Effect of the Duck Skin on Quality Characteristics of Duck Hams

Fresh duck breast (moisture content: 73.84%, protein content: 19.87%, fat content: 5.23%) and duck skin (moisture content: 32.17%, protein content: 9.21%, fat content: 58.41%) were purchased from a local processor 48 h post-mortem. The duck breast and duck skin were initially ground through an 8-mm plate. Five different duck meat batters were produced, and the experimental design and compositions are given in Table 1. Duck breast and duck skin in this study was used as meat and fat in general meat products. Duck breast and duck skin were added with different ratio to find the proper compounding ratio. Duck breast was homogenized, ground for 30 s in a silent cutter (Cutter Nr-963009, Hermann Scharfen GmbH & Co., Germany), then chilled in iced water (2°C). Sodium chloride (1.5%), sodium nitrite (0.02%), ascorbic acid (0.06%), sugar (1.7%), and isolated soy protein (1.7%) were added to the meat and mixed for 1 min. Duck skin was added after 3 min. The duck ham batters were homogenized for 5 min. A temperature probe (Kane-May, KM 330, UK) was used to monitor the temperature of the emulsion, which was maintained below 10°C throughout batter preparation. After emulsification, the meat batter was stuffed (Stuffer IS-8, Sirman, Italy) into cellulose casings. The duck meat batters were then heated to 75°C for 30 min in a water bath, and then cooled at room temperature (21°C) for 3 h. This procedure was performed in triplicate for each duck ham.

The compositional properties of the duck hams were determined using standard AOAC methods (2000). Moisture content (950.46B) was determined by weight loss after 12 h of drying at 105°C in a drying oven (SW-90D, Sang Woo Scientific Co., Korea). Fat content (960.69) was determined by the Soxhlet method using a solvent extraction system (Soxtec^® Avanti 2050 Auto System, Foss Tecator AB, Sweden) and protein content (981.10) was determined by the Kjeldahl method (Kjeltec^® 2300Analyzer Unit, Foss Tecator AB, Sweden). Ash was determined according to the AOAC method (920.153).

The duck hams were stuffed to the initial weight and after heat processing at 75°C for 30 min, cooked samples were cooled to room temperature (21°C) for 3 h. After cooling, the cooked duck ham samples were weighed and the cooking loss was deduced.

The samples of duck ham batters were analyzed for emulsion stability using the method of Blouka and Honikel (1992) with the following modifications: total expressible fluid and fat separated at the bottom of each graduated glass tube were measured and calculated (Choi et al., 2007).

The pH value of each sample was measured in a homogenate prepared with 5 g of sample and 20 mL distilled water using a Model 340 pH meter (Mettler-Toledo GmbH, Switzerland).

The color of each sample was determined using a colorimeter (Minolta Chroma meter CR-210, Minolta Ltd., Japan; illuminate C, calibrated with a white plate, L*=+97.83, a*=−0.43, b*=+1.98). CIE L*-value (lightness), CIE a*-value (redness), and CIE b*-value (yellowness) values were recorded.

The protein solubility was determined using the method of Joo et al. (1999). Sarcoplasmic protein solubility and total protein solubility were determined, and myofibrillar protein solubility was obtained by determining the difference between the total and sarcoplasmic protein solubilities.

The texture profile analysis (TPA) was performed at room temperature with a texture analyzer (TA-XT2i, Stable Micro Systems Ltd., England). The duck hams were stuffed into cellulose casings followed by heating, and the cooked samples were cooled to room temperature (21°C) for 30 h. Duck ham samples were taken from the central portion of each sample. TPA values (pre-test speed 2.0 mm/s, post-test speed 5.0 mm/s, maximum load 2.0 kg, head speed 2.0 mm/s, distance 8.0 mm, force 5.0 g) were obtained. Hardness (kg), springiness, cohesiveness, gumminess (kg), and chewiness (kg) were determined as described by Bourne (1978).

A trained thirty-member panel from the Food Processing Research Center at Korea Food Research Institute in Korea was used to evaluate the duck hams. Each duck ham was evaluated for color, flavor, tenderness, juiciness, and overall acceptability. The duck hams were heated at 75°C for 30 min, cooled to 21°C for 3 h, cut into quarters, and served to the panelists randomly. Panelists were instructed to cleanse their palates between samples using warm water. The qualities of the cooked samples were evaluated using a 9-point descriptive scale (1 = very undesirable, 9 = very desirable). This analysis was conducted using the Hedonic test described by Choi et al. (2008).

The apparent viscosity of duck meat batter was measured in triplicate with a rotational viscometer (HAKKE Viscotester^® 550, Thermo Electron Corporation, Germany) set at 10 rpm. The standard cylinder sensor (SV-2) was positioned in a 25 mL metal cup filled with duck ham batter and allowed to rotate under a constant shear rate (s⁻¹) for 60 s before each reading was taken. Apparent viscosity values in centipoises were obtained.

All tests were done at least three times for each experimental condition and mean values were reported. Statistical analysis was performed by using Statistical Analysis System (version 8.0, SAS Institute, USA) to calculate the average and standard deviation. When using Duncan’s multiple range test method, the significance test (p<0.05) was carried out through multiple tests.

The proximate compositions of the duck hams formulated with different ratio of duck breast meat and duck skin are shown in Table 2. The moisture content of duck hams decreased with increasing in the ratio of duck breast meat to duck skin (p<0.05), whereas the reverse correlation was observed in the fat content of duck hams (p<0.05), owing to the fat in duck skin. The protein and ash contents showed no significant differences between treatments (p>0.05). Choi et al. (2016) reported that the presence of skin resulted in significant increases in the fat content of meat products, which may be due to the higher fat content of chicken skin. Biswas et al. (2007) reported that a significant increase in fat levels of meat may be due to the addition of increased amounts of chicken skin in chicken sausage formulations. These results were consistent with those obtained by Kim et al. (2016) for the addition of chicken skin to chicken nuggets. These studies also showed that the moisture content of samples decreased and the fat content increased with an increase in chicken skin levels.

The results of cooking loss in the duck hams with different ratio of duck breast meat and duck skin are presented in Table 3. The smallest cooking loss was observed in T1 (12.25, p<0.05), while T5 had the highest cooking loss of 16.38% (p<0.05). Bhat et al. (2013) reported that meatballs containing higher levels of chicken skin tended to show decrease in cooking yield, while a control sample without chicken skin gave the highest cooking yield after heating. Consistent with the results of this study, chicken nuggets with chicken skin also showed higher cooking loss than those without chicken skin (Kim et al., 2016). These results may be attributable to the elution of fat and water from duck skin, and several studies have reported that cooking loss of meat products increased as skin content increased (Biswas et al., 2007; Fotjik and Mandigo, 1998; Kim et al., 2016).

The results for emulsion stability in duck meat batters are presented in Table 3. The highest level of fat separation in duck ham batters was observed in T4 and T5 with a value of 1.98% respectively, while T1 showed the lowest fat separation level (p<0.05; 80%). T4 (9.58%) and T5 (9.82%) also showed significantly higher levels of total fluid separation, whereas T1 (5.52%) and T2 (5.59%) showed lower values (p<0.05). This may be due to the high fat content of duck skin (Kang et al., 2014). Crehan et al. (2000) reported that emulsified sausage generally exhibited higher levels of fat separation and fluid separation as fat content increased, which was similar to our findings.

Table 4 shows the pH and CIE color values of duck hams depending on the ratio of duck breast meat and duck skin. Duck hams showed increasingly high pH values as the ratio of duck breast meat to duck skin decreased (p<0.05). Duck skin with a high fat content has a higher pH than duck breast meat, which could result in the increased pH of duck ham as duck skin content increased (Kang et al., 2014). According to a report by Lorenzo et al. (2011), the pH of dry cured duck sausage increased with the addition of fat, indicating that the high pH values in samples with a higher proportion of duck skin were related to the high fat content in duck skin.

Regarding lightness of duck hams (CIE L*), T5 was significantly lighter than other formulations, and lightness became significantly lower as the ratio of duck breast meat to duck skin increased (p<0.05). Bonifer et al. (1996) found that bologna sausages became lighter as the proportion of chicken skin increased. T1 and T2 exhibited significantly higher values of redness (CIE a*), while formulations with lower ratio of duck breast meat to duck skin showed lower values of redness (p<0.05). These results are consistent with those of previous studies showing that the addition of chicken skin to chicken nuggets reduced redness (Kim et al., 2016). There was no significant difference in yellowness (CIE b*) among the different formulations. Kang et al. (2014) reported that the tendency to show yellowness did not occur in duck meat sausage with or without duck skin.

The solubility of meat proteins is dependent on sarcoplasmic, myofibrillar, and stromal proteins, which can be extracted by changing water, phosphate content, salt concentration, and pH. Myofibrillar proteins are important for improving the texture and emulsifying and water-holding capacity of meat products, and higher protein levels are generally accompanied by improved protein solubility (Choi et al., 2015). The results of protein solubility analysis of duck hams in the study are presented in Table 5. For myofibrillar proteins, T1 and T2 showed the highest values, decreasing with the decreasing ratio of duck breast meat to duck skins resulted in the lowest values in T4 and T5 (p<0.05). Comparison of sarcoplasmic proteins between formulations revealed the highest value in T1 (p<0.05), and the sarcoplasmic protein contents significantly decreased as the ratio of duck breast meat to duck skin decreased (p<0.05). This is probably due to composition dependent effect by different ratio of duck breast meat and skin. Similarly, Chin et al. (1998) reported that protein solubility in low-fat sausages was higher than that in the control, which was attributed to the reduced fat content and increased protein levels.

Table 6 presents the texture profiles of the duck hams analyzed in the study, including hardness, springiness, cohesiveness, gumminess, and chewiness. A high content of duck skin affected hardness and springiness, resulting in the highest values for the two parameters in T1 and the lowest values in T5. There were no significant differences in hardness and springiness among T2, T3, T4, and T5 (p<0.05). Gomez and Lorenzo (2013) also reported that as fat content increased in semi-dried sausages, hardness decreased. Although cohesiveness significantly differed depending on proportion of duck breast meat and duck skin, there was consistent tendency. However, gumminess and chewiness significantly increased as duck breast meat content increased (p<0.05). This is in contrast with the work by Garcia et al. (2002), who reported no consistent differences in gumminess and chewiness of sausages depending on added fat amount. Because the high fat content of duck skin reduced the emulsifying capacity and protein solubility of duck hams, a higher amount of duck skin may decrease the texture profile.

Table 7 shows the sensory characteristics of duck hams manufactured with duck breast meats and duck skins. T1 and T2 showed significantly highest scores for color, whereas T5 had the lowest significant value (p<0.05). Cengiz and Gokoglu (2007) reported that the appearance of the frankfurter-type sausages generally increased in ratings as fat content decreased. There were no significant differences in flavor among the formulations of duck hams, while T1 and T2 exhibited higher tenderness scores than others (p<0.05). Despite significant differences in cooking loss between formulations, no significant difference in juiciness was observed (p>0.05). For overall acceptability, T1, T2, and T3 had significantly the highest scores, while T4 and T5 had the lowest scores. Biswas et al. (2007) reported that the texture and overall palatability of chicken sausages significantly decreased as the fat and skin contents of the sausages increased. According to Bonifer et al. (1996), the addition of skin to bologna-type sausages decreased textural properties, leading to reduced overall acceptability. Therefore, adding either 70% of duck breast meat and 30% of duck skin or 60% of duck breast meat and 40% of duck skin in duck hams may result in production of duck meat products with excellent quality characteristics.

Changes in viscosity of duck meat batter with the ratio of duck breast meat and duck skin at different times are presented in Fig. 1. Viscosity is affected by physical characteristics including water-holding capacity, protein solubility, and interactions between fats and proteins (Hamm, 1975). Initially, viscosity is high because of the high resistance caused by irregular arrays, and decreases as the molecular array becomes regular over time (Kim et al., 2009). T1 and T2 had the highest viscosity of duck hams, whereas T5 with the highest duck skin content showed the lowest value (p<0.05). Similarly, Hefnawy and Ramadan (2011) reported that emulsions with higher emulsion stabilities showed higher viscosity values, and Grigelmo-Miguel et al. (1999) also reported that emulsions with higher meat contents showed higher viscosity than those with higher fat content. Formulations with higher duck breast meat content had higher protein contents, which may have enhanced emulsifying capacity and binding capacity between emulsions, leading to high viscosity values.

Fig. 1. Changes in apparent viscosity of duck ham formulations with the different ratio of duck breast meat and duck skin.

Download Original Figure