El secuestro: la libertad retenida

The efficacy of high-pressure treatments for the inactivation of vegetative bacteria in foods has been reported previously (Cheftel, 1995a; Farkas & Hoover, 2000; Smelt, 1998; Yuste et al., 2001). However, there are limited studies on the evaluation of microbial safety and quality of pressurized products during chilled storage (Capri et al., 1999; Lopez-Caballero, Carballo, & Jimenez-Colmenero, 1999; Yuste, Pla, Capellas, Ponce, & Mor-Mur, 2000). Often, food composition can have a protective effect during pressurization, and it is therefore, important to evaluate microbial resistance to pressure in foods rather than in traditional buffer solutions (Yuste et al., 2001).

Marketing of convenience foods is increasing due to consumer demand. Slicing and packaging operations take place after cooking, and cross-contamination at these points is critical regarding the shelf-life and safety of the products. In this study, the initial microbial cell numbers of the HPP-treated (450 MPa and 600 MPa) cooked and vacuum- packaged chicken breast roast were at or below the level of detection for APCs (1 log10

cfu g-1) and LAB and Y&M (2.3 log10 cfu g-1), suggesting that the good manufacturing

practices observed in the production area were satisfactory. Results of the current study are similar to those reported by Patterson et al. (2010), where the initial microbial counts in the untreated and pressure-treated cooked chicken were reported to be <1 log10 cfu g-1.

In many cases, these products later became spoiled during storage (Bjorkroth & Korkeala, 1996; Chenoll, Macian, Elizaquivel, & Aznar, 2007; Hamasaki, Ayaki, Fuchu, Sugitama, & Morita, 2003). However, in this study, none of the pressure-treated poultry meat samples showed any sign of obvious spoilage, irrespective of the pressure level or hold time used. Patterson et al. (2010) also reported similar results, although in some cases, microbial numbers were at 7-8 log10 cfu g-1 after 35 days of storage at 4°C.

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In another study by Garriga et al. (2004), samples pressurized at 600 MPa for 6 min showed a significant delay in the growth of spoilage associated microorganisms compared to untreated samples, contributing to the maintenance of the sensory freshness of the products for at least 60 days after treatment. Even after storage of samples for 90 days at 4°C, the cell counts in the treated samples did not exceed >107 cfu g-1. The use of HPP process retained product quality in terms of off-odors, ropiness, and color changes. Capri et al. (1999) reported an extended shelf-life of sliced cooked ham treated at 600 MPa for 5 min up to 75 days stored at 4°C. The results achieved by Lopez-Caballero et al. (1999) with the same type of product but treated at lower pressures (200 MPa and 400 MPa) did not achieve the same degree of inactivation and the maximum shelf-life obtained was up to 21 days when stored at at 3°C, for sliced and pressurized (400 MPa for 20 min) cooked ham. The results of these previous studies agree with the generally accepted fact that the degree of inactivation is directly related to the level of pressure applied (Garriga et al., 2004).

Gola et al. (2000) demonstrated that use of pressures between 400 to 700 MPa caused significant reductions of eight mixed E.coli strains. Malicki, Sysak, and Bruzewicz

(2005) showed that pressures between 100 and 400 MPa efficiently reduced strains of

Salmonella. Styles, Hoover, and Farkas (1991) reported at least 7-log reductions in L. monocytogenes at a pressure of approximately 340 MPa and Patterson, Quinn, Simpson

and Gilmour (1995) reported a similar reduction of L. monocytogenes at 400 MPa.

Pressures of 450 MPa and 600 MPa were also reported to be very effective in increasing shelf-life of chicken breast fillet up to 14 days of storage at 4°C (Jo et al., 2011). Pressure treatments between 400 and 700 MPa were reported to increase shelf-life of minced meat under refrigeration conditions (Gola et al., 2000). The results obtained in this study clearly demonstrated that increased hydrostatic pressure (600 MPa) was able to inactivate microbial populations and extend the shelf-life of RTE sliced chicken breast roast to 16 weeks.

The cell populations of LAB and Y&M in the HPP-treated samples were below the detection limit (<1 log10 cfu g-1) throughout the storage period in this study. The results

were however in contrast with studies conducted by Garriga et al. (2004), where the LAB counts in samples pressurized at 600 MPa for 6 min were 2.65 ± 1.14 log10 cfu g-1 at 60

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days of storage and increased to 7.62 ± 0.97 log10 cfu g-1 by the end of storage period

(120 days).

High pressure processing conditions used in this study were effective against the growth of yeasts and Enterobacteriaceae, which can produce off-flavours and gas. Pressures between 300-600 MPa are known to inactivate most moulds (Tokusoglu et al., 2011). For microorganisms, the primary site of pressure damage is the cell membrane (Balasubramaniam, 2010). Pressures of 200-400 atmospheres can disrupt the stressed cell wall, and this may be a primary factor for yeasts (Hoover et al., 1989). Eukaryotic microorganisms are generally more sensitive to pressure than prokaryotic microorganisms. In general, Gram-negative bacteria are more sensitive to pressure than Gram-positive bacteria (Carlez, Rosec, Richard, & Cheftel, 1993; Shigehisa, Ohmori, Saito, Taji, & Hayashi, 1991). In fact, for all the sample treatments in this study, no recovery of yeasts or moulds was observed in HPP samples during the entire storage period.

In this study samples processed with HPP at 450 and 600 MPa for 3.5 and 5 min showed an increase in APCs during storage. This could be attributed to the germination of spores activated by the pressures used. Germination of bacterial spores at relatively low pressures (30-50 MPa) is not uncommon (Gao et al., 2006). However, the germinated spores can be subsequently killed by relatively mild heat treatments or high pressure treatments. Process temperatures in the range of 80-100°C in conjunction with a pressure of about 600 MPa have been used to inactivate bacteria such as Bacillus subtilis

propagated from inherent spores (Gao et al., 2006).

It has also been reported that pressure-assisted thermal processing (PATP) can inactivate spores, whereby the food product undergoes pre-heating and the adiabatic heat of compression is used to raise the temperature further (Balasubramaniam, 2010). When the pressure is released, the temperature rapidly drops, which minimizes the adverse effects of heating on the food product. Spores of Clostridium species have been associated with

raw and cooked poultry meats which are not handled appropriately (Skariyachan, Mahajanakatti, Biradar, Sharma, & Abhilash, 2010). High pressure alone does not necessarily kill significant populations of spores, but it does appear to induce

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germination. Spores can be inactivated by combination of both pressure and heat. According to reports by Doona & Feeherry (2007) and Balasubramaniam (2010), spores of Bacillus amyloliquefaciens and selected surrogates of pathogenic Clostridium and

Bacillus species were inactivated by application of high pressure and temperature

combined. However, the mode of action of HPP on bacterial spores is still unclear.

6.1.2 Effect of HPP+LAE on the microbiological quality of RTE sliced