Rheological, Textural And Film-forming Properties Of Tilapia Skin Gelatin

Gelatin has been widely applied in many fields, and its demands are increasing year by year. The traditional procedure of mammalian gelatin production could not meet the growing needs of gelatin. To avoid the security risk of infectious diseases and religious taboos of mammalian gelatin, as well as relieve the growth pressure of gelatin demands, it becomes the current trends for domestic and overseas to develop fish gelatin due to its safety and smaller religious dietary taboos, to broaden the sources of gelatin and thus increasing the yields of gelatin. Since the twenty- first century, profound research has been conducted in the extract of fish gelatin from fish processing waste(fish bones, scales, skin, etc.) and its properties and applications. In this study, the tilapia skin gelatin extracted by acid-alkaline method, and its basic physical and chemical properties were studied, such as the amino acid composition, molecular weight distribution, optical properties and functional properties. The results showed that tilapia skin was a good raw material for gelatin preparation. The yield of optimized acid-alkaline preparation process of tilapia skin gelatin was up to 23 %, and gelatin was almost free of fat and ash with the low moisture content of 5.59 %. The analysis of amino acid composition showed that the Gly-Pro-Hyp tripeptide structure content of fish gelatin was in line with Ι-collagen characteristics. SDS- polyacrylamide gel electrophoresis indicated that fish gelatin contained more α and β macromolecular chains. Absorbance, turbidity, color experiments revealed that fish gelatin exhibited excellent optical properties, which had a strong UV light barrier, substantially no absorption in the visible region, and the fish gelatin solution was clarified and transparent, almost colorless of solid film. Moreover, the gelatin had certain excellent functional properties, including foaming and foam stability, emulsification and emulsion stability. So the study of basic physical and chemical properties of gelatin will be widely used as a functional additive in the field of food, pharmaceutical and cosmetic industries.By the experiments of shear rate scanning, small amplitude oscillations temperature scanning, gel strength and uniaxial compression, we explored the factors of gelatin rheological and texture properties and gelatin sol-gel transition. The gelatin gel is thermally reversible, and the sol- gel reversible transition occur with temperature changing. Rheological and textural properties of fish gelatin mainly depended on its molecular structure, including the composition of amino acids, molecular weight distribution and content. In the study, one of the edible organic acids(citric or malic acid) and sugars(sucrose or fructose) was added into gelatin, resulting in the change of gelatin molecular structure, thus affecting the fish gelatin sol- gel transition, rheological and textural properties. The strong organic acids could destroy the molecular structures of gelatin, the β, γ chains could be broken and separated into single α-peptide chains, and the α-chain could be even broken down to α-chain fragments. With the change of structure, the molecular weight distribution and content of gelatin changed, resulting in the reduction of macromolecules and the increase of molecules concentration in the sol. These changes affected the gelling ability, and generated a weak structure of three-dimensional network with insufficient rigidity and toughness, making it difficult to form a high strength gel. The gel owed insufficient viscoelasticity, melting easily while heating, insufficient compressive hardness and toughness and easily broken while compressing. That was, the action of acid resulted in p H drastically reducing, the viscosity increasing, but did not changing the flow type of Newtonian fluid of fish gelatin solution, significantly reducing the sol- gel phase transition points, may slightly increasing the elastic modulus and viscous modulus of sol, but significantly decreasing the elasticity and viscous modulus of gel. At the same time, the gel strength, Young’s modulus and failure stress and strain of gelatin gel were obviously reduced. C itric acid, acidity slightly stronger than malic acid, therefore it had stronger damaging effects on fish gelatin.Neutral small molecule sugars containing a plurality of e-OH, can be together with water filled in the gap of fish gelatin molecules and formed hydrogen bonds with groups in the fish gelatin molecules, strengthened the fish gelatin hydration, increased the viscous and elastic properties of fish gelatin solution. When the gelatin was gelling, molecules aggregated closer to form a more complex, high-strength and stable three-dimensional network gel structure. Gel could not be melted easily while heating, viscoelasticity increased, compressing deformation resistance and hardness increased. That was, after the effect of sugar, no a significant change in p H, while a significant increase of the viscosity, but didn’t change the flow type of Newtonian fluid of the sol, and the sol- gel transition point as well as the elastic and viscous modulus of the gel increased dramatically, while slightly increased the elastic and viscous modulus of sol. Meanwhile, sugar increased the gel strength, Young’s modulus and broken stress and strain of gel. Sucrose, containing 6.3 e-OH more than the 3 e-OH of fructose, with an advantage to form stronger fish gelatin gel.Fish gelatin has good film- forming property, fish gelatin film having a high tensile strength, light transmittance, but its flexibility and water resistance are poor. Fish gelatin and polysaccharides can be compatible to prepare composite film. Pullulan is a linear mix of α-D-glucan consisting mainly of maltotriose repeating units interconnected by α-(1â†'6) linkages. Pullulan has good film-forming properties, and strong oxygen, nitrogen gas barrier properties, particularly suitable for food packaging. This paper studies showed that fish gelatin film or pullulan film with a single component(gelatin or pullulan), were with poor elongation and water barrier properties, not conducive to the application. To solve these shortcomings, we used glycerol and sorbitol as film plasticizer in pair to enhance the breaking enongation of the tilapia- gelatin film. By adding a chemical or biological cross- linking agent(glutaraldehyde or genipin), both containing two aldehyde groups could covalently cross- linked with free amino groups of the fish gelatin molecule to form a structure similar to Schiff base,so that to formed a more dense structure of film, thereby reduced the moisture permeability of fish gelatin film. Single factor experiments showed that adding pullulan concentration of 15-20%, the fish gelatin together with pullulan to prepare film could form a dense film structure, with more rigidity and toughness. Fish gelatin- pullulan composite film had the maximum tensile strength, elongation at break, water barrier properties, but also had a high light transmittance. When the fish gelatin and polysaccharide pullulan concentration ratio of 85:15, plasticizer concentration of 30 %, crosslinking agent concentration of 2 %. Fish gelatin- pullulan- glutaraldehyde cross- linked film(GP-GS-GTA) and fish gelatin- pullulan- genipin cross- linked film(GP-GS-GP), were retained the higher tensile strength and lower light resistance of fish gelatin film, under the premise, were increased the enongation and water barrier properties, was ideal natural biodegradable films. Genipin as a new type of protein cross- linking agent would replace the application of glutaraldehyde.