The functional properties and application of gelatin derived from the skin of channel catfish (Ictalurus punctatus)

Food Chemistry 239 (2018) 464–469

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The functional properties and application of gelatin derived from the skin of channel catfish (Ictalurus punctatus) Rui Duan a,b,⇑, Junjie Zhang a,b, Liping Liu a, Wenzhe Cui a, Joe M. Regenstein c a

College of Marine Life and Fisheries, Huaihai Institute of Technology, 59 Cangwu Road, Haizhou, 222005, China Jiangsu Key Laboratory of Marine Pharmaceutical Compound Screening, Huaihai Institute of Technology, 59 Cangwu Road, Haizhou, 222005, China c Department of Food Science, Cornell University, Ithaca, NY 14853-7201, USA b

a r t i c l e

i n f o

Article history: Received 11 January 2017 Received in revised form 30 May 2017 Accepted 27 June 2017 Available online 28 June 2017 Keywords: Catfish Gelatin Functional property

a b s t r a c t The objective of this work was to extract gelatin from skin of channel catfish (Ictalurus punctatus) and to study its functional properties and application in ice-cream and beer. SDS-PAGE patterns showed that channel catfish gelatin (CCG) had more high molecular weight components (b and c chains) than had calf bone gelatin (CBG). The viscosity of CCG was 42 mPa.s at 10 °C, which was three times more than that of CBG. Compared to CBG, CCG presented higher emulsion capacity and stability, as well as higher foaming stability. CCG conferred the ice-cream samples and beer with better mouth feel and clarification effect, respectively. The results indicated that CCG had great potential to be utilized in the food industry. Ó 2017 Published by Elsevier Ltd.

1. Introduction Gelatin is an irreversibly hydrolyzed form of collagen, wherein the hydrolysis results in the reduction of protein fibrils into smaller peptides (Wikipedia). Gelatin has broad applications in many fields, such as food, pharmaceutical, cosmetic, biomedical materials and other industries, due to its unique chemical and physical properties (Jamilah & Harvinder, 2002). Gelatins are produced on a large scale from skins and bones of land mammal (mainly bovine and porcine) by alkaline or acidic extraction (Veis, 1964). In recent years, the use of fish skin and bone to produce gelatin is gaining interest because of its greater safety, which is unlikely to be related to bovine spongiform encephalopathy (BSE) and transmissible spongiform encephalopathy (TSE). The gelatins from fish could also be accepted in some areas due to religious objections to those from bovine and porcine sources (Sadowska, Koladziejaka, & Niecikowska, 2003). Channel catfish (Ictalurus punctatus) is one of the most important freshwater fishes in the United States, with a catch of approximately 8 million per year (Keenan, Warner, Crowe, & Courtney, 2011). China first imported channel catfish from America in 1984. In 1993, China started to culture the fish in cages and in 2003 cultured on a large scale. In 2000 China started exporting the channel catfish to America. According to the U.S. Department of Agriculture, Chinese total cultured catfish production was ⇑ Corresponding author at: College of Marine Life and Fisheries, Huaihai Institute of Technology, 59 Cangwu Road, Haizhou, 222005, China. E-mail address: [email protected] (R. Duan). http://dx.doi.org/10.1016/j.foodchem.2017.06.145 0308-8146/Ó 2017 Published by Elsevier Ltd.

estimated at 610,000 tons in 2012, a rise from 598,000 tons in 2011 in response to dynamic domestic consumption. Current annual processing of channel catfish amounts to 150,000– 200,000 tons (Seafood News Aquaculture, 2013). Fish skin, the byproduct of fish processing needs to be used effectively. Several papers have dealt with the extraction and rheological properties (gel strength, viscosity and texture) of gelatin from skin and bone of catfish (Liu, Han, & Guo, 2009; Yang, Wang, Zhou, & Regenstein, 2008; Zhang, Liu, & Li, 2009). However, few reports could be found concerning functional properties and application of gelatin from catfish skin. In this study, gelatin from channel catfish skin was extracted and the functional properties and application of the gelatin in ice-cream and beer clarification were investigated. 2. Materials and methods 2.1. Materials Frozen channel catfish (Ictalurus punctatus) skin was provided by Yancheng Tianwei Aquatic Food Co., Ltd. (Yancheng city, Jiangsu, China), and stored at
25 °C until used. Calf bone gelatin (CBG) was purchased from Jialida Gelatin Co. (Cangnan, Zhejiang, China). 2.2. Methods 2.2.1. Channel catfish gelatin (CCG) extraction Before use, the skin was thawed below 10 °C and the adherent tissues of skin were scraped off manually. After thawing, the skins

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were cut into small pieces by scissors at the size of 1  1 cm and mixed well. The scissored skins were mixed with 0.1 M NaOH for 6 h with continuous stirring at a sample/alkali solution ratio of 1:8 (w/v) to remove non-collagenous proteins. The alkali solution was changed every 3 h. Then the samples were washed with cold distilled water, until neutral pH of washing water was obtained. The skins were then soaked in 10% butyl alcohol with a solid/solvent ration of 1:10 (w/v), overnight, to remove fat and then washed with cold distilled water repeatedly (Nagai et al., 2000). All procedures were carried out at 4 °C. Then gelatin was extracted from channel catfish skin by the method described by Jongjareonrak et al. (2010) with some modification. CCG was collected by centrifugation at 5000g for 30 min, and then the supernatant was lyophilized. Triplicate extractions were performed. 2.2.2. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) SDS-PAGE was performed by the method of Laemmli (1970). The gelatin samples were dissolved in distilled water. Then the samples were mixed with the sample buffer (0.5 M Tris-HCl, pH 6.8, containing 5% SDS, 20% glycerol) at 1:2 ratio. Electrophoresis was performed on 7.5% gels. High molecular weight markers (Sigma-Aldrich Chemical Co., St. Louis, Mo., USA) were used to estimate the molecular weight of proteins. 10 lg of protein were loaded in each well. 2.2.3. Viscosity changes upon cooling and heating The viscosity changes of gelatin samples upon cooling and heating were determined by the method described by Kittiphattanabawon, Benjakul, Visessanguan, Nagai, and Tanaka (2005) with some modification. Gelatin solution (1.4%, 10 ml) was prepared by dissolving lyophilized gelatin in distilled water at 40 °C for 15–20 min. The viscosity studies of gelatin solutions were performed on a circumvolving viscometer (model NDJ-7, Tongji university Labs Inc., Shanghai, China). Gelatin solution was first cooled from 40 to 5 °C, kept at 5 °C for 5 min, then heated from 5 to 40 °C. The cooling and heating rates were both 2 °C/min. At the designated temperature, the solution was held for 5 min prior to viscosity determination. The measurements were done in triplicate. 2.2.4. Emulsifying properties Emulsion activity index (EAI) and emulsion stability index (ESI) of gelatin were determined at 20 °C according to the method of Pearce and Kinsella (1978) with a slight modification. Soybean oil (2 ml) and gelatin solution (2%, 6 ml) were homogenized (Model PRO-250, PRO Scientific Inc. Monroe, CT, USA) at a speed of 20,000 rpm for 1 min. 100 ll emulsions were pipetted out at 0 and 10 min and diluted with 5 ml of 0.1% SDS. The mixture was mixed thoroughly for 10 s, using a homogenizer (Model PRO-250, PRO Scientific Inc. Monroe, CT, USA). A 500 ml quantity of the resulting dispersion was measured, using a spectrophotometer (UV-160, Shimadzu, Japan). EAI and ESI were calculated by the following formulae:

EAI ðm2 =gÞ ¼ 2:303  A0 =ð0:25  protein weight ðgÞÞ

ð1Þ

where A0 ¼ absorbance at 500 nm

ð2Þ

ESI ðminÞ ¼ A0  Dt=DA

ð3Þ

DA ¼ A0
A1 0min

ð4Þ

Dt ¼ 10 min

ð5Þ

465

2.2.5. Foaming properties Foam capacity (FC) and foam stability (FS) of gelatin samples were determined according to the method of Cho et al. (2004) with some modification. The gelatin solution, at the concentration of 2%, was prepared and then incubated in a water bath at 40 °C for 30 min. The sample was left at 20 °C for 20 min, followed by homogenization at a speed of 13,000 rpm, using a homogenizer (Model PRO-250, PRO Scientific Inc. Monroe, CT, USA) to incorporate air for 1 min. Then the whipped sample was immediately transferred into 25 ml cylinders and stood for 0 and 3 min. The foaming capacity was calculated according to the following equation:

FC ð%Þ ¼ ðVT
V0 Þ=V0  100

ð6Þ

where V0 is the original volume before whipping (ml); VT is total volume after whipping (ml). The whipped sample was allowed to stand at ambient temperature for 3 min and the volume of whipped sample was then recorded. Foam stability was calculated as follows:

FS ð%Þ ¼ ðVt
V0 Þ=V0  100

ð7Þ

where Vt is the total volume after leaving at 20 °C for 3 min. 2.2.6. Application of gelatins in ice-cream 2.2.6.1. Preparation of ice-cream samples. The milk, skimmed milk powder and cream were provided by Xinxiwang Dairy Co. Ltd. (Lianyungang, Jiangsu Province). Mix formulation was: 8.3% (w/ w) milkfat, 11.4% (w/w) milk solids not fat, 12.2% (w/w) sugar, 0.5% (w/w) gelatin and 67.4% (w/w) water. Water, skimmed milk powder and cream were mixed and temperature was increased to 50 °C; the ice cream mixture was divided into three parts of 1 L each (A, B, and C). The blends of CCG, along with sugar, were added to sample A while CBG, along with sugar, were added to sample B. Sugar was added to sample C. The resultant mixtures were flavoured with vanillin and homogenized with a first stage pressure of 13.8 MPa and a second stage pressure of 3.45 MPa (MFG Company, Chicago, IL). Then the mixtures were pasteurized at 75.0 °C for 10 min and, after pasteurization, the mixtures were cooled to 4 °C and aged for 18 h. Mixes were frozen in random order, using a portable vertical freezing machine of 1.5 L capacity (Caple ICE1510, Guangdong Province, China), then packaged in 200 ml polyfoam cups with cardboard lids, and stored in a hardening room at
30 °C until sensory tests were conducted. Prior to testing, samples were tempered at
18 °C for 15 h. 2.2.6.2. Sensory analysis. Panellists (6 males and 6 females) from the food engineering department, Huaihai Institute of Technology, were recruited for evaluation of the products. The ice cream samples were organoleptically assessed by the panellists under fluorescent white light, using the following score ratings: (a) eight attributes for flavour and taste (no criticism: 10, cooked flavour: 9–7, lack of sweetness and too sweet: 9–7, lack of flavour: 9–6, yogurt/probiotic flavour: 8–6, acidic/sour: 8–6, rancid and oxidized: 6–1, and other: 5–1), (b) seven characteristics of body and texture (no criticism: 10, crumbly: 8–4, coarse: 8–4, weak: 8–4, gummy: 8–4, fluffy: 6–2, sandy: 4–1) and (c) four terms describing colour and appearance (no criticism: 5, pale colour: 4–1, nonuniform colour: 4–1, unnatural colour: 3–1), as described by Homayouni, Azizi, Ehsani, Yarmand, and Razavi (2008) with some modification. Samples, labelled with 3-digit codes, were served at
10 °C in the polyfoam cups in which they were frozen. Judges were instructed to rinse with distilled water between samples. 2.2.7. Application of gelatins in beer clarification The raw beer (fresh beer without filtration and sterilization) was provided by Bioengineering Laboratory, Huaihai Institute of Technology (Lianyungang, Jiangsu, China) and transported to our

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lab by beer barrel (4 °C). 5 ml of beer were transferred into each test tube (12 mm  75 mm). CCG and CBG were dissolved with distilled water to obtain 2% (w/v) solutions, respectively. Then the gelatin solutions were added to beer at different concentrations and stirred with a vortex mixer (XW-80A, Huixi Qinpu. Inc. Shanghai, China) for 30 s. The samples were kept in the refrigerator (4 °C) for 6 h. One sample was kept for 24 h for natural precipitation. Then the supernatants of the beer were taken out and the transmittance was determined at 660 nm, using a 722 spectrophotometer (Shanghai Analytical Instrument Factory, China). The percent transmittance was considered a measure of clarity (Krop and Pilnik, 1974). The raw beer was employed as the control. 2.2.8. Statistical analysis One-way analysis of variance was carried out in some determinations, using the Tukey test of computer program SPSS-8 (SPSS Co., Chicago, Illinois). The confidence level was 95%. All experiments were conducted in triplicate. 3. Results and discussion 3.1. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) The subunit components of catfish skin gelatin and calf bone gelatin (CBG) were analyzed by polyacrylamide gel electrophoresis in the presence of SDS. As shown in Fig. 1, CCG had more high molecular weight components (b and c chains) than had CBG. Little degradation of the subunits could be observed for CCG, while the subunit components of CBG were obviously degraded, showing many fragments in the pattern. Protein bands with molecular weights lower than a chains (100 kDa) could be due to the degradation of a, b and c components during gelatin extraction. Therefore, the pattern of CCG was distinguished from that of CBG, showing little degradation and higher integrity of the gelatin molecules. 3.2. Viscosity changes upon cooling and heating The viscosity changes of gelatin solutions upon cooling and heating were recorded. Fig. 2 shows that, when the samples were

Fig. 2. Viscosity changes of CCG and CBG upon cooling and heating.

cooled from 40 to 5 °C, the viscosities increased obviously. Sharp increase of viscosity could be seen when the temperature decreased below 20 °C. Compared to CBG, CCG had more significant change of viscosity upon cooling and the viscosity values were always higher than those of CBG. The viscosity of CCG was 42 mPa. s at 10 °C, while that of CBG was 11 mPa.s, only 1/4 of the former. After being kept at 10 °C for 5 min, CCG began to form gel and it was hard to determine viscosity, but CBG still maintained a solution, showing viscosity of 23 mPa.s at 5 °C. When the samples were heated from 5 to 40 °C, viscosity of CCG and CBG decreased. However, CCG had a better viscosity property than had CBG during the heating period. Hence, for the viscosity property, CCG was superior to CBG. The viscosity of gelatin solutions varies with sources in terms of molecular weight and molecular size distribution of gelatin subunits, i.e. degradation extent (Sperling, 1985), concentration, pH (Cho, Jahncke, Chin, & Eun, 2006), as well as temperature (Arnesen & Gildberg, 2007). The difference between CCG and CBG could be explained by the different extents of degradation. For CBG, only a faint band of b and c components could be seen. The densities of a chains decreased, due to the obvious degradation. Conversely, CCG had almost no degradation, showing high contents of the subunit components. The excellent viscosity property of CCG may indicate its application in food products. 3.3. Emulsifying properties

Fig. 1. SDS-polyacrylamide gel electrophoresis of CCG and CBG. lane 1: CBG; lane 2: CCG; lane 3: high molecular weight marker.

The emulsion activity index (EAI) and emulsion stability index (ESI) of CCG and CBG are shown in Fig. 3A and B. The EAI was determined by the turbidity of the emulsion at a wavelength of 500 nm, estimating the ability of the protein to aid the formation and stabilization of newly created emulsion by giving units of area of the interface. Differences in EAI of the gelatins were observed. EAI of CCG was lower than that of CBG, but CCG showed much higher ESI than did CBG. This indicated that the emulsion of CCG was more stable than that of CBG. The mechanism to generate the emulsion system is attributed to the adsorption of peptides on the surface of freshly formed oil droplets during homogenization and the formation of a protective membrane that inhibits coalescence of the oil droplet (Dickinson & Lorient, 1994). Mutilangi, Panyam, and Kilara (1995) found higher contents of larger molecular weight peptides or more hydrophobic peptides contributing to the stability of the emulsion. Higher content of intact a and b chains resulted in the high stability of emulsion, caused by much more hydrophobic interaction with oil. Different EAI and ESI of CCG and CBG were as a result of different polypeptide

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Fig. 3. Emulsifying and foaming properties (EAI, ESI, FE and FS) of gelatins. (A) emulsifying capacity; (B) emulsifying stability; (C) foaming capacity; (D) foaming stability. Bars represent the standard deviation (n = 3). Data are shown as means ± SD (n = 3).

compositions of the two gelatins, i.e., little degradation of CCG may be responsible for the high emulsifying stability. Shorter polypeptides in CBG seemed more suitable for the dispersion of oil at the O/W interface. However, emulsion formed by CCG was more stable than that by CBG. 3.4. Foaming properties Fig. 3C and D present the foaming capability (FC) and foaming stability (FS) of CCG and CBG. The volume of the homogenized CCG solution was clearly larger than that of CBG, showing the foam capacities of CCG and CBG were 36.7% and 26.7%, respectively. It was noticeable that CCG exhibited a higher value of foam stability than did CBG. Foaming stability of CCG was o.33 min, while that of CBG was 0.13 min. Therefore, it was clear that CCG showed a

higher foaming capacity and stability than did CBG. Forming property is one of important characteristics of gelatin solutions. A protein must be capable of migrating rapidly to the air–water interface, unfolding and rearranging at the interface to express good foaming ability (Halling, 1981). Besides, foaming stability of protein solutions is generally positively correlated with molecular weight of peptides (Van der Ven, Gruppen, de Bont, & Voragen, 2002). Larger and longer peptides could stabilize the protein film at the interface more effectively. Shorter peptides cannot maintain stable foam (Shahidi, Han, & Synowiecki, 1995). Therefore, the foaming properties (FC and FS) of CBG were lower than those of CCG, which might be due to the intrinsic property as determined by the source of the protein and different degradation extents of polypeptide chains of gelatins (Phillips, Whitehead, & Kinsella, 1994; Wilde & Clark, 1996; Zayas, 1997). CCG with intact subunit

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components seemed to have more strongly formed flexible films around the air bubbles. 3.5. Application of gelatins in ice-cream The sensory scores of the ice cream samples are given in Table 1. Mean descriptive ratings for the 3 attributes across the 3 samples were compared. The points allocated for colour, body-texture and taste showed that the addition of gelatins had an effect on sensory properties of the ice cream. Colour differences were not observed among the three samples, but differences in body and texture were found. The mouth-feel of the ice-cream control sample (Sample C) was inferior to the mouth feels of other samples. Sensory descriptions of Sample C were icy, coarse and gritty, while the samples A and B were perceived more positively, with a thicker and smoother texture. The results illustrated that the addition of gelatin altered the textures of icecream samples. The viscosities of the mixes with added gelatin showed significant increases. The rating of the ice-cream with addition of CCG (sample A) was obviously higher than that of Sample B, which was accordant with the viscosity properties of CCG and CBG. Besides, the better emulsion stability and foam capacity and stability of CCG also contributed to the best texture of sample A. The texture of frozen dairy products is related to the ability of the protein–fat matrix to form a cohesive network to yield a uniform fat–liquid emulsion (Guinard et al., 1997). Gelatin could bind

water and form a gel network that would modify the rheology of the mix. The gelling properties of CCG at low temperature resulted in decreased rates of ice crystallization. Similarly, the increased stickiness of sample A could be related to the formation of a viscous gel matrix. However, sweetness of sample C was higher than those of samples A and B. Also, sample C had a higher vanilla flavour than had the other two samples, exhibiting the highest ratings of flavour and taste (Table 1). Total evaluations in term of colour, texture and taste of all samples were good, but the icecream with addition of CCG showed the highest total acceptability. 3.6. Application of gelatins in beer clarification The data in Table 2 indicate that the additions of CCG and CBG influence the % T of the beer. This suggested that there was an interaction of gelatin with constituents in the beer. Fig. 4 shows that the raw beer had a dense cloud, the % T of which was only 17.6%. After the sample was kept for 24 h for natural precipitation, an obvious increase in sedimentation was observed as evidenced by the aggregation of sedimentation at the bottom of the tube, showing % T of the supernatant was 68.7%. When CCG was added to the beer at the concentrations of 0.02% and 0.04%, the% T increased to 93.0% and to 91.1%, respectively. There appeared to be two distinct sections and the supernatants were very clear (Fig. 4). After the addition of CBG to the beer, an increase in % T and sedimentation was also observed. The % T

Table 1 Sensory properties of ice-cream. Samples

A B C

Colour and appearance*

Body and texture

Flavour and taste

Total acceptability

1–5

1–10

1–10

1–25

4.82 ± 0.08a 4.81 ± 0.12 a 4.82 ± 0.06 a

9.36 ± 0.10a 8.59 ± 0.08b 8.16 ± 0.21c

8.91 ± 0.22a 8.95 ± 0.19a 9.18 ± 0.08a

23.06 ± 0.85a 22.97 ± 0.37a 20.16 ± 0.48b

A: ice-cream with addition of 0.5% CCG; B: ice-cream with addition of 0.5% CBG; C: ice-cream free of gelatin. *Mean values from 12 panellists. a, b, c: Different superscripts in the same column indicate significant difference at p < 0.05.

Table 2 Transmittance of beer after clarification with CCG and CBG by using the raw beer as control. Samples

Transmittance (%) *

Raw beer

17.6

Beer NP*

68.7

Beer with addition of CCG

Beer with addition of CBG

0.02%

0.04%

0.06%

0.02%

0.04%

0.06%

93.0

91.1

88.3

81.6

79.8

78.5

Beer clarified by natural precipitation.

Fig. 4. Clarification of beer with addition of gelatins. (A) Control; (B) Beer of natural precipitation; (C) 0.02% CCG; (D) 0.02% CBG; (E) 0.04% CCG; (F) 0.04% CBG.

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values were 81.6% and 79.8% with the addition of CBG at 0.02% and 0.04%, respectively. It was obvious that, for beer clarification, the addition of gelatins was more effective than was natural precipitation. Furthermore, compared to CBG, CCG showed higher ability in beer clarification, which had a more transparent supernatant. There were decreases in % T when the concentration of CCG and CBG in the beer increased. In the presence of 0.06% CCG, the % T of beer decreased from above 93.0% T to about 88.3% T. A similar trend was also seen for CBG at 0.06%. Increased cloudiness could be observed at concentrations above 0.06%. The results showed that CCG could be used as a clarification reagent in beer, but controlling concentration was a key point. When the concentration was higher than 0.04%, the addition of gelatin would increase the viscosity of beer, which could result in slow sedimentation and cloudiness in beer. Liu et al. (2009) reported that the gelatin from head bone of channel catfish showed an ability for clarification of apple juice. For beer, gelatins could fluctuate the live yeasts and bind the protein into a jelly-like mass, which aggregated at the bottom of the containers (Fig. 4). The clarification function of gelatins was due to the flocculation effect of the peptides, which possessed a large amount of active groups. The active groups could react with live yeasts and other proteins by electric charge effects (Oberholster, Carstens, & du Toit, 2013). Based on the results, CCG extracted from channel catfish skin could be employed in the beer as a clarifying agent. 4. Conclusion Gelatin was extracted from skin of channel catfish. The functional properties (viscosity, emulsion and foaming properties) and the utilizations of CCG were studied. The SDS-PAGE patterns showed that CCG had more high molecular components than had CBG. CCG was superior to CBG in foaming capacity and stability, as well as emulsion stability. CCG was successfully used in icecream and beer, showing good mouth-feel and clarification effects, respectively. The study indicated that gelatin from channel catfish skin had great potential to be utilized in different food products. Acknowledgements This work was funded by the Open-end Funds of Jiangsu Key Laboratory of Marine Pharmaceutical Compound Screening (HY201604) and the project of the Priority Academic Program Development of Jiangsu Higher Education Institutions and by the grants from the Bureau of Science and Technology of Jiangsu Province through Research Project # BN2012016. References Arnesen, J. A., & Gildberg, A. (2007). Extraction and characterization of gelatin from Atlantic salmon (Salmo salar) skin. Bioresource Technology, 98, 53–57. Cho, S.-H., Jahncke, M. L., Chin, K. B., & Eun, J. B. (2006). The effect of processing conditions on the properties of gelatin from skate (Raja kenojei) skins. Food Hydrocolloids, 20, 810–816.

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