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Confectionery gels: Effects of low calorie sweeteners on the rheological properties and microstructure of fish gelatin
Food Hydrocolloids 67 (2017) 157e165
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Confectionery gels: Effects of low calorie sweeteners on the rheological properties and microstructure of fish gelatin Luyun Cai a, b, *, Jianhui Feng a, Joe Regenstein c, Yanfang Lv a, Jianrong Li a, b, ** a College of Food Science and Engineering, Bohai University, National & Local Joint Engineering Research Center of Storage, Processing and Safety Control Technology for Fresh Agricultural and Aquatic Products, Food Safety Key Lab of Liaoning Province, Jinzhou 121013, China b College of Food Science, Southwest University, Chongqing 400716, China c Department of Food Science, Cornell University, Ithaca, NY 14853-7201, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2016 Received in revised form 29 November 2016 Accepted 20 December 2016 Available online 26 December 2016
Low calorie sweeteners, such as xylitol and stevia, are being used for confectionery gels. Xylitol is digested using the human’s glucose pathway and stevia is a natural low calorie sweetener. However, the microstructure and rheological properties of fish gelatin (FG) confectionery gels with these sweeteners has not been studied. Therefore, the effects of xylitol or stevia at 0, 1, 3, 5, 10 and 20% on the rheological properties and microstructure of FG (6.67% w/w) were studied. The addition of xylitol at low concentrations (i.e., 3e5%) increased the gel strength. However, the gel strength decreased upon further addition of xylitol, likely due to the xylitol preventing gel network formation. The gel strength decreased as stevia increased, which might be attributed to an antagonism that prevents fish gelatin from forming three dimensional network structures. Fourier transform infrared (FTIR) spectroscopy showed that the overall intensities of Amides A, B, I, II, III and the Fingerprint region increased with increasing concentrations of xylitol and stevia, suggesting decreased molecular order. Xylitol had more pronounced effects than stevia, which probably was related to its greater solubility and number of eOH groups. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Fish gelatin Xylitol Stevia Confectionery gels Grass carp Ctenopharyngodon idellus
1. Introduction Gelatin is a water soluble, high molecular weight polypeptide derived from partial hydrolysis of collagen (Liu, Nikoo, Boran, Zhou, & Regenstein, 2015). Gelatin can be used as a thickener, stabilizer, emulsifier, foaming agent, and gelling agent (Karaman, Cengiz, Kayacier, & Dogan, 2016). Currently, most gelatin is obtained from mammalian by-products, such as cattle hides, beef bones and pork skins, but due to social, cultural and health-related concerns, there is an increasing demand for alternative sources (Karim & Bhat, 2009). With the decline in the harvest of saltwater fish species, more attention is being paid to using the by-products from
* Corresponding author. College of Food Science and Engineering, Bohai University, National & Local Joint Engineering Research Center of Storage, Processing and Safety Control Technology for Fresh Agricultural and Aquatic Products, Food Safety Key Lab of Liaoning Province, Jinzhou 121013, China. ** Corresponding author. College of Food Science and Engineering, Bohai University, National & Local Joint Engineering Research Center of Storage, Processing and Safety Control Technology for Fresh Agricultural and Aquatic Products, Food Safety Key Lab of Liaoning Province, Jinzhou 121013, China. E-mail addresses: [email protected] (L. Cai), [email protected] (J. Li). http://dx.doi.org/10.1016/j.foodhyd.2016.12.031 0268-005X/© 2016 Elsevier Ltd. All rights reserved.
freshwater fish processing as a potential alternative gelatin source. Fish gelatin (FG) can be obtained from fish by-products such as the skins, bones and scales (Zhang, Ma, Cai, Zhou, & Li, 2016). Grass carp (Ctenopharyngodon idellus) is a widely grown freshwater fish. Grass carp is one of the most abundant freshwater fish in China, as well as other Asian countries (Hema, Shakila, Shanmugam, & Jawahar, 2016). Although grass carp is eaten regularly in Asia, due to its small, widely distributed fish bones, it has not been used in the United States (US) for other than gefilte fish (a Jewish-style fish ball) manufacture (Regenstein, personal communication). In the US it is rapidly increasing in number and spreading more widely, and is considered an exotic species with ecological/ environmental concerns. So grass carp both in Asia and North America are readily available as a source of materials for fish gelatin extraction. Most food gel products are composite gels with sweeteners, where all of the components contribute to the structure and physical properties of the food. The interaction of gelatin with other food components has been well studied (Kuan, Nafchi, Huda, Ariffin, & Karim, 2016; Sow & Yang, 2015). Both groups determined that low concentrations of sugars decreased the gel strength, while higher levels of sugars increased the gelling and melting
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temperatures. Sweeteners are also used to impart specific flow behaviors, textures, appearances, and where required mouth-feel properties. Because of their wide spread use in confectionery gels, these systems have been well studied (Kuan et al., 2016), but it is only recently that sugar-free or reduced-sugar foods (using low calorie or no calorie sweeteners) have become popular although these compounds have not been as well studied in confectionary gels (Shankar, Ahuja, & Sriram, 2013). In confectionery gels, the sweetener is often the most critical factor for consumer acceptance due to its effects on both flavor and texture. Xylitol and stevia are two important low calorie sweeteners in the confectionery industry (Asif, 2015). Xylitol has been widely used in the food industry because of its numerous benefits such as fewer calories, lower viscosity in solution, good chemical stability and high sweetness. Stevia is also a natural low calorie sweetener with relative sweetness 250e300 times greater than sucrose (Alizadeh, Azizi-Lalabadi, & Kheirouri, 2014). Stevia has been used as an alternative sweetener and reported to have many benefits (Carbonell-Capella, Barba, Esteve, & Frígola, 2013; Kroyer, 2010). Among them are high sweetness as a natural low calorie sweetener which has attracted more attention. Xylitol and stevia are easily soluble in water at room temperature so their interaction with gelatin can be studied. Over the last few years, the application of low calorie sweeteners in food is of great importance. Consumers are more interested in new healthier products with decreasing amounts of sugar. Thus, xylitol and stevia have been used in confectionery gels instead of sugar, such as in sugar-free candies, cookies, and chewing gum. As the sweetness and texture of these new food products should not be changed too much, a study of their impact in gelatin mixtures seemed appropriate. The objective of this research was to study the effects of low calorie sweeteners on the physiochemical properties and microstructure of confectionery gel products that use fish gelatin, and to better understand the interaction between low calorie sweeteners and fish gelatin in gels. In this paper the physicochemical properties studied included the texture properties, gel strength, rheological properties and gelatin secondary structure to begin to understand the potential applications of fish gelatin in confectionery products. 2. Materials and methods 2.1. Materials Fresh grass carp (Ctenopharyngodon idellus) skins, which had been discarded after filleting, were obtained from a local aquaculture farm that processed on the farm in Jinzhou, Liaoning Province, China. The skins from recently harvested and processed fish were taken on ice to the Seafood Processing Laboratory of Bohai University within 0.5 h. On arriving at the laboratory, the visible fat was removed using a knife and the skins were washed with cold distilled water, then kept at 0 C until the next step of the FG extraction. Xylitol and stevia were obtained from Shandong Longli Bio Technologies Inc. (Qingdao, Shandong, China). All other reagents used were at least of analytical grade. 2.2. Methods 2.2.1. Extraction of gelatin Gelatin was extracted from clean skins after distilled water washing according to the methods of Songchotikunpan, Tattiyakul, and Supaphol (2008) with some modifications. The skins were cut with a scissor into small pieces (~2 2 cm). In the first step the small pieces of fish skins were treated with 0.1 M NaOH solution (1:30 w/v) and stirred using a magnetic stirrer at room temperature
(~25 C) for 4 h. The alkaline solution was changed every h. The samples were then washed with cold tap water until a neutral pH (<7.5) (pH-meter, Hangzhou Special Paper Co. Ltd., Zhejiang, China) of the wash water was obtained. In the second step, a 0.1 M HCl (1:30 w/v) pretreatment was done for 45 min with stirring at room temperature and again returned to a neutral pH (>6.5). The third step was heating skins in water (1:30 w/w) for 4 h at 50 C in a water bath to extract FG. The extracted gelatin solutions were centrifuged at room temperature (Sorvall Stratos Centrifuge, Thermo Fisher Scientific, Waltham, MA, USA) at 8000 g for 10 min and the sediment was discarded. The viscous supernatant was concentrated using a rotary evaporator (RE-2000, YaRong Biochemical Instrument Co. Ltd., Shanghai, China) at 85 C for 12 h and then freeze-dried overnight (FreeZone 2.5L, Labconco, Palo Alto, CA, USA). The dry matter was ground using a mortar and pestle. This gelatin powder was then stored in a desiccator at room temperature for up to 4 wks. The proximate compositions of the extracted gelatin were analyzed according to AOAC method (AOAC, 1997) using a Kjeldahl factor of 6.25. 2.2.2. Sample preparation The required amount of gelatin powder was dissolved in deionized water (Milli-Q Ultra Pure Water System, Millipore Inc., Billerica, MA, USA) until completely swollen to prepare a 6.67% FG solution at room temperature. A series of xylitol and stevia stock solutions were prepared at 1, 3, 5, 10 and 20% (w/v) prior to gelatin hydration. Then the appropriate amounts of xylitol and stevia were added to the FG solutions. The mixture of gelatin with xylitol or stevia was kept for 1 h at 60 C in a water bath (HH-6 Digital Display Thermostatic Bath, Changzhou Guohua Electric Appliance Co., Ltd., Jiangsu, China) with swirling until all of the FG was dissolved. After heating, the hot solutions (20 mL) were immediately poured into cylindrical-shaped flat bottom glass containers (21 mm diameter 36 mm height). They were then cooled to room temperature and kept at 4 C for about 18 h to form hydrogels, and then freeze-dried to obtain xerogels. Xerogels were individually packed in low O2 permeable polyethylene pouches and kept in a vacuum dryer over P2O5 (MZ250 Vacuum Desiccators, Shanghai Experimental Instrument Company, Shanghai, China) for up to 2 wk. Hydrogels were used for rheological and gel strength measurements, and xerogels were used to evaluate the secondary structure. 2.2.3. Fourier infrared (FTIR) spectroscopy The xerogels were ground into a powder using a mortar and pestle and mixed with KBr powder at a ratio of 1:50 (w/w) (Muyonga, Cole, & Duodu, 2004b). The spectra were automatically recorded using a near FT-IR spectrophotometer (Scimitar 2000, Madison, WI, USA) in the range of 4000 e 400 cm1 at a data resolution of 2 cm1 against a background spectrum recorded from the clean empty cell at room temperature (~25 C). Spectrum acquisition for each sample was repeated three times and an average spectrum was obtained using the software that came with the instrument. Intensity measurements (areas under the peaks) were also obtained using the software’s undefined algorithms. 2.2.4. Texture profile analysis and gel strength The textural properties of samples were evaluated at room temperature using a TA-XT Plus texture analyzer (Stable Micro Systems Ltd., Godalming, UK) equipped with a 50 mm diameter aluminum cylindrical probe (P/50). To keep the temperature consistent between samples, the testing was done immediately after the sample was removed from the 4 C refrigerator. Fish gelatin (6.67% w/w) with different xylitol and stevia concentrations were prepared as described previously. The samples were removed from the glass containers with a knife and a two cycle compression
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
was used (Supavititpatana, Wirjantoro, Apichartsrangkoon, & Raviyan, 2008). The detailed test settings were: Target mode: Distance; Distance of compression: 6 mm (30% of original gel height); Time: 10.0 s; Trigger type: Auto (force); Trigger force: 5.0 N; Tare mode: Auto; and Advanced Options: On. The pre-test, test and posttest speeds were set at 1.0 mm/s. The load cell (5 kg) was calibrated with a 5 kg weight. The hardness, springiness, cohesiveness and chewiness were calculated using the software supplied with the instrument. The hardness (N) was defined as the peak force during the first compression cycle. Cohesiveness was the area prior to reaching the peak force for the second compression divided by the same area for the first compression. Springiness was the time measured between the start of the second compression area and it reaching maximum force divided by the same parameter for the first compression. Resilience was defined as the negative force input after reaching peak force divided by the positive force during the first compression. Adhesiveness (N s) was the area below the baseline between the two compressions. Chewiness (N) was cohesiveness times springiness (Caine, Aalhus, Best, Dugan, & Jeremiah, 2003). All experiments were done in triplicate. Gel strength was done using the texture analyzer with a 12.5 mm flat bottomed aluminum cylindrical probe (P/0.5). Samples of fish gelatin with different xylitol and stevia were tested in the glass cylinders. The texture analyzer had the same load cell and a cross-head speed of 0.5 mm/s. The maximum force was determined when the probe penetrated 4 mm into the gel. Readings are the average of three determinations (Muyonga, Cole, & Duodu, 2004c). 2.2.5. Determination of gelation kinetics The gelation kinetics was studied using the method of Kuan et al. (2016) with a slight modification. The viscoelastic characteristics of each sample were measured using a stress-controlled rotational rheometer (Discovery HR-1, TA Instrument Ltd., New Castle, DE, USA). The samples were re-dissolved in distilled water at 35 C using a magnetic stirrer before each test. Each sample was transferred to the rheometer plate at ambient temperature and the excess material was wiped off with a plastic scraper. Before the test, the samples were equilibrated for about 30 min, and then the data recorded. A 40 mm diameter cone-plate geometry, 2 angle and 51 mm truncation gap were used. The temperature sweep was from 24 to 4 C at a rate of 1 C/min. The temperature of the rheometer was maintained with a Peltier plate attachment while the rheometer oscillated sinusoidally. Subsequently, the temperature was kept at 4 C for 3 h. The results were expressed in terms of the storage modulus (G0 ) and loss modulus (G00 ) as a function of the time sweep using the software supplied with the instrument. The strain sweep test for determination of the linear viscoelastic region (LVR) of the samples was carried out at a fixed frequency of 1 Hz and 4 C. Thereafter, a strain of 5%, which was within the LVR, was selected to do time and frequency sweep tests. For this part, the frequency (1 Hz) and strain (5%) were kept constant. The 5% strain was previously determined to be within the LVR for the gelatin gels. The analyses were done in triplicate. The results of G0 over time during the time sweep of gelation were fitted to a logarithmic equation of the following form (Kuan et al., 2016): Gt ¼ kgel ln(tgel)þC
(1)
where Gt is the value of G0 at time t, kgel is the gelation rate constant, tgel is the gelation time and C is a constant. FGX is the sample of fish gelatin with added xylitol and FGS is the sample of fish gelatin with added stevia. FG was used as the model to obtain the target storage modulus (G0 ) after 3 h of cooling and gelling (holding) at 4 C:
tmodel ¼ e
(G0 C)/kgel
159
(2)
where kgel values for all of the samples studied were obtained by fitting Equation (1) to the experimental data. Consequently, the time (tmodel) required for a gelling system to reach G0 ref (target storage modulus) was calculated using Equation (2). The expression was applied to describe the changes in gelation time. 2.2.6. Frequency sweeps Frequency sweeps give an indication of the cross-linking behavior of gelatin. The method as described by Sarbon, Badii, and Howell (2015) was used with slight modification. A dynamic frequency sweep was done at 4 C with a strain of 5% and frequency oscillated from 0.01 to 10 Hz, within the previously identified LVR of the material. Results of the changes were expressed in terms of G0 and G00 as a function of dynamic frequency sweep. Frequency of oscillation was varied, and the temperature and strain were kept constant. The analyses were done in triplicate. 2.2.7. Statistical analysis All experiments were done in triplicate to get mean values plus standard deviations. SPSS 19 for Windows (SPSS Inc., Chicago, IL, USA) was used. The data were subjected to one-way analysis of variance (ANOVA) and differences of mean values were evaluated using Duncan’s multiple range test with a confidence level of p < 0.05. 3. Results and discussion 3.1. Proximate component and FTIR spectroscopy analysis The proximate composition of the gelatin was moisture 7.7 ± 0.1%, protein 89.3 ± 0.2%, fat 0.22 ± 0.06% and ash 0.13 ± 0.02%. The FTIR spectra of the grass carp skin FG with xylitol or stevia are shown in Figs. 1 and 2. The positions of the FTIR absorption peaks were used to determine the effects of the added sweeteners. The differences in the secondary structure of FGX and FGS were compared in six regions of the spectra (Kuan et al., 2016): 3600e3200 cm1 (Amide A, reflecting the OH and NeH stretches coupled with H-bond stretching); [22] 3100e3000 cm1 (Amide B, reflecting the CeH antisymmetric and symmetric stretching); 1700e1600 cm1 (Amide I, reflecting the C]O stretching and NH bending) (Jongjareonrak, Benjakul, Visessanguan, & Tanaka, 2008); 1570e1335 cm1 (Amide II, reflecting the NeH bending and CH2 stretching) (Yakimets et al., 2005); 1300e1000 cm1 (Amide Ⅲ, reflecting the NH bending and stretching coupled CN stretching) (Li, Miao, Wu, Chen, & Zhang, 2014); and 1000e500 cm1 (Fingerprint region, reflecting the CeO skeletal stretching, CeH deformation vibrations and CeC skeletal stretching) (Sow & Yang, 2015). Fig. 1 and Table 1 show the effects of the sweeteners on FG and the spectra for samples with and without the addition of xylitol. All samples showed wide and strong absorption peaks in the Amide A region and showed a typical Amide A band between 3568 and 3375 cm1, which was associated with the stretching vibrations of NeH and OH groups. From Fig. 1, the Amide A band slowly tended to shift to a lower wavenumber from 3433 to 3379 cm1 with increasing xylitol from 0 to 20%, suggesting that hydrogen bonding occurred between xylitol and gelatin molecules consistent with the nez, Pe rez-Santin, Go mez-Guille n, Alema n, reported results of Gime and Montero (2011). The Amide I was the most useful to analyze the microstructure of proteins. The Amide I band located at 1654 cm1 reflected the structure of gelatin. A very low intensity at high concentrations suggested the loss of the ordered molecular structure when xylitol
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
1654
2914 3379
FG+20% xylito 2914
Absorbance
3568 3568
FG+10%xylitol
2927
3402
FG+3%xylitol
2936
3402
2935 3568
FG+1% xylitol
3423
3800
3600
1340 1340
1064 1012 FG+20% xylitol
FG control
3433
3400
3200 3000 wavenumber (cm-1)
2800
1654
1550
1450
1550
1450
1342 1338
1550
1700
Amide A
1650
1600
1340
1450
1550 1500 1450 wavenumber (cm-1)
Amide B
912 858 885 912 885858
921
746
522
746
522
850
921 1080
FG+5% xylitol FG+3%xylitol
1654
1654
2600
1062 1012
1550
FG+1% xylitol 2939
3568
1550 1550
FG+10%xylitol
1654
FG+5% xylitol Absorbance
3375
1654
Absorbance
160
1350
1300
FG+20% xylitol
455 FG+10% xylitol FG+5% xylitol
850
1080
FG+3% xylitol
1082 1082 1031
974
875
974
875
FG+1% xylitol
FG control
FG control
1400
455
1100
1000
900
800 700 wavenumber (cm-1)
600
500
400
Amide I Amide II Amide III Fingerprint
FG+20% xylitol
FG+10%xylitol
Absorbance
FG+5% xylitol
FG+3%xylitol
FG+1% xylitol
FG control
4000
3500
3000
2500 2000 1500 -1 wavenumber (cm )
1000
500
Fig. 1. FTIR spectra of fish gelatin (FG) containing 0, 1, 3, 5, 10 and 20% xylitol.
was added. In many studies it has been pointed out that the Amide I absorption band which is due almost entirely to the C]O stretch vibrations of the peptide linkages can be used to determine changes in the secondary structure of proteins (Ahmad & Benjakul, 2011). This was consistent with expected changes if the xylitol became embedded into the three-dimensional network structure of the gelatin. The locations and assignments with stevia are shown in Fig. 2 and Table 2. The Amide A again suggested increased H bonding. The Amide B band tended to shift to a higher wavenumber with increasing stevia concentrations suggesting a decrease in the molecular order of gelatin which may be related to the decreased gel
textural properties, particularly gel strength (Muyonga, Cole, & Duodu, 2004a). The results also suggested that stevia was not participating in the gelatin structure. The Amide II peak decreased and probably shifted to a lower wavenumber, where it became part of the Amide III band, which has been suggested to reflect hydration (Benjakul, Oungbho, Visessanguan, Thiansilakul, & Roytrakul, 2009). The slight upshift of the main Amide III peak was attributed to intermolecular interactions between FG and both sugars. From FTIR, it can be suggested that xylitol or stevia reduced the free hydrogens available for bonding with H2O. The loss of helix suggested decreased molecular order, which was greater with
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
3568 3568 3568 3568
0 3800
3600
FG+20% stevia
2931 3423
2929
FG+ 5% stevia FG+ 3% stevia
2931 3402 2933
3423
FG+1% stevia FG control
2939
3433
3400 3200 3000 wavenumber (cm-1)
1550 1654
3392
2800
Amide A
1550
1654
1550
1654
1550
1651 1654
0 1700
2600
1340
1450
1654
FG+10% stevia
1650
1550
1600
1340
1450
1340
1452
1340 1340
1450
1340
1448
1550 1500 1450 wavenumber (cm-1)
1078 1028
FG+10% stevia FG+ 5% stevia
1450
1076 1028
FG+20% stevia
FG+ 3% stevia
Absorbance
Absorbance
3568
1550
2927 3410
Absorbance
3568
161
1078 1028 1078 1028 1080 1031
FG+1% stevia
850
920
846 850
921 921
850
748
559
761
FG+20% stevia 559
759
FG+10% stevia
559
761
FG+ 5% stevia
559 FG+ 3% stevia
873
559
873
FG+1% stevia
559
1082 1031
FG control
FG control
1400
1350
1300
0 1100
1000
Amide I Amide II Amide III
Amide B
923
900
800 700 wavenumber (cm-1)
600
500
400
Fingerprint
FG+20% stevia
Absorbance
FG+10% stevia FG+ 5% stevia FG+ 3% stevia FG+1% stevia FG control
0 4000
3500
3000
2500 2000 1500 -1 wavenumber (cm )
1000
500
Fig. 2. FTIR spectra of fish gelatin (FG) containing 0, 1, 3, 5, 10 and 20% stevia.
stevia. The difference in the two low calorie sweeteners was related to the chemical structures of xylitol (C5H12O5) and stevia (C38H60O18). This should be reflected by poorer gel texture, in particular gel strength, hardness and chewiness of stevia gels (Sow & Yang, 2015). 3.2. TPA and gel strength TPA is used in confectionery factories to evaluate texture characteristic so that, for example, gummy jellies can be handled ~o n, 2015). without risk of deformation or breakage (Delgado & Ban TPA testing is intended to simulate the action of the tongue and teeth (Yang et al., 2007). The TPA parameters are shown in Table 3. The hardness, cohesiveness, chewiness, resilience and
adhesiveness in FGX gels were higher than the FGS gels (p < 0.05). Gel quality is generally better if these texture values are higher (Sow & Yang, 2015). However, there was no significant difference in springiness between FGX and FGS gels (p 0.05). The decrease of the FGS gel strength at 5% stevia was significantly (p < 0.05) less than the control. As stevia concentration increased to 5% hardness, cohesiveness and chewiness decreased and became significant (p < 0.05). These values were similar to those of Ramírez, Uresti, zquez (2011) who found that adding sugars Velazquez, and Va decreased gel network formation. However, xylitol concentration had little effect on gel texture even at higher concentrations suggesting that xylitol is a potential low calorie sweetener to replace sucrose in gummi-type candies. Gel strength results are shown in Fig. 3. Gel strength values for
162
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
Table 1 Location and assignment of the peaks identified in FTIR spectra for gels prepared with fish gelatin (FG) containing xylitol. Region
Amide A Amide B AmideⅠ Amide Ⅱ
Amide Ⅲ Fingerprint
Peak wavenumber (cm1)
Assignment and remarks
None (blank)
1% xylitol
3% xylitol
5% xylitol
10% xylitol
20% xylitol
3568 3433 2939 1654 1550 1450 1340 1242 1082 1031 974
3568 3423 2935 1654 1550 1450 1338 1240 1082 e 974 e
3568 3402 2936 1654 1550 1450 1342 1240 1080 e 921 e
3568 3402 2927 1654 1550 e e 1242 1080 e 921 e
e 3375 2914 1654 1550 e 1340 1244 1062 1012 912 885
e 3379 2914 1654 1550 e 1340 1255 1064 1012 912 885
OH stretch coupled with H-bond NeH stretch coupled with H-bond CH antisymmetric and symmetric stretching C]O stretch/hydrogen bond coupled with COO NH bend coupled with CN stretch CH2 bending (scissors) vibration CH2 wag of proline and glycine NH bend stretch coupled CeN stretch CeO skeletal stretch CeO skeletal stretch CeH deformation vibration (carbohydrate) CeH deformation vibration (carbohydrate)
e
875 e
850 e
850 e
858 746
858 746
CeH deformation vibration (carbohydrate) CeC Skeletal stretch
e
e
e
e
522
522
CeC Skeletal stretch
e
e
e
e
455
455
CeC Skeletal stretch
e 875
Table 2 Location and assignment of the peaks identified in FTIR spectra for gels prepared with FG containing stevia. Region
Amide A Amide B Amide I Amide II
Amide III Fingerprint
Peak wavenumber (cm1)
Assignment and remarks
None (blank)
1% stevia
3% stevia
5% stevia
10% stevia
20% stevia
3568 3433 2939 1654 1550 1450 1340 1242 1082 1031
3568 3423 2933 1651 1550 1450 1340 1240 1080 1031 e
3568 3402 2931 1654 1550 1452 1340 1240 1078 1028 921
3568 3392 2929 1654 1550 1450 1340 1242 1078 1028 921
3568 3423 2931 1654 1550 1450 1340 1242 1078 1028 920
3568 3410 2927 1654 1550 1448 1340 1240 1076 1028 923
OH stretch coupled with H-bond NeH stretch coupled with H-bond CH antisymmetric and symmetric stretching C]O stretch/hydrogen bond coupled with COO NH bend coupled with CN stretch CH2 bending (scissors) vibration CH2 wag of proline and glycine NH bend stretch coupled CN stretch CeO skeletal stretch CeO skeletal stretch CeH deformation vibration (carbohydrate)
873 e
846 761
850 759
846 761
850 748
CeH deformation vibration (carbohydrate) CeH deformation vibration (carbohydrate)
559
559
559
559
559
CeC Skeletal stretch
e 873 e 559
xylitol was higher than the control, and the sample at 3% xylitol had the highest gel strength. Above 3% xylitol gel strength decreased but was still higher than the control. The xylitol seems to favor network formation and strengthens the three-dimensional
FGS and FGX were within the range required for commercial products, i.e., 7.6e30.6 N (Tavakolipour, 2011). The gel strength of FGS at different concentrations of stevia was lower than the control. However, the gel strength of FGX at different concentrations of
Table 3 TPA parameters (hardness, cohesiveness, chewiness, springiness, resilience and adhesiveness) fish gelatin added with xylitol (FGX) and stevia (FGS). Concentration (%)
Hardness (N)
Cohesiveness
Chewiness (N)
FGX
FGS
FGX
FGS
FGX
FGS
0 1 3 5 10 20
2.20 ± 0.01aB 2.3 ± 0.1aB 2.2 ± 0.1aB 2.2 ± 0.1aB 2.37 ± 0.04aA 2.3 ± 0.1aAB
2.20 ± 0.01aA 2.00 ± 0.02bB 1.91 ± 0.01bC 1.76 ± 0.04bD 1.67 ± 0.03bE 1.6 ± 0.1bE
2.00 ± 0.00aB 2.0 ± 0.1aAB 2.0 ± 0.1aAB 2.0 ± 0.1aB 2.05 ± 0.04aA 2.0 ± 0.1aB
2.00 ± 0.00aA 1.83 ± 0.03bB 1.76 ± 0.02bC 1.65 ± 0.04bD 1.52 ± 0.01bE 1.4 ± 0.1bF
1.88 ± 0.02aB 1.9 ± 0.1aB 1.9 ± 0.1aB 1.9 ± 0.1aB 1.9 ± 0.1aA 1.9 ± 0.1aB
1.88 ± 0.02aA 1.71 ± 0.02bB 1.65 ± 0.00bBC 1.6 ± 0.1bC 1.43 ± 0.03bD 1.4 ± 0.1bD
Concentration (%)
Springiness FGX
0 1 3 5 10 20
0.94 0.94 0.93 0.93 0.93 0.94
Resilience FGS
± ± ± ± ± ±
0.01aA 0.00aA 0.01aA 0.00aA 0.00aA 0.01aA
0.94 0.93 0.94 0.93 0.94 0.94
FGX ± ± ± ± ± ±
0.01aAB 0.02aB 0.01aB 0.02aA 0.01aAB 0.01aAB
0.83 0.83 0.84 0.83 0.84 0.82
Adhesiveness (N s) FGS
± ± ± ± ± ±
0.01aB 0.01aB 0.01aAB 0.01aAB 0.01aA 0.01aB
0.83 0.84 0.82 0.82 0.81 0.81
FGX ± ± ± ± ± ±
0.01aAB 0.01bAB 0.03bB 0.02bA 0.00bB 0.01bB
0.91 0.90 0.91 0.90 0.91 0.90
FGS ± ± ± ± ± ±
0.00aA 0.01aAB 0.00aA 0.01aA 0.00aA 0.00aB
0.91 0.92 0.92 0.94 0.91 0.90
± ± ± ± ± ±
0.00aBC 0.01bB 0.01bB 0.01bA 0.01bBC 0.00bC
Data are means ± SD. Within each row, means with different lowercase letters are significantly different (p < 0.05) among different groups. Within each column, means with different capital letters are significantly different among different concentrations (p < 0.05).
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
FGX FGS
A D
2.5
a
E
B
C
b c
d
Gel Strength (N)
2.0
103
A CD e
102
f
1.5
1.0
Storage /Loss modulus (Pa)
3.0
163
0.5
y = 326Ln(x) + 505 2 R = 0.91
101
100
Storage modulus (G,) Loss modulus (G,,)
10-1
10-2
0.0 0
1%
3%
5%
Concentration
10%
10-3
20%
Fig. 3. Gel Strength of fish gelatin (FG) and fish gelatin added with different concentrations of xylitol (FGX) and stevia (FGS). Different capital letters mean significantly different (p < 0.05) with different FGX concentrations. Different lowercase letters mean significantly difference (p < 0.05) with different FGS concentrations. Bars represent standard deviations (n ¼ 3).
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Gelatin Times (hrs)
B
104
3.3. Gelation kinetics Fig. 4 shows the typical development of the storage modulus and loss modulus (G00 ) with time for FG, FGX and FGS at 4 C. The experimental data were fitted into a logarithmic function model to determine the gelation rates. G0 of the control group was calculated to obtain the target G0 . The time to reach the target G0 was then calculated for the various treatments. The model has one adjustable parameter (C) which was estimated by minimizing the R2 value. The detailed data for each system is summarized in Table 4. The kgel values for the FGS gelling system decreased upon the initial addition of stevia and then increased upon further addition of stevia at high concentrations. However, the kgel of FGX in the time sweep decreased from 1 to 20% xylitol. Prior research suggested that gelation was inhibited by the rearrangement of gelatin chains in the presence of sugars, as the addition of sugars increases the viscosity of bulk water around the sugar molecules in the continuous phase (Kuan et al., 2016). Uedaira, Ikura, and Uedaira (1989) suggested that sugar molecules containing a large number of OH groups have stronger stabilizing effects on the structure of water surrounding the sugar molecules, which is very much related to their solubility in water. Xylitol had many OH groups and had a greater influence on kgel than stevia. In addition, the solubility of xylitol in water is higher than that of stevia, so a more significant increase of kgel was observed for high concentrations of xylitol. Fig. 4 also shows that during the time sweeps the rate of G0 and G00 increased rapidly for about 0.5 h, the slope continued to increase at longer times but at a diminishing rate, which was consistent with a study by Boran, Mulvaney, and Regenstein (2010). G0 was much higher than G00 after 0.5 h of gelation in all samples, which indicated that the system at 4 C had more solid-like characteristics where the gel structures were been forming during the period of the time sweeps (Zaidel, Chronakis, & Meyer, 2012). The result suggested that as the gel network structures were forming, the rate of increase of G0 with time decreased. The changes in G0 and G00 probably reflected the inter-chain associations that lead to the formation of (G0 )
y = 126Ln(x) + 251 2 R = 0.93
102 101
Storage modulus (G,) Loss modulus (G,,)
100 10-1 10-2 10-3
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Gelatin Times (hrs)
103
C 102 Storage / Loss modulus (Pa)
network (Zhang et al., 2016), but excessive crosslinking may lead to a poorer gel. Gel strength showed a similar trend as the TPA.
Storage/Loss Modulus (Pa)
103
y = 148Ln(x) + 505 2 R = 0.92
101
100
Storage modulus (G,) Loss modulus (G,,)
10-1
10-2
10-3
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Gelatin Times (hrs) Fig. 4. Storage modulus (G0 ) and loss modulus (G00 ) for time sweeps at 4 C in gelatin dispersions of FG (control) (A), 20% FGX (B) and 10% FGS (C).
164
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
Table 4 Parameters for the analogue function model used to fit the gelation profiles of gelatin gelling systems containing xylitol and stevia for time sweeps. Gelling system
FG control FGþ1% xylitol FGþ3% xylitol FGþ5% xylitol FGþ10% xylitol FGþ20% xylitol FGþ1% stevia FGþ3% stevia FGþ5% stevia FGþ10% stevia FGþ20% stevia
Gelation rate G0 gel
R2
Kgel
C
t-model (hr)
827 827 827 827 827 827 827 827 827 827 827
0.91 0.92 0.92 0.92 0.92 0.93 0.93 0.92 0.92 0.92 0.92
326 154 161 154 150 126 181 161 159 148 144
505 526 547 527 544 251 613 572 529 505 492
3.00 7.20 5.70 7.06 5.99 96.5 3.25 4.86 6.48 8.77 10.2
3.4. Frequency sweeps
Logarithmic model equation: G0 ¼ kgel Ln(tgel)þC where G0 ¼ target storage modulus (Pa) obtained from the control (FG), kgel ¼ gelation rate constant, tgel ¼ gelation time (h) and C ¼ constant.
cross-links or junction zones (Kjøniksen, Hiorth, & Bo, 2005). The gelatin gels with various concentrations of xylitol had higher G0 values than the pure gelatin and gelatins with stevia, which
10
The results showed that the intensities of the six regions (Amides A, B, I, II, III and Fingerprint region) in the FTIR spectroscopy increased with increasing amounts of xylitol and stevia, suggesting decreased molecular order. The addition of sweeteners inhibited the gel network formation by lowering the kgel at higher concentrations of xylitol and stevia. In addition, the highest gel strength was found when xylitol was added at 3% concentration, and FGX had stronger gel strength than FGS, which could be attributed to a great number of eOH groups and high solubility of xylitol. Similar trends were observed for the effects of solutes on gelation kinetics. The results suggested that the addition of xylitol may have more potential applications than stevia in confectionery gels.
103 G’,G”(Pa)
FG FG+ 1% xylitol FG+ 3% xylitol FG+ 5% xylitol FG+ 10% xylitol FG+ 20% xylitol
102
Typical frequency sweeps at 4 C with different viscoelasticities in the range from 0.01 to 10 Hz are shown in Fig. 5 G0 and G00 showed changes dependent on the frequency. The G0 and G00 values of FGX decreased above 3% xylitol but were still higher than the pure gelatin. This suggested that the intermolecular interactions of the gelatin-xylitol mixture were stronger than that of the FG. The G0 values were higher than G00 which again suggested that the gelling systems at 4 C had more solid-like characteristics. The loss factor (tan d) decreased slightly upon the addition of stevia, suggesting that the three-dimensional network structure would aggregate slowly because of the increased viscosity of the continuous phase upon addition of sugars, thus kinetically slowing the approach of the gelatin chains to each other. 4. Conclusions
A
4
suggested that the addition of xylitol led to a synergistic effect on the viscoelastic properties of FG. The results were in accordance with the previous reports for FG gel formation with added sucrose (Kuan et al., 2016) and glucose (Evageliou, Mazioti, Mandala, & Komaitis, 2010).
FG FG + 1% xylitol FG + 3% xylitol FG + 5% xylitol FG + 10% xylitol FG + 20% xylitol
101
Acknowledgment
100 10-2
10-1
100
f (Hz)
101
This study was supported by the National Natural Science Foundation of China (31401478), the National Postdoctoral Science Foundation of China (2015M570760), China Scholarship Council (201508210023), the Postdoctoral Special Funding of Chongqing City (Xm2015021), the Research Project from Science & Technology Department of Liaoning Province of China (No. 2015103020).
B
103
References
2
G',G"(Pa)
10
FG FG+ 1% stevia FG+ 3% stevia FG+ 5% stevia FG+ 10% stevia FG+ 20% stevia
FG FG + 1% stevia FG + 3% stevia FG + 5% stevia FG + 10% stevia FG + 20% stevia
101
100 10-2
10-1
f (Hz)
100
101
Fig. 5. Storage modulus (G0 ) and loss modulus (G00 ) for frequency sweeps (0.01e10 Hz) at 4 C for FG containing 0, 1, 3, 5, 10 and 20% xylitol (A) and FG containing 0, 1, 3, 5, 10 and 20% stevia (B).
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L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165 Emerging Technologies, 18(18), 48e56. ~o n, S. (2015). Determining the minimum drying time of gummy Delgado, P., & Ban confections based on their mechanical properties. CyTA-Journal of Food, 13(3), 329e335. Evageliou, V., Mazioti, M., Mandala, I., & Komaitis, M. (2010). Compression of gellan gels. Part II: Effect of sugars. Food Hydrocolloids, 24(4), 392e397. nez, B., Pe rez-Santin, E., Go mez-Guille n, M. C., Alem Gime an, A., & Montero, P. (2011). Contribution of Leu and Hyp residues to antioxidant and ACE-inhibitory activities of peptide sequences isolated from squid gelatin hydrolysate. Food Chemistry, 125(2), 334e341. Hema, K., Shakila, R., Shanmugam, S., & Jawahar, P. (2016). Functional properties of restructured surimi gel product prepared from low valued short nose white tripod fish (Triacanthus brevirosterus). Journal of Food Processing & Technology, 7, 590e597. Jongjareonrak, A., Benjakul, S., Visessanguan, W., & Tanaka, M. (2008). Antioxidative activity and properties of fish skin gelatin films incorporated with BHT and atocopherol. Food Hydrocolloids, 22(3), 449e458. Karaman, S., Cengiz, E., Kayacier, A., & Dogan, M. (2016). Exposure to air accelerates the gelation of gelatin: Steady and dynamic shear rheological characterization to see the effect of air on the strength of gelatin gel. International Journal of Food Properties, 19(4), 721e730. Karim, A., & Bhat, R. (2009). Fish gelatin: Properties, challenges, and prospects as an alternative to mammalian gelatins. Food Hydrocolloids, 23(3), 563e576. Kjøniksen, A. L., Hiorth, M., & Bo, N. (2005). Association under shear flow in aqueous solutions of pectin. European Polymer Journal, 41(4), 761e770. Kroyer, G. (2010). Stevioside and stevia-sweetener in food: Application, stability and interaction with food ingredients. Journal für Verbraucherschutz und Lebensmittelsicherheit, 5(2), 225e229. Kuan, Y. H., Nafchi, A. M., Huda, N., Ariffin, F., & Karim, A. A. (2016). Effects of sugars on the gelation kinetics and texture of duck feet gelatin. Food Hydrocolloids, 58, 267e275. Li, J. H., Miao, J., Wu, J. L., Chen, S. F., & Zhang, Q. Q. (2014). Preparation and characterization of active gelatin-based films incorporated with natural antioxidants. Food Hydrocolloids, 37, 166e173. Liu, D., Nikoo, M., Boran, G., Zhou, P., & Regenstein, J. M. (2015). Collagen and gelatin. Annual Review of Food Science and Technology, 6, 527e557. Muyonga, J., Cole, C., & Duodu, K. (2004a). Characterisation of acid soluble collagen from skins of young and adult Nile perch (Lates niloticus). Food Chemistry, 85(1), 81e89. Muyonga, J., Cole, C., & Duodu, K. (2004b). Fourier transform infrared (FTIR)
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Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd
Confectionery gels: Effects of low calorie sweeteners on the rheological properties and microstructure of fish gelatin Luyun Cai a, b, *, Jianhui Feng a, Joe Regenstein c, Yanfang Lv a, Jianrong Li a, b, ** a College of Food Science and Engineering, Bohai University, National & Local Joint Engineering Research Center of Storage, Processing and Safety Control Technology for Fresh Agricultural and Aquatic Products, Food Safety Key Lab of Liaoning Province, Jinzhou 121013, China b College of Food Science, Southwest University, Chongqing 400716, China c Department of Food Science, Cornell University, Ithaca, NY 14853-7201, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 September 2016 Received in revised form 29 November 2016 Accepted 20 December 2016 Available online 26 December 2016
Low calorie sweeteners, such as xylitol and stevia, are being used for confectionery gels. Xylitol is digested using the human’s glucose pathway and stevia is a natural low calorie sweetener. However, the microstructure and rheological properties of fish gelatin (FG) confectionery gels with these sweeteners has not been studied. Therefore, the effects of xylitol or stevia at 0, 1, 3, 5, 10 and 20% on the rheological properties and microstructure of FG (6.67% w/w) were studied. The addition of xylitol at low concentrations (i.e., 3e5%) increased the gel strength. However, the gel strength decreased upon further addition of xylitol, likely due to the xylitol preventing gel network formation. The gel strength decreased as stevia increased, which might be attributed to an antagonism that prevents fish gelatin from forming three dimensional network structures. Fourier transform infrared (FTIR) spectroscopy showed that the overall intensities of Amides A, B, I, II, III and the Fingerprint region increased with increasing concentrations of xylitol and stevia, suggesting decreased molecular order. Xylitol had more pronounced effects than stevia, which probably was related to its greater solubility and number of eOH groups. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Fish gelatin Xylitol Stevia Confectionery gels Grass carp Ctenopharyngodon idellus
1. Introduction Gelatin is a water soluble, high molecular weight polypeptide derived from partial hydrolysis of collagen (Liu, Nikoo, Boran, Zhou, & Regenstein, 2015). Gelatin can be used as a thickener, stabilizer, emulsifier, foaming agent, and gelling agent (Karaman, Cengiz, Kayacier, & Dogan, 2016). Currently, most gelatin is obtained from mammalian by-products, such as cattle hides, beef bones and pork skins, but due to social, cultural and health-related concerns, there is an increasing demand for alternative sources (Karim & Bhat, 2009). With the decline in the harvest of saltwater fish species, more attention is being paid to using the by-products from
* Corresponding author. College of Food Science and Engineering, Bohai University, National & Local Joint Engineering Research Center of Storage, Processing and Safety Control Technology for Fresh Agricultural and Aquatic Products, Food Safety Key Lab of Liaoning Province, Jinzhou 121013, China. ** Corresponding author. College of Food Science and Engineering, Bohai University, National & Local Joint Engineering Research Center of Storage, Processing and Safety Control Technology for Fresh Agricultural and Aquatic Products, Food Safety Key Lab of Liaoning Province, Jinzhou 121013, China. E-mail addresses: [email protected] (L. Cai), [email protected] (J. Li). http://dx.doi.org/10.1016/j.foodhyd.2016.12.031 0268-005X/© 2016 Elsevier Ltd. All rights reserved.
freshwater fish processing as a potential alternative gelatin source. Fish gelatin (FG) can be obtained from fish by-products such as the skins, bones and scales (Zhang, Ma, Cai, Zhou, & Li, 2016). Grass carp (Ctenopharyngodon idellus) is a widely grown freshwater fish. Grass carp is one of the most abundant freshwater fish in China, as well as other Asian countries (Hema, Shakila, Shanmugam, & Jawahar, 2016). Although grass carp is eaten regularly in Asia, due to its small, widely distributed fish bones, it has not been used in the United States (US) for other than gefilte fish (a Jewish-style fish ball) manufacture (Regenstein, personal communication). In the US it is rapidly increasing in number and spreading more widely, and is considered an exotic species with ecological/ environmental concerns. So grass carp both in Asia and North America are readily available as a source of materials for fish gelatin extraction. Most food gel products are composite gels with sweeteners, where all of the components contribute to the structure and physical properties of the food. The interaction of gelatin with other food components has been well studied (Kuan, Nafchi, Huda, Ariffin, & Karim, 2016; Sow & Yang, 2015). Both groups determined that low concentrations of sugars decreased the gel strength, while higher levels of sugars increased the gelling and melting
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temperatures. Sweeteners are also used to impart specific flow behaviors, textures, appearances, and where required mouth-feel properties. Because of their wide spread use in confectionery gels, these systems have been well studied (Kuan et al., 2016), but it is only recently that sugar-free or reduced-sugar foods (using low calorie or no calorie sweeteners) have become popular although these compounds have not been as well studied in confectionary gels (Shankar, Ahuja, & Sriram, 2013). In confectionery gels, the sweetener is often the most critical factor for consumer acceptance due to its effects on both flavor and texture. Xylitol and stevia are two important low calorie sweeteners in the confectionery industry (Asif, 2015). Xylitol has been widely used in the food industry because of its numerous benefits such as fewer calories, lower viscosity in solution, good chemical stability and high sweetness. Stevia is also a natural low calorie sweetener with relative sweetness 250e300 times greater than sucrose (Alizadeh, Azizi-Lalabadi, & Kheirouri, 2014). Stevia has been used as an alternative sweetener and reported to have many benefits (Carbonell-Capella, Barba, Esteve, & Frígola, 2013; Kroyer, 2010). Among them are high sweetness as a natural low calorie sweetener which has attracted more attention. Xylitol and stevia are easily soluble in water at room temperature so their interaction with gelatin can be studied. Over the last few years, the application of low calorie sweeteners in food is of great importance. Consumers are more interested in new healthier products with decreasing amounts of sugar. Thus, xylitol and stevia have been used in confectionery gels instead of sugar, such as in sugar-free candies, cookies, and chewing gum. As the sweetness and texture of these new food products should not be changed too much, a study of their impact in gelatin mixtures seemed appropriate. The objective of this research was to study the effects of low calorie sweeteners on the physiochemical properties and microstructure of confectionery gel products that use fish gelatin, and to better understand the interaction between low calorie sweeteners and fish gelatin in gels. In this paper the physicochemical properties studied included the texture properties, gel strength, rheological properties and gelatin secondary structure to begin to understand the potential applications of fish gelatin in confectionery products. 2. Materials and methods 2.1. Materials Fresh grass carp (Ctenopharyngodon idellus) skins, which had been discarded after filleting, were obtained from a local aquaculture farm that processed on the farm in Jinzhou, Liaoning Province, China. The skins from recently harvested and processed fish were taken on ice to the Seafood Processing Laboratory of Bohai University within 0.5 h. On arriving at the laboratory, the visible fat was removed using a knife and the skins were washed with cold distilled water, then kept at 0 C until the next step of the FG extraction. Xylitol and stevia were obtained from Shandong Longli Bio Technologies Inc. (Qingdao, Shandong, China). All other reagents used were at least of analytical grade. 2.2. Methods 2.2.1. Extraction of gelatin Gelatin was extracted from clean skins after distilled water washing according to the methods of Songchotikunpan, Tattiyakul, and Supaphol (2008) with some modifications. The skins were cut with a scissor into small pieces (~2 2 cm). In the first step the small pieces of fish skins were treated with 0.1 M NaOH solution (1:30 w/v) and stirred using a magnetic stirrer at room temperature
(~25 C) for 4 h. The alkaline solution was changed every h. The samples were then washed with cold tap water until a neutral pH (<7.5) (pH-meter, Hangzhou Special Paper Co. Ltd., Zhejiang, China) of the wash water was obtained. In the second step, a 0.1 M HCl (1:30 w/v) pretreatment was done for 45 min with stirring at room temperature and again returned to a neutral pH (>6.5). The third step was heating skins in water (1:30 w/w) for 4 h at 50 C in a water bath to extract FG. The extracted gelatin solutions were centrifuged at room temperature (Sorvall Stratos Centrifuge, Thermo Fisher Scientific, Waltham, MA, USA) at 8000 g for 10 min and the sediment was discarded. The viscous supernatant was concentrated using a rotary evaporator (RE-2000, YaRong Biochemical Instrument Co. Ltd., Shanghai, China) at 85 C for 12 h and then freeze-dried overnight (FreeZone 2.5L, Labconco, Palo Alto, CA, USA). The dry matter was ground using a mortar and pestle. This gelatin powder was then stored in a desiccator at room temperature for up to 4 wks. The proximate compositions of the extracted gelatin were analyzed according to AOAC method (AOAC, 1997) using a Kjeldahl factor of 6.25. 2.2.2. Sample preparation The required amount of gelatin powder was dissolved in deionized water (Milli-Q Ultra Pure Water System, Millipore Inc., Billerica, MA, USA) until completely swollen to prepare a 6.67% FG solution at room temperature. A series of xylitol and stevia stock solutions were prepared at 1, 3, 5, 10 and 20% (w/v) prior to gelatin hydration. Then the appropriate amounts of xylitol and stevia were added to the FG solutions. The mixture of gelatin with xylitol or stevia was kept for 1 h at 60 C in a water bath (HH-6 Digital Display Thermostatic Bath, Changzhou Guohua Electric Appliance Co., Ltd., Jiangsu, China) with swirling until all of the FG was dissolved. After heating, the hot solutions (20 mL) were immediately poured into cylindrical-shaped flat bottom glass containers (21 mm diameter 36 mm height). They were then cooled to room temperature and kept at 4 C for about 18 h to form hydrogels, and then freeze-dried to obtain xerogels. Xerogels were individually packed in low O2 permeable polyethylene pouches and kept in a vacuum dryer over P2O5 (MZ250 Vacuum Desiccators, Shanghai Experimental Instrument Company, Shanghai, China) for up to 2 wk. Hydrogels were used for rheological and gel strength measurements, and xerogels were used to evaluate the secondary structure. 2.2.3. Fourier infrared (FTIR) spectroscopy The xerogels were ground into a powder using a mortar and pestle and mixed with KBr powder at a ratio of 1:50 (w/w) (Muyonga, Cole, & Duodu, 2004b). The spectra were automatically recorded using a near FT-IR spectrophotometer (Scimitar 2000, Madison, WI, USA) in the range of 4000 e 400 cm1 at a data resolution of 2 cm1 against a background spectrum recorded from the clean empty cell at room temperature (~25 C). Spectrum acquisition for each sample was repeated three times and an average spectrum was obtained using the software that came with the instrument. Intensity measurements (areas under the peaks) were also obtained using the software’s undefined algorithms. 2.2.4. Texture profile analysis and gel strength The textural properties of samples were evaluated at room temperature using a TA-XT Plus texture analyzer (Stable Micro Systems Ltd., Godalming, UK) equipped with a 50 mm diameter aluminum cylindrical probe (P/50). To keep the temperature consistent between samples, the testing was done immediately after the sample was removed from the 4 C refrigerator. Fish gelatin (6.67% w/w) with different xylitol and stevia concentrations were prepared as described previously. The samples were removed from the glass containers with a knife and a two cycle compression
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
was used (Supavititpatana, Wirjantoro, Apichartsrangkoon, & Raviyan, 2008). The detailed test settings were: Target mode: Distance; Distance of compression: 6 mm (30% of original gel height); Time: 10.0 s; Trigger type: Auto (force); Trigger force: 5.0 N; Tare mode: Auto; and Advanced Options: On. The pre-test, test and posttest speeds were set at 1.0 mm/s. The load cell (5 kg) was calibrated with a 5 kg weight. The hardness, springiness, cohesiveness and chewiness were calculated using the software supplied with the instrument. The hardness (N) was defined as the peak force during the first compression cycle. Cohesiveness was the area prior to reaching the peak force for the second compression divided by the same area for the first compression. Springiness was the time measured between the start of the second compression area and it reaching maximum force divided by the same parameter for the first compression. Resilience was defined as the negative force input after reaching peak force divided by the positive force during the first compression. Adhesiveness (N s) was the area below the baseline between the two compressions. Chewiness (N) was cohesiveness times springiness (Caine, Aalhus, Best, Dugan, & Jeremiah, 2003). All experiments were done in triplicate. Gel strength was done using the texture analyzer with a 12.5 mm flat bottomed aluminum cylindrical probe (P/0.5). Samples of fish gelatin with different xylitol and stevia were tested in the glass cylinders. The texture analyzer had the same load cell and a cross-head speed of 0.5 mm/s. The maximum force was determined when the probe penetrated 4 mm into the gel. Readings are the average of three determinations (Muyonga, Cole, & Duodu, 2004c). 2.2.5. Determination of gelation kinetics The gelation kinetics was studied using the method of Kuan et al. (2016) with a slight modification. The viscoelastic characteristics of each sample were measured using a stress-controlled rotational rheometer (Discovery HR-1, TA Instrument Ltd., New Castle, DE, USA). The samples were re-dissolved in distilled water at 35 C using a magnetic stirrer before each test. Each sample was transferred to the rheometer plate at ambient temperature and the excess material was wiped off with a plastic scraper. Before the test, the samples were equilibrated for about 30 min, and then the data recorded. A 40 mm diameter cone-plate geometry, 2 angle and 51 mm truncation gap were used. The temperature sweep was from 24 to 4 C at a rate of 1 C/min. The temperature of the rheometer was maintained with a Peltier plate attachment while the rheometer oscillated sinusoidally. Subsequently, the temperature was kept at 4 C for 3 h. The results were expressed in terms of the storage modulus (G0 ) and loss modulus (G00 ) as a function of the time sweep using the software supplied with the instrument. The strain sweep test for determination of the linear viscoelastic region (LVR) of the samples was carried out at a fixed frequency of 1 Hz and 4 C. Thereafter, a strain of 5%, which was within the LVR, was selected to do time and frequency sweep tests. For this part, the frequency (1 Hz) and strain (5%) were kept constant. The 5% strain was previously determined to be within the LVR for the gelatin gels. The analyses were done in triplicate. The results of G0 over time during the time sweep of gelation were fitted to a logarithmic equation of the following form (Kuan et al., 2016): Gt ¼ kgel ln(tgel)þC
(1)
where Gt is the value of G0 at time t, kgel is the gelation rate constant, tgel is the gelation time and C is a constant. FGX is the sample of fish gelatin with added xylitol and FGS is the sample of fish gelatin with added stevia. FG was used as the model to obtain the target storage modulus (G0 ) after 3 h of cooling and gelling (holding) at 4 C:
tmodel ¼ e
(G0 C)/kgel
159
(2)
where kgel values for all of the samples studied were obtained by fitting Equation (1) to the experimental data. Consequently, the time (tmodel) required for a gelling system to reach G0 ref (target storage modulus) was calculated using Equation (2). The expression was applied to describe the changes in gelation time. 2.2.6. Frequency sweeps Frequency sweeps give an indication of the cross-linking behavior of gelatin. The method as described by Sarbon, Badii, and Howell (2015) was used with slight modification. A dynamic frequency sweep was done at 4 C with a strain of 5% and frequency oscillated from 0.01 to 10 Hz, within the previously identified LVR of the material. Results of the changes were expressed in terms of G0 and G00 as a function of dynamic frequency sweep. Frequency of oscillation was varied, and the temperature and strain were kept constant. The analyses were done in triplicate. 2.2.7. Statistical analysis All experiments were done in triplicate to get mean values plus standard deviations. SPSS 19 for Windows (SPSS Inc., Chicago, IL, USA) was used. The data were subjected to one-way analysis of variance (ANOVA) and differences of mean values were evaluated using Duncan’s multiple range test with a confidence level of p < 0.05. 3. Results and discussion 3.1. Proximate component and FTIR spectroscopy analysis The proximate composition of the gelatin was moisture 7.7 ± 0.1%, protein 89.3 ± 0.2%, fat 0.22 ± 0.06% and ash 0.13 ± 0.02%. The FTIR spectra of the grass carp skin FG with xylitol or stevia are shown in Figs. 1 and 2. The positions of the FTIR absorption peaks were used to determine the effects of the added sweeteners. The differences in the secondary structure of FGX and FGS were compared in six regions of the spectra (Kuan et al., 2016): 3600e3200 cm1 (Amide A, reflecting the OH and NeH stretches coupled with H-bond stretching); [22] 3100e3000 cm1 (Amide B, reflecting the CeH antisymmetric and symmetric stretching); 1700e1600 cm1 (Amide I, reflecting the C]O stretching and NH bending) (Jongjareonrak, Benjakul, Visessanguan, & Tanaka, 2008); 1570e1335 cm1 (Amide II, reflecting the NeH bending and CH2 stretching) (Yakimets et al., 2005); 1300e1000 cm1 (Amide Ⅲ, reflecting the NH bending and stretching coupled CN stretching) (Li, Miao, Wu, Chen, & Zhang, 2014); and 1000e500 cm1 (Fingerprint region, reflecting the CeO skeletal stretching, CeH deformation vibrations and CeC skeletal stretching) (Sow & Yang, 2015). Fig. 1 and Table 1 show the effects of the sweeteners on FG and the spectra for samples with and without the addition of xylitol. All samples showed wide and strong absorption peaks in the Amide A region and showed a typical Amide A band between 3568 and 3375 cm1, which was associated with the stretching vibrations of NeH and OH groups. From Fig. 1, the Amide A band slowly tended to shift to a lower wavenumber from 3433 to 3379 cm1 with increasing xylitol from 0 to 20%, suggesting that hydrogen bonding occurred between xylitol and gelatin molecules consistent with the nez, Pe rez-Santin, Go mez-Guille n, Alema n, reported results of Gime and Montero (2011). The Amide I was the most useful to analyze the microstructure of proteins. The Amide I band located at 1654 cm1 reflected the structure of gelatin. A very low intensity at high concentrations suggested the loss of the ordered molecular structure when xylitol
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
1654
2914 3379
FG+20% xylito 2914
Absorbance
3568 3568
FG+10%xylitol
2927
3402
FG+3%xylitol
2936
3402
2935 3568
FG+1% xylitol
3423
3800
3600
1340 1340
1064 1012 FG+20% xylitol
FG control
3433
3400
3200 3000 wavenumber (cm-1)
2800
1654
1550
1450
1550
1450
1342 1338
1550
1700
Amide A
1650
1600
1340
1450
1550 1500 1450 wavenumber (cm-1)
Amide B
912 858 885 912 885858
921
746
522
746
522
850
921 1080
FG+5% xylitol FG+3%xylitol
1654
1654
2600
1062 1012
1550
FG+1% xylitol 2939
3568
1550 1550
FG+10%xylitol
1654
FG+5% xylitol Absorbance
3375
1654
Absorbance
160
1350
1300
FG+20% xylitol
455 FG+10% xylitol FG+5% xylitol
850
1080
FG+3% xylitol
1082 1082 1031
974
875
974
875
FG+1% xylitol
FG control
FG control
1400
455
1100
1000
900
800 700 wavenumber (cm-1)
600
500
400
Amide I Amide II Amide III Fingerprint
FG+20% xylitol
FG+10%xylitol
Absorbance
FG+5% xylitol
FG+3%xylitol
FG+1% xylitol
FG control
4000
3500
3000
2500 2000 1500 -1 wavenumber (cm )
1000
500
Fig. 1. FTIR spectra of fish gelatin (FG) containing 0, 1, 3, 5, 10 and 20% xylitol.
was added. In many studies it has been pointed out that the Amide I absorption band which is due almost entirely to the C]O stretch vibrations of the peptide linkages can be used to determine changes in the secondary structure of proteins (Ahmad & Benjakul, 2011). This was consistent with expected changes if the xylitol became embedded into the three-dimensional network structure of the gelatin. The locations and assignments with stevia are shown in Fig. 2 and Table 2. The Amide A again suggested increased H bonding. The Amide B band tended to shift to a higher wavenumber with increasing stevia concentrations suggesting a decrease in the molecular order of gelatin which may be related to the decreased gel
textural properties, particularly gel strength (Muyonga, Cole, & Duodu, 2004a). The results also suggested that stevia was not participating in the gelatin structure. The Amide II peak decreased and probably shifted to a lower wavenumber, where it became part of the Amide III band, which has been suggested to reflect hydration (Benjakul, Oungbho, Visessanguan, Thiansilakul, & Roytrakul, 2009). The slight upshift of the main Amide III peak was attributed to intermolecular interactions between FG and both sugars. From FTIR, it can be suggested that xylitol or stevia reduced the free hydrogens available for bonding with H2O. The loss of helix suggested decreased molecular order, which was greater with
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
3568 3568 3568 3568
0 3800
3600
FG+20% stevia
2931 3423
2929
FG+ 5% stevia FG+ 3% stevia
2931 3402 2933
3423
FG+1% stevia FG control
2939
3433
3400 3200 3000 wavenumber (cm-1)
1550 1654
3392
2800
Amide A
1550
1654
1550
1654
1550
1651 1654
0 1700
2600
1340
1450
1654
FG+10% stevia
1650
1550
1600
1340
1450
1340
1452
1340 1340
1450
1340
1448
1550 1500 1450 wavenumber (cm-1)
1078 1028
FG+10% stevia FG+ 5% stevia
1450
1076 1028
FG+20% stevia
FG+ 3% stevia
Absorbance
Absorbance
3568
1550
2927 3410
Absorbance
3568
161
1078 1028 1078 1028 1080 1031
FG+1% stevia
850
920
846 850
921 921
850
748
559
761
FG+20% stevia 559
759
FG+10% stevia
559
761
FG+ 5% stevia
559 FG+ 3% stevia
873
559
873
FG+1% stevia
559
1082 1031
FG control
FG control
1400
1350
1300
0 1100
1000
Amide I Amide II Amide III
Amide B
923
900
800 700 wavenumber (cm-1)
600
500
400
Fingerprint
FG+20% stevia
Absorbance
FG+10% stevia FG+ 5% stevia FG+ 3% stevia FG+1% stevia FG control
0 4000
3500
3000
2500 2000 1500 -1 wavenumber (cm )
1000
500
Fig. 2. FTIR spectra of fish gelatin (FG) containing 0, 1, 3, 5, 10 and 20% stevia.
stevia. The difference in the two low calorie sweeteners was related to the chemical structures of xylitol (C5H12O5) and stevia (C38H60O18). This should be reflected by poorer gel texture, in particular gel strength, hardness and chewiness of stevia gels (Sow & Yang, 2015). 3.2. TPA and gel strength TPA is used in confectionery factories to evaluate texture characteristic so that, for example, gummy jellies can be handled ~o n, 2015). without risk of deformation or breakage (Delgado & Ban TPA testing is intended to simulate the action of the tongue and teeth (Yang et al., 2007). The TPA parameters are shown in Table 3. The hardness, cohesiveness, chewiness, resilience and
adhesiveness in FGX gels were higher than the FGS gels (p < 0.05). Gel quality is generally better if these texture values are higher (Sow & Yang, 2015). However, there was no significant difference in springiness between FGX and FGS gels (p 0.05). The decrease of the FGS gel strength at 5% stevia was significantly (p < 0.05) less than the control. As stevia concentration increased to 5% hardness, cohesiveness and chewiness decreased and became significant (p < 0.05). These values were similar to those of Ramírez, Uresti, zquez (2011) who found that adding sugars Velazquez, and Va decreased gel network formation. However, xylitol concentration had little effect on gel texture even at higher concentrations suggesting that xylitol is a potential low calorie sweetener to replace sucrose in gummi-type candies. Gel strength results are shown in Fig. 3. Gel strength values for
162
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
Table 1 Location and assignment of the peaks identified in FTIR spectra for gels prepared with fish gelatin (FG) containing xylitol. Region
Amide A Amide B AmideⅠ Amide Ⅱ
Amide Ⅲ Fingerprint
Peak wavenumber (cm1)
Assignment and remarks
None (blank)
1% xylitol
3% xylitol
5% xylitol
10% xylitol
20% xylitol
3568 3433 2939 1654 1550 1450 1340 1242 1082 1031 974
3568 3423 2935 1654 1550 1450 1338 1240 1082 e 974 e
3568 3402 2936 1654 1550 1450 1342 1240 1080 e 921 e
3568 3402 2927 1654 1550 e e 1242 1080 e 921 e
e 3375 2914 1654 1550 e 1340 1244 1062 1012 912 885
e 3379 2914 1654 1550 e 1340 1255 1064 1012 912 885
OH stretch coupled with H-bond NeH stretch coupled with H-bond CH antisymmetric and symmetric stretching C]O stretch/hydrogen bond coupled with COO NH bend coupled with CN stretch CH2 bending (scissors) vibration CH2 wag of proline and glycine NH bend stretch coupled CeN stretch CeO skeletal stretch CeO skeletal stretch CeH deformation vibration (carbohydrate) CeH deformation vibration (carbohydrate)
e
875 e
850 e
850 e
858 746
858 746
CeH deformation vibration (carbohydrate) CeC Skeletal stretch
e
e
e
e
522
522
CeC Skeletal stretch
e
e
e
e
455
455
CeC Skeletal stretch
e 875
Table 2 Location and assignment of the peaks identified in FTIR spectra for gels prepared with FG containing stevia. Region
Amide A Amide B Amide I Amide II
Amide III Fingerprint
Peak wavenumber (cm1)
Assignment and remarks
None (blank)
1% stevia
3% stevia
5% stevia
10% stevia
20% stevia
3568 3433 2939 1654 1550 1450 1340 1242 1082 1031
3568 3423 2933 1651 1550 1450 1340 1240 1080 1031 e
3568 3402 2931 1654 1550 1452 1340 1240 1078 1028 921
3568 3392 2929 1654 1550 1450 1340 1242 1078 1028 921
3568 3423 2931 1654 1550 1450 1340 1242 1078 1028 920
3568 3410 2927 1654 1550 1448 1340 1240 1076 1028 923
OH stretch coupled with H-bond NeH stretch coupled with H-bond CH antisymmetric and symmetric stretching C]O stretch/hydrogen bond coupled with COO NH bend coupled with CN stretch CH2 bending (scissors) vibration CH2 wag of proline and glycine NH bend stretch coupled CN stretch CeO skeletal stretch CeO skeletal stretch CeH deformation vibration (carbohydrate)
873 e
846 761
850 759
846 761
850 748
CeH deformation vibration (carbohydrate) CeH deformation vibration (carbohydrate)
559
559
559
559
559
CeC Skeletal stretch
e 873 e 559
xylitol was higher than the control, and the sample at 3% xylitol had the highest gel strength. Above 3% xylitol gel strength decreased but was still higher than the control. The xylitol seems to favor network formation and strengthens the three-dimensional
FGS and FGX were within the range required for commercial products, i.e., 7.6e30.6 N (Tavakolipour, 2011). The gel strength of FGS at different concentrations of stevia was lower than the control. However, the gel strength of FGX at different concentrations of
Table 3 TPA parameters (hardness, cohesiveness, chewiness, springiness, resilience and adhesiveness) fish gelatin added with xylitol (FGX) and stevia (FGS). Concentration (%)
Hardness (N)
Cohesiveness
Chewiness (N)
FGX
FGS
FGX
FGS
FGX
FGS
0 1 3 5 10 20
2.20 ± 0.01aB 2.3 ± 0.1aB 2.2 ± 0.1aB 2.2 ± 0.1aB 2.37 ± 0.04aA 2.3 ± 0.1aAB
2.20 ± 0.01aA 2.00 ± 0.02bB 1.91 ± 0.01bC 1.76 ± 0.04bD 1.67 ± 0.03bE 1.6 ± 0.1bE
2.00 ± 0.00aB 2.0 ± 0.1aAB 2.0 ± 0.1aAB 2.0 ± 0.1aB 2.05 ± 0.04aA 2.0 ± 0.1aB
2.00 ± 0.00aA 1.83 ± 0.03bB 1.76 ± 0.02bC 1.65 ± 0.04bD 1.52 ± 0.01bE 1.4 ± 0.1bF
1.88 ± 0.02aB 1.9 ± 0.1aB 1.9 ± 0.1aB 1.9 ± 0.1aB 1.9 ± 0.1aA 1.9 ± 0.1aB
1.88 ± 0.02aA 1.71 ± 0.02bB 1.65 ± 0.00bBC 1.6 ± 0.1bC 1.43 ± 0.03bD 1.4 ± 0.1bD
Concentration (%)
Springiness FGX
0 1 3 5 10 20
0.94 0.94 0.93 0.93 0.93 0.94
Resilience FGS
± ± ± ± ± ±
0.01aA 0.00aA 0.01aA 0.00aA 0.00aA 0.01aA
0.94 0.93 0.94 0.93 0.94 0.94
FGX ± ± ± ± ± ±
0.01aAB 0.02aB 0.01aB 0.02aA 0.01aAB 0.01aAB
0.83 0.83 0.84 0.83 0.84 0.82
Adhesiveness (N s) FGS
± ± ± ± ± ±
0.01aB 0.01aB 0.01aAB 0.01aAB 0.01aA 0.01aB
0.83 0.84 0.82 0.82 0.81 0.81
FGX ± ± ± ± ± ±
0.01aAB 0.01bAB 0.03bB 0.02bA 0.00bB 0.01bB
0.91 0.90 0.91 0.90 0.91 0.90
FGS ± ± ± ± ± ±
0.00aA 0.01aAB 0.00aA 0.01aA 0.00aA 0.00aB
0.91 0.92 0.92 0.94 0.91 0.90
± ± ± ± ± ±
0.00aBC 0.01bB 0.01bB 0.01bA 0.01bBC 0.00bC
Data are means ± SD. Within each row, means with different lowercase letters are significantly different (p < 0.05) among different groups. Within each column, means with different capital letters are significantly different among different concentrations (p < 0.05).
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
FGX FGS
A D
2.5
a
E
B
C
b c
d
Gel Strength (N)
2.0
103
A CD e
102
f
1.5
1.0
Storage /Loss modulus (Pa)
3.0
163
0.5
y = 326Ln(x) + 505 2 R = 0.91
101
100
Storage modulus (G,) Loss modulus (G,,)
10-1
10-2
0.0 0
1%
3%
5%
Concentration
10%
10-3
20%
Fig. 3. Gel Strength of fish gelatin (FG) and fish gelatin added with different concentrations of xylitol (FGX) and stevia (FGS). Different capital letters mean significantly different (p < 0.05) with different FGX concentrations. Different lowercase letters mean significantly difference (p < 0.05) with different FGS concentrations. Bars represent standard deviations (n ¼ 3).
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Gelatin Times (hrs)
B
104
3.3. Gelation kinetics Fig. 4 shows the typical development of the storage modulus and loss modulus (G00 ) with time for FG, FGX and FGS at 4 C. The experimental data were fitted into a logarithmic function model to determine the gelation rates. G0 of the control group was calculated to obtain the target G0 . The time to reach the target G0 was then calculated for the various treatments. The model has one adjustable parameter (C) which was estimated by minimizing the R2 value. The detailed data for each system is summarized in Table 4. The kgel values for the FGS gelling system decreased upon the initial addition of stevia and then increased upon further addition of stevia at high concentrations. However, the kgel of FGX in the time sweep decreased from 1 to 20% xylitol. Prior research suggested that gelation was inhibited by the rearrangement of gelatin chains in the presence of sugars, as the addition of sugars increases the viscosity of bulk water around the sugar molecules in the continuous phase (Kuan et al., 2016). Uedaira, Ikura, and Uedaira (1989) suggested that sugar molecules containing a large number of OH groups have stronger stabilizing effects on the structure of water surrounding the sugar molecules, which is very much related to their solubility in water. Xylitol had many OH groups and had a greater influence on kgel than stevia. In addition, the solubility of xylitol in water is higher than that of stevia, so a more significant increase of kgel was observed for high concentrations of xylitol. Fig. 4 also shows that during the time sweeps the rate of G0 and G00 increased rapidly for about 0.5 h, the slope continued to increase at longer times but at a diminishing rate, which was consistent with a study by Boran, Mulvaney, and Regenstein (2010). G0 was much higher than G00 after 0.5 h of gelation in all samples, which indicated that the system at 4 C had more solid-like characteristics where the gel structures were been forming during the period of the time sweeps (Zaidel, Chronakis, & Meyer, 2012). The result suggested that as the gel network structures were forming, the rate of increase of G0 with time decreased. The changes in G0 and G00 probably reflected the inter-chain associations that lead to the formation of (G0 )
y = 126Ln(x) + 251 2 R = 0.93
102 101
Storage modulus (G,) Loss modulus (G,,)
100 10-1 10-2 10-3
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Gelatin Times (hrs)
103
C 102 Storage / Loss modulus (Pa)
network (Zhang et al., 2016), but excessive crosslinking may lead to a poorer gel. Gel strength showed a similar trend as the TPA.
Storage/Loss Modulus (Pa)
103
y = 148Ln(x) + 505 2 R = 0.92
101
100
Storage modulus (G,) Loss modulus (G,,)
10-1
10-2
10-3
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Gelatin Times (hrs) Fig. 4. Storage modulus (G0 ) and loss modulus (G00 ) for time sweeps at 4 C in gelatin dispersions of FG (control) (A), 20% FGX (B) and 10% FGS (C).
164
L. Cai et al. / Food Hydrocolloids 67 (2017) 157e165
Table 4 Parameters for the analogue function model used to fit the gelation profiles of gelatin gelling systems containing xylitol and stevia for time sweeps. Gelling system
FG control FGþ1% xylitol FGþ3% xylitol FGþ5% xylitol FGþ10% xylitol FGþ20% xylitol FGþ1% stevia FGþ3% stevia FGþ5% stevia FGþ10% stevia FGþ20% stevia
Gelation rate G0 gel
R2
Kgel
C
t-model (hr)
827 827 827 827 827 827 827 827 827 827 827
0.91 0.92 0.92 0.92 0.92 0.93 0.93 0.92 0.92 0.92 0.92
326 154 161 154 150 126 181 161 159 148 144
505 526 547 527 544 251 613 572 529 505 492
3.00 7.20 5.70 7.06 5.99 96.5 3.25 4.86 6.48 8.77 10.2
3.4. Frequency sweeps
Logarithmic model equation: G0 ¼ kgel Ln(tgel)þC where G0 ¼ target storage modulus (Pa) obtained from the control (FG), kgel ¼ gelation rate constant, tgel ¼ gelation time (h) and C ¼ constant.
cross-links or junction zones (Kjøniksen, Hiorth, & Bo, 2005). The gelatin gels with various concentrations of xylitol had higher G0 values than the pure gelatin and gelatins with stevia, which
10
The results showed that the intensities of the six regions (Amides A, B, I, II, III and Fingerprint region) in the FTIR spectroscopy increased with increasing amounts of xylitol and stevia, suggesting decreased molecular order. The addition of sweeteners inhibited the gel network formation by lowering the kgel at higher concentrations of xylitol and stevia. In addition, the highest gel strength was found when xylitol was added at 3% concentration, and FGX had stronger gel strength than FGS, which could be attributed to a great number of eOH groups and high solubility of xylitol. Similar trends were observed for the effects of solutes on gelation kinetics. The results suggested that the addition of xylitol may have more potential applications than stevia in confectionery gels.
103 G’,G”(Pa)
FG FG+ 1% xylitol FG+ 3% xylitol FG+ 5% xylitol FG+ 10% xylitol FG+ 20% xylitol
102
Typical frequency sweeps at 4 C with different viscoelasticities in the range from 0.01 to 10 Hz are shown in Fig. 5 G0 and G00 showed changes dependent on the frequency. The G0 and G00 values of FGX decreased above 3% xylitol but were still higher than the pure gelatin. This suggested that the intermolecular interactions of the gelatin-xylitol mixture were stronger than that of the FG. The G0 values were higher than G00 which again suggested that the gelling systems at 4 C had more solid-like characteristics. The loss factor (tan d) decreased slightly upon the addition of stevia, suggesting that the three-dimensional network structure would aggregate slowly because of the increased viscosity of the continuous phase upon addition of sugars, thus kinetically slowing the approach of the gelatin chains to each other. 4. Conclusions
A
4
suggested that the addition of xylitol led to a synergistic effect on the viscoelastic properties of FG. The results were in accordance with the previous reports for FG gel formation with added sucrose (Kuan et al., 2016) and glucose (Evageliou, Mazioti, Mandala, & Komaitis, 2010).
FG FG + 1% xylitol FG + 3% xylitol FG + 5% xylitol FG + 10% xylitol FG + 20% xylitol
101
Acknowledgment
100 10-2
10-1
100
f (Hz)
101
This study was supported by the National Natural Science Foundation of China (31401478), the National Postdoctoral Science Foundation of China (2015M570760), China Scholarship Council (201508210023), the Postdoctoral Special Funding of Chongqing City (Xm2015021), the Research Project from Science & Technology Department of Liaoning Province of China (No. 2015103020).
B
103
References
2
G',G"(Pa)
10
FG FG+ 1% stevia FG+ 3% stevia FG+ 5% stevia FG+ 10% stevia FG+ 20% stevia
FG FG + 1% stevia FG + 3% stevia FG + 5% stevia FG + 10% stevia FG + 20% stevia
101
100 10-2
10-1
f (Hz)
100
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Fig. 5. Storage modulus (G0 ) and loss modulus (G00 ) for frequency sweeps (0.01e10 Hz) at 4 C for FG containing 0, 1, 3, 5, 10 and 20% xylitol (A) and FG containing 0, 1, 3, 5, 10 and 20% stevia (B).
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