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Oligosaccharides and monosaccharides were obtained from agro industrial and agricultural residues by hydrothermal treatment

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  • Save Food and Public Health 2014, 4(3): 123-139 DOI: 10.5923/j.fph.20140403.08 Obtaining Oligo- and Monosaccharides from Agroindustrial and Agricultural Residues Using Hydrothermal Treatments Fiorella P. Cardenas-Toro, Sylvia C. Alcazar-Alay, Tânia Forster-Carneiro, M. Angela A. Meireles* LASEFI/DEA/FEA (School of Food Engineering)/UNICAMP (University of Campinas), Rua Monteiro Lobato, 80; Campinas, SP; CEP: 13083-862, Brazil Abstract Agricultural and agroindustrial residues are major sources of cellulose, hemicellulose, and starch that can be converted into bioactive compounds, such as oligosaccharides and monosaccharides, using various chemical and biological methods. These bioactive compounds can be used as raw materials by food, cosmetic and pharmaceutical industries, as well as in the production of intermediate products and the development of biomaterials by chemical industries. In Brazil, the major industrial residues, which are corn residues, soybean residues, sugarcane bagasse, palm and coconut fibers, and grape and tomato seeds, among others, are produced at a rate of approximately of 600 million tons per year. Thus, the utilization of these residues using sustainable technology is of great interest. Hydrothermal treatment is a green technology that includes autohydrolysis as well as subcritical and supercritical hydrolysis, in which water is used at high pressures and temperatures to recover polysaccharides from complex vegetal matrices. The hydrolytic mechanisms can be improved by changing the ionic product or the polarity and electrical conductivity of water in subcritical and supercritical states. These properties promote the selective dissolution of the starch, hemicellulose and cellulose in the residues. The conversion of starch and hemicellulose into oligosaccharides and monosaccharides is preferentially performed at temperatures of less than 200°C. In contrast, the conversion of cellulose into oligosaccharides is promoted at temperatures greater than 200°C, with the highest amount oligosaccharide formation occurring at close to the critical point. In this article, the main biomass components, the properties of water under subcritical and supercritical conditions, and the latest studies of polysaccharide conversion in biomasses using hydrothermal treatments are reviewed. Keywords Monosaccharides, Oligosaccharides, Biomass, Subcritical water, Supercritical water, Hydrothermal treatment 1. Introduction The continuous population increase and the rapid consumption of non-renewable resources in the world have promoted extensive research of sustainable alternatives that could help to preserve the ecosystem for future generations [1]. A lignocellulosic biomass is a complex vegetal matrix composed mainly of cellulose, hemicellulose, starch and lignin. It is one of the most abundant raw materials in earth and it can be converted into marketable products without competition with the food supply chain. Recently, studies of biomass transformation have focused on the recovery of carbohydrates from starch, hemicellulose and cellulose to produce oligosaccharides, monosaccharides and ethanol [2]. The depolymerization of hemicellulose and cellulose into * Corresponding author: (M. Angela A. Meireles) Published online at Copyright © 2014 Scientific & Academic Publishing. All Rights Reserved oligosaccharides, monosaccharides and their degradation products using conventional techniques, such as acid hydrolysis, alkaline hydrolysis and enzymatic hydrolysis, has been extensively studied [3]. However, these techniques present disadvantages in processing operations such as equipment corrosion, necessary neutralization steps and long reaction periods. In addition, the development of new economically feasible technologies for the production of target compounds with high yield and less pollution is required. Hydrothermal technologies are green technologies that are suitable for the recovery of valuable compounds from biomass in which water is used at high temperatures and pressures without the addition of catalysts. Among these technologies, autohydrolysis, subcritical hydrolysis and supercritical hydrolysis are the most studied techniques. Water at a high temperature and pressure acts as a reaction medium for chemical processing of biomass because of the change in its physicochemical properties compared to water under normal conditions. This change of properties includes a reduced dielectric constant which increases the solubility 124 Fiorella P. Cardenas-Toro et al.: Obtaining Oligo- and Monosaccharides from Agroindustrial and Agricultural Residues Using Hydrothermal Treatments of organic compounds, an increased ionic product which promote acid-base reactions and a greater density which improves solubility. These conditions led water to be considered as a suitable solvent and reaction medium for the decomposition of biomass. Several studies using model compounds composed of starch, hemicellulose and cellulose have shown that the use of water at mild temperature conditions (less than 200°C) led to the complete degradation of starch and hemicellulose into oligosaccharides, whereas supercritical water promoted the complete depolymerization of cellulose during short residence periods. In this paper, a review of the state of the art of hydrolysis of the cellulose, hemicellulose and starch present in agricultural and agroindustrial biomasses is presented, aimed at a better understanding of the operational conditions that promote the formation of oligosaccharides and monosaccharides. 2. Biomasses The production of plant biomasses, which are typically the non-food parts of plants, is based on the photosynthetic process, during which polymeric carbohydrates with a ratio of [CH1.4O0.6]n are generated. The chemical components of biomass are cellulose, hemicellulose, starch, lignin, ash and small amounts of extractable compounds, such as lipids and proteins. The main sources of lignocellulosic biomass can be classified as follows: forest residues; agricultural and agro-industrial residues; energy crops; municipal solid waste and organic industrial waste [3]. Cellulose, hemicellulose and starch are macromolecules composed of different types of sugars, whereas lignin is an aromatic polymer synthesized from phenylpropanoid precursors. Additionally, these components, including oligosaccharides, pectins, gums and waxes, are elements of dietary fiber, which is recognized for its health benefits and is included in the processing of fiber-rich products by food industry [4]. Table 1 shows the relative amounts of cellulose, hemicellulose, starch and lignin in various agricultural and agro-industrial lignocellulosic residues. 2.1. Major Polysaccharide Components of Biomasses 2.1.1. Cellulose Cellulose is the most abundant biopolymer in nature. It is a linear homopolymer of β-(1→4)-D-glucopyranose with a high degree of polymerization (DP), of between 200 and 12,000. These DP values depend on the origin of the cellulose and the pretreatment of the biomass and affect its properties. Cellobiose is the basic structural unit of cellulose, consisting of two units of 4-O-β-D-glucopyranosyl -β-D-glucopyranose. Figure 1 shows the chemical structure of cellulose in a chair conformation (the most common conformation for cellulose). The C-2, C-3 and C-6 of each glucopyranose unit are linked to hydroxyl groups that undergo the characteristic reactions of primary and secondary alcohols. Moreover, the hydroxyl groups that are linked to C-1 and C-4 at the end of cellulose chain are the most reactive groups for intra- and intermolecular iterations and degradative reactions because they have reducing and non-reducing properties, respectively [5]. Cellulose constitutes up to 30% of the primary cell wall and 40 to 90% of the secondary cell wall [6], [7]. The basic units of cellulose are linked by intermolecular hydrogen bonds to form microfibrils with a diameter of between 3-25 nm [8], [9]. The strong hydrogen bonding between the cellulose units is mainly due to the close interaction between the hydroxyl groups in the equatorial orientation and hydrogen atom in the axial orientation, which results in high crystallinity and low solubility in aqueous media (8-14% solubility in water under normal conditions). Moreover, the microfibrils represent amorphous regions (15-20%) that are more vulnerable to chemical and enzymatic attack than are the crystalline regions [10]. Cellulose is not digested by human gastrointestinal enzymes and it has been used as a bulking additive in food manufacturing due to its water-absorbing ability and low solubility [4], [11], [12]. HO HO 4 3 5 2 OH 6 O 1 O H HO OH OH O HO O OH OH O OH OH 4 O HO 3 5O 21 OH 6 OH n-2 cellobiose unit Figure 1. Molecular structure of cellulose (adapted from Klemm et al. [5]) Food and Public Health 2014, 4(3): 123-139 125 O Xyl O Xyl O Ara O Ara Fer COOH O Xyl O Xyl O Xyl O Xyl O Xyl O Gluc Acid O O O Ac Ara Ara Ara Fer a) O Fuc O Xyl Xyl O Xyl O O Glu Xyl O O Glu Gal O O Glu Xyl O Xyl O O Glu O Glu Xyl O O Glu Xyl O O Glu b) O Glu Gal O Gal O Gal O Gal O O Man O Glu O Man O Man O Glu O Man O Man O Man c) Figure 2. Schematic representation of the main types of hemicellulose in plants: a) glucuronoarabinoxylan, b) xyloglucan and c) galactoglucomannan (adapted from Scheller and Ulvskov [13] 2.1.2. Hemicellulose Hemicellulose is a heterogeneous branched polymer with a backbone composed of glucose, mannose and xylose joined by β-(1→4) linkages in an equatorial configuration, and in some cases, linkages with uronic acid. Hemicellulose has structural and physic-chemical properties that are different from those of cellulose, pectin and starch. There are several classes of hemicelluloses, such as xylans, xyloglucans, mannans, glucomannans, arabinoxylans, glucoroarabinoxylans, 4-O-methyl-glucuronoxylans and β-(1→3,1→4)-glucans, as shown in Figure 2. In general, their structures resemble a long rod with branches and side chains linked by hydrogen bonds, and they have lower molecular weights than cellulose. In addition, hemicelluloses comprise a soluble fraction and an insoluble fraction, which can be degraded by intestinal microflora [13]. Hemicelluloses have many applications in the food industry, for example, seeds such as guar gum (galactomannans) and tamarind gum (xyloglucan) are used directly in products as bulking agents. β-(1→3,1→4)glucans, a major component of barley and oat cells, contributes to the reduction of serum cholesterol concentrations. This compound is used in cereal products as soluble dietary fiber, and a daily intake of this hemicellulose has been recommended by the Food and Drug Administration [11], [13], [46], [47]. The hydrolysis of xylan to yield xylooligosaccharides (non-digestible oligosaccharides) using acid and enzymatic hydrolysis has been proposed to produce new prebiotics and soluble fibers. Xylooligosaccharides are used as dietary sweeteners and for food for diabetics [48]. 2.1.3. Starch Starch is one of the most abundant carbohydrates that is synthetized by plants and it can be found in roots, cereals and fruits. The most important sources of commercial starch are corn, potatoes, wheat, rice, cassava (tapioca) and sorghum, which provide the major energy sources in human and animal diets. Additionally, starch is the second major component of biomass, after cellulose. Starch is mainly composed of two homopolymers of glucose: amylose and amylopectin. Generally, normal starch (cereal starches) consists of 25% amylose and 75% amylopectin, waxy starches (maize, rice, and sorghum) consist of 0-8% amylose and high-amylose starches (maize and barley) consist of 126 Fiorella P. Cardenas-Toro et al.: Obtaining Oligo- and Monosaccharides from Agroindustrial and Agricultural Residues Using Hydrothermal Treatments 40-70% amylose [11], [49], [50]. Furthermore, in nutritional terms, starch can be divided into the three following categories: rapidly digestible starch (RDS), slowly digestible starch (SDS) and RS (resistant starch). RDS is starch that is rapidly and completely digested in the small intestine, tends to be rapidly degraded to glucose and has a high glycemic index. SDS is starch that is digested in the small intestine at a lower rate than RDS. RS is the starch that is fermented in the large intestine by the gut microflora. This classification helps to predict the glycemic response to foods. Numerous physiological effects have been attributed to resistant starch, leading to its use as a dietary fiber and as a prebiotic ingredient, and include its contribution to preventing colon cancer and its hypoglycemic effect [51-53]. Table 1. Composition of agricultural and agroindustrial residues (% dry basis) Biomass Silver bird Betula pendula Hardwood pine chips Japanese beech Fagus crenata Sugi wood Cryptomeria japonica Fern fronds Pteris vittatal Switchgrass (Panicum virgatum) Beech wood Eucalyptus globulus wood Rapeseed straw Country Finland United States Japan Japan France United States Turkey Spain Poland Cellulose 43.1a 40.0a 45.0 43.0 34.0 42.9a 45.1 46.3 49.2 Hemicellulose 20.9b 17.0b 29.0 23.0 34.0 23.5b 31.5 20.7 14.6 Starch Pectin - - - - - - - - - - - - - - - - - - Corn stalks China 17.7 30.7 - - Corn stalks China 22.8 43.0 - - Corn cobs Spain 36.8 30 - - Corn cobs Spain 34.3 31.1 - - Corn stover United States 34.5 27.7 - - Rice husks Spain 34.0a 15.9b - - Rice husks China 25.4 49.6 - - Defatted rice bran Japan - - - - Maize straw China 25.5 30.3 - - Rye straw Germany 43.6a 20.0b - - Wheat straw China 23.5 12.6 - - Wheat bran China 38.6 21.5 - - Wheat husks India 36.0 18.0 - - Babassu flour Brazil 13.5 8.0 60.1 - Sugarcane bagasse Japan 36.4 Water lettuce biomass China 53.0 Ginger bagasse after CO2 extraction Brazil - Citrus junos peels after CO2 extraction Japan 29.0 Cassava flour waste Thailand 16.0 37.2 - - 15.0 - - - 56.5 - 11.7 - 32.2 5.0 60.0 - Cassava pulp Indonesia - Brewer’s spent grain Portugal - Almond shells Spain 26.8 - 79.5 - - - - 32.5 - - Oil palm fronds Malaysia 44 30 - - Empty fruit bunches Korea 28.7 27.4 - - Oil palm shells Malaysia 39.7 21.8 - - Pressed palm fibers Brazil - - - - Reported as: a glucose, b xylose, c nitrogen-free soluble substance, d crude fiber Carbohydrate - - - - - 54.6 - - 50 48.6 Soluble sugars 2.6 Lignin Protein Ash Ref. 8.2 - - [14] - 31.0 - 2 [15] - 26.7 - - [16] - 32.0 - - [17] - 35.0 - - [18] - 21.8 - 2.4 [19] - 22.3 [20] - 22.9 - - [21] - 21.6 - - [22] 29.0 8.5 - 5.1 [23] - 15.6 - - [24] - 23.1 - 1.0 [25] - 18.8 3.3 1.3 [26] - 17.8 - 5.6 [27] - 23.3 2.5 11.2 [28] - 22.8 - 17.0 [29] - 11.1 19.3 13.6 [30] - 18.3 - 3.6 [29] - 22.0 - - [31] - 36.1 - 9.9 [23] - 23.8 - 4.1 [29] - 16.0 - - [32] - 15.8 3.0 - [33] - 16.8 - - [34] - 6.0 14.0 2.3 [35] 6.1 - 12.0 5 [36] - - - - [37] - 19.0 - - [38] - - 5.4 0.7 [39] - - 30 5 [40] - 27.4 - 2.8 [41] - 15.4 - 3.2 [42] - 26.5 - 3.1 [43] - 32.5 - - [44] - 32.1 6.3 3.1 [45] Food and Public Health 2014, 4(3): 123-139 127 Figure 3 shows schematic representations of amylose and amylopectin. Amylose is defined as a linear polymer of between 500 and 6000 units of α-(1,4)-D-glucopyranose with minor branches of a few molecules via α-(1,6) linkages [54]. The α-(1,4)-glycosidic bonds confer a random helical conformation of the molecule, in which the hydroxyl and hydrocarbon groups of anhydroglucose units are joined to form helices with parallel or antiparallel orientations. Analysis of the structure of amylose using X-ray diffraction revealed the presence of starch types A and B. These types are based on the hexagonal and antiparallel orientation of helical molecules, where in which type A is a helical molecule with eight molecules of water in the central region and type B has 36 water molecules in the central region. Amylopectin is a branched polymer with chains of α-(1,4)-D-glucopyranose connected by α-(1,6)-Dglucopyranose linkages. Amylopectin has a higher molecular weight than amylose and consists of between hundreds of thousands to tens of millions of anhydroglucose units. Chains of 15 to 20 units of glucose between the branching points are arranged in clusters, and they are linked by longer chains into several clusters. This polymodal distribution results in a starch granule having crystalline and amorphous regions [52], [55-57]. OH O HO O H OH OH OHO O OH O HO OH O OH O a) OH O O OH HO OH O HO O OH OH O HO O H O OH OHO O OH O HO OH O OH O b) Figure 3. Schematic representation of the basic unit of a) amylose and b) amylopectin in starch (adapted from [57]) Acid hydrolysis of starch causes fragmentation of its chains, with the subsequent formation of oligosaccharides and monosaccharides of glucose. Due to its α-linkages, starch is easier to hydrolyze than are cellulose and hemicellulose [58]. Heating aqueous suspensions of starch granules causes them to swell, leading to disorganization of their structures, which is characterized by the loss of their birefringence, and the leaching of amylose and a portion of amylopectin, producing a viscous suspension (gelatinization). In addition, cooling this suspension promotes the realignment of linear chains of amylopectin in a process called retro gradation. Gelatinization improves the enzymatic hydrolysis of starch [11]. Commercial starches, mainly those from maize, tapioca, the potato and waxy maize, are chemically modified to improve their functional properties (viscosity, shelf stability, processing parameters, textures, appearance and emulsifiability). Enzymatic hydrolysis of starch produces non-digestible oligosaccharid es, such as maltooligosaccharide, isomaltooligosaccharide, and cyclodextrin, which are used as low-calorie sweeteners by the food industry [59]. 2.2. Biomasses in Brazil Agricultural and agroindustrial residues are obtained mainly from industrial and commercial enterprises, and they are considered potential raw materials for the production of value-added chemicals. In Brazil, the soybean, corn, wheat, rice, cotton, cassava, citrus, coconut and forestry industries generate approximately 600 million tons of waste per year. Therefore, new solutions for their utilization must be proposed. Table 2 shows the current availability of the main agricultural and agroindustrial residues [60], [61]. Table 2. Availability of agricultural and agroindustrial residues in Brazil [60], [61] Residue Soybean waste (stalks, leaves) Corn waste (stalks, leaves) Rice straw Forest residue Corn cobs Citrus waste (bagasse, husks and seeds) Cotton stalks Sugarcane bagasse 1000 tDM/year 118029 31007 14102 14511 10442 10384 8561 6400 Wheat straw Cassava bagasse Sawdust Sorghum leaves Rice husks Coconut husks Palm fibers Almond shells Sawdust 4966 4214 3950 1922 1772 452 123 71 3950 Regional availability Midwest, Southeast South, Southeast, Midwest South, Midwest North, South, Southeast South, Southeast, Midwest n.s. Midwest Southeast, Midwest, Northeast South, Southeast, Midwest n.s. Southeast, South Southeast, Midwest South, Southeast North, Northeast, Southeast North, Northeast Southeast, Midwest, Northeast Southeast, South tDM: ton of dry matter; n.s.: not specified The residues obtained after sugarcane processing consist of leaves and immature plants, which are considered potential raw materials for second-generation ethanol production. In contrast, the residues of corn crops and those 128 Fiorella P. Cardenas-Toro et al.: Obtaining Oligo- and Monosaccharides from Agroindustrial and Agricultural Residues Using Hydrothermal Treatments generated by corn industries are mainly used as cattle feed. The residues generated by the palm-oil industry contain oils rich in alpha- and beta-carotenes, which are precursors of vitamin A, and have a high carbohydrate content. The extraction of palm fibers produces an oil rich in carotenoids, and the further conversion of its carbohydrate fraction into oligosaccharides and monosaccharides is a sustainable use of this residue [45], [62]. The residues generated by extracting antioxidant compounds from Brazilian ginseng roots contain high quantities of starch and cellulose. These residues can be used in hydrolytic reactions for saccharide production [63]. 3. Chemical Conversion of Biomasses Many technologies have been developed for the transformation of biomasses into valuable products for the food, pharmaceutical and chemical industries. In general, the chemical conversion of a biomass has been classified into three main categories, as follows: biochemical, mechanical and thermochemical conversion processes. The biochemical processes includes anaerobic digestion, saccharification and fermentation. Anaerobic digestion converts solid and liquid biomass into a mixture of methane and CO2 in the absence of O2. Saccharification and fermentation is a process that consists of the enzymatic hydrolysis of the cellulosic fraction into fermentable sugars under the mild conditions of 45-50°C and their subsequent fermentation by yeast at 37°C to yield ethanol. The mechanical processes are mechanical extractions of biomass seeds, such as canola, palm, sunflower and cotton seeds, to produce oil, which is further converted into biodiesel. Several sources have been employed to produce biodiesel, such as canola oil, palm oil, sunflower oil, soybean oil and recycled oil by mechanical processes. The thermochemical conversion processes include combustion, pyrolysis, gasification, liquefaction and chemical hydrolysis. Combustion processes are employed to produce electrical and thermal energy through burning the chemical energy stored in a biomass at temperatures of approximately 800-1000°C. Pyrolysis converts a biomass into liquid (bio-oil), coal and non-condensable gases by heating it at a temperature higher than 400°C and a pressure of between 0.1 and 0.5 MPa in the absence of air. Gasification is the partial oxidation of compounds at a high temperature, between 800 and 900°C that enhances the production of a combustible gas mixture of CO, H2, CH4, CO2 and N2. Liquefaction uses water temperatures of between 250 and 350°C and pressures of between 5 and 20 MPa to produce gases and aqueous and oily products. Chemical hydrolysis consists of the cleavage of the polymeric bonds of a biomass using acids, bases or water (hydrothermal treatment) to yield oligosaccharides with various degrees of polymerization, monosaccharides and platform molecules, such as lactic acid, glycerol and levulinic acid [64-68]. 4. Hydrothermal Treatment of Cellulose, Hemicellulose and Starch in Biomasses Hydrothermal treatment is a process in which water at a high temperature and pressure is used to catalyze hydrolytic reactions. Water is one of the most useful green solvents in the chemical industry due to its non-toxicity, non-flammability, and wide availability in nature. This type of process entails autohydrolysis, hydrothermolysis, aqueous liquefaction, liquid hot-water pretreatment, subcritical water hydrolysis and supercritical water hydrolysis. These processes use water at temperatures and pressures higher than 100°C and 0.1 MPa, respectively, which are classified as subcritical and supercritical water hydrolysis [15],[69-71]. In the hydrothermal conversion of polysaccharides, subcritical or supercritical water is used to produce insoluble oligosaccharides, oligosaccharides (DP < 10), monosaccha rides, such as glucose, fructose and xylose, and polyols, such as xylitol and sorbitol. In addition, degradation products, such as phenolic and furan derivatives, which are toxic to humans and microorganisms, are formed [72]. Therefore, optimization of the hydrolytic process in terms of the monomers and oligomers yielded has been studied by several researchers for its use in the food and chemical industries [73]. In general, hydrolysis of a biomass using subcritical water (temperatures of 150°C < T < 374°C and pressures of P > Psat) for a few minutes causes the depolymerization of hemicellulose and starch, the partial depolymerization of cellulose and the improved susceptibility of the crystalline cellulose fraction to subsequent hydrolysis [36], [74]. Supercritical hydrolysis of a biomass (T > 374°C, P > 22.1 MPa) causes the depolymerization of the crystalline cellulose and the separation of the hydrolytic products of lignin [73]. Hydrothermal treatments have advantages over other conventional treatments, such as acidic and enzymatic hydrolysis, because the mild pH conditions result in fewer problems due to corrosion compared to acidic hydrolysis, the steps for the waste management and recycling acid are avoided, and the reaction conditions provide excellent selectivity with respect to the degradation of polysaccharides, resulting in a simpler procedure. 4.1. Physicochemical Properties of Water under Subcritical and Supercritical Conditions Subcritical water, which is also called superheated water or pressurized hot water, is defined as water with a temperature of between 100°C and 374°C (critical temperature) and a pressure higher than that of its vapor saturate, up to 22.1 MPa, to maintain the water in its liquid state. Supercritical water is a fluid with a temperature and pressure higher than those at its critical point (374°C, 22.1 MPa) [75]. Figure 4 shows the subcritical and supercritical regions of water [76]. The physicochemical properties of subcritical and supercritical water, such as the density, viscosity, dielectric constant and ionic product, vary Food and Public Health 2014, 4(3): 123-139 129 considerably with the increase of temperature and pressure compared with the properties under normal conditions (Figure 5). P (MPa) 50 40 30 20 10 0 -50 Subcritical water Supercritical water Tc, Pc 374 °C, 22. 1 MPa Tb, 100°C 150 350 550 T (°C) concentration is reached, and the formation of dehydration products such as furfural. The acidic medium promotes the partial solubilization of the soluble lignin without considerable modification of the insoluble cellulose and lignin. The depolymerization behavior of hemicellulose at temperatures below 200°C has allowed investigation into separating it from the other structural components of the biomass, such as cellulose and lignin [28]. Tables 3 and 4 show the results of experimental studies of subcritical and supercritical hydrolysis of model compounds (cellulose, hemicellulose, and starch) and agricultural and agricultural residues, respectively. -10.0 a) -12.5 Ionic product (log Kw) Figure 4. Pressure-temperature phase diagram of water: Tc = 374°C (critical temperature), Pc = 22.1 MPa (critical pressure), Tb = 100°C (boiling point). (data extracted from [76]) -15.0 The physicochemical properties of water depend strongly upon its temperature; for example; ionization of hydrogen bonds at high temperatures causes a decrease in the dielectric constant from 35 at 200°C to 10 at the critical temperature of 374°C. This feature makes water in subcritical and supercritical states suitable for organic reactions and the miscibility of organic compounds increases with temperature. Moreover, the content of the ionic product (Kw = [H+][OH-]) of water is three orders of magnitude higher under these conditions than its value under normal conditions, so that it acts as acid-base catalyst. In addition, water’s density decreases with the increase in temperature, from 1000 kg/m3 at 25°C to 820 kg/m3 at 250°C and 25 MPa. The combination of these phenomena causes the dependence of solvation power on temperature, in which high temperatures, near the supercritical point, lead to a high level of water dissociation and reduced density. These variations affect the kinetics of chemical reactions and the selectivity of a reaction within a network of reactions, and in many cases are considered catalytic [77], [78]. Dielectric constant ( ) -17.5 -20.0 0 b) 60 50 40 30 20 10 0 0 c) P = Psat P = 250 bar P = 500 bar 100 200 300 400 500 T (°C) P = Psat P = 250 bar P = 500 bar 100 200 300 400 500 T (°C) 4.2. Hydrothermal Treatment of Lignocellulosic Biomasses Different types of lignocellulosic biomasses were proposed as raw materials for the recovery of value-added compounds. This selection was based mainly on their abundance in the geographic region of study and their availability for conversion into value-added products. Treating a biomass with subcritical water at mild temperatures simultaneously catalyzes a number of chemical reactions, such as the progressive degradation of polysaccharides into xylooligomers, xylose monomers and degradation compounds (such as acetic acid and organic acids that decrease the pH of the medium). Higher temperatures and reaction periods promote the fragmentation of oligomers into monomers until a maximal monomer 1200 1000 Density (Kg/m3) 800 600 400 P = Psat P = 250 bar 200 P = 500 bar 0 0 100 200 300 400 500 T (°C) Figure 5. Physicochemical properties of water under subcritical and supercritical conditions (data extracted from [76],[79],[80]) 130 Food and Public Health 2014, 4(3): 123-139 1 Fiorella P. Cardenas-Toro et al.: Obtaining Oligo- and Monosaccharides from Agroindustrial and Agricultural Residues Using Hydrothermal Treatments Table 3. Subcritical hydrolysis of model polysaccharide compounds in a biomass Compound Size Temp. (°C) Pressure (MPa) Cata-lyst τ S/F Flow rate System Reactor dimension Optimal conditions Ref. OS: Cellulose 20-100 µm 320-400 25 None 0.05-10 s 2% w/w 10 mL/min Continuous Tubular reactor, 316 SS 72.2% (400°C, 0.01 s) MS: 42.6% (400°C, 0.15 s) [92] 25.3% (350°C, 3.5 s) Cellulose 40-100 µm 200-400 25 None 2 min 120 mg/5.7 mL 10 mL/min Semi-continuous 5.7 mL, 0.85 cm i.d. x 10 cm length, 316 SS MS: 18% (300°C, 340 s) [93] MS: Cellobiose n.s. 225-275 10 None 0-263 s 0.003 M n.s. Continuous Tube reactor of various L/D, 316 SS Glucose: 31% (225°C, 263 s) DP: 5-HMF: 18%, Glycolaldehyde: 9% [94] (275°C, 263 s) Cellulose n.s. 380 → 280-360 22→10 None 16 s → 15-50 s 60 mg/2.5 mL - Batch 5 mL, 7 x 2.5mm i.d., 13 cm length. 316 SS OS: 28.1% (380°C, 16 s) MS: 39.5% (280°C, 44 s) [95] OS: 25% (380°C, 0.25 s) Cellulose n.s. 250-380 25 None 0.25-0.75 s 2.5% w/w Re: 1904-55 00 Continuous 390 µL, high-alloy MS: stainless steel Glucose: 18% (360°C, 0.50 s) [81] (SAE designation Fructose: 4% (360°C, 0.50 s) type 4744) DP: 5-HMF: 5% Furfural: 2% (360°C, 0.75 s) Cellulose 75-106 µm 280-380 10 None 0-60 min n.s. 10-40 mL/min Semicontinuous OS: n.s. Oligomers with DP of 28 [82] (270°C) 250 mL, Xylan n.s. 180-300 > Psat None 0-30 min 2 g/100 mL water - Batch 316 SS, 65 mm i.d. x 70 MS: 20% (220 °C, 2 min) [96] mm Xylan n.s. 200 > Psat With and without CO2 15 min 0.03 g/3 g acid solution - Batch 3.6 mL, 316 SS MS: 5% (without CO2) 15% (with CO2) [97] OS 90.5% (without CO2) Starch n.s. 200 > Psat With and without CO2 15 min 0.03 g/3 g acid solution - Batch 3.6 mL, 316 SS 34% (with CO2) MS: [97] 4.2% (without CO2) 54% (with CO2) OS: oligosaccharide, MS: monosaccharide, DP: degradation product (dehydration and decomposition) n.s.: not specified Food and Public Health 2014, 4(3): 123-139 131 Table 4. Subcritical hydrolysis of agricultural and agroindustrial residues Biomass Size Temp. (°C) Pressure (MPa) Catalyst τ S/F System Reactor dimension Bean dregs < 140 mesh 220-300 5.4-9.4 CO2 2-10 min 1 g /200 mL Batch 200 mL reactor, 316 SS Rice husks n.s. 180 > Psat None 15-40 1 kg/8 kg min water Batch n.s. Corn stalks (pre-washe d) < 40 mesh 384 → 280 > Psat None 16-24 40 mg / 2.5 mL water Batch 5 mL (i.d. 7 x 2.5 mm, length = 130 mm) Wheat straw (pre-washe d) < 40 mesh 384 → 280 > Psat None 16-21 20 mg / 2.5 mL water Batch 5 mL (i.d. 7 x 2.5 mm, length = 130 mm) Barley husks > 1 mm 190-220 > Psat None 0.55-0. 1 kg/10 88 h kg water Batch 3.8 L 316 SS Sugarcane bagasse > 40 mesh 200 > Psat None 0-20 min 0.5-10 g/100 g water. Batch 9 mL Empty fruit bunches > 50 mesh 160-180 > Psat 0-40 min 1 g / 8 mL Batch 9 mL, 316 SS Eucalyptus globulus > 8 mm 180-220 > Psat None 0-1 h n.s. wood Batch 0.6 and 3.8 L, 316 SS Corn cobs > 0.3 mm 179 > Psat None 23 min 625 g/5 L Batch 10 L, Material: ANSI 304 and SS 316 Almond shell > 300 µm 179 > Psat OS: oligosaccharide, MS: monosaccharide None 23 min 833 g/5 L Batch 10 L, Material: ANSI 304 and SS 316 Optimal conditions OS and MS 50% cellulose (without CO2) 65% cellulose (with CO2) (300°C, 360 s) OS: 70% xylan (30 min) MS: 9% xylan (30 min) SupW OS: 17.2% d.b. MS: 23.24% d.b. (384°C, 17 s) SubW OS: 11.41% d.b. MS: 27.4% d.b. (280°C, 27 s) SupW OS: 10.37% d.b. MS: 1.60% d.b. (384°C, 19 s) SubW OS: 3.02% d.b. MS: 6.67% d.b. (280°C, 54 s) OS: Xylo-OS: 18% d.b. Ara-OS: 2.2% d.b. (200°C, 0.7 h) MS: 4.5% d.b. (220°C, 0.7 h) OS: 86% xylan MS: 13% xylan (S/F: 0.5, t: 10 min) MS: Xylose: 18 g/L (180°C, 10 min) Glucose: 0.8 g/L (180°C, 10 min) OS: 62% xylan (212°C, 0.3 h for 0.6 L, 0.9 h for 3.8 L) MS: 8% xylan (212°C, 0.3 h for 0.6 L and 0.9 h for 3.8 L) OS: 64% initial xylan, 11% initial cellulose MS: 9% initial xylan, 5% initial cellulose OS: 56% initial xylan, 3% initial cellulose MS: 10% initial xylan, 2% initial cellulose Ref. [98] [28] [23] [23] [99] [100] [43] [21] [41] [41] 132 Fiorella P. Cardenas-Toro et al.: Obtaining Oligo- and Monosaccharides from Agroindustrial and Agricultural Residues Using Hydrothermal Treatments Biomass Table 4. Subcritical hydrolysis of agricultural and agroindustrial residues (Cont.) Size Temp. (°C) Pressure (MPa) Catalyst τ S/F System Reactor dimension Almond shell > 300 µm 179 > Psat None 23 min 833 g/5 L Batch 10 L, Material: ANSI 304 and SS 316 Rice husk > 300 µm 179 > Psat None 23 min 833 g/5 L Batch 10 L, Material: ANSI 304 and SS 316 Ginger bagasse after CO2 supercritical n.s. 176-200 15 extraction CO2 0-15 min 3 g/7 g of water Batch 5 mL Beet fiber n.s. 160-180 > Psat OS: oligosaccharide, MS: monosaccharide None 5-15 min 3.8 g/35 g of water Batch 50 mL Optimal conditions OS: 56% initial xylan, 3% initial cellulose MS: 10% initial xylan, 2% initial cellulose OS: 30% initial xylan 12% initial cellulose MS: 6% initial xylan, 1% initial cellulose OS: 10000 – 100000 Da: 30-40% hydrolyzate (176°C) 160 – 8400 Da: 60-75% hydrolyzate (188°C) MS: 18% starch (200°C, 11 min) OS 84% d.b. (160°C, 12 min) Ref. [41] [41] [36] [101] 4.2.1. Hydrothermal Treatment of Cellulose Several studies of the depolymerization of cellulose in subcritical and supercritical water have been performed at temperatures above 200°C and pressures above its saturated pressure, and particularly in near-critical water, with the objective of providing a suitable reaction environment for the breakdown of its glycosidic bonds, which are highly resistant to chemical attack [15], [16], [81-95]. Hydrolysis of cellulose results in oligomers with various degrees of polymerization, including insoluble oligomers (DP < 200) and soluble oligomers (DP < 6). Short residence times promoted the formation of oligosaccharides [81], [92]. These oligosaccharides are further converted into a monosaccharide, glucose, which is epimerized to form fructose. Glucose is further degraded to form several compounds, such as 5-HMF, erythrose, glycolaldehyde and acidic organic compounds. Insoluble residues of cellulose with predominantly aromatic carbon structures are also formed [73], [85], [102]. The scheme of the reaction pathways is shown in Figure 6. Moeller et al. [85] studied the effect of the degree of cellulose crystallinity (38% to 74% crystallinity index) on its conversion and the saccharide yield at 205°C. A high level of conversion (approximately 40%) was obtained using cellulose with low crystallinity values (33% and 42% crystallinity index) due to the presence of larger amorphous regions. Moreover, no effect of the crystallinity on glucose and 5-HMF formation was found. Tolonen et al. [81] studied the degradation of cellulose using water in sub- and supercritical media (280-380°C). The decrease of the dielectric constant of water as well as those of the polar solvents and the increase in the ionic product of water with values higher than those under normal conditions increased the solvation of hydrophobic compounds. The products obtained were insoluble oligosaccharides, water-soluble oligosaccharides, fructose, glucose, 5-HMF and furfural. The highest level of oligosaccharide formation (25% oligosaccharides) was attained at the highest temperature and the shortest reaction period (380°C, 0.25 s). Additionally, other researchers have reported that cellulose dissolution occurred at temperatures greater than 250°C [73], [91-93], [103]. A comparative study of the hydrolysis of cellulose under subcritical conditions (280°C, 40 MPa), supercritical conditions (400°C, 40 MPa), and using a combined treatment was performed to evaluate the variation in the reaction rate behaviors. Hydrolysis in supercritical media produced the highest yield of hydrolyzate compounds (47%, < 1 min), the rate of formation of oligomers was higher than the rates at which monomers and fragmentation products of cellulose, such as erythrose and glycolaldehyde, were formed. Subcritical hydrolysis resulted in the lowest yield (22.4%, 4 min), and the rate of formation of monomers was higher than

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