Extraction of agar from Gracilaria lemaneiformis based on microwave method and its physico-chemical properties

In order to find the optimal conditions of microwave extraction method in the extraction process of agar from Gracilaria lemaneiformis and investigate microwave's effects on the physico-chemical properties of agar, we extracted agar from G. lemaneiformis (cultivated in Weihai, Shandong) under laboratory conditions by microwave extraction procedure. The effects of the different microwave power (160, 480, 800 W) and microwave time (2, 4, 6, 8, 10 min) on agar yield, gel strength, sulfate content, 3,6-AG content, gelling temperature, melting temperature and other physico-chemical properties were investigated. The results showed that the optimal conditions of the extraction method were as follows: solid-liquid ratio was 1∶60 (W/V), microwave power was 480 W, microwave time was 6 min. The agar yield was 16.20% and the gel strength was 1003.6 g/cm² under these conditions. The effects of microwave power and microwave time on physico-chemical properties of agar were significant, with the increase of the power and the time, the growth of agar yield and gel strength displayed inverted U type changes, and maximum value appeared in the best extraction conditions. 3,6-AG content increased at first and then decreased, while the sulfate content had been declining, and there was a negative correlation between sulphate content and 3,6-AG content in a certain range. Both gelling temperature and melting temperature demonstrated a trend of increasing at first and then decreasing, and the respective maximum values of 30.0 and 97.4 °C appeared in the conditions of 480 W and 6 min. The test results showed that the microwave power and time were important factors in microwave extraction of agar from G. lemaneiformis and they both had significant effects on the physico-chemical properties of agar. In conclusion, this research provided a theoretical evidence for the microwave extraction of agar from G. lemaneiformis.