Comparison of bacterial inactivation with novel agitating retort and static retort after mild heat treatments

Food Control 43 (2014) 150e154 Contents lists available at ScienceDirect Food Control journal homepage: www.elsevier.com/locate/foodcont Comparison of bacterial inactivation with novel agitating retort and static retort after mild heat treatments Mehmet Baris Ates a, b, *, Dagbjørn Skipnes a, Tone Mari Rode a, Odd-Ivar Lekang b a b Nofima AS, Richard Johnsens Gate 4, P.O. Box 8034, N-4068 Stavanger, Norway The Norwegian University of Life Sciences, Department of Mathematical Science and Technology, P. O. Box 5003, N-1432 Ås, Norway a r t i c l e i n f o a b s t r a c t Article history: Received 17 September 2013 Received in revised form 21 January 2014 Accepted 8 March 2014 Available online 20 March 2014 Lower thermal load on foods is desirable for food producers and consumers as the food gets higher quality. With reduced thermal load, the investigation of food safety is of importance. In this study, microbial inactivation efficacy of a new retort process with high frequency longitudinal agitation was compared to static retort process which was used as a benchmark. As a model, fish soup samples, inoculated with approximately 108 cells/ml Listeria innocua, was exposed to mild heat treatments at 62, 65 and 68  C. Results clearly demonstrated that agitating mode can provide equivalent lethality to the model organism L. innocua within significantly shorter heating times compared to static mode. Bacteria were not detected on TSA-YE plates after 11.5, 6.8 and 5.5 min processing in agitating mode; 77, 67 and 52 min processing in static mode at 62, 65 and 68  C respectively. Bacterial inactivation in agitating mode was generally correlated with estimated inactivation based on product core temperature, D- and z-values for L. innocua. This may indicate that distribution of the heat load over the soup was enhanced through agitation. Results showed that utilization of high frequency longitudinal agitation mechanism in retorts is promising for reducing the thermal load on food products without compromising on food safety related to non-spore forming pathogens. Ó 2014 Elsevier Ltd. All rights reserved. Keywords: Agitating retort Thermal inactivation Listeria innocua 1. Introduction Thermal processing in hermetically sealed containers is a common method to produce foods with extended shelf-life utilized historically since the Napoleonic era. Static retorts have been widely used for industrial applications of thermal processing. Early versions of these retorts used steam as the heating medium but later on water, steam/air, raining and spray water systems were also developed as heating media. In 1920s, agitating retorts were introduced for the first time where agitation mechanism was based on axial rotation (rolling of cans) (Eisner, 1988). Rotary retort was later developed with end-over-end rotation principle (Clifcorn, Peterson, Boyd, & O’Neil, 1950). Development of agitating retorts was the outcome of a need to overcome some weaknesses of conventional static retorts. The weaknesses included differing temperature zones within heated product, over-cooking, lack of consistency in texture and flavor in processed products and slow heat penetration (Eisner, 1988). On * Corresponding author. Nofima AS, Richard Johnsens Gate 4, P.O. Box 8034, N4068 Stavanger, Norway Tel.: þ47 96 95 27 94. E-mail addresses: baris.ates@nofima.no, brsates@gmail.com (M.B. Ates). http://dx.doi.org/10.1016/j.foodcont.2014.03.006 0956-7135/Ó 2014 Elsevier Ltd. All rights reserved. the other hand, agitation of packed foods has enabled more uniform distribution of heat and process time reduction. These benefits often give higher quality food products while being only applicable for liquid and semi-liquid foods (Rosnes, Skara, & Skipnes, 2011). Furthermore, product consistency, headspace, fillin weight and rotation speed has to be strictly controlled to prevent under-processing as these parameters influence the heat transfer effectiveness of agitating process (Awuah, Ramaswamy, & Economides, 2007). Recently, an agitating retort with high frequency longitudinal agitation mechanism was developed in 2006 (Fig. 1). The new process allows rotation speeds beyond the 20e 40 rpm range with the help of reciprocating agitation mechanism (Rosnes et al., 2011). Although critical factors for the new retort process are believed to be same as rotary retorts, investigation of food safety through microbial inactivation studies is of importance with such novel processes (Walden, 2008). Microbial inactivation studies with artificially inoculated foods are typically used for food safety studies with novel processes. Lower number of survivors than the hazardous level in the food product over a determined shelf-life period after thermal processing is desirable (NACMF, 2010). Traditionally, process lethality calculations have been based on thermal resistance data for M.B. Ates et al. / Food Control 43 (2014) 150e154 151 over the whole food product with agitating retort process in comparison to static process which was used as a benchmark. This was done by conducting bacterial inactivation experiments through a large set of heat treatments with fish soup samples inoculated with L. innocua model organism. The product was intended to have limited shelf life at refrigerated temperatures lower than 3.3  C. 2. Materials and methods 2.1. Fish soup preparation Fig. 1. Principle for the new retort system: Products (1) within the basket are agitated in longitudinal direction. The corresponding patent can be referred to for descriptions on each number shown in the figure modified from (Walden & Ferguson, 2007). bacteria and spores. Thermal resistance has been mathematically expressed by decimal reduction time (D-value) and z-value. D- and z-values are based on the assumption that thermal inactivation of bacteria follows log-linear kinetics. D value is the duration of heat treatment at a specific temperature necessary to kill 90% of the microbial population and z value is the temperature change required to shift D value by 1 log unit (Stumbo, 1973). Other available thermal inactivation models for microorganisms have been reviewed by (Smelt & Brul, 2014) for interested readers. Milder heat treatment is generally applied on foods designed to have short shelf-life under refrigeration temperatures. For such foods, Listeria monocytogenes contamination is a large problem as the bacterium can cause lethal diseases. Compared to other nonspore forming food-borne pathogens, Listeria is generally reported to have higher heat resistance as well as being able to grow at temperatures from 1.5  C up to 44  C (Hudson & Mott, 1993). Furthermore, L. monocytogenes has been reported to grow in foods from aw of 0.91e0.93 and pH value of 4.2 (FAO/WHO, 2004). It is generally agreed that sufficient pasteurization can eliminate L. monocytogenes. Mild heated products are required to be heated for at least 2 min at 70  C (at the coldest point) in order to achieve 6-log kill effect on L. monocytogenes (FAO, 1999, p. 34; Rosnes et al., 2011). In research studies, Listeria innocua is proposed as a potential surrogate microorganism for L. monocytogenes. This is because L. innocua is safer to work with as well as having major phenotypic similarity with L. monocytogenes (Kamat & Nair, 1996; Lorentzen, Ytterstad, Olsen, & Skjerdal, 2010; Miller, Gil, Brandão, Teixeira, & Silva, 2009). In a recent review, Milillo et al. (2012) recommended more precise selection of surrogate microorganisms for L. monocytogenes based on experimental conditions. Although several strains of L. innocua have been found to be more heat tolerant than L. monocytogenes (Friedly et al., 2008; O’Bryan, Crandall, Martin, Griffis, & Johnson, 2006; Sorqvist, 2003), these results may sometimes vary with process conditions and product matrix (Murphy, Duncan, Beard, & Driscoll, 2003). Lorentzen et al. (2010) compared the survival of L. innocua ATCC 33090, L. monocytogenes NCTC 11994 and No. 4006 and found ATCC 33090 to be the most heat tolerant one. L. innocua is apparently a useful model organism for inactivation studies but should not be directly used for thermal validation purposes. To the best of our knowledge, there are no studies published regarding microbial inactivation efficacy of the retort process with high frequency longitudinal agitation mechanism. Therefore, objective of this study was to investigate bacterial inactivation A common recipe for making fish soup was used. Ingredients consisted of approximately 2% fish bouillon, 8e8.5% fat (from butter, milk and cream), 0.66% salt, and the rest being mainly water. The soup was cooked, packed and treated with a sterilization pro10C  3 min based on core temperature. As a next step, gram of F121 C the soup was packed in polypropylene plastic trays (Promens, Kristiansand, Norway) with dimensions 9  4  13.2 cm and sealed with a plastic film. Samples were then stored at 1  C until the day of experiments. 2.2. Culture preparation and inoculation into fish soup samples L. innocua ATCC 33090 (Oxoid, Hampshire, U.K) was stored in Microbank (Pro-Lab Diagnostics, Canada) at 80  C. L. innocua was initially grown in Tryptic Soy Broth (Oxoid) supplemented with 0.6% w/w Yeast Extract (Merck, Darmstadt, Germany) (TSB-YE) at 37  C for 20 h at 150 rpm. The overnight culture was then subcultured in TSB-YE with 20 h incubation at 30  C, 150 rpm. Resulting cell density was 108e109 cells/ml. In order to concentrate the cells further, bacteria were centrifuged at 3500  g for 4 min and cell pellets were collected and re-suspended in peptone water (Merck) to obtain 1010e1011 cells/ml. Bacteria were then added to fish soup samples to obtain an initial concentration of approximately 108 cells per ml fish soup. Finally, inoculated fish soup samples, each having 350 g weight, were separately packed and shaken thoroughly to distribute the bacteria evenly before subsequent heat treatments. 2.3. Heat treatments A batch retort (Steriflow, Roanne, France) was used for all heat treatments since it was possible to run the retort both in agitating and static heating modes. The retort was previously calibrated and checked for even heat distribution and the heat transfer medium was steam and raining water. The range used in agitating mode was 80e100 strokes per minute (spm). There were two process batches where three replicates of inoculated fish soup were heated in each (n ¼ 6). Core temperature histories of three additional soup samples were recorded during treatments. A negative control (without bacteria) was also processed and analyzed in each process batch. For a detailed assessment of bacterial inactivation pattern with longitudinal agitating heating mode in comparison to static mode, three different mild temperatures (62, 65 and 68  C) were selected for investigation. Retort program phases were (1) come-up period for retort water temperature; (2) heating; (3e4) cooling (Fig. 2). Phases 1, 3 and 4 were always kept constant for each thermal treatment. Only the effect of selected timeetemperature combinations for heating phase (2) on bacterial inactivation was studied. For this purpose, time intervals analyzed in agitating mode were 4e 9.5 min (at 62  C), 3e4.8 min (at 65  C) and 1.5e3.5 min (at 68  C). In static mode, time intervals 40e70 min (at 62  C), and 30e65 min (at 65  C) and 25e50 min (at 68  C) were investigated. Additionally, 152 M.B. Ates et al. / Food Control 43 (2014) 150e154 dynamic heating condition was the case for all treatments, timee temperature history data was incorporated to isothermal heat inactivation kinetics to calculate the lethality (L) of each thermal treatment based on Equation (1) where Dref ¼ 2.4 min; Tref ¼ 60  C; z ¼ 4.71  C; t and Tcore were treatment time (min) and core temperature ( C) respectively (Monfort, Sagarzazu, Gayán, Raso, & Álvarez, 2012): L ¼ Fig. 2. Retort programs for thermal treatments in agitating and static modes (1: comeup period for retort water temperature, 2: heating period at constant retort temperature, 3e4: cooling periods). pre-heating of retort water temperature was 2 min for agitating and static heating modes for all treatments. Detailed settings of phases in agitating and static heating setups were as follows:  (1) Retort water heated up to set temperature (62, 65 or 68  C). Process time kept constant as 2 min with 80 strokes per minute (spm) shaking frequency in agitating mode whereas process time was 2 min in static mode without shaking,  (2) various temperature-time combinations at 100 spm shaking frequency in agitating mode and without agitation in static mode,  (3) retort water cooled down to 30  C in 2 min at 100 spm shaking frequency in agitating mode and in 3 min without shaking in static mode,  (4) retort water further cooled down to 20  C within 5 min at 80 spm shaking frequency in agitating mode and within 15 min without shaking in static heating mode The reason for choosing a longer cooling time for static process was to ensure fish soup samples were below ca. 45  C since cooling was slower in static process compared agitated cooling. The retort was otherwise too warm to touch before the samples could immediately be placed on ice-water. 2.4. Calculation of process lethality and core temperature data acquisition 1 Dref Zt Tcore Tref z 10 dt (1) 0 2.5. Enumeration of survivors Along with positive and negative controls, triplicate samples of 2 ml processed fish soup (taken from 350 g aseptically opened fish soup packs) were diluted in 18 ml peptone water and vortexed. Further ten-fold serial dilutions were prepared for subsequent surface plating on Tryptic Soya Agar (Oxoid) supplemented with 0.6% yeast extract (TSA-YE). Additionally, dilutions were plated on Brilliance Listeria Agar added with selective and differential supplements (Oxoid). The latter medium, that is selective for Listeria, can give information about cell injury, as injured cells have difficulties to grow on this medium (Hansen & Knochel, 2001; Miller, Brandão, Teixeira, & Silva, 2006). Eddy Jet spiral plater instrument (IUL Instruments, Barcelona, Spain) was used for surface plating procedure unless manual plating was necessary to count lower amounts of bacteria. All plates were incubated at 30  C for 2e5 days. 2.6. Statistical analysis of data L. innocua inactivation data was calculated as log10 (N0/N), where N0 is the initial bacteria concentration and N number of survivors on TSA-YE and Brilliance plates after heat treatment. Detection limit was 102 cells/ml. If there were no survivors on plates with lowest dilution, all bacteria were considered killed after the heat treatment. Results for each data point were obtained by calculating the mean-value and standard deviation from six replicates. The general linear modeling (GLM) and Tukey’s HSD test were used to compare means at significance level p < 0.05 using Minitab Statistical Software v15 (Minitab Ltd., Coventry, UK). 3. Results and discussion Process lethality was estimated only on agitating process samples based on coldest spot. Lethality was not calculated on static process samples since different temperature zones within the product causes misleading results. Classical thermal death model was used for process lethality calculations. Heat resistance data for the L. innocua culture was previously obtained as D60 C ¼ 2.40 min with a standard deviation (SD) of 0.04 and z ¼ 4.71  C with an SD of 0.08 after experiments made in TSB-YE medium. As a comparison, in previous studies D60 C was found 2.43  0.17 min for L. innocua ATCC 33090 in peptone water medium (Ahn & Balasubramaniam, 2007). Based on experiments with various food media, z-values for L. innocua were obtained 5.8  0.8  C (Sorqvist, 2003). Heat resistance data available for L. innocua culture was used for estimating microbial inactivation (in log units) based on lowest core temperature values (0.05  C accuracy). The temperature data was obtained via E-Val Flex thermocouple system (Ellab A/S, Hilleroed, Denmark) integrated to PC software (Valsuite, Ellab A/S). Threshold temperature value was set to 50  C where L. innocua inactivation was assumed to begin. Since 3.1. Heat penetration data Agitating mode caused faster product heating than the static mode for all experiments. As a representative for other heat penetration curves, selected heat penetration values and simulated core temperature data are shown (Fig. 3). 3.2. Microbial inactivation data Results showed agitating heating mode shortens process time needed to inactivate the model organism L. innocua compared to static mode. No colonies were detected after 11.5, 6.8 and 5.5 min processing in agitating mode compared to 77, 67 and 52 min processing in static mode at 62, 65 and 68  C respectively (Figs. 4 and 5). Another study concluded that the heat resistance of L. innocua in liquid medium decreases at higher heating rates shown with nonisothermal heating experiments at 1.5, 1.8 and 2.6  C/min from 20 to 65  C (Miller et al., 2011). Similar cases were also reported for L. monocytogenes (Quintavalla & Campanini, 1991) and Salmonella M.B. Ates et al. / Food Control 43 (2014) 150e154 153 Fig. 3. Timeetemperature history of fish soup and retort water temperature during agitating heating mode (62  C, 11.5 min process) and static heating mode (62  C, 72 min process). typhimurium (Mackey & Derrick, 1987). In the present study, the phenomenon is likely a synergistic factor for microbial inactivation since agitating heating mode provides faster heating rates in comparison to static mode. Bacterial inactivation data from TSA-YE plates and calculated log kill (based on product coldest point and first order model) after agitated processing was similar (Fig. 4). This may indicate the thermal load was rather homogenously distributed over the product in agitated process. Nevertheless, using traditional methods for thermal process validation is more convenient since the heat resistance among different bacterial strains may vary. L. innocua cells were sublethally injured after agitated processing for 8, 9, 9.5 and 10 min at 62  C; 5.5 min at 65  C; 4.5 min at 68  C and after static processing for 47, 32 and 27 min at 62, 65 and 68  C respectively (Figs. 4 and 5). Occurrence of cell injury was determined by the difference between colony counts on TSA-YE and Brilliance plates (p < 0.05). Highest difference observed was in the range of 4e5 log units suggesting that only 0.01e0.001% of the survivors were not sublethally injured in those cases. The possible explanation is that under insufficient heat treatment, injured cells appear to be able to repair themselves and grow on TSAYE medium but not on selective medium as similar cases were reported in previous studies (Crawford, Beliveau, Peeler, Donnelly, & Bunning, 1989; Mackey, Boogard, Hayes, & Baranyi, 1994). Competitive microflora, composition and temperature history of a model food product should be considered for inactivation studies. In the present study, potential effects from the competitive microflora were knocked out by using sterile fish soup. However, since Listeria has higher heat-resistance than other non-sporulating bacteria, survival risk of other pathogens such as Salmonella spp. and pathogenic Escherichia coli would be eliminated along with Listeria under sufficient heat treatment (Farber, 1989). Furthermore, cells may develop modified heat resistance based on food composition (high concentration of salt, acids, sugar and inhibitors) and prior heat shocks which have been reviewed in the literature (Doyle, Mazzotta, Wang, Wiseman, & Scott, 2001; Farber & Brown, 1990). This factor should be taken into account for such inactivation studies. Finally, attention should be given when processing liquid products with large solid particles. Underestimation of L. monocytogenes inactivation in ground beef with log-linear and Weibull model during dynamic heating was reported (Huang, 2009). In the present study, the product had neutral pH as well as being absent of solid particles and excessive amounts of salt. Fig. 4. Inactivation of L. innocua exposed to agitating heating at 62  C (a), 65  C (b) and 68  C (c). Black bars: inactivation data (log10) obtained from TSA-YE plates; gray bars: data from Brilliance plates; and white bars with dots show mathematically calculated log inactivation based on lowest core temperature data. Data are means from six replicate samples. Error bars represent standard deviation of each mean value. Retort cooling times were 7 and 18 min for agitating and static modes (not shown). 4. Conclusions Results clearly demonstrated that agitating retort process could deliver equivalent lethality to the model organism L. innocua within significantly shorter times compared to static process. No colonies were observed on TSA-YE plates after heat treatments of 11.5, 6.8 and 5.5 min in agitating mode; 77, 67 and 52 min in static mode at 62, 65 and 68  C respectively. Results could be relevant for food producers interested in minimal food processing. Furthermore, estimated bacterial inactivation was generally correlated with real inactivation with samples treated in agitating mode. The result was supportive of the assumption that the heat load through the soup was homogenously distributed with the help of agitation. Apparently, further inactivation studies using more heat-resistant L. monocytogenes strains (with 70  C/2 min process) and spores are also needed for understanding the efficacy of the agitating process at higher temperature ranges. Last but not the least, 154 M.B. Ates et al. / Food Control 43 (2014) 150e154 Fig. 5. Inactivation of L. innocua exposed to static heating mode at 62  C (a), 65  C (b) and 68  C (c). Black bars: inactivation data (log10) obtained from TSA-YE plates, gray bars show data from Brilliance plates; and white bars with dots show mathematically calculated log inactivation based on lowest core temperature data. Data are means from six replicate samples. 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