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Freeze-thaw stability of duck surimi-like materials with different cryoprotectants added
Freeze-thaw stability of duck surimi-like materials with different cryoprotectants added K. Ramadhan,* N. Huda,*1 and R. Ahmad† *Fish and Meat Processing Laboratory, Food Technology Programme, School of Industrial Technology, Universiti Sains Malaysia, Penang 11800, Malaysia; and †Advanced Medical and Dental Institute, Universiti Sains Malaysia, Penang 11800, Malaysia ABSTRACT In this study, the effect of the addition of different cryoprotectants on the freeze-thaw stability of duck surimi-like material (DSLM) was tested. A 6% (wt/wt) low-sweetness cryoprotectant (i.e., polydextrose, trehalose, lactitol, or palatinit) was added to a 3-kg portion of DSLM, and the mixture was subjected to freeze-thaw cycles during 4 mo of frozen storage. The DSLM with no cryoprotectant added (control) and with a 6% sucrose-sorbitol blend (high-sweetness cryoprotectant) added also were tested. The polydextrose-added sample had the highest water-holding capacity among the sample types tested (P < 0.05), and it retained its higher value during frozen storage. The protein solubility of the cryoprotectant-added samples decreased significantly (P < 0.05) from 58.99 to 59.60% at initial frozen storage (0 mo) to 48.60 to 54.61% at the end of the experiment (4 mo). The gel breaking force of all
samples significantly decreased (P < 0.05) at 1 mo; this breaking force then stabilized after further frozen storage for the cryoprotectant-added samples, whereas it continued to decrease in the control samples. Gel deformation fluctuated during frozen storage and was significantly lower (P < 0.05) at the end of experiment than at the beginning. The presence of cryoprotectants reduced the whiteness of DSLM. Samples containing polydextrose, trehalose, lactitol, and palatinit were able to retain the protein solubility, gel breaking force, and deformation of DSLM better than control samples after 4 mo of frozen storage and exposure to freeze-thaw cycles. The effects of these low-sweetness cryoprotectants are comparable to those of sucrose-sorbitol, thus, these sugars could be used as alternatives in protecting surimi-like materials during frozen storage.
Key words: duck surimi-like material, low-sweetness cryoprotectant, freeze-thaw cycle, frozen storage 2012 Poultry Science 91:1703–1708 http://dx.doi.org/10.3382/ps.2011-01926
INTRODUCTION Duck meat is the third most widely produced poultry meat in the world after chicken and turkey. Worldwide duck meat production has continuously increased, and it reached more than 3.8 million tons in 2009 (Food and Agriculture Organization of the United Nations, 2010). However, utilization of this protein source is limited compared with chicken meat due to factors such as low cooking yield and emulsion stability, high cooking loss, and darker color (Biswas et al., 2006; Ali et al., 2007). Use of surimi-like material produced from under-used sources of meat has become popular due to development of processing methods that improve its quality attributes (Desmond and Kenny, 1998; Nowsad et al., 2000; Ensoy et al., 2004; Jin et al., 2010). For example, several studies reported that duck surimi-like materi©2012 Poultry Science Association Inc. Received October 8, 2011. Accepted March 10, 2012. 1 Corresponding author: [email protected]
al (DSLM) has significantly improved water-holding capacity (WHC), color, and textural properties compared with untreated duck meat (Nurkhoeriyati et al., 2011; Ramadhan et al., 2011). Prolonged frozen storage may damage the quality of meat proteins due to ice crystal growth, which can affect osmotic removal of water and cause denaturation of proteins (Thyholt and Isaksson, 1997). Moreover, the temperature fluctuations that can occur during transportation, retail display, and consumption of meat also can adversely affect the product (Srinivasan et al., 1997). Freeze-thaw cycles are destabilizing forces for muscle proteins because they cause proteins to unfold and the meat quality to deteriorate (Benjakul and Bauer, 2000). Cryoprotectants are used to protect the myofibrillar proteins of surimi during the freezing process and frozen storage (Carvajal et al., 2005). The most commonly used cryoprotectant added to surimi is sucrose-sorbitol at a 1:1 ratio and concentration of 8% (wt/wt). This compound, however, imparts a sweet taste that is undesirable to some consumers (Carvajal et al., 1999). In an
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effort to replace sucrose, many researchers have studied the use of various types of low-sweetness sugars as cryoprotectants; for example, polydextrose, trehalose, palatinit, and lactitol (Sych et al., 1990; Sultanbawa and Li-Chan, 1998; Pan et al., 2010). Polydextrose is a low-molecular weight randomly bonded polysaccharide of glucose. This low-calorie bulking agent frequently is used to replace sugar in reduced-calorie foods and contains zero relative sweetness compared with sucrose (Craig et al., 1996; O’Donnell, 2005; Auerbach et al., 2006). Trehalose, which has 0.45 relative sweetness compared with sucrose, is a disaccharide consisting of 2 glucose moieties linked through their respective anomeric carbon atoms (1,1) by an α-glycosidic bond (O’Donnell, 2005; Lindley, 2006). Palatinit is a type of sugar alcohol with a relative sweetness value of 0.45 to 0.6 compared with sucrose (Wijers and Sträter, 2001). Lactitol is a sugar alcohol (polyol), and its relative sweetness value is 0.40 compared with sucrose (Young, 2006). Five freeze-thaw cycles often are used to determine the freeze-thaw stability of muscle proteins (Srinivasan et al., 1997; Benjakul and Bauer, 2000; Xia et al., 2009; Liu et al., 2011). Although many studies have described the activity of cryoprotectants during frozen storage of surimi, studies of the cryoprotective effects of lowsweetness sugars during frozen storage and freeze-thaw cycles are lacking. Thus, the goal of this study was to evaluate the effects of different cryoprotectants on the stability of DSLM during both frozen storage and during freeze-thaw cycles.
MATERIALS AND METHODS Preparation of DSLM Carcasses of 8-wk-old Pekin duck broilers were mechanically deboned at a commercial processing plant (Fika Food Corporation Sdn. Bhd., Penang, Malaysia) using a deboning machine with a pore size of 0.9 mm (Meat Maker Deboner, Prince Industries Inc., Murrayville, GA). The mechanically deboned duck meat was stored at –20°C in frozen meat block form before production of DSLM. To produce DSLM, meat blocks were cut into smaller sizes using a meat bone saw (model P79-SS, Powerline Equipment, Norwalk, CT) and ground using a meat grinder (model EVE/ALL-12, Rheninghaus Srl, Torino, Italy). The DSLM preparation followed procedures described by Ensoy et al. (2004) with modification of the mixing time and dewatering technique. Briefly, 2 kg of meat were mixed with 6 L of cold tap water (<5°C) for 5 min using a universal mixer (GB-30 Orimas, Yungkang, Taiwan). The mixture was centrifuged at 4,043 × g for 15 min at <4°C using a Union 5KR centrifuge (Hanil Science Industrial, Co., Ltd., Incheon, Korea). The upper layer of fat and aqueous supernatant was discarded from the meat concentrate. The washing process was repeated 3 times.
The cryoprotectant treatments were created as follows. Using a silent cutter (Hobart Corp., Troy, OH), one of the following was added to 3 kg of DSLM: 6% (wt/wt) sucrose-sorbitol blend (1:1), 6% (wt/wt) polydextrose, 6% (wt/wt) trehalose, 6% (wt/wt) lactitol, or 6% (wt/wt) palatinit. The DSLM without cryoprotectant added was used as the control. The treated DSLM samples and control each were placed in a plastic box (20 × 15 × 10 cm) and frozen in an air-blast freezer (Irinox S.p.A, Corbanese, Italy) at –27°C for 3 h and then kept at –20°C before analysis.
Freeze-Thaw Stability During Frozen Storage Freeze-thaw stability of the samples was assessed following procedures described by Nowsad et al. (2000) with modification of the number of cycles. The frozen DSLM samples were kept at –20°C and subjected to 5 freeze-thaw cycles. A freeze-thaw cycle was conducted once every month during the 4-mo storage duration. At 0 mo, the frozen samples were thawed inside the chiller at 5 to 7°C for 15 h, and subsamples were taken from thawed samples. The WHC and protein solubility of each subsample was measured, and a 300-g portion of each subsample was processed into a cooked gel for the analysis of gel breaking force, deformation, and degree of whiteness. The remainder of each thawed sample was refrozen at –20°C. The same freeze-thaw process was conducted after 1, 2, 3, and 4 mo of storage.
WHC The WHC was analyzed according to procedures described by Babji and Gna (1994) with modification of the calculation method. About 20 g of DSLM were mixed with 40 mL of distilled water using a homogenizer (T25 digital Ultra-Turrax, Staufen, Germany). Approximately 10 g of homogenate were weighed into a centrifuge tube and centrifuged at 1,229.8 × g for 5 min. The supernatant was discarded and the precipitate was weighed. The WHC was calculated using the following formula and expressed as grams of water per grams of protein of the meat:
WHC =
weightprecipitate − weightdry matters of meat weighthomogenate ×%Protein meat
.
Protein Solubility Protein solubility was determined following procedures described by Venugopal et al. (1996). One gram of DSLM was added to 40 mL of 3% NaCl solution in a 50-mL centrifuge tube. A Stuart SA7 vortex mixer (Bibby Scientific Limited, Staffordshire, UK) was used to homogenize the sample for 2 min. Aliquots were centrifuged at 5,918.9 × g for 5 min using a Heraeus Multifuge X1R centrifuge (Thermo Electron LED GmbH,
FREEZE-THAW STABILITY OF DUCK SURIMI-LIKE MATERIAL
Langenselbold, Germany). The supernatant was collected for protein analysis using the Kjeldahl method. Protein solubility was calculated on the basis of whole protein content of the meat and expressed as a percentage: Protein solubility (%) =
%Protein supernatant %Protein meat
× 100% %.
tested (control and DSLM with 5 different cryoprotectants added), and there were 2 replicates for each treatment. Each parameter was measured in triplicate for each treatment. The data obtained were subjected to ANOVA (ANOVA), after which Duncan’s multiplerange test was used to determine the significance of differences among the treatments. SPSS software (SPSS Statistics 17.0 for Windows, SPSS Inc., Chicago, IL; SPSS, 2008) was used to conduct the statistical tests.
Preparation of Cooked Gels Cooked gels were prepared for analysis of gel breaking force, deformation, and degree of whiteness. First, 300 g of thawed DSLM were mixed with 3% (wt/wt) salt using a cutter mixer (Blixer, Robot Coupe USA, Inc., Jackson, MS) for 1 min until the mixture formed a meat batter. The batter was stuffed into plastic casing cylinders (diameter 2 cm, height 15 cm) using a manual sausage stuffer (Amrapali Ltd., Bihar, India). Stuffed batters were incubated in a water bath (WiseBath, Witeg Labortechnik GmbH, Wertheim, Germany) at 36°C for 30 min followed by 90°C for 10 min until the gel had formed. The samples then were immersed in ice flakes until they reached a temperature of ~20°C.
Breaking Force and Deformation Gel breaking force (g) and deformation (mm) were measured using a TA-XT plus texture analyzer (Stable Micro Systems Ltd., Surrey, UK) following procedures described by Balange and Benjakul (2009). Cooked gels were cut into 3-cm thick and 2-cm diameter pieces. A piece of the gel was placed perpendicularly on the platform surface, where it was penetrated by a spherical probe (type P/0.25) at a constant 1 mm/s until 11 mm depth was reached. The trigger force used was 5 g, with 1 mm/s of pretest speed and 10 mm/s of posttest speed. The load cell capacity of the texture analyzer was 5 kg, and the return distance was 35 mm. Breaking force was obtained from the highest peak of the graph (y-axis), and deformation was calculated as the distance between the penetration starting point and the highest peak point (x-axis).
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RESULTS AND DISCUSSION WHC Figure 1 shows the WHC of the different DSLM samples during 4 mo of frozen storage and exposure to freeze-thaw cycles. At 0 mo, the addition of polydextrose resulted in a significantly higher WHC (P < 0.05) compared with all other treatments. Except for the DSLM treated with polydextrose, the WHC of the treated samples and the control did not differ significantly (P > 0.05). Polydextrose is highly soluble in water and has high viscosity and hygroscopicity, thus it is able to hold more water (Craig et al., 1996). The WHC of all samples decreased gradually in conjunction with longer storage time. The DSLM with polydextrose added exhibited the lowest difference in WHC between the initial (0 mo) and final (4 mo) measurements. Kovačević et al. (2011) previously reported that polydextrose interacts with proteins of chicken surimi-like material in accordance with the cryoprotecting mechanism; hence it retained WHC at a higher level compared with the control sample. The WHC of the control sample decreased significantly at 1 mo, and it continued to decline until the end of the experiment. The control sample exhibited the greatest WHC reduction among all samples tested (P < 0.05). This is because protein denaturation occurs when cryoprotecant
Degree of Whiteness Color properties of the cooked gels were measured using a Minolta CM-3500d spectrophotometer (Konica Minolta Sensing Americas Inc., Ramsey, NJ) to obtain the L* (lightness), a* (redness), and b* (yellowness) values. Furthermore, whiteness was calculated using the following formula (Park and Lin, 2005): 100 – [(100 – L*)2 + a*2 + b*2]1/2.
Statistical Analysis This experiment was set up using a completely randomized design. Six different DSLM treatments were
Figure 1. Water-holding capacity of duck surimi-like material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
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is not present. Protein denaturation involves conformational changes of myosin followed by exposure of hydrophobic groups of amino acids and protein aggregation. This results in poor WHC and an inferior gel network (Benjakul et al., 2003; Zhou et al., 2006). Samples containing the sucrose-sorbitol blend, trehalose, and palatinit maintained a relatively constant WHC with no significant changes from 0 to 2 mo of storage (P > 0.05), but the WHC then decreased significantly at 3 and 4 mo of storage (P > 0.05). The lactitoladded sample maintained the WHC with no significant declines until 3 mo of storage (P > 0.05). Sych et al. (1990, 1991) reported that the cryoprotective activities of lactitol and palatinit are mainly due to the presence of numerous hydroxyl groups that interact with protein chains, leading to protein hydration and decreased protein-protein interaction. Thus, polydextrose, lactitol, trehalose, and palatinit showed cryoprotective activities comparable to those of the sucrose-sorbitol blend. Zhou et al. (2006) reported that trehalose prevented protein denaturation of tilapia surimi until 6 mo of frozen storage. The duration of cryoprotective activity of the sugars tested in the current study was shorter, possibly because exposure to freeze-thaw cycles might have caused more rapid protein denaturation.
Protein Solubility A decline in protein solubility is a main indicator that freeze-induced protein denaturation has occurred. This denaturation takes place due to the formation of hydrogen or hydrophobic bonds as well as disulfide bonds and ionic interactions during frozen storage (Zhou et al., 2006). Insolubilization of myofibrillar protein also may be related to conformational changes in myosin (Tironi et al., 2007). Protein solubility of all samples decreased significantly (P < 0.05) after 4 mo of storage and exposure to freeze-thaw cycles (Figure 2). At initial storage, the protein solubility ranged from 58.99 to 59.60%. The cryoprotectant-added samples exhibited decreased protein solubility at 1 mo, but it tended to remain stable from 2 mo onwards. At 4 mo, the cryoprotectant-added samples had protein solubility ranging from 48.60 to 54.61%. Protein solubility of samples treated with sucrose-sorbitol, polydextrose, trehalose, lactitol, and palatinit was similar (P > 0.05) and remained higher (P < 0.05) than the control throughout the 4 mo of the experiment. This result emphasizes the effective cryoprotective activity of the low-sweetness cryoprotectants used in this study. Similar results were reported by Sych et al. (1990), who found that the addition of 8% (wt/wt) polydextrose, lactitol, or palatinit to cod surimi was comparable to the addition of sucrose-sorbitol in providing cryoprotection for myofibrillar proteins after 3 mo of storage. Sych et al. (1991) also noted that the addition of 5.7 to 6.4% (wt/wt) lactitol provided a cryoprotective effect in cod surimi after 2 mo of storage. Pan et al.
(2010) reported that the addition of 6% (wt/w) trehalose to grass carp surimi was comparable to adding 8% sucrose-sorbitol to provide cryoprotection of myofibrillar proteins after 6 mo of frozen storage.
Gel Breaking Force and Deformation Changes in gel breaking force that occurred during the freeze-thaw cycles and frozen storage are shown in Figure 3. There was no significant difference (P > 0.05) in breaking force among sample types at 0 mo (range, 830.05–911.90 g). At 1 mo of storage, the gel breaking force values of all samples decreased significantly (P < 0.05; ~34.86%, range, 574.51–614.82 g), except for the sucrose-sorbitol-added sample. It decreased by only 19.6% and thus was significantly higher than the gel breaking force of the other samples (P < 0.05). Nowsad et al. (2000) also detected a drastic decline in gel breaking force in chicken surimi-like material with cryoprotectant added after 1 mo of frozen storage. In chicken myofibrillar protein isolate without cryoprotectant added, gel hardness declined to 48% after 4 wk of frozen storage (Uijttenboogaart et al., 1993). Gel breaking force continued to decrease at 2 mo, and the range of values of the sample types was narrower at this time point (535.26–587.04 g). At 3 and 4 mo of storage, no significant changes were detected in the cryoprotectant-added samples, whereas the gel breaking force in the control sample had decreased by 53.63% at 4 mo compared with 0 mo of storage. The deterioration intensity in the current study was lower than that reported by Sultanbawa and Li-Chan (1998), who found that ling cod surimi without added cryoprotectant could not be formed into a gel after 4 mo of frozen storage. Decreased gel-forming ability is caused by protein aggregation via disulfide bonds or hydrophobic interactions (Zhou et al., 2006). The result of the current study indicated that the presence of cryoprotectants inhibited further deterioration and protected myofibrillar proteins of DSLM during frozen storage.
Figure 2. Protein solubility of duck surimi-like material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
FREEZE-THAW STABILITY OF DUCK SURIMI-LIKE MATERIAL
Figure 3. Gel breaking force of cooked gels made from duck surimi-like material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
Figure 4 shows the changes in gel deformation that occurred during the freeze-thaw cycles and frozen storage. At the initial time of storage (0 mo), gel deformation values were 10.43 to 10.96 mm, with no significant differences among sample types (P > 0.05). During frozen storage, however, gel deformation fluctuated irregularly, and then finally decreased at 4 mo. Nowsad et al. (2000) also reported fluctuations in gel deformation of spent hen surimi-like material. In the current study, the control sample exhibited the lowest decline in gel deformation; this indicates that cryoprotective activities were present in the samples treated with cryoprotectants.
Whiteness Figure 5 shows the changes in whiteness that occurred during the freeze-thaw cycles and frozen storage. The control sample had the highest whiteness values, whereas the samples treated with cryoprotectants had lower values. The whiteness range (63.13–65.96) observed in this study is lower than that reported in a study of spent hen surimi with cryoprotectants added (64.07–67.11; Jin et al., 2010). The lower whiteness values found in samples treated with cryoprotectants indicate that the addition of cryoprotectants reduced the whiteness of DSLM. This difference was caused by the Maillard browning that occurred in samples treated with cryoprotectants due to the presence of sugar, which reacted with proteins and resulted in browning and a decrease in whiteness (Auh et al., 1999). Whiteness values of all samples did not significantly change (P > 0.05) from 0 to 3 mo, but then they decreased significantly (P < 0.05) at 4 mo. Samples containing polydextrose, trehalose, lactitol, and palatinit had the lowest whiteness values at the last measurement time, and these values were not significantly different from each other. Overall, the decrease in the whiteness value for a given sample type was not very large between the initial and the final measurement
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Figure 4. Gel deformation of cooked gels made from duck surimilike material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
times (i.e., from 63.13 to 65.96 to 59.37–62.36). The decrease in whiteness of cooked gels is influenced by pigment protein, especially pigment in muscle protein that becomes oxidized during the cooking process. Additionally, lipid oxidation in muscle during frozen storage induces cross-linking of pigment and muscle proteins through the free-radical process, which in turn results in decreased whiteness of cooked gels (Benjakul et al., 2005).
Conclusions In conclusion, when added to DSLM, polydextrose, trehalose, lactitol, and palatinit were able to maintain stable protein solubility, gel breaking force, and deformation of the material better than control samples after 4 mo of storage and exposure to freeze-thaw cycles. The values of these parameters were comparable to those of DSLM with sucrose-sorbitol added. Polydextrose-
Figure 5. Whiteness of cooked gels made from duck surimi-like material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
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added material had the highest WHC, and addition of cryoprotectants reduced the whiteness of DSLM. The results of this study indicate that these low-sweetness cryoprotectants could be used as alternatives to sucrose-sorbitol in protecting surimi-like materials during frozen storage.
ACKNOWLEDGMENTS The authors acknowledge with gratitude the support given by the Universiti Sains Malaysia. This research was conducted with aid from a research grant (304/ PTEKIND/650462/K132) provided by the Malayan Sugar Manufacturing Company Berhad (Malaysia).
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samples significantly decreased (P < 0.05) at 1 mo; this breaking force then stabilized after further frozen storage for the cryoprotectant-added samples, whereas it continued to decrease in the control samples. Gel deformation fluctuated during frozen storage and was significantly lower (P < 0.05) at the end of experiment than at the beginning. The presence of cryoprotectants reduced the whiteness of DSLM. Samples containing polydextrose, trehalose, lactitol, and palatinit were able to retain the protein solubility, gel breaking force, and deformation of DSLM better than control samples after 4 mo of frozen storage and exposure to freeze-thaw cycles. The effects of these low-sweetness cryoprotectants are comparable to those of sucrose-sorbitol, thus, these sugars could be used as alternatives in protecting surimi-like materials during frozen storage.
Key words: duck surimi-like material, low-sweetness cryoprotectant, freeze-thaw cycle, frozen storage 2012 Poultry Science 91:1703–1708 http://dx.doi.org/10.3382/ps.2011-01926
INTRODUCTION Duck meat is the third most widely produced poultry meat in the world after chicken and turkey. Worldwide duck meat production has continuously increased, and it reached more than 3.8 million tons in 2009 (Food and Agriculture Organization of the United Nations, 2010). However, utilization of this protein source is limited compared with chicken meat due to factors such as low cooking yield and emulsion stability, high cooking loss, and darker color (Biswas et al., 2006; Ali et al., 2007). Use of surimi-like material produced from under-used sources of meat has become popular due to development of processing methods that improve its quality attributes (Desmond and Kenny, 1998; Nowsad et al., 2000; Ensoy et al., 2004; Jin et al., 2010). For example, several studies reported that duck surimi-like materi©2012 Poultry Science Association Inc. Received October 8, 2011. Accepted March 10, 2012. 1 Corresponding author: [email protected]
al (DSLM) has significantly improved water-holding capacity (WHC), color, and textural properties compared with untreated duck meat (Nurkhoeriyati et al., 2011; Ramadhan et al., 2011). Prolonged frozen storage may damage the quality of meat proteins due to ice crystal growth, which can affect osmotic removal of water and cause denaturation of proteins (Thyholt and Isaksson, 1997). Moreover, the temperature fluctuations that can occur during transportation, retail display, and consumption of meat also can adversely affect the product (Srinivasan et al., 1997). Freeze-thaw cycles are destabilizing forces for muscle proteins because they cause proteins to unfold and the meat quality to deteriorate (Benjakul and Bauer, 2000). Cryoprotectants are used to protect the myofibrillar proteins of surimi during the freezing process and frozen storage (Carvajal et al., 2005). The most commonly used cryoprotectant added to surimi is sucrose-sorbitol at a 1:1 ratio and concentration of 8% (wt/wt). This compound, however, imparts a sweet taste that is undesirable to some consumers (Carvajal et al., 1999). In an
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effort to replace sucrose, many researchers have studied the use of various types of low-sweetness sugars as cryoprotectants; for example, polydextrose, trehalose, palatinit, and lactitol (Sych et al., 1990; Sultanbawa and Li-Chan, 1998; Pan et al., 2010). Polydextrose is a low-molecular weight randomly bonded polysaccharide of glucose. This low-calorie bulking agent frequently is used to replace sugar in reduced-calorie foods and contains zero relative sweetness compared with sucrose (Craig et al., 1996; O’Donnell, 2005; Auerbach et al., 2006). Trehalose, which has 0.45 relative sweetness compared with sucrose, is a disaccharide consisting of 2 glucose moieties linked through their respective anomeric carbon atoms (1,1) by an α-glycosidic bond (O’Donnell, 2005; Lindley, 2006). Palatinit is a type of sugar alcohol with a relative sweetness value of 0.45 to 0.6 compared with sucrose (Wijers and Sträter, 2001). Lactitol is a sugar alcohol (polyol), and its relative sweetness value is 0.40 compared with sucrose (Young, 2006). Five freeze-thaw cycles often are used to determine the freeze-thaw stability of muscle proteins (Srinivasan et al., 1997; Benjakul and Bauer, 2000; Xia et al., 2009; Liu et al., 2011). Although many studies have described the activity of cryoprotectants during frozen storage of surimi, studies of the cryoprotective effects of lowsweetness sugars during frozen storage and freeze-thaw cycles are lacking. Thus, the goal of this study was to evaluate the effects of different cryoprotectants on the stability of DSLM during both frozen storage and during freeze-thaw cycles.
MATERIALS AND METHODS Preparation of DSLM Carcasses of 8-wk-old Pekin duck broilers were mechanically deboned at a commercial processing plant (Fika Food Corporation Sdn. Bhd., Penang, Malaysia) using a deboning machine with a pore size of 0.9 mm (Meat Maker Deboner, Prince Industries Inc., Murrayville, GA). The mechanically deboned duck meat was stored at –20°C in frozen meat block form before production of DSLM. To produce DSLM, meat blocks were cut into smaller sizes using a meat bone saw (model P79-SS, Powerline Equipment, Norwalk, CT) and ground using a meat grinder (model EVE/ALL-12, Rheninghaus Srl, Torino, Italy). The DSLM preparation followed procedures described by Ensoy et al. (2004) with modification of the mixing time and dewatering technique. Briefly, 2 kg of meat were mixed with 6 L of cold tap water (<5°C) for 5 min using a universal mixer (GB-30 Orimas, Yungkang, Taiwan). The mixture was centrifuged at 4,043 × g for 15 min at <4°C using a Union 5KR centrifuge (Hanil Science Industrial, Co., Ltd., Incheon, Korea). The upper layer of fat and aqueous supernatant was discarded from the meat concentrate. The washing process was repeated 3 times.
The cryoprotectant treatments were created as follows. Using a silent cutter (Hobart Corp., Troy, OH), one of the following was added to 3 kg of DSLM: 6% (wt/wt) sucrose-sorbitol blend (1:1), 6% (wt/wt) polydextrose, 6% (wt/wt) trehalose, 6% (wt/wt) lactitol, or 6% (wt/wt) palatinit. The DSLM without cryoprotectant added was used as the control. The treated DSLM samples and control each were placed in a plastic box (20 × 15 × 10 cm) and frozen in an air-blast freezer (Irinox S.p.A, Corbanese, Italy) at –27°C for 3 h and then kept at –20°C before analysis.
Freeze-Thaw Stability During Frozen Storage Freeze-thaw stability of the samples was assessed following procedures described by Nowsad et al. (2000) with modification of the number of cycles. The frozen DSLM samples were kept at –20°C and subjected to 5 freeze-thaw cycles. A freeze-thaw cycle was conducted once every month during the 4-mo storage duration. At 0 mo, the frozen samples were thawed inside the chiller at 5 to 7°C for 15 h, and subsamples were taken from thawed samples. The WHC and protein solubility of each subsample was measured, and a 300-g portion of each subsample was processed into a cooked gel for the analysis of gel breaking force, deformation, and degree of whiteness. The remainder of each thawed sample was refrozen at –20°C. The same freeze-thaw process was conducted after 1, 2, 3, and 4 mo of storage.
WHC The WHC was analyzed according to procedures described by Babji and Gna (1994) with modification of the calculation method. About 20 g of DSLM were mixed with 40 mL of distilled water using a homogenizer (T25 digital Ultra-Turrax, Staufen, Germany). Approximately 10 g of homogenate were weighed into a centrifuge tube and centrifuged at 1,229.8 × g for 5 min. The supernatant was discarded and the precipitate was weighed. The WHC was calculated using the following formula and expressed as grams of water per grams of protein of the meat:
WHC =
weightprecipitate − weightdry matters of meat weighthomogenate ×%Protein meat
.
Protein Solubility Protein solubility was determined following procedures described by Venugopal et al. (1996). One gram of DSLM was added to 40 mL of 3% NaCl solution in a 50-mL centrifuge tube. A Stuart SA7 vortex mixer (Bibby Scientific Limited, Staffordshire, UK) was used to homogenize the sample for 2 min. Aliquots were centrifuged at 5,918.9 × g for 5 min using a Heraeus Multifuge X1R centrifuge (Thermo Electron LED GmbH,
FREEZE-THAW STABILITY OF DUCK SURIMI-LIKE MATERIAL
Langenselbold, Germany). The supernatant was collected for protein analysis using the Kjeldahl method. Protein solubility was calculated on the basis of whole protein content of the meat and expressed as a percentage: Protein solubility (%) =
%Protein supernatant %Protein meat
× 100% %.
tested (control and DSLM with 5 different cryoprotectants added), and there were 2 replicates for each treatment. Each parameter was measured in triplicate for each treatment. The data obtained were subjected to ANOVA (ANOVA), after which Duncan’s multiplerange test was used to determine the significance of differences among the treatments. SPSS software (SPSS Statistics 17.0 for Windows, SPSS Inc., Chicago, IL; SPSS, 2008) was used to conduct the statistical tests.
Preparation of Cooked Gels Cooked gels were prepared for analysis of gel breaking force, deformation, and degree of whiteness. First, 300 g of thawed DSLM were mixed with 3% (wt/wt) salt using a cutter mixer (Blixer, Robot Coupe USA, Inc., Jackson, MS) for 1 min until the mixture formed a meat batter. The batter was stuffed into plastic casing cylinders (diameter 2 cm, height 15 cm) using a manual sausage stuffer (Amrapali Ltd., Bihar, India). Stuffed batters were incubated in a water bath (WiseBath, Witeg Labortechnik GmbH, Wertheim, Germany) at 36°C for 30 min followed by 90°C for 10 min until the gel had formed. The samples then were immersed in ice flakes until they reached a temperature of ~20°C.
Breaking Force and Deformation Gel breaking force (g) and deformation (mm) were measured using a TA-XT plus texture analyzer (Stable Micro Systems Ltd., Surrey, UK) following procedures described by Balange and Benjakul (2009). Cooked gels were cut into 3-cm thick and 2-cm diameter pieces. A piece of the gel was placed perpendicularly on the platform surface, where it was penetrated by a spherical probe (type P/0.25) at a constant 1 mm/s until 11 mm depth was reached. The trigger force used was 5 g, with 1 mm/s of pretest speed and 10 mm/s of posttest speed. The load cell capacity of the texture analyzer was 5 kg, and the return distance was 35 mm. Breaking force was obtained from the highest peak of the graph (y-axis), and deformation was calculated as the distance between the penetration starting point and the highest peak point (x-axis).
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RESULTS AND DISCUSSION WHC Figure 1 shows the WHC of the different DSLM samples during 4 mo of frozen storage and exposure to freeze-thaw cycles. At 0 mo, the addition of polydextrose resulted in a significantly higher WHC (P < 0.05) compared with all other treatments. Except for the DSLM treated with polydextrose, the WHC of the treated samples and the control did not differ significantly (P > 0.05). Polydextrose is highly soluble in water and has high viscosity and hygroscopicity, thus it is able to hold more water (Craig et al., 1996). The WHC of all samples decreased gradually in conjunction with longer storage time. The DSLM with polydextrose added exhibited the lowest difference in WHC between the initial (0 mo) and final (4 mo) measurements. Kovačević et al. (2011) previously reported that polydextrose interacts with proteins of chicken surimi-like material in accordance with the cryoprotecting mechanism; hence it retained WHC at a higher level compared with the control sample. The WHC of the control sample decreased significantly at 1 mo, and it continued to decline until the end of the experiment. The control sample exhibited the greatest WHC reduction among all samples tested (P < 0.05). This is because protein denaturation occurs when cryoprotecant
Degree of Whiteness Color properties of the cooked gels were measured using a Minolta CM-3500d spectrophotometer (Konica Minolta Sensing Americas Inc., Ramsey, NJ) to obtain the L* (lightness), a* (redness), and b* (yellowness) values. Furthermore, whiteness was calculated using the following formula (Park and Lin, 2005): 100 – [(100 – L*)2 + a*2 + b*2]1/2.
Statistical Analysis This experiment was set up using a completely randomized design. Six different DSLM treatments were
Figure 1. Water-holding capacity of duck surimi-like material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
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is not present. Protein denaturation involves conformational changes of myosin followed by exposure of hydrophobic groups of amino acids and protein aggregation. This results in poor WHC and an inferior gel network (Benjakul et al., 2003; Zhou et al., 2006). Samples containing the sucrose-sorbitol blend, trehalose, and palatinit maintained a relatively constant WHC with no significant changes from 0 to 2 mo of storage (P > 0.05), but the WHC then decreased significantly at 3 and 4 mo of storage (P > 0.05). The lactitoladded sample maintained the WHC with no significant declines until 3 mo of storage (P > 0.05). Sych et al. (1990, 1991) reported that the cryoprotective activities of lactitol and palatinit are mainly due to the presence of numerous hydroxyl groups that interact with protein chains, leading to protein hydration and decreased protein-protein interaction. Thus, polydextrose, lactitol, trehalose, and palatinit showed cryoprotective activities comparable to those of the sucrose-sorbitol blend. Zhou et al. (2006) reported that trehalose prevented protein denaturation of tilapia surimi until 6 mo of frozen storage. The duration of cryoprotective activity of the sugars tested in the current study was shorter, possibly because exposure to freeze-thaw cycles might have caused more rapid protein denaturation.
Protein Solubility A decline in protein solubility is a main indicator that freeze-induced protein denaturation has occurred. This denaturation takes place due to the formation of hydrogen or hydrophobic bonds as well as disulfide bonds and ionic interactions during frozen storage (Zhou et al., 2006). Insolubilization of myofibrillar protein also may be related to conformational changes in myosin (Tironi et al., 2007). Protein solubility of all samples decreased significantly (P < 0.05) after 4 mo of storage and exposure to freeze-thaw cycles (Figure 2). At initial storage, the protein solubility ranged from 58.99 to 59.60%. The cryoprotectant-added samples exhibited decreased protein solubility at 1 mo, but it tended to remain stable from 2 mo onwards. At 4 mo, the cryoprotectant-added samples had protein solubility ranging from 48.60 to 54.61%. Protein solubility of samples treated with sucrose-sorbitol, polydextrose, trehalose, lactitol, and palatinit was similar (P > 0.05) and remained higher (P < 0.05) than the control throughout the 4 mo of the experiment. This result emphasizes the effective cryoprotective activity of the low-sweetness cryoprotectants used in this study. Similar results were reported by Sych et al. (1990), who found that the addition of 8% (wt/wt) polydextrose, lactitol, or palatinit to cod surimi was comparable to the addition of sucrose-sorbitol in providing cryoprotection for myofibrillar proteins after 3 mo of storage. Sych et al. (1991) also noted that the addition of 5.7 to 6.4% (wt/wt) lactitol provided a cryoprotective effect in cod surimi after 2 mo of storage. Pan et al.
(2010) reported that the addition of 6% (wt/w) trehalose to grass carp surimi was comparable to adding 8% sucrose-sorbitol to provide cryoprotection of myofibrillar proteins after 6 mo of frozen storage.
Gel Breaking Force and Deformation Changes in gel breaking force that occurred during the freeze-thaw cycles and frozen storage are shown in Figure 3. There was no significant difference (P > 0.05) in breaking force among sample types at 0 mo (range, 830.05–911.90 g). At 1 mo of storage, the gel breaking force values of all samples decreased significantly (P < 0.05; ~34.86%, range, 574.51–614.82 g), except for the sucrose-sorbitol-added sample. It decreased by only 19.6% and thus was significantly higher than the gel breaking force of the other samples (P < 0.05). Nowsad et al. (2000) also detected a drastic decline in gel breaking force in chicken surimi-like material with cryoprotectant added after 1 mo of frozen storage. In chicken myofibrillar protein isolate without cryoprotectant added, gel hardness declined to 48% after 4 wk of frozen storage (Uijttenboogaart et al., 1993). Gel breaking force continued to decrease at 2 mo, and the range of values of the sample types was narrower at this time point (535.26–587.04 g). At 3 and 4 mo of storage, no significant changes were detected in the cryoprotectant-added samples, whereas the gel breaking force in the control sample had decreased by 53.63% at 4 mo compared with 0 mo of storage. The deterioration intensity in the current study was lower than that reported by Sultanbawa and Li-Chan (1998), who found that ling cod surimi without added cryoprotectant could not be formed into a gel after 4 mo of frozen storage. Decreased gel-forming ability is caused by protein aggregation via disulfide bonds or hydrophobic interactions (Zhou et al., 2006). The result of the current study indicated that the presence of cryoprotectants inhibited further deterioration and protected myofibrillar proteins of DSLM during frozen storage.
Figure 2. Protein solubility of duck surimi-like material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
FREEZE-THAW STABILITY OF DUCK SURIMI-LIKE MATERIAL
Figure 3. Gel breaking force of cooked gels made from duck surimi-like material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
Figure 4 shows the changes in gel deformation that occurred during the freeze-thaw cycles and frozen storage. At the initial time of storage (0 mo), gel deformation values were 10.43 to 10.96 mm, with no significant differences among sample types (P > 0.05). During frozen storage, however, gel deformation fluctuated irregularly, and then finally decreased at 4 mo. Nowsad et al. (2000) also reported fluctuations in gel deformation of spent hen surimi-like material. In the current study, the control sample exhibited the lowest decline in gel deformation; this indicates that cryoprotective activities were present in the samples treated with cryoprotectants.
Whiteness Figure 5 shows the changes in whiteness that occurred during the freeze-thaw cycles and frozen storage. The control sample had the highest whiteness values, whereas the samples treated with cryoprotectants had lower values. The whiteness range (63.13–65.96) observed in this study is lower than that reported in a study of spent hen surimi with cryoprotectants added (64.07–67.11; Jin et al., 2010). The lower whiteness values found in samples treated with cryoprotectants indicate that the addition of cryoprotectants reduced the whiteness of DSLM. This difference was caused by the Maillard browning that occurred in samples treated with cryoprotectants due to the presence of sugar, which reacted with proteins and resulted in browning and a decrease in whiteness (Auh et al., 1999). Whiteness values of all samples did not significantly change (P > 0.05) from 0 to 3 mo, but then they decreased significantly (P < 0.05) at 4 mo. Samples containing polydextrose, trehalose, lactitol, and palatinit had the lowest whiteness values at the last measurement time, and these values were not significantly different from each other. Overall, the decrease in the whiteness value for a given sample type was not very large between the initial and the final measurement
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Figure 4. Gel deformation of cooked gels made from duck surimilike material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
times (i.e., from 63.13 to 65.96 to 59.37–62.36). The decrease in whiteness of cooked gels is influenced by pigment protein, especially pigment in muscle protein that becomes oxidized during the cooking process. Additionally, lipid oxidation in muscle during frozen storage induces cross-linking of pigment and muscle proteins through the free-radical process, which in turn results in decreased whiteness of cooked gels (Benjakul et al., 2005).
Conclusions In conclusion, when added to DSLM, polydextrose, trehalose, lactitol, and palatinit were able to maintain stable protein solubility, gel breaking force, and deformation of the material better than control samples after 4 mo of storage and exposure to freeze-thaw cycles. The values of these parameters were comparable to those of DSLM with sucrose-sorbitol added. Polydextrose-
Figure 5. Whiteness of cooked gels made from duck surimi-like material with or without cryoprotectant during frozen storage and freeze-thaw cycles.
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added material had the highest WHC, and addition of cryoprotectants reduced the whiteness of DSLM. The results of this study indicate that these low-sweetness cryoprotectants could be used as alternatives to sucrose-sorbitol in protecting surimi-like materials during frozen storage.
ACKNOWLEDGMENTS The authors acknowledge with gratitude the support given by the Universiti Sains Malaysia. This research was conducted with aid from a research grant (304/ PTEKIND/650462/K132) provided by the Malayan Sugar Manufacturing Company Berhad (Malaysia).
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