The effect of added salts on the viscoelastic properties of fish skin gelatin
Introduction
Gelatin can be used as an ingredient to enhance the elasticity, consistency and stability of food products, and therefore the quality of a food grade gelatin depends to a large extent on its rheological properties. Although mammalian and avian gelatins have been extensively studied, less is known about fish gelatin (Grossman & Bergman, 1992, Gudmundsson & Hafsteinsson, 1997, Holzer, 1996, Kim & Cho, 1996, Norland, 1990, Osborne, Voigt & Hall, 1990). It is generally known that gelatins from warm blooded animals are characterized by having considerably higher melting and gelling points than cold-water fish gelatins (Leuenberger, 1991); moreover, the gels are also stronger, which is directly related to the fact that hydroxyproline content is higher in the former (Ledward, 1992, Norland, 1990). The interest in the utilisation of fish skins lies, not only in the exploitation of by-products, but also, from a socio-cultural standpoint, as an alternative to mammal gelatin. For many applications, good rheological properties are required, and these could be attained by using gelatin-modifying materials. One possible means of manipulating the characteristics of a given gelatin is to trigger interactions by the addition of solutes, for instance, salts (Elysée-Collen & Lencki, 1996). As reviewed by Asghar and Henrickson (1982), electrolytes in general have a decisive influence on the biophysical properties (swelling, solubility, gelation, viscosity and water-binding capacity) of a protein at different ionic strengths and pH values (Hermansson, 1975). There are two points of view as regards possible interaction between collagen (or gelatin) molecules and saline ions. Some workers believe in the possibility of direct-ion binding to the peptide backbone of collagen, while others believe that ions affect collagen folding indirectly by interacting with structurally bound water molecules (Asghar & Henrickson, 1982). According to Fennema (1977), ions may be classified as “water structure formers” or “water structure breakers” on the basis of their ability to alter the net structure of water by its polarizing power, which results in modification of water viscosity. Hydration, caused by the interaction of ions of neutral salts with non-ionic bonds (e.g. hydrogen bonds) of collagen is described as “lyotropic hydration”. Thus, lyotropic agents may alter water structure around collagen molecules, interrupt internal hydrogen bonds, or interact with internal hydrophobic bonds by direct binding at some sites of protein chains (Asghar & Henrickson, 1982). It has been stated that the effect of salt concentration on protein stability is very ion-specific, with stabilizing or destabilizing effects typically following the Hofmeister series (Von Hippel & Wong, 1962). However, salts with the same cation but different anions, as well as those with the same anion and different cations, did not necessarilly function according to the lyotropic order (Hamm, 1958).
The effect of different salts on the rigidity or melting temperature of animal gelatins has been known for a long time (Harrington & Von Hippel, 1961). Other substances, such as dextran dialdehydes, have been tested to improve crosslinking of gelatin (Schacht, Nobels, Vansteenkiste, Demeester, Franssen & Lemahiev, 1993). More recently ammonium sulphate has been found to reduce the solubility of gelatin as a consequence of both, protein and salt competing for water to hydrate (Elysée-Collen & Lencki, 1996). However, little information is available in relation to the effect of salts in food grade gelatins obtained from fish skins, which differ greatly from gelatins of animals.
The aim of this work was to examine the effect of several salts, at high (0.5 M) and low (0.1 M) concentration, and at two pH levels (pH 5 and 8), on the viscoelastic properties of a class A gelatin (obtained with a mild acid pretreatment) from megrim (Lepidorhombus boscii) skins. This species may be classified as half-way between a cold water species and a typical warm water species such as tilapia, which has been used for several fish gelatin patents (Grossman & Bergman, 1992, Holzer, 1996). A comparison was made between megrim and tilapia gelatins on the basis of their amino acid composition and viscoelastic properties, and the effect of addition of the different salts was also compared.
Section snippets
Materials and methods
Fresh megrim [Lepidorhombus boscii (Risso)] skins were obtained from a local market in Madrid, and were stored at −20°C until use. A commercial tilapia (Oreochromis spp) skin gelatin was supplied by Croda Ltd. (UK). All reagents used were analytical grade.
Results and discussion
A comparison was made of viscoelastic properties of megrim gelatin powder dissolved in different saline solutions, at both pH 5 and 8, and at 0.5 and 0.1 M salt concentration. Fig. 1 shows the elastic modulus (G′), viscous modulus (G″) (both measured at 5°C), and melting temperature, of gelatin dissolved at 0.5 M salt concentration (pH 5 and 8). A class A gelatin (mild acid pretreatment) will carry a net positive charge in all food uses, irrespective of the pH of the medium (Stainsby, 1987),
Conclusions
The addition of MgSO4, (NH4)2SO4 or NaH2PO4 at high concentration (0.5 M) to megrim gelatin was critical in considerably raising the melting point, whereas chloride salt acted to reduce it. At 0.1 M concentration and pH 5, MgSO4 was the only salt that improved melting point and elastic modulus although with the drawback of considerably prolonging the setting time.
A commercial gelatin from tilapia presented a similar melting temperature to megrim gelatin, but a higher G′, which was related to
Acknowledgements
This research was financed by the EC under project FAIR CT97.3055, and by Spanish CyCIT under project ALI 98 1215 CE.
References (23)
Investigation of viscosity and gelation properties of different mammalian and fish gelatins
Food Hydrocolloids
(1991)- et al.
Some aspects of the crosslinking of gelatin by dextran dialdehydes
Polymer Gels and Networks
(1993) - Asghar, A., & Henrickson, R.L. (1982). Chemical, biochemical, functional, and nutritional characteristics of collagen...
- et al.
Effect of ethanol, ammonium sulfate, and temperature on the phase behaviour of type B gelatin
J. Agric. Food Chem.
(1996) - Fennema, O. (1976). Water and ice. In O. Fenemma, Principles of food science. Part 1 (pp. 13). New York:...
Water and protein hydration
- Grossman, S., & Bergman, M. (1992). Process for the production of gelatin from fish skins. US Patent 5,093,...
- et al.
Gelatin from cod skins as affected by chemical treatments
Journal of Food Scence
(1997) The chemistry and reactivity of collagen
(1956)The biochemistry of meat salting
Zeitschrift fuer Lebensmittel-Untersuchung und-Forschung.
(1958)
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