Food Hydrocolloids 20 (2006) 810–816 www.elsevier.com/locate/foodhyd
The effect of processing conditions on the properties of gelatin from skate (Raja Kenojei) skins Soung-Hun Choa, Michael L. Jahnckec, Koo-Bok Chinb, Jong-Bang Euna,* a
Department of Food Science and Technology and Institute of Biotechnology, Chonnam National University, 300 Yongbong-dong Buk-gu, Gwangju 500-757, South Korea b Department of Animal Science, Chonnam National University, 300 Yongbong-dong Buk-gu, Gwangju 500-757, South Korea c Virginia Tech. and Virginia Seafood, Agricultural Research and Extension Center, Hampton, VA, 23669, USA
Abstract Effects of several conditions (liming concentrations, extraction solution pH, extraction temperature and extraction time) to extract gelatin from skate skin on the yield and quality properties were investigated. The optimum conditions for gelatin extraction are as follows; place skin in a lime solution of 1.5% (w/v) calcium hydroxide, extract with three volumes of water (pH 6.0) for 4 h at 50%, filter gelatin through activated carbon (250–350 mesh, 3%) and freeze-dry the colloidal suspension. The functional properties of skate skin gelatin produced by optimum extraction conditions were: gelling point 16.12 8C; melting point 19.30 8C; isoelectric point 6.45; and turbidity 6.98. q 2005 Elsevier Ltd. All rights reserved. Keywords: Skate (Raja Kenojei) skin; Physicochemical properties; Gelatin
1. Introduction Gelatin has broad applications in for food, pharmaceutical and photographic industries. Gelatin is commercially derived from collagen using a controlled acid or alkaline hydrolysis; and the source, age and type of collagen each influence the properties of the gelatins derived from different sources (Johnston-Banks, 1990). Currently, most of available gelatins are manufactured from mammalian resources, either pigskins or cowhides (Gomez-Guillen, Turnay, Fernadez-Diaz, Ulmo, Lizarbe and Montero, 2002). There is growing interest to develop alterative sources of raw materials such as using chicken and fishery by-products (Lim, Oh, & Kim, 2001). In South Korea, a few studies have been carried out to investigate the feasibility of using chicken feet as a resource to in place of cowhides for Jokpyun (traditional Korean gel-type food) (Jun, Lee, Lee, & Kim, 2000). Research on fish gelatin was reported using cod skin (Gudmundsson & Hafsteinsson, 1997), tilapia skin (Grossman & Bergman, 1992), shark, lungfish and carp skin
* Corresponding author. Tel.: C82 62 530 2145; fax: C82 62 530 2149. E-mail address:
[email protected] (J.-B. Eun).
0268-005X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2005.08.002
(Ward & Courts, 1997) as well as conger eel and arrow squid (Kim & Cho, 1996). Fish resources are important, since fish processing wastes after filleting can account for as much as 75% of the total catch weight (Shahidi, 1994). Skates (Raja Kenojei) are a popular food in South Korea. Approximately, 30% of processing waste from skate consists of skin and bone, which contain high collagen concentrations. In addition, the demand for skate (Raja Kenojei) has increased dramatically, due to its unique characteristics and health benefits (Nam & Lee, 1995). In South Korea, fermented skate is more popular than fresh skate. The main drawback of fish gelatins, however, is that the gels are less stable and have poor physiological properties compared with mammalian gelatins (Leuenberger, 1991), which is due to lower concentrations of proline (Ledward, 1986). Gelatin gels are formed by the entanglement of molecular chains by hydrogen bonds, ionic bonds and hydrophobic bonds between the chains or the segments. These gels have a reversible sol–gel transition through change in temperature, solvent composition, pH, etc. (Yoshimura et al., 2000). The physicochemical properties of a gelatin depend on the species, and tissues used and by the severity of the manufacturing method (Johnston-Banks, 1990). The objectives of this study were to investigate the optimum conditions for gelatin extraction from skins of
S.-H. Cho et al. / Food Hydrocolloids 20 (2006) 810–816
fermented skate, and to evaluate the physicochemical properties.
2. Materials and methods 2.1. Materials Skates (Raja Kenojei) were obtained from a local skate processing plant (Naju, South Korea) and the skins were collected after fermenting them for 2 weeks at storage at 10 8C known as good fermentation conditions through anecdotal information without any physical and chemical treatments. The fermented skins were immediately frozen and stored at K20 8C until used. All reagents were of analytical grade. All experiments were performed in duplicate with at least three replicated samples. The results were expressed as mean and standard deviations. 2.2. Preparation of gelatins The extraction procedures are shown in Fig. 1. The skate skins were thoroughly rinsed in excess water to remove impurities. They were soaked at different concentrations of
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calcium hydroxide solution followed by washing in excess water to remove alkali. The extraction was carried out in hot distilled water with different pH values and extraction times. The extracts were centrifuged at 12,000 g for 15 min in a centrifuge (J2-21, Beckman Instrument Ltd, Palo Alto, CA, USA), and the supernatant was immediately collected. The aliquots of the extract were then filtered through Whatman No.1 filter paper and activated carbon (250–350 mesh) and freeze-dried. 2.3. Yield The yield was calculated as dry weight gelatin/wet weight skate skins!100. 2.4. Proximate composition The moisture, crude protein, crude lipid and ash contents of the extracted gelatin derived from the fermented skate skin were determined in triplicate (AOAC, 1990). Crude protein of the gelatin was expressed as 5.4!nitrogen content (Johnston-Banks, 1990). All values were calculated on a percent wet weight basis. 2.5. Viscosity Gelatin solutions (10% (w/v)) were made by dissolving the dry powder in distilled water and heating at 60 8C. Viscosity as a function of temperature was determined using a computerized Brookfield digital viscometer (Model DV— II, Brookfield Engineering, USA) equipped with a No. 1 spindle (Model RVT) at 60 rpm starting at 40G1 8C (Kim, Byun, & Lee, 1994). 2.6. Gel strength Gel strength was determined according to the method described by Johnston-Banks (1990) on a gelatin gel of 6.67% concentration, formed by dissolving dried skate skin gelatin in 50 ml distilled water. The solution was cooled at 10G1 8C for 16–18 h. Measurements were conducted at 8G1 8C using a Texture Analyzer (TA.XT2, Stable Microsystems LTD, UK) for a 4 mm depression at a rate of 0.5 mm/s using a probe 2 cm in diameter. The gelling and the melting point of gelatin solution was determined visually observing changes in appearance (fluidity) and sinking loaded using a magnetic stirrer bar (1 g) on the top surface of the gelatin sample, respectively. 2.7. Color
Fig. 1. Procedures for preparation of the gelatin from the fermented skate skin.
Gelatin solutions (6.67% (w/v)) were cooled in a refrigerator at 10G1 8C for 16–18 h. Color was measured by composition with standards using the Hunter Colorimeter (CM-3500d, Minolta Co., Japan). The results were
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expressed as L (lightness), a (redness), and b (yellowness) values. 2.8. Gelling and melting point The gelling and the melting point were measured according to the method described by Kim et al. (1994) using the 10% (w/v) gelatin solutions dissolved in test tubes (15 mm!178 mm). Gelatin maturated 12 h at 5 8C was subjected to temperature ramps from 5 to 50 8C and back to scans at 50–5 8C and were performed at 0.5 8C/min. 2.9. Isoelectric point Determination of isoelectric point was performed in accordance with Kang, Jeon, Kim, and Song (1992) using strong acid (Dowex 50WX8-400, Aldrich Chemical Co., USA) and base (Dowex 1X8-400, Sigma Chemical Co., St Louis, MO, USA) resins for deionization. 2.10. Turbidity After dissolving the dried gelatins for 30 min in distilled water at 60 8C (at 0.1% w/w) and making a standard curve with Kaolin (100 mg/L) (USP Ke-500, Fisher Sci., USA) prepared at concentrations of 2, 4, 6, 8, 10, 12 ppm. Turbidity was determined by measuring absorbance at 660 nm using a spectrophotometer (UV-1201 Spectrophotometer, Shimadzu Co., Japan) (Kim, Byun and Lee, 1994).
3. Results and discussions 3.1. Proximate composition of the fermented skate skin gelatin Data on proximate composition of the fermented skate skin gelatin extracted in optimum conditions was expressed as grams (g) per 100 g gelatin (Table 1). Proximate analysis of the fermented skate skin gelatin showed 4.52% moisture, 92.31% protein, 0.35% lipid and 1.42% ash. The low lipid and ash contents suggest that the extraction processes was effective. 3.2. Effects of liming conditions Frozen skate skins were cut into small pieces (approximately 5!5 cm2) to facilitate liming and then soaked in 0.5, 1.0, 1.5 and 2.0% (w/v) calcium hydroxide solution for 2 days at 5G1 8C. Calcium hydroxide solution was used to remove impurities (proteoglycan, blood, mucins, sugars, fat, etc.) and change collagen to the optimal type for gelatin extraction (Johnston-Banks, 1990). After liming, the skate skins were washed in distilled water (pH 7.0) for 12 h and extracted at 50 8C for 3 h in a pH 7.0 extraction solution. The extraction yields of the gelatin at different liming concentrations are shown in Fig. 2. The highest gelatin extraction yield from skate skins was 16.80 at a 1.5% liming concentration. This yield was higher compared with tilapia spp. (Jamilah & Harvinder, 2002) and cod skin Table 1 Proximate composition of the gelatin derived from the fermented skate skin
2.11. Amino acid composition Amino acid analysis was performed by acid hydrolysis of dry gelatins using an amino acid analyzer (S-433H; Sycam). Samples were hydrolyzed for 24 h at 110 8C in 6 N HCl in sealed ampoules under vacuum.
Dry gelatina Moisture (%) Crude protein (%) Crude lipid (%) Crude ash (%) a
4.52G0.18 92.31G0.33 0.35G0.09 1.42G0.19
MeanGstandard deviation (Four replicated samples).
2.12. Sensory evaluation Sensory attributes were evaluated using a 15 member panel. Panelists were able to detect off odors by dissolving 0.5 g of gelatin in 7 ml of distilled water (6.67% (w/v)). Odor intensity was evaluated using a six point scale: 0, no odor; and 5, very strong and very offensive odor. 2.13. Statistical analysis One-way analysis of variance (ANOVA) was conducted using SAS (SAS Institute Inc., Cary, NC, USA). Data were analyzed using the Tukey test to determine significant differences between means. Paired-T tests were performed comparing mean values. The level of significance was P! 0.05.
Fig. 2. Yields of fermented skate skin gelatins extracted at different liming concentrations (under extraction conditions of pH 7.0 solution, 50 8C and 3 h). Different letters (a, b, c) indicate significant (P!0.05) differences.
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Table 2 Effects of liming concentrations on the physicochemical properties of gelatins extracted from the fermented skate skin Conc. (%)
0.5 1.0 1.5 2.0
Viscosity (cP)a
Gel strength (N)a
Color L
a
b
18.71b 19.02b 22.25a 20.53ab
0.43d 0.62c 0.75a 0.70b
25.96c 36.44a 37.13a 32.93b
0.03c 0.12bc 0.17b 0.46a
0.53b 0.18c 0.25c 0.87a
Different letters (a, b, c, d) in the same column indicate significant (P!0.05) differences among different concentrations of calcium hydroxide. a See the text for Section 2.
(Gudmundsson & Hafsteinsson, 1997). Table 2 shows the viscosity, gel strength, and color (L, a and b) values of the skate skin gelatins. Viscosity is the second most important commercial physical property of a gelatin (Ward & Courts, 1997). The viscosity of the gelatins from skate skins using several liming concentrations (0.5. 1.0, 1.5 and 2.0% (w/v) calcium hydroxide) was small (P!0.05). Viscosity is partially controlled by molecular weight and molecular size distribution (Sperling, 1985). For many gelatins, minimum viscosity occurs at pH 6–8 (Stainsby, 1952). The minimum viscosity occurs at the isoelectric point (Ward & Courts, 1997). Therefore, the viscosity may be improved by maintaining pH at 2–3. Gel strength varied with different liming treatments (P! 0.01) (i.e. 0.43, 0.62, 0.75 and 0.70 N at 0.5, 1.0, 1.5 and 2.0% liming conc., respectively). The highest gel strength occurred at 1.5% liming concentration. Gudmundsson and Hafsteinsson (1997) reported that gel strength is dependent on the isoelectric point and it can be controlled, by adjusting the pH. More compact and stiffer gels are formed by adjusting the pH of the gelatin at or near its isoelectric point (Jamilah & Harvinder, 2002).
Fig. 3. Yields of fermented skate skin gelatins extracted at different pH of solution for the extraction (under extraction conditions of 1.5% CaOH, 50 8C and 3 h). Different letters (a, b) indicate significant (P!0.05) differences.
3.4. Effects of extraction temperature After liming in 1.5% calcium hydroxide, the skins were washed in distilled water (pH 7.0) for 12 h and extracted temperatures ranging from 40 to 70 8C for 3 h at pH 6. The extraction yields at different temperatures are shown in Fig. 4. The higher the temperature, the higher levels of gelatin extracted. Table 4 shows the physicochemical Table 3 Effects of pH of solution for the extraction on the physicochemical properties of gelatins extracted from the fermented skate skin PH
4 5 6 7 8 9
Viscosity (cP)
Gel strength (N)
Color L
a
b
19.51c 20.31bc 24.25a 22.42ab 17.75c 20.25bc
0.61c 0.67b 0.74a 0.71a 0.65b 0.65b
40.51cd 39.68d 42.87ab 43.79a 41.58bc 39.89d
0.36a 0.33a 0.21b 0.23b 0.08c 0.21b
0.43c 0.36e 0.37e 0.58a 0.39d 0.48b
3.3. Effects of pH of solution for the extraction
Different letters (a, b, c, d, e) in the same column indicate significant (P!0.05) differences among different pH.
After liming the sliced skate skins in 1.5% calcium hydroxide, they were washed in distilled water (pH 7.0) for 12 h and extracted at 50 8C for 3 h at a pH of 4–9 pH adjustment was done by adding small quantities of 0.1 M HCl. Extraction yields differed by pH (Fig. 3). The highest gelatin yield was at pH 4. In contrast, Kim, Byun and Lee (1994) reported increasing yields at pH 6–7. Table 3 shows the physicochemical properties of gelatins extracted at different pH values. Both viscosity and gel strength were highest at pH 6–7 (i.e. 24.25 cP and 0.74 N in pH 6, 22.42 cP and 0.71 N in pH 7, respectively). Gelatin polymers are closer to being neutrally charged and form more compact and stiffer gels when the pH of the fermented skate skin gelatin is near its isoelectric point (6.45) (Gudmundsson & Hafsteinsson, 1997). Stainsby (1952) reported that minimum gelatin viscosity occurred at a pH of 6–8.
Fig. 4. Yields of fermented skate skin gelatins extracted at different temperatures (under extraction conditions of 1.5% CaOH, pH 6.0 solution and 3 h). Different letters (a, b, c) indicate significant (P!0.05) differences.
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Table 4 Effects of extraction temperature on the physicochemical properties of gelatins extracted from the fermented skate skin
Table 5 Effects of extraction time on the physicochemical properties of gelatins extracted from the fermented skate skin
Temp. (8C)
Viscosity (cP)
Gel strength (N)
Color
Time (h)
L
a
b
40 50 60 70
22.45a 20.45b 18.59c 19.08bc
0.73a 0.66b 0.61c 0.53d
27.47d 43.71a 42.12b 30.86c
0.39c 0.84b 0.82b 1.40a
1.35d 2.14c 2.82b 3.64a
3 4 5 6
Viscosity (cP)
Gel strength (N)
Color L
a
b
21.34b 23.68a 20.07bc 18.31c
0.66c 0.73a 0.69b 0.53d
34.48ab 41.73a 36.12b 33.49b
0.43c 0.94ab 1.13a 0.91b
2.67d 3.28c 3.65b 3.89a
Different letters (a, b, c, d) in the same column indicate significant (P!0.05) differences among different extraction temperatures.
Different letters (a, b, c, d) in the same column indicate significant (P!0.05) differences among different extraction time.
properties of gelatins at different extraction temperatures. Both viscosity and gel strength at 40 8C were higher (22.45 cP and 0.73 N, respectively) compared with other temperatures. These results agree with Ward and Courts (1997), who reported that at temperatures higher than 50 8C, gel forming ability and the physical properties decreased, due to breakage of hydrogen bonds and free amino acid hydroxyl groups. The yield at 50 8C was much better than at 40 8C despite of slightly low quality.
temperatures greater than 40 8C, due to hydrolysis of cross-linkages in collagen and other proteins.
3.5. Effects of extraction time After liming in 1.5% calcium hydroxide, the skins were washed in distilled water (pH 7.0) for 12 h and extracted at 50 8C for 3 h at pH 6. Extraction yields increased gradually with increasing extraction time up to 6 h (Fig. 5). The highest yield occurred at 6 h of extraction (17.48%). Kim, Byun and Lee (1994) reported that yields of gelatin slightly increased with increasing extraction times. Table 5 shows the physicochemical properties at different extraction times. The physicochemical properties were the highest at 4 h extraction time (i.e. viscosity, 23.68 cP; gel strength, 0.73 N; lightness (L), 41.72). According to Kim, Byun and Lee (1994), physical properties declined at extraction
3.6. Functional properties of gelatin from skate skin Table 6 shows the gelling and melting point, isoelectric point and turbidity of the gelatin. The values of functional properties are as follows: gelling point, 16.12 8C; melting point, 19.30 8C; isoelectric point, 6.45; and turbidity, 6.98. The gelling and the melting points were lower compared with Alaska Pollack skin and commercial bovine skin (Kim, Byun and Lee, 1994), but were higher compared with cod skins (Gudmundsson & Hafsteinsson, 1997). Fish gelatins have lower melting points compared with mammalian gelatins and especially, fish gelatin extracted from cold water species, which represent the majority of the industrial fisheries, (Leuenberger, 1991; Norland, 1990). Johnston-Banks (1990) reported that limed or alkaline processed commercial gelatins have isoelectric points typically around pH 4.8–5.0. Gelatins produced at pH 6.0, near the isoelectric point, had higher gel strengths. Gudmundsson and Hafsteinsson (1997) reported that gel strength is dependent on isoelectric point. The turbidity of gelatin from skate skins was also less compared with Alaska Pollack skin, but was higher compared with commercial bovine skin (Kim, Byun and Lee, 1994). The higher turbidity in skate gelatin reflects its poorer quality compared with commercial gelatins (Montero, Fernandez-Diaz, & Gomez-Guillen, 2002; Yang, Ryu, Moon, Kim, & Kim, 1994). 3.7. Amino acid composition The amino acid composition of the fermented skate skin gelatin extracted in optimum conditions is shown in Table 7. Imino acid and glycine contents of the fermented skate skin Table 6 Functional properties of the gelatin extracted from the fermented skate skin under optimum extraction conditions
Fig. 5. Yields of fermented skate skin gelatins extracted at different extraction time (under extraction conditions of 1.5% CaOH, pH 6.0 solution, 50 8C). Different letters (a, b, c) indicate significant (P!0.05) differences.
Valuesa a
Gelling point (8C)
Melting point (8C)
Isoelectric point
Turbidity (ppm)
16.12G1.02
19.3G0.86
6.45G1.34
6.98G0.43
MeanGstandard deviation (three replicated samples).
S.-H. Cho et al. / Food Hydrocolloids 20 (2006) 810–816 Table 7 Amino acid composition of the gelatin from the fermented skate skin (g/100 g) Amino acid
Fermented skate skin gelatin
Aspartic acid (Asp)
10.14G1.05
Threonine (Thr) Serine (Ser) Glutamic acid (Glu) Hydroxy Proline (Hyp)a Proline (Pro)a Glycine (Gly) Alanine (Ala) Valine (Val) Methionine (Met) Isoleucine (Ile) Leucine (Leu) Phenylalanine (Phe) Histidine (His) Lysine (Lys) Arginine (Arg)
6.73G1.21 1.79G0.32 16.30G0.28 6.92G1.02 10.38G0.89 20.49G1.02 10.30G1.39 1.54G0.11 1.25G0.09 0.98G0.03 1.70G0.05 1.19G0.09 1.38G0.08 2.88G0.40 6.03G0.85
Data presented as meanGstandard deviation of three replicated samples. a Amino acid.
gelatin were 17.30 and 20.49%, respectively. The stability of collagen and gelatins is proportional to their total imino acid (ProCHyp) and glycine contents (Lehninger, Nelson, & Cox, 1993). Ultimate gel strength is related to its imino acid and glycine content. Gelatins derived from fish collagens are weak with low melting points compared with mammalian gelatins. Mammalian gelatins have a high proportion of proline, hydroxyproline and glycine (Johnston-Banks, 1990). The low gel strength of the fermented skate skin gelatin may be due to its lower amino acid (i.e. proline and glycine) concentrations (Cho, Jahncke, & Eun, 2004). Calf skin gelatins and warm water fish gelatins from tilapia, they contain approximately 23.2 and 18.9% imino acid (Piez & Gross, 1960). This is higher compared with fermented skate skin gelatin. 3.8. Sensory evaluation Sensory scores of off odor between fermented skate skin and bovine commercial gelatin showed no any difference. The odor scores for fermented skate skin gelatin was 0.93G 0.07, similar to commercial gelatin (0.80G0.04), suggesting that the activated carbon treatment (250–350 mesh, 3%) and freeze-drying eliminated fishy odors. In addition, the fermented skate skin gelatin had the best snowy white color of all gelatins, which is a positive attribute.
4. Conclusions Gelatins from fermented skate skin have similar physicochemical properties, and higher yields compared with gelatins produced from skins from other marine
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species. Additional research is needed to improve the overall quality of gelatins produced from the fermented skate skins.
Acknowledgements This study was financially supported by Chonnam National University in the program, 2001. We thank the donor and express our heartfelt gratitude.
References A.O.A.C. (1990). Official methods of analysis (15th ed.). Washington, DC: Association of Official Analytical Chemists p. 132. Cho, S. H., Jahncke, M. L., & Eun, J. B. (2004). Nutritional composition and microflora of the fresh and fermented skate (Raja Kenojei) skins. International Journal of Food Science and Nutrition, 55(1), 45–51. Gomez-Guillen, M. C., Turnay, J., Fernadez-Diaz, M. D., Ulmo, N., Lizarbe, M. A., & Montero, P. (2002). Structural and physical properties of gelatin extracted from different marine species: A comparative study. Food Hydrocolloids, 16, 25–34. Grossman, S., Bergman, M. (1992). Process for the production of gelatin from fish skin. United States Patent No. 5,093,474. Gudmundsson, M., & Hafsteinsson, H. (1997). Gelatin from cod skin as affected by chemical treatments. Journal of Food Science, 62(1), 37–39. Jamilah, B., & Harvinder, K. G. (2002). Properties of gelatins from skins of fish—black tilapia (Oreochromis mossambicus) and red tilapia (Oreochoomis nilotica). Food Chemistry, 77, 81–84. Johnston-Banks, F. A. (1990). Gelatine. In P. Harris (Ed.), Food gels (pp. 250–259). London: Elsevier Applied Science Publishing Co., Inc.. Jun, M., Lee, J. M., Lee, K. S., & Kim, K. O. (2000). The effect of preparation conditions on the properties of Jokpyun (traditional Korean gel type food) model system. Food Science and Biotechnology, 9(27), 27–31. Kang, T. J., Jeon, Y. J., Kim, S. K., & Song, D. J. (1992). Investigation of pretreatment method for gelatin preparation from flounder skin. Bulletin of Korean Fish Society, 25(2), 93–102. Kim, J. S., & Cho, S. Y. (1996). Screening for raw material of modified gelatin in marine animal skins caught in coastal offshore water in Korea. Agricultural and Chemical Biotechnology, 39, 134–139. Kim, S. K., Byun, H. G., & Lee, E. H. (1994). Optimum extraction conditions of gelatin from fish skins and its physical properties. Journal of Korean Industrial and Engineering Chemistry, 5(3), 547–559. Ledward, D. A. (1986). Gelation of gelatin. In J. R. Mitchell, & D. A. Ledward (Eds.), Functional properties of food macromolecules (pp. 171–201). London: Elsevier Applied Science Publishers. Lehninger, Albert L., Nelson, David L., & Cox, Michael M. (1993). Threedimensional structure of proteins Principles of biochemistry (2nd ed.). New York: Worth Publishers, Inc. pp. 160–197. Leuenberger, B. H. (1991). Investigation of viscosity and gelatin properties of different mammalian and fish gelatins. Food Hydrocolloids, 5(4), 353–361. Lim, J. Y., Oh, S. S., & Kim, K. O. (2001). The effects of processing conditions on the properties of chicken feet gelatin. Food Science and Biotechnology, 10(6), 638–645. Montero, P., Fernandez-Diaz, M. D., & Gomez-Guillen, M. C. (2002). Characterization of gelatin gels induced by high pressure. Food Hydrocolloids, 16, 197–205. Nam, H. K., & Lee, M. K. (1995). Studies on the fatty acids and cholesterol level of Raja skate. Journal of Korean Oil Chemists’ Society, 12(1), 55–58.
816
S.-H. Cho et al. / Food Hydrocolloids 20 (2006) 810–816
Norland, R. E. (1990). Fish gelatin. In M. N. Voigt, & J. K. Botta (Eds.), Advances in fisheries and biotechnology for increased profitability (pp. 325–333). Lancaster, PA: Technomic. Piez, K. A., & Gross, J. G. (1960). The amino acid composition of some fish collagens: The relation between composition and structure. Journal of Biological Chemistry, 235(4), 995–998. Shahidi, F. (1994). Seafood processing by-products. In F. Shahidi, & J. R. Botta (Eds.), Seafoods chemistry, processing, technology and quality (pp. 320–334). Glasgow: Blackie Academic and Professional. Sperling, L. H. (1985). Introduction to physical polymer science. New York: Wiley.
Stainsby, G. (1952). Viscosity of diluted gelatin solutions. Nature, 169, 662–665. Ward, A. G., & Courts, A. (1997). The science and technology of gelatin. London: Academic press. Yang, S. T., Ryu, B. H., Moon, Y. H., Kim, D. S., & Kim, H. S. (1994). Effects of protein concentration, heat treatment and pH on the functional properties of gelatin. Bulletin of KyungSang National University, 15(4), 79–88. Yoshimura, K., Terashima, M., Hozan, D., Ebato, T., Nomura, Y., Ishii, Y., et al. (2000). Physical properties of shark gelatin compared with pig gelatin. Journal of Agricultural and Food Chemistry, 48, 2023–2027.
