Optimization of microwave-vacuum drying for detoxified starfish

Starfish is widely distributed in inshore aquaculture zones in China and it is the predator for many aquaculture animals such as oyster, scallop and abalone. Since starfish has strong fecundity and regenerative capacity, the number of starfish has been growing in recent years with the development of inshore aquaculture. The annual yield of starfish has reached 50 000 tons in Dalian, which seriously imperils the development of aquaculture industry. Starfish is rich in nutrients such as protein and polysaccharide, but it contains some toxic substances (such as saponin) as well. It is accordingly a potential resource for feed processing industries after being detoxified. The drying of detoxified starfish has been paid more attention because the traditional drying methods are inefficient and hardly for product quality controlling. In this paper, microwave-vacuum drying of detoxified starfish was carried out and the drying rate, product quality and energy consumption were evaluated. The influences of microwave power density (3-7 W/g), pulse time (20-60 s) and relative vacuum degree (between -0.070 and -0.090 MPa) on the drying characteristics, energy consumption and protein retention rate were firstly studied by single-variable experiments. Results showed that when the microwave power density increased from 4 to 7 W/g, the drying time reduced from 18 to 11 min, the average drying rate increased from 2.95%/min to 4.87%/min, and the energy consumption decreased from 0.0130 to 0.0054 kW·h/g, but the protein retention rate dropped to 83.06% from 92.77%; meanwhile, when drying under the pulse time of 60 s, the average drying rate, energy consumption and protein retention rate would be respectively 1.17 times, 75.16% and 97.47% of that when drying under 20 s pulse time. Similarly, drying under -0.090 MPa relative vacuum degree, the average drying rate, energy consumption and protein retention rate were 1.19 times, 68.41% and 1.09 times of that drying under -0.070 MPa, respectively. It would be more favorite by controlling the microwave power density, pulse time and relative vacuum degree respectively within 4-7 W/g, 30-60 s and -0.080--0.090 MPa. In addition, the response surface methodology (RSM) was employed for the multi-variable experiments, and the regression model was accordingly established. Results of RSM experiments demonstrated that the drying process of detoxified starfish was significantly influenced by microwave power density, followed by pulse time and relative vacuum degree. The microwave power density and the pulse time had significant effect on the comprehensive score (P<0.01). An interaction worked between the microwave power density and the pulse time on comprehensive score (P<0.05). The average drying rate of starfish, energy consumption, protein retention rate and comprehensive score would be 2.95%/min, 0.0118 kW·h/g, 93.80% and 0.751, respectively, under the drying conditions of 4 W/g microwave power density, 60 s pulse time and -0.090 MPa relative vacuum degree. Under this condition, the energy consumption of 1 kg detoxified starfish dried with microwave vacuum drying method could be saved by 8 kW·h compared to hot air drying and the drying cost of starfish could reduce by 4 RMB. Meanwhile, a higher protein retention rate and good quality of the dried detoxified starfish could be obtained by microwave vacuum drying. Microwave-vacuum drying is an effective method for detoxified starfish processing and it is a promising approach for the industrial utilization of starfish.