Effect of Thawing Methods and Storage Time on Physicochemical Characteristic and Fatty Acid and Amino Acid Content of Frozen Duck Meat

The samples obtained from seventy-two 8-wk-old Korean native ducks (average body weight 2.8±0.35 kg) were used. Breast meats were collected within 30 minutes after slaughter, vacuum packed, and stored in a deep freezer (−50°C).

The samples were divided into 8 treatments (3 replications/ treatment, 3 samples/replication), with 2 × 4 complex factors of 2 thawing methods (with running water at 12°C for 3 h or in a refrigerator at 5°C for 24 h) and 4 storage periods (1, 3, 6, and 12 mon). Thawing was terminated when the temperature of the mass reached 4～6°C.

Meat color, CIE L* (lightness), CIE a* (redness), and CIE b* (yellowness), was measured using a colorimeter (CR-300 Chroma Meter, Minolta, Tokyo, Japan). The colorimeter was standardized using a white calibration plate of Y=92.40, x=0.3136, y=0.3196. The measurements were recorded in triplicate per sample.

Shear force, cooking loss, and water holding capacity were analyzed by the method of Chae et al. (2006).

For the analysis of shear force, the meat was heated at 70°C for 10 min to collect samples (average weight 60 g) in the direction of the muscle fiber with a core of 1.27 cm in diameter and then measured with a shear force meter (Warner-Bratzler shear force meter, Tallgrass Solutions Inc., USA).

To measure cooking loss, the samples were cut from skinless breast meat and the initial weight was measured. These samples were put in polyethylene bags and cooked in an 80°C water bath for 1 h and cooled at room temperature for 30 min before weighing. Cooking loss was measured as the weight difference before and after cooking. The test was performed in triplicate.

To measure the water holding capacity (WHC) by centrifugation, about 0.5 g of the sample, from which the fat and fascia were removed from the tube, was measured and heated in a water bath at 80°C for 20 min. After cooling for 10 min, the samples were centrifuged at 2,000 rpm, at 10°C, for 10 min (Hitachi SCR 20BA; Hitachi, Tokyo, Japan). After centrifugation, the meat was removed from the tube, and the weight of the centrifuge tube before and after drying was measured. WHC was evaluated in triplicate and expressed as the percentage of moisture that remained in the meat.

The chemical components (moisture, fat, protein, and ash) were analyzed according to the feed standard analysis method (NLRI, 2001) and the AOAC (2012) method.

Lipids were extracted using the method by Folch et al. (1957). BHT 50 μL and Folch solution (chloroform:methanol =2:1) were homogenized at 14,000 rpm for 30 s, after which 50 mL of 0.88% NaCl was added to 25 g of the sample, followed by stirring. The mixture was centrifuged at 3,000 rpm for 10 min. The bottom layer was collected, concentrated, and the solvent left under N₂ was removed.

About 80 mg of the extracted lipids was hydrolyzed at 90°C for 7 min in 1 mL of 0.5 N NaCl, followed by cooling at room temperature for 5 min. One milliliter of 14% boron trifluoride methanol solution (BF₃ methanol; Sigma-Aldrich, St. Louis, Mo, USA) was added, and the free fatty acids were methylated at 90°C for 15 min and then cooled at room temperature for 30 min. Hexane 2 mL and distilled water 10 mL were added and the upper layer 1 mL was sampled for fatty acid analysis. Fatty acid composition was analyzed using a gas chromatography (Agilent/HP 6890N; HP, Palo Alto, CA, USA) with a flame ionization detector and a 60 m × 0.32 mm Supelcowax^® 10 column.

Amino acid content was measured using an amino acid analyzer (Hitachi L-850-A, Hitachi, Korea) after cutting the lean portion of the breast meat and hydrolyzing them in 6 N HCl at 110°C for 16 h.

The physicochemical characteristics, fatty acid content and amino acid content before freezing duck meat are shown in Tables 1, 2, and 3.

All data from this study were analyzed using the GLM program by SAS (2012), and the mean value of each treatment interval was tested at a 95% and 99% confidence level using Duncan’s multiple test (Duncan, 1955).

Table 4 shows the meat color and physical properties of duck meat according to storage period and thawing method. The meat color and water holding capacity (WHC) were similar to the result of fresh meat (Table 1), but shear force and cooking loss were higher than those of fresh meat.

CIE L* and CIE a* did not differ based on storage time; however, CIE b* was significantly lower at 6 mon of storage among the storage periods and was not significantly different between at 1 and 12 mon of storage (P<0.05). There was no difference in meat color depending on thawing method.

Frozen stored meat is used after thawing, which results in many changes in meat quality that occur during the thawing process (Kang et al., 2006). Jeong et al. (2006) and Park et al. (2012) reported that various mechanisms in the cell do not work properly due to damage of cells in the process of freezing and thawing of pork, which causes oxidation of myoglobin and affects the meat color. Park et al. (2012) reported that the redness of pork was lower after thawing, and Kang et al. (2006) and Heo et al. (2016) stated that frozen meat showed increased CIE L* due to the physical damage of the muscle fiber and the leakage of water.

Moon et al. (2010) reported that meat color of Hanwoo beef was not affected by storage time at −20°C. In the present study, however, a significant change was observed in CIE b* of duck meat by storage time. Moon et al. (2010) also reported that meat color was changed during refrigeration after thawing but no color change was detected in this experiment.

No significant difference was observed in cooking loss base on storage time; however, it was significantly lower when the fast thawing treatment was applied (P<0.05). The WHC significantly decreased after 3 mon of storage (P<0.05), but was not affected by thawing method. There was no significant difference in shear force based on storage time or thawing method.

The cooking loss and WHC of duck breast meat used in this study are different according to storage period and thawing method. The reason is that the higher WHC of thawed frozen meat, the less moisture reduced when heated.

Moon (2013) showed that the level of cooking loss was higher when refrigeration time was longer. In this study, however, cooking loss of meat refrigerated for 24 h was significantly higher. Jung (2016) suggested that thawing loss depends on the degree of cell damage during freezing and storage. However, Taylor and Dant (1971) reported that, when the meat was frozen rapidly, the thawing loss was reduced. In the case of shear force and WHC, Jung (2016) reported no significant difference according to storage period and thawing method, although WHC tended to decrease when the running water thawing method was used. Similar results were obtained in this study; cooking loss and WHC decreased with storage time and showed no difference based on thawing method.

Table 5 shows the chemical properties of duck meat according to storage period and thawing method. Among these results, moisture, crude protein and crude ash contents were similar to those of fresh meat (Table 1), but crude fat content was lower than that of fresh meat.

Crude protein content showed a significant difference based on storage period time (P<0.05). The protein content of meat stored for 3 mon was higher than that of meat stored for 1, 6, and 12 mon. Moisture, crude fat, and crude ash did not show any difference based on storage time. The chemical composition of duck meat was not affected by thawing method.

There have been few studies on the changes in chemical properties of meat during freezing, storage, and thawing. Lee (1989) reported that water-soluble proteins and amino acids were lost along with juice during thawing. In this study, the difference in protein content, except at 3 mon of storage, is probably due to rapid freezing of meat; however, the cause of protein content increase at 3 mon of storage could not be explained. Love and Pearson (1971) reported that the oxidation of fat during freezing or storage has a direct effect on the discoloration of meat and on the oxidation and reduction of myoglobin.

The fatty acid content of duck meat used in this study is shown in Tables 6 and 7. The content of most of fatty acids in thawed meat was similar to that of fresh meat; however, the content of arachidonic acid (C20:4n6) was higher than that of fresh meat. In contrast to this result, Al-Sabagh et al. (2016) reported that the level of linolenic acid (C18:3) of frozen duck meat decreased when compared to fresh meat. According to Tomás and Anón (1990), the time and rate of lipid oxidation is affected by degree of fatty acid saturation, storage period and storage temperature.

There was a significant difference in the content of γ-linoleic acid (C18:3n6) and eicosenoic acid (C20:1n9) based on storage period. The content of γ-linoleic acid (C18:3n6) and eicosenoic acid (C20:1n9) decreased significantly by 3 mon of storage (P<0.01). Palmitoleic acid (C16:1n7) tended to be lower by 3 mon of storage; however, there was no significant difference based on storage periods. The content of all fatty acids were not different according to thawing methods. The contents of saturated and unsaturated fatty acids were not different according to the storage periods and thawing methods.

Park et al. (1999) reported that the content of oleic acid was related to the taste of meat. Westering and Hedrick (1979) reported a negative correlation between linoleic acid and the flavor of meat. However, the effects of storage period on fatty acids content related to the taste of meat were not observed in this study. Heo et al. (2016) reported that palmitoleic acid content of duck meat was the highest when the meat was defrosted by refrigeration, but no difference was observed in palmitoleic acid content based on thawing method in this study.

Zymon et al. (2007) reported that palmitoleic acid level of veal was significantly reduced during 3 mon of frozen storage. De Pedro and Murillo (2000) also reported that palmitoleic acid content decreased in subcutaneous fat in frozen pork stored for 32 mon. These previous findings are similar to the result of present study. It is assumed that certain changes may occur during first 3 mon frozen storage, while the lipid oxidation and hydrolysis proceed very slowly and to a small extent, presumably as a result of the inactivation of tissue enzymes and microorganisms. In the present study, however, palmitoleic acid level increased after 6 mon and 12 mon of frozen storage. Further investigation is needed to identify the change of fatty acid composition during frozen storage of duck meat.

The amino acid content of duck meat used in this study is shown in Tables 8 and 9. Most of the essential amino acids and aspartic acid (among non-essential amino acids) in thawed meat were similar to those of fresh meat, but histidine and arginine (in essential amino acids) and most of nonessential amino acids were higher than those of fresh meat. These results were in agreement with the study of Heo et al. (2016) who also observed higher non-essential amino acid content in thawed duck meat when compared to fresh meat. However, the correlation between thawing methods, storage time and amino acid content was not explained in the present study.

There was a significant difference in the content of essential amino acids, except methionine, based on storage time (P<0.05). Methionine content showed no significant change based on storage time. Most non-essential amino acids also showed a significant difference between the storage periods (P<0.05, P<0.01); tyrosine content did not change significantly during the storage period. Despite the significant difference of amino acid content among the treatments, no constant tendency of change was detected. There was no significant effect of thawing method on the levels of either essential or non-essential amino acids.