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PET trays coated with Citrus extract exhibit antioxidant activity with cooked turkey meat
LWT - Food Science and Technology 47 (2012) 471e477
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PET trays coated with Citrus extract exhibit antioxidant activity with cooked turkey meat C. Contini a, M.G. Katsikogianni b, F.T. O’Neill b, M. O’Sullivan a, D.P. Dowling b, F.J. Monahan a, * a b
UCD School of Agriculture and Food Science, University College Dublin, Belfield, Dublin 4, Ireland UCD School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2011 Received in revised form 2 February 2012 Accepted 7 February 2012
A natural Citrus extract with potential antioxidant activity was evaluated as an ingredient for the production of active food packaging. The extract, in methanol, was sprayed onto the surface of polyethylene terephthalate (PET) trays. For comparison, a second set of trays were prepared using atocopherol as the coating. The effectiveness of the two types of packaging in delaying lipid oxidation in cooked turkey meat slices, stored at 4 C over 4 days, was compared using 2-thiobarbituric acid-reactive substances (TBARS) and hexanal assays. TBARS and hexanal values, for meat stored on the Citrus extract coated trays, were significantly lower (P < 0.01) than those of meat stored on uncoated (control) trays while a-tocopherol coated trays exhibited no significant effect compared to control trays (P > 0.05). The effectiveness of the Citrus extract coating, compared to the a-tocopherol coating, was attributed to its higher surface roughness, demonstrated by optical profilometry, and the higher level of release (solubility) of the antioxidant in water. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Active packaging Citrus extract a-Tocopherol Lipid oxidation Meat
1. Introduction As demand for food products capable of maintaining their sensory and nutritional quality over longer period increases, new advances in food packaging are required. Among the options, active packaging can improve the organoleptic properties and prolong the shelf life of packaged food products (De Kruijf et al., 2002). One specific application of active packaging is to improve sensory and nutritional quality by reducing the oxidative deterioration of foods without the direct addition of antioxidants, including synthetic antioxidants, to the food. It is well established that oxidation in muscle foods during storage has detrimental effects on food flavor (Gray & Pearson, 1987), texture (Lund, Lametsch, Hviid, Jensen, & Skibsted, 2007) and healthiness (Kanner, 2007). A potential solution is an active package consisting of food contact surfaces coated with natural antioxidant compounds. Indeed, antioxidants have been incorporated into food packaging by including them in film forming solutions which are subsequently cast on plates (Park & Zhao, 2004; Siripatrawan & Harte, 2010) or including them (rosemary extract, BHT) in polyolefin films (Nerín et al., 2006; SotoCantú, Graciano-Verdugo, Peraltra, Islas-Rubio, & González-
* Corresponding author. Tel.: þ353 1 716 2842. E-mail addresses: [email protected] (D.P. Dowling), [email protected] (F.J. Monahan). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2012.02.009
Córdova, 2008). The release of the antioxidant from the film and its effectiveness in real food system was reported in the study of SotoCantú et al. (2008) using cheese packed in a BHT-containing film. In the present study, a Citrus extract containing a mixture of bioflavonoids was examined as a potential ingredient for the development of active antioxidant packaging, in which the natural antioxidant is applied as a coating on the food contact surface. Citrus fruits contain bioflavonoids which are known to act as antioxidants by scavenging the free radicals formed during oxidative processes, particularly during the propagation of oxidative reactions. Specifically, some bioflavonoids known to be present in Citrus fruit have been shown to have antioxidant potential by monitoring DPPH (1.1-diphenyl-2-picrylydrazyl) radical scavenger ability (Lee, Lee, Kim, & Jeong, 2009; Wilmsen, Spada, & Salvador, 2005). However, to-date no studies have been published on the effectiveness of a Citrus extract in reducing lipid oxidation in foods when used in an active packaging application. The objective of this study was to investigate the efficacy of Citrus extract and a-tocopherol, sprayed (nebulized) onto the surface of recycled PET food trays, in preventing lipid oxidation in cooked meat. a-Tocopherol was chosen as a second natural antioxidant for comparison with Citrus extract, because of its proven efficiency in delaying oxidative processes when added directly to fatty foods (Djenane, Sánchez-Escalante, Beltran, & Roncalés, 2002; O’Sullivan, Lynch, Lynch, Buckley, & Kerry, 2004), or when incorporated into animal-derived foods via animal diets (Monahan et al.,1992). Slices of
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cooked turkey meat were stored on coated trays at 4 C for 4 days and the ability of the Citrus extract and a-tocopherol coatings to reduce lipid oxidation compared. Turkey meat was chosen as test food due to its relative abundance of PUFA (Pignoli, Bou, Rodriguez-Estrada, & Decker, 2009) which make it suitable for the evaluation of oxidative processes (Caprioli, O’Sullivan, & Monahan, 2009). 2. Material and methods 2.1. Meat Fresh turkey breasts were obtained from IGWT Poultry Service Ltd., County Monaghan, Ireland. The bones and the visible fat were removed. The turkey breasts were divided into two portions of similar size (w1.2 kg), vacuum packed, stored at 20 C and used within 6 months. 2.2. Reagents
a-Tocopherol (96%) was obtained from SigmaeAldrich Ireland Ltd, Dublin, Ireland. Citrus extract, containing a mixture of bioflavonoids with potential antioxidant activity, was obtained from a commercial source (Citrox Ltd, Middlesbrough, England). The composition of the Citrus extract, as per the manufacturer’s specification, was narangin 3.6%, neohesperidin 1.9%, rhoifolin 0.4%, poncirin 0.3%, naringenin 0.2%, hesperidin 0.2%, malic acid 15%, ascorbic acid 15%, and citric acid 15%. Hydrochloric acid (37%), 2thiobarbituric acid (TBA, 98%), 1,1,3,3,-tetra-ethoxypropane (TEP 96%) and acetic acid were supplied by SigmaeAldrich Ireland Ltd, Dublin, Ireland. Screw-cap plastic tubes (10 mL) and PYREX screw-cap test tubes (10 mL) were supplied by Sarstedt Ltd., Wexford, Ireland. Recycled polyethylene terephthalate (PET) trays (100 150 mm) were supplied by Holfeld Plastics (Arklow, Co Wicklow, Ireland). Low-density polyvinylchloride (PVC) catering film (thickness 7.0 mm; O2 transmission 2000 cm3 m2 d1 bar1) was supplied by Western Plastic Ltd., Galway, Ireland. Polydimethylsiloxane/divinylbenzene (PDMS/DVB) solid phase microextraction (SPME) fibers (65 mm) were obtained from Supelco (SigmaeAldrich Ireland Ltd, Dublin, Ireland). 2.3. Deposition of a-tocopherol and Citrus extract coatings on PET trays
a-Tocopherol and Citrus extract were dissolved in ethanol and methanol, respectively, at 100 mg mL1 and deposited on recycled PET trays (Holfeld Plastic Ltd, Wicklow, Ireland) using a handmade Teflon nebulizer mounted on a computer numerical control (CNC) system (cncGraf4, Auto Grav, Boenigh Electronics, Bonn, Germany). The nebulizer was moved over the substrate in a raster pattern with a line speed of 7 mm s1. The solutions of a-tocopherol or Citrus extract were pumped from a remote syringe pump to the nebulizer, at a flow rate of 100 mL min1, and converted into an aerosol using the flow of helium through the nebulizer at a flow rate of 5 L min1. The resultant helium-aerosol mix was directed to the substrate through a 23 mm long and 15 mm diameter teflon tube. The substrate to tube orifice distance was fixed at 2 mm. The nebulization was repeated two times. A coating density of 0.45 mg cm2 for the Citrus extract and 0.28 mg cm2 for the a-tocopherol was calculated as the difference in weight of a tray before and after the coating procedure. 2.4. Surface characterization of the a-tocopherol and Citrus extract coatings on PET trays Surface morphology, roughness and film thickness of the atocopherol and Citrus extract coatings were evaluated using a Wyko
NT1100 optical profilometer in vertical scanning interferometry (VSI) mode, which allows a 3 dimensional image of the coating to be built over several hundred nanometers. Three roughness parameters were evaluated: the average surface roughness (Ra), which is the arithmetic mean of peaks and valleys or deviations from the centerline over the sampling length, the root mean square roughness (Rq or RMS), which is the root mean square measurement of the peaks and valleys deviations from the centerline and the maximum distance between the highest peak and the lowest groove (Rt). Line plots of the surface profile over a step height indicated the variation of the coating thickness and permitted the estimation of an average thickness value over the scanned surface, at an instrumental resolution of around 5 nm. Two magnification levels were used; 25 for measuring the thickness and the roughness of the Citrus extract coatings and the roughness of the a-tocopherol coatings, and 5 for measuring the thickness of the a-tocopherol coatings. Surface wettability was examined by measuring water contact angles (WCA) on the coatings using the sessile drop technique at room temperature using an optical contact angle measuring instrument OCA 20 (Dataphysics Instruments GmbH, Filderstadt, Germany). To confirm the presence, in the antioxidant coatings, of molecular groups consistent with a-tocopherol or bioflavonoids (from the Citrus extract), Fourier transform infrared spectroscopy (FTIR) measurements were carried out in transmission mode on coatings deposited on 500 mm thick silicon wafers, using a Bruker Vertex-70 system (Bruker Optics Gmbh, Ettlingen, Germany). The coated silicon wafers were placed in a sample chamber, initially purged with N2 and 64 scans were applied in the range of 400e4000 cm1 using a spectral resolution of 4 cm1. The surface characterization measurements were repeated three times on each of three independently prepared substrates. 2.5. Release of antioxidants from the coatings into water Coated trays were cut into 10 cm 2.5 cm segments and the mean of three weighing on a 5 decimal place precision balance (Sartorius, CP225D, Goettingen, Germany) was calculated. After the initial weighing, each segment was immersed in 400 mL of deionized water at 4 C in a Schott bottle (SCHOTT DURANÒ, Germany). After 4 days, the PET segments were removed from the water, carefully dried under a stream of air, weighed again and the mean weight of each segment determined. The total release of coating from the PET trays was calculated as the difference between initial and final weights and was expressed as a % of the total weight of deposited coating (Section 2.3). 2.6. Preparation of the meat samples A fresh turkey breast (w1.2 kg) was wrapped in aluminum foil, placed in an oven preheated to 190 C and cooked to an internal temperature of 73 C (w2 h), measured with a thermocouple Digitron 2038T (Sifam Instrumentals Ltd., Torquay, England) inserted in the deepest portion of the muscle. Immediately after cooking, the meat was packed under vacuum into polyethylene bags (20 cm 30 cm) and rapidly cooled (4 C) in an ice bath (2.5 h). A preliminary evaluation of the antioxidant properties of Citrus extract and a-tocopherol was carried out by adding them, in alcoholic solutions, directly to raw and cooked turkey meat ground through a plate with 4 mm diameter holes (OMAS Food machinery, Varese, Italy). Each antioxidant was added to the ground meat at an amount equivalent to the coating in contact with the meat on PET trays (1.35 mg g1 of meat and 0.84 mg g1 of meat for Citrus extract and a-tocopherol, respectively). The antioxidant was mixed
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with the ground meat using a glass rod and then stored in screwcap plastic tubes. Lipid oxidation was measured immediately (day 0) and after 1, 2 and 4 days of storage in a refrigerator at 4 C. For the experiment with antioxidant coated PET trays, the meat was cut into 5 mm thick slices, using a meat slicer (Medoc, S.Q. Poligino Cantabria l. Logroños) and the slices further cut into square shaped pieces (3 cm 3 cm) of approximately 3 g. Samples were randomly placed on the prepared PET trays which were immediately overwrapped with a catering film. Samples were removed immediately (day 0) and after 1, 2 and 4 days of storage in a refrigerator at 4 C for measurement of lipid oxidation. 2.7. Measurement of lipid oxidation 2.7.1. TBARS assay Meat samples (3 g) were homogenized in 25 mL of distilled water for 1 min at 8000 rpm using an Ultraturrax T25 (Janke & Kunkel IKA-Labortechnik and 2-thiobarbituric acid-reactive substances (TBARS)) were measured following the distillation procedure of Tarladgis, Watts, and Younathan (1960). The instrumental response was calibrated with standard solutions of 1,1,3,3,tetra-ethoxypropane (TEP) in a concentration range of approximately 1e20 nmol mL1. The recovery of TEP using the distillation procedure, calculated as w73%, was obtained from the difference between the absorbance of a distilled solution containing TEP, at a concentration corresponding to the median point of the calibration curve, and the absorbance of the same solution not distilled. The TBARS values were calculated from the calibration curve and expressed as mg malonaldehyde (MDA) kg1 of meat, the results were in accordance with the calculation obtained by multiplying the absorbance values by a distillation constant k, according to the method of Tarladgis et al. (1960), where the constant k was calculated as follows:
kðdistillationÞ ¼
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(ANOVA) was performed on the data and Duncan’s test was used to identify significant differences between samples. 3. Results and discussion 3.1. Direct addition of antioxidants The raw meat had an initial low level of lipid oxidation (w0.3 mg MDA kg1 meat) (Fig. 1a) and TBARS values subsequently increased to reach a maximum at day 4. Initial (day 0) TBARS values did not differ between treatments but values were significantly lower (P < 0.01) in the a-tocopherol and Citrus extract treatments compared to the control meat after 1, 2 and 4 days of storage. At each time point a-tocopherol and the Citrus extract were shown to be equally effective as antioxidants with no significant differences in lipid oxidation between these treatments. The cooked turkey meat data confirmed the results obtained for raw meat (Fig. 1b). It is well established that the cooking process increases the propensity of muscle lipids to oxidize (Gray & Pearson, 1987). While initial (day 0) TBARS values were higher than for the raw meat, as expected, the trend in development of lipid oxidation thereafter was similar to that in the raw meat, with a-tocopherol and Citrus extract shown to be equally effective as antioxidants compared to the control. 3.2. Antioxidant effect of trays coated with Citrus extract and atocopherol Since turkey muscle, because of its high polyunsaturated fatty acid content, is prone to lipid oxidation at high temperatures, particular care was taken during the cooking and cooling phases. The preparation was optimized to maintain initial oxidation of meat samples at a value of w2 mg MDA kg1 meat (Fig. 2a), which
ðmoles MDA=3 mL distillateÞ mol wt: MDA 107 100 Abs at 532 nm sample weight % recovery
2.7.2. Analysis of hexanal The amount of hexanal produced by lipid oxidation in the meat samples was determined by a solid phase micro-extractionegas chromatography (SPMEeGC) using the method described by Brunton, Cronin, Monahan, and Durcan (2000) with 2methylpentanal as internal standard (IS). Levels of hexanal in the turkey samples were calculated as follows:
Hexanal ðmg=kgÞ ¼
corresponded to values obtained in previous studies (Brunton et al., 2000; Caprioli et al., 2009). The TBARS values of samples on the control trays increased significantly (P < 0.01) at days 1 and 2, to reach a value of w6 mg kg1 meat, which remained relatively unchanged thereafter. In contrast to the effect of direct addition of a-tocopherol to meat (Section 3.1), meat on the a-tocopherol coated trays did not
Peak area of hexanal=½Peak area IS Weight of IS ðmgÞ Response factorðRFÞ Sample weight ðkgÞ
The response factor (RF) was calculated as the ratio of the peak area of hexanal and internal standard 2-methylpentanal (10 mg mL1 each) in aqueous mixtures. 2.8. Statistical analysis Each experiment was conducted in triplicate and results were expressed as the mean value of the three replicates. Statistical analysis of the data was performed using SPSS. Analysis of variance
show any reduction in TBARS values, compared to the control samples, over the 4 days of the experiment. However, TBARS values for turkey samples stored on the Citrus extract coated trays were significantly lower (P < 0.01) than those of samples on the control trays and a-tocopherol coated trays after 1, 2 and 4 days of storage. While TBARS values provide a useful index of lipid oxidation the assay itself is non-specific (Kosugi, Kato, & Kikugawa, 1987) and measurement of a specific oxidation product, such as hexanal,
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Fig. 1. Effect of the direct addition of a-tocopherol (0.84 mg g1 meat) and Citrus extract (1.35 mg g1 meat) on lipid oxidation (TBARS value) in raw (a) and cooked (b) turkey meat stored for 4 days at 4 C. Black column, turkey meat without antioxidant (control); grey column, turkey meat with a-tocopherol; white column, turkey meat with Citrus extract. Bars indicate mean SD. Mean separation was performed by applying Duncan’s test. a,bWithin each storage time, bars with different letters are significantly different (P < 0.01) due to treatment. x,y,zWithin each treatment, bars with different letters are significantly different due to storage time.
is useful to validate TBARS results (Kosugi et al., 1987; Wu & Shaldon, 1988). The trend in lipid oxidation revealed by TBARS was confirmed by the results of hexanal analysis (Fig. 2b). The hexanal content of meat stored in uncoated trays rapidly increased over the first 2 days of storage. The meat stored on atocopherol coated trays had a similar level of lipid oxidation to that of the control meat and the hexanal content of the meat stored on Citrus extract coated trays was significantly lower compared to the control meat (P < 0.01) at day 1, day 2 and day 4. The TBARS and hexanal values obtained were in agreement with the results of previous studies on the measurement of lipid oxidation in meat products (Brunton et al., 2000; Mielnik, Olsen, Vogt, Adeline, & Skrede, 2006). The results of TBARS and hexanal analysis clearly showed a strong antioxidant effect of the active packaging prepared with Citrus extract in preventing the oxidative spoilage of cooked meat. Although TBARS values increased over time in meat samples stored on the Citrus extract coated trays, lipid oxidation was almost two fold lower than that of samples on the uncoated trays. The lower TBARS values of meat on the Citrus extract coated trays was shown to be due to an antioxidant effect of the packaging, and not interference of the antioxidant in the TBARS assay as investigated by Caprioli, O’Sullivan, and Monahan (2010) (data not shown).
Fig. 2. Effect of coating of PET trays with a-tocopherol and Citrus extract on lipid oxidation (TBARS value) (a) and on hexanal level (b) in cooked turkey meat slices stored for 4 days at 4 C. Black column, turkey meat slices on PET without antioxidant (control); grey column, turkey meat slices with a-tocopherol coated PET; white column, turkey meat slices with Citrus coated PET. Bars indicate mean SD. Mean separation was performed by applying Duncan’s test. a,bWithin each storage time, bars with different letters are significantly different due to treatment (P < 0.01). x,y,zWithin each treatment bars with different letters are significantly different due to storage time (P < 0.01).
The results of TBARS analysis of samples stored on the atocopherol coated trays showed that this packaging was ineffective in delaying lipid oxidation. This is at odds with the results obtained in the first experiment (Fig. 1) and previously, where the antioxidant effect of a-tocopherol added to fatty foods (Kanatt, Paul, D’Souza, & Thomas, 1998; Mielnik, Aaby, & Skrede, 2003) or as an ingredient in an active packaging application (Lee, An, Lee, Park, & Lee, 2004) was demonstrated. However, the antioxidant effectiveness of a-tocopherol is quite likely to be affected by its mode of interaction with the food, for example, a-tocopherol is believed to be particularly effective when incorporated into muscle tissue through dietary absorption; this is because of its deposition in close proximity to membranal phospholipids which are particularly susceptible to oxidative deterioration (Monahan et al., 1992). In order to understand differences in the relative antioxidant effectiveness of the Citrus extract and a-tocopherol when added directly to meat, compared to their application as a coating to a PET surface, the surface physical properties (Section 3.3) and water solubility (Section 3.4) of the coatings were investigated. 3.3. Surface physical properties Optical profilometry analysis showed marked differences in morphology and roughness values for the two coatings (Fig. 3). The
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Fig. 3. Optical profilometry images of the surface of PET coated with Citrus extract (a) and a-tocopherol (b).
Citrus extract coating on PET was irregularly distributed, with a rough surface, while PET coated with a-tocopherol showed a more even distribution of the antioxidant film and a smoother surface, except for the presence of a small number of holes or pitting on the surface. Table 1 summarizes the mean values of Ra, Rq and Rt for the two coatings and the results show that roughness values of the control PET and the a-tocopherol coated PET were substantially lower than those of the Citrus extract coated PET. Thickness values of the two coatings (Fig. 4 and Table 1) were also different, with the a-tocopherol coating being thicker than the Citrus extract coating. The profile of thickness values of Citrus extract coating was irregular (Fig. 4a), with an average thickness value of 1.7 0.6 mm. In contrast the profile of the thickness values of the a-tocopherol coating was more regular, with an average
Table 1 Mean value (standard deviation) of average surface roughness (Ra), root mean square roughness (Rq), the maximum distance between the highest peak and the lowest groove (Rt), coating thickness and water contact angle (q) of the various substrates. Sample
Ra (mm)
Rq (mm)
Rt (mm)
Thickness q Water (mm) (deg)
PET (control) 0.11 0.04 0.14 0.05 1.34 0.34 e Citrus extract 1.13 0.18a 1.85 0.47a 22.91 5.99a 1.7 0.6a a-Tocopherol 0.19 0.08 0.22 0.1 0.86 0.35 4.0 0.9
82 0.4 22 0.6a 75 0.5b
a P < 0.01 when the values of the Citrus extract coating were compared to the values of the control PET and the a-tocopherol coating. b P < 0.01 when the values of the a-tocopherol coating were compared to the values of the control PET.
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Fig. 4. Thickness of (a) Citrus extract coating (evaluated over the step height indicated by the grey area in the figure) and (b) a-tocopherol coating on PET surfaces. The average thickness is obtained by measuring the difference in height between the PET surface and the average coating height of Citrus extract or the top of the coating for atocopherol.
thickness value of 4 0.9 mm, measured by using the step height arising from pitting on the surface of the coating (Fig. 4b). Table 1 also presents the water contact angle (WCA) measurements, q in deg, of the a-tocopherol coated PET and Citrus extract coated PET surface. The results show that the WCA for the Citrus extract coating was lower than that of the a-tocopherol coating and the control PET. This indicates that the PET coated with the Citrus extract presents a more hydrophilic surface to the meat. Given the high moisture content of the cooked turkey meat (approximately 64%; Food Standards Agency, 2002) this may allow better contact between the matrices, facilitating release of water soluble antioxidant components into the meat. In contrast the relatively hydrophobic a-tocopherol coating may have interacted less readily with the meat. While WCA values are indicative of the surface energy of the coating, material properties (e.g. solubility, roughness) (Kamusewitz & Possart, 2003) are also known to strongly affect the WCA values. Therefore, comparisons between the WCA of the Citrus extract, which is partially soluble in water (Section 3.4), and atocopherol, which is insoluble in water, should be carefully considered. The FTIR spectroscopic analysis confirmed the presence of the Citrus extract and a-tocopherol coatings on the surfaces (Fig. 5). In the case of Citrus extract coating FTIR confirmed the presence of flavonoids in the samples (Fig. 5a). The characteristic bands (aromatic between 1480 and 1637 cm1, eOHephenolic at 1205, 1293, 1439 and 3476 cm1, methoxylic at 1248 cm1 and carbonylic at 1657 cm1) are in good agreement with the FTIR spectra of hesperidin, naringenin and naringin (Ficarra et al., 2002). In the case of a-tocopherol (Fig. 5b) the band at 3472 cm1 is associated
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3.5. Mechanism of antioxidant activity of the coatings The surface characteristics (Section 3.3) and water solubility (Section 3.4) results for the two coatings suggest possible explanations for the antioxidant activity of the Citrus extract coating and the apparent lack of activity in the case of the a-tocopherol coating. The difference in the solubility of the Citrus extract and the a-tocopherol may explain the difference in antioxidant effect as it will affect the ability of antioxidant molecules to diffuse into the aqueous phase within turkey muscle and thereby exert an antioxidant effect. In contrast the low fat content of the turkey meat (approximately 2%; Food Standards Agency, 2002) makes it less likely that the lipophilic a-tocopherol will diffuse into the meat and, specifically, that it would be deposited in close proximity to unsaturated membranal lipids and cholesterol to delay their oxidation. It is also possible that, in the case of Citrus extract, the availability of active hydrophilic polyphenolic groups at the interface between the meat and the coating is greater than in the case of the a-tocopherol, in which the active phenolic group of a-tocopherol may be separated from the meat by the hydrophobic phytol sidechain of a-tocopherol. These hypotheses are the subject of ongoing investigations in the laboratory. 4. Conclusion The findings point to the promising characteristics of a Citrus extract as a component of a novel active packaging for reduction of lipid oxidation during storage of cooked turkey meat. Active packaging of this type may result in a useful extension of the shelf life of cooked meat products, which is limited principally by lipid oxidation. This novel packaging has the further advantage that the active component is a natural plant extract and, as such is less likely to suffer consumer resistance than similar products based on synthetic antioxidants. Fig. 5. FTIR spectra of Citrus extract (a) and a-tocopherol coating (b).
with eOH as has been reported previously by Che Man, Ammawath, and Mirghani (2005). Moreover, bands at 2924 and 2863 cm1 represent the asymmetric and symmetric stretching vibrations of eCH2e and eCH3, respectively, 1465 cm1 for phenyl skeletal and methyl asymmetric bending, 1388 cm1 associated with methyl symmetric bending, 1081 cm1 attributed to plane bending of phenyl and 922 cm1 representing trans ]CH2 stretching (Naghibzadeh et al., 2010). Thus the coating process appears to have preserved the chemical functionality of both the citrus extract and the a-tocopherol layers. Therefore the differences observed in antioxidant activity cannot be attributed to selective loss of a-tocopherol antioxidant function during deposition of the coatings. 3.4. Release of antioxidants from the coatings in water The results of the tests of the 4 day immersion of coated PET segments in water showed a mean total release of 6.72 0.77 mg (58.8% of the coating) for the Citrus extract coating and 1.41 0.50 mg (20.1% of the coating) for the a-tocopherol coating. The greater water solubility of the Citrus extract suggests that certain components could be released from the PET surface into the meat samples to exert an antioxidant effect, whereas for atocopherol coating this occurred to a lesser extent or the components that were released did not have antioxidant activity.
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*Nerín, C., Tovar, L., Djenane, D., Camo, J., Salafranca, J., Beltrán, J. A., et al. (2006). Stabilization of beef meat by new active packaging containing natural antioxidants. Journal of Agricultural and Food Chemistry, 54, 7840e7846. O’Sullivan, C. M., Lynch, A. M., Lynch, P. B., Buckley, D. J., & Kerry, J. P. (2004). Assessment of the antioxidant potential of food ingredients in fresh, previously frozen and cooked chicken patties. International Journal of Poultry Science, 3, 337e344. Park, S., & Zhao, Y. (2004). Incorporation of a high concentration of mineral or vitamin into chitosan-based films. Journal of Agricultural and Food Chemistry, 52, 1933e1939. Pignoli, G., Bou, R., Rodriguez- Estrada, M. T., & Decker, E. A. (2009). Suitability of saturated aldehydes as lipid oxidation markers in washed turkey meat. Meat Science, 83, 412e416. Siripatrawan, U., & Harte, B. R. (2010). Physical properties and antioxidant activity of an active film from chitosan incorporated with green tea extract. Food Hydrocolloids, 24, 770e775. Soto-Cantú, C. D., Graciano-Verdugo, A. Z., Peraltra, E., Islas-Rubio, A. R., & GonzálezCórdova, A. (2008). Release of butylated hydroxytoluene from an active film packaging to Asadero cheese and its effect on oxidation and odor stability. Journal of Dairy Science, 91, 11e19. Tarladgis, B. G., Watts, B. M., & Younathan, M. T. (1960). A distillation method for the quantitative determination of malonaldehyde in rancid food. The Journal of the American Oil Chemists’ Society, 37, 44e48. Wilmsen, P. K., Spada, D. S., & Salvador, M. (2005). Antioxidant activity of the flavonoid hesperidin in chemical and biological systems. Journal of Agricultural and Food Chemistry, 53, 4757e4761. Wu, T. C., & Sheldon, B. W. (1988). Flavour components and factors associated with the development of off-flavours in cooked turkey rolls. Journal of Food Science, 53, 49e54.
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LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
PET trays coated with Citrus extract exhibit antioxidant activity with cooked turkey meat C. Contini a, M.G. Katsikogianni b, F.T. O’Neill b, M. O’Sullivan a, D.P. Dowling b, F.J. Monahan a, * a b
UCD School of Agriculture and Food Science, University College Dublin, Belfield, Dublin 4, Ireland UCD School of Mechanical and Materials Engineering, University College Dublin, Belfield, Dublin 4, Ireland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 June 2011 Received in revised form 2 February 2012 Accepted 7 February 2012
A natural Citrus extract with potential antioxidant activity was evaluated as an ingredient for the production of active food packaging. The extract, in methanol, was sprayed onto the surface of polyethylene terephthalate (PET) trays. For comparison, a second set of trays were prepared using atocopherol as the coating. The effectiveness of the two types of packaging in delaying lipid oxidation in cooked turkey meat slices, stored at 4 C over 4 days, was compared using 2-thiobarbituric acid-reactive substances (TBARS) and hexanal assays. TBARS and hexanal values, for meat stored on the Citrus extract coated trays, were significantly lower (P < 0.01) than those of meat stored on uncoated (control) trays while a-tocopherol coated trays exhibited no significant effect compared to control trays (P > 0.05). The effectiveness of the Citrus extract coating, compared to the a-tocopherol coating, was attributed to its higher surface roughness, demonstrated by optical profilometry, and the higher level of release (solubility) of the antioxidant in water. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Active packaging Citrus extract a-Tocopherol Lipid oxidation Meat
1. Introduction As demand for food products capable of maintaining their sensory and nutritional quality over longer period increases, new advances in food packaging are required. Among the options, active packaging can improve the organoleptic properties and prolong the shelf life of packaged food products (De Kruijf et al., 2002). One specific application of active packaging is to improve sensory and nutritional quality by reducing the oxidative deterioration of foods without the direct addition of antioxidants, including synthetic antioxidants, to the food. It is well established that oxidation in muscle foods during storage has detrimental effects on food flavor (Gray & Pearson, 1987), texture (Lund, Lametsch, Hviid, Jensen, & Skibsted, 2007) and healthiness (Kanner, 2007). A potential solution is an active package consisting of food contact surfaces coated with natural antioxidant compounds. Indeed, antioxidants have been incorporated into food packaging by including them in film forming solutions which are subsequently cast on plates (Park & Zhao, 2004; Siripatrawan & Harte, 2010) or including them (rosemary extract, BHT) in polyolefin films (Nerín et al., 2006; SotoCantú, Graciano-Verdugo, Peraltra, Islas-Rubio, & González-
* Corresponding author. Tel.: þ353 1 716 2842. E-mail addresses: [email protected] (D.P. Dowling), [email protected] (F.J. Monahan). 0023-6438/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.lwt.2012.02.009
Córdova, 2008). The release of the antioxidant from the film and its effectiveness in real food system was reported in the study of SotoCantú et al. (2008) using cheese packed in a BHT-containing film. In the present study, a Citrus extract containing a mixture of bioflavonoids was examined as a potential ingredient for the development of active antioxidant packaging, in which the natural antioxidant is applied as a coating on the food contact surface. Citrus fruits contain bioflavonoids which are known to act as antioxidants by scavenging the free radicals formed during oxidative processes, particularly during the propagation of oxidative reactions. Specifically, some bioflavonoids known to be present in Citrus fruit have been shown to have antioxidant potential by monitoring DPPH (1.1-diphenyl-2-picrylydrazyl) radical scavenger ability (Lee, Lee, Kim, & Jeong, 2009; Wilmsen, Spada, & Salvador, 2005). However, to-date no studies have been published on the effectiveness of a Citrus extract in reducing lipid oxidation in foods when used in an active packaging application. The objective of this study was to investigate the efficacy of Citrus extract and a-tocopherol, sprayed (nebulized) onto the surface of recycled PET food trays, in preventing lipid oxidation in cooked meat. a-Tocopherol was chosen as a second natural antioxidant for comparison with Citrus extract, because of its proven efficiency in delaying oxidative processes when added directly to fatty foods (Djenane, Sánchez-Escalante, Beltran, & Roncalés, 2002; O’Sullivan, Lynch, Lynch, Buckley, & Kerry, 2004), or when incorporated into animal-derived foods via animal diets (Monahan et al.,1992). Slices of
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cooked turkey meat were stored on coated trays at 4 C for 4 days and the ability of the Citrus extract and a-tocopherol coatings to reduce lipid oxidation compared. Turkey meat was chosen as test food due to its relative abundance of PUFA (Pignoli, Bou, Rodriguez-Estrada, & Decker, 2009) which make it suitable for the evaluation of oxidative processes (Caprioli, O’Sullivan, & Monahan, 2009). 2. Material and methods 2.1. Meat Fresh turkey breasts were obtained from IGWT Poultry Service Ltd., County Monaghan, Ireland. The bones and the visible fat were removed. The turkey breasts were divided into two portions of similar size (w1.2 kg), vacuum packed, stored at 20 C and used within 6 months. 2.2. Reagents
a-Tocopherol (96%) was obtained from SigmaeAldrich Ireland Ltd, Dublin, Ireland. Citrus extract, containing a mixture of bioflavonoids with potential antioxidant activity, was obtained from a commercial source (Citrox Ltd, Middlesbrough, England). The composition of the Citrus extract, as per the manufacturer’s specification, was narangin 3.6%, neohesperidin 1.9%, rhoifolin 0.4%, poncirin 0.3%, naringenin 0.2%, hesperidin 0.2%, malic acid 15%, ascorbic acid 15%, and citric acid 15%. Hydrochloric acid (37%), 2thiobarbituric acid (TBA, 98%), 1,1,3,3,-tetra-ethoxypropane (TEP 96%) and acetic acid were supplied by SigmaeAldrich Ireland Ltd, Dublin, Ireland. Screw-cap plastic tubes (10 mL) and PYREX screw-cap test tubes (10 mL) were supplied by Sarstedt Ltd., Wexford, Ireland. Recycled polyethylene terephthalate (PET) trays (100 150 mm) were supplied by Holfeld Plastics (Arklow, Co Wicklow, Ireland). Low-density polyvinylchloride (PVC) catering film (thickness 7.0 mm; O2 transmission 2000 cm3 m2 d1 bar1) was supplied by Western Plastic Ltd., Galway, Ireland. Polydimethylsiloxane/divinylbenzene (PDMS/DVB) solid phase microextraction (SPME) fibers (65 mm) were obtained from Supelco (SigmaeAldrich Ireland Ltd, Dublin, Ireland). 2.3. Deposition of a-tocopherol and Citrus extract coatings on PET trays
a-Tocopherol and Citrus extract were dissolved in ethanol and methanol, respectively, at 100 mg mL1 and deposited on recycled PET trays (Holfeld Plastic Ltd, Wicklow, Ireland) using a handmade Teflon nebulizer mounted on a computer numerical control (CNC) system (cncGraf4, Auto Grav, Boenigh Electronics, Bonn, Germany). The nebulizer was moved over the substrate in a raster pattern with a line speed of 7 mm s1. The solutions of a-tocopherol or Citrus extract were pumped from a remote syringe pump to the nebulizer, at a flow rate of 100 mL min1, and converted into an aerosol using the flow of helium through the nebulizer at a flow rate of 5 L min1. The resultant helium-aerosol mix was directed to the substrate through a 23 mm long and 15 mm diameter teflon tube. The substrate to tube orifice distance was fixed at 2 mm. The nebulization was repeated two times. A coating density of 0.45 mg cm2 for the Citrus extract and 0.28 mg cm2 for the a-tocopherol was calculated as the difference in weight of a tray before and after the coating procedure. 2.4. Surface characterization of the a-tocopherol and Citrus extract coatings on PET trays Surface morphology, roughness and film thickness of the atocopherol and Citrus extract coatings were evaluated using a Wyko
NT1100 optical profilometer in vertical scanning interferometry (VSI) mode, which allows a 3 dimensional image of the coating to be built over several hundred nanometers. Three roughness parameters were evaluated: the average surface roughness (Ra), which is the arithmetic mean of peaks and valleys or deviations from the centerline over the sampling length, the root mean square roughness (Rq or RMS), which is the root mean square measurement of the peaks and valleys deviations from the centerline and the maximum distance between the highest peak and the lowest groove (Rt). Line plots of the surface profile over a step height indicated the variation of the coating thickness and permitted the estimation of an average thickness value over the scanned surface, at an instrumental resolution of around 5 nm. Two magnification levels were used; 25 for measuring the thickness and the roughness of the Citrus extract coatings and the roughness of the a-tocopherol coatings, and 5 for measuring the thickness of the a-tocopherol coatings. Surface wettability was examined by measuring water contact angles (WCA) on the coatings using the sessile drop technique at room temperature using an optical contact angle measuring instrument OCA 20 (Dataphysics Instruments GmbH, Filderstadt, Germany). To confirm the presence, in the antioxidant coatings, of molecular groups consistent with a-tocopherol or bioflavonoids (from the Citrus extract), Fourier transform infrared spectroscopy (FTIR) measurements were carried out in transmission mode on coatings deposited on 500 mm thick silicon wafers, using a Bruker Vertex-70 system (Bruker Optics Gmbh, Ettlingen, Germany). The coated silicon wafers were placed in a sample chamber, initially purged with N2 and 64 scans were applied in the range of 400e4000 cm1 using a spectral resolution of 4 cm1. The surface characterization measurements were repeated three times on each of three independently prepared substrates. 2.5. Release of antioxidants from the coatings into water Coated trays were cut into 10 cm 2.5 cm segments and the mean of three weighing on a 5 decimal place precision balance (Sartorius, CP225D, Goettingen, Germany) was calculated. After the initial weighing, each segment was immersed in 400 mL of deionized water at 4 C in a Schott bottle (SCHOTT DURANÒ, Germany). After 4 days, the PET segments were removed from the water, carefully dried under a stream of air, weighed again and the mean weight of each segment determined. The total release of coating from the PET trays was calculated as the difference between initial and final weights and was expressed as a % of the total weight of deposited coating (Section 2.3). 2.6. Preparation of the meat samples A fresh turkey breast (w1.2 kg) was wrapped in aluminum foil, placed in an oven preheated to 190 C and cooked to an internal temperature of 73 C (w2 h), measured with a thermocouple Digitron 2038T (Sifam Instrumentals Ltd., Torquay, England) inserted in the deepest portion of the muscle. Immediately after cooking, the meat was packed under vacuum into polyethylene bags (20 cm 30 cm) and rapidly cooled (4 C) in an ice bath (2.5 h). A preliminary evaluation of the antioxidant properties of Citrus extract and a-tocopherol was carried out by adding them, in alcoholic solutions, directly to raw and cooked turkey meat ground through a plate with 4 mm diameter holes (OMAS Food machinery, Varese, Italy). Each antioxidant was added to the ground meat at an amount equivalent to the coating in contact with the meat on PET trays (1.35 mg g1 of meat and 0.84 mg g1 of meat for Citrus extract and a-tocopherol, respectively). The antioxidant was mixed
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with the ground meat using a glass rod and then stored in screwcap plastic tubes. Lipid oxidation was measured immediately (day 0) and after 1, 2 and 4 days of storage in a refrigerator at 4 C. For the experiment with antioxidant coated PET trays, the meat was cut into 5 mm thick slices, using a meat slicer (Medoc, S.Q. Poligino Cantabria l. Logroños) and the slices further cut into square shaped pieces (3 cm 3 cm) of approximately 3 g. Samples were randomly placed on the prepared PET trays which were immediately overwrapped with a catering film. Samples were removed immediately (day 0) and after 1, 2 and 4 days of storage in a refrigerator at 4 C for measurement of lipid oxidation. 2.7. Measurement of lipid oxidation 2.7.1. TBARS assay Meat samples (3 g) were homogenized in 25 mL of distilled water for 1 min at 8000 rpm using an Ultraturrax T25 (Janke & Kunkel IKA-Labortechnik and 2-thiobarbituric acid-reactive substances (TBARS)) were measured following the distillation procedure of Tarladgis, Watts, and Younathan (1960). The instrumental response was calibrated with standard solutions of 1,1,3,3,tetra-ethoxypropane (TEP) in a concentration range of approximately 1e20 nmol mL1. The recovery of TEP using the distillation procedure, calculated as w73%, was obtained from the difference between the absorbance of a distilled solution containing TEP, at a concentration corresponding to the median point of the calibration curve, and the absorbance of the same solution not distilled. The TBARS values were calculated from the calibration curve and expressed as mg malonaldehyde (MDA) kg1 of meat, the results were in accordance with the calculation obtained by multiplying the absorbance values by a distillation constant k, according to the method of Tarladgis et al. (1960), where the constant k was calculated as follows:
kðdistillationÞ ¼
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(ANOVA) was performed on the data and Duncan’s test was used to identify significant differences between samples. 3. Results and discussion 3.1. Direct addition of antioxidants The raw meat had an initial low level of lipid oxidation (w0.3 mg MDA kg1 meat) (Fig. 1a) and TBARS values subsequently increased to reach a maximum at day 4. Initial (day 0) TBARS values did not differ between treatments but values were significantly lower (P < 0.01) in the a-tocopherol and Citrus extract treatments compared to the control meat after 1, 2 and 4 days of storage. At each time point a-tocopherol and the Citrus extract were shown to be equally effective as antioxidants with no significant differences in lipid oxidation between these treatments. The cooked turkey meat data confirmed the results obtained for raw meat (Fig. 1b). It is well established that the cooking process increases the propensity of muscle lipids to oxidize (Gray & Pearson, 1987). While initial (day 0) TBARS values were higher than for the raw meat, as expected, the trend in development of lipid oxidation thereafter was similar to that in the raw meat, with a-tocopherol and Citrus extract shown to be equally effective as antioxidants compared to the control. 3.2. Antioxidant effect of trays coated with Citrus extract and atocopherol Since turkey muscle, because of its high polyunsaturated fatty acid content, is prone to lipid oxidation at high temperatures, particular care was taken during the cooking and cooling phases. The preparation was optimized to maintain initial oxidation of meat samples at a value of w2 mg MDA kg1 meat (Fig. 2a), which
ðmoles MDA=3 mL distillateÞ mol wt: MDA 107 100 Abs at 532 nm sample weight % recovery
2.7.2. Analysis of hexanal The amount of hexanal produced by lipid oxidation in the meat samples was determined by a solid phase micro-extractionegas chromatography (SPMEeGC) using the method described by Brunton, Cronin, Monahan, and Durcan (2000) with 2methylpentanal as internal standard (IS). Levels of hexanal in the turkey samples were calculated as follows:
Hexanal ðmg=kgÞ ¼
corresponded to values obtained in previous studies (Brunton et al., 2000; Caprioli et al., 2009). The TBARS values of samples on the control trays increased significantly (P < 0.01) at days 1 and 2, to reach a value of w6 mg kg1 meat, which remained relatively unchanged thereafter. In contrast to the effect of direct addition of a-tocopherol to meat (Section 3.1), meat on the a-tocopherol coated trays did not
Peak area of hexanal=½Peak area IS Weight of IS ðmgÞ Response factorðRFÞ Sample weight ðkgÞ
The response factor (RF) was calculated as the ratio of the peak area of hexanal and internal standard 2-methylpentanal (10 mg mL1 each) in aqueous mixtures. 2.8. Statistical analysis Each experiment was conducted in triplicate and results were expressed as the mean value of the three replicates. Statistical analysis of the data was performed using SPSS. Analysis of variance
show any reduction in TBARS values, compared to the control samples, over the 4 days of the experiment. However, TBARS values for turkey samples stored on the Citrus extract coated trays were significantly lower (P < 0.01) than those of samples on the control trays and a-tocopherol coated trays after 1, 2 and 4 days of storage. While TBARS values provide a useful index of lipid oxidation the assay itself is non-specific (Kosugi, Kato, & Kikugawa, 1987) and measurement of a specific oxidation product, such as hexanal,
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Fig. 1. Effect of the direct addition of a-tocopherol (0.84 mg g1 meat) and Citrus extract (1.35 mg g1 meat) on lipid oxidation (TBARS value) in raw (a) and cooked (b) turkey meat stored for 4 days at 4 C. Black column, turkey meat without antioxidant (control); grey column, turkey meat with a-tocopherol; white column, turkey meat with Citrus extract. Bars indicate mean SD. Mean separation was performed by applying Duncan’s test. a,bWithin each storage time, bars with different letters are significantly different (P < 0.01) due to treatment. x,y,zWithin each treatment, bars with different letters are significantly different due to storage time.
is useful to validate TBARS results (Kosugi et al., 1987; Wu & Shaldon, 1988). The trend in lipid oxidation revealed by TBARS was confirmed by the results of hexanal analysis (Fig. 2b). The hexanal content of meat stored in uncoated trays rapidly increased over the first 2 days of storage. The meat stored on atocopherol coated trays had a similar level of lipid oxidation to that of the control meat and the hexanal content of the meat stored on Citrus extract coated trays was significantly lower compared to the control meat (P < 0.01) at day 1, day 2 and day 4. The TBARS and hexanal values obtained were in agreement with the results of previous studies on the measurement of lipid oxidation in meat products (Brunton et al., 2000; Mielnik, Olsen, Vogt, Adeline, & Skrede, 2006). The results of TBARS and hexanal analysis clearly showed a strong antioxidant effect of the active packaging prepared with Citrus extract in preventing the oxidative spoilage of cooked meat. Although TBARS values increased over time in meat samples stored on the Citrus extract coated trays, lipid oxidation was almost two fold lower than that of samples on the uncoated trays. The lower TBARS values of meat on the Citrus extract coated trays was shown to be due to an antioxidant effect of the packaging, and not interference of the antioxidant in the TBARS assay as investigated by Caprioli, O’Sullivan, and Monahan (2010) (data not shown).
Fig. 2. Effect of coating of PET trays with a-tocopherol and Citrus extract on lipid oxidation (TBARS value) (a) and on hexanal level (b) in cooked turkey meat slices stored for 4 days at 4 C. Black column, turkey meat slices on PET without antioxidant (control); grey column, turkey meat slices with a-tocopherol coated PET; white column, turkey meat slices with Citrus coated PET. Bars indicate mean SD. Mean separation was performed by applying Duncan’s test. a,bWithin each storage time, bars with different letters are significantly different due to treatment (P < 0.01). x,y,zWithin each treatment bars with different letters are significantly different due to storage time (P < 0.01).
The results of TBARS analysis of samples stored on the atocopherol coated trays showed that this packaging was ineffective in delaying lipid oxidation. This is at odds with the results obtained in the first experiment (Fig. 1) and previously, where the antioxidant effect of a-tocopherol added to fatty foods (Kanatt, Paul, D’Souza, & Thomas, 1998; Mielnik, Aaby, & Skrede, 2003) or as an ingredient in an active packaging application (Lee, An, Lee, Park, & Lee, 2004) was demonstrated. However, the antioxidant effectiveness of a-tocopherol is quite likely to be affected by its mode of interaction with the food, for example, a-tocopherol is believed to be particularly effective when incorporated into muscle tissue through dietary absorption; this is because of its deposition in close proximity to membranal phospholipids which are particularly susceptible to oxidative deterioration (Monahan et al., 1992). In order to understand differences in the relative antioxidant effectiveness of the Citrus extract and a-tocopherol when added directly to meat, compared to their application as a coating to a PET surface, the surface physical properties (Section 3.3) and water solubility (Section 3.4) of the coatings were investigated. 3.3. Surface physical properties Optical profilometry analysis showed marked differences in morphology and roughness values for the two coatings (Fig. 3). The
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Fig. 3. Optical profilometry images of the surface of PET coated with Citrus extract (a) and a-tocopherol (b).
Citrus extract coating on PET was irregularly distributed, with a rough surface, while PET coated with a-tocopherol showed a more even distribution of the antioxidant film and a smoother surface, except for the presence of a small number of holes or pitting on the surface. Table 1 summarizes the mean values of Ra, Rq and Rt for the two coatings and the results show that roughness values of the control PET and the a-tocopherol coated PET were substantially lower than those of the Citrus extract coated PET. Thickness values of the two coatings (Fig. 4 and Table 1) were also different, with the a-tocopherol coating being thicker than the Citrus extract coating. The profile of thickness values of Citrus extract coating was irregular (Fig. 4a), with an average thickness value of 1.7 0.6 mm. In contrast the profile of the thickness values of the a-tocopherol coating was more regular, with an average
Table 1 Mean value (standard deviation) of average surface roughness (Ra), root mean square roughness (Rq), the maximum distance between the highest peak and the lowest groove (Rt), coating thickness and water contact angle (q) of the various substrates. Sample
Ra (mm)
Rq (mm)
Rt (mm)
Thickness q Water (mm) (deg)
PET (control) 0.11 0.04 0.14 0.05 1.34 0.34 e Citrus extract 1.13 0.18a 1.85 0.47a 22.91 5.99a 1.7 0.6a a-Tocopherol 0.19 0.08 0.22 0.1 0.86 0.35 4.0 0.9
82 0.4 22 0.6a 75 0.5b
a P < 0.01 when the values of the Citrus extract coating were compared to the values of the control PET and the a-tocopherol coating. b P < 0.01 when the values of the a-tocopherol coating were compared to the values of the control PET.
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Fig. 4. Thickness of (a) Citrus extract coating (evaluated over the step height indicated by the grey area in the figure) and (b) a-tocopherol coating on PET surfaces. The average thickness is obtained by measuring the difference in height between the PET surface and the average coating height of Citrus extract or the top of the coating for atocopherol.
thickness value of 4 0.9 mm, measured by using the step height arising from pitting on the surface of the coating (Fig. 4b). Table 1 also presents the water contact angle (WCA) measurements, q in deg, of the a-tocopherol coated PET and Citrus extract coated PET surface. The results show that the WCA for the Citrus extract coating was lower than that of the a-tocopherol coating and the control PET. This indicates that the PET coated with the Citrus extract presents a more hydrophilic surface to the meat. Given the high moisture content of the cooked turkey meat (approximately 64%; Food Standards Agency, 2002) this may allow better contact between the matrices, facilitating release of water soluble antioxidant components into the meat. In contrast the relatively hydrophobic a-tocopherol coating may have interacted less readily with the meat. While WCA values are indicative of the surface energy of the coating, material properties (e.g. solubility, roughness) (Kamusewitz & Possart, 2003) are also known to strongly affect the WCA values. Therefore, comparisons between the WCA of the Citrus extract, which is partially soluble in water (Section 3.4), and atocopherol, which is insoluble in water, should be carefully considered. The FTIR spectroscopic analysis confirmed the presence of the Citrus extract and a-tocopherol coatings on the surfaces (Fig. 5). In the case of Citrus extract coating FTIR confirmed the presence of flavonoids in the samples (Fig. 5a). The characteristic bands (aromatic between 1480 and 1637 cm1, eOHephenolic at 1205, 1293, 1439 and 3476 cm1, methoxylic at 1248 cm1 and carbonylic at 1657 cm1) are in good agreement with the FTIR spectra of hesperidin, naringenin and naringin (Ficarra et al., 2002). In the case of a-tocopherol (Fig. 5b) the band at 3472 cm1 is associated
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3.5. Mechanism of antioxidant activity of the coatings The surface characteristics (Section 3.3) and water solubility (Section 3.4) results for the two coatings suggest possible explanations for the antioxidant activity of the Citrus extract coating and the apparent lack of activity in the case of the a-tocopherol coating. The difference in the solubility of the Citrus extract and the a-tocopherol may explain the difference in antioxidant effect as it will affect the ability of antioxidant molecules to diffuse into the aqueous phase within turkey muscle and thereby exert an antioxidant effect. In contrast the low fat content of the turkey meat (approximately 2%; Food Standards Agency, 2002) makes it less likely that the lipophilic a-tocopherol will diffuse into the meat and, specifically, that it would be deposited in close proximity to unsaturated membranal lipids and cholesterol to delay their oxidation. It is also possible that, in the case of Citrus extract, the availability of active hydrophilic polyphenolic groups at the interface between the meat and the coating is greater than in the case of the a-tocopherol, in which the active phenolic group of a-tocopherol may be separated from the meat by the hydrophobic phytol sidechain of a-tocopherol. These hypotheses are the subject of ongoing investigations in the laboratory. 4. Conclusion The findings point to the promising characteristics of a Citrus extract as a component of a novel active packaging for reduction of lipid oxidation during storage of cooked turkey meat. Active packaging of this type may result in a useful extension of the shelf life of cooked meat products, which is limited principally by lipid oxidation. This novel packaging has the further advantage that the active component is a natural plant extract and, as such is less likely to suffer consumer resistance than similar products based on synthetic antioxidants. Fig. 5. FTIR spectra of Citrus extract (a) and a-tocopherol coating (b).
with eOH as has been reported previously by Che Man, Ammawath, and Mirghani (2005). Moreover, bands at 2924 and 2863 cm1 represent the asymmetric and symmetric stretching vibrations of eCH2e and eCH3, respectively, 1465 cm1 for phenyl skeletal and methyl asymmetric bending, 1388 cm1 associated with methyl symmetric bending, 1081 cm1 attributed to plane bending of phenyl and 922 cm1 representing trans ]CH2 stretching (Naghibzadeh et al., 2010). Thus the coating process appears to have preserved the chemical functionality of both the citrus extract and the a-tocopherol layers. Therefore the differences observed in antioxidant activity cannot be attributed to selective loss of a-tocopherol antioxidant function during deposition of the coatings. 3.4. Release of antioxidants from the coatings in water The results of the tests of the 4 day immersion of coated PET segments in water showed a mean total release of 6.72 0.77 mg (58.8% of the coating) for the Citrus extract coating and 1.41 0.50 mg (20.1% of the coating) for the a-tocopherol coating. The greater water solubility of the Citrus extract suggests that certain components could be released from the PET surface into the meat samples to exert an antioxidant effect, whereas for atocopherol coating this occurred to a lesser extent or the components that were released did not have antioxidant activity.
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