Production of agar with low-temperature solubility and high gel strength from Gracilaria by screw extrusion

Abstract: Agar is extracted from red seaweed (Rhodophyceae). It is insoluble in cold water and hydrates. With good characteristics of reversibility of gel and security, agar is widely used in food, bio-engineering, medicine and other fields. Because some foods are sensitive to high temperature, the need for fully hydrate agar by boiling may limit its use in gel applications. It was reported that the agar was mixed with water and other materials such as sugar, locust bean gum, guar gum or other dispersants to improve its solubility. In order to obtain the optimal processing conditions for agar which dissolves at 60℃ with high gel strength, agar from Chinese Gracilaria Lemaneiformis was extruded and processed without adding other substances by a twin screw extruder, then the production was crushed into powder, sieved, sealed and stored. First, processing temperature, liquid to solid ratio, screw revolution speed and nozzle diameter were optimized by single factor experiment for analyzing their influences on the solubility properties at low temperature and gel strength of agar. The process conditions of agar that dissolves at 60℃ with high gel strength were optimized by Box-Behnken design and response surface methodology. Texture analyzer and advanced rheometer were used to determine gel strength and melting-gelling point respectively. The data were analyzed using SPSS 18.0 and Design-Expert 8.0. Finally the physical indexes of agar were determined. The results showed that processing temperature was the most significant factor affecting the solubility properties of agar at low temperature. Because the surface of agar formed porous expanded state at appropriate processing temperature and the particle size of agar after crushing and sieving was moderate, agar with good flowability and dispersibility dissolved at low temperature. Processing temperature and liquid to solid ratio had significantly influence on the gel strength of agar, while screw revolution speed was the least significant factor. Also, processing temperature and liquid to solid ratio had significant interaction. The optimized technological parameters were as follows: processing temperature 126℃, liquid to solid ratio 0.37 mL/g, screw revolution speed 150 r/min and nozzle diameter 6 mm, the measured average gel strength(mass concentration 1%) was 683.6 g/cm2, which was consistent well with the predicted value 693.4 g/cm2. The physical index of agar were as follows: the gelling temperature 31℃, the melting point 72℃, moisture content 15.2%, pH value 5.64. This study can provide a reference for factory production. Gracilaria is the main species utilised for commercial agar production in China. The improved solubility and high gel strength allow the gelling agent agar to be used more widely in food applications. The production of agar by this method exhibits low solution temperature, high gel strength and saving the industrial production of energy, but it requires 7-10 min to dissolve under stirring. The instant agar at low temperature will be the main direction of the further development.