The effects of process conditions of an alkaline seaweed extraction process on yield and on biostimulant activity of the extract

Abstract

Seaweed extracts (SWEs) comprise the single largest category of agricultural crop biostimulants. The global biostimulants market is much smaller, but faster growing than the markets for either traditional mineral fertilizers or agrochemicals. They are applied at low volumes and concentrations and stimulate plant growth, nutrient use efficiency and stress responses, thus contributing significantly to sustainable crop production.

Alkaline extraction is the most often used process to produce SWEs. It can be operated at ambient or elevated temperatures and pressures. The process studied in this work is the Ambient Alkaline Extraction (AAE) process. It benefits from lower energy costs and simpler equipment than the higher temperature processes, at the expense of longer residence times of days rather hours. The seaweed used as biomass raw material was Australian Bull Kelp Durvillaea potatorum.

The experimental work was conducted on an existing, successful commercial AAE production plant, as well as pilot (1/64th) and bench (1/1800th) scale plants. The process conditions and relationships have, as far as is known, not been published before for any commercial alkaline extraction plant, and for bench scale work only at elevated temperatures. Measurements were developed to track process conditions relating to overall process yield. In addition, six biostimulant properties of the SWE product, as assessed by the tomato seed germination and growth bio-assay were measured at different SWE concentrations and combinations of process conditions (temperature, alkali concentration, water:biomass ratio and residence times). Statistically significant (P<0.05) biostimulant effects of the SWE were reported for the vast majority of process conditions tested, but also statistically significant effects of process conditions on the quantum of growth stimulation.

The results can be used in the design of new production plants or retrofits to existing plants, and in selecting optimal process conditions not only for process yield and throughput, but also for biostimulant effects of the SWE product. In general, there is of course not only a single set of optimal conditions, since conditions for optimal outcomes on different biostimulant parameters can be different, and these can also be different from those for optimal production outcomes such as throughput and yield. Optimal conditions therefore depend on the relative desirability of the different (and in some cases, competing) outcomes.