
| high aw)||Sliced cooked ham||300 MPa, 20 °C, 15 min||Aerobic mesophiles: 0.3 immediately after processing||López-Caballero and others (2002a)|
| ||Sliced cooked ham||400 MPa, 17 °C, 10 min||Inoculated Listeria monocytogenes: 1.8 immediately after processing||Aymerich and others (2005)|
| ||Cooked ham||600 MPa, 31 °C, 6 min||Aerobic mesophiles: >2.45 immediately after processing||Garriga and others (2004)|
| ||Cooked ham||600 MPa, 20 °C, 10 min||Aerobic mesophiles: 1.5||Karlowski and others (2002)|
| ||Blood sausages||600 MPa, 15 °C, 10 min||Aerobic mesophiles: 2.62 the day after processing||Diez and others (2008)|
| ||Frankfurters||400 MPa, 30 °C, 10 min||Total viable count: 2.16 immediately after processing||Ruiz-Capillas and others (2007c)|
| ||Fried minced pork meat||400 MPa, 20 °C, 60 min||Inoculated Bacillus stearothermophilus: 2 immediately after processing||Moerman and others (2001)|
The composition of the food matrix has been shown to influence the lethality of a high-pressure treatment despite the fact that the effect of each food constituent on pressure resistance is not well known. First, the microbial reduction is always lower in food than in a buffer system (Smelt 1998; Patterson 2005; Considine and others 2008), and the D-values are increased in ham (Tassou and others 2007) and fish (Panagou and others 2007) in comparison with a PBS buffer. It has also been demonstrated that a low aw decreases the efficiency of high-pressure treatments. In products with aw≤ 0.92 cells are protected against pressure (Garriga and others 2004; Ruiz-Capillas and others 2007b). For example, in the study by Jofré and others (2009b), dry-cured ham with aw of 0.918 showed lower inactivation levels of inoculated microorganisms after high-pressure treatment (600 MPa, 6 min, 31 °C) than did cooked ham and marinated beef loin. However, this undesirable protective effect against pressure seems to be compensated for by the inhibition of the recovery of the cells during storage (Jofré and others 2009b). For example, in cooked ham (aw= 0.98) and fermented sausages (aw= 0.90), the same high-pressure treatment (400 MPa, 10 min, 17 °C) produced completely different results. While the high-pressure treatment produced an immediate 1.8-log CFU/g reduction of L. monocytogenes spiked into cooked ham (Aymerich and others 2005), the pathogen was not significantly reduced in fermented sausages (Jofré and others 2009a). However, a progressive decrease in the counts of L. monocytogenes was observed during subsequent storage of this latter product, probably due to the progressive death of sub-lethally injured bacteria. Pressure resistance of microorganisms at a low aw, like heat resistance in the same conditions, is probably due to the more stable state of macromolecules at low water content (Corry 1975). Furthermore, the aw of a food product is also dependent on the concentrations of solutes, such as sugar and salt, the nature of which can significantly affect cell survival after a pressure treatment (as discussed in the reviews of Considine and others 2008 and Smelt 1998). The barotolerance observed at elevated levels of osmolarity could be due to the microbial uptake of compatible solutes (such as betaine or carnitine) from the external environment (Smiddy and others 2004). These solutes play the role of stabilizers for enzyme functions or of osmotic balancers (Hill and others 2002). However, this has only been shown in studies carried out in model media.
Park and others (2001) compared the inactivation of Lactobacillus viridescens in Man Rogosa Sharp (MRS) and in protein-fortified MRS broth and reported that the addition of proteins decreased the inactivation after a high-pressure treatment at 400 MPa for 5 min at 20 °C. However, these authors did not provide the aw values of the media tested. The influence of nutrient composition on microbial inactivation was evaluated by Moerman and others (2001) who compared the high-pressure-induced reduction of different microorganisms in fried chicken and in mashed potatoes (two products with similar pH values of 5.9 and aw values of 0.98) using an experimental design defined over 0 to 400 MPa, 20 to 80 °C and 1 to 60 min. The effect of the medium was shown to be negligible and, thus, this study did not reveal any protective action of the major nutrient fractions (carbohydrates, fat, and proteins). Nevertheless, the microbial protection of fat has been mentioned several times. Rubio and others (2007a, b) observed that high pressure (500 MPa for 5 min at 18 °C) did not produce an inhibitory effect on the mesophilic count in dry sausages, whereas the same treatment efficiently delayed the growth of spoilage microorganisms in dry-cured beef. These authors hypothesized that the different behaviors of microorganisms in dry sausages and dry-cured beef might be caused by the protective effect of fat in dry sausages. However, no significant difference in the reduction of the total aerobic count was observed between low-fat (90 g/kg) and high-fat sausages (247 g/kg) that were treated for 20 min at 300 MPa (Jiménez-Colmenero and others 1997). According to Escriu and Mor-Mur (2009), this effect is dependent on both the type of fat and the type of microorganism tested. These authors showed that immediately after a high-pressure treatment (400 MPa, 20 °C, 2 min), the Listeria innocua count was more reduced in chicken meat mixed with olive oil than in the same meat mixed with tallow. In addition, depending on the type and percentage of fat content, Listeria innocua and Salmonella Typhimurium did not recover in the same way after 60 d of cold storage; however, it was not possible to show any clear relationship with either fat content or fat quality. Rubio and others (2007a) also failed to establish a clear relationship between the fatty acid composition of a meat product and the effectiveness of high-pressure treatment. These authors evaluated the microbiological quality of 3 types of sausages with different compositions of fat (control, high oleic, and high linolenic) after treatment at 500 MPa for 5 min at 18 °C. Thus, it appears that the effect of fat is not simple and may depend on its composition, its location in the product, and its interactions with the other components of the matrix. For example, olive oil may contain antimicrobial phenolic compounds (Medina and others 2007) that could explain the higher cell reduction observed in meat supplemented with olive oil in comparison with meat supplemented with tallow (Escriu and Mor-Mur 2009).
Cell recovery during subsequent storage is another important item to consider. Many authors have reported low recovery of microorganisms in high-pressure-treated products during subsequent storage, and, in most cases, bacterial growth is delayed by a high-pressure treatment at a sufficient pressure level (≥400 MPa) (Yuste and others 2000a; Garriga and others 2004; Jofré and others 2009b). For example, Patterson and others (2010) showed that the total viable count in cooked poultry meat treated at 600 MPa at 18 °C for 10 min could be stabilized to 3 log CFU/g during 35 d of cold storage. Sometimes the microbial count is not reduced immediately after the pressure treatment but shows a significant decrease during cold storage, as for enterobacteria in blood sausages treated at 300 to 600 MPa for 10 min at 15 °C (Diez and others 2008) or for S. aureus spiked in dry-cured ham treated at 600 MPa for 6 min at 31 °C (Jofré and others, 2009b). However, for cooked products, problems of fast microbial recovery during subsequent storage have sometimes been described (Garriga and others 2002, 2004). Recovery of Escherichia coli and LAB reached the level of the control after less than 20 d of chilled storage in cooked ham homogenized with water and treated at 400 MPa for 10 min at 17 °C (Garriga and others 2002). In sliced cooked ham high-pressure-treated at 600 MPa for 6 min at 31 °C, total aerobic count increased during chilled storage, mainly due to LAB growth, but no recovery was observed in dry-cured ham or in marinated beef loin treated in the same conditions (Garriga and others 2004). This may be due to the negative effect of high pressure on the water-holding capacity of cooked products that thus produce rich exudates (Pietrzak and others 2007) and to the fact that cooked products do not display hurdles against microbiological growth during cold storage (Garriga and Aymerich 2009). Besides permitting a fast recovery, rich exudates could also display a protective effect against inactivation as a higher survival of Listeria monocytogenes was observed in cooked chicken and beef mince in comparison with raw meat (Simpson and Gilmour 1997).
Microbial reduction and recovery during storage in high-pressure-treated food products depend on the type of product tested. In products with low aw (≤0.9) cells are protected against pressure, but recovery is inhibited during storage. In cooked products, fast recovery during subsequent storage can be observed. There is a significant impact of food composition on microbial reduction and the effect of protein and fat is complex. Finally, differences in lethality and recovery rates may be due to how the food matrix tolerates pressure treatments and to the ways in which the interactions of all components affect this matrix and are not restricted to the pressure effects on a single component.
Influence of the microorganism species on the inactivation under pressure
Enterobacteria and LAB are the main components of the deterioration flora in meat. Low-temperature alterations are provoked by psychrotrophs (mainly Pseudomonas). Concerning pathogenic microorganisms, Salmonella, Listeria monocytogenes, and some specific strains of Escherichia coli are the greatest threats. Additionally, in industrialized countries, Campylobacter spp. (especially jejuni) are a major cause of enteritis and are found mainly in poultry and to a lesser extent in pork (Belloc and others 2004). The risk of the development of the dreaded toxin-producing anaerobic Clostridium botulinum continues to exist with long-term storage at high temperature (25 to 40 °C) of some meat products.
Recent results of bacterial reduction in meat products are presented in Table 2 for alteration flora and in Table 3 for pathogenic flora. A high-pressure treatment (400 to 600 MPa, 7 to 18 °C, 5 to 10 min) is generally effective for reducing the number of enterobacteria below the level of detection in high aw products, such as cooked ham, marinated beef loin, or blood sausages (López-Caballero and others 1999; Garriga and others 2004; Diez and others 2008). Escherichia coli is a potentially pathogenic enterobacterium, and the effects of high pressure on some relevant species have been studied. E. coli has also been shown to have a high sensitivity to pressure. In cooked ham, dry-cured ham, and marinated beef loin inoculated at 3.5 log CFU/g, and in marinated beef with an endogenous load of 1.18 log CFU/g, E. coli was reduced below the level of detection during 120 d of chilled storage by a high-pressure treatment at 600 MPa for 6 min at 31 °C (Garriga and others 2004; Jofré and others 2009b). In the study of Garriga and others (2002), a mixture of 2 strains of E. coli displayed a 4.5-log CFU decline 24 h after a high-pressure treatment (400 MPa, 10 min, 17 °C). A high-pressure treatment of 400 MPa at 12 °C for 20 min was sufficient to give a reduction of 2.45 log CFU/g (of 7 log initially present) of a pressure-resistant strain of the serotype O157:H7 in ground beef (Morales and others 2008). Porto-Fett and others (2010) showed a total reduction of the initial 5 log CFU/g of E. coli O157:H7 inoculated into dry-fermented salami after a high-pressure treatment at 483 MPa at 19 °C for 5 min. A total reduction was also reported by Gola and others (2000) in raw minced meat that was inoculated with a mixture of 8 strains of E. coli O157:H7 and treated at 700 MPa at ambient temperature for 5 min.
Table 2. Table 2–Recent results obtained for inactivation of alteration flora in meat and meat products treated by high-pressure process (HP = high-pressure-treated sample).
|Family||Genus/Species||Gram||Treatment||Meat or meat product||Microbial load (log CFU/g)||Reference|
|Enterobacteria|| ||−||400 MPa, 5 °C, 5 min||Cooked ham||After 35 d of chilled storage Control: 6.36; HP: <1||Lopez-Caballero and others (1999)|
| || || ||600 MPa, 16 °C, 6 min||Cooked ham||After 90 d of chilled storage control: 3.71; HP: <1||Garriga and others (2004)|
| || || || ||Marinated beef loin||After 90 d of chilled storage control: 5.16; HP: <1|| |
| || || ||300 MPa, 15 °C, 10 min||Blood sausages||After 28 d of chilled storage: control: 1.92; HP<1||Diez and others (2008)|
| ||Pseudomonas sp.||−||450 MPa, 20 °C, 20 min||Fresh minced meat||Control: 5; HP: <1 immediately after high pressure processing but total recovery after 15 d of chilled storage||Carlez and others (1994)|
| ||P. fluorescens isolated from pork meat|| ||400 MPa, 20 °C, 10 min||Culture broth||Control: 7.8; HP: <1 immediately after high pressure processing but total recovery after 16 d of storage at 20 °C||Lopez-Caballero and others (2002b)|
| ||Pseudomonas sp.|| ||300 MPa, 15 °C, 10min||Blood sausages||After 21 d of chilled storage, control: 3.30; HP: <2||Diez and others (2008)|
|Lactic acid bacteria|| ||+||400 MPa, 5 °C, 20 min||Cooked ham||After 7 d of chilled storage, control: 3.78; HP: <1 after 35 d of chilled storage, control: 7.28; HP: 2.66||Lopez-Caballero and others (1999)|
| ||Lactobacillus sakei and Leuconostoc carnosum|| ||400 MPa, 17 °C, 10 min||Model meat system||Immediately after high pressure processing, control: 8.5; HP: <2 after 6 d of chilled storage, recovery to 6 for HP||Garriga and others (2002)|
| || || ||600 MPa, 16 °C, 6 min||Cooked ham||After 30 d of chilled storage, control: 7.84; HP: <2 after 120 d of chilled storage, control: 8.71; HP: 2.65||Garriga and others (2004)|
| || || || ||Beef loin||After 120 d of chilled storage, control: 8.68; HP: <2|| |
| || || ||500 MPa, 18 °C, 5 min||Cecina de Leon (smoked and dried beef meat) cuts||After 15 d of chilled storage, control: 3.83; HP: <1||Rubio and others (2007b)|
| || || ||600 MPa, 15 °C, 10 min||BIood sausages||Immediately after high pressure processing, control: 5.46; HP: 5.41 after 28 d of chilled storage, control: 8.67; HP: 8.50||Diez and others (2008)|
| || || ||300 MPa, 27 °C, 10 min||Sliced ham||After 20 d of chilled storage, control: 6.5; HP: 2 after 40 d of chilled storage, control: 6.5; HP: 6||Slongo and others (2009)|
Table 3. Table 3–Recent results obtained for inactivation of pathogenic flora (given as the reduction immediately after high-pressure treatment) in meat and meat products.
|Genius/Species||Gram||Treatment||Meat or meat product||Reduction (log CFU/g)||Reference|
|Listeria monocytogenes||+||375 MPa, 18 °C, 20 min||Raw chicken mince Cooked chicken mince Model meat system||4 of 8.7 inoculated 1.5 of 8.7 inoculated||Simpson and Gilmour (1997)|
| || ||400 MPa, 17 °C, 10 min|| ||6.5 of 8 inoculated, total recovery after 20 d of chilled storage||Garriga and others (2002)|
| || ||400 MPa, 17 °C, 10 min||Sliced cooked ham||4 of 4 inoculated and recovery to about 8 log CFU/g after 40 d of chilled storage||Aymerich and others (2005), Marcos and others (2008a)|
| || ||500 MPa, 20 °C, 1 min||Turkey breast meat||0.9 of 7 inoculated.||Chen (2007)|
| || ||500 MPa, 25 °C, 10 min||Sliced cooked ham||5 of 5 inoculated but total recovery after 70 d of chilled storage||Koseki and others (2007)|
| || ||600MPa, 10 °C, 5 min||Cooked ham||3.5 of 4 inoculated||Jofré and others (2008b)|
| || ||600 MPa, 18 °C, 5 min||Salami||1.6 to 6 of 7 inoculated, depending on fermentation and drying conditions. No or slight recovery under chilled storage||Porto Fett (2010)|
|Listeria innocua|| ||400 MPa, 20 °C, 2 min||Chicken breast||1.5 to 3 depending on the composition||Escriu and Mor-Mur (2009)|
|Staphylococcus aureus|| ||600 MPa, 31 °C, 6 min||Marinated beef loin Dry cured ham Cooked ham,||2.5 of 3.5 inoculated 0.5 of 3.5 inoculated 1.1 of 3.5 inoculated||Jofré and others (2009b)|
| || ||400 MPa, 17 °C, 10 min||Model meat system||No significant reduction of 8 inoculated||Garriga and others (2002)|
| || ||400 MPa, 20 °C, 30 min||Pork Marengo||no significant reduction of 4.6 inoculated||Moerman 2005|
|Salmonella Typhimurium||−||400 MPa, 20 °C, 2 min||Minced chicken||3.26 to 4.35 (depending on the composition), total recovery after 25 d of chilled storage||Escriu and Mor-Mur (2009)|
|Salmonella enterica|| ||400 MPa, 17 °C, 10 min||Meat model||6 of 8 inoculated. No recovery during 60 d of chilled storage||Garriga and others (2002)|
| || ||400 MPa, 17 °C, 10 min||Fermented sausages||2 of 2.7 inoculated. Inactivation to <1 log CFU/g after 20 d of chilled storage||Jofré and others (2009a)|
|Salmonella enteritidis|| ||500 MPa, 50 °C, 10 min||Poultry sausages||7.16 of 8 inoculated.||Yuste and others (2000b)|
|Escherichia coli O157 H7|| ||700 MPa, 20 °C, 5 min||Raw minced meat||Total inactivation||Gola and others (2000)|
|Escherichia coli|| ||400 MPa, 17 °C, 10 min||Model meat system||4.5 of 8 inoculated. Recovery after 10 d of chilled storage||Garriga and others (2002)|
|Endogenous Escherichia coli|| ||600 MPa, 16 °C, 6 min||Marinated beef loin||Total inactivation of 1.18 initially present during 120 d of chilled storage||Garriga and others (2004)|
|Escherichia coli O157 H7|| ||400 MPa, 12 °C, 20 min||Ground beef patties||2.45 of 7 inoculated.||Morales and others (2008)|
|Escherichia coli|| ||600 MPa, 31 °C, 6 min||Cooked ham, dry cured ham and marinated beef loin||Total inactivation of 4 inoculated. Slight recovery only for cooked ham||Jofré and others (2009b)|
|Escherichia coli O157 H7|| ||483 MPa, 19 °C, 5 min||Dry fermented salami||5 of 5 inoculated||Porto Fett (2010)|
|Campylobacter jejuni|| ||200 MPa, 25 °C, 10 min||Chicken meat||2 of 8 inoculated||Martinez-Rodriguez and Mackey (2005)|
| || ||450 MPa, 15 °C, 1 min||Chicken slurry||7 of 7 inoculated||Lori and others (2007)|
| || ||600 MPa, 31 °C, 6 min||Cooked ham, dry cured ham and marinated beef loin||3.5 of 3.5 inoculated||Jofré and others (2009b)|
Psychrotrophic microorganisms are also pressure-sensitive and are more susceptible to pressure than are mesophiles. Yuste and others (2001) showed a reduction of 4.74 log CFU/g for psychrotrophs and of 3.7 log CFU/g for mesophiles in mechanically recovered poultry meat treated at 450 MPa for 15 min at 20 °C. Garriga and others (2004) reported a reduction in the psychrotrophic bacteria in high-pressure treated sliced dry-cured ham and sliced cooked ham (600 MPa, 16 °C, 6 min) to levels below the level of detection during 60 d of subsequent storage, whereas mesophilic bacteria were less reduced by the treatment and recovered during storage. One possible explanation is that when most psychrotrophs are subjected to high pressure, they lose their ability to grow at low temperatures, preventing their recovery during subsequent chilled storage. Psychrotrophs in meat are mainly composed of bacteria from the genus Pseudomonas (Jay and others 2003; Ercolini and others 2009). Processing meat at pressures between 300 and 450 MPa appears to be sufficient to completely inactivate indigenous Pseudomonas (Carlez and others 1994; López-Caballero and others 2002a). However, after a lag period, cells from Pseudomonas spp. can be detected again and resume growth, as was observed by Carlez and others (1994), in fresh minced meat. The same phenomenon was observed by López-Caballero and others (2002b) for Pseudomonas fluorescens isolated from pork meat and inoculated at 8 log CFU/g in culture broth. Even with a total reduction just after the pressure treatment (400 MPa, 20 °C, 10 min), after 8 d of incubation, the cell counts were similar for high-pressure-treated and untreated samples. In the study by Diez and others (2008) on blood sausages treated between 300 and 600 MPa at 15 °C, Pseudomonas was the most pressure-sensitive microbial group (along with Enterobacteria), and the bacterial count remained below the level of detection during 28 d of cold storage.
Conversely, the baroresistance of LAB has been widely reported, although the resistance and ability of LAB to recover from a high-pressure treatment can be positive or negative depending on whether the strain is used for its technological properties or is a spoilage strain. Different behaviors of LAB after high-pressure treatment have been observed depending on the strain and the food matrix. When 2 inoculated LAB strains were treated at 400 MPa at 17 °C for 10 min in a meat model, immediate reductions of 8.5 log were observed. However, after 20 d at 4 °C, both strains reached levels of at least 6 log CFU/g (Garriga and others 2002). Garriga and others (2004) observed the ability of endogenous LAB present in cooked ham to recover during storage of the product at 4 °C after a 600-MPa treatment. In contrast, the authors found no recovery of LAB in high-pressure-treated dry-cured ham and in beef loin during 120 d of storage at 4 °C. In dry-cured beef, a reduction of 3 log CFU was attained one day after a high-pressure treatment at 500 MPa for 5 min at 18 °C (Rubio and others 2007b), and the bacterial growth was delayed during the subsequent chilled storage. In blood sausages, LAB counts were only slightly reduced by a high-pressure treatment at 600 MPa for 10 min at 15 °C (Diez and others 2008). In sliced cooked ham, a significant reduction in the LAB population and a marked delay in their recovery can be obtained with a high-pressure treatment at 400 MPa for 20 min at 7 °C (López-Caballero and others 1999) or for 5 min at 27 °C (Slongo and others 2009). This treatment delays the use-by date from 19 to 85 d despite a recovery of LAB (Slongo and others 2009). All of these data confirm the fact (as mentioned by various authors, such as Patterson 2005 and Escriu and Mor-Mur 2009) that Gram+ bacteria (LAB) are more resistant than gram-bacteria (Enterobacteria, Pseudomonas); this is probably a result of a more robust cell envelope in Gram+ bacteria that contains a high percentage of peptidoglycan and teichoic acids, as suggested by Escriu and Mor-Mur (2009).
The higher pressure resistance of Gram+ bacteria is confirmed by the comparison of the resistance of the 2 main pathogens of meat, namely Listeria (Gram+) and Salmonella (Gram−) (Garriga and others 2002; Escriu and Mor-Mur 2009). The contamination of meat products with Listeria monocytogenes is a major public health problem. Thus, numerous studies have been devoted to the effect of high-pressure treatment on this pathogen (Simpson and Gilmour 1997; Garriga and others 2002; Hayman and others 2004; Aymerich and others 2005; Chen 2007; Koseki and others 2007; Marcos and others 2008a, b; Porto-Fett and others 2010). Generally, a treatment at 400 MPa is necessary to significantly decrease the Listeria load (Simpson and Gilmour 1997; Chen 2007). According to Porto-Fett and others (2010), a reduction of 1.6 to ≥ 5 log CFU/g can be achieved in salami by high-pressure treatment at 600 MPa and 18 °C for 1 to 7 min or at 483 MPa and 18 °C for 5 to 12 min, depending on the aw of the product and on the treatment strength. Garriga and others (2004) also emphasized the importance of the type of product processed for reducing the safety risks associated with Listeria monocytogenes. According to these authors, a pressure of 600 MPa for 6 min at 31 °C is sufficient for sliced marinated beef loin and for dry-cured ham. In chicken batters, containing a different type of fat, a reduction of 1.5 to 3 log CFU/g of Listeria innocua was achieved after a high-pressure treatment at 400 MPa and 20 °C for 2 min (Escriu and Mor-Mur 2009). However, regardless of the immediate reductions in the cell count, the main risk associated with Listeria is its potential recovery during cold storage. According to Marcos and others (2008a), pressure treatment of cooked ham at 400 MPa and 17 °C for 10 min and cold storage cannot prevent Listeria recovery; 600 MPa and 10 °C for 5 min followed by cold storage below 6 °C is necessary to ensure cooked ham's safety for up to 3 mo (Jofré and others 2008b).
The same treatments that can remove Listeria risk are also generally effective at inactivating Salmonella spp., which show less potential for recovery during subsequent storage (Garriga and others 2002; Jofré and others 2009b). For example, a treatment at 600 MPa for 6 min at 31 °C permitted researchers to reduce a cocktail of inoculated salmonella strains from 3.5 log CFU/g to <10 CFU/g in cooked ham, dry-cured ham, and marinated beef loin (Jofré and others 2009b). A treatment at 500 MPa and 50 °C for 10 min can lead to a reduction of more than 7 log CFU/g in inoculated poultry sausages (Yuste and others 2000b).
Among food-borne pathogens, Staphylococcus aureus appeared to be the most resistant to high pressure in comparison with L. monocytogenes, Salmonella, Yersinia enterocolitica, and Campylobacter jejuni (Jofré and others 2009b). After the application of a 600-MPa treatment (for 6 min at 31 °C), the reduction in S. aureus counts in meat products spiked at 3.5 log CFU/g was 2.7 log units in beef loin and 1.1 log units in cooked ham. In dry-cured ham, the pathogen only decreased by 0.5 log units after the high-pressure treatment. Treatment at 700 MPa for 5 min was necessary to achieve complete inactivation of the pathogen in a buffer initially inoculated at 4.5 CFU/mL, although no investigation of the possible recovery of sub-lethally injured cells was carried out (Yuste and others 2004). In a meat model, 2 different strains of Staphylococcus aureus were resistant to a treatment of 400 MPa at 17 °C for 10 min (Garriga and others 2002), and in pork Marengo, S. aureus was affected little by a pressure treatment at 400 MPa and 20 °C for 30 min (Moerman 2005).
Investigations have also been carried out with the Gram-Campylobacter and more frequenty with C. jejuni. Martinez-Rodriguez and Mackey (2005) treated (200 to 400 MPa, 25 °C, 10 min) different Campylobacter jejuni strains inoculated into chicken meat. This microorganism has a relatively high sensitivity to pressure depending on the strain, as a 200-MPa treatment could sometimes reduce the microbial count by 2 log CFU/g. The low resistance to pressure of C. jejuni was confirmed by Lori and others (2007) who showed that treatment at 450 MPa and 15 °C for 30 seconds was sufficient to inactivate more than 6 log CFU/g in chicken slurry. According to Jofré and others (2009b), Campylobacter jejuni was the most inactivated microorganism among Salmonella, Listeria, Staphylococcus, Escherichia coli, and LAB that were spiked into different types of meat products. As of yet, no study has addressed Campylobacter recovery after a high-pressure inactivation.
Whatever the bacterial species, it must be highlighted that the obtained inactivation is highly dependent on the strain; sometimes a difference of 3 log CFU can be observed in the inactivation of two strains of the same bacterial species for the same treatment (Simpson and Gilmour 1997). This finding suggests the importance of using a cocktail of strains as target bacteria in further studies of meat and meat products (Garriga and others 2002; Jofré and others 2009b). In addition, endogenous microbiota are believed to be more resistant than inoculated collection strains (Carlez and others 1994; Yuste and others 2001).
Increasing numbers of studies now address the development and the composition of product microbiota during storage after high-pressure treatment (Yuste and others 2000a; Garriga and others 2002; Tuboly and others 2003; Patterson and others 2010). Patterson and others (2010) showed that a high-pressure treatment could result in the selection of barotolerant species in meat products, which can result in a beneficial effect on the quality of the product during storage. The authors found that in cooked poultry meat treated at 500 to 600 MPa for 1 to 10 min at 18 °C, only bacteria identified as the LAB Weissela viridescens grew during the cold storage. Despite a high Weissela count, no signs of spoilage were observed on the pressure-treated samples during 35 d of chilled storage, probably due to antimicrobial activity of the bacteria against a wide range of microorganisms.
Microbial reduction following a high-pressure treatment is highly dependent on the bacteria species. Gram- bacteria (Enterobacteria, bacteria of the genus Pseudomonas and Campylobacter) are generally more pressure-sensitive than Gram+ bacteria (LAB, Listeria monocytogenes, Staphylococcus aureus). Thus, high-pressure treatment modifies the development and the composition of the microbiota in meat products during their subsequent storage.
The same factors that influence microbial inactivation in high-pressure-treated meat products may also alter the resulting quality of the products. Thus, an essential challenge for the industry is to find strategies to optimize microbial inactivation while providing meat products with desirable physicochemical qualities and sensory attributes.