Polysaccharides from Bioagro-Waste for New Biomolecules

Abstract

Polysaccharides are the main chemical components of several kinds of agro-wastes including food processing residues and agricultural wastes. These types of biomass are produced in huge amounts every year and therefore represent a global environmental issue. The exploitation of agro-wastes as rich sources of valuable polysaccharides, according to the biorefinery approach, could afford both a strategy for waste minimizing and a more sustainable production of energy and chemicals. As a matter of fact, indeed polysaccharides are traditionally used in a wide range of production chains, for example, in food, pharmaceutical, material, or packaging industries.

In this chapter, besides a brief overview of more abundant agro-wastes produced in the world and of their annual production, some remarkable examples of already implemented or possible biotechnological uses of waste polysaccharides are presented.

1 Polysaccharides from Bioagro-Wastes: A Sustainable Source of Chemicals and Energy

Polysaccharides’ market is continuously increasing because of the wide range of potential applications of this class of natural polymers that indeed, as such or by means of chemical and/or biological transformations, can constitute the main feedstock in many industrial activities such as food, materials, chemicals, and energy production.

Polysaccharides are produced by animals, microorganisms, and plants, but the latter are the main source of these biomolecules: indeed about 90 % of total natural polysaccharides produced on Earth can be found in the vegetables. Polysaccharides are the main constituents of vegetable biomass that, in turn, is currently exploited for the production of chemicals, materials, and energy: many examples of biorefinery facilities (a new production system resembling the petroleum refinery for the production of chemicals and fuels, but that is based on renewable feedstock like vegetables) can be found such as corn or sugarcane refineries that produce food and biofuel.

Notably polysaccharides are massively processed for different production chains; they are indeed the sources of the sugars that in turn are the starting materials for the production of those chemical compounds that have been included in the list of the so-called “building block” molecules , according to the National Renewable Energy Laboratory (NREL) of the United States. These “building block” substances (i.e., succinic, fumaric, and malic acids; 2,5-furandicarboxylic acid, 3-hydroxypropionic acid, aspartic acid, glucaric acid, glutamic acid, itaconic acid, levulinic acid, 3-hydroxybutyrolactone, glycerol, sorbitol, and xylitol/arabinitol) are fundamental feedstock for the production of secondary and intermediate chemicals that are then used in several industrial sectors such as cosmetic, pharmaceutical, material, food, transportation, industry, etc. (Pacific Northwest National Laboratory 2004).

For these reasons in the last years, an increasing demand of plant polysaccharide-derived goods (energy and foods) has caused an increase in the intensity of acreage, agricultural production, and agro-industrial activities that in turn generate million tons of wastes. Such wastes, i.e., bioagro-wastes, comprise all the residues produced from horticulture and/or agricultural activities during cultivation, postharvest, and processing of plants. Indeed all the different phases of manufacturing of vegetable materials (e.g., selection of fruits and vegetables for the whole market or food industries , industrial processing of crops for the production of chemicals, energy, food, and so on) produce every year huge amounts of highly heterogeneous residues. Depending on the feedstock and on the production chain considered, such waste biomass is constituted by unemployed parts of plants, like roots, straw, leaves, cobs, etc., or by vegetable transformation residues like exhausted pulps, peels, and seeds, etc.

The proper disposal of such residual biomass represents a critical environmental concern and an economical problem that all agro-industries have to face with. Nonetheless, since bioagro-wastes still contain a variety of value-added chemicals, they could be considered as starting material for other production chains rather than residual matter. Indeed significant amounts of proteins, polysaccharides and fibers , polyphenols, carotenoids, fatty acids, etc., that are lost in the discarded materials could be used for the production of a variety of goods like biomaterials , food additives, nutraceuticals, antioxidant and antimicrobial agents, bioenergy, etc. The reuse and then the valorization of bioagro-wastes could be implemented in the frame of an integrated biorefinery approach to biomass exploitation, in which the exploitation of full plant feedstock could be a solution to the issues of sustainability and of waste disposal.

Therefore, on these bases, vegetable biomass like bioagro-wastes is under investigation as sources of biologically and biotechnologically useful polysaccharides (Sanchez-Vazquez et al. 2013).

In the next sections, after a brief overview of main plants’ polysaccharide structures, some of the principal agro-waste sources of polysaccharides and the most diffused and promising biotechnological applications of such biomolecules are discussed.

2 Polysaccharides of Vegetable Origin: Chemical Structure and Classification

Polysaccharides are carbohydrate polymers made up of different monosaccharide units and are mainly located in plants that indeed produce more than 90 % of the total polysaccharides on Earth.

Generally referred to also as “glycans ,” polysaccharides are very complex polymers: indeed they can be defined as “polydisperse” molecules (due to the wide range of molecular weights they can have) and as “polymolecular” since they exhibit different fine structures, depending on their natural source.

Their classification can be made on the basis of chemical and structural features or depending on their biological role.

On the basis of structural criteria, the following groups can be distinguished:

• Homoglycans (made up of one type of monomer unit) and heteroglycans (made up of two or more types of monomer units)

• Linear and branched (with different degrees of branching, i.e., with few and very long branches regularly or irregularly spaced, with short branches grouped to form clusters, or with branch-on-branch structures, i.e., with bush-like structures)

• Neutral or charged (cationic or anionic)

Finally, on the basis of their biological role, they can also be distinguished in:

• Structural elements

• Energy-reserve polysaccharides

Following the latter classification, in this section the main polysaccharides that can be found in higher plants and therefore in the different kinds of bioagro-wastes are described.

2.1 Structural Polysaccharides in Higher Plants

The main and most abundant structural polysaccharide that is found in plant kingdom is cellulose , a homoglycan constituted by β-(1→4)-linked D-glucopyranose (Glcp) units. Cellulose is a high-molecular-weight polysaccharide made up of repeating cellobiose units (n) forming a linear structure in which both intra-chain and interchain molecular hydrogen bonds occur to link the chains.

The intermolecular hydrogen bonds give a sheetlike nature to the native polymer that possesses a crystalline structure whose organization varies from source to source (Klemm et al. 2005).

The cellulose chains are organized to give supramolecular structures, namely, elementary fibrils (with a length between 1.5 and 3.5 nm), microfibrils (between 10 and 30 nm), and microfibrillar bands whose length can be on the order of several hundred nm (Klemm et al. 2005). Nevertheless, the physical organization of cellulose polymers into such fibrillar structures varies depending on the plant source (Reddy and Yang 2005).

Cellulose is found both in primary and secondary plant cell wall in tight association with other polysaccharides, namely, the group of hemicelluloses . Historically identified as those polysaccharides extractable from higher plant tissues by means of hot aqueous alkaline solutions, hemicelluloses are a group of structurally different polysaccharides that can be distinguished in xylans , mannans , β-glucans , and xyloglucans (Ebringerova et al. 2005).

Xylans are the most abundant polysaccharides that can be found in the secondary cell walls of both mono- and dicotyledon (hardwood, grasses, cereals), and they include both homopolymers (the less frequent ones) and heteroglycans containing glucuronic acid and arabinose units. Xylans can be distinguished in: glucuronoxylans (GXs), arabinoglucuronoxylans (AGXs) and glucuronoarabinoxylans (GAXs), and arabinoxylans (AXs).

GXs possess a linear backbone of β-(1→4)-linked D-xylanopyranose (Xylp) units decorated by single 4-O-methyl-α-D-glucopyranosyl uronic acid (MeGlcA) residues linked at C-2 of Xylp monomers (Figs. 1 and 2).

Fig. 1

Structure of cellulose backbone with the cellobiose repeating unit in brackets

Fig. 2

Structure of glucuronoxylans (GXs) (Ebringerova et al. 2005)

AGXs (Fig. 3) and GAXs are made up of a linear β-(1→4)-D-Xylp unit chain branched with MeGlcA at position 2 and α-L-arabinofuranose (L-Araf) at position 3 of the backbone units.

Fig. 3

Structure of arabinoglucuronoxylans (AGXs) (Ebringerova et al. 2005)

Finally AXs are polysaccharides consisting of a linear chain of β-(1→4)-D-Xylp-linked units bearing branches of α-L-Araf at position 2 or 3 of xylan residues (Fig. 4).

Fig. 4

Structure of arabinoxylans (AXs) (Ebringerova et al. 2005)

The second group of xylans, namely, mannans , are the main components of the secondary cell walls of softwoods, although some of them can be found also as reserve polysaccharides. Mannoglycans can in turn be divided in: galactomannans (GaMs), glucomannans (GMs), and galactoglucomannans (GGMs).

GaMs (Fig. 5) are composed of linear chains of β-(1→4)-linked D-mannopyranose (Manp) units substituted at position 6 with galactopyranose (Galp) residues with the Man:Gal ratio ranging from 1:1 to 5.7:1.

Fig. 5

Structure of galactomannans (GaMs) (Ebringerova et al. 2005)

GMs (Fig. 6) possess a backbone consisting of β-(1→4)-D-Manp and β-(1→4)-D-Glcp units that, like in the case of Konjac’s mannans, can be acetylated at two or three position of some Manp residues.

Fig. 6

Structure of glucomannan (GM) (Ebringerova et al. 2005)

GGMs are characterized by a backbone made up of alternating segments of β-(1→4)-linked D-Manp and segments made up of both β-(1→4)-linked D-Manp and (1→4)-D-Glcp units, with branches of Galp units linked at two, three, or six position of Manp and at three or six position of Glcp units. Either GMs or GGMs are also present as storage carbohydrates in the seeds or roots of some plants such as Konjac.

β-Glucans , also named (1→3, 1→4)-β-D-glucans, are the hemicellulose components of cereal grains. They are unbranched homopolymers in which about 70 % Glc units are (1 →4) linked and the remaining 30 % are (1→3) linked, mainly assembled as repeating units of cellotriosyl, cellotetraosyl, or less frequently of chains of 9–14 Glc residues, separated by single (1→3) links.

Xyloglucans (XGs) are the most abundant polysaccharides that can be found in the primary cell walls of dicotyledon. They present a cellulose-like backbone made up of (1→4) β-D-Glcp units decorated with α-D-Xylp residues linked at 6 position of Glc monomers (Fig. 7), with a degree of xylosylation ranging from 30–40 % in grasses to 60–75 % in some dicotyledons.

Fig. 7

Structure of xyloglucan (XG) (Ebringerova et al. 2005)

XGs can be distinguished in two types: XXXG, in which three xylosylated Glc units are separated by one free Glc unit, and XXGG, in which there are alternating sequences of two xylosylated and two non-xylosylated Glc units.

The other main group of structural polysaccharides that can be found in plants is represented by the complex of pectic polysaccharides that are constituted by both neutral and acidic sugars. They comprise the homogalacturonans (HGs) also called the “smooth region” of pectins , and the heteropolymeric rhamnogalacturonans (RGs) and arabinogalactans (AGs), the “hairy regions” of pectins.

Homogalacturonans (HGs) , the most abundant members of pectins, are homopolymers of α-(1→4)-D-GalAp units partially methyl esterified at C-6 and O-acetyl esterified at C-2 or C-3 (Fig. 8), depending on the source. Depending on the degree of methyl esterification (DM), HGs can be distinguished in “high methyl-esterified HGs” (DM >50 %) or “low methyl-esterified Hgs” (DM <50 %) (Yapo 2011).

Fig. 8

Structure of homogalacturonans (HGs) (Ochoa-Villarreal et al. 2012)

The “hairy region” of pectins is composed of arabinogalactans (AGs) linked to rhamnogalacturonans (RG) chains. AGs can be distinguished in two groups: type I arabinogalactan (AG-I) (Fig. 9) and type II arabinogalactan (AG-II) (Fig. 10). AG-I, also defined as arabino-4-galactans, are constituted by a β-(1→4)-Galp backbone (Fig. 9) with side chains, linked at C-3 of Gal, made by arabinans. Arabinans are linear or branched chains of 1→5-linked L-Araf units (Fig. 9): linear ones are substituted by single L-Araf residues at C-2 or C-3 of the chain.

Fig. 9

Structure of arabinogalactans I (AG-I) (Wong 2008)

Fig. 10

Structure of rhamnogalacturonans I (RG-I) (Wong 2008)

Type II arabinogalactans (AG-II), also defined as arabino-3,6-galactans, present a linear backbone of 1 →3- and 1→6-linked Galp units, branched at 1, 3, or 6 position with arabinan chains. Both AG-I and AG-II are the constituents of RG-I pectins.

Rhamnogalacturonans (RGs) , also defined as the real pectins, are heteropolymers of galactopyruronic acid (GalAp) and rhamnopyranose (Rhap) branched with AGs chains. They also are divided in two groups: rhamnogalacturonans I and II (RG-I and RG-II). RG-I possess a linear backbone of alternating α-1,4-linked GalAp units and α-1,2-Rhap units, the latter being the branching point to which (on position 4) arabinans or AGs chains are linked (Fig. 10).

Rhamnogalacturonans II (RG-II) are constituted by a homogalacturonan (HG) backbone of about 9–10 GalAp monomers, partially methyl esterified, and branched at position 3 or 4 by four different types of oligosaccharide (Fig. 11).

Fig. 11

Structure of rhamnogalacturonans II (RG-II) (Wong 2008)

The side chains of RG-II present some glycosyl residues rarely found in other polysaccharides such as apiose (3-C-hydroxymethyl-β-D-erytrose), AceA (3-C-carboxyl-5-deoxy-L-xylofuranose), 3-deoxy-D-manno-octulosonic acid (KDO), 2-O-methylfucose, 2-O-methylxylose, 3-deoxy-D-lyxo-2-heptulosaric acid (DHA), aceric acid, and L-Gal (Paulsen and Barsett 2005).

2.2 Energy-Reserve Polysaccharides in Higher Plants

Starch is the main energy-storage polysaccharide that can be found in higher plants: it is composed of two glucose homopolymers, namely, the linear amylose and the branched amylopectin . Amylose is a linear chain of α-(1→4)-linked Glcp units, while amylopectin has a linear backbone of α-(1→4)-linked Glcp units with branches at C-6 made up of linear chains similar to amylose chains (Fig. 12). The degree of branching of amylopectin can vary, depending on the source of starch, from 4.2 in maize to 5.2 in oats. Similarly, the ratio of amylose vs. amylopectin component is different in the diverse natural sources: in general the branched component is the prevailing one since it can range from 75 % in maize or 77 % in wheat or 78 % in potatoes or 80 % in rice till up to 100 % in the mutant waxy maize (Robyt 2008).

Fig. 12

Structure of amylose and amylopectin

Although starch is the main energy-reserve carbohydrate, other glycans such as fructans can be found as reserve in about 15 % of flowering plants. Fructans include both linear and branched polymers made up mainly of fructofuranose (Fruf) units that can be linked by β-(2→1) or β-(2→6) or both glycosidic bonds. The most studied fructans are the inulin -type fructans that possess a linear backbone of β-(2→1)-linked Fruf units attached to an initial sucrose (Suc) unit (Fig. 13).

Fig. 13

Structure of inulin

Notably, as mentioned in the previous section, some structural glycans can also serve as energy-reserve polysaccharides in higher plants, such as galactomannans in some legumes, mannans and glucomannans, and β-glucans in cereal grains.

3 Main Bioagro-Waste Sources of Polysaccharides

Agro-industrial wastes’ chemical composition includes a complex of diverse polysaccharides that, in the frame of the biorefinery approach, could be converted in chemicals and/or energy by means of chemical and/or biochemical treatments.

Agro-wastes that potentially could afford sources of polysaccharides can be divided in two groups, namely, food wastes , i.e., residues of processing (canning, squeezing, peeling, milling, etc.) of fruits, cereals, and vegetables for food production, and agricultural residues , i.e., residues of harvesting and postharvesting phases.

3.1 Food Wastes

Food wastes that are currently used or that could potentially be exploited for polysaccharide production include mainly vegetable processing residues like seeds, peels and skins, husks, exhausted pulps, unripe or damaged fruits, etc. (Fig. 14).

Fig. 14

Some main food wastes: a apple pomace, b lemon residues, c tomato peels, d potato peels

Such residues are rich in structural carbohydrates (cellulose , hemicellulose , or pectins ) or in other kinds of glycans and/or dietary fibers (i.e., a mixture of nondigestible carbohydrates comprising cellulose, hemicelluloses, pectins, and inulin in association with other nonsugar molecules like lignin, waxes, and polyphenols).

Several examples are available in literature concerning the chemical investigation of polysaccharide fraction of different vegetable wastes: in Table 1 some of them that are already exploited for industrial purposes and others that could be promising sources of valuable polysaccharides are listed.

Table 1 Main food waste sources of polysaccharides (Partially adapted from Poli et al. 2011)

Apple pomace annual production can rise up to 3.42 Mtons since nearly 50 % of the fruit is discarded after industrial processing for fruit juice production (Min et al. 2011; Walia et al. 2013). Depending on the extraction method employed, such wastes can be an interesting source of pectins and arabinans that can account from 4.6 % to 11.7 % or of hemicellulose and cellulose (Nawirska and Kwasniewska 2005) that in turn can be employed as soluble dietary fibers.

Banana is among the main agricultural resources in tropical and subtropical countries, and its cultivation and consumption generate about 10.9 Mtons of wastes per year (Sanchez-Vazquez et al. 2013). Banana wastes, i.e., peels, account for about 40 % of the fresh fruit weight and are composed of 50 % of dietary fibers of which 21.7 % are pectin polysaccharides, 3.8 % are hemicellulose, and 1.3 % are cellulose (Das and Singh 2004).

Pomace is also the main kind of residue remaining after the industrial processing of other fruits: some examples are constituted by black currants, cherries, chokeberries, and pears that are used in several European countries for the production of juices and beverages. These wastes are almost composed of dietary fibers (comprising besides polysaccharides like pectins, hemicellulose, and cellulose also lignin as main noncarbohydrate fraction) whose amount for the above-listed fruits is equal to 90.8 %, 91.37 %, 90.3 %, and 94 % of dry matter weight, respectively. The pomaces of the listed fruits present variable amounts of pectins, hemicellulose, and cellulose (Nawirska and Kwasniewska 2005), as reported in Table 1.

World citrus fruit production reaches more than 80 Mtons every year [Pourbafrani et al. 2010], and about 50–60 % of this biomass is discarded after squeezing for juices or peeling for liquor or jams production. Such wastes (mainly coming from oranges, mandarins, grapefruit, lemons, and limes transformation) include peels, seeds, and pulps that contain different polysaccharides among which pectins are the most abundant. The total pectins’ amount that can be recovered from citrus fruit residues depends upon both the extraction method employed and the citrus species: as shown in Table 1, the extraction yields can vary from 18 % to 25 % of dry matter for orange fruits (Kratchanova et al. 2004; Pourbafrani et al. 2010) to 23.0 % of mandarin peels (Sanchez-Vazquez et al. 2013) or finally to 14.3 % in the case of lemon residues (Poli at al. 2011). With regard to lemon residues, they account for about 50 % of starting material; therefore, since the total world production of lemons is ≈5 Mtons y⁻¹, a potential quantity of ≈2.5 Mtons of such residues could be exploited as source of pectins. In addition also other structural glycans could be recovered from citrus fruits, for example, hemicellulose and cellulose that share a significant portion of the polysaccharides extracted from orange and mandarin wastes (Table 1).

Exhausted pulps are produced also in the conversion of vegetables like carrots whose world production reaches about 33.6 Mtons per year (Sanchez-Vazquez et al. 2013): about 30–40 % of such amount is lost as by-products. Notably, 54.2 % of carrot pulp, on weight basis, is constituted by dietary fibers in which the main polysaccharides are represented by pectins, hemicellulose, and cellulose (see Table 1).

Corn is one of the main cereal crop productions indeed; according to data from the Food and Agriculture Organization of the United Nations (FAO, http://faostat3.fao.org/faostat-gateway/go/to/home/E), its world production is more than 800 Mtons per year. Corn bran and fibers are the main residues from processing of grains for food production: bran is the residue of dry milling for meals and floor production, while fibers are left after wet milling of grains for starch and oil recovery. Since corn grains account for about 15 % of total harvested maize, and considering that bran represents 6.5 %, while fiber is 9.5 % of total corn grain, a total production per year of about 7.8 Mtons and 11.4 Mtons of bran and fibers can be hypothesized, respectively. Corn fibers still contain interesting quantities of starch, while bran can afford besides starch also significant yields of cellulose and hemicellulose.

Among the most abundant vegetable food wastes, also potato and tomato residues have to be mentioned. Potato wastes, mainly peels, are about 80 Mtons y⁻¹ and contain besides starch, as main polysaccharide, also appreciable quantities of cellulose and hemicellulose polymers. Tomato residues from canning industry (more than 11 Mtons of peels , seeds, and exhausted pulps , according to FAO data), besides the structural glycans of cell wall (cellulose and hemicellulose), are also an interesting source of new polysaccharides like the xyloglucan fraction isolated from peels and seeds by means of alkaline extraction (Tommonaro et al. 2008).

Husks from coffee and peanuts are also food waste sources of polysaccharides. About 55 % of coffee wastes are produced in Latin America, and husks contain pectins as main polysaccharide, accounting for more than 10 % of waste dry weight. Peanut is also cultivated and used in several countries; indeed its world production is about 34 Mtons: the by-products, mainly husks, are a rich source of arabinans, galactans, and mannans accounting in total for about 1 % of residues.

3.2 Agricultural Residues

Agro-wastes that are potential sources of polysaccharides encompass all the residues that are produced during the cultivation, harvesting, and postharvesting steps of several crops such as sugarcane or cereal crops like corn, barley, rice, wheat, etc. (Fig. 15). Such wastes are also classified as “lignocellulosic residues”: lignocellulose is the complex of cellulose, hemicellulose, and lignin that are characteristic of plant cell wall.

Fig. 15

Some main agricultural residues: a corn stover, b rice straw, c wheat straw, d sugarcane bagasse

Among the most abundant agricultural and lignocellulosic residues, sugarcane bagasse and cereal straw are particularly interesting polysaccharide sources because of their total annual production and significant lignocellulose content.

According to FAOSTAT data, the world total sugarcane production in 2012 has been more than 1,000 Mtons. Sugarcane bagasse, i.e., the residue of crushing of canes for sugar juice extraction, represents about 30 % of the plant, and it is composed of cellulose (40–45 %) and hemicelluloses (30–35 %), besides lignin (Cardona et al. 2010).

Cassava is a tuber crop that represents the basic food for more than 700 million people in Asia (where it is known as tapioca), Latin America, and Africa. According to FAO data, its world total production in 2012 has been about 263 Mtons y⁻¹. Cassava processing for flour production and starch extraction from tubers results in the production of peels and of a solid residue, the bagasse, that still retains high levels of starch that indeed (on a weight basis) constitute about 50 % of the biomass (Pandey and Nigam 2009).

With regard to cereal crops, the main residues left after harvesting are represented by straws that comprise more than 50 % of the crops: since in late years the world production of cereals on average has been about 2600 Mtons (FAO data), it can be estimated that potentially 1,300 Mtons of these lignocellulosic residues are produced and can be exploited as valuable carbohydrate polymer sources.

Corn world production, as mentioned in the previous section, equals about 800 Mtons per year: only the grains are harvested and used for food applications, while the other components (leaves, shell, stalks) are left on the ground. Corn residues from harvesting, i.e., corn stover, account for about 85 % of the plant; therefore, the total amount of such wastes is nearly 700 Mtons per year. These wastes are particularly rich in cellulose and hemicellulose thus representing the main renewable source of world lignocellulose (Sanchez-Vazquez et al. 2013).

Barley, oat, rice, sorghum, and wheat world production in 2012, according to FAOSTAT data, were ≈13, 21, 719, 57, and 670 Mtons, respectively.

Barley grain is used as animal feed, as malt, and for human food: after harvesting, the main residue is constituted by straw whose chemical composition reveals that more than 60 % of this biomass is made of valuable polysaccharides, namely, cellulose and hemicellulose . Since cereal postharvest residues account on average for 50–75 % of crop, therefore in the case of barley potentially, 5.6–9.8 Mtons could be available for polysaccharide recovery.

Oat is diffused in temperate regions, and it is mainly used as breakfast cereal as flakes or for porridge; indeed it is not suitable for baking since it lacks gluten. Its by-products still retain significant quantities of both cellulose and hemicellulose, as reported in Table 2. The potential waste from oat that is yearly produced should be about 42 Mtons.

Table 2 Main agro-industrial waste sources of polysaccharides (PS)

Together with corn and wheat, rice is one of the main cereal crops in the world with China as the main producer; notably it also represents the principal staple food for more than 50 % of the world’s population. After harvesting, the rice is dehusked, and the resulting straw is usually left onto the soil or burned: nevertheless it is an interesting lignocellulose biomass since it still retains appreciable amounts of cellulose and hemicellulose (Table 2).

Sorghum is native to Africa, but nowadays it is cultivated in Africa, the United States, and Asia where Nigeria, Mexico, the United States, and India are the main producers, respectively. Sorghum is mainly exploited as animal feed or for alcohol production, but in poorest areas it also serves for human nutrition. Sorghum straw is among the lignocellulosic biomass under investigation due to its polysaccharide fraction that is mainly constituted by cellulose and hemicellulose, as shown in Table 1. On the basis of sorghum total world production, its straw yield is estimated to be between about 28 and 43 Mtons per year.

Wheat is mainly cultivated in Asia (43 %) and Europe (32 %) followed by North America as the third producer with 15 % of total global production. The main part of wheat is used for food production, with a minor fraction for animal feeding. The remainder part is estimated to be about 550 Mton y⁻¹ and is also a biomass rich in cellulose and hemicellulose that could be further exploited for other purposes.

4 Brief Phytochemistry

In the assessment of polysaccharide structure, two stages can be distinguished: firstly, the determination of primary structure, which means the polysaccharide composition, the configuration, and position of glycosidic linkages and the ring configuration, and, secondly, the determination of spatial structure that can be obtained through the knowledge of bond lengths, bond angles, and torsional angles overall regarding the glycosidic torsional angles and the exocyclic torsional angle. Besides the primary structure, a detailed knowledge of the spatial structure is essential in order to assess structure-function or structure-property relations. The clarification of polysaccharide structures is very important to explain the physicochemical and biological properties of these biopolymers and to attribute and in some cases to predict the biotechnological applications of these biomolecules. The rheological properties of these polymers are surely influenced by the primary conformation. Furthermore, the ordered secondary configuration frequently takes the form of aggregated helices. Moreover, the presence or also the absence of specific acyl groups, for example, O-acetyl or O-succinyl esters or pyruvate ketals, can influence the formation of ordered helical aggregates (Poli et al. 2011). Several chemical and physical techniques are used to determine the primary structure of polysaccharides: chemical degradation and derivatization, in association with chromatographic methods and mass spectrometry analysis, are used to determine the sugar composition, their absolute configuration, and the presence and the position of possible substituents. In details, molecular size analyses are carried out by Sepharose CL-6B column using a mixture of dextrans for calibration curves (Pazur 1994). Fourier transform infrared (FT-IR) spectroscopy spectra of EPS are obtained with FT-IR spectrometer between 400 and 4,000 wave numbers (cm⁻¹). Thermogravimetric analysis (TGA) of EPS is obtained with TGA apparatus where a known amount of polysaccharide sample in H₂O is heated from 30 to 400 °C at a rate of 20 °C min⁻¹ under a constant flow of nitrogen. For carbohydrate analysis, the purified polysaccharide fractions are hydrolyzed with 2 M trifluoroacetic acid (TFA) at 120 °C for 2 h. Sugar components are firstly detected by thin-layer chromatography (TLC), using a mix of acetone/butanol/H2O (8:1:1, v/v/v) as mobile phase and standard monosaccharides for qualitative determination. Afterward monosaccharide composition is defined by high-pressure anion exchange-pulsed amperometric detector (HPAE-PAD) . Sugars are eluted isocratically with 16 mM NaOH and identified by comparison with reference standards (Poli et al. 2011). The linkage positions of the monosaccharides are determined by methylation analysis: the samples are methylated with CH₃I in dimethyl sulfoxide and NaOH, and the products are hydrolyzed using TFA 1 M at 70 °C for 45 min. After reduction with NaBD₄, the samples are acetylated, and then the sugar derivatives are analyzed by GC-MS. The absolute configuration of the sugars is determined by gas chromatography of the acetylated (S)-2-octylglycosides (Manzi and van Halbeek 2009).

The scrutiny of unknown structures still lies in the crucial point of polysaccharide research, and the nuclear magnetic resonance (NMR) technique persistently continues leading this procedure. Even today, when important technological advances occur, the unambiguous identification remains a challenging task (Halabalaki et al. 2014). Toward this direction, computational tools might hold a key role in the future. Expert systems equipped with versatile algorithms enabling structure or spectra prediction are able to give recourses for the structure elucidation of natural molecules in complicated mixtures. Comprehensive and qualified open access natural products databases support considerably the identification of unknown molecules (Halabalaki et al. 2014). NMR contribute to the determination of the repeating unit of polysaccharides by using in particular two-dimensional ¹H- and ¹³C-NMR. For conformational analysis it is important that the primary structure of polysaccharide being studied is known and that as many signals as possible have been assigned unambiguously. In order to make possible NMR or mass spectrometry analysis, it is necessary to carry out a chemical or enzymatic digestion of polysaccharides to obtain smaller fragments that are more easily analyzed. Two types of homonuclear 2D NMR spectroscopy could be used for structural analysis of oligosaccharides: the first is characterized by magnetization transfer through scalar coupling (COSY type); in experiments of the second type, the magnetization is transferred through space (NOESY type) at short interproton distances. The most useful method of the COSY type is the 2D homonuclear Hartmann-Hahn (2D HOHAHA) spectroscopy. Advantages of 2D HOHAHA over other COSY-type techniques are higher sensitivity; pure in-phase magnetization, which avoids signal canceling by antiphase magnetization in conjunction with large line widths, as in 2D double-quantum filtered COSY (DQF-COSY); and the possibility to obtain complete subspectra by multistep magnetization transfer. In some cases also NOESY-type experiments are useful for assignment purposes often in combination with COSY-type experiments (Halabalaki et al. 2014).

Other assignment techniques exploit the large spread in chemical shift in ¹³C NMR spectra. Since the proton-detected heteronuclear multiple-quantum coherence (HMQC) experiment renders information about direct (one bond) ¹H-¹³C connectivities, it is used for the assignment of either the ¹H or the ¹³C spectrum. Long-range heteronuclear correlation experiments such as the proton-detected heteronuclear multiple-bond correlation (HMBC) spectroscopy afford linkage information and therefore useful for the determination of carbohydrate sequences. In addition, the HMBC experiment makes it possible to assign carbon atoms that are not accessible by HMQC spectroscopy, namely, quaternary carbon atoms (Halabalaki et al. 2014).

In addition, a solid-state NMR has been adopted as uniquely suited for the examination of insoluble and complex macromolecule, or, for example, to study whole-cell systems and also to check the integrity of the polysaccharide structures (Finore et al. 2014). Solid-state NMR spectroscopy allows investigation of molecules in the solid state and reveals information about crystal packing using a noninvasive and nondestructive analysis. This method can provide not only chemical information but also chemical environment and ultrastructural details that are not easily accessible by other nondestructive high-resolution spectral techniques (Foston 2014; Serra et al. 2012). The solid-state NMR methodologies result is particularly useful when studying structural problems in complex biological systems, for example, in the study of the mechanisms of biosynthesis and deconstruction for lignocellulosic biomass or individual plant cell wall components. The most frequently used solid-state NMR techniques are ¹³C CPMAS, a double-bearing cross-polarization magic-angle spinning NMR spectroscopy that is the combination of three techniques: firstly, cross polarization; secondly, magic-angle spinning; and, thirdly, high-power decoupling (Foston 2014).

Recently, a bioanalytically probing technique has been developed to study biomaterials at cellular and molecular level within intact tissue (Yu 2011). This technique named synchrotron radiation Fourier transform infrared microspectroscopy (SR-IMS) takes advantage of bright synchrotron light which is a million times brighter than sunlight (Marinkovic and Chance 2006). It is able to have simultaneous information about tissue chemistry, tissue composition, tissue environment, and tissue structure by a noninvasive and nondestructive tool. An appropriate example of this technique comes from the study of microstructural features of the embryo (germ) in sorghum (Sorghum bicolor L.) seeds in which the intensity and distribution of the various chemical functional groups (including cellulosic compounds) could be chemically mapped (Yu 2011) (Fig. 16).

Fig. 16

Main steps and methods for purification and structural characterization of polysaccharides

5 Biotechnological Approaches to Waste Polysaccharide Exploitation

Polysaccharides from renewable and sustainable sources, like food and agricultural wastes, can play a central role in the emerging biobased economy , i.e., the new economy system shifting from fossil resources of energy and chemicals to renewable resources such as biomass. Indeed the wide variety of structures and biological functions of polysaccharides that can be recovered from agro-wastes make them biotechnologically useful biopolymers that either are already exploited for several purposes or are under investigation for new applications (Fig. 17).

Fig. 17

Main industrial applications of polysaccharides for biomaterials and biodegradable plastics (a, b), for food packaging (c), for pharmaceutical industry (d), for fuel ethanol (e) and enzyme (f) production (Photos’ web sources: (a) www.jetsongreen.com; (b, c) www.novamont.com; (d) www.bookdepository.com (e) http://www.indianaenvironmentallaw.com/agriculture/)

Since a great share of industrial production of chemicals and energy is based on the exploitation of plant polysaccharides, like starch, the search for new biotechnologies and strategies for exploiting and employing waste materials in several industrial fields is therefore the object of growing interest. Such an approach to polysaccharide waste biomass exploitation comes under the frame of the so-called biorefinery , i.e., a new production system in which a variety of goods (fuels, chemicals, and biomaterials) can be recovered from renewable feedstock by using a single or a combination of chemical, physical, and biological treatments.

Biorefinery of biomass is a multistep process that can be divided in: choice of starting material and of suitable pretreatment method, in order to render it more prone to the following processes, and conversion of the pretreated biomass by means of one or combined biological, chemical, and physical techniques to obtain energy or value-added chemicals and/or building blocks for further production processes.

Polysaccharide rich waste biomass is the ideal feedstock for biorefinery since they are renewable, not in competition with the food chain and not impacting on land use and biodiversity.

In addition, such materials are available in significant amounts every year (see Tables 1 and 2) and therefore are under investigation for several industrial applications, for example, production of green chemicals, biomaterials, second-generation biofuel, etc.

In the following section some remarkable examples of useful products that can be obtained from polysaccharides recovered from agro-industry waste biomass by means of physical, chemical, and biological transformation are described.

5.1 Building Blocks for Composites and Biomaterials

Waste polysaccharides like cellulose and starch are valuable building blocks for the material’s preparation like composites or bioplastics .

Cellulose is the main glycan found in the lignocellulosic agro-wastes that in turn are emerging as potential renewable sources of fibers for paper, material, and textile industry. Cellulose can be recovered by waste biomass fractionation by means of different pretreatment techniques such as steam explosion, ionic liquid-based fractionation, and acidic treatment followed by shear mechanical treatment. The cellulose polymers that in such ways can be recovered, for example, from wheat (Dufresne et al. 1997), rice straw (Ping and Hsieh 2012), and from sugarcane bagasse (Mandal and Chakrabarty 2011), are essentially nanocrystalline cellulose, i.e., a cellulose polymer that is formed by rigid rodlike particles whose widths and lengths are from 5–70 nm to 100 nm up to micrometers in range. Cellulose nanofibers, extracted from wheat straw by these methods, have been used for the reinforcing of polypropylene composites, for the preparation of biocomposites and of thermoplastic starch-based nanocomposites, and for the production of composition panels that have shown to be particularly resistant to earthquake (Kalia et al. 2011). Also other several applications have been identified for nanocrystalline cellulose from wastes such as in nanocomposites as filler, improving mechanical and barrier properties, as building block for selectively permeable membranes, as foams or aerogels, as adhesive or to prepare adhesive materials, and as a reinforcing agent for polymer electrolytes in lithium batteries (Brinchi et al. 2013), but further and maybe still unknown applications in these fields are possible.

Other examples of application of waste cellulose for composites are represented by: wheat straw cellulose that has been used as a natural filler to reinforce composites of a polyolefin and of a biodegradable polyester (Le Digabel et al. 2004); by rice straw cellulose that has been used to prepare composite boards for construction that afforded good acoustical and electrical insulation, besides possessing anti-caustic and anti-rot properties (Yang et al. 2004a); by banana peels fibers that have been employed to prepare composites with polyester matrix (Pothan et al. 2007); by rice residual cellulose that has been investigated as reinforcing filler for polypropylene-based thermoplastic composites (Yang et al. 2004b); and finally by bagasse wastes that, like the previously mentioned waste sources of cellulose, were employed to produce composite materials by mixing with low-density polyethylene, acid stearic, or maleated low-density polyethylene (Habibi et al. 2008).

Starch is also very promising for new biodegradable polymers that can find several applications in materials science; after blending with other natural polymers like gutta-percha or natural rubber, they can afford more suitable composite materials (Yu et al. 2006). With particular regard to cassava bagasse starch, it has been used with different approaches to produce biocomposites: it has been mixed with Kraft paper to obtain a composite similar to cardboard that showed mechanical properties resembling the recycled paper (Matsui et al. 2004); it has also been used to produce thermoplastic starches using glycerol as plasticizer (de Morais Teixeira et al. 2005).

Also food wastes like tomato wastes showed to be potential sources of polysaccharides for bioplastics : indeed a glucan isolated from tomato canning wastes, after addition with glycerol as plasticizer agent, showed to possess promising properties as bioplastic for mulching or solarization applications (Tommonaro et al. 2008).

5.2 Biomaterials for Food Packaging

Several examples of polysaccharide-based edible films and coatings are available in literature, but most of them are based on the exploitation of polysaccharides from non-waste sources. Recently several examples of the reuse of agro-industrial residues for the preparation of edible and biodegradable films appeared: in this section a survey of very recent examples of applications of waste polysaccharides to this issue will be presented.

Hemicellulose polymers from both food wastes and agricultural residues can find applications in packaging; indeed, for example, arabinoxylans (AXs) isolated by alkali extraction from barley residues were used to prepare edible films by water casting and showed to have interesting mechanical properties (Mikkonen and Tenkanen 2012). On the other hand also AXs recovered from corn residues and blended with glycerol, propylene glycol, or sorbitol were used to produce stable films that showed to be able to afford moisture barrier in a preservation experiment with grapes samples (Zhang and Whistler 2004); AXs from corn bran were emulsified with fats (palmitic acid, oleic acid, triolein, a hydrogenated palm oil) and then used to prepare water vapor-permeable films (Peroval et al. 2002).

Cellulose from sugarcane bagasse has been exploited to prepare a cellulosic film fully biodegradable, with very high tensile strength and good water vapor permeability that potentially could be used for food packaging (Ghaden et al. 2014); cellulose nanocrystals obtained from the same agro-waste were used to reinforce starch-based films that showed in this way higher water resistance and water barrier properties (Slavutsky and Bertuzzi 2014).

Residual starch from cassava bagasse proved to have potential application in bioactive food packaging since it was used to prepare biodegradable films impregnated with antimicrobial agents and possessing favorable water sorption and permeability properties (de Souza et al. 2014); cassava bagasse polysaccharides (cellulose and starch mixture, see Table 2 for chemical composition) have also been mixed with cassava starch to prepare a thermoplastic starch matrix (de Morais Teixeira et al. 2009).

Also pectins from food wastes have been studied for their potential exploitation in food packaging : pectins indeed have been used to produce bioactive films and coatings that are useful for prolonging food’s shelf life. Such materials, besides providing a semipermeable barrier to oxidizing agents, can also be added with antimicrobial, antioxidant, and anti-softening agents, thus preventing food deterioration (Raybaudi-Massilia and Mosqueda-Melgar 2012).

5.3 Human Nutrition, Food Additives, and Prebiotics

Pectins, cellulose, and hemicellulose are part of human nutrition since they constitute the so-called dietary fibers: thanks to their chemical properties, during human digestion, they reach the small intestine where they have beneficial effects on the microflora.

High dietary fiber powders can be obtained from both food and agricultural residues. Dietary fibers can be recovered from orange peels, in peach residues, orange and lemon peels, or apple pomace. Agricultural residues like corn, wheat, and rice bran are also a good source of such valuable compounds. Thanks to their physicochemical features, dietary fibers can find application in food industry for improving the texture, viscosity, and shelf life of foods: several reports are found in literature concerning their beneficial effects when added to jams, bread, pasta, frozen foods, dairy products, etc. (Elleuch et al. 2011).

Pectins are the most popular polysaccharide from food wastes that, for generations, have been used as food ingredients or additives in all countries of the world. Their chemical structure, characterized from the coexistence of polar and nonpolar regions, makes them suitable for incorporation in different kinds of foods . Commercially employed pectins are mainly derived from apple pomace and citrus peels, and their principal application is in food industry as gelling agents, thickeners, water binders, and stabilizers. Pectins are indeed exploited for the industrial preparation of jam and jellies, to which they are added as a powder during the cooking step; fruit juices, where they act as clouding agents; soft drinks, where they counterbalance the deprivation of sugar; yogurts, where they prevent floatation of fruit pieces; frozen foods, where they exert a great firming effect and retard ice crystal growth thus improving food texture (Thakur et al. 1997).

Polysaccharides from food wastes have also a great biotechnological potential as sustainable sources of prebiotics . The latter are a group of food ingredients that resist digestion in the intestine and stimulate the growth and activities of indigenous bacteria (commonly Bifidobacteria and Lactobacilli) that constitute the beneficial gut microflora. Oligosaccharides are among the most known molecules that are able to exert a prebiotic effect, and some of them can be produced starting from food wastes by means of enzymatic modifications. Pectic oligosaccharide with prebiotic properties has been prepared from commercial pectins by using an enzyme membrane reactor and proved to be effective against toxins produced by Escherichia coli and to be able to induce apoptosis in human adenocarcinoma cells. Moreover, pectic oligosaccharides recovered from orange peel were able to favor Bifidobacteria sp. and Escherichia rectale growth (Pandey and Nigam 2009). Cellulose oligosaccharides also showed prebiotic properties: indeed the cellulosic fraction of apple pomace was used to produce prebiotic oligosaccharides like glucooligosaccharides, xylooligosaccharides, and arabinooligosaccharides by digestion using an enzyme cocktail (cellulose and cellobiase from Trichoderma reesei and Aspergillus niger) (Pandey and Nigam 2009).

5.4 Feedstock for the Production of Chemicals, Enzymes, and Biopolymers by Microbial Fermentation

The polysaccharide components of residual agricultural raw materials can be used, by means of bioconversion, for the production of chemicals like organic acids (citric acid and lactic acid) or building block molecules, enzymes, and biopolymers that in turn are employed in a wide range of industrial fields.

With regard to organic acids , citric acid has a great commercial importance since it has many applications in food industry (as antimicrobial, preservative, or pH adjuster for soft drinks, confectionery, dairy, marmalades, fats, and oil production), in pharmaceutical industry (as anticoagulant, acidulant, or effervescent), in cosmetic industry (as antioxidant, metal chelator, or pH adjuster), and finally for other miscellaneous sectors (textile, paper industry, tobacco production, waste treatment, etc.) (Pandey and Nigam 2009). Thanks to their high polysaccharide content, citrus wastes and apple pomace have been evaluated as carbon sources for the production of citric acid employing A. niger strains in both submerged fermentation (SmF) and solid-state fermentation (SSF) conditions (Pandey and Nigam 2009). Citric acid can be produced by means of SmF with different A. niger species using several agro-residues including apple pomace, orange and carrot wastes, coffee husks, and sugarcane and cassava bagasse. Lactic acid is another organic acid that can be produced using waste polysaccharides. It is also used for several purposes in food industry (as preservative, for production of fermented foods and dairy products), in pharmaceutical industry (as blood coagulant or for preparation of anti-inflammatories), and for the synthesis of biodegradable and biocompatible plastics (by polymerization to polylactic acid, PLA). Lactic acid is produced in SSF conditions by means of fungi (Rhizopus species) or bacteria using cassava bagasse, wheat straw, or carrot wastes (Pandey and Nigam 2009).

With regard to the building block molecules production (Pacific Northwest National Laboratory 2004), some hemicellulosic wastes have been investigated for the production of xylitol, a natural sweetener substitute of sucrose that is widely used in food industry because it is anticariogenic and also suitable for diabetes patients (Canilha et al. 2005). Xylitol can be produced by fermentation of xylose recovered from hemicellulose hydrolysates from several food and agricultural wastes such as: barley and corn wastes, by Debaryomyces hansenii fermentation (Cruz et al. 2000); corn cobs, by a xylitol-producing yeast (Domínguez et al. 1997), Candida sp. 11–2; and peanut husks by A. niger fermentation (Mudaliyar et al. 2011).

With regard to enzyme production, agro-wastes are also valuable feedstock for their industrial production by means of microbial fermentation. A remarkable example is represented by residual cellulose polysaccharides that can be employed as sole carbon sources for the fermentation of biotechnological strains that in turn are usually exploited for the production of industrially useful enzymes . This is the case, for example, of cellulose and hemicellulose from sugarcane bagasse that have been exploited to produce: cellulase enzymes by fermentation of Streptomyces, Trichoderma, and Aspergillus species, xylanase that can be generated in SSF conditions by cocultured T. reesei and A. niger or A. phoenicis or by Penicillium janthinellum and Trichoderma viride, and inulinase, by fermentation of Kluyveromyces marxianus (Pandey and Nigam 2009). Also pectins in food wastes like citrus and apple pomace can be exploited as fermentation medium for the production of useful enzymes. Such wastes have been investigated, both in SmF and SSF conditions, as growth substrate for the production of bacteria, yeasts, or fungi producing pectinase activities. Such enzymes are the object of interest since they are used in different fields like in food and alcoholic drinks production, in textile and paper industry, for wastewater treatment, etc. (Jayani et al. 2005). Moreover polysaccharide fraction in apple pomace has been exploited also to produce by fermentation other useful enzymes like β-fructofuranosidase, xylanase, β-glucosidase, peroxidase, and cellulose activities. Similarly it has been reported for citrus waste polysaccharides that they can be used to produce α-amylase, protease, xylanase, and cellulose enzymes (Pandey and Nigam 2009).

Finally, with regard to microbial biopolymers’ production, an interesting example is given by residual starch from cassava that has been employed also for xanthan gum’s production by means of Xanthomonas campestris SmF on cassava acid hydrolysate as fermentation medium. Other studies showed the benefit in using residual pectin, cellulose, and hemicellulose recovered from citrus wastes to produce xanthan , by means of X. campestris fermentation (Pandey and Nigam 2009). Commercial uses of xanthan gum are mainly as food additives in salad dressing and sauces or for the preparation of low-fat and gluten-free foods. Acid hydrolysates from sugarcane bagasse have been used for the synthesis of microbial biopolymers like polyhydroxyalkanoates (PHAs) , a biopolyester that is the object of interest as biodegradable plastic. Bagasse polysaccharides have been used to produce PHA by aerobic bacterium Ralstonia eutropha.

5.5 Pharmaceutical Applications

Several kinds of polysaccharides already find numerous applications in the pharmaceutical industry, for example, cellulose that since many years is usually used for drug delivery, or starch that is the starting material for cyclodextrins’ production. Notably, also polysaccharides from food and agricultural residues recently have received attention for their biological properties that make them potentially useful for different pharmaceutical applications .

Pectins can be exploited for pharmaceutical purposes, for example, for the preparation of mucoadhesive polymers for gastrointestinal adhesion, in combination with other polymers such as Carbopol and chitosan; against poisoning by lead and mercury; as antihemorrhagic and coagulating agent; for overeating control; and for drug delivery, for controlled release of active principles in the treatment of diseases like ulcerative colitis, Crohn’s disease, or colon carcinomas (Srivastava and Malviya 2011). Pectic polysaccharides from citrus wastes are able to bind and thus inhibit galectin-3 (GAL3), a prometastatic protein that is overexpressed in many cancer types.

By means of enzymatic and/or alkali treatment, it is possible to produce chemically modified pectins that have also been claimed to be potential anticancer agents . Some modified citrus pectins, patented as PectaSol (commercially available as supplement) and as GCS-100 (Maxwell et al. 2012), showed indeed to be active against human prostate cancer and to be effective in the treatment of patients showing solid tumors, respectively. The encouraging results obtained in human studies have been confirmed also in animals and tumor cell lines. Oral intake of modified citrus pectins in mice caused a decrease in colon cancer; on the other hand modified apple pectins showed to be able to lower inflammation and to prevent tumor formation in a mouse model of colitis-associated colon cancer. Several studies performed with cancer cell lines showed that modified pectins from citrus fruits are able to inhibit GAL3 and thus to induce apoptosis in murine endothelial cells (Maxwell et al. 2012); moreover, modified citrus pectins have been shown to be active against cultured leukemic cells (Ramachandran et al. 2011).

Hemicellulose and in particular xylans from agricultural wastes can be useful for biomedical applications: xylans recovered from corn stover and wheat straw have indeed been reported to inhibit the growth rate of sarcoma-180 and other tumors, and on the other hand water-soluble AGXs isolated from corn residues showed to possess in vitro mitogenic and co-mitogenic activities (Ebringerova et al. 1999).

Finally a glucan isolated from tomato wastes showed interesting biological properties as a potential anti-inflammatory agent . Indeed in an in vitro experiment of J774 macrophages stimulation with bacterial lipopolysaccharide, the tomato waste polysaccharide proved to be able to inhibit NF-κB activation and iNOS expression by preventing the production of reactive oxygen species. Therefore it could be a promising molecule for controlling oxidative stress and/or inflammation processes (De Stefano et al. 2007).

5.6 Feedstock for the Production of Second-Generation Bioethanol

Polysaccharides from wastes and more specifically from cellulosic agricultural residues represent the main and most important feedstock for the production of second-generation bioethanol , since they are not in competition with food chain. Lignocellulosic wastes could afford a less environmentally impacting production of ethanol because their production do not require further exploitation of land (they are the residues of “primary crops”) and also because they are available at zero cost in significant amounts (see also Tables 1 and 2).

The most studied waste biomass for second-generation bioethanol are some lignocellulosic residues, like cereal straw or sugarcane bagasse: the biotechnological potential of cellulosic and hemicellulosic residues for renewable energy production has been indeed the object of various researches carried out in the last 40 years. Due to the complex chemical composition and to the recalcitrance of lignocellulosic biomass, such studies have been focused on the problems connected to the different stages of the industrial process for conversion of biomass to fermentable sugars and finally to ethanol, i.e., the initial delignification treatment step, necessary to remove the nonsugar lignin fraction from the carbohydrate complex matrix, the fractionation of the complex polysaccharide matrix made of cellulose embedded in the hemicellulose envelope, the production of highly concentrated monomer sugar mixtures by means of enzymatic hydrolysis of polysaccharides (the so-called saccharification ), the fermentation of sugar syrups, and the final distillation phase to gain pure ethanol.

The pretreatment step that is necessary to enable enzymes to degrade the glycosidic bonds in the polymer, by enhancing the surface area, can be accomplished by steam explosion, ammonia fiber explosion (AFEX), SO₂/H₂SO₄ or CO₂ explosion, hot liquid water, dilute acid or alkaline treatment, or finally ionic liquid or ligninolytic enzyme treatment. After the first step, hydrolysis of polysaccharides (saccharification) is carried out by using cellulose enzymes: the most studied include cellulose and β-glucosidase enzymes produced by fungi such as Trichoderma reesei, Trichoderma viride, and Aspergillus niger. The mixture of sugar resulting from cellulose and hemicellulose depolymerization is therefore fermented by ethanologenic microorganisms like Saccharomyces cerevisiae and Zymomonas mobilis. The main challenge associated to the fermentation in bioethanol production based on cellulosic feedstock is represented by the conversion of pentoses (deriving from hemicellulose fraction degradation) that are not fermented by the conventional yeast species. For this reason many recent researches have been devoted to identify a microorganism able to ferment both C6 and C5 sugars generated from hydrolysis of lignocellulosic biomass. Among the most interesting species is also Z. mobilis that has shown to be able to ferment both glucose and xylose derived from corn stover.

The biotechnological process by which biomass after pretreatment is converted in ethanol can be accomplished in different configurations, and it is the object of study too. The basic process is represented by separate hydrolysis and fermentation (SHF) that allows the optimal temperature conditions either for cellulose enzymes or for fermenting microorganisms, but that can be less efficient due to the glucose inhibition of cellulose enzymes; therefore, a simultaneous saccharification and fermentation process is under investigation to afford higher saccharification yields. Although the latter could be a more sustainable and efficient process, some problems are still to be solved, such as the different temperature operation’s conditions required from enzymes and yeasts. For these reasons recently the so-called consolidated bioprocessing (CBP) has received increasing attention: in such a biotechnological process, biomass would be converted in ethanol by using a single engineered strain able to carry out the cellulase enzyme production, the saccharification of cellulose, and the fermentation of monomer sugars to ethanol (Cheng and Wang 2013). It is noteworthy to underline that a commercially available microorganism able to perform a CBP process has not yet been reported, but research in this field is still going on (Pandey and Nigam 2009).

6 Conclusion

Vegetable biomass is one of the most promising renewable sources of energy and chemicals . Its exploitation for biofuel and commodity chemicals is indeed the focus of continuous researches since many years. In the last decades the gradual shift toward the so-called biobased economy (an economy system based on exploitation of renewable resources such as biomass) has determined a global increasing demand for goods derived mainly from polysaccharide components of vegetable biomass. However this phenomenon is causing many environmental, social, and economical problems such as the intensive exploitation of land (e.g., for energy crops), the competition with food chain, and the consequent increase of foods prices, especially dramatic for developing countries. In order to ensure a more sustainable productivity and the global food security, wastes coming from biomass cultivation, harvesting, and processing of vegetables are more currently considered as an alternative source for polysaccharides that are used for energy and chemical production.

Indeed different kinds of agro-wastes such as food residues (coming from industrial processing of cereals, fruits, and vegetables) or such as agricultural wastes (resulting from harvest and postharvest operations of sugar or cereal crops) are massively produced every year. Therefore several million tons of such residual biomass are available as valuable sources of polysaccharides that in turn could be exploited as feedstock for the production of a wide range of value-added chemicals and for renewable energy generation.

Several biotechnological strategies for the conversion of agro-waste polysaccharides are available for the production of food additives (dietary fibers, stabilizers, preservatives, sweeteners, etc.), of biomaterials (biocomposites, edible films for food packaging, or biodegradable plastics), of bioactive compounds (prebiotic or anticancer agents), of enzymes and value-added compounds (organic acids or building block chemicals), and of renewable energy (second-generation bioethanol) (Fig. 18).

Fig. 18

Schematic representation of potential biotechnological uses of agro-wastes

Some of these production systems are already an industrial reality, while others are under investigation for the scaling up of laboratory scale processes. A remarkable example of the biotechnological potential of agro-wastes is represented by polysaccharides from tomato canning residues. Indeed in a recent study, it showed promising features (Fig. 19) since it was able to inhibit inflammatory processes, but also it could be used as a building block for biodegradable plastic suitable for agricultural uses.

Fig. 19

Schematic representation of potential uses for tomato waste polysaccharides

The future biobased economy development will be limited only by the availability of biomass; therefore, in such a scenario polysaccharides from agro-wastes can represent not only a more environmentally friendly and sustainable source of goods but also the economical solution to supplementing feedstock for a wide range of industrial activities.