Jerky, consumed as a traditional and popular dried meat product, is prepared as whole-muscle jerky and restructured jerky . In particular, restructured jerky is capable of mass production of standardized products during the manufacturing process . Restructured jerky is economic and environmentally friendly, as it may be prepared using the unused parts of pork or those discarded during production . However, restructured jerky has a tough texture from the drying process . Studies conducted to address this problem have employed modified drying conditions (semi-dried condition) or added ingredients that enhance water holding capacity [4–7].
In restructured jerky, hydrocolloids are useful functional ingredients that improve tenderness . One of the most common hydrocolloid gums in the meat industry is carrageenan . Carrageenan, a mucopolysaccharide obtained from red algae, is composed of D-galactose and 3,6-anhydro-D-galactose linked with a β-1-4 linkage or an α-1-3 linkage . In the preparation of meat products, carrageenan dissolved during thermal processing forms a gel upon cooling . It enhances water retention, texture, and consistency of meat products when used in combination with gelatin and other ingredients [11,12]. Pietrasik and Li-Chan  indicated that carrageenan in meat batter improved the water holding capacity and the cooking loss of meat gels.
In food industries, Gelatin known as hydrocolloid has been used widely due to its unique functional properties . Its gel formation ability is one of the most useful characteristics when preparing restructured meat products . Gelatin is derived from the partial hydrolysis of collagen from cowhide or pigskin . However, new alternative sources of gelatin are in demand owing to the outbreak of foot-and-mouth disease and bovine spongiform encephalopathy . Poultry by-products, especially skin and feet, have abundant gelatin and may serve as a replacement for the existing gelatin sources . Gelatin from chicken or duck feet has been studied and the physicochemical properties of restructured jerky containing that gelatin was improved [14,17]. However, little data is available on the effect of duck skin gelatin and its combination with carrageenan on restructured jerky.
In this direction, here we conduct this study to increase the value of discarded duck skin by extracting gelatin and determining its effects with or without carrageenan on the quality of semi-dried restructured jerky.
MATERIALS AND METHODS
Duck skin gelatin was prepared according to the method of Kim et al.  with some modifications. Duck skin (Pekin duck, Cherry Valley strain; purchased from Farm duck, Jeongeup, Korea) was soaked for five times in a solution that was adjusted pH 1 to facilitate swelling. After incubation for 24 h at ambient temperature (20 ± 2°C), the swollen skin was washed in flowing tap water for 48 h at ambient temperature. Then, 1 kg of swollen skin was heated using a super-heated steamer (QF-5200C, Naomoto, Osaka, Japan; oven temperature: 150°C, steam temperature: 150°C). When duck skin was melted, it was filtered at a holding time of 12 h at 2°C to separate fat and non-fat ingredients. Separated non-fat ingredients, called crude gelatin, were adjusted at pH 7 with 0.1 N NaOH and dehydrated using a spray dryer (B-290, Buchi, New Castle, DE, USA). Drying conditions were as follows: inlet temperature of 120°C, aspirator 100%, pump 15%. Duck skin gelatin had 0.47% moisture, 91.28% protein, 1.94% fat, and 6.32% ash. The color of duck skin gelatin was CIE L* 86.63, CIE a* −0.27, and CIE b* 10.71.
Duck skin gelatin powder and phosphate were mixed in distilled water (DW) at 60°C to obtain 0.5% and 1.0% gelatin solutions (w/w), which were cooled down until the central temperature reached 4°C.
Fresh pork ham was obtained from a local market (Jeonju, Korea) and ground (Φ-8 mm). Table 1 showed the experimental design and formulation of the semi-dried restructured jerky. Each batch of samples comprised restructured jerkies with different levels of duck skin gelatin (0%, 0.5%, and 1.0%) with or without carrageenan (0% and 0.3%). The ground lean meat and gelatin solution were homogenized in a Nr-963009 silent cutter (Hermann Scharfen GmbH & Co., Witten, Germany) for 1 min. Other ingredients such as sugar, salt, ascorbic acid, and carrageenan were added to the silent cutter and those were mixed for 2 min. The homogenized meat jerky batter was stuffed into Φ-20 mm size of cellulose casing (Viskase Sales, Chicago, IL, USA). Each preparation was cut into 20 cm long pieces. Then, the sample was dried at 55°C for 90 min, followed by the removal of casing. Drying condition was at 55°C for 30 min, 65°C for 180 min, and 80°C for 60 min (MAXi3501 chamber, Kerres, Postfach, Germany). This procedure was conducted in triplicates for each sample jerky.
The moisture content (Drying oven method), protein content (Kjeldahl method), fat content (Soxhlet method), and ash content (Muffle furnace method) were measured using the standard method of AOAC .
Restructured jerky for water activity analysis was ground, and their water activity was measured in duplicates using a water activity meter (Novasina, Labmaster-aw, Lachen, Switzerland).
The processing yield was determined by Triyannanto and Lee  method. The change in the weight before and after the drying procedure was used to calculate the processing yield.
The semi-dried restructured jerky (5 g) was homogenized (8,000 rpm) with DW (20 mL). The pH of each homogenate was measured using a Mettler-Toledo GmbH pH meter (Schwerzenbach, Switzerland).
The CIE L* value, CIE a* value, and CIE b* values of restructured jerky were determined using a CR-410 colorimeter (Minolta, Tokyo, Japan) calibrated using a white plate (Illuminate C observer 2°).
Shear force value (test speed: 2 mm/s) was determined as described by De Huidobro et al.  using a TA-XT plus texture analyzer (Stable Micro Systems, Surrey, UK). The samples were cut into 2 cm (diameter) × 3 cm (height) pieces.
The rehydration experiments were carried out according to the method of Kim et al.  with suitable modifications. A 100 mL beaker was filled with semi-dried restructured jerky samples and DW. The weights of soaked samples were measured after 15, 30, 45, and 60 min.
Small pieces of semi-dried restructured jerky were used for SEM analysis. Each sample was fixed with 2 mL Karnovsky’s fixative at 4°C for overnight and washed thrice with 0.05 M sodium cacodylate buffer at room temperature for 10 min each. Samples were fixed with osmium tetroxide (2%) in 0.1 M sodium cacodylate buffer for 2 h at 4°C and washed with DW twice. Then, samples were dehydrated using gradually increasing ethyl alcohol concentrations for 10 min (30%, 50%, 70%, 80%, 90%, and 99.8%). Each samples was covered with aluminum stubs and coated with a layer of platinum under vacuum (E-1010, HITACHI, Tokyo, Japan). Micrographs of the semi-dried restructured jerky were obtained under a SEM (S-2380N, HITACHI, Tokyo, Japan).
A sensory panel comprising 24 members from Korea Food Research Institute (KFRI) evaluated the sensory properties of restructured jerky samples. Panelists tasted the samples and cleansed their mouths with warm water. The following sensory items were evaluated using a 9-point descriptive scale (9, extremely desirable; 1, extremely undesirable): appearance, flavor, texture, and overall acceptability.
All experimental data were analyzed using SPSS statistical software program (SPSS Ver. 20.0, IBM, Chicago, IL, USA). One-way analysis of variance was performed using the general linear model procedure (GLM) to investigate the addition effect of duck skin gelatin and carrageenan. Significance of differences among mean values was determined by Duncan’s multiple range tests with the confidence level of p < 0.05. Then, mean values and standard deviations were presented.
RESULTS AND DISCUSSION
In Table 2, the proximate composition of semi-dried restructured jerky with duck skin gelatin and carrageenan is shown. Both duck skin gelatin and carrageenan had significant effect on moisture content of semi-dried restructured jerky (p < 0.001) (Table 5). The moisture content is the one of the factors which determine intermediate moisture food and the range of moisture content from 20% to 50% is known as a standard value of intermediate moisture food (IMF) [23–25]. The semi-dried restructured jerkies produced in this study had a moisture content of 40% to 44%. Concerning the amount of gelatin added, G1 group had the highest (p < 0.05) moisture content, owing to the high water absorption property of gelatin . Carrageenan is reported to exhibit a high water retention capability . The jerkies containing carrageenan showed a higher moisture content compared to those without carrageenan (p < 0.05). The protein content of the G1 group was significantly (p < 0.05) lower than that of the control group (G0), even gelatin known a protein (Table 2). Table 5 also shows that duck skin gelatin has significant effect on protein content (p < 0.001). The decrease in the protein content following treatment may be associated with the fat content of duck skin gelatin. G0.5 and G1 groups showed high-fat content, while G1 groups showed high ash content compared to the control group (G0). The duck generally has abundant fat and ash contents in skin and these contents might be contained in the crude extracted gelatin even removed a separated fat . Table 2 presents the difference in the water activity of semi-dried restructured jerky with duck skin gelatin and carrageenan. Water activity is related to the thermodynamic equilibrium state of jerky . Jerky should have stable water activity value for avoiding its quality changes during storage . In case of semi-dried jerky, the water activity was within the range of 0.88–0.91 , and the jerkies from G0C, G0.5, and G0.5C groups satisfied this standard. The water activity of jerkies from G1 and G1C groups was the highest, while that of the jerkies from G0.5 group was the lowest (p < 0.05). According to Table 5, water activity of semi-dried restructured jerky was significantly affected by the addition of duck skin gelatin (p < 0.05), while not the addition of carrageenan. Similar observations were reported by Kim et al.  who found that the water activity of duck jerky decreased when the amount of konjac was increased while that of collagen decreased.
Table 3 presents the pH and color of semi-dried restructured jerky prepared from duck skin gelatin and carrageenan. The pH values of semi-dried restructured jerky significantly (p < 0.05) increased with an increase in duck skin gelatin concentration and the addition of carrageenan. Both according to Kim et al. , the process of pH adjustment during gelatin neutralization affects the product. In the present study, the addition of duck skin gelatin neutralized to pH 7 affected the pH of the semi-dried restructured jerky. When we measured the pH of 0.3% carrageenan in DW, the value was 6.98 (data not shown). That pH might be affected by the pH of the semi-dried restructured jerky. The lightness and yellowness values of semi-dried restructured jerky significantly increased after the addition of duck skin gelatin and carrageenan, while the value of redness significantly decreased (Tables 3 and 5). Considering the amount of gelatin added, these results could be explained by color of duck skin gelatin. These observations are in line with those previously reported , wherein surimi gel prepared with duck foot collagen showed a significant increase in all color parameters as compared to those prepared using dark flesh or white flesh fish. Similar results were also reported by Demirci et al.  who found that the lightness and yellowness of meatball increased with an increase in carrageenan levels.
Fig. 1 shows the processing yield of semi-dried restructured jerky with duck skin gelatin and carrageenan. No significant (p > 0.05) difference was observed in the processing yield according to the addition of duck skin gelatin. Schilling et al.  reported that the cooking loss for boneless cured ham manufactured with pork collagen was not different from that reported for the control ham. Prabhu et al.  noted no difference in cooking yield following the addition of 0.5% pork collagen. The G1C, G0.5C, and G0C respectively was higher (p < 0.05) processing yield compared to the same amount of gelatin. Table 5 also shows that the addition of carrageenan significantly affects the processing yield. These results are consistent with those of Trius et al.  that studied the interaction between carrageenan and meat proteins used in meat products. These authors reported that carrageenan helps in water retention of meat products by holding water in the interstitial spaces of protein gel. A similar trend was reported in the study by Candogan and Kolsarici , wherein low-fat beef frankfurters were made with pectin and carrageenan. These authors showed an increase in processing yield with the addition of carrageenan, owing to the improvement in the hydration and binding abilities of meat products.
Fig. 2 presents the effect of duck skin gelatin and carrageenan on the shear force values of semi-dried restructured jerky. Texture has an important role in the organoleptic property of jerky and affects the consumer preference . Shear force is defined as the force that transforms the food shape  and correlates with muscle fiber, processing yield, and moisture content [29,39]. Duck skin gelatin and carrageenan had significant effects on shear force of semi-dried restructured jerky, respectively (p < 0.001, Table 5). In Fig. 2, the shear force values of semi-dried restructured jerky with duck skin gelatin and carrageenan was lower (p < 0.05) than that of the control (G0), and the lowest (p < 0.05) shear force was observed for the semi-dried restructured jerkies from G0.5C and G1C groups (Fig. 2). Kim et al.  found that the increase in the addition of chicken feet gelatin to semi-dried chicken jerky resulted in a decrease in shear force. Similar results were also reported by Kim et al.  who showed that the shear force values of duck jerky made with the collagen and konjac were lower than that of duck jerky made with only collagen. These authors convinced that the reduced shear force values owing to both collagen and konjac addition positively improved the tenderness of jerky. Thus, the addition of duck skin gelatin and carrageenan to semi-dried restructured jerky improved its processing yield and tenderness.
The effect of duck skin gelatin and carrageenan on the rehydration capacity of semi-dried restructured jerky is presented in Fig. 3. Rehydration capacity indicates hysteresis during rehydration owing to cellular and structural disturbances while drying . Therefore, it is one of the most important factors affect the sensory properties such as tenderness during mastication . G1C, G0.5C, and G0.5 groups showed higher rehydration capacity than the control (G0) group (p < 0.05). Some researchers have reported no effect of 1% gelatin on the rehydration capacity of jerky [6,29]. Our results are in agreement with those of Kim et al.  who observed that the rehydration capacity of duck jerky made with a composite of konjac and collagen (40/60 and 60/40) was higher compared to that of the jerky treated with either konjac or collagen.
SEM images of semi-dried restructured jerky prepared from duck skin gelatin and carrageenan are shown in Fig. 4. When gelatin was added in the jerky, a spherical gel-type structure was identified and which was not shown in G0. Additionally, the size of the gel-type structure increases when carrageenan was added with gelatin. The gel-type structure of G1C showed the largest size and the smallest number. Andrès et al.  reported gel-type structures in the micrograph of chicken sausage treated with whey protein and guar gum. These authors suggested that gel-type structures improved the texture properties such as cohesiveness. Similar results were reported by Eyiler Yilmaz et al.  who found gel structures in the micrograph of low-fat frankfurter containing kappa-carrageenan. The authors suggested that these structures were related to hardness because low-fat frankfurter treated with kappa-carrageenan had lower hardness than those from the control group.
The effect of duck skin gelatin and carrageenan of on the sensory properties of semi-dried restructured jerky is presented in Table 4. In sensory values, the appearance score of semi-dried restructured jerky was unaffected by the addition of duck skin gelatin. Meanwhile, the addition of gelatin affects the flavor, texture, and overall acceptability (p < 0.001), and the addition of carrageenan affects appearance, flavor, and texture (p < 0.05, Table 5). The flavor, texture, and overall acceptability scores were the highest (p < 0.05) for the G1C group (Table 4). Several researchers reported that the texture property of jerky is the most important sensory attribute [2, 8, 17, 29]. Kim et al.  found that semi-dried jerky showed an increase in tenderness score with an increase in the addition of chicken feet gelatin. These results are agreed with those of the study by Demirci et al. , that the hardness of meatballs treated with 0.5% carrageenan was lower than that of control meatballs. Therefore, the semi-dried restructured jerky prepared with duck skin gelatin and carrageenan in our study showed improved sensory properties.
1) Appearance, flavor, overall acceptability (1, extremely undesirable; 9, extremely desirable), texture (1, extremely tough; 9, extremely tender) were expressed for evaluating sensory attributes.
We evaluated the effect of duck skin gelatin and carrageenan on the quality of semi-dried restructured jerky. As a result, the processing yield and rehydration capacity of jerkies from G0.5C and G1C groups were higher than those reported for jerkies from other treatment groups. The shear force was the lowest for G0.5C and G1C groups, while overall acceptability scores of G1C were the highest. The results of this study demonstrated that the addition of 1.0% duck skin gelatin and 0.3% carrageenan to restructured jerky formulations may result in optimized quality characteristics.