# Production and carbon footprint of microbial oil from waste lemon peel extract

## TL;DR

This paper shows how to convert waste lemon peel into microbial oil using yeast, with a green extraction method and a carbon footprint analysis.

## Contribution

A novel fermentation process using waste lemon extract and a green solvent-based lipid extraction method for microbial oil production.

## Key findings

- Autoclaving waste lemon extract increases glucose and fructose concentrations to 28.77 g/L and 25.68 g/L.
- Cutaneotrichosporon oleaginosum produced 0.47 g oil/g cell dry weight in a 2 L bioreactor.
- Green solvents extracted 35% of microbial oil from cell dry weight with a carbon footprint of 71–434 kgCO2eq/kg.

## Abstract

The agricultural sector is one of the leading producers of agro-industrial solid organic waste. This waste, mainly disposed of by incineration or landfilled, could be used for the production of high-value chemicals. In this study, a fermentation process for the production of microbial oil from waste lemon extract (LE), an aqueous side-stream deriving from waste lemon peel and pulp processing, was developed and assessed for its impact. Microbial oil can have many diverse applications, from plasticizers in plastic and rubber compounds to moisturizers in cosmetic formulations.

Characterization of LE revealed that its autoclaving process is effective for increasing the concentration of readily available glucose and fructose, reaching 28.77 ± 0.08 g L
-1 and 25.68 ± 0.27 g L
-1. Nitrogen content was measured too, revealing a C/N ratio of 85, optimal for triggering lipid accumulation in the selected microbial cell factory. Therefore, the oleaginous yeast
Cutaneotrichosporon oleaginosum was cultivated in an unmodified LE-based medium in 2 L bioreactors, resulting in a lipid accumulation of 0.47 ± 0.08 g
oil g
CDW
-1. Finally, a new lipid extraction method using green solvents was developed, which allowed to extract and purify 11.29 g of oil, corresponding to 35% of the cell dry weight. The carbon footprint of this laboratory-scale production was estimated to be 71 – 434 kgCO
2eq kg
-1 of microbial oil, with electricity consumption of the fermentation step as the main factor. Simulation of the process in a 300 L fermenter suggests that the overall impact can be drastically reduced with scale-up.

The proposed process is promising in terms of production and does not compete with edible resources and land use. However, the microbial oil yield and the downstream must be optimized to make the process sustainable.

This work is part of the results from the European project “Agro2Circular”, which aims at the development of circular economy examples in the European territory by giving new life to the waste of the agricultural sector. In this study, we developed a strategy to convert lemon juice residues into microbial oil. Microbial oil is not made from vegetables (such as olive oil or seed oils), but from microorganisms, such as yeasts, which are similar to the one we use for beer, bread and wine. This microbial oil can be used to substitute vegetable oils in the cosmetic industry, which have a huge environmental impact. To avoid the use of non-renewable chemicals, we also developed a green and safe extraction method. Finally, we performed a carbon footprint analysis, which allowed us to understand the climate impacts and how to further modify the process to improve its sustainability from an environmental point of view.