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Since the bovine spongiform encephalopathy crisis, there has been a growing interest for finding an alternative source of raw materials for gelatin production. Gelatin produced from fish processing by-products is a potential alternative to mammalian gelatin. Fish processing generates solid wastes that can be as high as 50-80% of the original raw material. These wastes are an excellent raw material for preparation of high protein foods. About 30% of the wastes consists of skin and bone with a high collagen content. Fish gelatin can be obtained by hydrolysis of collagen the principal protein found in skin and bone. Fish skin and bone gelatin can be prepared with bloom strength similar to that obtained from mammalian sources. Fish gelatin has numerous applications, particularly, in the food, pharmaceutical, and photographic industries due to its unique chemical and physical properties. This review presents how fish processing by-products can be utilized in the manufacture of gelatin.

The influence of using an endo-peptidase (alcalase) on the functional properties of hydrolysis products from Nile perch, Grass carp, and Nile tilapia skin was studied. Reaction conditions were controlled at pH 8.25, 60°C, and the enzyme was added on the basis of standard activity units at an enzyme to substrate ratio of 1.7 g/100 g protein. The reaction was allowed to proceed for 85 min and enzyme was inactivated by heat. The soluble protein fractions were recovered and lyophilized. All freeze-dried fish skin hydrolysates powders had a light yellow color and contained up to 90% protein. Nitrogen solubility varied from 95.93 to 98.72% and was not significantly different at 5% probability level. The water and oil holding capacities of the skin hydrolysates were good in the range of 2.8 to 3.2 mL/g and 3.4 to 3.8 mL/g, respectively. Emulsification capacity varied from 11.3 to 21 mL/0.5 g with Nile perch skin hydrolysate having the highest score while Nile tilapia skin hydrolysate was the lowest. Grass carp skin hydrolysate was not able to form stable foam, unlike the Nile perch and Nile tilapia skin hydrolysate. Alcalase treated freshwater fish skin exhibited satisfactory functional properties hence may play an important role as an ingredient in the food and pharmaceutical industry.

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Protein hydrolysate was produced from grass carp skin using Alcalase. Hydrolysis conditions were optimized by using a response surface methodology (RSM). A model equation was proposed with regard to effects of temperature (T), pH, enzyme/substrate (E/S) ratio (%v/w of minced skin) and time (t) on degree of hydrolysis (%DH). The optimum values for T, pH, E/S ratio and t were found to be 59.74C, 8.25, 1.70% and 83.83 min, respectively. Regression coefficients indicated that all linear forms T, pH, t and E/S ratio plus one quadratic form t2 and one cross‐product interaction T · t were significant (α = 0.05). The model showed a good fit in experimental data because 90.7% of the variability within the range of values studied could be explained by it. The freeze‐dried hydrolysate contained high protein content (90.8%). The hydrolysate produced was highly water‐soluble with good water‐holding, oil‐binding and emulsifying properties. Thus, the production of a hydrolysate with desired functional properties is possible from grass carp skin using Alcalase. PRACTICAL APPLICATIONS Considerable amounts of protein‐rich by‐products from seafood processing plants are discarded without any attempt to carry out recovery. The preference in many Western countries for bone‐free fillets necessitates the removal of flesh from fish, and a substantial amount of original weight is considered a waste, of which the skin is inclusive. It would be of great interest to put these by‐products that are high in protein to use in human foods instead of in fish meals for animal feed. Enzymatic hydrolysis of food proteins generally results in profound changes in the functional properties of proteins treated. The choice of substrate, the protease enzyme employed and the degree of hydrolysis can greatly affect the physicochemical properties of the resultant hydrolysate. Through the control of the process parameters such as pH, time and enzyme/substrate ratio, it is possible to produce hydrolysates whose components may present some interesting properties. Fish protein hydrolysates have numerous applications, particularly in the food and pharmaceutical industries.