Optimal conditions for production of Khai-Nam Wolffia globosa

Study on optimal conditions for the production of Khai-nam (water meal), Wolffia globosa, were carried out. Khai-nam was collected from the natural pond in Mueang district, Sakon Nakhon province, Thailand. According to morphological investigation, it was identified as Wolffia globosa. Under natural condition, asexual reproduction of Khai-nam took approximately 96 hours or 4 days for each generation. Highest density of W. globosa in natural pond was 65.18 g dry weight m-2 in July and the lowest density was 2.45 g dry weight m-2 in February. The highest productivity, 1.05 g dry weight m-2d-1, was found in June and no growth of Khai-nam was detected in August. Nutritional analysis of W. globosa from native pond was 33.3% protein with complete essential amino acids profile, 5.0% fat and 10.4% crude fiber. Bacterial analysis showed that Khai-nam from natural source had low bacterial contamination and therefore accepted for human consumption. Growth optimization of W. globosa was carried out under laboratory conditions. It was found that Hutner’s medium provided high yield of 0.18±0.04 fronds ml-1d-1 with 17.37±2.9 days of life span. With outdoor cultivation, high light intensity during day time did not affect photosynthesis efficiency (quantum yield of PSII) measure by chlorophyll fluorescence technique but high temperature at 40℃ significantly reduced photosynthesis efficiency. The optimum pH for W. globosa was between 5 and 7 and the optimum initial density for W. globosa cultivation was 5 to 20% water surface area coverage. A factorial experiment on light intensity, initial pH and initial density on growth performance and quality of W. globosa indicated that 10,000 lux light intensity, initial pH of 6 and 15% initial density provided the highest yield. Finally, five outdoor culture systems for production of W. globosa were evaluated. The culture systems included (1) static tank, (2) vertical aeration tank, (3) horizontal surface agitation tank, (4) tank with top water spraying, and (5) layer culturing system with top water spraying. The cultivation period was 28 days. The results showed that the tank with horizontal circulation provided the highest yield of 1.52±0.04 g DW m-2 d-1. The biomass produced had 48.2% protein with complete essential amino acids, 9.6% fat, and 14.5% crude fiber with low bacterial contamination.