Obtaining Cellulose Fibers from Almond Shell by Combining Subcritical Water Extraction and Bleaching with Hydrogen Peroxide.

Almond shell (AS) represents about 33% of the almond fruit, being a cellulose-rich by-product. The use of greener methods for separating cellulose would contribute to better exploitation of this biomass. Subcritical water extraction (SWE) at 160 and 180 °C has been used as a previous treatment to purify cellulose of AS, followed by a bleaching step with hydrogen peroxide (8%) at pH 12. For comparison purposes, bleaching with sodium chlorite of the extraction residues was also studied. The highest extraction temperature promoted the removal of hemicellulose and the subsequent delignification during the bleaching step. After bleaching with hydrogen peroxide, the AS particles had a cellulose content of 71 and 78%, with crystallinity index of 50 and 62%, respectively, for those treated at 160 and 180 °C. The use of sodium chlorite as bleaching agent improved the cellulose purification and crystallinity index. Nevertheless, cellulose obtained by both bleaching treatments could be useful for different applications. Therefore, SWE represents a promising green technique to improve the bleaching sensitivity of lignocellulosic residues, such as AS, allowing for a great reduction in chemicals in the cellulose purification processes.

Optimization and characterization of reinforced biodegradable cellulose-based aerogels via polylactic acid/polyhydroxybutyrate coating.

Vine shoots (VS) and waste eucalyptus paperboard (EP) have been used as cellulose sources (in the form of cellulose nanocrystals -CNCs- and cellulosic fibers respectively) for developing cellulose-based aerogels. Two different parameters including cellulose concentration (0.5 % and 2 % w/v) and freezing temperatures (-20 °C and -80 °C) were tested to evaluate differences in the porosity of the aerogels via Brunauer-Emmett-Teller (BET) and thermal conductivity analyses. In addition, a supplementary coating was applied to the raw aerogels by means of dipping the materials in either polylactic acid (PLA) or polyhydroxybutyrate (PHB) solutions (1 % w/v). Their microstructure was observed via SEM and the reinforcing capacity provided by the coating was measured by means of mechanical compressive tests (~10-fold improvement) and water resistance (contact angle >100°). Finally, aerogels' biodegradability was also confirmed according to the standard ISO 20200 thus providing a sustainable and high-performance alternative to conventional materials also following circular economy principles.

Subcritical Water Extraction for Valorisation of Almond Skin from Almond Industrial Processing.

Almond skin (AS) is an agro-industrial residue from almond processing that has a high potential for valorisation. In this study, subcritical water extraction (SWE) was applied at two temperatures (160 and 180 °C) to obtain phenolic-rich extracts (water-soluble fraction) and cellulose fibres (insoluble fraction) from AS. The extraction conditions affected the composition and properties of both valorised fractions. The dry extracts obtained at 180 °C were richer in phenolics (161 vs. 101 mg GAE. g-1 defatted almond skin (DAS)), with greater antioxidant potential (1.063 vs. 1.490 mg DAS.mg-1 DPPH) and showed greater antibacterial effect (lower MIC values) against L. innocua (34 vs. 90 mg·mL-1) and E. coli (48 vs. 90 mg·mL-1) than those obtained at 160 °C, despite the lower total solid yield (21 vs. 29%) obtained in the SWE process. The purification of cellulose from the SWE residues, using hydrogen peroxide (H2O2), revealed that AS is not a good source of cellulose material since the bleached fractions showed low yields (20-21%) and low cellulose purity (40-50%), even after four bleaching cycles (1 h) at pH 12 and 8% H2O2. Nevertheless, the application of a green, scalable, and toxic solvent-free SWE process was highly useful for obtaining AS bioactive extracts for different food, cosmetic, or pharmaceutical applications.