## 1 Introduction  
The reduction and valorisation of by-products is one of the biggest challenges of the food industry. Nowadays, there is an increasing concern for the achievement of a more sustainable food production chain, specifically, in reducing the environmental impact of its by-products or the costs associated with its proper treatment [1]. Besides the economic and environmental consequences, there is also an ethical and moral dimension within the general concept of global food security, since 805 million people across the globe suffer from hunger [2]. These negative impacts are even more flagrant in growing industries, like the olive oil industry (mainly in the Mediterranean countries), that produce large quantities of by-products and wastes [1, 3]. Portugal is one of the Mediterranean countries where olive oil production has been rising (with an estimated increase of 30% in olive oil production in 2019 compared to the 2018 campaign) [4], which also implies the treatment of more tonnes of waste. Between all the wastes and by-products of the olive industry, the olive pomace is the most representative [5]. Olive pomace is a combination of olive husk and pulp, crushed olive stone and olive mill wastewater with a moisture content of 65% [6] that derives from the modern two-phase extraction method, in which no water is added (more eco-friendly than the three-phase process) [7]. Olive pomace is phytotoxic and non-biodegradable biomass [8] that is challenging to treat due to its richness in moisture (higher energy demand and consequently higher costs during the drying process) and organic compounds (adverse effects on soil) [9]. Nonetheless, olive pomace is also a significant source of bioactive compounds like fibre [10], minerals, polyunsaturated fatty acids and phenolic compounds [5]. Commonly, olive pomace is sent to pomace oil extraction mills, in which, after a drying process, it is used to extract oil using hexane. This process generates pomace oil but also dried pomace that is mainly used as a solid fuel [3] due to its high calorific power [11]. However, dried olive pomace contains oil residues and principally low-weight particles (pulp and olive stones \(<\) 1 mm), which are considered to be harmful compounds related to uncontrolled emission, corrosion and slagging, compromising the performance of olive pomace as solid biofuel [12]. To a lesser extent, olive pomace can also be composted to avoid the harmful effects of the organic compounds and used as a fertiliser [9], or even used to generate or cogenerate electricity [13]. However, these traditional olive pomace treatments were considered a wastage of bioactive compounds with high value-added, as these treatments are not the most sustainable options because of their use of solvents and high carbon dioxide equivalent CO\({}_{\text{2-eq}}\) emissions [9; 13]. Besides, under the aim of a transition from a non-sustainable linear economy ('take-make-dispose of') to a circular bioeconomy (maintain the value of products and materials for as long as possible while minimising resource use and waste generation) of the olive oil sector, higher-value uses should be prioritised over the current energy and compost valorisation [14]. Bioactive compounds of olive pomace have been described as potent food antioxidants and antimicrobials and can even contribute to the prevention of chronic diseases such as cardiovascular disease and cancer [7]. Having this in consideration, olive pomace should be firstly employed as food source ingredients and only after as an energy generator [14]. The adoption of valorisation approaches to increase the value of olive pomace biomass is a crucial target to permit the olive oil sector to enhance its economic and environmental sustainability. A considerable number of valorisation methods of olive pomace have been studied in the last years, but none of the proposed methods appear to answer entirely to the principal problem.