Pilot-scale biorefinery of agri-food residual biomass oriented to valuable bioproducts

Resumen

Our society is based on a series of products dependent on crude oil. This model is problematic and unsustainable for three main reasons: (1) the clear depletion of fossil resources, (2) the environmental problems arising from their use as raw material and fuel, and (3) the increasing population that demands more and more products and energy. Among the collateral problems are the dumping of plastics and the global warming related to the emission of greenhouse gases. Plastic is one of the most widely produced materials worldwide due to its application in all industrial sectors. It is a material that stands out for its excellent barrier, mechanical and technical properties, making it very versatile. The main problem with plastic is that it is produced from non-renewable fossil fuels and is not biodegradable, which leads to its accumulation in the environment causing problems for living beings. As an alternative to the use of fossil fuels, the use of renewable sources such as lignocellulosic biomass stands out. Lignocellulosic biomass from vegetal waste is an abundant and inexpensive material containing interesting compounds. The biorefinery concept is equivalent to that of a traditional refinery, but instead of starting from fossil sources, lignocellulosic biomass is used. High value-added products are produced applying fractionation and several purification and transformation processes. The processes applied in biorefineries aim to be environmentally friendly, giving rise to a production model that fits in with the concept of circular economy and sustainability. The most abundant components of lignocellulosic biomass are the extractives (non-structural components) and the structural biopolymers, namely cellulose, hemicellulose, pectin, and lignin. All biomass components can be valorized and transformed through biorefinery processes. Extractives are first extracted and can be formulated if they are bioactive compounds of interest, or they can be transformed into other compounds by e.g., biotechnology. Structural biopolymers can be extracted in their polymeric form from biomass and used as substitutes for traditional polymers from fossil sources. Once biopolymers are extracted, separated, and purified, if their properties are suitable, they can lead to high value-added products such as bioplastics. Bioplastics are biodegradable materials from biomass that have similar properties to plastics and are therefore potential substitutes with enormous environmental and economic advantages. Agricultural and agri-food residues are among the most abundant lignocellulosic biomass residues worldwide. They represent a double problem as they imply an economic loss for producers and are also an environmental issue due to their difficult management. These wastes are often characterized by a high moisture content, which makes them more expensive to transport and accelerates their degradation. Incineration of this waste is discarded because the moisture content makes it inefficient. It is usual for a small proportion of the waste to be used as animal feed and the majority ends up in landfill sites, causing environmental problems in the area. The use as raw material in biorefinery is attractive due to the compounds of interest that this type of waste contains and because they are easily harvested as they are located in specific places (cultivation land, processing plant, etc.). The biomass used in this thesis was the agri-food residue (spent coffee grounds) and the agriculture/agri-food residue (discarded carrots). This thesis demonstrates that a pilot-scale biorefinery can be applied to agricultural and agri-food residues to produce valuable products. We have applied hydrothermal treatment in a multi-reactor system with downstream processes such as ultrafiltration/diafiltration, spray drying, encapsulation, and fermentation. As a result, several intermediate products have been obtained, e.g., carotenoids, sugars, hemicellulose, pectin and lignin-containing cellulose nanofibers. These intermediates have been successfully used to produce biofilms, encapsulated pigments, and fermentation products (lactic acid and ethanol). In Chapter 1, the valorization of spent coffee grounds on a laboratory scale was studied by applying environmentally friendly processes. The first step was to extract the oil, as it is the most abundant extractive. The extraction was performed with supercritical CO2, which is a cheap and non-toxic compound compared to the usual organic solvents. The extracted oil was characterized for potential applications in the food, cosmetic and pharmaceutical industries. The defatted solid was subjected to hydrothermal treatment in a flow-through reactor system for the extraction of the hemicellulose biopolymer, leaving the cellulose and lignin biopolymers in the solid. The extraction of hemicelluloses was studied over time operating at 140 and 160 °C. The hydrothermal extracts were characterized by hemicelluloses with a wide molecular weight distribution and some byproducts such as free sugars, organic acids, and degradation compounds. Therefore, the extracts were subjected to a concentration, separation, and purification process through multistage ultrafiltration and diafiltration using membranes of different molecular weight cut-off (30, 10 and 5 kDa). This process allowed the fractionation of each extract into three liquid products. Certain groups of hemicelluloses were concentrated by a factor of up to 5 with respect to the concentration of the extract. The diafiltration system allowed to purify the hemicelluloses reducing the retention of by-products from 45.6 % to 8.7 %. In total, six hemicellulose fractions were obtained with purities ranging from 83.7-97.8 % and molecular weights from 1641 to 49,733 Da. In Chapter 2, the lignocellulosic biomass used was discarded carrots, which accounted for 30 % of the total carrots harvested. Due to their high moisture content of around 95 %, the first step was the separation of juice and pulp. Most of the extractives from the carrot went into the juice, and this Chapter focused on the valorization of the pulp. The pulp was subjected to hydrothermal treatment for the extraction of the biopolymers hemicellulose and pectin, leaving the cellulose and lignin in the solid. The extraction was performed in a flow-through reactor system operating at 140, 160 and 180 °C. The extracted components reached 211 g/kg dry pulp of free sugars, 29.13 g/kg dry pulp of homogalacturonan pectin, and 70.45 g/kg dry pulp of arabinogalactan hemicellulose. The residual pulp had a majority cellulose content (57.5 %) followed by a lignin content of 15.7 %. It is therefore an interesting material for applications where cellulose is used while presenting the advantages of lignin, such as hydrophobicity. The liquid extracts were characterized by a low biopolymer purity (high content of free sugars) and a wide molecular weight distribution. Consequently, it is interesting to subject these extracts to conditioning for the utilization of the extracted biopolymers, which was developed in Chapter 4. Chapter 3 focused on the valorization of the juice of discarded carrots, complementing Chapter 2, which focused on the valorization of the pulp. The juice was obtained in a similar weight proportion to the pulp and was characterized by a high content of free sugars (sucrose, glucose and fructose) and by the carotenoids content in the form of pulp microparticles. To valorize these two components, the juice was subjected to a physical separation process by means of several cycles of diafiltration with a 30 kDa MWCO membrane. This separation yielded a suspension rich in carotenoids (4996.4 μg/g) and with a very low sugar content. The other fraction was a solution containing free sugars (84.83 ± 3.26 g/L) and nutrients such as minerals and vitamins. Carotenoids are very interesting compounds with potential applications in the food and pharmaceutical industry. Therefore, their encapsulation was studied using gum Arabic and spray drying and freeze drying as drying methods. Encapsulation using spray drying reduced the degradation of the carotenoids by 51.9 % compared to non-encapsulation, and increased the stability of the carotenoids in water resulting in a more stable suspension. The sugar-rich fraction was valorized through three types of fermentation: with autochthonous microorganisms, with lactic acid bacteria, and with yeasts. Fermentation with autochthonous microorganisms and lactic acid bacteria resulted in lactic acid production (up to 17.64 ± 1.54 g/L), while fermentation with yeasts resulted in ethanol production (up to 49.46 ± 0.28 g/L). The addition of 6 % (w/v) NaCl to the medium as an additive prevented contamination by external microorganisms and allowed pure lactic acid to be obtained with both autochthonous microorganisms and lactic acid bacteria. The yeast fermentation resulted in a total consumption of sugars, while the lactic fermentation did not reach total consumption, probably due to the marked decreased in pH, a variable that should be controlled if a higher lactic acid production is desired. In Chapter 4, the raw material were the three hydrothermal extracts obtained in Chapter 2, resulting from the hydrothermal treatment at 140, 160 and 180 °C. Given the compounds present in the extracts and the molecular weight distribution, the biopolymers present in these extracts were concentrated, separated, and purified by means of ultrafiltration and multistage diafiltration processes with membranes. The membranes were of larger scale than those of Chapter 1, and with MWCO of 30, 10, 5 and 1 kDa. The 140 and 160 °C extracts were subjected to a cascade treatment (30-10- 5-1 kDa) versus the 180 °C extract which was subjected to a mixed configuration (5-10- 1 kDa). The highest molecular weight hemicelluloses and pectins increased in concentration by a factor of up to 5 in the cascade configuration and by a factor of up to 16.67 in the mixed configuration. The application of at least five diafiltration cycles on each retentate, with extra cycles reusing previous diafiltration waters, resulted in high removal of free sugars from the fractions (98.9-99.5 %) as well as of by-products (94.2-99.2 %) through the diafiltration waters and through the 1 kDa permeate. The system allowed going form feed with molecular weight, polydispersity and purity in the ranges 9.02-18.83 kDa, 16.2-31.6, and 30.12-33.51 % (w/w) to fractions with values in the ranges 2.59-102.75 kDa, 1.2-4.0, and 73.1-100 % (w/w). The purified solid fractions were dried using freeze drying and stored. Chapter 5 developed the processes investigated in Chapters 2 and 4 on a larger scale to obtain fractions in sufficient quantity for further processing into products. Approximately 11.32 kg of fresh carrots were valorized in each experiment, applying hydrothermal treatment at 140 and 180 °C by means of cycles of several flow-through reactors operating in series at the same time. Free sugars, hemicelluloses and pectins were extracted with maximum yields of 379.51 g/kg dry pulp, 81.01 g/kg dry pulp and 5.35 g/kg dry pulp, respectively. The extraction yield of hemicelluloses reached 96.1 % (w/w). The two target extracts were conditioned applying ultrafiltration and several diafiltration cycles with membranes of 10 and 30 kDa (140 °C) and 10, 30 and 1 kDa (180 °C). By-products were removed at 99.8-100 % and free sugars at 98.7-100 %. Biopolymers from the 140 °C extract were recovered in two fractions (10-30 kDa and > 30 kDa) which yielded 31.4 % of the extracted hemicelluloses and 32.4 % of the extracted pectins. Biopolymers from the 180 °C extract were recovered in three fractions (1-10, 10-30 and > 30 kDa) with percentages of 36.8 % of the extracted hemicelluloses and 97.9 % of the extracted pectins. Fouling during membrane operation was evaluated. The membrane system allowed to go from feeds with molecular weight, polydispersity and purity values of 8.08-14.77 kDa, 18.2-19.2 and 14.9-22.2 % (w/w) to five fractions with the following values: 1) 14.77 kDa, 19.2 and 22.2 % (140 °C) and 2) 8.08 kDa, 18.2 and 14.9 % (180 °C), to fractions with values of 1) 80.36 kDa, 2.4 and 100 % (140 °C), 2) 9.85 kDa, 2.1 and 100 % (140 °C), 3) 67.77 kDa, 3.8 and 100 % (180 °C), 4) 5.23 kDa, 1.3 and 64.5 % (180 °C), and 5) 3.86 kDa, 1.5 and 66.8 % (180 °C). The five fractions were obtained in sufficient quantity to be dried using both freeze drying and spray drying. The solids were stored for further processing into biodegradable films. Chapter 6 focused on the formation of biodegradable films whose main ingredient was the purified solid fractions of hemicelluloses and pectins obtained in Chapter 4 and 5. The residual pulp obtained from the two hydrothermal treatments (140 and 180 °C) applied in Chapter 4 was studied as an additive in the films. A small percentage of residual pulp addition (< 5 %) decreased the oxygen permeability through the film (up to 29 %) but increased the water vapor permeability and worsened the tensile properties. Higher residual pulp content (5-25 %) allowed the film to regain properties similar to those of the reference film (the one without residual pulp) and significantly improved the surface hydrophobicity by increasing the water contact angle from 79.9° (0 % residual pulp) to 125.8° (25 % residual pulp). Glycerol was used as a plasticizing agent in the films at a content of 35 %. The influence of the molecular weight (67.77- 102.75 kDa) and the composition of the purified hemicellulose and pectin fractions on films containing 1 % residual pulp was studied. Higher molecular weight decreased oxygen permeability (from 48.18 to 41.14 cm3·μm/m2/kPa/day), increased water vapor permeability (from 21.56 to 24.01 g·mm/m2/kPa/day), and decreased hydrophobicity (from 86.84° to 71.10°). Tensile strength was higher for higher pectin content and lower molecular weight (from 1.13 to 2.84 MPa), and elongation was higher for higher hemicellulose content (from 5.92 to 15.28 %). The films had acceptable properties for application in the food packaging industry, their main strengths being: 1) 100 % origin from an agri-food waste, 2) production through environmentally friendly processes, 3) non-toxicity, 4) high hydrophobicity without the need for chemical modification, and 5) predictable high biodegradability due to no chemical modification.