Effects of steam and lactic acid treatments on inactivation of Listeria innocua surface-inoculated on chicken skins
Introduction
Raw poultry carcasses are often heavily contaminated with pathogens belonging to the genera Listeria spp., Salmonella spp. and Campylobacter spp. (Uyttendaele et al., 1998). Among the currently available decontamination treatments, heat and acid treatments have so far shown particular promise (James and James, 1997, Avens et al., 2002).
Since most poultry products are sold raw, mild heat treatments with low steam temperatures of between 71 °C and 75 °C have been applied in order for the quality of the product to be retained (Klose et al., 1971). However, the treatment times used of several minutes were too long to fit in with food processing production rates. The same kind of steam temperature has been applied shortly (18 s) on beef carcasses at industrial scale leading to a 1–2 log reduction of the initial microbial population (Corantin et al., 2005). A European project aimed to analyse kinetically the effect of hot air flows and steam flows on bacteria inactivation (James and Evans, 2006) proved that steam inactivated bacteria more efficiently than hot air. Microbial inactivation kinetics have been evaluated for Escherichia coli O157:H7 and for Salmonella typhimurium DT104 at the surface of chicken skin subjected to a steam flow of 100 °C (McCann et al., 2006), and have shown an exponential reduction of the microbial population with a rapid 3 to 4-log10 destruction within the first 20 to 30 s of application of the treatment. After the European project, inactivation of Campylobacter jejuni AR6 and E. coli K12 on the surface of poultry carcasses by steam at atmospheric pressure and hot water treatments have been analysed in relation to product damaging at three different treatment times: 10, 12 and 20 s respectively (James et al., 2007). Maximum decontamination (about 3 log10 cfu cm− 2) without skin damaging was found by combining steam, hot water and a rapid cooling of product surface. Jets of steam, superheated or non-superheated, have also been applied for a period up to 60 s to decontaminate disks of poultry skin, surface-inoculated with Listeria innocua (Kondjoyan and Portanguen, 2008b). Superheated steam was clearly more effective against the pathogen than non-superheated steam, leading to an average reduction of more than 5 log10 cfu cm− 2 after 30 s of treatment. The use of superheated steam may prove useful in industrial settings for treatment durations of 10 s to 30 s. This procedure can be a highly effective surface decontamination intervention that does not result in cooking the meat, and a process patent has thus been filed (No. 05 53451).
Another method of decontaminating the surface of food products involves organic acids solutions. Lactic acid is, at first sight, a good candidate as it is authorised for food products, is inexpensive, and in dilute solution does not have an unpleasant odour or taste. A 5-min treatment with a 1% solution of lactic acid gives an immediate reduction of the order of 0.5 to 1 log10 for Listeria monocytogenes inoculated on chicken skin (Gonzalez-Fandos and Dominguez, 2006). The reduction is of the order of 1 log10 when the concentration of lactic acid is increased to 2% or 4%. Increasing the time to 15 min and the solution temperature to 55 °C leads to a reduction of 2.38 log10 on chicken breast meat (Goncalves et al., 2005). There are hardly any results on solutions with concentrations of lactic acid above 5%, and no systematic study has been conducted to evaluate the effect of contact time on decontamination.
An approach combining heat treatment and organic acid treatments has been used in the case of vacuum-packed, dried or marinated meat (Koutsoumanis and Sofos, 2004, Ikeda et al., 2003, Calicioglu et al., 2002, Calicioglu et al., 2003) or frankfurters (Murphy et al., 2005). Heat treatments give immediate and significant reductions in bacterial counts, while acid treatments lead to bactericidal or bacteriostatic effects that are maintained during storage. Thus an approach combining heat treatment and organic acid treatments seems also particularly promising for carcasses and fresh products which have not been vacuum-packed, marinated, or dried.
This study aimed at identifying the best way of combining steam and lactic acid treatments in order to obtain a method for decontaminating raw poultry carcasses that would be suitable for use at food processing plants, and that would present maximum efficiency without adversely affecting the sensory qualities of the raw product (taste, appearance, texture, colour, and odour). Such a decontamination method should achieve an immediate reduction of the initial bacterial load, and also provide an effective protection of food products during cold storage. Studies have shown that Listeria spp. strains are more resistant to decontamination treatments than most other pathogens, in particular Salmonella spp. and Campylobacter spp. (Hwang and Beuchat, 1995, Sörqvist, 2003, Fernandez et al., 2007). Since L. monocytogenes is a major human pathogen it was not reasonable to use this bacterium in an experimental procedure which was expected to be transferred at a pilot scale in a processing plant. Thus instead of L. monocytogenes, L. innocua was used in this study; a non-pathogenic species, which has been proved to be very close physiologically to L. monocytogenes (Begot et al., 1997, Vaz-Velho et al., 2001) and has been used as a model of the pathogen in several other tested processes (Antwi et al., 2007). The strains chosen in this study L. innocua CLIP 20595 and L. monocytogenes 14 were isolated from a meat processing plant and thus found appropriate to study decontamination treatments which have to be applied on a meat and in abattoirs. Preliminary experiments were done to compare the thermal resistance of L. innocua CLIP 20595 to that of L. monocytogenes 14.
Section snippets
Inoculation
The strains of L. innocua CLIP 20595 and L. monocytogenes 14 were stored on microspheres at − 18 °C. It was then transferred to an inclined TSA agar tube (Difco, USA) and incubated for 8 h at 37 °C. Two inoculating loops were then transferred to 100 ml of meat medium (meat peptone 10 g/l from Merck, Germany; yeast extract 5 g/l from Difco, USA; Glucose 5 g/l from Prolabo). This culture was incubated for 20 h at 20 °C, and then used to inoculate the chicken skin samples or to compare thermal
Comparison L. innocua to monocytogenes
During the preliminary experiments performed in a liquid medium L. innocua CLIP 20595 was found to be more heat resistant than L. monocytogenes 14 with D60 °C values of 49 s and 28 s respectively. This result was confirmed on Teflon® slides leading to average reductions of L. innocua and L. monocytogenes numbers at 70 °C of 2.7 log10 cfu cm− 2 and 1.5 log10 cfu cm− 2 respectively. Thus it was considered that the reduction of the population of L. innocua CLIP 20595 will, at least, lead to the same
Conclusion
The results obtained demonstrated that, as would be expected, the efficacy of heat treatments increased with surface temperature or treatment duration. The most marked reductions in bacterial count were obtained by applying a 1-min treatment at 70 °C; shorter treatments at higher temperatures also gave significant reductions. However, heat treatment used alone did not prevent risks of later bacterial growth during storage, as observed After 7 days of storage at 4 °C for the most effective heat
Acknowledgments
This work was supported by the European Social Fund (ESF) and the Reunion Island Regional Authority. Thanks to A.T.T. for the translation of this text.
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