A systematic literature review of environmental assessments of citrus processing systems, with a focus on the drying phase.

Citrus processing results in a considerable quantity of citrus waste that must be treated in an environmentally friendly manner to minimise its overall impact and address the issue of resource circularity. The phase having the greatest environmental impact is the drying phase, since it requires a significant input of energy and time. This phase is essential for many valorisation strategies, as it facilitates the storage and transportation of citrus waste. To gain insight into the Life Cycle Assessment (LCA) of drying techniques, a Systematic Literature Review (SLR) was conducted in this study based on PRISMA statement. Eleven articles were reviewed, to understand the energy requirements and the environmental impacts associated with the process, along with the economical profitability. To that end, attention was focussed upon single- and multi-indicator LCAs. Results from the SLR showed that the drying phase makes citrus waste low valuable for energy valorisation employing gasification, with heat generation efficiency between 21 % and 25 %, or to produce biodiesel, with the impact greater than in conventional diesel usage for a vehicle. Sustainable practices in which the drying phase has a lesser impact include incineration and animal feed. In both cases, dehydration was performed with an industrial direct kiln, requiring the consumption of fossil fuels. Indeed, citrus waste has low thermal potential (low heating value<4 MJ*kg), and its use for thermal energy would result in lower environmental performance than natural gas. Moreover, using citrus waste as a feed additive for livestock is feasible, provided that the distance between the livestock and citrus processing facilities is assessed for sustainability.