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Effect of irradiation and storage time on lipid oxidation of chilled pork
Radiation Physics and Chemistry 80 (2011) 475–480
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Effect of irradiation and storage time on lipid oxidation of chilled pork Anwei Cheng a, Fachun Wan b, Tongcheng Xu a, Fangling Du a,n, Wenliang Wang a, Qingjun Zhu a a b
Institute of Atomic Energy Application in Agriculture, Shandong Academy of Agricultural Science, Jinan 250100, China Institute of the Animal Science and Veterinary Medicine, Shandong Academy of Agricultural Science, Jinan 250100, China
a r t i c l e in f o
abstract
Article history: Received 19 August 2009 Accepted 17 October 2010
The effects of g-irradiation with different doses (0, 2, 4, 6, 8 and 10 kGy) and storage time (0–30 days) on the lipid oxidation of chilled pork and the combined effect of irradiation and antioxidant on the lipid oxidative stability during storage at 4 1C were investigated. The results indicated that irradiation treatment increased lipid oxidation, measured as peroxide (PV) and thiobarbituric acid reactive substance (TBARS) values. Lipid oxidation was increased with the increase in storage time. The addition of tea polyphenol (TP) was effective in controlling the lipid oxidation of chilled pork after irradiation during cold storage. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Irradiation Storage time Lipid oxidation Antioxidant Chilled pork
1. Introduction Irradiation has been widely used in food industry as preservation technology, and irradiated foodstuffs are approved in many countries such as Belgium, France, USA and the Netherlands (Alfaia et al., 2007; Grolichova´ et al., 2004). Nutritive values and physical–chemical properties of irradiated foodstuffs could be preserved much longer than those of non-irradiated foodstuffs. Meat irradiation is recognized as a safe and effective method to attain meat preservation (Kanatt et al., 2006). The use of high-energy gamma rays or accelerated electrons to irradiate fresh meat may extend shelf-life and inhibit proliferation of pathogenic bacteria (Kwon et al., 2008). Therefore, there have been attempts to utilize irradiation not only for food safety but also for technological purposes. Fats are among the least stable food components, and are very susceptible to ionizing radiation, which may induce autoxidation. Lipid oxidation is a complex process in which unsaturated fatty acids react with molecular oxygen via a free radical chain mechanism and form fatty acyl hydroperoxides (Mexis and Kontominas, 2009). Radiation processing generates free radicals and accelerates the oxidation of unsaturated fatty acids that may induce some biochemical changes in meat and thus influence its quality, such as the nutritional value (Du et al., 2000). Polyunsaturated fatty acids (PUFAs) of the phospholipid fraction, which represent 0.5–1.0% of the total lipids in meat, are most susceptible to irradiation, and are the major contributors to the development of rancidity during meat storage (Giroux and Lacroix, 1998). Several papers have pointed out that irradiation doses lower than 10 kGy may control the growth of pathogenic and spoilage bacteria on meat and meat products (Shenoy et al., 1998). However, meats are n
Corresponding author. Tel.: + 86 531 83179137; fax: + 86 531 88960332. E-mail address: [email protected] (F. Du).
0969-806X/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2010.10.003
generally susceptible to oxidative deterioration principally due to the oxidation of PUFAs present in phospholipids. By enhancing free radical reactions, irradiation probably can lead to color changes, lipid oxidation and odor generation in meat products, which can result in the negative responses from consumers to the quality deterioration (Ahn et al., 2000; Jo and Ahn, 2000; Du et al., 2002). In addition, the irradiation effect (mainly lipid oxidation) could be increased during the storage process, particularly during post-irradiation storage. Free radicals generated from the radiolysis of water, such as hydroxyl radicals (OHd), hydrated electrons and hydrogen ion (H + ) (Merrit et al., 1978), attack food components (proteins, amino acids, lipids, etc.), and lead to an increased rate of lipid oxidation. These free radicals appear in aqueous systems and also in meat, because over 75% of muscle cells are composed of water and surrounded by lipid bilayers (Thakur and Singh, 1994). Although many publications have evaluated the quality of irradiated bovine, ovine, swine and poultry meat (Ahn et al., 2000; Du et al., 2000), the information on changes of chilled pork is limited. Pork has the maximum consumption in China due to its unique delicacy. The short shelf-life of pork, mainly derives from the development of rancidity, is a drawback of pork related products, and lipid oxide is a main cause of the degeneration of pork. Thus, the objective of the present study was to determine the effect of g-irradiation (0, 2, 4, 6, 8 and 10 kGy) and storage time (0–30 days) on lipid oxidation of chilled pork.
2. Materials and methods 2.1. Sample preparation, storage and irradiation Fresh marbled meat was bought from the local supermarket in February 2009. We selected the internal part of pork and removed
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the external part that was exposed in the air. The internal part of marbled meat was divided into 50 g portions (including the nonirradiated sample), which had the equal surface area as possible, and the control represents uncut pork that would not be oxidized primarily by the effects of irradiation. All the samples were rapidly wrapped in polystyrene bags under aerobic conditions and stocked at 4 1C. Tea polyphenol (TP) was dissolved in distilled water, the concentrations were 50 10 6 and 100 10 6 mol/L. One milliliter of TP solution was mixed with 50 g of the chilled pork by using the spray method. The chilled pork with adding 1 mL distilled water was also prepared as control. Irradiation was carried out at room temperature in the Institute of Atomic Energy Application in Agriculture (Jinan, China). Samples were irradiated with a 300 kCi 60Co source at 0, 2, 4, 6, 8 and 10 kGy or at 0, 5 and 10 kGy. To minimize variations in radiation-dose absorption, pouches were rotated 1801 half way during their radiation process. Irradiation doses were measured using the Harwell Perspex Polymethylmethacrylate Amber 3042 dosimeters. The absorbance signal was measured using a Camspec M 201-UV spectrometer at 640 nm. The average dose rate was 1 kGy/h for all irradiated samples.
2.2. Materials Thiobarbituric acid (TBA), trichloroacetic acid (TCA), diethyl ether, acetic acid, isooctane and malonaldehyde (MDA) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Tea polyphenol (TP) was from Luoshi Co. Ltd. (Beijing, China).
2.3. Determination of peroxide value (PV) The samples of pork were analyzed by the method of GB/T 5009.44-2003, National Standard of the People’s Republic of China. Crushed pork (5 g) was transferred into a 250 mL separate funnel with 100 mL of diethyl ether and 10 mL of distilled water. The funnel was agitated for 2 min and subsequently left to equilibrate for 24 h. Fifty milliliter of sample was transferred to a crystallizing dish and diethyl ether evaporated in a water bath at 40 1C. The extracted oil was dried in an oven at 105 1C for 3 min, and the residue was used to determine the peroxide value. Peroxide value was determined according to GB/T 5009.3-1996, National Standard of the People’s Republic of China.
3. Results and discussion 3.1. Effect of irradiation dose on lipid oxidation of chilled pork Because the storage time of pork at room temperature was very short and equal to the difference treated samples as possible, the material was fresh. We chose the internal part of pork as the experimental samples. In theory, there is not a significant difference in times at which the different portions of pork are at room temperature. Therefore in the following experiment, we ignored the initial oxidative state of the pork, which was different, and thought that the initial oxidative state was similar. 3.1.1. Peroxide value (PV) As shown in Fig. 1, PV increased from an initial value of 1.64 to a value of 9.84 meq O2/kg pork after irradiation at a dose of 10 kGy. It is obvious that increasing doses of irradiation enhanced lipid oxidation and represented peroxide formation. It is worth mentioning that PV was statistically different (po0.05) between control and irradiated samples, after irradiation at a dose of 2 kGy. The observed lag in PV between 8 and 10 kGy may be possibly related to the simultaneous formation and destruction of hydroperoxides for the production of secondary oxidation products at intermediate irradiation doses. Lipid peroxidation is a complicated autocatalytic process running in two phases. During the first phase, primary products of oxidation occur—hydroperoxides and conjugated dienes. These primary products are unstable and decompose to generate various secondary products, such as aldehydes (hexanal), ketones, alcohols and hydrocarbons (Bakalivanova et al., 2009). During the second phase the hydroperoxides, because of their high reactivity, are transformed into secondary products of oxidation, resulting in the formation of trienes, aldehydes, ketones, volatile fatty acids, etc. (Bakalivanova et al., 2009). 3.1.2. Thiobarbituric acid reactive substances (TBARS) values The effect of irradiation on lipid oxidation of pork during the storage at 4 1C was monitored using TBARS, as depicted in Fig. 2. TBARS values of chilled pork increased in a dose-dependent manner associated with the increase in irradiation dose. TBARS increased from an initial value of 0.44 to 0.73 mg MDA/kg pork after irradiation at a dose of 10 kGy. Irradiated samples had relatively higher TBARS values than the control. The increase in rate of TBARS values was relatively slower in samples irradiated at 8 kGy than in samples irradiated at 2 kGy. During storage, TBARS values in all 12
TBARS were determined according to the method of Buege and Aust (1978). Briefly, samples (5 g) were homogenized with 25 mL of TBARS solution (0.375% thiobarbituric acid, 15% trichloroacetic acid, and 0.25 N HCl). The mixture was boiled for 10 min to develop a pink color. Then the mixture was cooled with running water and centrifuged at 5500 rpm for 25 min. The absorbance of the supernatant was measured at 532 nm, using a model UV-1601 spectrophotometer. TBARS values were calculated from the standard curve of malonaldehyde (MDA) and expressed as mg MDA/kg sample.
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All determinations were performed in triplicate. The results were expressed as mean 7standard deviations (S.D.). Data were analyzed using the Analysis of Variance (SAS 9.0) and T-test to determine the statistical significance (p o0.05).
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Fig. 1. Changes in PV (Meq O2/kg) of chilled pork irradiation at 0, 2, 4, 6, 8 and 10 kGy during storage at 4 1C for 24 h. Bars represent the standard deviations from triplicate determinations.
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Fig. 2. Changes in TBARS (mg MDA/kg) of chilled pork irradiation at 0, 2, 4, 6, 8 and 10 kGy during storage at 4 1C for 24 h. Bars represent the standard deviations from triplicate determinations.
3.2. Effect of storage time on the lipid oxidation of chilled pork 3.2.1. Peroxide value (PV) The effects of storage time on PV in chilled pork were shown in Fig. 3. Non-irradiated samples showed significantly lower PV than those of irradiated at 5 and 10 kGy (p o0.05), and PV in irradiated samples was dose-dependent and increased with the irradiation dose. Non-irradiated samples had small changes during the storage for 15 days, and after 15 days, these samples presented slimy surface as a result of mold growth (data not shown). It was postulated to be due to the loss of volatile oxidation compounds from pork stored for a longer time. Irradiation has been known to initiate the normal process of lipid oxidation, which gives rise to rancid off-flavors (Ahn and Nam, 2004). In addition, samples irradiated at 5 kGy had an obviously increase in PV when storage time increased. After 20 days of storage, PV was decreased with the storage time increasing. Among all samples, pork samples irradiation at 10 kGy showed the greatest increase during the storage for 0–15 days. After a marked decrease was generally found with
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Fig. 3. Changes in PV of chilled pork irradiation at 0 (B), 5 (’) and 10 kGy (m) during storage for 0, 5, 10, 15, 20, 25 and 30 days. Bars represent the standard deviations from triplicate determinations.
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samples increased continuously. The highest TBARS value was observed in chilled pork irradiated at 10 kGy. Sohn et al. (2009) reported that TBARS values of ground beef irradiated with 5 and 10 kGy were significantly higher than the control. Similar results were found by Ramo´n et al. (2009), who observed that the TBARS values in dry cured loin slices from pigs increased markedly with the increase in irradiation dose. Irradiation has been reported to increase TBARS in different meat species, packaging and storage conditions (Hampson et al., 1996; Ahn et al., 1998a; Du et al., 2000). Although free radicals are known to accelerate lipid oxidation in meat (Jo and Ahn, 2000), the effect of irradiation was obvious in pork. Irradiated meats produced more volatiles than the nonirradiated ones regardless of meat species, but the degree of volatile change varied significantly among meats (Kwon et al., 2008). Irradiation was reported to induce lipid oxidation and oxidation progresses during storage (Kanatt et al., 1998), which was consistent to our findings in chilled pork. The damage of the muscle structure caused by radicals that exposed fatty acids to oxygen and catalysing factors, such as iron and heme, might be associated with the accelerated lipid oxidation in samples irradiated at higher doses (Morrisey et al., 1998). Lipid oxidation was responsible for the reduction in nutritional quality as well as changes in flavor (Aguirrez´abal et al., 2000). From these results, it appeared that lipid oxidation in pork samples could be stimulated by irradiation at a higher dose.
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Fig. 4. Changes in TBARS of chilled pork irradiation at 0 (B), 5 (’) and 10 kGy (m) during storage for 0, 5, 10, 15, 20, 25 and 30 days. Bars represent the standard deviations from triplicate determinations.
increase in storage time, the extent of decrease was obviously greater than that irradiation at 5 kGy or the non-irradiation control. Irradiation samples treated at 10 kGy showed the highest PV at day 15. In general, the rate of increase in PV was greater in samples irradiated with higher doses (10 kGy) during the first 15 days of the storage, than in control or samples irradiated at 5 kGy. 3.2.2. Thiobarbituric acid reactive substances (TBARS) values The effect of storage time on TBARS was shown in Fig. 4. During storage, TBARS values in all samples increased continuously. The highest TBARS value was observed in chilled pork irradiated at 10 kGy when stored for 25 days. After this, the TBARS value decreased in chilled pork irradiated at 10 kGy. Non-irradiated samples had small changes during storage for 15 days, and after 15 days, these samples presented slimy surface as a result of mold growth (data not shown). During the storage of 15 days, the changes in TBARS values of samples irradiated at 5 and 10 kGy were not obvious compared with those of the controls. When the storage time was over 15 days, the increased extent of TBARS values of irradiated samples (5 and 10 kGy) was relatively greater than the
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in chilled pork treated with 0, 50 10 6 and 100 10 6 mol/L doses of TP. Regardless of TP, no differences in PV of chilled pork with and without irradiation were observed among all samples at day 0. In addition, the irradiated samples showed marked increase in PV compared with non-irradiated control. Samples irradiated at 10 kGy had the highest PV in all samples. Changes in PV of pork without irradiation were shown in Fig. 5(A). After 15 days, nonirradiated samples presented slimy surface as a result of mold growth. PV of pork without TP was the highest in all samples during storage for 5 days. When the concentration of TP was 50 10 6 mol/L, the storage time had no effect on the PV of non-irradiated samples. Fig. 5(B) showed that PV in pork treated TP was lower than that in untreated samples. With irradiation of 5 kGy, the highest values of PV in pork treated with TP (0, 50 10 6 and 100 10 6 mol/L) were 26.73, 31.29 and 20.40 meq O2/kg, respectively, and the highest PV of samples untreated TP was observed after being stored for 15 days, but that of treated TP was delayed to 20 days. Thereafter, a decrease in PV was noticeable. Fig. 5(C) showed that, with irradiation of 10 kGy, the highest PV of sample untreated TP was 41.78 meq O2/kg at the storage for 10 days.
first 15 days of storage. From the results, lipid oxidation preceded during the extended storage, especially with irradiation samples. Sohn et al. (2009) reported that, as storage time increased, TBARS values in ground beef increased significantly. However, there was no significance at day 7 of the irradiated samples. Ahn and Nam (2004) also pointed out that as the storage time increased, over all lipid oxidation of irradiated beef increased and scored higher TBARS value than that of non-irradiated beef. The formation of TBARS did not show a continuous tendency during storage (Fig. 4). These variations can be explained as a result of the different phases of peroxide decomposition, the information of carbonyls and the interaction compounds with nucleophilic molecules present in the muscles (Aubourg et al., 2004). Throughout the storage, TBARS significantly increased in the middle 15–25 days of refrigerated storage. Subsequently, TBARS values decreased and showed fluctuations for sample irradiation at 10 kGy. This could be attributed either to the interaction of TBA-reactive products with other tissue constituents, or to malonaldehyde deutilization by surviving microflora (Kasimoglu et al., 2003). Highly unsaturated fatty acids are more readily oxidized than less unsaturated fatty acids. In lipids, particularly those containing unsaturated fatty acids, radiolytic decomposition occurs and induces the formation of some volatile compounds responsible for off-odors (Riebroy et al., 2007). Ionizing radiation produces free radicals that can accelerate oxidative processes and produce radiolytic products from meat components. From the results, lipid oxidation proceeded during the extended storage, especially with the irradiated samples. Storage tended to increase PV and TBARS values in chilled pork, especially in higher dose irradiation samples.
3.3.2. Thiobarbituric acid reactive substances (TBARS) values The effect of TP on TBARS values of pork was shown in Fig. 6. During the storage time, antioxidant (TP) significantly decreased TBARS values of pork with and without irradiation. TP decreased TBARS in a dose-dependent fashion. Samples treated with higher dose of TP (100 10 6 mol/L) had lower TBARS values than those treated with lower dose of TP (50 10 6 mol/L) or the untreated control. During refrigerated storage, TBARS values showed relatively smaller changes compared with PV in chilled pork treated with TP. Fig. 6(A) showed that, in samples untreated with TP, no significant differences were found as a result of non-irradiation treatment at days 0, 5, 10 and 15 of refrigerated storage. A similar trend was found in the samples treated with the higher dose of TP (100 10 6 mol/L). While TBARS in pork treated with TP (50 10 6 mol/L) increased with the increase in storage time. Fig. 6(B)
3.3. Effect of antioxidant on the lipid oxidation of chilled pork 3.3.1. Peroxide value (PV) The effects of tea polyphenol (TP) on PV of pork, without and with irradiation during the storage at 4 1C, were shown in Fig. 5. The results showed that a dose-dependent decrease in PV was observed
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Fig. 6. Effects of tea polyphenol with the concentration of 0 (B), 50 10 6 (’) and 100 10 6 mol/L (m) on the TBARS values of pork irradiated with 0 (A), 5 (B) and 10 kGy (C) during storage at 4 1C. Bars represent the standard deviations from triplicate determinations.
showed that TBARS values were increased in time-dependent manners treated or untreated with TP. Fig. 6(C) showed that the higher TBARS values in samples with untreated TP were detected during the storage for 15–30 days, and the highest TBARS values was observed at day 25 in all samples. While samples treated with TP has no obvious differences in TBARS values during the storage time for 0–30 days. The effect of antioxidant in chilled pork was better after 5 days of storage than the initial stage of storage, which was consistent with the effects of 5 and 10 kGy samples in this study. It was suggested that the addition of antioxidant such as TP might help to retard the lipid oxidation of irradiated pork during cold storage. Our results were in agreement with the result reported on other kinds of antioxidant (such as a-tocopherol), which indicated that antioxidant might help retard the lipid oxidation and increase the shelf-life of irradiated pork (Sohn et al., 2009). Ahn et al. (1998b) reported that TBARS in irradiated turkey breast gradually decreased as dietary tocopherol acetate level increased. Antioxidants can minimize irradiation-induced peroxidation of lard and tallow (Kyong et al., 1998; 1999). Ascorbyl palmitate (AP) was extremely effective in minimizing irradiation-induced oxidation in tallow, lard and linolenic acid in a concentration-dependent fashion (Lee et al., 1999). Due to the energy input, irradiation can induce sequential reactions among food components, including oxidation of ions, reduction of carbonyls to hydroxy derivatives, elimination of double bonds, decrease in aromaticity, hydroxylation of aromatic and heterocyclics and generation of free radicals that can be oxidized to various peroxides (Brewer, 2009). Many researchers have found that irradiated meat generated a characteristic odor, which has been described as metallic, sulfide, wet dog, wet grain or brunt (Huber et al., 1953). The undesirable odor compounds in irradiated meats were hydrophilic and contained nitrogen and/or
sulfur and the produced sulfur-containing compounds, such as methyl mercaptan and sulfur dioxide, were responsible for some of the irradiation off-odor (Sohn et al., 2009). In this article, we studied only the effect of irradiation and storage time on the primary characters of lipid oxidation (PV and TBARS) in the chilled pork during storage at 4 1C. Further work is ongoing to delineate the different characteristic odors from the irradiated meat compared with the non-irradiated pork and to investigate the underlying mechanism of irradiated oxidation.
4. Conclusion In the present study, irradiation at doses of 0, 2, 4, 6, 8 and 10 kGy induced changes in lipid oxidation of chilled pork immediately after irradiation, and affected the PV and TBARS values during refrigerated storage. However, gamma irradiation could retard microbial growth in pork, leading to the retardation of spoilage and putridity. Shelf-life of marble meat from landrace pig could be extended by irradiation at 2–10 kGy in combination with the refrigerated storage. In addition, antioxidant (TP) can decrease the lipid oxidation of chilled pork after irradiation, prolong the shelf-life and improve the safety of chilled pork. Therefore, irradiation in combination with antioxidant addition may be usable in the pork industry to produce a prime quality product with prolonged shelf-life. References Aguirrez´abal, M.M., Meteo, J., Domı´nguez, M.C., Zumalaca´rregui, J.M., 2000. The effect of paprika, garlic and salt on rancidity in dry sausage. Meat Sci. 54, 77–81. Ahn, D.U., Olson, D.G., Jo, C., Chen, X., Wu, C., Lee, J.I., 1998a. Effect of muscle type, packaging, and irradiation on lipid oxidation, volatile production, and color in raw pork patties. Meat Sci. 47, 27–39.
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Effect of irradiation and storage time on lipid oxidation of chilled pork Anwei Cheng a, Fachun Wan b, Tongcheng Xu a, Fangling Du a,n, Wenliang Wang a, Qingjun Zhu a a b
Institute of Atomic Energy Application in Agriculture, Shandong Academy of Agricultural Science, Jinan 250100, China Institute of the Animal Science and Veterinary Medicine, Shandong Academy of Agricultural Science, Jinan 250100, China
a r t i c l e in f o
abstract
Article history: Received 19 August 2009 Accepted 17 October 2010
The effects of g-irradiation with different doses (0, 2, 4, 6, 8 and 10 kGy) and storage time (0–30 days) on the lipid oxidation of chilled pork and the combined effect of irradiation and antioxidant on the lipid oxidative stability during storage at 4 1C were investigated. The results indicated that irradiation treatment increased lipid oxidation, measured as peroxide (PV) and thiobarbituric acid reactive substance (TBARS) values. Lipid oxidation was increased with the increase in storage time. The addition of tea polyphenol (TP) was effective in controlling the lipid oxidation of chilled pork after irradiation during cold storage. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Irradiation Storage time Lipid oxidation Antioxidant Chilled pork
1. Introduction Irradiation has been widely used in food industry as preservation technology, and irradiated foodstuffs are approved in many countries such as Belgium, France, USA and the Netherlands (Alfaia et al., 2007; Grolichova´ et al., 2004). Nutritive values and physical–chemical properties of irradiated foodstuffs could be preserved much longer than those of non-irradiated foodstuffs. Meat irradiation is recognized as a safe and effective method to attain meat preservation (Kanatt et al., 2006). The use of high-energy gamma rays or accelerated electrons to irradiate fresh meat may extend shelf-life and inhibit proliferation of pathogenic bacteria (Kwon et al., 2008). Therefore, there have been attempts to utilize irradiation not only for food safety but also for technological purposes. Fats are among the least stable food components, and are very susceptible to ionizing radiation, which may induce autoxidation. Lipid oxidation is a complex process in which unsaturated fatty acids react with molecular oxygen via a free radical chain mechanism and form fatty acyl hydroperoxides (Mexis and Kontominas, 2009). Radiation processing generates free radicals and accelerates the oxidation of unsaturated fatty acids that may induce some biochemical changes in meat and thus influence its quality, such as the nutritional value (Du et al., 2000). Polyunsaturated fatty acids (PUFAs) of the phospholipid fraction, which represent 0.5–1.0% of the total lipids in meat, are most susceptible to irradiation, and are the major contributors to the development of rancidity during meat storage (Giroux and Lacroix, 1998). Several papers have pointed out that irradiation doses lower than 10 kGy may control the growth of pathogenic and spoilage bacteria on meat and meat products (Shenoy et al., 1998). However, meats are n
Corresponding author. Tel.: + 86 531 83179137; fax: + 86 531 88960332. E-mail address: [email protected] (F. Du).
0969-806X/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2010.10.003
generally susceptible to oxidative deterioration principally due to the oxidation of PUFAs present in phospholipids. By enhancing free radical reactions, irradiation probably can lead to color changes, lipid oxidation and odor generation in meat products, which can result in the negative responses from consumers to the quality deterioration (Ahn et al., 2000; Jo and Ahn, 2000; Du et al., 2002). In addition, the irradiation effect (mainly lipid oxidation) could be increased during the storage process, particularly during post-irradiation storage. Free radicals generated from the radiolysis of water, such as hydroxyl radicals (OHd), hydrated electrons and hydrogen ion (H + ) (Merrit et al., 1978), attack food components (proteins, amino acids, lipids, etc.), and lead to an increased rate of lipid oxidation. These free radicals appear in aqueous systems and also in meat, because over 75% of muscle cells are composed of water and surrounded by lipid bilayers (Thakur and Singh, 1994). Although many publications have evaluated the quality of irradiated bovine, ovine, swine and poultry meat (Ahn et al., 2000; Du et al., 2000), the information on changes of chilled pork is limited. Pork has the maximum consumption in China due to its unique delicacy. The short shelf-life of pork, mainly derives from the development of rancidity, is a drawback of pork related products, and lipid oxide is a main cause of the degeneration of pork. Thus, the objective of the present study was to determine the effect of g-irradiation (0, 2, 4, 6, 8 and 10 kGy) and storage time (0–30 days) on lipid oxidation of chilled pork.
2. Materials and methods 2.1. Sample preparation, storage and irradiation Fresh marbled meat was bought from the local supermarket in February 2009. We selected the internal part of pork and removed
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the external part that was exposed in the air. The internal part of marbled meat was divided into 50 g portions (including the nonirradiated sample), which had the equal surface area as possible, and the control represents uncut pork that would not be oxidized primarily by the effects of irradiation. All the samples were rapidly wrapped in polystyrene bags under aerobic conditions and stocked at 4 1C. Tea polyphenol (TP) was dissolved in distilled water, the concentrations were 50 10 6 and 100 10 6 mol/L. One milliliter of TP solution was mixed with 50 g of the chilled pork by using the spray method. The chilled pork with adding 1 mL distilled water was also prepared as control. Irradiation was carried out at room temperature in the Institute of Atomic Energy Application in Agriculture (Jinan, China). Samples were irradiated with a 300 kCi 60Co source at 0, 2, 4, 6, 8 and 10 kGy or at 0, 5 and 10 kGy. To minimize variations in radiation-dose absorption, pouches were rotated 1801 half way during their radiation process. Irradiation doses were measured using the Harwell Perspex Polymethylmethacrylate Amber 3042 dosimeters. The absorbance signal was measured using a Camspec M 201-UV spectrometer at 640 nm. The average dose rate was 1 kGy/h for all irradiated samples.
2.2. Materials Thiobarbituric acid (TBA), trichloroacetic acid (TCA), diethyl ether, acetic acid, isooctane and malonaldehyde (MDA) were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). Tea polyphenol (TP) was from Luoshi Co. Ltd. (Beijing, China).
2.3. Determination of peroxide value (PV) The samples of pork were analyzed by the method of GB/T 5009.44-2003, National Standard of the People’s Republic of China. Crushed pork (5 g) was transferred into a 250 mL separate funnel with 100 mL of diethyl ether and 10 mL of distilled water. The funnel was agitated for 2 min and subsequently left to equilibrate for 24 h. Fifty milliliter of sample was transferred to a crystallizing dish and diethyl ether evaporated in a water bath at 40 1C. The extracted oil was dried in an oven at 105 1C for 3 min, and the residue was used to determine the peroxide value. Peroxide value was determined according to GB/T 5009.3-1996, National Standard of the People’s Republic of China.
3. Results and discussion 3.1. Effect of irradiation dose on lipid oxidation of chilled pork Because the storage time of pork at room temperature was very short and equal to the difference treated samples as possible, the material was fresh. We chose the internal part of pork as the experimental samples. In theory, there is not a significant difference in times at which the different portions of pork are at room temperature. Therefore in the following experiment, we ignored the initial oxidative state of the pork, which was different, and thought that the initial oxidative state was similar. 3.1.1. Peroxide value (PV) As shown in Fig. 1, PV increased from an initial value of 1.64 to a value of 9.84 meq O2/kg pork after irradiation at a dose of 10 kGy. It is obvious that increasing doses of irradiation enhanced lipid oxidation and represented peroxide formation. It is worth mentioning that PV was statistically different (po0.05) between control and irradiated samples, after irradiation at a dose of 2 kGy. The observed lag in PV between 8 and 10 kGy may be possibly related to the simultaneous formation and destruction of hydroperoxides for the production of secondary oxidation products at intermediate irradiation doses. Lipid peroxidation is a complicated autocatalytic process running in two phases. During the first phase, primary products of oxidation occur—hydroperoxides and conjugated dienes. These primary products are unstable and decompose to generate various secondary products, such as aldehydes (hexanal), ketones, alcohols and hydrocarbons (Bakalivanova et al., 2009). During the second phase the hydroperoxides, because of their high reactivity, are transformed into secondary products of oxidation, resulting in the formation of trienes, aldehydes, ketones, volatile fatty acids, etc. (Bakalivanova et al., 2009). 3.1.2. Thiobarbituric acid reactive substances (TBARS) values The effect of irradiation on lipid oxidation of pork during the storage at 4 1C was monitored using TBARS, as depicted in Fig. 2. TBARS values of chilled pork increased in a dose-dependent manner associated with the increase in irradiation dose. TBARS increased from an initial value of 0.44 to 0.73 mg MDA/kg pork after irradiation at a dose of 10 kGy. Irradiated samples had relatively higher TBARS values than the control. The increase in rate of TBARS values was relatively slower in samples irradiated at 8 kGy than in samples irradiated at 2 kGy. During storage, TBARS values in all 12
TBARS were determined according to the method of Buege and Aust (1978). Briefly, samples (5 g) were homogenized with 25 mL of TBARS solution (0.375% thiobarbituric acid, 15% trichloroacetic acid, and 0.25 N HCl). The mixture was boiled for 10 min to develop a pink color. Then the mixture was cooled with running water and centrifuged at 5500 rpm for 25 min. The absorbance of the supernatant was measured at 532 nm, using a model UV-1601 spectrophotometer. TBARS values were calculated from the standard curve of malonaldehyde (MDA) and expressed as mg MDA/kg sample.
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Fig. 1. Changes in PV (Meq O2/kg) of chilled pork irradiation at 0, 2, 4, 6, 8 and 10 kGy during storage at 4 1C for 24 h. Bars represent the standard deviations from triplicate determinations.
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Fig. 2. Changes in TBARS (mg MDA/kg) of chilled pork irradiation at 0, 2, 4, 6, 8 and 10 kGy during storage at 4 1C for 24 h. Bars represent the standard deviations from triplicate determinations.
3.2. Effect of storage time on the lipid oxidation of chilled pork 3.2.1. Peroxide value (PV) The effects of storage time on PV in chilled pork were shown in Fig. 3. Non-irradiated samples showed significantly lower PV than those of irradiated at 5 and 10 kGy (p o0.05), and PV in irradiated samples was dose-dependent and increased with the irradiation dose. Non-irradiated samples had small changes during the storage for 15 days, and after 15 days, these samples presented slimy surface as a result of mold growth (data not shown). It was postulated to be due to the loss of volatile oxidation compounds from pork stored for a longer time. Irradiation has been known to initiate the normal process of lipid oxidation, which gives rise to rancid off-flavors (Ahn and Nam, 2004). In addition, samples irradiated at 5 kGy had an obviously increase in PV when storage time increased. After 20 days of storage, PV was decreased with the storage time increasing. Among all samples, pork samples irradiation at 10 kGy showed the greatest increase during the storage for 0–15 days. After a marked decrease was generally found with
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Fig. 3. Changes in PV of chilled pork irradiation at 0 (B), 5 (’) and 10 kGy (m) during storage for 0, 5, 10, 15, 20, 25 and 30 days. Bars represent the standard deviations from triplicate determinations.
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samples increased continuously. The highest TBARS value was observed in chilled pork irradiated at 10 kGy. Sohn et al. (2009) reported that TBARS values of ground beef irradiated with 5 and 10 kGy were significantly higher than the control. Similar results were found by Ramo´n et al. (2009), who observed that the TBARS values in dry cured loin slices from pigs increased markedly with the increase in irradiation dose. Irradiation has been reported to increase TBARS in different meat species, packaging and storage conditions (Hampson et al., 1996; Ahn et al., 1998a; Du et al., 2000). Although free radicals are known to accelerate lipid oxidation in meat (Jo and Ahn, 2000), the effect of irradiation was obvious in pork. Irradiated meats produced more volatiles than the nonirradiated ones regardless of meat species, but the degree of volatile change varied significantly among meats (Kwon et al., 2008). Irradiation was reported to induce lipid oxidation and oxidation progresses during storage (Kanatt et al., 1998), which was consistent to our findings in chilled pork. The damage of the muscle structure caused by radicals that exposed fatty acids to oxygen and catalysing factors, such as iron and heme, might be associated with the accelerated lipid oxidation in samples irradiated at higher doses (Morrisey et al., 1998). Lipid oxidation was responsible for the reduction in nutritional quality as well as changes in flavor (Aguirrez´abal et al., 2000). From these results, it appeared that lipid oxidation in pork samples could be stimulated by irradiation at a higher dose.
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Fig. 4. Changes in TBARS of chilled pork irradiation at 0 (B), 5 (’) and 10 kGy (m) during storage for 0, 5, 10, 15, 20, 25 and 30 days. Bars represent the standard deviations from triplicate determinations.
increase in storage time, the extent of decrease was obviously greater than that irradiation at 5 kGy or the non-irradiation control. Irradiation samples treated at 10 kGy showed the highest PV at day 15. In general, the rate of increase in PV was greater in samples irradiated with higher doses (10 kGy) during the first 15 days of the storage, than in control or samples irradiated at 5 kGy. 3.2.2. Thiobarbituric acid reactive substances (TBARS) values The effect of storage time on TBARS was shown in Fig. 4. During storage, TBARS values in all samples increased continuously. The highest TBARS value was observed in chilled pork irradiated at 10 kGy when stored for 25 days. After this, the TBARS value decreased in chilled pork irradiated at 10 kGy. Non-irradiated samples had small changes during storage for 15 days, and after 15 days, these samples presented slimy surface as a result of mold growth (data not shown). During the storage of 15 days, the changes in TBARS values of samples irradiated at 5 and 10 kGy were not obvious compared with those of the controls. When the storage time was over 15 days, the increased extent of TBARS values of irradiated samples (5 and 10 kGy) was relatively greater than the
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in chilled pork treated with 0, 50 10 6 and 100 10 6 mol/L doses of TP. Regardless of TP, no differences in PV of chilled pork with and without irradiation were observed among all samples at day 0. In addition, the irradiated samples showed marked increase in PV compared with non-irradiated control. Samples irradiated at 10 kGy had the highest PV in all samples. Changes in PV of pork without irradiation were shown in Fig. 5(A). After 15 days, nonirradiated samples presented slimy surface as a result of mold growth. PV of pork without TP was the highest in all samples during storage for 5 days. When the concentration of TP was 50 10 6 mol/L, the storage time had no effect on the PV of non-irradiated samples. Fig. 5(B) showed that PV in pork treated TP was lower than that in untreated samples. With irradiation of 5 kGy, the highest values of PV in pork treated with TP (0, 50 10 6 and 100 10 6 mol/L) were 26.73, 31.29 and 20.40 meq O2/kg, respectively, and the highest PV of samples untreated TP was observed after being stored for 15 days, but that of treated TP was delayed to 20 days. Thereafter, a decrease in PV was noticeable. Fig. 5(C) showed that, with irradiation of 10 kGy, the highest PV of sample untreated TP was 41.78 meq O2/kg at the storage for 10 days.
first 15 days of storage. From the results, lipid oxidation preceded during the extended storage, especially with irradiation samples. Sohn et al. (2009) reported that, as storage time increased, TBARS values in ground beef increased significantly. However, there was no significance at day 7 of the irradiated samples. Ahn and Nam (2004) also pointed out that as the storage time increased, over all lipid oxidation of irradiated beef increased and scored higher TBARS value than that of non-irradiated beef. The formation of TBARS did not show a continuous tendency during storage (Fig. 4). These variations can be explained as a result of the different phases of peroxide decomposition, the information of carbonyls and the interaction compounds with nucleophilic molecules present in the muscles (Aubourg et al., 2004). Throughout the storage, TBARS significantly increased in the middle 15–25 days of refrigerated storage. Subsequently, TBARS values decreased and showed fluctuations for sample irradiation at 10 kGy. This could be attributed either to the interaction of TBA-reactive products with other tissue constituents, or to malonaldehyde deutilization by surviving microflora (Kasimoglu et al., 2003). Highly unsaturated fatty acids are more readily oxidized than less unsaturated fatty acids. In lipids, particularly those containing unsaturated fatty acids, radiolytic decomposition occurs and induces the formation of some volatile compounds responsible for off-odors (Riebroy et al., 2007). Ionizing radiation produces free radicals that can accelerate oxidative processes and produce radiolytic products from meat components. From the results, lipid oxidation proceeded during the extended storage, especially with the irradiated samples. Storage tended to increase PV and TBARS values in chilled pork, especially in higher dose irradiation samples.
3.3.2. Thiobarbituric acid reactive substances (TBARS) values The effect of TP on TBARS values of pork was shown in Fig. 6. During the storage time, antioxidant (TP) significantly decreased TBARS values of pork with and without irradiation. TP decreased TBARS in a dose-dependent fashion. Samples treated with higher dose of TP (100 10 6 mol/L) had lower TBARS values than those treated with lower dose of TP (50 10 6 mol/L) or the untreated control. During refrigerated storage, TBARS values showed relatively smaller changes compared with PV in chilled pork treated with TP. Fig. 6(A) showed that, in samples untreated with TP, no significant differences were found as a result of non-irradiation treatment at days 0, 5, 10 and 15 of refrigerated storage. A similar trend was found in the samples treated with the higher dose of TP (100 10 6 mol/L). While TBARS in pork treated with TP (50 10 6 mol/L) increased with the increase in storage time. Fig. 6(B)
3.3. Effect of antioxidant on the lipid oxidation of chilled pork 3.3.1. Peroxide value (PV) The effects of tea polyphenol (TP) on PV of pork, without and with irradiation during the storage at 4 1C, were shown in Fig. 5. The results showed that a dose-dependent decrease in PV was observed
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Fig. 6. Effects of tea polyphenol with the concentration of 0 (B), 50 10 6 (’) and 100 10 6 mol/L (m) on the TBARS values of pork irradiated with 0 (A), 5 (B) and 10 kGy (C) during storage at 4 1C. Bars represent the standard deviations from triplicate determinations.
showed that TBARS values were increased in time-dependent manners treated or untreated with TP. Fig. 6(C) showed that the higher TBARS values in samples with untreated TP were detected during the storage for 15–30 days, and the highest TBARS values was observed at day 25 in all samples. While samples treated with TP has no obvious differences in TBARS values during the storage time for 0–30 days. The effect of antioxidant in chilled pork was better after 5 days of storage than the initial stage of storage, which was consistent with the effects of 5 and 10 kGy samples in this study. It was suggested that the addition of antioxidant such as TP might help to retard the lipid oxidation of irradiated pork during cold storage. Our results were in agreement with the result reported on other kinds of antioxidant (such as a-tocopherol), which indicated that antioxidant might help retard the lipid oxidation and increase the shelf-life of irradiated pork (Sohn et al., 2009). Ahn et al. (1998b) reported that TBARS in irradiated turkey breast gradually decreased as dietary tocopherol acetate level increased. Antioxidants can minimize irradiation-induced peroxidation of lard and tallow (Kyong et al., 1998; 1999). Ascorbyl palmitate (AP) was extremely effective in minimizing irradiation-induced oxidation in tallow, lard and linolenic acid in a concentration-dependent fashion (Lee et al., 1999). Due to the energy input, irradiation can induce sequential reactions among food components, including oxidation of ions, reduction of carbonyls to hydroxy derivatives, elimination of double bonds, decrease in aromaticity, hydroxylation of aromatic and heterocyclics and generation of free radicals that can be oxidized to various peroxides (Brewer, 2009). Many researchers have found that irradiated meat generated a characteristic odor, which has been described as metallic, sulfide, wet dog, wet grain or brunt (Huber et al., 1953). The undesirable odor compounds in irradiated meats were hydrophilic and contained nitrogen and/or
sulfur and the produced sulfur-containing compounds, such as methyl mercaptan and sulfur dioxide, were responsible for some of the irradiation off-odor (Sohn et al., 2009). In this article, we studied only the effect of irradiation and storage time on the primary characters of lipid oxidation (PV and TBARS) in the chilled pork during storage at 4 1C. Further work is ongoing to delineate the different characteristic odors from the irradiated meat compared with the non-irradiated pork and to investigate the underlying mechanism of irradiated oxidation.
4. Conclusion In the present study, irradiation at doses of 0, 2, 4, 6, 8 and 10 kGy induced changes in lipid oxidation of chilled pork immediately after irradiation, and affected the PV and TBARS values during refrigerated storage. However, gamma irradiation could retard microbial growth in pork, leading to the retardation of spoilage and putridity. Shelf-life of marble meat from landrace pig could be extended by irradiation at 2–10 kGy in combination with the refrigerated storage. In addition, antioxidant (TP) can decrease the lipid oxidation of chilled pork after irradiation, prolong the shelf-life and improve the safety of chilled pork. Therefore, irradiation in combination with antioxidant addition may be usable in the pork industry to produce a prime quality product with prolonged shelf-life. References Aguirrez´abal, M.M., Meteo, J., Domı´nguez, M.C., Zumalaca´rregui, J.M., 2000. The effect of paprika, garlic and salt on rancidity in dry sausage. Meat Sci. 54, 77–81. Ahn, D.U., Olson, D.G., Jo, C., Chen, X., Wu, C., Lee, J.I., 1998a. Effect of muscle type, packaging, and irradiation on lipid oxidation, volatile production, and color in raw pork patties. Meat Sci. 47, 27–39.
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