Combined Chitosan-Thyme Treatments with Modified Atmosphere Packaging on a Ready-to-Cook Poultry Product

Transcription

1 663 Journal of Food Protection, Vol. 73, No. 4, 2010, Pages Copyright G, International Association for Food Protection Combined Chitosan-Thyme Treatments with Modified Atmosphere Packaging on a Ready-to-Cook Poultry Product V. GIATRAKOU, A. NTZIMANI, AND I. N. SAVVAIDIS* Laboratory of Food Chemistry and Food Microbiology, Department of Chemistry, University of Ioannina, Ioannina 45110, Greece MS : Received 27 October 2009/Accepted 17 December 2009 ABSTRACT In the present study, natural antimicrobials chitosan and thyme, and their combination, were evaluated for their effect on the shelf life of a ready-to-cook (RTC) chicken-pepper kebab (skewer) stored under modified atmosphere packaging (MAP) conditions at 4 0.5uC for 14 days. The following treatments were examined: control samples stored under aerobic packaging (A), samples stored under MAP (M), samples treated with 1.5% chitosan (vol/wt) and stored under MAP (M-CH), samples treated with 0.2% thyme essential oil (vol/wt) (M-T), and samples treated with 1.5% chitosan (vol/wt) and 0.2% thyme essential oil (vol/wt) and stored under MAP (M-CH-T). Treatment M-CH-T significantly affected aerobic plate counts and counts of lactic acid bacteria, Pseudomonas spp., Brochothrix thermosphacta, Enterobacteriaceae, and yeasts and molds during the entire storage period. Similarly, lipid oxidation of the RTC product was retarded (M-CH-T treatment) during storage, whereas redness was maintained in M-T, M-CH, and M-CH-T samples. Based primarily on sensory data (taste attribute), M-CH and M-T treatments extended RTC product shelf life by 6 days, whereas M-CH-T treatment resulted in a product with a shelf life of 14 days that maintained acceptable sensory characteristics (shelf life of the control was 6 days). Retail refrigerated ready-to-cook (RTC) food products are gaining popularity for their convenience and freshness and consist of one or several fresh, uncooked elements. Chicken-pepper kebab is a popular RTC Greek product consisting of fresh chicken meat (chunks) and cut green bell peppers fixed on a skewer. This RTC product requires minimal (4 to 5 min) of cooking before consumption, usually in a conventional grill and/or in a microwave oven at home. However, the problem with RTC products prepared (usually manually) with poultry meat (chunks) plus vegetables (mostly cut bell peppers) is the relatively short shelf life (6 days). The high initial microbial load often found in these products (12, 23) may result from (i) initial contamination of the raw material, (ii) contamination associated with preparation (i.e., manual handling and mixing of components), (iii) poor personal hygiene of food handlers, and (iv) inadequate cleaning of processing equipment. The challenge for poultry processing companies is to develop new methods that will extend the shelf life and ensure the safety of such poultry products. Because of the increasing interest by consumers in fresher and more natural meals, food industries have been faced with the need to develop new preservation methods using natural additives. These new meals and products should be of restaurant quality, should project a green image, should be readily available at the supermarket, and most importantly should have adequate shelf life and safety. Several studies have revealed the beneficial effects of natural antimicrobials individually and/or in combination * Author for correspondence. Tel: z ; Fax: z ; isavvaid@uoi.gr. when applied in food systems. Roller et al. (18) developed a novel preservation system involving the combined use of chitosan glutamate, carnocin, and sulfite for the preservation of pork sausages. Zivanovic et al. (28) incorporated oregano oil in chitosan films to enhance the antimicrobial properties of the film against two pathogens inoculated onto bologna slices. Georgantelis et al. (5, 6) studied the effect of rosemary extract, a-tocopherol, and chitosan on microbial parameters and lipid oxidation of fresh pork sausages and frozen beef burgers. Chitosan is a product that is generally recognized as safe by the U.S. Food and Drug Administration (22), and it has broad-spectrum antimicrobial activity against grampositive and gram-negative bacteria and fungi (15). Potential applications of chitosan as a biopreservative have been investigated in various meat products (3, 5, 6, 14, 15, 18, 19, 21, 26). With regard to chitosan applications in meat preservation, Yingyuad et al. (26) studied the combined effect of chitosan coating and vacuum packaging on the quality of refrigerated grilled pork, and Ye et al. (24) tested the effectiveness of chitosan-coated plastic films incorporating antimicrobials for inhibition of Listeria monocytogenes on cold-smoked salmon. Essential oils (EOs) possess antibacterial, antiviral, antifungal, and antioxidant properties (2, 8). Among the EOs, thyme oil has increasingly attracted the interest of researchers as a potential natural antimicrobial to be used in food processing (2). Generally, thyme oil has strong antibacterial activity against both foodborne pathogens and spoilage organisms because of its high percentage of phenolic compounds such as thymol, p-cymene, carvacrol, and c-terpinene (2, 8).

2 664 GIATRAKOU ET AL. J. Food Prot., Vol. 73, No. 4 To our knowledge, the use of chitosan as a natural antimicrobial, either individually or in combination with EOs such as thyme oil, has not been studied in fresh poultry meat and poultry products. Thus, the objective of the present work was to determine the effect of chitosan and thyme oil, applied individually or in combination, on microbiological, physicochemical, and sensory parameters of an RTC product (fresh chicken-pepper kebab) during refrigerated storage under modified atmosphere packaging. MATERIALS AND METHODS Preparation, storage conditions, and packaging of samples. The chicken-pepper kebabs (an RTC product of ca g each) were supplied by a local poultry processing company (PINDOS, S.A., Rodotopi, Ioannina, Greece). The production of the RTC samples involved fresh chicken pieces and chopped bell peppers manually fixed on a wooden stick (skewer). RTC samples were transported to the laboratory in insulated polystyrene boxes on ice within 1 h after processing and were subsequently kept under refrigeration (4 0.5uC) until the addition of the antimicrobials. Preparation of chitosan and thyme solutions. Chitosan of low molecular weight (340) in powder form made from crab shells was purchased from Aldrich Company (Athens, Greece). Moisture content was less than 10%, and chitosan had a deacetylation degree of 75 to 85% (manufacturer s data). A stock solution of chitosan was prepared by dissolving 2 g in 100 ml of 1% (wt/vol) glacial acetic acid and stirring overnight at room temperature. The final chitosan concentration was 2% (wt/vol). Pure thyme oil from Thymus vulgaris (Mane Fils Company, Le Bar Sur Loup, France) consisted of 57.7% thymol, 18.7% p-cymene, and 2.8% carvacrol (major components, manufacturer s data). Application of the antimicrobials to the RTC samples. The antimicrobials were added to the RTC product, either singly or sequentially using the following procedure. An RTC skewer ( g) was transferred aseptically into an open pouch made of a sandwiched material: low density polyethylene, polyamide, and low density polyethylene (LDPE-PA-LDPE; VER PACK, Thessaloniki, Greece). Chitosan (final concentration of 1.5%, vol/wt) was sprayed directly onto the product, and undiluted thyme oil (final concentration of 0.2%, vol/wt) was added with a micropipette. For all treatments, the antimicrobials were massaged with gloved fingers (to avoid cross-contamination of samples and transmission of foodpoisoning organisms) into the product, including both chicken and pepper pieces, to achieve even distribution of the antimicrobials. Treatments. Control samples were packaged under aerobic conditions without antimicrobial agents (treatment A). Test samples were stored under modified atmosphere packaging (MAP) in the absence of antimicrobials (treatment M), under MAP with 0.2% thyme oil (treatment M-T), under MAP with 1.5% chitosan (treatment M-CH), and under MAP treated with 1.5% chitosan and 0.2% thyme oil (treatment M-CH-T). The final optimum concentration of each antimicrobial applied to the RTC samples was established following preliminary microbiological analysis, i.e., determination of aerobic plate counts (APCs) and sensory evaluation of the RTC samples treated with these antimicrobials (results not shown). Packaging of samples. The RTC samples (one chickenpepper skewer per pouch) were packaged in barrier pouches of LDPE-PA-LDPE that were 75 mm thick with an oxygen permeability of 52.2 cm 3 /m 2 /day/atm at 75% relative humidity and 23uC, a carbon dioxide permeability of 191 cm 3 /m 2 /day/atm at 0% relative humidity and 23uC, and a water vapor permeability of 2.4 g/m 2 /day at 100% relative humidity and 23uC. Control samples were conventionally packaged in the LDPE- PA-LDPE pouches under atmospheric conditions and heat sealed with a BOSS N48 packaging machine (Helmut Boss Verpackungsmaschinen KG, Bad Homburg, Germany). Pouches containing RTC samples to be stored under MAP (M, M-T, M-CH, and M-CH-T treatments) were first evacuated and then immediately injected with the gas mixture (30% CO 2 and 70% N 2 ) produced by a model 9000 gas mixer (PBI-Dansensor, Ringsted, Denmark) and heat sealed with the packaging machine connected to the gas mixer. Three samples from each experiment were analyzed at each sampling time. All samples (control and treated) were kept under refrigeration (4 0.5uC ) for 14 days. Sampling was carried out at predetermined intervals: 0, 2, 4, 6, 8, 10, 12, and 14 days. Microbiological analyses. Three samples of 25 g each (consisting of equal amounts of chicken and pepper pieces) were aseptically removed from the RTC chicken-pepper skewer at 2-day intervals for microbial examination. Samples were homogenized with 225 ml of 0.1% (wt/vol) sterile buffered peptone water (Merck, Darmstadt, Germany) by blending for 1 min on high speed in a stomacher (Stomacher 400 Lab Blender, Seward Medical, London, UK). Determination of APCs and counts of Pseudomonas spp., Brochothrix thermosphacta, lactic acid bacteria, Enterobacteriaceae, and yeasts and molds. Serial decimal dilutions of homogenized samples made in 9-ml tubes of buffered peptone water were used for enumeration and differentiation of microorganisms. APCs were determined using plate count agar (Merck) after incubation for 48 to 72 h at 30uC. Pseudomonas spp. were enumerated on Pseudomonas agar base (Oxoid, Basingstoke, UK) supplemented with cetrimide, fucidine, and cephaloridine supplements (SR 103, Oxoid), providing a selective isolation medium for Pseudomonas spp. Colonies were counted after 2 days at 30uC, and an oxidase test was performed. B. thermosphacta was enumerated on STAA medium supplemented with streptomycin sulfate, thallous acetate, and cycloheximide (actidione) made from basic ingredients in the laboratory, as described by Atlas (1), and incubated at 30uC for 48 h. Confirmed B. thermosphacta colonies were positive for the Gram and catalase tests and whereas negative for the oxidase test. As a reference, one strain of B. thermosphacta 847 (ATCC 11589) was used. Lactic acid bacteria (LAB) were enumerated by the pour plate technique on de Man Rogosa Sharpe agar (code CM361, Oxoid) and incubated at 30uC for 3 days. LAB colonies were confirmed using the catalase test (negative for Lactobacillus) and Gram test (positive for Lactobacillus). Enterobacteriaceae counts were determined using the pour plate technique on violet red bile glucose agar (Oxoid) after incubation at 37uC for 24 h. Yeasts and molds were enumerated on rose Bengal chloramphenicol selective agar (Merck) after incubation for 3 to 5 days at 30uC. Average counts were calculated on the basis of quadruplicate values for each time point. Physicochemical analyses: determination of ph. One RTC sample (25 g) was homogenized thoroughly with 225 ml of distilled water, and the homogenate was used for ph determination using a ph meter (HI 9219, Hanna Instruments, Woonsocket, RI). Determination of lipid oxidation. The 2-thiobarbituric acid (TBA) assay was carried out according to the procedure of

3 J. Food Prot., Vol. 73, No. 4 EFFECT OF CHITOSAN AND THYME OIL ON A POULTRY PRODUCT 665 Schmedes and Hølmer (20). TBA values were expressed as milligrams of malondialdehyde (MDA) per kilogram of chicken sample. Color measurement. Superficial color alterations were monitored with a colorimeter (model DP-9000, HunterLab, Cambridge, MA) as described by Du et al. (4). Color parameters were measured directly on the surface of the RTC product after opening the packages (on day 0 of storage and every 2 days throughout the storage period): a* (redness), b* (yellowness), and L* (lightness) and the total color difference DE*, defined by equation DE* ~ [(L* 2 L 0 ) 2 z (a* 2 a 0 ) 2 z (b* 2 b 0 ) 2 ] 1/2, where L 0, a 0, and b 0 are the L*, a*, and b* values of the control sample on day 0. Sensory analyses. The attributes of microwave-cooked RTC product (odor and taste) were evaluated by a panel of seven experienced judges on each day of sampling. Samples were cooked individually in a microwave oven at high power (800 watts) for 5 min and immediately presented to the panelists. Freshly thawed RTC samples (previously stored at 30uC ) also were presented to the panelists (reference sample). Sensory evaluation was conducted in individual booths under controlled conditions of light, temperature, and humidity. Panelists were asked to score odor and taste of cooked RTC samples using a 0 to 9 intensity scale from least liked to most liked, where 9 represents excellent, 8 represents very good, 7 represents good, 6 represents acceptable, and,6 represents unacceptable (17). The product was defined as unacceptable (a score of,6) when it developed any off-odor. Statistical analyses. Experiments were replicated twice on different occasions with different samples, and three samples were taken per replicate (n ~ 3 2). Data were averaged, converted to log CFU per gram, and subjected to an analysis of variance (ANOVA) using Stat Graphics software (Statistical Graphics Corp., Rockville, MD). Means and standard deviations (SDs) were calculated, and when F values were significant at the P, 0.05 level, mean differences were separated by the least significant difference procedure. RESULTS AND DISCUSSION Microbiological changes in the RTC product stored under MAP with and without antimicrobials. Changes in APCs and counts of LAB, pseudomonads, B. thermosphacta, Enterobacteriaceae, and yeasts and molds of all control (treatment A) and treated (M, M-CH, M-T, and M-CH-T) RTC samples at 4uC are shown in Figure 1A, 1B, 1C, 1D, 1E, and 1F, respectively. The ANOVA revealed a significant effect of storage time and packaging conditions on the APC (P, 0.05) (Fig. 1A). APCs for the RTC samples reached or exceeded 7.0 log CFU/g, considered the upper limit of acceptability for fresh meat (9), on day 4 (control samples, treatment A), day 6 (treatment M), and day 9 (treatments M-T and M-CH) of storage. M-CH-T samples reached the limit of 7.0 log CFU/g after 13 days of storage. Thus, compared with the control samples (A), the microbiological shelf-life extension was 2 and 5 days for M and M-T and M-CH samples, respectively, and 9 days for M-CH-T samples. The 5-day shelf-life extension for M-T and M-CH samples could be due to the antimicrobial action of the components of thyme oil (especially thymol) and chitosan acting on spoilage microorganisms (2, 7, 8, 16). Of the treatments examined in the present study, M-CH-T was the most effective for inhibiting the growth of total mesophilic aerobic bacteria (as indicated by APCs) throughout the storage period. In other related studies, the combined use of rosemary extract and chitosan on the preservation of fresh pork sausages led to a reduction in the APC (by ca. 1 to 2 log CFU/g), extending their shelf life at 4uC (5). Zivanovic et al. (28) reported that the addition of oregano EO to the chitosan film both improved the film s antimicrobial properties and strongly reduced pathogens on bologna slices. LAB in all treatments (Fig. 1B) increased during storage. M-CH and M-T samples had similar LAB growth patterns (P. 0.05), demonstrating the similar antimicrobial effect of chitosan and thyme oil when applied individually under MAP conditions. However, M-CH-T samples had lower LAB populations (P, 0.05) than did M-CH and M-T samples during the storage period, final LAB populations were 4-log lower in M-CH-T samples than in the control and M samples. Georgantelis et al. (5) reported that throughout refrigerated storage of Greek-style sausages containing chitosan and rosemary extract, the LAB counts were approximately 2.0-log lower than those for the control samples. Pseudomonas spp. initial counts (day 0) were 5.05 log CFU/g and increased to reach a final population of ca. 9.8 log CFU/g (A samples), whereas respective counts for M, M-T, and M-CH samples were approximately 1 to 2.3 log lower (P, 0.05) than those of the control samples (Fig. 1C). As previously found for APCs and LAB counts, Pseudomonas spp. populations were significantly lower (P, 0.05) for M-CH-T samples than for all other treatments, resulting in populations of approximately 6 log CFU/g. Of all the treatments examined in the present study, M-CH-T appears to be the most effective for the inhibition of pseudomonads in RTC samples, probably because of the additive or synergistic antimicrobial action of the chitosan plus thyme combination. B. thermosphacta reached high final counts in M samples, indicating that MAP was not effective when applied individually to inhibit the growth of this microorganism (Fig. 1D). Treatments M-T and M-CH were effective for inhibiting the growth of this gram-positive meat spoilage bacterium, reducing final populations significantly (P, 0.05) by approximately 2 to 3 log CFU/g compared with the control (A) samples. Treatment M-CH-T attained a low final population (4.7 log CFU/g), thus achieving a significant 4.5-log reduction (P, 0.05) compared with the control (A) samples. Soultos et al. (21) applied 0.5 and 1% (wt/vol) chitosan to fresh pork sausages and reduced B. thermosphacta counts by 0.5 to 1.5 log CFU/g in the first day of chilled storage, and final populations were less than those of the control samples. Initial Enterobacteriaceae counts (4.2 log CFU/g; Fig. 1E) indicate inadequate processing conditions during handling and preparation of the RTC chicken-pepper kebab samples. M-T and M-CH treatments produced lower Enterobacteriaceae counts than found in the control

4 666 GIATRAKOU ET AL. J. Food Prot., Vol. 73, No. 4 FIGURE 1. Changes in APC (A), lactic acid bacteria (LAB) (B), Pseudomonas spp. (C), Brochothrix thermosphacta (D), Enterobacteriaceae (E), and yeasts and molds (F) of a ready-to-cook (RTC) chicken-pepper kebab (skewer) during storage at 4 0.5uC under aerobic packaging (treatment A) and under modified atmosphere packaging (MAP) alone (treatment M), with thyme oil (treatment M-T), with chitosan (treatment M-CH), and with chitosan plus thyme oil (treatment M-CH-T). Each point is the mean of three samples taken from two replicate experiments (n ~ 3 2 ~ 6). Error bars show the SD. samples during storage, whereas the M-CH-T treatment had a significant effect (P, 0.05) on this bacterial group, reducing its viability by approximately 4 log CFU/g by the end of the storage period (day 14) as compared with the control samples. Similarly, Roller et al. (18) reported that chitosan in combination with low sulfite content selectively inactivated these bacteria to an undetectable level (as determined by the plate count method) in fresh pork sausages stored at 4uC. For yeasts and molds, all of the antimicrobial treatments examined in the present study produced significantly lower counts (P, 0.05) than were found in the control samples, especially after day 6 of storage (Fig. 1F). The combination treatment (M-CH-T) suppressed the growth of these species, and their counts were below 3.0 log CFU/g at the end of storage period. The combination of antimicrobials under MAP conditions led to approximately 4.0- and 2.5-log reductions compared with the A and M samples, respectively. Physicochemical changes. The initial ph of the RTC poultry product was ca. 6.2 (results not shown), in agreement with ph values (6.3 to 6.4) of Greek-style sausages containing chitosan with rosemary extract (5). RTC samples stored in air (treatment A) had the largest ph increase (P, 0.05), whereas the largest drop in ph values (P, 0.05) was found in M samples. For M-CH and M-CH-T treatments, ph values were 6.1 to 6.3 (not significantly differences, P. 0.05) during storage. These values are lower (P, 0.05) than those recorded for the control samples

5 J. Food Prot., Vol. 73, No. 4 EFFECT OF CHITOSAN AND THYME OIL ON A POULTRY PRODUCT 667 FIGURE 2. Changes in TBA (milligrams of MDA per kilogram) of a ready-to-cook (RTC) chicken-pepper kebab (skewer) during storage at 4 0.5uC under aerobic packaging (treatment A) and under modified atmosphere packaging (MAP) alone (treatment M), with thyme oil (treatment M-T), with chitosan (treatment M-CH), and with chitosan plus thyme oil (treatment M-CH-T). Each point is the mean of three samples taken from two replicate experiments (n ~ 3 2 ~ 6). Error bars show the SD. (treatment A) probably because of the presence of chitosan, as also reported by Lin and Chao (14) for reduced-fat sausages containing chitosan. Changes in lipid oxidation of the RTC product stored at 4uC under MAP with and without antimicrobials are shown in Figure 2. M-CH, M-T, and M-CH-T treatments produced the lowest MDA values (P, 0.05) compared with the control or M samples. Lipid oxidation of RTC samples treated with the M-CH-T antimicrobial combination during the 14-day storage period resulted in lower values (by ca. 1.0 to 2.0 mg MDA/kg) than those of the control samples. Chitosan may retard oxidative rancidity in muscle foods by acting as a chelator of transition metal ions, such as ferrous ions, which can initiate lipid peroxidation and start chain reactions that lead to deterioration of flavor and taste in foods (25). To our knowledge, there are no reports of the combined effect of MAP, chitosan, and thyme oil on lipid oxidation in poultry products. In other studies, chitosan prevented lipid oxidation in sausages and beef burgers containing rosemary extract (5, 6) and in pork salamis with mint extract (11). Changes in color parameters L*, a*, and b* and the total color difference DE* are shown in Figure 3A, 3B, 3C, and 3D, respectively. A, M, and M-T samples had the lowest L* values, whereas samples with added chitosan (M-CH and M-CH-T treatments) had higher values at the end of the storage period. RTC samples with added chitosan had higher L* values (P, 0.05) than did A and M samples, indicating that the presence of chitosan resulted in an increase in lightness of the RTC product compared with the control samples. However, contradicting results have been reported in the literature, and in one study Jo et al. (10) reported that sausages containing chitosan had higher L* values than did the control samples during the storage period, whereas Darmadji and Izumimoto (3) found that meat samples containing chitosan had lower L* values than did the controls. Hunter color a* values are shown in Figure 3B. During the fist 8 days of storage, these values did not show any significant changes among the various treatments (P. 0.05). However, from day 8 to day 14 of storage, a* values increased for M-T and M-CH samples, followed by M-CH- T samples, whereas respective values for control and M samples progressively decreased. Jo et al. (10) and Youn et al. (27) reported that chitosan affected the a* values of sausages, resulting in a redder surface color. Georgantelis et al. (6) noted that the combination of chitosan and rosemary extract acted synergistically and improved the redness of beef burgers during frozen storage, whereas individual use of chitosan or rosemary extract improved color stability compared with that of the controls. The b* (yellowness) values varied from 6.44 to 10.4, with no specific pattern produced by any of the treatments (Fig. 3C). Overall, samples stored under aerobic or MAP conditions without the addition of antimicrobial agents had the highest total color differences (P, 0.05) compared with their initial color values, according to DE* calculations (Fig. 3D). However, samples containing both chitosan and thyme oil maintained their color values near initial levels for a longer period than did sample from other treatments, as shown by their lower DE* values. Both chitosan and thyme oil may act as antioxidants in food products, including poultry meat. With regard to EOs, thyme oil has a high total phenol concentration and thus could act as an efficient antioxidant, preventing oxidation of red heme pigments to brown metmyoglobin (13). Chitosan also has antioxidant properties and may maintain redness in muscle foods because of its ability to act as a chelator of transition metal ions, which catalyze oxidative reactions (25) such as oxidation of myoglobin and oxymyoglobin. Sensory changes of the RTC product stored under MAP without and with antimicrobials. The results of the sensory evaluation (taste and odor) of cooked RTC samples stored under MAP without and with antimicrobials are presented in Figure 4A and 4B, respectively. Generally, the taste attribute was a more sensitive parameter than odor; therefore, in our study the taste attribute was used for the determination of the shelf life of cooked RTC products, both untreated (treatment A) and treated (treatments M, M-T, M-CH and M-CH-T) samples. On day 0 of storage, RTC A samples (control untreated) had a pleasant taste and odor. The ANOVA gave a shelf life (based on taste attribute data) of 6 days for A samples and M samples, whereas samples containing thyme oil (M-T) or chitosan (M-CH) reached the limit of sensory acceptability (sensory score of 6.0) after 12 days. Samples containing chitosan plus thyme oil (M-CH-T) were rejected only at the end of the storage period (14 days; Fig. 4A). The presence of chitosan in M-CH and M-CH-T samples produced a very pleasant taste and odor in the RTC products, promoting the natural freshness and aroma of the chicken-pepper kebabs. Thyme acted additively or synergistically with chitosan (M-CH-T samples), resulting in a spicy and desirable (organoleptically acceptable) product. According to Soultos et al. (21) and Roller et al. (18), the addition of chitosan to sausages gave more acceptable odor and flavor compared with the untreated samples.

6 668 GIATRAKOU ET AL. J. Food Prot., Vol. 73, No. 4 FIGURE 3. Changes in color parameters L* for lightness (A), a* for redness (B), b* for yellowness (C) and in the total color difference DE* (D) of a ready-to-cook (RTC) chicken-pepper kebab (skewer) during storage under aerobic packaging (treatment A) and under modified atmosphere packaging (MAP) alone (treatment M), with thyme oil (treatment M-T), with chitosan (treatment M-CH), and with chitosan plus thyme oil (treatment M-CH-T). Each point is the mean of three samples taken from two replicate experiments (n ~ 3 2 ~ 6). Error bars show the SD. Yingyuad et al. (26) found that aerobically stored and vacuum-packaged Thai-style grilled pork reached unacceptable sensory scores after 7 and 14 days, respectively, whereas vacuum-packaged samples coated with chitosan maintained acceptable sensory quality throughout the storage period of 28 days at 2uC. In our study, addition of thyme oil (0.2%, vol/wt) in cooked M-T and M-CH-T samples produced a distinct and organoleptically acceptable odor, and the product was well received by the panelists. Both chitosan and EOs are regarded as natural food antimicrobials, are relatively cheap to use, and most importantly may improve the sensory characteristic of food products (as shown in our study). These factors should be exploited to develop new and preservative-free products. Results of the present study indicate that the combined use of chitosan and thyme EO under MAP conditions can (i) inhibit growth of microbial spoilage flora, (ii) retard lipid oxidation, (iii) maintain redness, and (iv) improve the sensory quality of an RTC poultry product (chicken-pepper kebab). FIGURE 4. Changes in taste (A) and odor (B) of a ready-to-cook (RTC) chicken-pepper kebab (skewer) during storage at 4 0.5uC under aerobic packaging (treatment A) and under modified atmosphere packaging (MAP) alone (treatment M), with thyme oil (treatment M-T), with chitosan (treatment M-CH), and with chitosan plus thyme oil (treatment M-CH-T). Each point is the mean of three samples taken from two replicate experiments (n ~ 3 2 ~ 6). Error bars show the SD.

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