Optimization of Autohydrolysis of Olive Pomaces to Obtain Bioactive Oligosaccharides: The Effect of Cultivar and Fruit Ripening

Abstract

The valorisation of agro-industrial residues presents a challenge in obtaining economically sustainable and environmentally friendly industrial processes. Olive pomace is a by-product generated in large quantities, from olive oil extraction. This residue mostly consists of lignocellulosic materials. The aim of this study was to evaluate the potential use of extracted olive pomaces (EOP) obtained from olives with different ripening indexes (RI) and from different cultivars (Cobrançosa; RI = 2.5; 3.3 and 4.7; and Galega Vulgar; RI = 1.8; 2.9 and 4.8), to produce bioactive oligosaccharides from hemicelluloses by autohydrolysis. The hydrothermal treatment conditions were optimized by Response Surface Methodology, following a central composite rotatable design (CCRD), as a function of temperature (T: 142–198 °C) and time (t: 48–132 min), corresponding to severity factor (SF) values from 3.2 to 4.9. For all pomace samples, soluble sugar production was described by concave surfaces as a function of temperature and time. Autohydrolysis with SF equal or higher than 4.0 produced higher sugar yields, with maximum values around 180 g glucose equivalent/kg EOP for SF of 4.7 (190 °C/120 min) or 4.9 (198 °C/90 min). These values were similar for both cultivars and were not dependent on the ripening stage of the olives. Maximum oligosaccharide (OS) yields of 98% were obtained by autohydrolysis with SF of 4.0. The increase in SF to 4.9 resulted in a decrease in OS yield to 86–92%, due to the release of monomeric sugars. The monosaccharides were mostly xylose (55.8–67.7% in Galega; 50.4–69.0% in Cobrançosa liquid phases), and glucose, galactose, arabinose and rhamnose, in smaller quantities. Therefore, the production of bioactive xylo-oligosaccharides (XOS) from olive pomaces mainly depends on the hydrothermal conditions used.

Towards the Integral Valorization of Olive Pomace‐Derived Biomasses through Biorefinery Strategies

“…S1 shows the chemical composition (extractives, cellulose, hemicellulose, lignin, protein, and fat content) of different olive pomace-derived biomasses, which is crucial to consider in biorefinery cascading schemes in order to select the best valorization route in each case. The olive cultivar, farming practices, and fruit ripening can also contribute to variation in the chemical composition of these biomasses [40][41][42]. This explains, at least in part, the observed variation of the data within the same biomass.…”

“…S1). Certain variations in these contents have been found in olive pomace of different origins [40], which can be attributed to the olive cultivar, ripening degree, and processing practices [40,42].…”

“…Their qualitative and quantitative composition will depend on the biomass type [15]. Other crucial factors are the olive cultivar, fruit collection mode, and olive oil extraction technology, among others [11,42]. This review has focused on sugar derivatives, phenolic compounds, and triterpenic acids owing to the current industrial interest.…”

“…Therefore, they present great potential for formulating functional foods and pharmaceutical products [72]. The hemicellulose fraction of lignocellulosic biomass is known to be a rich source of oligosaccharide production [73], most notably xylo-oligosaccharides (XOS) that derive from xylans [42]. In this type of oligomers, D-xylose residues are linked via β-1,4-glycosidic bonds [74], e.g., Fig.…”

“…For an environmentally sustainable extraction process, selecting the solvent is also a crucial factor, especially when looking for alternatives to conventional, toxic organic solvents [42]. Water or steam, ethanolic water solutions, and supercritical CO 2 are currently affordable and greener than solvents of fossil origin.…”