Utilization and Management of Horticultural Waste

C H A P T E R

19 Utilization and Management of Horticultural Waste M.G. Lobo*, E. Dorta† *

Postharvest & Food Technology Laboratory, Tropical Fruits Department, Canarian Institute of Agriculture Research, Canary Islands, Spain †Pontifical Catholic University of Chile, Santiago de Chile, Chile

19.1 INTRODUCTION According to the International Society of Horticultural Sciences (ISHS), horticulture includes the production of vegetables in small areas and other crops such as floriculture, olericulture, fruit growing, arboriculture, as well as the cultivation of aromatic and medicinal species. Horticultural products cover all products, raw or processed, that arise from the horticultural industry. The principal crops generally accepted by researchers and educators in horticultural science are: • tree, bush, and perennial vine fruits • perennial bush and tree nuts • vegetables (roots, tubers, shoots, stems, leaves, fruits, and flowers of edible and mainly annual plants) • aromatic and medicinal foliage, seeds, and roots (from annual or perennial plants) • cut flowers, potted ornamental plants, and bedding plants (involving both annual or perennial plants) • trees, shrubs, turf, and ornamental grasses propagated and produced in nurseries for use in landscaping or for establishing fruit orchards or other crop production units. In Canada, honey and maple syrup are classified as horticultural crops although they are produced by animals. Nowadays, climate change, environment, resource efficiency, and raw materials are the main concerns throughout the world. Therefore the search for alternatives to these problems is of great interest for the scientific community. Globally, 140 billion metric tons of waste is Postharvest Technology of Perishable Horticultural Commodities https://doi.org/10.1016/B978-0-12-813276-0.00019-5

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19. UTILIZATION AND MANAGEMENT OF HORTICULTURAL WASTE

generated in the world every year. One-third of the food produced in the world for human consumption (1.3 billion tons) gets lost or wasted, with fruits, vegetables, roots, and tubers among the highest wastage rates. Thus 30% of cereals; 40%–50% of root crops, fruits, and vegetables; 20% of oil seeds, meat, and dairy; and 35% of fish are lost. Fig. 19.1 shows the world production of the main crops versus the hypothetical residues that would be generated (FAOSTAT, 2018). In Europe and North America the consumer waste per capita is between 95 and 115 kg a year, while in sub-Saharan Africa, south, and southeastern Asia, it is only 6–11 kg. Globally, 40%, of losses occur at postharvest and processing levels in developing countries while the same losses occurred in industrialized countries at retail and consumer levels. Various processes in the horticulture industry (e.g., processing, packaging, transport, and storage) generate important volumes of waste, but the reuse of these wastes by another industry allows for a circular economy contributing to environmental protection, thus reducing the dependency on raw material imports and generating growth and jobs. The horticulture wastes are mainly organic and characterized by a high biological oxygen demand (BOD), chemical oxygen demand (COD), high water content and variations in composition. In such conditions, it is very likely that bacteria contaminate them, which is an environmental problem. Therefore, there exists a high interest in their valorization. This chapter discusses horticultural waste generation and describes measures to minimize and/or to use these wastes in an environmentally sustainable manner. 3.00E+009 2.50E+009

World production Residues

2.00E+009 1.50E+009 1.00E+009 5.00E+008 80,000,000 70,000,000 60,000,000 50,000,000 40,000,000 30,000,000 20,000,000 10,000,000

R oo t

s

an d

tu be rs

al nu ts W

Le gu m es

le s Ve ge ta b

Fr ui ts

C itr us

C er ea ls

0

FIG. 19.1 World production of main crops and their hypothetical residues associated (values in tons, FAOSTAT, 2018).

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19.2 VALORIZATION AND APPLICATIONS OF HORTICULTURAL WASTE

19.2 VALORIZATION AND APPLICATIONS OF HORTICULTURAL WASTE The valorization of horticulture waste consists of obtaining different compounds from them or through their use in a process. As mentioned above the activity in horticulture crops are frequently associated with the production of large amounts of waste with several potential applications. Nevertheless, horticulture waste without a correct treatment will trigger environmental and health risks. Nowadays, different strategies have been developed to value and reuse the horticulture waste. This chapter includes four blocks: chemical, biological, biofuels, and thermal valorization (Fig. 19.2).

19.2.1 Chemical Valorization Horticultural waste constitutes a largely under-exploited residue from which a variety of valuable chemicals can be derived. 19.2.1.1 Starch, Pectin, and Cellulose Starch is a carbohydrate composed of amylose and amylopectin, which are glucose building blocks present in fruit and vegetable waste. The principal uses of starch are obtaining sugar or biogas. Recently, starch has been used, due to its low cost, as a substrate for microbial growth in order to produce bioenergy (biohydrogen and biomethane). The combination of microwave and ultrasound-assisted procedure, generates sugars through the catalytic conversion of starch-based industrial wastes. Pectin is a structural heteropolysaccharide present in the cell wall of all terrestrial plants and is basically composed of α (1,4) linked D-galactrounic acid residues. They can be found in most fruit pomaces and after extraction and purification can be added as gelling agents in numerous food products such as jams, fillings, and sweets. Pectin is highly valued as a functional food ingredient as it contains dietary fibers, lactic acid, pigments, vinegar, natural sweeteners, and cellulose. Today, it finds wide pharmaceutical applications. Different sources of pectin have been described and in general all came from waste (e.g., peel from passion fruit, pomegranate, mango, Jackfruit, biowaste from banana and tomato, and even

Chemical

Biological

Biofuels

Thermal

• Starch, pectins, cellulose

• Animal feeding

• Bioethanol

• Incineration

• Natural colorants

• Composting

• Biogas

• Pyrolysis

• Dietary nutrients or fibre

• Vermiculture

• Bioactive compounds

• Substrate for microbial growth

FIG. 19.2 Strategies to value and reuse horticultural waste.

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cladodes of Opuntia ficus indica. In recent years the relevance of pectin has increased due to its important health effects, including lowering serum cholesterol levels, immune protection and ability to promote growth of probiotics, inhibition of tumor growth, and regulation of oncogenes. This perspective highlights the importance of the reuse of biological wastes in the production of this type of highly valuable compounds. Cellulose is an abundant polymer in nature and occurs in various plants (e.g., wood, cotton, flax, hemp, ramie, fruit peels, or vegetables) and in living organisms. It is environmentally friendly, cheap, and biodegradable. It is localized in the cell wall, where it forms microfibrils. The cellulose production from horticulture waste, especially from plant fibers, create an excellent environmental sustainability alternative to minimize the impact of conventional polymers in nature. Cellulose fibers are composed of nanofiber assemblies with a diameter ranging from 2 to 20 nm and a length of more than a few micrometers. Table 19.1 presents the source of cellulose fibers from horticulture waste. 19.2.1.2 Natural Colorants The search for new sources of natural pigments has increased in the last years, mainly because of the negative effects produced by the chemical colorants, such as hypersensitivity reactions involving urticaria, angioneurotic edema, and asthma. Anthocyanins and carotenoids present in vegetables and fruits are considered as natural colorants, and their antioxidant properties make them very interesting. Therefore in the last years, researchers have been interested in determining novel natural compounds sources to produce natural food colorants. Horticultural products are extremely rich in all types of pigments (chlorophylls, carotenoids, anthocyanins, and betalanins). The major photosynthetic pigments of higher plants are chlorophylls (a and b which occur in a ratio of approximately 3:1) and carotenoids (carotenes and xantophylls). Chlorophyll gives plants, vegetables, or fruits their green color and may hide the other pigments present. Chlorophyll absorbs all colors of visible light except green, which it reflects to be detected by our eyes. Most carotenoids are yellow, red, or orange, but some are green, pink, and even black. Many of the bright colors found in flower petals, fruits, or vegetables are due to the presence of carotenoids (although some are due to anthocyanins). TABLE 19.1 Sources of Cellulose Natural Fiber From Horticultural Waste Source Potato peel Spines and peel from fruits from Opuntia ficus-indica Lemon and maize Coconut husk Banana rackis and peel Pineapple leaf and peel Wood Yard trimmings

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The yellow colors of fall foliage are due primarily to the preferential destruction of the green chlorophylls, revealing the carotenoid color. Anthocyanin-based colorants are attractive for their red, purple, and blue colors and are commercially manufactured for food use from horticultural crops (e.g., fruits, vegetables, flowers, and cereal grains) and from processing wastes. Eggplant peels, red cabbage, red grapes, strawberry, red carrot, and roselle flowers are very rich in anthocyanin. Betalains are plant-derived natural pigments (red, yellow, or purple) extracted from red beetroot, amaranth, prickly pear and red pitahaya. Some of these pigments can be used to functionalize other foods due to their antioxidant, anticancer, antilipidemic and antimicrobial activity. The use of fungi, yeasts, bacteria, microalgae, and actinomycetes capable of growing on horticultural waste leads to the production of microbial pigments. Penicillium, Fusarium and Monascus produce water-soluble pigments. Rhodotorula sp., widely distributed in nature, can biosynthesize specific carotenoids such as β-carotene, torulene, and torularhodin, in different proportions when grown on apple pomace. An apple pomace-based basic medium can be used to produce Rhodotorula sp. (pink color), Chromobacter (dark red), and Micrococcus sp. (light yellow) (Mata-Go´mez et al., 2014). 19.2.1.3 Dietary Nutrients or Fiber Dietary fiber has been defined as a plant part that resists digestion against gut enzymes in human beings. Chemically, they are composed of homopolysaccharides, heteropolysaccharides, resistant starches, oligosaccharides, lignin, gums, and mucilages. On the basis of water solubility, these can be grouped into two classes: soluble and insoluble dietary fibers (SDF and IDF, respectively). Plant cellulose, hemicellulose, and lignin material are categorized as IDF and provide structural strength to plant material, whereas, gums, mucilages, pectins, β-glucan, galactomannans, and glucomannans are considered as soluble fibers and have a gel formation capacity. The growing interest in maintaining organism health has led to a greater interest in the food constituents. In this context, dietary fiber (DF) plays an important role in the human diet, thus providing numerous health benefits: improve gastrointestinal health, glucose tolerance and insulin response, reduce the risk of developing some cancers, favors lipid digestion, and therefore in some degree weight control. Peel and seeds are interesting horticultural waste because of their DF. For example the extrusion of citrus residues allows recovering about 40%–50% of the DF for use as a potential high-fiber ingredient in the food industry. Recently, studies with by-products of tropical fruits has shown that the total DF content (g/100 g) was 31.57  0.46 for mango, 69.64  0.28 for pineapple and 56.93  0.51 for passion fruit, making these by-products an interesting source of DF. Interestingly the chemical composition of the different DF by-products sources depends on the composition of the raw material (e.g., pomegranate, citrus, and tiger nuts), the industrial source (e.g., juice extraction, ice cream, jam, canned, and sauces), and the coproducts processing into DF extracts. During the processing of coproducts into fiber, the raw material undergoes two critical steps: scalding (which includes washing) and drying. In general terms the content of ash, sugar, and protein decreased, while the content of fat increased after this treatment (Lo´pez-Marcos et al., 2015). Table 19.2 shows the content of different fiber extracted by different by-products.

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Composition

LDF (g/kg)

GDF (g/kg)

PDF (g/kg)

TNDF (g/kg)

MPDF (100 g/g)

TDF (g/kg)

OPDF (100 g/g)

BPDF (100 g/g)

TBDF (100 g/g)

Moisture

78.8  5.2

69.5  4.2

79.4  9.0

77.1  6.1

66.37  0.8

101.4  5.4

62.12  1.31

86.75  2.54

6.47  0.11

Total DF

667.1  4.2

691.5  5.2

518.0  4.1

597.1  5.5

42.50  1.80

495.3  24.3

49.20  0.40

50.60  0.80

67.43  1.14

Sugars

74.1  5.8

48.7  3.6

52.2  2.5

65.6  4.6

nd

95.4  5.1

nd

nd

nd

Protein

80.7  8.6

117.2  8.2

102.1  1.0

47.5  3.6

3.10  0.12

176.6  7.1

5.03  0.03

4.64  0.11

3.90  0.05

Lipids

27.7  1.1

4.8  0.5

209.6  8.2

98.5  5.2

2.12  0.42

96.6  4.7

1.45  0.74

4.51  0.16

0.65  0.10

Ash

43.3  1.5

56.9  4.1

25.0  4.1

29.9  1.5

3.91  0.24

36.4  5.2

5.72  0.03

12.37  0.08

4.08  0.03

DF, dietary fiber; LDF, lemon dietary fiber; GDF, grapefruit dietary fiber; PDF, pomegranate dietary fiber; TNDF, tiger nut fiber; MDF, mango peel dietary fiber; TDF, tomato dietary fiber; OPDF, orange peel dietary fiber; TBDF, tamarind bagasse dietary fiber; nd, not determinated.

19. UTILIZATION AND MANAGEMENT OF HORTICULTURAL WASTE

TABLE 19.2 Chemical Composition of Different Rich Fiber Extracts From Horticultural By-Products

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19.2.1.4 Bioactive Compounds Horticulture waste presents high amounts of bioactive compounds, especially in fruit industrial processing, where the amount of waste generated depends on fruit (mango 30%–50%, banana 20%, pomegranate 40%–50%, and citrus 30%–50%). Many compounds have been identified in these biowastes, such as phenolic acids, cinnamic acids, flavonoids, proanthocyanidines, anthocyanidins, carotenoids, sterols, tocopherols, among others (AyalaZavala et al., 2011) (Table 19.3). Most of the bioactive compounds obtained from horticultural waste are polyphenols, which have been considered important due to their ability to scavenge free radicals and prevent oxidative reactions in food. The biological basis to explain the health benefits of polyphenols relates to their well-established antioxidant properties. From a structural point of view, polyphenols can be classified into two major distinguishable classes. The first and largest one is the flavonoids, whose basic structure includes 2 benzene rings (A and B) that are linked through 3 carbon atoms that frequently form an oxygenated pyran heterocyclic ring (C) (Fig. 19.3). The second class of polyphenols, defined as nonflavonoid phenolics, includes a more heterogeneous group of compounds with an important subclass, the phenolic acids. TABLE 19.3

Bioactive Compounds Identified in Different Fruits and Vegetable By-Products

Biowaste

Bioactive Compounds

Avocado peel and seed

Phenols, carotenoids

Tomato peel

Flavonols, phenolic acids, flavonones, carotenoids

Banana peel

Phenols, flavonoids, carotenoids, flavonols

Mango peel and seed

Phenolic acids, flavonoids, flavonols, gallotannins, carotenoids, bioactive lipids

Pineapple by-products

Phenols, cinnamic acid, amino acids, proteins

Citrus peel and seed

Flavonones

Pomegranate peel and seed

Bioactive lipids, anthocyanins, ascorbic acid

Orange peel

Phenolic acids

Watermelon peel

Anthocyanins

Papaya peel

Carotenoids, amino acids, proteins

Phenolic acids 3′ 2′ 8

1

O

7

A

C

5

4

6

FIG. 19.3

1

O

O

4′

B

OH

OH

5′

2 6′ 3

Benzoic acid

Cinnamic acid

Basic structure of flavonoids, two benzene rings (A and B), and an oxygenated pyran heterocycle ring (C). Structure of two phenolic acids, benzoic, and cinnamic acid.

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The addition of natural bioactive compounds as a source of antioxidants to replace synthetic additives is of great interest for the food industry. Antioxidants from banana peels, mangoes, tomatoes, and pineapple by-products have been incorporated to juice, biscuits, buffalo ghee, vegetable oils, potato chips, dry-cured sausage, hamburgers, etc. Thus food industries can use horticultural waste as natural ingredients for the formulation of functional foods.

19.2.2 Biological Valorization Horticultural waste can be used in animal feed, composting, vermiculture and as substrate for microbial growth, to minimize environmental pollution and waste accumulation, being an excellent raw material of low cost for industries. 19.2.2.1 Animal Feeding Animal feeding is one of the ways of utilizing a substantial portion of the enormous potential agroresidues. Today, it is well recognized that in animal husbandry, feeding is a critical factor to obtain animal with excellent development. Therefore the production of animal feed from agroindustrial biowaste will be one of the most sustainable technologies that will allow obtaining income to the agro community and a better management of the environmental pollution (Ajila et al., 2012). Horticulture wastes used as animal feed can be classified into two main groups, plant origin and fermentation industry (Table 19.4). Son et al. (2017) determinate the digestible and metabolizable energy of different by-products to feed pigs: 9 by-products from the oil extraction process and 2 by-products from the distillation process. Protein concentration and its biological value, digestibility, level of energy, fats and carbohydrates, quantitative and qualitative composition of amino acids, vitamin and mineral content, and the amount of fiber and the presence of hazardous or toxic substances all determine the feasibility of by-products as animal feed. TABLE 19.4 Horticulture Wastes Used as Animal Feed Plant Origin

Fermentation Industry

Bran

Grain

Waste flour

Sugarcane industry (molasses, bagasse)

Wastes from grain-cleaning process

Potato distillers soluble

Wheat

Brewery waste

Corn

Bacteria and fungi biomass

Rye germs

Winemaking industry (grape pomace)

By-products of oil industry

Citrus by-products (molasses, citrus-activated sludge)

By-products of sugar and starch industry

Anthocyanins

By-products of fruit and vegetable industry

Effluents from biogas production

Plant by-products (husk and pods)

Dairy industry

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19.2.2.2 Composting Composting is defined as the biological degradation process of heterogeneous solid organic materials under controlled moist, self-heating, and aerobic conditions to obtain a stable material that can be used as organic fertilizer. In Europe, almost 50% of the whole amount of compost produced is used in agriculture and constitutes a way to care the soil conditions, thus ensuring their nutrients and organic matter content (Cesaro et al., 2015). Therefore composting supposes a tool in the reutilization of organic waste into valuable products with different applications, which include soil fertility and the suppression of some phytopathogens. The main requirement to safely use compost for agricultural purposes is its degree of maturity and stability. Recently the European Compost Network (ECN) elaborated a scheme to separate collection and treatment of biowaste carried out by different European member countries (Fig. 19.4). Countries such as Austria, Switzerland, Germany, the Netherlands, Flanders (Belgium), Sweden, and Norway, have relied upon separate biowaste collection and

Selective collection and composting/digestion of biowaste Selective collection and composting/digestion of biowaste Only limited collection of biowaste

FIG. 19.4 European map related to waste management by European Members Countries. Adapted by European Compost Network (ECN).

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treatment systems for over 15 years, while countries, such as the United Kingdom, Italy, Finland, Ireland, Slovenia, Estonia, and France have made significant advances during this period. On the other hand, considerable potential for expansion remains in a number of countries, such as Bulgaria, Greece, Croatia, Latvia, Lithuania, Malta, Poland, Portugal, Romania, Slovakia, Spain, Czech Republic, Hungary, and Cyprus. Today, an interesting use of compost is as a replacement for peat. The peat is used as a growing medium in the nurseries due to their good agronomic characteristics but it is rather expensive. In addition the extraction of peat from wetland ecosystems is an environmental problem of international concern. In fact, there is an international effort to evaluate alternative organic substrates to peat, so compost is an excellent alternative. Different residues and by-products from the fruit and vegetable processing industry are used to produce addedvalue compost with the objective to reduce peat use and fungicides against a soil-borne pathogen (Fusarium oxysporum sp. melonis, FOM). Morales et al. (2017) demonstrate that compost had suitable components of mixed compost-peat growing media, providing a 50% substitution of peat; some of them had added value as a suppressive organic medium against Fusarium wilt in muskmelon seedling. This work is an example that demonstrates the possibility in achieving a circular economy. 19.2.2.3 Vermiculture Vermicomposting system is a biological process in which detritivore earthworms interact with microorganisms, thus accelerating the stabilization of the organic matter and greatly modifying its physical and biochemical properties. Fig. 19.5 shows two phases involved in vermocomposting: (I) Active phase: The earthworm interacts with by-products and produce changes in the physicochemical characteristics and microbial composition of the biological residues. (II) Maturation phase: The earthworm displaces to a new layer of fresh substrate and leaves room for the proliferation of microorganisms responsible for decomposing the biological material processed previously by the earthworm. Compared to traditional waste management approaches, such as landfills, fermentation, farmland use, and incineration, vermiculture technology is simpler, requires less investment, can have a better treatment effect, and causes less secondary pollution. New tendencies consist in an integrated system formed by cropping, vermiculture, and organic waste Vegetable and fruit byproducts

Acve phase • Physicochemical characteriscs • Microbial composion

FIG. 19.5

Maturaon period • Displacement a new layers offresh substrate • Microorganism decompose the processed maer by earthworm

Principal vermocomposting phases. Adapted by Garcı´a-Sa´nchez, M., Tausˇnerova´, H., Hancˇ, A., Tlustosˇ, P., 2017. Stabilization of different starting materials through vermicomposting in a continuous-feeding system: changes in chemical and biological parameters. Waste Manage. 62, 33–42.

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treatment. In this sense, Wu et al. (2012) compared this integrated system with conventional cultivation in term of effectiveness organic waste treatment, system production, and soil enhancement. Four treatments were used: cattle dung (CD), sewage sludge (SS), mushroom residue (MR), and conventional cultivation (CC). The average annual amount of organic waste input and the amount of vermcompost output differed significantly among treatments, with CD as the best (2093 m3/ha) because the C:N value was 30:1, a factor suitable for the growth and reproduction of earthworms. CD presented the best values in nitrogen, phosphorus, potassium, organic matter contents, and mineral content of vermicompost. Thus depending on the waste type used, the system can increase yields, provide economic benefits, and improve physical soil properties, as well as produce soil fertilizer. 19.2.2.4 Substrate for Microbial Growth One of the systems for obtaining value-added compounds is the use of microorganisms in a system called solid-state bioprocessing, where high-value substances are generated by the action of microbial growth. Solid-state bioprocessing refers to the use of water insoluble substrates for microbial growth and is generally carried out in solid or semisolid systems in the absence or near absence of water. The production of enzymes by biotransformation of wastes are very interesting due to their uses in the industry including manufacturing of food and feedstuff, cosmetics, medicinal products, and as a tool for research and development. Kitcha and Cheirsilp (2014) obtained enzymes and lipids using oleaginous fungi (Microsphaeropsis sp., Aspergillus oryzae A-4, Colletotrichum sp., Alternaria sp., Microcyclus elongate PFY, Aspergillus tubingensis TSIP9) by the solid-state fermentation of palm-pressed fibers and palm-empty fruit bunches. The newly isolated oleaginous fungus A. tubingensis TSIP9 exhibited satisfactory enzymes and lipid production, in a solid-state fermentation of palm-pressed fibers and palm-empty fruit bunches. The results showed excellent cellulose (26.1 U/dry substrate, gds) and xylanase activity, along with a lipid yield of 88.5 mg/gds. Another source of enzymes, such as lipases, laccases, Mn-dependent peroxidases, and pectinases, were the olive by-products (Mateo and Maicas, 2015).

19.2.3 Biofuels The conversion of horticulture waste into fuels and platform chemicals has become a key issue of the development of fossil-independent production of carbon-based consumer products. The term biofuel or biorenewable fuel is referred to as solid, liquid, or gaseous fuels that are predominantly produced from biomass. The by-products can be treated and pressed into the form of solid fuel to obtain energy, converted into biogas or into heat and steam from incinerated waste (see Section 19.2.4.1). Waste-to-energy (WTE) technologies that produce fuels are referred to as waste-to-fuel technologies (Demirbas et al., 2011). Advanced WTE technologies can be used to produce biogas (methane and CO2), syngas (hydrogen and CO), liquid biofuels (ethanol and biodiesel), or pure hydrogen. These fuels can be used for electricity production, in transports, industrial processes, heating and cooling. 19.2.3.1 Bioethanol Bioethanol is the most widely used biofuel for transportation worldwide. About 95% of ethanol produced in the world is from agricultural products. A major problem with the

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production of bioethanol is the availability of raw materials, as it depends on the starch (60% bioethanol produced) and sugar (40%) content of the biomass employed. Therefore the manufacture of bioethanol depends on the availability of raw materials, which depends on the season and locations; this will affect the cost of bioethanol production. The main producers of bioethanol are the United States with 9000 million gallons and Brazil with 6472 (Mussatto et al., 2010). Biofuel can be produced from direct fermentation of simple sugars or polysaccharides (e.g., starch or cellulose) that can be converted into sugars. Three stages are usually performed in their production: (I) consist in obtaining a solution of fermentable sugars; (II) fermentation of sugars into ethanol; and (III) ethanol separation and purification, usually by distillation–rectification–dehydration. The first stage is the main difference between the ethanol production processes from simple sugar, starch, or lignocellulosic material. Table 19.5 shows the main factors involved in the production of bioethanol from different biomass sources. Pure sugar, followed by rice grain and corn biomass, presented better bioethanol yields than the biomass from sugar beet, sugarcane, and sweet sorghum. 19.2.3.2 Biogas The biogas is generated by the breakdown of organic matter by anaerobic bacteria. In contrast with the process for biofuel production the step for product separation is unnecessary, as the biogas distillates off by itself from the liquid. Biogas typically contains 50%–75% methane, 25%–50% carbon dioxide, 1%–5% water vapor, 0%–5% nitrogen, smaller amounts of hydrogen sulfide (0–5000 ppm), ammonia (0–500 ppm), and trace concentrations of hydrogen and carbon monoxide. Due to the biogas characteristics, it has been defined as one of the most energy-efficient and environmentally beneficial technology for bioenergy production. The degradation process can be divided into four phases: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In each individual phase, different groups of facultative or obligatory anaerobic microorganisms are involved. The spatial separation of anaerobic digestion process TABLE 19.5 Main Factors in the Production of Bioethanol From Different Biomass Biomass

Fermentation Type

Sugar or Starch Content (%)

Extraction Efficiency (%)

Bioethanol Yield (Gal/Ton of Feedstock)

Barley

Starch

50–55

89.5

70.1–77.5

Corn

Starch

72

86.9 (dried minced)

98.0–98.6

Grain sorghum

Starch

67.0–73.8

78.0–90.4

81.8–105.1

Oats

Starch

64

89.5

89.7–90.2

Pure sugar

Sugar

100

100

141.1–141.9

Rice grain

Starch

74.5

89.5

104.5–105.0

Sugar beet

Sugar

16.0–17.3

87.9

19.8–21.6

Sugarcane

Sugar

10–12

93.1–97.0

15.5–18.6

Sweet sorghum

Sugar

13

86.9

15.9–16.0

Wheat

Starch

57.9

95.0–97.3

86.1–88.7

Data from Szulczyk, K.R., McCarl, B.A., Cornforth, G., 2010. Market penetration of ethanol. Renew. Sustain. Energy Rev. 14, 394–403.

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(AD) is in two-stage system: combined system (CS), composed of a first stage, named dark fermentation (DF), where hydrogen is obtained, and a second one that is mostly methanogenic (Corneli et al., 2016). In addition, to optimize biogas production, new strategies, such as aerobic pretreatments for the substrate preparation, have been developed before the anaerobic digestion process. The goal of this aerobic pretreatment is to reduce the amount of total polyphenols in the biomass that are responsible for inhibiting biomethanization. In recent research the biogas generated in an AD plant was used in the motor pumps employed in the same industry facilitating a significant reduction in the greenhouse gases (Moreno et al., 2017).

19.2.4 Thermal Valorization Fig. 19.2 shows the main methodology for converting horticulture waste into desirable products with high added value. The thermochemical conversion technology consists of combustion, gasification, pyrolysis, and liquefaction and has several advantages, such as applicability to a wide range of feedstocks, conversion of both carbohydrate and lignin to high added-value products, faster reaction rates, and the ability to produce diverse fuels. 19.2.4.1 Incineration Incineration is a mature technology that involves the combustion and conversion of biowaste to produce electrical energy and heat. Incinerators can reduce the volume of solid wastes up to 80%–85%. On the other hand the combustion of horticulture wastes is an old technique, and its use as a viable waste management strategy is still not fully accepted. The reluctance of some countries to rely on waste incineration is related to toxic air emissions containing dioxins and heavy metals generated from the earlier equipments and technologies. 19.2.4.2 Pyrolysis Pyrolysis is a thermochemical conversion strategy used to convert lignocellulosic biomass or organic waste materials into available energy, such as liquids or gases from fast pyrolysis and char from the conventional slow pyrolysis process. The pyrolysis of the horticulture waste, using temperatures between 400°C and 800°C in the absence of oxygen or in an atmosphere of inert gas, converts the material from the solid state into liquid products (so-called pyrolysis oil) and/or gas (syngas). They can be used as fuels or become the source of secondary raw materials to subsequent chemical processes. The solid carbon residues can be further refined by providing products, such as activated carbon (bio-char). The products of pyrolysis are therefore gaseous, liquid, and solid and their proportion depends upon the pyrolysis method and the reaction parameters (Fig. 19.6). Nowadays, pyrolysis is being used as an alternative to combustion in waste management. One of the objectives of the biorefinery is the maximum valorization of lignocellulosic materials that include nonfoods, such as wood residues and agricultural wastes; from this point of view the horticulture by-products suppose an excellent alternative. In this context, recently Chen et al. (2016) investigated the use of pyrolysis with cotton by-products (e.g., cotton exocarp and cotton seed) to obtain bio-oil, phenolic compounds, acetic acid and biochar as products from pyrolysis.

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FIG. 19.6

General scheme for pyrolysis process from biomass by-products.

19.3 COMPOSITION AND POTENTIAL VALUE OF WASTE FROM SELECTED FRUITS AND VEGETABLES Fruits and vegetables are usually consumed fresh, but in the last 25 years the demand of processed products has increased because consumers are more committed in eating products that enhance their well-being, resulting in a large amount of waste products. Nevertheless, food industry waste can be good sources of potentially valuable bioactive compounds. Gustavsson et al. (2011) indicated that every year about 1.3 billion tons of food is wasted worldwide, which represents 1/3 of the total food industry production (0.5 billion tons of fruits and vegetables). The waste material obtained from processing depends on the product, the cultivation and harvest techniques, and the process used to transform the fruit or vegetable. The procedures involved often include cleaning, sorting, grading, peeling, size reduction, separation, and mixing; among these, peeling and separation are the major steps of waste production. Fruits and vegetables yield between 25% and 30% of nonedible products, but in some cases the nonedible portion can reach 50%–75% of fresh weight (e.g., pineapple, passion fruit, mango, etc.).

19.3.1 Fruits The main industrial products obtained from fruits are minimally processed, puree, juice, nectar, canned, frozen, in syrup, pickles, chutney, etc. generating skins, rinds, peels, husk, seeds, kernel, stones, bagasse, membranes, vesicles, etc. The valorization of residues requires knowledge of their chemical composition.

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19.3.1.1 Citrus The citrus processing industry generates huge amounts of wastes every year; citrus peel waste alone accounts for almost 50% of the wet fruit mass. Oranges, lemons, limes, grapefruits, mandarins, and tangerines generate residue when processed into juice that can be used for the production of enzymes, ethanol, nutraceuticals, essential oils (EOs), fertilizers, and animal feed, as well as the biosorption of heavy metals. Orange peel waste contains 16.9% soluble sugars, 9.21% cellulose, 10.5% hemicelluloses, 42.5% pectins, 0.84% lignin, 3.75% starch, 6.50% proteins, 1.95% fats and 3.50% ashes. This by-product shows great potential for use in products with high added value obtained through chemical or enzymatic hydrolysis and subsequent biological conversion. Nighojkar et al. (2006) found that the orange peel is a good substrate and inducer in the production of polygalacturonase by microorganisms, and Mamma et al. (2008) observed that the fungal strains of the genera Aspergillus, Fusarium, Neurospora, and Penicillium were able to generate multienzyme activity (pectinolytic, cellulolytic, and xylanolytic) using a simple growth medium of a solid by-product of the citrus processing industry (orange peel). Widmer et al. (2010) studied the use of citrus waste for ethanol production by saccharification and fermentation processes and processing and observed that the ethanol yields based on the sugar content after enzymatic hydrolysis following 48 h of simultaneous saccharification and fermentation ranged from 76% to 94%. There is a growing acceptance that phenols, amino acids, essential oils, pectin, carotenoids, flavonoids, and vitamin C exert beneficial effects in the prevention of degenerative diseases. The dried peel of tangerine is used as nutraceutical ingredient in dietary supplements and functional and conventional foods. Tangeritin is a bioflavonoid that has been known to strengthen epithelial cells in a manner that inhibits the metastasis of cancer cells. Tangerine peel tea contains salvestrols that promote the cellular death of breast, lung, ovarian, and prostate cancer cells while not causing any problems with healthy tissues. Natural prenyloxycoumarins such us auraptene, bergamottin, imperatorin, heraclenin, oxypeucedanin, etc. have been isolated from citrus juice and peel extracts. Citrus EOs are economic, ecofriendly, and natural alternatives to chemical preservatives and other synthetic antioxidants, such as sodium nitrites, nitrates, or benzoates, that are commonly used for the preservation of fruits, vegetables, meat, fish, and processed foodstuffs (Mahato et al., 2017). They can be obtained from the peels discarded as wastes by citrus juice production plants. From an economic and environmental perspective the extraction of orange essential oil is a high value-added option for the valorization and use of the orange peel. Ayala et al. (2017) obtained a yield of essential oil of 5.23% v/w with a concentration of 74.43% of limonene, 4.27% of p-myrcene, 3.26% of sabinene, 1.54% of β-pinene, and 1.54% of linalool. Recent studies showed that citrus peels have been used successfully for the effective removal of heavy metals and chemical compounds from wastewaters generated by the chemical and textile industries. The maintenance of adequate levels of nutrients in soil is essential for healthy plant growth. The peels of pomegranates, sweet limes, and oranges are highly rich in potash, ion, zinc, etc. and can be applied for plant growth. The solid orange waste can be used as an ingredient for animal feed due to its high content of sugars, fibers, and other residual substances. Citrus juice pulp is the semisolid product obtained from the industrial centrifugation of juices to obtain a clear juice that can be used as animal feed. Anaerobic digestion is another possible way to treat and revalorize abundant orange peel waste. This process is a biological conversion of organic material to a variety of end products

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including “biogas,” whose main constituents are methane (65%–70%) and carbon dioxide. Nevertheless, terpenes in citrus waste often make the biological waste treatment quite difficult because of their genotoxic effect on various prokaryotic and eukaryotic organisms. Rapid acidification and inhibition by D-limonene are major challenges of biogas production from citrus waste. As limonene is a hydrophobic chemical, this challenge was encountered using hydrophilic polyvinylidine difluoride (PVDF) membranes in a biogas reactor (Wikandari et al., 2014). Citrus seeds amount to 0.1%–5% of the fruit mass depending on the variety and can be used for oil extraction and the recovery of terpenoids, while the meal remaining from the extraction is a good source of proteins. 19.3.1.2 Pomes Apple pomace is the main by-product of the apple cider and juice processing industries. It accounts for about 25% of the original fruit mass and typically contains 66.4%–78.2% moisture 26.4% dry matter, 4.0% proteins, 9.5%–22.0% carbohydrates, 3.6% sugars, 6.8% cellulose, 0.38% ash, 0.42% acid and calcium, 8.7 mg/100 g of wet apple pomace (Shalini and Gupta, 2010). Pectin is an important by-product from apple or pear processing waste that can be used for jam and jelly making. Pectin consists of 10%–15% of apple pomace on a dry weight basis recovered by acid extraction and precipitation. Its gelling properties are higher than the citrus pectin but present a brown color that may limit its incorporation into light-color foods. Citric acid can be obtained growing Aspergillus niger on apple pomace under controlled conditions. Shojaosadati and Babaeipour (2002) reported a production of 128 g per kg of apple pomace at 78% moisture level in a packed-bed bioreactor. Phenolics, total flavonoids, total flavan-3-ols, and some individual phenolic compounds contribute significantly to the antiradical activities of apple pomace. The polyphenol content of apples, in particular flavonol glycosides, is higher in the peel waste than in the pomace and varies with genotype. Flavonols (quercetin glycosides), cinnamic acids (chlorogenic and caffeic acids), flavanols (catechin and epicatechin), procyanidins, dihydrochalcones (phloridzin), and anthocyanins (cyanidin glycosides) are present. The phenolic components of apples have been linked with the inhibition of colon cancer in vitro. Apple extracts are able to inhibit the proliferation of CaCo-2 cells in a dose-dependent manner, and the inhibitory effect is greater in extracts containing apple peels. Solid-state fermentated apple pomace can be used for ethanol recovery. 19.3.1.3 Melons The watermelon flesh constitutes approximately 68% of the total weight while the rind and the seeds approximately 30% and 2%, respectively. Watermelon rind contains vitamin C, dietary fiber, citrulline, cucurbitacin, triterpenes, sterols, potassium, a small amount of vitamin B-6, and alkaloid compounds. The citrulline has antioxidant effects and converts to arginine, an amino acid vital to the heart, circulatory system, and immune system. Watermelon rind can be used as an adsorbent for the removal of various pollutants, such as dye, heavy metals and ammonia nitrogen, from wastewater. Watermelon seeds constitute about 1%–4% of total fruit weight. Jadhav et al. (2017) analyzed 11 varieties and found significant variation in their proximate composition: crude protein (19.04%–25.16%), crude fat (24.19%–9.87%), crude fiber (19.78%–30.78%), carbohydrates (18.19%–24.61%), and ash content (2.97%–3.94%). Cardiac glycosides and saponins were the most concentrated

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phytochemicals; also present were alkaloids, phenols, flavonoids, and a positive radical scavenging ability. The melons (Cucumis melo, Cucurbitaceae) are a diverse group of fresh dessert fruits including C. melo (var. cantalupensis) with a rough and warty skin, not netted (cantaloupe), C. melo var. inodorus, with a green, yellow or orange skin (winter melons), C. melo var. reticulatus, and with a netted skin (true muskmelon). C.melo rind has potential as an adsorbent material for the removal of Fe and Pb ions from groundwater as well as acts as a source of pectin. Nutritional analysis suggests that honeydew waste products possess energy-rich content that could be the cheapest and valuable bio-resource to cultivate probiotics lactobacilli for the production of bioactive metabolites. Petkova and Antova (2015) analyzed three Bulgarian honeydew melon seeds for proximate and lipid composition. They showed that melon seeds contain fat (41.6%–44.5%), proteins (34.4%–39.8%), crude fiber (4.5%–8.5%), carbohydrates (8.2%–12.7%), soluble sugars (3.7%–4.2%), and minerals (4.6%–5.1%) while the lipid fractions include sterols, phospholipids, and tocopherols. The major fatty acid in lipids is linoleic (51.1%–58.5%) and oleic acid (24.8%–25.6%). The trilinolein (31.3%–32.2%), oleoyl dilinolein (31.0%–34.0%), and palmitoyl dilinolein (14.9%–22.3%) are the major triglycerides. β-carotene and phytoene are also present. The seeds provide opportunities to be developed as valueadded products, dietary supplements, and medicines. 19.3.1.4 Drupes Peaches, apricots, plums, coconuts, mangoes, olives, and cherries are drupes. Solid and liquid wastes generated during the production of drupes are rich in protein, pectin, and polyphenol, with a good gelling property due to a high degree of methoxylation in pectin. Peach and apricot fruits are mainly used to produce juices and canned products. Phenolic compounds that belong to flavonoids (luteolin and prunin), phenolic acids (cafeic acid), lactones (D-decalactone), and triterpens (3-epi maslinic acid) are present. From the peach and apricot kernels, good yields of kernel oil can be obtained. This oil is also known as “persic oil” and has commercially high added value in the cosmetics industry. The fatty acid composition in peach stone oil includes myristic acid (0.1%), palmitic acid (6.6%), palmitoleic acid (0.6%), margaric acid (0.1%), margaroleic acid (0.1%), stearic acid (1.9%), oleic acid (72,6%), linoleic acid (17.7%), arachidic acid (0.1%), eicosenoic acid (0.1%), and lignoceric acid (0.1%), whereas the apricot stone oil contains palmitic acid (5.7%), palmitoleic acid (0.8%), margaric acid (0.1%), margaroleic acid (0.1%), stearic acid (1.3%), oleic acid (67.4%), and linoleic acid (24.8%) (Sa´nchez-Vicente et al., 2009). The classic production of olive oil generates three phases and two wastes: olive oil (20%); solid waste called “orujo” or “olive oil cake” (30%), which is a combination of olive pulp and stones; and aqueous liquor or “alpechin” or “olive mill wastewater” (50%) comes from the vegetation water and soft tissues of the olive fruits, with water added during processing. The use of a modern two-phase processing technique in which no water is added generates oil and a new by-product that is a combination of liquid and solid waste called “alperujo, alpeorujo” or “two-phase olive mill waste” (Ferna´ndez-Bolan˜os et al., 2006). This by-product is a high-humidity residue with thick sludge consistency that contains 80% of the olive fruit, including skin, seed, pulp, and pieces of stones, which are later separated and often used as solid fuel. The polyphenols (oleuropein and hydroxytyrosol), squalene, tocopherols and triterpenes (erythrodiol, oleanolic, and maslinic acid) present in olive waste are potent

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antioxidants and radical scavengers with antitumor and anti-inflammatory properties. Pectins, oligosaccharides, mannitols, and polymerins are other compounds that can be extracted. The sorption of Pb and Cd ions from aqueous solutions can be performed using plum stones. Moreover, desorption tests confirm that the adsorbed heavy metals can be recovered and the sorbent materials after regeneration can be reused. Waste plum stones are a valuable raw material for obtaining fatty oil for the use as alternative feedstock in biodiesel. The endocarp of a drupe is the hardened inedible portion of the fruit that encases the seed and represents the highest lignin content commonly up to 50%. As a biofuel, lignin has approximately double a higher energy density than cellulose. Coconut shell constitutes 15%–20% of the fruit. The coconut fiber-reinforced composites can be used as building materials and in furniture, fishnets, and other household appliances. 19.3.1.5 Berries World wine production is 259.5 million hectolitres (mhl). Italy is the main world producer country, followed by France and Spain. One ton of processed grape will approximately produce 0.13 t marc, 0.06 t lees, 0.03 t of stalks, and 1.65 m3 of wastewater (Olivera and Duarte, 2016). Grape pomace from wine production is composed of seeds, stems, and skins and contains up to 15% sugars, 0.9% pigments and phenolics, up to 1% tartrate acid, and up to 40% fiber. Grape seeds are very rich in linoleic acid and omega-6 fatty acids, phenolics, vitamins E and C, ß- carotene, and steroids (campesterol, ß-sitosterol, and stigmasterol) (Perez-Bibbins et al., 2015). Wine waste is a source for obtaining natural antioxidants, can be potentially used as soil conditioner, fertilizer, and as an adsorbent for the absorption of heavy metals from aqueous solutions and for pullulan. The grape marc is distilled to recover ethanol, tartaric acid, and extracted for antioxidant flavanols used as a nutritional supplement. Mixtures of dehydrated waste grape skins are an innovative way to improve the color and polyphenol profile of red wines. 19.3.1.6 Tropical Fruits The main industrial products obtained from tropical fruits are puree, slices in syrup, nectar, fruit leather, pickles, canned slices, and chutney. Banana industry produces >57.6 million metric tons of banana peels annually. Banana peels contribute about 40% of total fresh weight of banana being a good source of pectin (10%–21%), lignin (6%–12%), cellulose (7%–9.6%), hemicelluloses (6.4%–9.4%), and galacturonic acid (Mohapatra et al., 2010). Banana waste (peels, trunks, pseudo-stems, leaves, and piths) is an effective material as an adsorbent against various water-soluble pollutants such as dyes, heavy metals, pesticides, oils, organic compounds, etc. Moreover, it is a good source of pectins, natural bioactive compounds, starch, nutraceuticals for livestock feed, natural fibers, and nanocellulose fiber, can be used as feedstock for bioenergy (in the form of biogas biohydrogen and bio-ethanol), and for the production of citric acid by Aspergillus niger, and cellulase by Rhizopus sp. The major by-products from mango processing are peels and seeds (35%–60% of the total weight of the fruit, depending on the cultivar and the products made). The peel composes approximately 7%–24% of the total mango weight and are rich sources of valuable compounds, such as phytochemicals, polyphenols, carotenoids, enzymes, vitamin E, and vitamin

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C, which have predominant functional and antioxidant properties. Moreover, Sogi et al. (2013) reported that mango peels are rich in dietary fiber (40.6%–72.5%, being galactose, glucose, and arabinose the major neutral sugars in insoluble and soluble dietary fibers), cellulose, hemicellulose, lipids, protein, enzymes, and pectin. Mango peels can be used for various food applications, particularly in flour, which is a functional ingredient in many food products, such as macaroni, noodles, bread, sponge cakes, biscuits, and other bakery products. During industrial processing, mango seeds are also discarded as by-products. Depending on the variety the kernel represents 45%–85% of the seed and approximately 20% of the whole fruit (Arogba, 1997). Starch, crude fats (7.1%–15%) and protein are the major components. The oil of mango seed kernel consists of about 44%–48% saturated fatty acids (mainly stearic and palmitic acids), 52%–56% unsaturated (mainly oleic and linoleic acids). This fat has attracted considerable interest from scientists due to its unique physical and chemical characteristics, which are similar to those of cocoa butter, illipe, shea, kokum and sal butter. It is a promising, safe and natural source of edible fats, as it does not contain any trans fatty acids. The content of protein is low but contains the most essential amino acids (leucine, valine, and lysine). Mango seed kernels are a good source of polyphenols (gallates and gallotannins; flavonoids, mainly quercetin derivatives; ellagic acid and derivatives; xanthones, principally mangiferin; benzophenones and derivatives, such as maclurin derivatives) (Dorta et al., 2014), phytosterols as campesterol, sitosterol and tocopherols. They can be used in the preparation of functional foods and cosmetics, and for antimicrobial compounds production. The mango stone obtained after the decortication of mango seed can be utilized as adsorbent (Kittiphoom, 2012). Reports have shown that 40%–80% of pineapple fruit is discarded as waste. On the other hand the increasing production of pineapple processed items (juice and canned) results in massive waste generations. The peel represents the largest portion (30%–42%), followed by the core (9%–10%), stem (2%–5%, w/w), and crown (2%–4%) (Dorta and Sogi, 2017). Bromelain enzyme is primarily present in the fruit stem as well as in the fruit and in the waste. It has wide applications in pharmaceutical industry (antiedematous, antiinflammatory, antithrombotic, fibrinolytic activities and anticancer agent), and in food (meat tenderizer). Pineapple peel contains 35% cellulose, 19% hemicellulose, and 16% lignin on a dry basis and can be used as low-cost raw material for the production of ethanol, methane, and hydrogen generation (Khedkar et al., 2017a). The sugars present in large quantities in pineapple peel can be used as nutrients in fermentation processes. Li et al. (2014) characterized the major polyphenolics in pineapple peel and their antioxidant interactions. The core is used for the production of pineapple concentrates, juices, alcoholic beverages, or vinegar. Pineapple is employed as substrates for microbes to obtain single-cell proteins or protein-rich products. Pineapple peel and sugar cane juice are used as carbon sources in the production of bacterial cellulose with Gluconacetobacter swingsii sp. or Acetobacter xylinum, as well as for the lactic and citric acids production through submerged and solid state fermentation. Lactic acid has applications in food processing, and it functions in the chemical industry as a pH regulator, preservative, and buffer. Lactic acid is also used as raw material for the production of biodegradable polymers. Lun et al. (2014) obtained vanillic acid and vanillin from pineapple cannery waste. The high proportion of insoluble dietary fiber in pineapple by-products is considered an advantage in the food industry, as it has beneficial health effects related to increased satiety and the volume and weight of fecal mass, thus promoting improved

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functioning of the digestive system. Pineapple leaves, peels, and cores have a high cellulose content responsible for the strength of the nonwood materials used in paper making industries. Moreover, the residues of the pineapple leaves, obtained after scratching during fiber production, can be used as starting material for vermicomposting.

19.3.2 Vegetables Vegetable waste is produced in large quantities during harvesting, poor and inadequate transportation, storage facilities, marketing practices, and processing industries. Vegetable production has been increasing for the last 10 years. 1.1 billion tons were produced worldwide in 2014 and so do the waste (approximately 45%) (FAOSTAT, 2018). These wastes contain high amounts of water; organic matter such as proteins, carbohydrates, and lipids; and bioactive compounds such as phenolics, carotenoids, anthocyanins, dietary fiber, vitamins, etc. Vegetables are mostly consumed as fresh but in the last decade consumers have demand ready-to-eat or prepare foods, thus increasing the generation of waste. This waste might be converted into biofertilizers, renewable energy, and ingredients for functional foods. 19.3.2.1 Leaves Organic solid waste from leafy vegetables, such as lettuce, spinach, cabbage, Brussels sprouts, etc., can be processed into compost in order to increase soil fertility and thus agricultural productivity. Lettuce waste can be used in a variety of ways: as feedstock; for acetone, butanol, and ethanol production because it is rich in sugars; and in polyphenols. Perino et al. (2016) applied microwave hydrodiffusion and a gravity technique in a pilot-scale, solvent-free for the extraction of polyphenols from lettuce with the aim to save time and energy. The polyphenols composition profile was similar to the classic methods, though a bit lower in total content, while the energy consumption in the optimized procedure (30 min) was 1 W/g of fresh matrix. The basic composition of cabbage waste is crude proteins (9.8%), crude fiber (5.5%), and ash (6.9%). It can be a source for biofuel production through anaerobic digestion. Mu et al. (2017) enhanced methane production by the semicontinuous mesophilic codigestion of potato waste and cabbage waste. Choi et al. (2002) used Chinese cabbage as a substrate for yeast biomass production. Glucosinolates are secondary metabolites of Brassica vegetables that are associated with health benefits and can be extracted from waste. Gonzales et al. (2015) first discriminate of the phenolic and glucosinolate profiles of the extractable and nonextractable polyphenols, using metabolomics techniques, from red cabbage and Brussels sprout waste streams. 19.3.2.2 Bulbs Onions (Allium cepa L.) are the second most produced vegetable worldwide (behind tomatoes). Onion industries produce large amounts of waste during processing. The main onion waste includes onion skins, two outer fleshy scales and roots generated during industrial peeling, and undersized, malformed, or damaged bulbs. These wastes represent an environmental problem because they are not suitable for fodder in high concentration due

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to the onion’s characteristic aroma. Onions should not be used as an inorganic fertilizer because of the rapid development of phytogenetic agents, and they should also not be used to feed dogs, cats, guinea pigs, or other animals in any form due to their toxicity during digestion. Nevertheless, the bioactive compounds present make this waste very interesting as an ingredient for functional foods. Bello et al. (2013) found that moisture (9.21%), protein (8.76%), and ash (11.46%) are highest in the top-bottom part, and carbohydrates (66.12%), fat (15.71%), and fiber (26.84) are highest in the outer scale. X-ray spectrometry revealed that the outer scale had the highest level of calcium (3.05%), followed by the onion bulb (2.98%), and least in the top-bottom part (2.08%). Gas chromatographic analysis of the extracted oil revealed that the oil from the outer scale part has the highest percentage of linoleic acid, 52.87%. The brown skin and the top-bottom could be potentially used as functional ingredients rich in dietary fiber, mainly in insoluble fraction, total phenolics, flavonoids, fructooligosaccharides, alkyl or alkenyl cystein sulphoxides, and other sulfur compounds, which are related with healthy properties (Benı´tez et al., 2011). Onion is a potent cardiovascular and anticancer agent, with hypocholesterolemic, antithrombotic and antiplatelet activity, and antioxidant, antiasthmatic, and antibiotic effects. Onion and garlic (Allium sativum L.) wastes can be used for the adsorption of Pb2+, Sn2+, Fe2+, Hg2+, As3+, and Cd2+ from both synthetic and industrial effluents (Negi et al., 2012). Garlic husks, which contributes 25% of garlic bulb, is regarded as agricultural waste, but they contain phenolic compounds with antioxidant and antibacterial activities, which can be applied in food and pharmaceutical. 19.3.2.3 Roots Carrot residue (carrot pomace and peels) is a by-product obtained during carrot juice processing that accounts for 12% of fresh carrot weight. The juice yield in carrots is only 60%–70% and even up to 80% of carotene may be lost with leftover carrot pomace (Bohm et al., 1999). Carrot residue contain a large amount of fiber (cellulose 51.6%; lignin 32.2%; hemicellulose 12.3%, and pectin 3.88%) (Nawirska and Kwasniewska, 2005). It also has good residual amount of all the vitamins and minerals but it is quite perishable due to the high moisture (88%  2%). Dried carrot pomace has carotene and ascorbic acid in the range of 9.87 to 11.57 mg and 13.53 to 22.95 mg per 100 g, respectively (Upadhyay et al., 2008). A promising way is to store the carrot pomace in dried form and utilize it in the development of bakery products, specifically extrudates, which are becoming more popular than other bakery products in the ready-to-eat food category. Carrot pulp represents another important agrofood waste. It is rich in phenolic compounds, dietary fiber, and anthocyanins and carotenoids. Its phenolic acids have a strong antioxidant potential, and anthocyanins have been proven to reduce cardiovascular heart disease by decreasing the inflammation and lipid oxidation. The primary object in growing sugar beets is the production of refined sugar and several by-products that occurred during the manufacture. These consist in beet tops (leaves and crowns), pulp, waste molasses, and lime cake. Alcohol can be made from waste molasses as it contains 50% by weight of sugar, and the leftover molasses, rich in nitrogen, as either commercial fertilizer or animal feed (Finkenstadt, 2014). Important functional ingredients can be also extracted from the sugar beet pulp such as pectic oligosaccharides (arabinan a prebiotic in human gut), phenolic compounds as antioxidants, dietary fiber, etc.

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19.3.2.4 Tubers Potato manufacturing industries (potato chips, frozen French fries, dehydrated mashed potatoes, dehydrated diced potatoes, potato flake, potato starch, potato flour, canned white potatoes, prepeeled potatoes, etc.) generate a huge volume of potato waste. Potato waste can be used for ethanol production after fermentation with Saccharomyces cerevisiae or cocultures of Aspergillus niger and S. cerevisiae (Izmirlioglu and Demirci, 2017). Potato starch is 100% biodegradable and widely used by the pharmaceutical, textile, wood, and paper industries as an adhesive, binder, texture agent, and filler by oil drilling firms to wash boreholes and a substitute for polystyrene and other plastics in disposable plates and cutlery. Traditionally, potato peel waste is used for producing animal feed and fertilizer as well as for the extraction of phytochemicals with antioxidant, antibacterial, apoptotic, chemopreventive and antiinflammatory properties. The peel of potatoes has higher amounts of phenolic compounds (phenolic acids and flavonoids including flavonols, flavanols and anthocyanins) than the flesh. Potato peel extracts are very efficient in reducing the lipid peroxidation for fish oil and oil-in water emulsions (Koduvayur Habeebullah et al., 2010) and limiting lipid oxidation in meat (Kanatt et al., 2005). Glycoalkaloids are natural compounds produced in potatoes during germination with both adverse and beneficial effects. These compounds can cause death at certain concentrations, such as a >330 mg/kg sample (Liu, 2013), so it is very important to check the presence of these compounds in the extracts. However, depending on their concentration, they can also have an anticarcinogenic effect against human cervical, liver, lymphoma and stomach cancer cells (Friedman et al., 2005). The fermentation of potato peels has multiple uses, including their use as adsorbents, biocomposites, and packaging materials. These products can also be used in ethanol and energy production, biopolymer film development, corrosion inhibition, and the synthesis of cellulose nanocrystals to improve the mechanical and barrier properties of a polymer (Pathak et al., 2017). 19.3.2.5 Flowers Cauliflower waste is composed of 17.32% cellulose, 9.12% hemicellulose, and 5.94% lignin and phenolic acids, carotenoids, vitamins, and minerals. Cauliflower waste for acetonebutanol-ethanol can be performed by Clostridium acetobutylicum NRRL B 527 (Khedkar et al., 2017b). β-galactosidase production, an enzyme used in cheese manufacture, can be enhanced when whey is supplemented with 20% of dry cauliflower waste (Oberoi et al., 2008). The addition of cauliflower waste to ready-to-eat snacks enhances the dietary fiber in the finished product, the protein content, and the water adsorption index. During harvesting >70% of the fresh weight is discarded, including florets, leaves, and stalks. Broccoli waste contains amino acids; fatty acids; vitamins (C, E, A, and K); phenolic compounds (chlorogenic acid and sinapic acid derivatives and flavonoids); minerals (N, P, S, Na, K, Ca, Mg, Fe, Mn, and Zn); and glucosinolates. Campas-Baypoli et al. (2009) studied the biochemical composition and physicochemical properties of three different flours prepared from florets, leaves, and stalks waste of broccoli dried at 60°C. The floret flour showed the highest protein content; ash was higher in leaves flour, and the lipid content was similar in the flours of leaves and stalks. The stalks flour had high crude fiber content and low protein content. All flours presented a high water absorption index. The amino acids tyrosine, aspartic acid, glutamic acid, proline, and valine were found in large concentrations. The most abundant fatty acids in the lipids were

19.3 COMPOSITION AND POTENTIAL VALUE OF WASTE FROM SELECTED FRUITS AND VEGETABLES

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linolenic acid, palmitic acid, and linoleic acid. Broccoli flours are good source of nutrients and can be utilized as dietary supplements. The peroxidase isoenzyme purified from the stem by Duarte-Va´zquez et al. (2007) is active in cafeic and ferulic acid substrates for lignin production. Artichoke waste is a good source for the recovery of valuable phenolic compounds (particularly rich in mono- and di-caffeoylquinic acids and flavones) and the production of renewable bioenergy. During plant development, phenolic compounds tend to accumulate in the peripheral parts (peels, leaves, and stems) where they perform their biological functions. Throughout the industrial processing of artichoke, about 80%–85% of the total plant biomass is discarded (Pandino et al., 2011). Artichoke wastes are a good source for animal feedstuff and nutraceuticals and can be recovered to produce functional ingredients for food supplement and cosmetic industries. 19.3.2.6 Vegetable Fruits Tomatoes are the second most consumed vegetable in the world. Around 40% of fresh tomatoes produced is considered waste because they do not achieve the standards of commercialization. Even more, one of the largest-growing tomato regions in Australia (Bundaberg) rejected up to 87% undamaged, edible harvested tomato because of their cosmetic appearance. Approximately 30% of tomatoes is consumed as processed products, originating skin and seeds wastes that are richer in polyphenolic compounds than the pulp. Tomato pomace is composed of 61.5% skin and pulp and 38.5% seeds. The proximate composition of the dry pomace is 59.03% fiber, 25.73% total sugars, 19.27% protein, 7.55% pectins, 5.85% total fat, and 3.92% minerals (Del Valle et al., 2006). Torres et al. (2005) identified important amounts of flavonoids such as quercertin, kaempferol, and naringenin enantiomers in tomato seeds. ß-Carotene, and lutein are the major carotenoid and xantophyll pigments, respectively. The oil content (13.3%–19.3%) is high in linoleic acid (20.8–39.9 mg/mL), followed by palmitic acid (6.3–19.3 mg/mL), oleic acid (2.5–14.2 mg/mL), linolenic acid (0.7–4.9 mg/mL), stearic acid (0.1–0.8 mg/mL), palmitoleic acid (0.03–0.5 mg/mL), arachidic acid (0.08–0.4 mg/mL), myristic acid (0.05–0.2 mg/mL), and margaric acid (0.02–0.11 mg/mL) (Botineştean et al., 2015). Tomato waste and rotten tomatoes can be used to generate energy and electricity, as well as to recover valuable bioactive compounds and pigments. Pumpkin seed are usually a by-product of pumpkin pulp constituting 2.9% in weight of the fresh fruit, while in their dry form, they account for 32% of the weight (Sedano-Castro et al., 2005). Pumpkin seeds are excellent sources of protein (25.4%–35.4%) and oil (41.5%–49.1%) (Rodrı´guez-Miranda et al., 2012, 2014). Pumpkin seeds have been reported to have a positive effect on human health, including antidiabetic, antihypertensive, antitumor, antibacterial, antihypercholesterolemic and anti-inflammatory actions. 19.3.2.7 Seeds Pea (Pisum sativum L.) is becoming one of the appropriate alternatives to protein products of animal origin because of its high nutritive quality, good technofunctional properties, and low cost. Green pea processing (canning and freezing) yields a mixture of leaves, stems, and empty pods resulting from the cleaning process. Pea seeds contain about 22%–23% proteins being the majority globulins and albumins (80% of total seed protein content); albumins 18%–25% and globulins 55%–65% of total proteins (Tsoukala et al., 2006). Pea peel waste is

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the outer cover of the pea rich in lignocellulosic biomass that can serve as raw material for cellulase production. It is very rich in dietary fiber, particularly insoluble dietary fiber and their extractable polyphenols demonstrate antioxidant activity The pea starch industry yields pea protein (a protein concentrate) and two by-products: pea pulp (a wet product containing cell walls an residual starch), and pea solubles. 19.3.2.8 Stems and Shoots One-half of asparagus is removed while making juice or canning and contain 7.18–15.10% of soluble sugar, 5.27%–6.56% of neutral sugar, 9.06%–17.04% of uronic acid, 18.7%–25.3% of cellulose, 12.3–16.3 of protein and 11.0%–18.1% of klason lignin on dry weight basis (FuentesAlventosa et al., 2013). Asparagus is a good source of inulin, a soluble dietary fiber with prebiotic effects, and contains asparagine a natural diuretic, which is why it is used in slimming diets.

19.4 IMPACT OF PREPARATION ON WASTES The main limiting factor for the effective use of wet waste is the low dry matter content, which hinders the transport, storage, and preservation of this residue, as it can provide ideal conditions for the development of microorganisms, such as filamentous fungi and yeasts, thus promoting the degradation of the waste. Thermal treatment is effective in pasteurizing fruits and vegetable wastes, but the concentration and biological activity of most healthrelated compounds are dramatically reduced as thermal treatment intensity increases. Thus a number of alternative technologies allowing low-temperature processing such as pulsed electric field treatment (PEF), microwave (MW), vacuum drying, freezing, irradiation, etc. have emerged. These treatments should have the ability to inactivate microorganisms and enzymes, all while avoiding the degradation of heat-labile components and consequently preserving the nutritional quality of the waste. Freeze-drying is able to stabilize the antioxidant activity of mango peels and seeds, improve the antiradical capacity of the mango peel against ABTS/+, scavenge free radicals and inhibit lipid peroxidation in the mango kernel. Meanwhile, hot-drying methods (e.g., oven, hot air, infrared) diminish the antioxidant capacity (Dorta et al., 2012; Sogi et al., 2013). The method used to extract essential oils (Eos) affects their chemical profile (number and stereo chemistry of extracted molecules), hence the choice of extraction method depends also on the purpose of the final use. Besides their extraction procedure, EOs can vary in quality, quantity, and composition according to climate, soil type, plant organ, age, and vegetative cycle stage of plant Therefore in order to obtain EOs with a constant composition, all those factors of variability have to be taken into account.

19.5 CONCLUSIONS Ready-to-eat and drink food consumption has increased considerably during the last 25 years, generating a large amount of waste that causes environmental problems and high costs to eliminate them. Taking into account the interesting composition of these wastes (e.g., dietary fiber, antioxidants, vitamins, enzymes, etc.), a circular economy has to be

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developed in order to achieve a sustainable agriculture. The circular economy is a system of resource exploitation where the reduction, reuse, and recycling of the elements are the priority. Thus waste becomes a resource: the biodegradable material returns to nature and the one that is not biodegradable is reused. Products that no longer correspond to the initial consumer needs are introduced into the economic circuit. Finally, the circular economy helps to reduce the environmental impacts of our production and consumption.

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