Studies on the fishy odour removal and physicochemical properties of gelatin from Nile tilapia (<I>Oreochromis niloticus</I>) skin

Abstract

Nile tilapia (Oreochromis niloticus) aquaculture is expanding throughout the world,most notably in China. According to the statistics of FAO,annual China’s production of tilapia by 2008 had risen to nearly 1.2 million tons,which accounts for about 50% of tilapia production in the world. During processing of tilapia to fillets,large quantities of the by-products such as skin are produced. On one hand,collagen contents in tilapia skins are rich. On the other hand,the bovine spongiform encephalopathy (BSE) episode,as well as religious concerns,has led to intensive research to identify and develop alternatives to mammal-derived gelatin. Therefore,tilapia skin can be a good resource of extracting gelatin. However,gelatin extraction from tilapia skin usually had a strong fishy odour,which would limit its utilization. In order to be applied to food and pharmaceutical industries,some methods must be adopted to make the gelatin odorless. In the present study,methods of active carbon absorption,yeast and lactobacillus fermentation were used to remove fishy odour in the gelatin,in which their roles to diminish fishy odour were compared with each other by sensory analysis. And the effects of active carbon absorption conditions on the transmittance and sensory scores of the gelatin solution were studied by orthogonal experiments. After removing fishy odour,the physicochemical properties of the gelatin were studied and its volatile components were evaluated by simultaneous distillation-extraction (SDE) and gas chromatography-mass spectrometry (GC-MS). Sensory analysis showed that the effects of fishy odour removal of gelatin were significantly different among methods of active carbon absorption,yeast and lactobacillus fermentation (P0.05). Among these methods,the best one was active carbon absorption. The results of orthogonal experiment indicated that the optimal conditions were the ratio of active carbon addition to 5% (w/v) gelatin solution 1.5% (w/v),and incubated at 40 ℃ for 30 min. After removal treatment,fishy odour in the tilapia skin gelatin was barely detectable. The content of crude protein in the gelatin was 91.3% and its gel strength was high up to 301 g. The transmittance of the tilapia skin gelatin solution was higher than before treatment,and the content of water-undissolved matter was diminished. A total of 33 volatile components in tilapia skin gelatin solution before removing fishy odour were detected by simultaneous distillation-extraction (SDE) and gas chromatography-mass spectrometry (GC-MS). The volatile compounds of the gelatin solution were predominantly esters,followed by alcohols,ketones and olefines. After removing fishy odour,a total of volatile components in gelatin solution have been diminished to 26 kinds,which were mainly olefines and ketones. The research shows that active carbon absorption appears as a suitable technique to remove fishy odour in tilapia skin gelatin and to improve their physicochemical properties. So,it can be used for industrial production of fishy odour free tilapia skin gelatin.