Growth of indigenous flora in vacuum packed smoked turkey stored under different temperatures
The LAB in the control samples steadily increased from 1.3 log CFU/g to the spoilage limit (> 7 log CFU/g) at 50.95, 22.91, 10.5 and 6.5 days of storage at 0, 5, 10 and 15 °C, respectively. In the samples treated with L. monocytogenes, the initial TVC in the VP smoked turkey was similar to the pathogen population of ca. 6 log CFU/g. TVC in smoked turkey reached the spoilage limit (> 7 log CFU/g) after 23, 8, 2.5 and 1.5 days of storage at 0, 5, 10 and 15° C, respectively, while the LAB in the samples reached these levels after 29, 12, 6 and 4 days of storage under the same conditions (Fig. 1).
Under MAP and MAPEO, the initial levels of LAB were ca. 5 log CFU/g. The LAB had similar patterns growth under MAP and MAPEO, and reached the spoilage limit after 11.75, 4.75, 1.63 and 0.63 days of storage at 0, 5 10 and 15 °C, respectively (Figs. 3, 4).
Changes in the pH and redox values in smoked turkey under different storage temperatures
The evaluation of pH and redox potential in smoked turkey showed similar patterns under the three packaging combinations and different temperature regimes. A slight change in pH and redox potential was observed during the shelf life of the product in all of the control and treated smoked turkey samples. At the end of storage, a high drop in the pH and high increase in redox potential were observed under all storage temperatures and packaging conditions (Figs. 3, 4, 6, 7).
Changes in gases in MAP and MAPEO samples
The concentration of gases in the packages throughout the storage period showed only little change from the beginning until the end of storage, highlighting the effectiveness of the mechanical barriers for gas exchange.
The behaviour of L. monocytogenes in ready-to-eat (RTE) smoked turkey
From the control experiment on smoked turkey (without inoculation of L. monocytogenes Scott A), no L. monocytogenes were found throughout the period of storage (96 days) at 0, 5, 10 and 15 °C, which demonstrate that L. monocytogenes was likely not initially present in the smoked turkey used. During the shelf life of the product inoculated with L. monocytogenes, the maximum increases in the growth of L. monocytogenes were by 0.54, 0.72 and 0.74 log CFU/g at 0 °C, 0.78, 0.20 and 0.64 log CFU/g at 5 °C, 1.33; 0.72 and 0.78 log CFU/g at 10 °C; and 2.41, 1.02 and 0.32 log CFU/g) at 15 °C, under packaging with VP, MAP and MAPEO, respectively (Fig. 5). The application of MAP and MAPEO is within the U.S. Department of Agriculture, Food Safety and Inspection Service (2004) guidelines as it did not allow the pathogen to increase by more than 1 log CFU/g during the shelf life of the product, but the application of VP to samples did allow such an increase.
Under vacuum packaging
The initial level of the pathogen in the VP smoked turkey was 5.95 log CFU/g. The levels of L. monocytogenes increased to ≥ 7 log CFU/g after 76, 23, 2.5 and 1.5 days of storage at 0, 5, 10 and 15 °C, respectively. Storage at 0 and 5 °C allowed slower growth of L. monocytogenes than at 10 and 15 °C, but it did not allow the pathogen to increase more than 1 log CFU/g, while storage at 10 and 15 °C allowed the pathogen to increase by ca. 2 log CFU/g during the shelf life of smoked turkey. The ability of L. monocytogenes to grow on VP raw and industrially processed (frankfurters) beef and pork meat has been documented by other studies (Grau and Vanderlinde 1990a, b; Porto et al. 2002; Samelis et al. 2002). Therefore, a combination of low-temperature storage in the presence of packaging film was found to provide suitable condition for the survival of L. monocytogenes on smoked turkey for a period longer than 5 months. Another study indicated that VP enhances the survival of L. monocytogenes on meat products compared to aerobic storage (unpackaged products), especially at low temperatures (Gounadaki et al. 2007).
The initial levels of the pathogen in the smoked turkey were 5.28 log CFU/g. Indeed, MAP smoked turkey did not allow the pathogen to increase in the product by > 1 log CFU/g throughout the period of storage under all storage conditions. Rutherford et al. (2007) reported that CO2 packaging of ready-to-eat shrimp and storage at 3 °C not only had a great influence on growth of L. monocytogenes and other psychrotrophic bacteria but also increased the shelf life in comparison to vacuum packaging. In addition, storage temperatures higher than 3.3 °C did not inhibit the pathogen. The combination of MAP with all the storage regimes seemed to effectively inhibit growth and cause the death of L. monocytogenes, whose numbers decreased by > 1 log CFU/g after 116.83, 80.83, 64.83 and 40.83 days of storage at 0, 5, 10 and 15 °C, respectively. Carbon dioxide inhibition of L. monocytogenes is effective even at abusive temperatures as illustrated by Avery et al. (1995).
Under MAP with oregano essential oil
The initial level of L. monocytogenes Scott A in the smoked turkey was 5.26 log CFU/g. The MAPEO samples under different storage temperatures did not permit the growth of L. monocytogenes by more than log 1 CFU/g throughout the period of storage. Under MAPEO, the inoculum in the product was suppressed by ca. 5 log CFU/g (0 and 5), 10 and 15 °C after (150.79 days), 93.71 and 64.83 days of storage, respectively. A strong decrease in viable cells was noticed under MAPEO compared to MAP and VP under all temperatures. In another study by Mytle et al. (2006) on RTE frankfurters and clove essential oil, Scott A exhibited growth of 1.0 log10 CFU/g in 14 days when a high inoculum of 6.0 log10 CFU/g was applied at 5 °C. In contrast, Hao et al. (1998a) observed no growth at high populations of L. monocytogenes (Scott A) in the controls at 5 °C. Mytle et al. (2006) stated that the essential oil treatments exhibited a reduction in final L. monocytogenes populations at 5 °C and 15 °C storage as compared to the control in all treatments including Scott A. The essential oils do have greater effect in broth culture medium than in food as observed by Hao et al. (1998b).
Results assured that the inhibition of L. monocytogenes in RTE smoked turkey under MAP and MAPEO (at 0 and 5 °C) was enhanced compared to VP during the shelf life of the product. The MAPEO system could be efficiently used to control the growth of the pathogen when strict temperature control is difficult as during processing, transportation, retail display, or home use. By contrast, VP under different temperatures allowed for multiplication of the pathogen, with 1–3 log10 CFU/g.