| In the last two decades, with the rapid development of large fresh-water fish breeding, food processing industry are also speeding up to product, getting rid of plenty of byproducts such as skins, innards, bones, fins and so on. Focusing on how to effectively transform these by-products into high value-added products is an effective way to increase profits and realize industrialization of fresh-water fish processing. This study chose viscera and skin separately from catfish and grass carp, both of which are major breeding and processing in our country, as objects of research and development. Firstly crude protease were extracted from catfish viscera, which were then used to extract collagen from grass carp skin. Physical and chemical properties of collagen obtained in last step were analyzed to provide valuable theoretical support for comprehensive utilization of protease and collagen from fish by-product sources.In this paper, general components of grass carp skin and its amino acid contents were investigated. Then surface methodology was conducted to optimize the extraction conditions of protease from catfish viscera, which was then used to extract collagen from skin of grass carp. The properties of both collagen and collagen films were investigated to provide theoretical support for their future comprehensive utilization.The results showed the high protein content(26.84±1.91)g/100g(wet weight) and high fat content of(11.16±2.78)g/100g(wet weight) in the skin of grass carp. Automatic amino-acid analyzer showed that there were 15 different kinds of amino acids, including 6 kinds of essential amino acids(EAA) which account for 18.3%, and 5 kinds of flavor amino acids(FAA) accounting for 53.7%. Glycine was the most abundant amino acid in the grass carp skin, followed by proline, which indicates that collagen is abundant in grass carp skin.Protease from viscera of catfish was employed to extract collagen from skin of grass carp, the extraction conditions were optimized firstly. Single-factor tests showed that protease from stomach was more active than from intestine. On the basis of single-factor tests, surface methodology was carried out to obtain the optimal extraction conditions. The results showed that protease extracted from stomach was significantly affected by the interaction between ultrasonic intensity and extraction time, as well as protease extracted from intestine was significantly affected by Na Cl concentration and ultrasonic intensity, while their interaction was not obvious affection. The optimal conditions of protease from stomach were as follows: 0.3% Na Cl solution(added with a ratio of material to liquor which was 1:5), ultrasonic intensity 90 W, 30 min of processing time and at 35℃, with an activity of 130U/m L; the optimum conditions of protease from intestine of catfish were: 0.25% Na Cl solution(added with a ratio of material to liquor which was 1:5), ultrasonic intensity 100 W, 40 min of processing time and at 35℃, with an activity of 110U/m L.The crude enzyme extracted from stomach and intestine of catfish were employed to extract collagen from grass carp skin. An extraction rate of 18.79% and 0.7%(wet weight) were obtained, with protease from stomach and intestine respectively. Then the physicochemical properties of collagen extracted by stomach protease were analyzed. The results showed that the appearance of collagen was spongiform, smooth and white, with a feeling of toughness. The collagen was studied with good emulsifying property and emulsifying stability in proper p H values and temperature. The emulsifying capacity is as high as 94%, which shows its potential as natural emulsifying agent. Thermal denaturation temperature of this collagen was analyzed to be 20.36℃.In addition, films made in different collagen concentrations were analyzed, the results are shown as following: film of collagen in the concentration of 1.5% showed the best transmittance( 58.2%), film of collagen in the concentration of 2.5% showed the best water solubility(62.73%) and water vapor permeability(0.62 g mm/KPa h m2). Considering heat resistance as an important target of film, thermal denaturation temperature of collagen film was analyzed to be 23.84 ℃, protein decomposition temperature was studied to be 117.80℃, and glass-transition temperature was measured to be 151.43℃, indicating better heat resistance property than most collagen film from deep sea fish skin. Scanning electron microscope(SEM) was performed which showed that the microstructure of film had a flat surface, as well as a homogeneous and compact internal structure. |
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