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Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat
INNFOO-01194; No of Pages 6 Innovative Food Science and Emerging Technologies xxx (2014) xxx–xxx
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Innovative Food Science and Emerging Technologies journal homepage: www.elsevier.com/locate/ifset
Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat Yan Dai a,b,c, Yi Lu a,c, Wei Wu a,c, Xiao-ming Lu b, Zhao-peng Han b, Yi Liu a,c, Xing-min Li a,c, Rui-tong Dai a,c,⁎ a b c
College of Food Science and Nutritional Engineering, China Agricultural University, 17 Qinghua East Road, Beijing, 100083, China Chinese National Egg Engineering Research Center, Beijing DQY Agricultural Technology Co. ‘Ltd.’, Beijing, 102115, China Beijing Higher Institution Engineering Research Center of Animal Product, Beijing China, 100081, China
a r t i c l e
i n f o
Article history: Received 2 March 2014 Accepted 13 June 2014 Available online xxxx Editor Proof Receive Date 14 July 2014 Keywords: Ohmically Water bath TBARS Oxidation
a b s t r a c t In the present work, changes in lipid and protein oxidation, non-heme iron (NHI) content, color and texture parameters during refrigerated storage (7 days/4 °C) of ohmically (OH) and water bath (WB)-cooked pork meat were studied. A significant effect of refrigerated storage duration and cooking method on color, texture, lipid oxidation and carbonyl content was detected for OH-cooked and WB-cooked meat. The OH-cooked meat had better color appearance, lipid oxidative stability, slightly enhanced hardness and protein oxidation levels during 7 days of storage having lower values of TBARS, center lightness (L⁎), center/surface brownness (b⁎) and free thiol groups; however, there were higher values of center/surface redness (a⁎), non-heme iron, protein carbonyl and hardness compared to those from WB-cooked meat. Industrial relevance: The conventional cooking protocols always result in quality deteriorations and reduced shelf-life in meat products due to longer cooking times. In this study, the ohmically-cooked meat showed comparable or improved oxidation levels, color appearance and texture attributes during 7 days of refrigerated storage. From an anti-oxidation perspective, the available data are provided for the application of ohmic cooking in Chinese commercial productions of cooked meat with high quality requirements that could be established. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction In ohmic or electroconductive heating, foods are heated by passing alternating current through them (Sarang, Sastry, & Knipe, 2008). Ohmic heating generates heat by direct energy application in a volumetric fashion which reduces the long cooking times associated with conventional methods (Zell, Lyng, Cronin, & Morgan, 2010a). Apart from microbial spoilage, lipid and protein oxidation is considered as one of the main causes for functional, sensory and nutritional quality deteriorations in meat and meat products (Ventanas, Estevez, Tejeda, & Ruiz, 2006). Changes associated with lipid oxidation include the development of unpleasant odors and tastes, deteriorations in color, rheological properties as well as potential formation of toxic compounds. On the other hand, protein oxidation induces increasing water losses and protein fragmentation or aggregation, decreasing protein digestibility and solubility as well as weaker protein gels or unstable emulsions of meat and meat products (Ramirez & Cava, 2007; Ventanas et al., 2006). Concerning oxidation promoters in animal
⁎ Corresponding author. Tel.: +86 1062737547; fax: +86 1062839300. E-mail address: [email protected] (R. Dai).
foodstuffs, iron is thought to have high catalytic activity (Estévez & Cava, 2004). Non-heme iron (NHI) is considered as the most important oxidation promoter. An increase in the amount of NHI as a result of thermal processes or subsequent refrigerated storage on meat systems has been linked to the decrease of heme iron (HI) as a consequence of the breakdown of the heme molecule and oxidative deterioration of the porphyrin ring of myoglobin (Estévez & Cava, 2004). Meat is always cooked before consumption, and thermal treatments greatly accelerate oxidations through increasing the free radical productions and decreasing in parallel the antioxidant protection during subsequent refrigerated storage (Gatellier, Kondjoyan, Portanguen, & Santé-Lhoutellier, 2010; Zell et al., 2010a). Currently, meat products are mainly thermally processed by steam or hot water immersion. A major problem in the cooking of meats in this way results in longer cooking times and a reduction in the quality of such products (McKenna, Lyng, Brunton, & Shirsat, 2006; Zell, Lyng, Cronin, & Morgan, 2010b). Previous work on ohmically-cooked meat showed promising results in terms of product quality (Zell et al., 2010a). Nevertheless, the study on the occurrence of protein and lipid oxidations, sensory (color and texture) deteriorations in meat during ohmic cooking are still lacking during refrigerated storage. The aim of the present study was to determine the suitability of ohmically-cooked pork meat with particular reference to its effects on selected physicochemical parameters, such as lipid and protein
http://dx.doi.org/10.1016/j.ifset.2014.06.009 1466-8564/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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Y. Dai et al. / Innovative Food Science and Emerging Technologies xxx (2014) xxx–xxx
oxidation, levels of NHI, deteriorations in color and texture attributes, and comparison of ohmically-cooked pork with conventionallycooked-ones. 2. Materials and methods 2.1. Sample preparation M. longissimus thoracis et lumborum muscles were obtained from a local abattoir in Beijing, China. Briefly, carcasses from four (large white crossbred) pigs about six months of age were slaughtered at a live weight of 110 ± 5 kg and chilled at 4 °C for one day in accordance with EU regulations. Longissimus thoracis et lumborum muscles (from the 12th thoracic to the 5th lumbar vertebrae) were removed from both sides of each carcass, then trimmed of visible fat and connective tissue, individually vacuum packaged. Each muscle (approximately 500 g) was assigned to water bath (WB) and ohmic cooking (OH) groups. Each treatment was carried out in four replicates (i.e., meat from an individual animal equates to one replicate). Samples were thawed at 4 °C for 10 h before treatment. 2.2. Heating equipment 2.2.1. Ohmic cooking A modified ohmic heating device designed according to Dai et al. (2013) with slightly modification, was connected to a 3.5 kW power supply (15 A, 0–250 V, 50 Hz) (Wocen Power Ltd., China) to cook pork muscles of M. longissimus thoracis et lumborum. The whole muscle (about 500 ± 10 g) was clamped into the cooking cell, high-grade (316) stainless steel electrodes were connected to the sample at each end of the cooking cell, a compression fitting was designed to ensure full contact of electrodes to the meat sample. Temperature measurements were monitored by using three type T thermocouples (Industrial Temperature Sensors Ltd., Beijing, China) from nine different sections of the sample in the cooking cell (the geometrical center, 1.3 cm apart from the electrodes on the radial axis and 3.5 cm apart from the electrodes at the sample surface). The pork meat samples were ohmically-cooked to a minimum end-point temperature (EPT) of 95 °C (range 95 °C–115 °C) at the geometric center (cold spot) by applying a 10 V cm−1 voltage gradient (Zell et al., 2010b). The ohmic heater was immediately switched off when the temperature reached the final EPT (approximately 13 ± 2 min of cooking time in total). 2.2.2. Conventional cooking Briefly, the intact pork muscle about (500 ± 10 g) was packed using strings and then cooked in a water bath described by Dai et al. (2013) with slight modifications. The dimensions of these samples were similar with a 4.2 ± 1 cm diameter and 10 ± 1 cm length. During cooking, three temperature probes (Industrial Temperature Sensors Ltd., Beijing, China) were inserted into the geometric core of each muscle to monitor the internal temperature. The meat samples were immediately removed from the stainless steel cell when the core temperature reached 95 °C (range 95 °C–98 °C) as previously described (Laycock, Piyasena, & Mittal, 2003). The cooking time was approximately 83 ± 4 min in total. The OH and WB-cooked samples were vacuum packed, stored at 4 °C in a refrigerator and analyzed at intervals of 1, 3, 5 and 7 days respectively. 2.3. Quality parameters 2.3.1. Physico-chemical and cooking loss analysis The percentage of cooking loss (CL) was calculated by obtaining the difference between the green and cooked weights (post-cooling) and divided by the raw weight of OH and WB-cooked meat (Zell et al., 2010a), and the proximate analysis of these samples was carried out using the methods described by Zell et al.(2010a). All measurements were performed four times.
2.3.2. Color measurement A Minolta colorimeter (Model No. CR-400, Konica Minolta Sensing Americas Inc., Ramsey, NJ, USA) calibrated for internal light (D65) before analysis was used to measure L⁎ (lightness), a⁎ (redness/greenness) and b⁎ (yellowness/brownness) values. Color measurements were evaluated at four points near the center and at four points on the surfaces of the four cooked pork samples (Zell et al., 2010a). 2.3.3. Texture measurement A TA-XT2 texture analyzer (Stable Micro Systems, Godalming, UK) equipped with a 250 N load cell was used for all texture measurements and data was analyzed using the Texture Expert program. TPA analysis was conducted using the procedure described by Zell et al. (2010a). About four rectangular shaped samples (1.5 × 1.7 × 1.5 cm) were removed from each muscle of OH and WB-cooked meat parallel to the muscle fiber, then prepared and wrapped in cling film and equilibrated for 30 min at 25 °C prior to measurement. The samples were placed centrally under a 50 mm diameter plunger and compressed to 50% of their original height in a double compression cycle using a crosshead speed of 50 mm min− 1. Hardness, cohesiveness, springiness, gumminess and chewiness parameters were calculated for each sample. TPA parameters of treated meat were measured in four replicates for each treatment. 2.3.4. Thiobarbituric acid-reactive substances (TBARS) TBARS indicate the changes of lipid oxidation in muscle foods during processing and refrigerated storage (Weber, Bochi, Ribeiro, Victório, & Emanuelli, 2008). The amounts of TBARS in OH and WB-cooked meat samples were determined according to the method of Soyer, Özalp, Dalmış, and Bilgin (2010). The minced sample (1 g) was homogenized with 10 ml deionized water, and an aliquot of the sample (1 ml) was added to 2 ml of trichloroacetic acid/thiobarbituric acid (TCA/TBA) stock solution consisting of 15% TCA (w/v) and 0.375% TBA (w/v) in 0.25 M HCl and 3 ml 2% butylated hydroxytoluene (BHT) (w/v) prepared in absolute ethanol in a test tube, then immediately mixed thoroughly with a vortex mixer. The mixture was incubated in a 100 °C water bath for 15 min to develop color. After cooling in cold water, the sample was centrifuged at 3000 ×g for 10 min. The absorbance of the resulting upper layer was measured at 532 nm. The amounts of TBARS were expressed as mg of malondialdehyde per kg of meat using a molar extinction coefficient of 1.56 × 105 M−1 cm−1. 2.3.5. Non-heme iron (NHI) Non-heme iron concentration of the cooked meat was determined according to Igene, King, Pearson, and Gray (1979) and South, Lei, and Miller (2000) with some modifications. 1.0 g muscle sample and 10 ml 20% trichloroacetic acid in 3 M HCl were added to a plastic 20 ml centrifuge tube. 40 μl of 0.39% NaNO2 was added to stabilize against release of iron from the heme complex. Samples were incubated at 65 °C for 20 h. After incubation, tubes were cooled to room temperature and centrifuged (3600 × g) for 10 min. The concentration of nonheme iron in the supernatant was determined using an atomic absorption spectrophotometer (Varian SpectrAA 40, Varian Techtron Pty., Mulgrave, Victoria, Australia). The results were expressed as μg of non-heme iron per gram of meat. 2.3.6. Protein carbonyl content Carbonyl measurement is the most common method for determining protein oxidation. Protein carbonyls were measured by estimation of total carbonyl groups according to the method of Soyer et al. (2010) with some modifications. From two fractions of 500 μl myofibrillar protein samples, one aliquot was treated with 2 ml of 2.0 N HCl (control) and the other was treated with 2.0 ml of 10 mM 2,4-dinitrophenylhydrazine (DNPH) in 2.0 N HCl for 1 h at room temperature. After incubation, the two fractions were then precipitated with 2.0 ml of 20% trichloroacetic acid. The precipitate was washed three times with 1.0 ml of ethanol: ethylacetate (1:1, v/v) solution to remove unreacted DNPH and blow-dried. The pellet
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
Y. Dai et al. / Innovative Food Science and Emerging Technologies xxx (2014) xxx–xxx Table 1 Proximate chemical compositions and cooking loss of OH and WB-cooked meat. Treatment
Moisture%
Fat%
Protein%
Cooking loss%
OH WB
59.86 ± 0.10x 58.69 ± 0.04y
2.05 ± 0.34 2.57 ± 0.36
33.31 ± 1.31 35.38 ± 0.58
36.29 ± 1.47 37.56 ± 0.80
x–y
in the same column with different Values are means ± SE of four replicates. superscripts are significant differences (P b 0.05).
was then dissolved in 2 ml of 6.0 M guanidine hydrochloride with 20 mM potassium phosphate buffer (pH 6.5). Absorbance was measured at 370 nm. The amount of protein carbonyl content was expressed as nmol DNPH of mg protein using an absorption coefficient of 2.2 × 104 M− 1 cm− 1 for protein hydrazones.
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3. Results and discussion 3.1. Proximate analysis Ohmic (OH) and water bath (WB) cooked pork meat showed no significant difference (P N 0.05) in protein, fat content and cooking loss (Table 1). No significant differences in cooking loss (36.29%, 37.56%) were detected in OH and WB-cooked meat samples partly due to considerably higher temperature reached (range 95 °C–115 °C). Significant differences (P b 0.05) were found in the moisture content with 59.86% in OH-cooked meats and 58.69% in the WB-cooked ones, reflecting the higher moisture loss of the latter. Slightly higher values for moisture contents of cooked beef meat during ohmic cooking were also found by Zell et al. (2010b). 3.2. Color measurements
2.3.7. Protein free thiol groups 5,5′-Dithiobis(2-nitrobenzoic acid) (DTNB) was used for determination of free thiol groups in proteins (Lund, Lametsch, Hviid, Jensen, & Skibsted, 2007). Briefly, 2.0 g muscle was homogenized in 50 ml of 5.0% SDS in 0.10 M tris buffer (pH 8.0). The homogenates were placed in a water bath at 80 °C for 30 min and centrifuged at 5000 × g for 20 min. Protein concentration of the filtrate was determined by measuring absorbance at 280 nm and calculated from a standard curve prepared from 0 to 3 mg/ml BSA. The filtrates did not absorb light N300 nm and myoglobin was therefore found not to interfere with the assay. The filtrates were diluted to a concentration of 1.5 mg/ml with homogenization buffer and assayed according to Lund et al. (2007) and Soyer et al. (2010) by mixing 0.50 ml sample, 2.0 ml of 0.10 M tris buffer (pH 8.0) and 0.50 ml of 10 mM DTNB in 0.10 M tris buffer (pH 8.0). Absorbance at 412 nm was measured after 30 min against a reference solution of 0.50 ml of 5% SDS and 2.50 ml of 0.10 M tris buffer (pH 8.0). The thiol content was calculated using a molar extinction coefficient of 11,400 M−1 cm−1 for 5,5′-dithiobis at this wavelength. Results were expressed as nmol of total thiol groups per milligram of protein.
2.4. Statistical analysis Data were analyzed using the SPSS 16.0 system. The unpaired Student's t-test was used to determine the levels of statistical significance between OH and WB-cooked meat with P b 0.05. Data obtained for the initial characterization of the cooked meat were submitted to analysis of variance (ANOVA) and Duncan's multiple range test was applied to determine significant differences in the refrigerated storage time (1–7 days) with P b 0.05. To assess the effects of refrigerated time, cooking methods and their interactions, data were also analyzed by a two-way ANOVA with P b 0.05.
The influence of cooking methods and storage time on color changes in ohmically and conventionally-cooked pork meat is illustrated in Table 2. Color parameters showed significant difference (P b 0.05) in the effects of storage time and cooking methods (Table 4). Lightness (L⁎-values) gradually increased in OH and WB-cooked pork meat. At days 3 and 5, WB-cooked meats showed a significantly (P b 0.05) lighter (i.e. higher L⁎ values) center color (range 76.34–77.11) relative to OHcooked ones (range 74.55–75.83). The OH and WB-cooked meat samples tended to lower redness (a⁎) values, and the surface/center a⁎-values from the WB-cooked samples were significantly (P b 0.05) lower than OH-cooked ones during refrigerated storage. In general, yellowness/brownness (b⁎) values in cooked meat products increased significantly throughout refrigerated storage (Estévez, Ventanas, & Cava, 2006; Ganhão, Morcuende, & Estévez, 2010). At days 3 to 7, WBcooked meat exhibited significantly (P b 0.05) higher center b⁎-values (range 18.02–18.63) than OH-cooked ones (range 16.43–17.47). Increased L⁎ and decreased a⁎-values measured on WB and OH-cooked meat might have been associated with nitrosopigment degradations and oxidations (Estévez et al., 2006; Fuentes, Ventanas, Morcuende, Estévez, & Ventanas, 2010; Ganhão et al., 2010). The WB-cooked meat displayed significantly (P b 0.05) higher L⁎ and b⁎-values but lower a⁎-values than OH-cooked meat. Similar color results for OH-cooked and WB-cooked turkey meat were also noted by Zell et al. (2010a). A higher exposure to oxygen and longer cooking time particularly in the WB cooking could have caused more increases in the oxidation of lipids and myoglobin pigments thus inducing eventual discolorations in cooked meat (Fuentes et al., 2010; Ganhão et al., 2010). 3.3. Texture deterioration Results for texture profile analysis of WB and OH-cooked meat are presented in Table 3. Both cooking methods (CM) and storage time
Table 2 Instrumental color measured during refrigerated storage of WB and OH cooked pork M. longissimus dorsi. Treatments L⁎ a⁎ b⁎ L⁎ a⁎ b⁎
Day 1 OH (surface) WB (surface) OH (surface) WB (surface) OH (surface) WB (surface) OH (center) WB (center) OH (center) WB (center) OH (center) WB (center)
70.78 70.65 1.02 −0.44 17.38 20.09 69.79 73.79 1.48 −0.39 15.48 16.72
Day 3 ± ± ± ± ± ± ± ± ± ± ± ±
0.18a 0.91a 0.23bx 0.47by 0.33ax 0.50by 2.23a 0.50a 0.68b 0.14c 0.78a 0.56a
72.45 71.94 −0.10 −2.51 18.92 18.90 74.55 76.34 −0.59 −3.72 16.43 18.02
Day 5 ± ± ± ± ± ± ± ± ± ± ± ±
0.75b 0.83ab 0.52bx 0.39ay 0.33b 0.17a 0.59bx 0.39by 0.58ax 0.09by 0.46abx 0.30by
74.20 73.14 −1.52 −3.35 19.66 20.46 75.83 77.11 −2.33 −4.42 17.47 18.49
Day 7 ± ± ± ± ± ± ± ± ± ± ± ±
0.68c 0.55b 0.42ax 0.22ay 0.41b 0.12b 0.23bx 0.13by 0.61ax 0.08ay 0.12bx 0.22by
74.78 72.64 −2.24 −3.35 19.88 20.48 76.47 77.27 −2.55 −4.34 17.25 18.63
± ± ± ± ± ± ± ± ± ± ± ±
0.20cx 0.50aby 0.49a 0.19a 0.38b 0.17b 1.29b 0.39b 0.57ax 0.14ay 0.41bx 0.18by
Values are means ± SE of four replicates. L⁎, lightness; a⁎, redness; b⁎, yellowness; a–d in the same row with different superscripts indicate significant differences (P b 0.05). A,B in the same column with different superscripts are significant differences (P b 0.05).
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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Table 3 Texture profiles of OH and WB-cooked pork M. longissimus dorsi during refrigerated storage. Treatments
Day 1
Hardness 1 (N)
OH WB OH WB OH WB OH WB OH WB OH WB
Hardness 2 (N) Cohesion Springiness (mm) Chewiness (N mm) Gumminess (N)
106.96 103.82 97.27 91.40 0.49 0.45 4.01 4.04 211.74 191.42 52.50 46.81
Day 3 ± ± ± ± ± ± ± ± ± ± ± ±
7.91a 11.63a 7.51a 10.98a 0.01bx 0.01bcy 0.11 0.16 24.51 27.33ab 4.92a 6.63ab
121.50 96.66 111.13 84.12 0.53 0.43 4.00 3.94 254.53 160.07 64.15 41.75
Day 5 ± ± ± ± ± ± ± ± ± ± ± ±
6.79ax 4.30ay 6.53abx 3.35ay 0.01cx 0.01aby 0.10 0.20 14.15x 19.58ay 4.37abx 1.79ay
145.92 103.91 130.47 91.81 0.45 0.47 4.01 4.19 261.98 210.41 65.82 49.69
Day 7 ± ± ± ± ± ± ± ± ± ± ± ±
7.83bx 2.51ay 7.17bcx 2.55ay 0.01a 0.01c 0.07 0.13 16.12x 14.29aby 4.69abx 2.34aby
148.04 136.15 133.74 118.66 0.46 0.40 3.80 4.38 267.90 240.18 69.74 54.78
± ± ± ± ± ± ± ± ± ± ± ±
6.91b 4.73b 6.39c 4.62b 0.01abx 0.01ay 0.06x 0.12y 16.14 18.66b 3.79bx 2.78by
Values are means ± SE of four replicates. a–d in the same row with different superscripts indicate significant differences (P b 0.05). x–y in the same column with different superscripts are significant differences (P b 0.05).
(ST) had significant effect (P b 0.05) on meat texture parameters except springiness (Table 4). In general, hardness, chewiness and gumminess increased significantly (P b 0.05) in the cooked pork meat at end of refrigerated storage (Table 3). Hardness increases during refrigerated storage of cooked meat products has been reported (Ganhão et al., 2010). The increase of hardness and chewiness in cooked meat during refrigerated storage is highly undesirable as this could have a great impact on consumer acceptability (Ganhão et al., 2010). According to Ganhão et al. (2010), it is plausible that oxidative damage to proteins led to an increase of hardness in cooked meat through the formation of protein carbonyls and cross-links between proteins, the loss of protein thiol groups and protein functionality which was consistent with our present study. At days 3 and 5, all texture parameters except springiness for OH-cooked pork were significantly (P b 0.05) higher when compared to WB-cooked samples. In relation to hardness, ohmically-cooked samples were in significant (P b 0.05) higher values (111.13 N–145.92 N) during 5 days of storage (Table 3). This is in agreement with the findings of Zhang, Lyng, and Brunton (2004) and Zell et al. (2010a) who reported that radio frequency (RF) and ohmicallycooked (OH) meat products were firmer than conventional cooked ones. Heat-affected changes in meat myofibrillar proteins, muscle cytoskeleton, intramuscular connective tissue and protein oxidative damages are largely responsible for textural attributes of cooked meat (Ganhão et al., 2010; Wills, Dewitt, Sigfusson, & Bellmer, 2006). According to Wills et al. (2006) and Zell et al. (2010a), higher heating temperatures achieved for the rapid OH-cooking method would likely result in more collagen shrinking and toughening in meat compared with the slow WB-cooking.
Table 4 Table of significances.
L⁎ (surface) a⁎ (surface) b⁎ (surface) L⁎ (center) a⁎ (center) b⁎ (center) Hardness 1 Hardness 2 Cohesion Springiness Chewiness Gumminess TBARS NHI Carbonyls Free thiol groups
Storage time
CM
Day × CM
b0.001 b0.001 b0.001 b0.001 b0.001 0.001 b0.001 b0.001 0.002 NS 0.039 0.032 b0.001 b0.001 b0.001 b0.001
0.041 b0.001 b0.001 0.009 b0.001 b0.001 b0.001 b0.001 b0.001 0.050 0.002 b0.001 b0.001 0.020 0.038 NS
NS NS 0.002 NS NS NS NS NS b0.001 NS NS NS NS 0.009 0.013 b0.001
CM, cooking methods; NHI, non-heme iron; NS, not significant (P N 0.05).
3.4. Lipid oxidation Lipid oxidation is a major factor reducing quality and acceptability of meat products (Fuentes et al., 2010). Lipid oxidation, which results in the production of (oxy- and lipid-) free radicals, is closely coupled with pigment oxidation, since lipid oxidation is a promoter of myoglobin oxidation (Kim, Huff-Lonergan, Sebranek, & Lonergan, 2010). The influence of cooking methods and storage time on the development of lipid oxidation in ohmically and conventionally-cooked pork meat is illustrated in Fig. 1a. Significant differences (P b 0.05) were detected in the effects of storage time and cooking methods on the TBARS value of treated-meat (Table 4). A rapid increase in TBARS data was evident for all cooked samples during 5 days of storage which is typical of stored cooked meats (Zell et al., 2010a). During 7 days of storage, values in the range of 0.20–0.67 MDA per kg meat and 0.41–1.00 mg MDA per kg meat were obtained for OH and WB-cooked meat respectively. Heat may affect development of oxidative rancidity in several ways including cooking method, cooking temperature, cooking time and end-point temperature (EPT) of the meat products (Wills et al., 2006). Compared to WB-cooked meat samples, the TBAR-values for all OH-cooked samples were significantly (P b 0.05) lower and the rate of increase with storage time was relatively slower. The WB-products gave higher TBAR-values than the OH-products which maybe most likely due to prolonged exposure to higher temperatures at the surface regions of the meat and the cooperative effects of thermal damage to the membrane phospholipids and heat denaturation of proteins (Zell et al., 2010a). This, in turn, leads to rapid lipid oxidation when exposed to air (Zell et al., 2010a). Similar findings were reported by Zell et al. (2010a) who compared rapid radio frequency, ohmic and conventional cooking of encased turkey meat and found the highest levels of lipid oxidation in the conventional cooked products. The significant (P b 0.05) increases in TBARS values and the decreases in a⁎ values of the WB-cooked meat might be consistent with the coupling reaction of lipid and myoglobin oxidations. 3.5. Non-heme iron content Non-heme iron (NHI) is one of the major catalysts of lipid oxidation (Lombardi-Boccia, Martinez-Dominguez, & Aguzzi, 2002). NHI displayed significant difference (P b 0.05) among storage time, cooking methods and their interactions effects (Table 4). NHI content increased significantly (P b 0.05) during refrigerated storage (Fig. 1b). The amount of NHI increased from 3.28 to 4.32 μg/g and from 2.06 to 5.02 μg/g from day 1 to day 7 in samples from OH and WB-cooked meat, respectively. OH-cooked meat had significantly higher (P b 0.05) amounts of NHI than WB-cooked meat at first 3 days. These results suggest that some disruptions of the porphyrin ring could have occurred during cooking or storage and led to the release of iron (Estévez et al., 2006). Passage of current could stimulate oxidation or excessive membrane
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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destruction through electron transfer (Wills et al., 2006). The increased reaction rates of electron transfer and higher heating temperature could explain why ohmically-cooked samples developed greater NHI values than water bath-cooked ones (Wills et al., 2006). But no significant differences (P N 0.05) of NHI changes were found in OH and WB-cooked meat after 3 days of refrigerated storage. Therefore, OH-cooked meat with increasing amounts of NHI might not have severely increased oxidative instability in our present study.
3.6. Protein oxidation
Fig. 1. Changes in (a) TBARS and (b) NHI content in OH and WB-cooked pork meat during refrigerated storage. Values are means ± SE of four replicates measurements. Error bars represent positive standard errors of the mean. MDA, malonaldehyde; NHI, non-heme iron.
Oxidation of numerous amino acids leads to the formation of carbonyls groups. Moreover, carbonyls can react with free amino groups of non-oxidized amino acids of proteins to form amide bonds (Gatellier et al., 2010). These reactions lead to negative impacts on nutritional value and protein digestibility of meat and meat products (Gatellier et al., 2010). The results from the analysis of the carbonyl contents of OH and WB-cooked pork meat during refrigerated storage are shown in Fig. 2a. Protein oxidation analysis showed significant difference (P b 0.05) among storage time and the interactions between cooking methods and storage time (Table 4). The amount of carbonyls from protein oxidation significantly (P b 0.05) increased in OH and WB-cooked meat after 3 days of storage time, with this increase being significantly (P b 0.05) higher in WB-cooked meat than in OH-cooked samples at day 3. Similar increases in carbonyl groups of other meat products during refrigerated storage have been reported (Estévez et al., 2006). Since carbonyl groups are the principal products of autoxidation, increases in total carbonyls indicate that oxidative changes occurred in cooked meat during refrigerated storage. At day 3, OHcooked meat had significantly (P b 0.05) higher amounts of carbonyls (4.03 nmol carbonyls/mg protein) than WB-cooked samples (2.96 nmol carbonyls/mg protein). No significant differences (P N 0.05) between OH-cooked and WB-cooked meat were detected for the carbonyls
Fig. 2. Changes in (a) carbonyl content and (b) free thiol groups in OH and WB427 cooked pork meat during refrigerated storage. Values are means ± SE of four replicates measurements. Error bars represent positive standard errors of the mean.
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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Y. Dai et al. / Innovative Food Science and Emerging Technologies xxx (2014) xxx–xxx
content since all cooked meat contained similar amounts of carbonyls after 5–7 days of refrigerated storage. Although the rapid ohmic cooking might cause oxidations of myofibrillar proteins thus inducing more formation of carbonyls and harder texture than conventional cooking methods (Wills et al., 2006), the results clearly showed that duration of refrigerated storage has no strong impact on carbonyl contents in OHcooked meat. Protein oxidation is also associated with a decrease in free thiol groups, which are converted into disulfides. The formation of disulfides promotes protein aggregation and impacts negatively on the nutritional value of meat (Gatellier et al., 2010; Lund et al., 2007; Santé-Lhoutellier, Astruc, Marinova, Greve, & Gatellier, 2008). Cooking method and duration of refrigerated storage had significant (P b 0.05) effects on the free thiol content of cooked-meat, with the mean content decreasing from 60.98 to 32.04 nmol/mg protein in OH-cooked meat and from 48.29 to 34.28 nmol/mg protein in WB-cooked meat during 7 days of storage (Table 4, Fig. 2b). A decrease in free thiol content has been reported previously in frozen chicken meat, microsomal membranes from turkey muscle, raw pork meat and minced fish during refrigerated storage (Gatellier et al., 2010; Lund et al., 2007). Santé-Lhoutellier et al. (2008) found cooking could cause a decrease of free thiol groups in meat. Losses of free thiol groups in OH-cooked and WB-cooked meat generally increased with duration of refrigerated storage, with the differences in OH-cooked meat stored for 5 and 7 days being more pronounced than WB-cooked meat. In addition, the results from the thiol determination strongly supports the explanation of harder texture based on protein cross-linking as a decreased content of free thiols was found especially in OH-meat during refrigerated storage (Lund et al., 2007). 4. Conclusion In summary, the refrigerated storage time (range 1–7 days) greatly influenced oxidation stability, color and texture qualities in OH and WB-cooked meat. The OH-cooked meat, which was cooked almost 8 times more rapidly than the WB product, had improved color qualities and developed lipid oxidation at a slower rate during refrigerated storage. However, the protein oxidation of meat during ohmic cooking was slightly higher than those obtained in water bath cooking thus resulting in higher hardness values (harder texture) during refrigerated storage. Therefore, from an anti-oxidation perspective, OH-cooking had considerable industrial potential to yield meat product with comparable or improved quality attributes to those achieved by WB-treated ones. Future research might shed light on oxidation mechanisms and optimizations of the ohmic cooking procedures thus producing meat products of high qualities. Acknowledgments This work was supported by grants from the National Natural Science Foundation of China (No. 31271894) and the Ministry of Agriculture of China (No. 200903012).
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Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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Innovative Food Science and Emerging Technologies journal homepage: www.elsevier.com/locate/ifset
Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat Yan Dai a,b,c, Yi Lu a,c, Wei Wu a,c, Xiao-ming Lu b, Zhao-peng Han b, Yi Liu a,c, Xing-min Li a,c, Rui-tong Dai a,c,⁎ a b c
College of Food Science and Nutritional Engineering, China Agricultural University, 17 Qinghua East Road, Beijing, 100083, China Chinese National Egg Engineering Research Center, Beijing DQY Agricultural Technology Co. ‘Ltd.’, Beijing, 102115, China Beijing Higher Institution Engineering Research Center of Animal Product, Beijing China, 100081, China
a r t i c l e
i n f o
Article history: Received 2 March 2014 Accepted 13 June 2014 Available online xxxx Editor Proof Receive Date 14 July 2014 Keywords: Ohmically Water bath TBARS Oxidation
a b s t r a c t In the present work, changes in lipid and protein oxidation, non-heme iron (NHI) content, color and texture parameters during refrigerated storage (7 days/4 °C) of ohmically (OH) and water bath (WB)-cooked pork meat were studied. A significant effect of refrigerated storage duration and cooking method on color, texture, lipid oxidation and carbonyl content was detected for OH-cooked and WB-cooked meat. The OH-cooked meat had better color appearance, lipid oxidative stability, slightly enhanced hardness and protein oxidation levels during 7 days of storage having lower values of TBARS, center lightness (L⁎), center/surface brownness (b⁎) and free thiol groups; however, there were higher values of center/surface redness (a⁎), non-heme iron, protein carbonyl and hardness compared to those from WB-cooked meat. Industrial relevance: The conventional cooking protocols always result in quality deteriorations and reduced shelf-life in meat products due to longer cooking times. In this study, the ohmically-cooked meat showed comparable or improved oxidation levels, color appearance and texture attributes during 7 days of refrigerated storage. From an anti-oxidation perspective, the available data are provided for the application of ohmic cooking in Chinese commercial productions of cooked meat with high quality requirements that could be established. © 2014 Elsevier Ltd. All rights reserved.
1. Introduction In ohmic or electroconductive heating, foods are heated by passing alternating current through them (Sarang, Sastry, & Knipe, 2008). Ohmic heating generates heat by direct energy application in a volumetric fashion which reduces the long cooking times associated with conventional methods (Zell, Lyng, Cronin, & Morgan, 2010a). Apart from microbial spoilage, lipid and protein oxidation is considered as one of the main causes for functional, sensory and nutritional quality deteriorations in meat and meat products (Ventanas, Estevez, Tejeda, & Ruiz, 2006). Changes associated with lipid oxidation include the development of unpleasant odors and tastes, deteriorations in color, rheological properties as well as potential formation of toxic compounds. On the other hand, protein oxidation induces increasing water losses and protein fragmentation or aggregation, decreasing protein digestibility and solubility as well as weaker protein gels or unstable emulsions of meat and meat products (Ramirez & Cava, 2007; Ventanas et al., 2006). Concerning oxidation promoters in animal
⁎ Corresponding author. Tel.: +86 1062737547; fax: +86 1062839300. E-mail address: [email protected] (R. Dai).
foodstuffs, iron is thought to have high catalytic activity (Estévez & Cava, 2004). Non-heme iron (NHI) is considered as the most important oxidation promoter. An increase in the amount of NHI as a result of thermal processes or subsequent refrigerated storage on meat systems has been linked to the decrease of heme iron (HI) as a consequence of the breakdown of the heme molecule and oxidative deterioration of the porphyrin ring of myoglobin (Estévez & Cava, 2004). Meat is always cooked before consumption, and thermal treatments greatly accelerate oxidations through increasing the free radical productions and decreasing in parallel the antioxidant protection during subsequent refrigerated storage (Gatellier, Kondjoyan, Portanguen, & Santé-Lhoutellier, 2010; Zell et al., 2010a). Currently, meat products are mainly thermally processed by steam or hot water immersion. A major problem in the cooking of meats in this way results in longer cooking times and a reduction in the quality of such products (McKenna, Lyng, Brunton, & Shirsat, 2006; Zell, Lyng, Cronin, & Morgan, 2010b). Previous work on ohmically-cooked meat showed promising results in terms of product quality (Zell et al., 2010a). Nevertheless, the study on the occurrence of protein and lipid oxidations, sensory (color and texture) deteriorations in meat during ohmic cooking are still lacking during refrigerated storage. The aim of the present study was to determine the suitability of ohmically-cooked pork meat with particular reference to its effects on selected physicochemical parameters, such as lipid and protein
http://dx.doi.org/10.1016/j.ifset.2014.06.009 1466-8564/© 2014 Elsevier Ltd. All rights reserved.
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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oxidation, levels of NHI, deteriorations in color and texture attributes, and comparison of ohmically-cooked pork with conventionallycooked-ones. 2. Materials and methods 2.1. Sample preparation M. longissimus thoracis et lumborum muscles were obtained from a local abattoir in Beijing, China. Briefly, carcasses from four (large white crossbred) pigs about six months of age were slaughtered at a live weight of 110 ± 5 kg and chilled at 4 °C for one day in accordance with EU regulations. Longissimus thoracis et lumborum muscles (from the 12th thoracic to the 5th lumbar vertebrae) were removed from both sides of each carcass, then trimmed of visible fat and connective tissue, individually vacuum packaged. Each muscle (approximately 500 g) was assigned to water bath (WB) and ohmic cooking (OH) groups. Each treatment was carried out in four replicates (i.e., meat from an individual animal equates to one replicate). Samples were thawed at 4 °C for 10 h before treatment. 2.2. Heating equipment 2.2.1. Ohmic cooking A modified ohmic heating device designed according to Dai et al. (2013) with slightly modification, was connected to a 3.5 kW power supply (15 A, 0–250 V, 50 Hz) (Wocen Power Ltd., China) to cook pork muscles of M. longissimus thoracis et lumborum. The whole muscle (about 500 ± 10 g) was clamped into the cooking cell, high-grade (316) stainless steel electrodes were connected to the sample at each end of the cooking cell, a compression fitting was designed to ensure full contact of electrodes to the meat sample. Temperature measurements were monitored by using three type T thermocouples (Industrial Temperature Sensors Ltd., Beijing, China) from nine different sections of the sample in the cooking cell (the geometrical center, 1.3 cm apart from the electrodes on the radial axis and 3.5 cm apart from the electrodes at the sample surface). The pork meat samples were ohmically-cooked to a minimum end-point temperature (EPT) of 95 °C (range 95 °C–115 °C) at the geometric center (cold spot) by applying a 10 V cm−1 voltage gradient (Zell et al., 2010b). The ohmic heater was immediately switched off when the temperature reached the final EPT (approximately 13 ± 2 min of cooking time in total). 2.2.2. Conventional cooking Briefly, the intact pork muscle about (500 ± 10 g) was packed using strings and then cooked in a water bath described by Dai et al. (2013) with slight modifications. The dimensions of these samples were similar with a 4.2 ± 1 cm diameter and 10 ± 1 cm length. During cooking, three temperature probes (Industrial Temperature Sensors Ltd., Beijing, China) were inserted into the geometric core of each muscle to monitor the internal temperature. The meat samples were immediately removed from the stainless steel cell when the core temperature reached 95 °C (range 95 °C–98 °C) as previously described (Laycock, Piyasena, & Mittal, 2003). The cooking time was approximately 83 ± 4 min in total. The OH and WB-cooked samples were vacuum packed, stored at 4 °C in a refrigerator and analyzed at intervals of 1, 3, 5 and 7 days respectively. 2.3. Quality parameters 2.3.1. Physico-chemical and cooking loss analysis The percentage of cooking loss (CL) was calculated by obtaining the difference between the green and cooked weights (post-cooling) and divided by the raw weight of OH and WB-cooked meat (Zell et al., 2010a), and the proximate analysis of these samples was carried out using the methods described by Zell et al.(2010a). All measurements were performed four times.
2.3.2. Color measurement A Minolta colorimeter (Model No. CR-400, Konica Minolta Sensing Americas Inc., Ramsey, NJ, USA) calibrated for internal light (D65) before analysis was used to measure L⁎ (lightness), a⁎ (redness/greenness) and b⁎ (yellowness/brownness) values. Color measurements were evaluated at four points near the center and at four points on the surfaces of the four cooked pork samples (Zell et al., 2010a). 2.3.3. Texture measurement A TA-XT2 texture analyzer (Stable Micro Systems, Godalming, UK) equipped with a 250 N load cell was used for all texture measurements and data was analyzed using the Texture Expert program. TPA analysis was conducted using the procedure described by Zell et al. (2010a). About four rectangular shaped samples (1.5 × 1.7 × 1.5 cm) were removed from each muscle of OH and WB-cooked meat parallel to the muscle fiber, then prepared and wrapped in cling film and equilibrated for 30 min at 25 °C prior to measurement. The samples were placed centrally under a 50 mm diameter plunger and compressed to 50% of their original height in a double compression cycle using a crosshead speed of 50 mm min− 1. Hardness, cohesiveness, springiness, gumminess and chewiness parameters were calculated for each sample. TPA parameters of treated meat were measured in four replicates for each treatment. 2.3.4. Thiobarbituric acid-reactive substances (TBARS) TBARS indicate the changes of lipid oxidation in muscle foods during processing and refrigerated storage (Weber, Bochi, Ribeiro, Victório, & Emanuelli, 2008). The amounts of TBARS in OH and WB-cooked meat samples were determined according to the method of Soyer, Özalp, Dalmış, and Bilgin (2010). The minced sample (1 g) was homogenized with 10 ml deionized water, and an aliquot of the sample (1 ml) was added to 2 ml of trichloroacetic acid/thiobarbituric acid (TCA/TBA) stock solution consisting of 15% TCA (w/v) and 0.375% TBA (w/v) in 0.25 M HCl and 3 ml 2% butylated hydroxytoluene (BHT) (w/v) prepared in absolute ethanol in a test tube, then immediately mixed thoroughly with a vortex mixer. The mixture was incubated in a 100 °C water bath for 15 min to develop color. After cooling in cold water, the sample was centrifuged at 3000 ×g for 10 min. The absorbance of the resulting upper layer was measured at 532 nm. The amounts of TBARS were expressed as mg of malondialdehyde per kg of meat using a molar extinction coefficient of 1.56 × 105 M−1 cm−1. 2.3.5. Non-heme iron (NHI) Non-heme iron concentration of the cooked meat was determined according to Igene, King, Pearson, and Gray (1979) and South, Lei, and Miller (2000) with some modifications. 1.0 g muscle sample and 10 ml 20% trichloroacetic acid in 3 M HCl were added to a plastic 20 ml centrifuge tube. 40 μl of 0.39% NaNO2 was added to stabilize against release of iron from the heme complex. Samples were incubated at 65 °C for 20 h. After incubation, tubes were cooled to room temperature and centrifuged (3600 × g) for 10 min. The concentration of nonheme iron in the supernatant was determined using an atomic absorption spectrophotometer (Varian SpectrAA 40, Varian Techtron Pty., Mulgrave, Victoria, Australia). The results were expressed as μg of non-heme iron per gram of meat. 2.3.6. Protein carbonyl content Carbonyl measurement is the most common method for determining protein oxidation. Protein carbonyls were measured by estimation of total carbonyl groups according to the method of Soyer et al. (2010) with some modifications. From two fractions of 500 μl myofibrillar protein samples, one aliquot was treated with 2 ml of 2.0 N HCl (control) and the other was treated with 2.0 ml of 10 mM 2,4-dinitrophenylhydrazine (DNPH) in 2.0 N HCl for 1 h at room temperature. After incubation, the two fractions were then precipitated with 2.0 ml of 20% trichloroacetic acid. The precipitate was washed three times with 1.0 ml of ethanol: ethylacetate (1:1, v/v) solution to remove unreacted DNPH and blow-dried. The pellet
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
Y. Dai et al. / Innovative Food Science and Emerging Technologies xxx (2014) xxx–xxx Table 1 Proximate chemical compositions and cooking loss of OH and WB-cooked meat. Treatment
Moisture%
Fat%
Protein%
Cooking loss%
OH WB
59.86 ± 0.10x 58.69 ± 0.04y
2.05 ± 0.34 2.57 ± 0.36
33.31 ± 1.31 35.38 ± 0.58
36.29 ± 1.47 37.56 ± 0.80
x–y
in the same column with different Values are means ± SE of four replicates. superscripts are significant differences (P b 0.05).
was then dissolved in 2 ml of 6.0 M guanidine hydrochloride with 20 mM potassium phosphate buffer (pH 6.5). Absorbance was measured at 370 nm. The amount of protein carbonyl content was expressed as nmol DNPH of mg protein using an absorption coefficient of 2.2 × 104 M− 1 cm− 1 for protein hydrazones.
3
3. Results and discussion 3.1. Proximate analysis Ohmic (OH) and water bath (WB) cooked pork meat showed no significant difference (P N 0.05) in protein, fat content and cooking loss (Table 1). No significant differences in cooking loss (36.29%, 37.56%) were detected in OH and WB-cooked meat samples partly due to considerably higher temperature reached (range 95 °C–115 °C). Significant differences (P b 0.05) were found in the moisture content with 59.86% in OH-cooked meats and 58.69% in the WB-cooked ones, reflecting the higher moisture loss of the latter. Slightly higher values for moisture contents of cooked beef meat during ohmic cooking were also found by Zell et al. (2010b). 3.2. Color measurements
2.3.7. Protein free thiol groups 5,5′-Dithiobis(2-nitrobenzoic acid) (DTNB) was used for determination of free thiol groups in proteins (Lund, Lametsch, Hviid, Jensen, & Skibsted, 2007). Briefly, 2.0 g muscle was homogenized in 50 ml of 5.0% SDS in 0.10 M tris buffer (pH 8.0). The homogenates were placed in a water bath at 80 °C for 30 min and centrifuged at 5000 × g for 20 min. Protein concentration of the filtrate was determined by measuring absorbance at 280 nm and calculated from a standard curve prepared from 0 to 3 mg/ml BSA. The filtrates did not absorb light N300 nm and myoglobin was therefore found not to interfere with the assay. The filtrates were diluted to a concentration of 1.5 mg/ml with homogenization buffer and assayed according to Lund et al. (2007) and Soyer et al. (2010) by mixing 0.50 ml sample, 2.0 ml of 0.10 M tris buffer (pH 8.0) and 0.50 ml of 10 mM DTNB in 0.10 M tris buffer (pH 8.0). Absorbance at 412 nm was measured after 30 min against a reference solution of 0.50 ml of 5% SDS and 2.50 ml of 0.10 M tris buffer (pH 8.0). The thiol content was calculated using a molar extinction coefficient of 11,400 M−1 cm−1 for 5,5′-dithiobis at this wavelength. Results were expressed as nmol of total thiol groups per milligram of protein.
2.4. Statistical analysis Data were analyzed using the SPSS 16.0 system. The unpaired Student's t-test was used to determine the levels of statistical significance between OH and WB-cooked meat with P b 0.05. Data obtained for the initial characterization of the cooked meat were submitted to analysis of variance (ANOVA) and Duncan's multiple range test was applied to determine significant differences in the refrigerated storage time (1–7 days) with P b 0.05. To assess the effects of refrigerated time, cooking methods and their interactions, data were also analyzed by a two-way ANOVA with P b 0.05.
The influence of cooking methods and storage time on color changes in ohmically and conventionally-cooked pork meat is illustrated in Table 2. Color parameters showed significant difference (P b 0.05) in the effects of storage time and cooking methods (Table 4). Lightness (L⁎-values) gradually increased in OH and WB-cooked pork meat. At days 3 and 5, WB-cooked meats showed a significantly (P b 0.05) lighter (i.e. higher L⁎ values) center color (range 76.34–77.11) relative to OHcooked ones (range 74.55–75.83). The OH and WB-cooked meat samples tended to lower redness (a⁎) values, and the surface/center a⁎-values from the WB-cooked samples were significantly (P b 0.05) lower than OH-cooked ones during refrigerated storage. In general, yellowness/brownness (b⁎) values in cooked meat products increased significantly throughout refrigerated storage (Estévez, Ventanas, & Cava, 2006; Ganhão, Morcuende, & Estévez, 2010). At days 3 to 7, WBcooked meat exhibited significantly (P b 0.05) higher center b⁎-values (range 18.02–18.63) than OH-cooked ones (range 16.43–17.47). Increased L⁎ and decreased a⁎-values measured on WB and OH-cooked meat might have been associated with nitrosopigment degradations and oxidations (Estévez et al., 2006; Fuentes, Ventanas, Morcuende, Estévez, & Ventanas, 2010; Ganhão et al., 2010). The WB-cooked meat displayed significantly (P b 0.05) higher L⁎ and b⁎-values but lower a⁎-values than OH-cooked meat. Similar color results for OH-cooked and WB-cooked turkey meat were also noted by Zell et al. (2010a). A higher exposure to oxygen and longer cooking time particularly in the WB cooking could have caused more increases in the oxidation of lipids and myoglobin pigments thus inducing eventual discolorations in cooked meat (Fuentes et al., 2010; Ganhão et al., 2010). 3.3. Texture deterioration Results for texture profile analysis of WB and OH-cooked meat are presented in Table 3. Both cooking methods (CM) and storage time
Table 2 Instrumental color measured during refrigerated storage of WB and OH cooked pork M. longissimus dorsi. Treatments L⁎ a⁎ b⁎ L⁎ a⁎ b⁎
Day 1 OH (surface) WB (surface) OH (surface) WB (surface) OH (surface) WB (surface) OH (center) WB (center) OH (center) WB (center) OH (center) WB (center)
70.78 70.65 1.02 −0.44 17.38 20.09 69.79 73.79 1.48 −0.39 15.48 16.72
Day 3 ± ± ± ± ± ± ± ± ± ± ± ±
0.18a 0.91a 0.23bx 0.47by 0.33ax 0.50by 2.23a 0.50a 0.68b 0.14c 0.78a 0.56a
72.45 71.94 −0.10 −2.51 18.92 18.90 74.55 76.34 −0.59 −3.72 16.43 18.02
Day 5 ± ± ± ± ± ± ± ± ± ± ± ±
0.75b 0.83ab 0.52bx 0.39ay 0.33b 0.17a 0.59bx 0.39by 0.58ax 0.09by 0.46abx 0.30by
74.20 73.14 −1.52 −3.35 19.66 20.46 75.83 77.11 −2.33 −4.42 17.47 18.49
Day 7 ± ± ± ± ± ± ± ± ± ± ± ±
0.68c 0.55b 0.42ax 0.22ay 0.41b 0.12b 0.23bx 0.13by 0.61ax 0.08ay 0.12bx 0.22by
74.78 72.64 −2.24 −3.35 19.88 20.48 76.47 77.27 −2.55 −4.34 17.25 18.63
± ± ± ± ± ± ± ± ± ± ± ±
0.20cx 0.50aby 0.49a 0.19a 0.38b 0.17b 1.29b 0.39b 0.57ax 0.14ay 0.41bx 0.18by
Values are means ± SE of four replicates. L⁎, lightness; a⁎, redness; b⁎, yellowness; a–d in the same row with different superscripts indicate significant differences (P b 0.05). A,B in the same column with different superscripts are significant differences (P b 0.05).
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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Table 3 Texture profiles of OH and WB-cooked pork M. longissimus dorsi during refrigerated storage. Treatments
Day 1
Hardness 1 (N)
OH WB OH WB OH WB OH WB OH WB OH WB
Hardness 2 (N) Cohesion Springiness (mm) Chewiness (N mm) Gumminess (N)
106.96 103.82 97.27 91.40 0.49 0.45 4.01 4.04 211.74 191.42 52.50 46.81
Day 3 ± ± ± ± ± ± ± ± ± ± ± ±
7.91a 11.63a 7.51a 10.98a 0.01bx 0.01bcy 0.11 0.16 24.51 27.33ab 4.92a 6.63ab
121.50 96.66 111.13 84.12 0.53 0.43 4.00 3.94 254.53 160.07 64.15 41.75
Day 5 ± ± ± ± ± ± ± ± ± ± ± ±
6.79ax 4.30ay 6.53abx 3.35ay 0.01cx 0.01aby 0.10 0.20 14.15x 19.58ay 4.37abx 1.79ay
145.92 103.91 130.47 91.81 0.45 0.47 4.01 4.19 261.98 210.41 65.82 49.69
Day 7 ± ± ± ± ± ± ± ± ± ± ± ±
7.83bx 2.51ay 7.17bcx 2.55ay 0.01a 0.01c 0.07 0.13 16.12x 14.29aby 4.69abx 2.34aby
148.04 136.15 133.74 118.66 0.46 0.40 3.80 4.38 267.90 240.18 69.74 54.78
± ± ± ± ± ± ± ± ± ± ± ±
6.91b 4.73b 6.39c 4.62b 0.01abx 0.01ay 0.06x 0.12y 16.14 18.66b 3.79bx 2.78by
Values are means ± SE of four replicates. a–d in the same row with different superscripts indicate significant differences (P b 0.05). x–y in the same column with different superscripts are significant differences (P b 0.05).
(ST) had significant effect (P b 0.05) on meat texture parameters except springiness (Table 4). In general, hardness, chewiness and gumminess increased significantly (P b 0.05) in the cooked pork meat at end of refrigerated storage (Table 3). Hardness increases during refrigerated storage of cooked meat products has been reported (Ganhão et al., 2010). The increase of hardness and chewiness in cooked meat during refrigerated storage is highly undesirable as this could have a great impact on consumer acceptability (Ganhão et al., 2010). According to Ganhão et al. (2010), it is plausible that oxidative damage to proteins led to an increase of hardness in cooked meat through the formation of protein carbonyls and cross-links between proteins, the loss of protein thiol groups and protein functionality which was consistent with our present study. At days 3 and 5, all texture parameters except springiness for OH-cooked pork were significantly (P b 0.05) higher when compared to WB-cooked samples. In relation to hardness, ohmically-cooked samples were in significant (P b 0.05) higher values (111.13 N–145.92 N) during 5 days of storage (Table 3). This is in agreement with the findings of Zhang, Lyng, and Brunton (2004) and Zell et al. (2010a) who reported that radio frequency (RF) and ohmicallycooked (OH) meat products were firmer than conventional cooked ones. Heat-affected changes in meat myofibrillar proteins, muscle cytoskeleton, intramuscular connective tissue and protein oxidative damages are largely responsible for textural attributes of cooked meat (Ganhão et al., 2010; Wills, Dewitt, Sigfusson, & Bellmer, 2006). According to Wills et al. (2006) and Zell et al. (2010a), higher heating temperatures achieved for the rapid OH-cooking method would likely result in more collagen shrinking and toughening in meat compared with the slow WB-cooking.
Table 4 Table of significances.
L⁎ (surface) a⁎ (surface) b⁎ (surface) L⁎ (center) a⁎ (center) b⁎ (center) Hardness 1 Hardness 2 Cohesion Springiness Chewiness Gumminess TBARS NHI Carbonyls Free thiol groups
Storage time
CM
Day × CM
b0.001 b0.001 b0.001 b0.001 b0.001 0.001 b0.001 b0.001 0.002 NS 0.039 0.032 b0.001 b0.001 b0.001 b0.001
0.041 b0.001 b0.001 0.009 b0.001 b0.001 b0.001 b0.001 b0.001 0.050 0.002 b0.001 b0.001 0.020 0.038 NS
NS NS 0.002 NS NS NS NS NS b0.001 NS NS NS NS 0.009 0.013 b0.001
CM, cooking methods; NHI, non-heme iron; NS, not significant (P N 0.05).
3.4. Lipid oxidation Lipid oxidation is a major factor reducing quality and acceptability of meat products (Fuentes et al., 2010). Lipid oxidation, which results in the production of (oxy- and lipid-) free radicals, is closely coupled with pigment oxidation, since lipid oxidation is a promoter of myoglobin oxidation (Kim, Huff-Lonergan, Sebranek, & Lonergan, 2010). The influence of cooking methods and storage time on the development of lipid oxidation in ohmically and conventionally-cooked pork meat is illustrated in Fig. 1a. Significant differences (P b 0.05) were detected in the effects of storage time and cooking methods on the TBARS value of treated-meat (Table 4). A rapid increase in TBARS data was evident for all cooked samples during 5 days of storage which is typical of stored cooked meats (Zell et al., 2010a). During 7 days of storage, values in the range of 0.20–0.67 MDA per kg meat and 0.41–1.00 mg MDA per kg meat were obtained for OH and WB-cooked meat respectively. Heat may affect development of oxidative rancidity in several ways including cooking method, cooking temperature, cooking time and end-point temperature (EPT) of the meat products (Wills et al., 2006). Compared to WB-cooked meat samples, the TBAR-values for all OH-cooked samples were significantly (P b 0.05) lower and the rate of increase with storage time was relatively slower. The WB-products gave higher TBAR-values than the OH-products which maybe most likely due to prolonged exposure to higher temperatures at the surface regions of the meat and the cooperative effects of thermal damage to the membrane phospholipids and heat denaturation of proteins (Zell et al., 2010a). This, in turn, leads to rapid lipid oxidation when exposed to air (Zell et al., 2010a). Similar findings were reported by Zell et al. (2010a) who compared rapid radio frequency, ohmic and conventional cooking of encased turkey meat and found the highest levels of lipid oxidation in the conventional cooked products. The significant (P b 0.05) increases in TBARS values and the decreases in a⁎ values of the WB-cooked meat might be consistent with the coupling reaction of lipid and myoglobin oxidations. 3.5. Non-heme iron content Non-heme iron (NHI) is one of the major catalysts of lipid oxidation (Lombardi-Boccia, Martinez-Dominguez, & Aguzzi, 2002). NHI displayed significant difference (P b 0.05) among storage time, cooking methods and their interactions effects (Table 4). NHI content increased significantly (P b 0.05) during refrigerated storage (Fig. 1b). The amount of NHI increased from 3.28 to 4.32 μg/g and from 2.06 to 5.02 μg/g from day 1 to day 7 in samples from OH and WB-cooked meat, respectively. OH-cooked meat had significantly higher (P b 0.05) amounts of NHI than WB-cooked meat at first 3 days. These results suggest that some disruptions of the porphyrin ring could have occurred during cooking or storage and led to the release of iron (Estévez et al., 2006). Passage of current could stimulate oxidation or excessive membrane
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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destruction through electron transfer (Wills et al., 2006). The increased reaction rates of electron transfer and higher heating temperature could explain why ohmically-cooked samples developed greater NHI values than water bath-cooked ones (Wills et al., 2006). But no significant differences (P N 0.05) of NHI changes were found in OH and WB-cooked meat after 3 days of refrigerated storage. Therefore, OH-cooked meat with increasing amounts of NHI might not have severely increased oxidative instability in our present study.
3.6. Protein oxidation
Fig. 1. Changes in (a) TBARS and (b) NHI content in OH and WB-cooked pork meat during refrigerated storage. Values are means ± SE of four replicates measurements. Error bars represent positive standard errors of the mean. MDA, malonaldehyde; NHI, non-heme iron.
Oxidation of numerous amino acids leads to the formation of carbonyls groups. Moreover, carbonyls can react with free amino groups of non-oxidized amino acids of proteins to form amide bonds (Gatellier et al., 2010). These reactions lead to negative impacts on nutritional value and protein digestibility of meat and meat products (Gatellier et al., 2010). The results from the analysis of the carbonyl contents of OH and WB-cooked pork meat during refrigerated storage are shown in Fig. 2a. Protein oxidation analysis showed significant difference (P b 0.05) among storage time and the interactions between cooking methods and storage time (Table 4). The amount of carbonyls from protein oxidation significantly (P b 0.05) increased in OH and WB-cooked meat after 3 days of storage time, with this increase being significantly (P b 0.05) higher in WB-cooked meat than in OH-cooked samples at day 3. Similar increases in carbonyl groups of other meat products during refrigerated storage have been reported (Estévez et al., 2006). Since carbonyl groups are the principal products of autoxidation, increases in total carbonyls indicate that oxidative changes occurred in cooked meat during refrigerated storage. At day 3, OHcooked meat had significantly (P b 0.05) higher amounts of carbonyls (4.03 nmol carbonyls/mg protein) than WB-cooked samples (2.96 nmol carbonyls/mg protein). No significant differences (P N 0.05) between OH-cooked and WB-cooked meat were detected for the carbonyls
Fig. 2. Changes in (a) carbonyl content and (b) free thiol groups in OH and WB427 cooked pork meat during refrigerated storage. Values are means ± SE of four replicates measurements. Error bars represent positive standard errors of the mean.
Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009
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Y. Dai et al. / Innovative Food Science and Emerging Technologies xxx (2014) xxx–xxx
content since all cooked meat contained similar amounts of carbonyls after 5–7 days of refrigerated storage. Although the rapid ohmic cooking might cause oxidations of myofibrillar proteins thus inducing more formation of carbonyls and harder texture than conventional cooking methods (Wills et al., 2006), the results clearly showed that duration of refrigerated storage has no strong impact on carbonyl contents in OHcooked meat. Protein oxidation is also associated with a decrease in free thiol groups, which are converted into disulfides. The formation of disulfides promotes protein aggregation and impacts negatively on the nutritional value of meat (Gatellier et al., 2010; Lund et al., 2007; Santé-Lhoutellier, Astruc, Marinova, Greve, & Gatellier, 2008). Cooking method and duration of refrigerated storage had significant (P b 0.05) effects on the free thiol content of cooked-meat, with the mean content decreasing from 60.98 to 32.04 nmol/mg protein in OH-cooked meat and from 48.29 to 34.28 nmol/mg protein in WB-cooked meat during 7 days of storage (Table 4, Fig. 2b). A decrease in free thiol content has been reported previously in frozen chicken meat, microsomal membranes from turkey muscle, raw pork meat and minced fish during refrigerated storage (Gatellier et al., 2010; Lund et al., 2007). Santé-Lhoutellier et al. (2008) found cooking could cause a decrease of free thiol groups in meat. Losses of free thiol groups in OH-cooked and WB-cooked meat generally increased with duration of refrigerated storage, with the differences in OH-cooked meat stored for 5 and 7 days being more pronounced than WB-cooked meat. In addition, the results from the thiol determination strongly supports the explanation of harder texture based on protein cross-linking as a decreased content of free thiols was found especially in OH-meat during refrigerated storage (Lund et al., 2007). 4. Conclusion In summary, the refrigerated storage time (range 1–7 days) greatly influenced oxidation stability, color and texture qualities in OH and WB-cooked meat. The OH-cooked meat, which was cooked almost 8 times more rapidly than the WB product, had improved color qualities and developed lipid oxidation at a slower rate during refrigerated storage. However, the protein oxidation of meat during ohmic cooking was slightly higher than those obtained in water bath cooking thus resulting in higher hardness values (harder texture) during refrigerated storage. Therefore, from an anti-oxidation perspective, OH-cooking had considerable industrial potential to yield meat product with comparable or improved quality attributes to those achieved by WB-treated ones. Future research might shed light on oxidation mechanisms and optimizations of the ohmic cooking procedures thus producing meat products of high qualities. Acknowledgments This work was supported by grants from the National Natural Science Foundation of China (No. 31271894) and the Ministry of Agriculture of China (No. 200903012).
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Please cite this article as: Dai, Y., et al., Changes in oxidation, color and texture deteriorations during refrigerated storage of ohmically and water bath-cooked pork meat, Innovative Food Science and Emerging Technologies (2014), http://dx.doi.org/10.1016/j.ifset.2014.06.009