| Gelatin is a kind of natural polymer that has some unique physiochemical properties like gelling, water holding, film forming, emulsifying and foaming properties, which has been widely used in food, pharmaceutical and chemical industries. In recent years, the demand for gelatin is increasing in the world, however, its application was limited due to the security risks like religious and vegetarian etc. As a result, to broaden the sources of gelatin and increase yield is the current trends in the development of gelatin both domestic and international. China is rich in fish resources, a wide range of use of waste(such as skin, fish bones and scales, etc.) left in the production process to extract gelatin, can not only reduce environmental pollution, but also increase economic efficiency. In recent decades, extensive and profound studies was launched to boost the development and application of gelatin. The tilapia skin gelatin was prepared with the optimized method in this study and it’s basic physical and chemical properties were analyzed. Due to tilapia skin gelatin has a strong fishy aroma, powdered activated carbon, diatomaceous earth and betacyclodextrin were compared in an attempt to confirmed the preferable purified agent and the purification process. The effect of several acids that commonly used in food(lactic acid, citric acid and malic acid) on the p H, gel strength, and rheological properties of gelatin and its impact on yogurt acidity, syneresis, texture and rheological properties of yogurt were studied:1. The basic physical and chemical properties of tilapia skin gelatinSodium hydroxide and acetic acid were used respectively to pre-treat tilapia skin, and gelatin was obtained after water extraction and drying. The p H, clarity, gel strength, the gel point, melting point and molecular weight distribution of gelatin were determined. The results showed that the yield of gelatin can reach to 21.3 ± 0.2%, which was significantly higher than those of some other fish gelatin. Its transmittance was 85.1 ± 0.23% and the visual appearance was light yellow, its clarity could improve by optimizing the filtration process or using the appropriate method to handle with gelatin solution. The tilapia skin gelatin prepared had high gel strength(387.8g), gelling point(21.8 ℃) and melting point(29.3 ℃), which were significantly higher than the cold water fish gelatin and very close to mammal gelatin, indicating that tilapia skin gelatin is a potential alternative to mammalian gelatin in industry.2. The volatile compounds of tilapia skin gelatin and the fishy odor removingThe gelatin solutions was treated by β-CD(1.0-3.0% w/w), powdered active carbon(0.5-2.0% w/w) and diatomaceous earth(0.1-0.5% w/w) at different conditions. Results showed that the clarity of gelatin solutions were increased from 85.1 ± 0.23% to 92.4 ± 0.15%, 91.5 ± 0.38% and 92.6 ± 0.32%, respectively, the visual appearance was from yellow to colorless. Sensory evaluation results showed that all of these three substances can remove the fish aroma in certain degrees and the most preferable purified agent was b-cyclodextrin(3%, 50 oC, 30 min). The volatile compounds of tilapia gelatin was detected by means of solid phase micro-extraction and GC-MS. A total of 18 kinds of volatile components were confirmed, namely hexanal, heptanal, octanal, nonanal, decanal, undecanal, dodecanal,(E)-2-octenal,(E)-2-nonenal,(Z)-2-decenal, 2-undecenal 1-octanol, 2, 4-decadien-1-ol, 1-octen-3-ol, cedrol, cetic acid, methoxy-phenyl-oxime, and 2-pentyl-furan. A total of 3, 9 and 12 compounds were detected respectively after β-CD, powdered activated carbon and diatomaceous earth treatment. It could be concluded from sensory evaluation and instrumental analysis that nonanal, hexanal, heptanal, octanal and(E)-2- nonenal were responsible for the fishy odor.3. The effect of malic acid, citric acid and lactic acid on the rheological properties of tilapia skin gelatinThe effect of different acid(lactic, citric and malic acid) concentration(0.5-2.0% w/w) on the p H, gel strength, and rheological properties of gelatin was studied. The results showed that these three acids could reduce the p H, gel strength, the gelling point, melting point, elastic modulus and loss modulus of gelatin. Due to the acidity of citric and malic acid is stronger than lactic acid, thus the effect on gelatin is more obvious, the effect of a concentration of 2.0% citric and malic acid on p H and gel strength of gelatin showed no significant difference(P > 0.05). When the concentration of citric and lactic acid was up to 2.0%, the gel strength of gelatin decreased by about 16.0%(P<0.05), and p H was in the range of 5.2-2.9. The gelling point of 1.5-6.67%(w/w) gelatin solution were 21.8, 19.5, 14.8 and 11.8 ℃ and the melting point were 29.3, 28.2, 26.9 and 26.0 ℃ respectively. The relationship of gelatin concentration and the gelling point or melting point fitted very well in a logarithmic relationship(R2=0.9964-0.9985) and Eldridge-Ferry model(R2=0.9669-0.9960), then the gelling point or melting point of other gelatin concentrations could be calculated by using this relationship. While lactic, citric and malic acid was induced in 6.67% gelatin solution, the gelling point and the melting point were in the range of 20.2-21.8 ℃ and 28.8-29.3 ℃ for lactic acid, 18.7-21.8 ℃ and 26.8-29.3 ℃ for citric and malic acid, respectively. The effect of acid on the gelling and melting point of lower gelatin concentration was more obvious, 1.5% gelatin did not gelled in the temperature of 2-40 ℃ while any acid and its concentration was induced. The higher the gelatin concentration, the greater the elastic modulus and loss modulus. The induce of acid could reduce both the elastic and loss modulus, and the higher the acid concentration, the lowing effect was more remarkable. When the temperature did not surpass the melting point of gelatin, A nearly linear relation was found between the G’ or G’’ values and the square of gelatin concentration was good linear relationship(R2 = 0.9959-0.9981),, the relationship was still logical(R2 = 0.9967-0.9998) after acid was added.4. Effect of tilapia skin gelatin on the texture profile and rheological properties of yogurtThe effect of different concentrations of tilapia skin gelatin solution(0.2-0.8% w/w) on the syneresis, titratable acidity, hardness, adhesiveness, and rheological properties of yogurt was studied. The results showed that the addition of gelatin can significantly reduce the syneresis of yogurt, and increase the acidity, hardness and adhesiveness. When the gelatin concentration was in the range of 0.2-0.4%, the syneresis significantly reduced but the hardness and adhesiveness showed no significant increase. while gelatin concentration up to 0.6-0.8%, no syneresis was observed, and there is a significant increase in hardness and adhesiveness, it led to a very dense and compact structure which could impair palatability, yogurt was a jellylike rather than thick fluid. Therefore, the optimal concentration of tilapia skin gelatin 0.4%. Therefore, the level of additional gelatin at 0.4%(w/v) was suggested for milk yogurt making. However, this optimum concentration will depend on the bloom specification of the gelatin used. |
PDF Full Text Request