Introduction
Aquaculture is the fastest growing food industry for decades, a large user of fish oil (FO) and fishmeal for fish feed production. FO resources are found in finite supplies, and partially replaced by vegetable oils (VO). The addition of VO to fish feed has limitations such as competition with food production, high carbon footprint and may indirectly result in rain forest cuttings (note that in most European aquaculture industries, rapeseed oil is used, rarely or not palm- or soya oil). Single cell oil produced by cultivation of oleaginous yeasts on lignocellulose sources has the potential to solve several of the problems related to VO. However, utilisation of lignocellulose for growth of oleaginous yeasts requires thermochemical pretreatment, which requires energy. Energy is also required for fermentation and other processes required for production of microbial oil. Moreover, pretreatment of lignocellulose may generate persistent organic pollutants (POPs) from the polyaromatic compound lignin. POPs can also be taken up by the plant from the environment. These compounds could be biomagnified through the food chain and result in metabolic changes/unwanted side effects. The aim of this study was to evaluate food safety aspects, as well as energy use, of a novel sustainable fish feed ingredient produced from yeast cultivation on lignocellulose (wheat straw) hydrolysate and to determine whether its use would be advantageous compared to using VO.
Materials and Methods
Cultivated yeast was screened for the presence of persistent organic compounds of concern by GC/MS. Analytical methods were optimized and validated. POPs quantification was performed by the isotope dilution method; isotopically labelled standards were be used for each compound.
Arctic char (Salvelinus alpinus) was fed a diet in which VO and parts of the protein content were replaced by biomass of the oleaginous yeast Rhodotorula toruloides (Brunel et al. 2022). Growth of the fish was measured and the lipid composition of the fish was determined and compared to a control. Additionally, impact of new feed on fish was identified by measuring the catalytic activities of cytochromes 1A1 enzymes. Sensory analysis was performed with the triangle test method (Sinkinson, 2017) at the Swedish University of Agricultural Sciences (SLU) in November 2019 with 34 untrained volunteers.
In the system analysis, the outcome was a mass and energy balance for the yeast oil-producing biorefinery (Karlsson et al. 2016). The mass balance was represented by the yield of farmed salmon, yeast oil and other valuable outputs per tonne of straw input. The energy balance covered the fossil primary energy demand per tonne of salmon, per tonne of yeast oil produced and per tonne of wheat straw used, which was the sum of all non-renewable primary energy used for inputs in the manufacturing process. Four scenarios representing different process designs of the biorefinery were also tested and themass and energy balance modelling was carried out in Aspen Plus.
Results and Discussion
The investigated heavy metals and organic pollutants in the yeast biomass were below the limits of the regulations defined by the European Union. Moisture and ash contents, fatty acid profile and protein contents were similar between feed containing yeast oil and the control, confirming previous results with another oleaginous yeast species (Blomqvist et al. 2018). No significant differences were found in the hepatic activity of 7-ethoxyresorufin-O-deethylase. Sensory analysis did not reveal any perceptible sensory difference.
System analysis showed that production of 1 tonne of yeast oil would require 9.2 tonne of straw, 14.7 GJ in fossil primary energy demand, 14.6 GJ of process electricity and 13.3 GJ of process heat, while 21.5 GJ of biomethane (430 kg) and 6 GJ of excess power would be generated simultaneously. By applying economic allocation, the fossil primary energy demand was estimated to 11.9 GJ per tonne oil. In comparison with the commonly produced rapeseed oil, the primary energy demand of the oil among the scenarios tested, was 10-38% lower.
The results show that at least partial replacement of VO by oil from oleaginous yeasts grown on lignocellulose hydrolysate in aquaculture feed is safe and can be a sustainable alternative.
References
Blomqvist, J., Pickova, J., Tilami, S.K., Sampels, S., Mikkelsen, N., Brandenburg, J., Sandgren, M., Passoth, V., 2018. Oleaginous yeast as a component in fish feed. Sci Rep. 8: 15945
Brunel, M., Burkina, V., Pickova, J., Sampels, S., & Moazzami, A.A., 2022. Oleaginous yeast Rhodotorula toruloides biomass effect on the metabolism of Arctic char (Salvelinus alpinus). Frontiers in Molecular Bioscience, 9: 931946
Karlsson, H., Ahlgren, S., Sandgren, M., Passoth, V., Wallberg, O., Hansson, P.A. 2016. A systems analysis of biodiesel production from wheat straw using oleaginous yeast: process design, mass and energy balances. Biotechnol Biofuels 9: 229
Sinkinson, C., 2017. Triangle Test. Discrimination Testing in Sensory Science: A Practical Handbook, 153–170. doi:10.1016/B978-0-08-101009-9.00007-1.