Abstract
Duckweed is a potential alternative protein source for food and feed. However, little is known about the nitrate accumulation in this plant. A high nitrate level in vegetables can indirectly lead to an elevated intake of nitrites and N-nitroso compounds, increasing the risk of diseases for humans and animals. This research hypothesizes that the nitrate accumulation of Lemna minor differs between growing media. Additionally, it evaluates whether legal safety levels of nitrate for human and animal intake are exceeded. The duckweed was grown on (i) rainwater, and (ii) three synthetic media containing different nutrient levels. Furthermore, (iii) biological effluent of swine manure treatment and (iv) aquaculture effluent from pikeperch production were used, as these are potential media for closing nutrient loops in the agriculture sector. It was found that nitrate levels increased with the increasing availability of macronutrients in the water, and pH showed a particularly strong negative correlation with the nitrate levels in the plant. Nevertheless, nitrate content never exceeded 530 mg NO3 kg−1 fresh weight. To conclude, Lemna minor's nitrate content was below safety limits for human consumption in all tested growing media; however, a potential risk for ruminants was observed as these are more sensitive to nitrate conversions in their gastro-intestinal track.
Highlights
The growth of the world population and the improvement of living standards have increased the demand for animal-derived protein (Boland et al, 2013; United Nations, 2015), which has considerable environmental implications (de Beukelaar et al, 2019)
There is no significant difference in the nitrate content of DSN and BE, and of DSN and PE, but the nitrate content of PE is significantly lower than that of BE
Rainwater was added in the middle of the experiment to reduce the effect of evaporation, but at the end of the experiment the containers had a mass of 13.7 ± 0.1 kg, which is an evaporation loss of 2.8 ± 0.1 kg of the growing medium
Summary
The growth of the world population and the improvement of living standards have increased the demand for animal-derived protein (Boland et al, 2013; United Nations, 2015), which has considerable environmental implications (de Beukelaar et al, 2019). One potential protein alternative is duckweed (Lemna minor) This is a highly productive plant that has been intensively investigated for its value as a protein ingredient in food and feed (Appenroth et al, 2017; Culley and Epps, 1973; Putra and Ritonga, 2018). These small floating macrophytes occur all over the world and are the most rapidly growing Angiosperms, following a quasi-exponential growth rate (Ziegler et al, 2015). The low amount of fibre makes it readily digestible for monogastric animals and fish (Aslam et al, 2017)
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