High-Value-Added Compound Recovery with High-Temperature Hydrothermal Treatment and Steam Explosion, and Subsequent Biomethanization of Residual Strawberry Extrudate

Abstract

This study was on the comparison of hydrothermal treatments at 170 °C (steam injection) and 220 °C (steam explosion) to solubilize the organic matter contained in residual strawberry extrudate, focusing on phenolic compounds that were susceptible to be extracted and on sugars. After the extraction step, the remaining strawberry extrudate phases were subjected to anaerobic digestion to generate biogas that would compensate the energy requirements of the suggested hydrothermal treatments and to stabilize the remaining waste. Hydrothermal treatment at 220 °C allowed the recovery of 2053 mg of gallic acid eq. per kg of residual strawberry extrudate. By contrast, after hydrothermal treatment at 170 °C, only 394 mg of gallic acid eq. per kg of residual strawberry extrudate was recovered. Anaerobic digestion processes were applied to the de-phenolized liquid phase and the solid phase together, which generated similar methane productions, i.e., around 430 mL CH4/g volatile solids, after both 170 °C and 220 °C hydrothermal treatments. Considering the latest observation, hydrothermal treatment at 220 °C is a preferable option for the valorization of residual strawberry extrudate (RSE) due to the high solubilization of valuable phenolic compounds that can be recovered.

Mesophilic Semi-Continuous Anaerobic Digestion of Strawberry Extrudate Pretreated with Steam Explosion

“…The extraction of the phenolic compounds was carried out with a column, 4.5 cm in diameter and 140 cm in height, filled with 1 L of adsorbent resin Amberlite XAD-16, which was described by Cubero-Cardoso [ 4 ]. The extraction of the phenolic compounds retained in the Amberlite was carried out according to Fernández-Bolaños et al [ 13 ].…”

“…During the industrial process for obtaining strawberry concentrate, the strawberries are extruded through sieves with different mesh sizes, in such a way that a residual fraction, formed by the fibrous part and the achenes, is retained, called the strawberry extrudate (SE) [ 3 ]. SE is, to some extent, used as animal feed, but most of it is landfilled [ 4 ]. Alternatives for the management of the SE are necessary to avoid severe environmental impacts caused by landfilling, such as negative effects on agricultural soil quality, atmospheric contamination, and/or the polluting of aquatic ecosystems.…”

“…Cubero-Cardoso et al [ 4 ] proposed the recovery of bioactive compounds from SE using heat treatments. These bioactive compounds can be used in the pharmaceutical, food, and/or cosmetic industries [ 2 , 3 ].…”

“…Under steam-explosion conditions, the lignin is highly degraded, and the hemicellulose is hydrolysed into its component sugars [ 3 ]. Cubero-Cardoso et al [ 4 ] showed that the steam-explosion treatment of SE at 220 °C allowed the recovery of up to 2 g of gallic acid equivalents per kg of residual SE.…”

“…Cubero-Cardoso et al [ 4 ] also studied the anaerobic digestion of the steam-exploded and de-phenolised SE in batch mode. The anaerobic digestion of lignocellulosic materials, such as SE, is a promising alternative to valorising these wastes because this treatment allows generating renewable and sustainable energy in the form of biogas [ 8 , 9 ].…”