IV. Plant‑Based Phytochemicals: Extraction, Isolation, and Healthcare

Plant Secondary Metabolites: Commercial Extraction, Purification, and Health Benefits

ABSTRACT

Plant secondary metabolites include bioactive compounds, such as terpenes, alkaloids, polyphenols, glucosinolates, and carotenoids. Apart from conventional extraction methods (e.g., soxlilet and maceration), several novel technologies with basic extraction processes have been developed to extract secondary metabolites from various plant parts and sources. Such extraction methods are high pressure-assisted (HPA), negative pressure cavitation- assisted (NPCA), high-pressure homogenization-assisted (HPHA), high voltage electrical discharge-assisted (HVEDA), microwave-assisted (MA), ultrasound-assisted (UA), moderate electric field-assisted extraction (MEFA) and pulsed electric field-assisted (PEFA), enzyme-assisted (EA), and chemical pretreatment methods. Therefore, applications of plant secondary metabolites in pharmaceuticals, foods, and cosmetics are noteworthy. This chapter focuses on the characteristics of selected plant secondary metabolites, technologies for their industrial production, and applications.

INTRODUCTION

Historically, plants or herbs are used as a functional food, source of pharmaceuticals or medicine, and ingredients of cosmetics and fragrance [60, 101]. Primary metabolites of plants include low molecular weight sugar, polysaccharides, amino acids, proteins, intermediates in the Krebs cycle, and nucleic acids produced by glycolysis, Krebs cycle, photosynthesis, and associated pathways.

Examples of plant secondary metabolites are terpenes, alkaloids, polyphenols, carotenoids, glucosinolates, etc., [77]. These are stored at intra- and inter-cellular spaces in leaf, bark, root, stem, seed, etc. Isolation and extraction of these secondary metabolites has received significant attention due to their several health benefits [5]. Figure 9.1 indicates various metabolic pathways in plants.

However, plant secondary metabolites are important but are not essential for the growth of a plant. They are useful as nutrients for their growth under adverse situations, protect from abiotic stresses, often used for defense purpose and impart characteristics to plant, such as, the development of fragrance, color of flower and fruit, etc. Furthermore, secondary plant metabolites, such as, hormones, are important to regulate and signal the metabolic pathways and take a significant role in the overall plant development. However, there is no fixed rule to classify these plant metabolites.

Metabolic cycle for the synthesis of plant metabolites. Source

FIGURE 9.1 Metabolic cycle for the synthesis of plant metabolites. Source: Self-developed with concepts from Ref. [77].

Based on the origin of biosynthesis, plant secondary metabolites are grouped into four major classes, such as: (a) terpene, (b) alkaloid, (c) polyphenol (d) carotenoid and (d) sulfur-containing compound, i.e., gluco- sinolates. Either plant secondary metabolites in plant extracts or as a pure compound are able to cure diseases and reduce risk of several health hazards [114]. Several unique biological activities of plant secondary metabolites are: antioxidant, anti-inflammatory, antibacterial, antiviral, antiallergic, antithrombotic, antiaging, and carcinogenicity, etc., [23].

This chapter focuses on selected plant secondary metabolites (such as: terpenes, alkaloids, polyphenols, carotenoids, and glucosinolates); their industrial production, purification, and applications in food, biopharmaceuticals, and cosmetics.

CLASSES OF PLANT SECONDARY METABOLITES

9.2.1 TERPENE

The term ‘terpene’ is originated from the word ‘turpentine,’ which also signifies’ resin acids.’ Chemically all terpenes are derived from 5-carbon isoprene units and are assembled in different ways. Terpenes are classified based on the number of isoprene units in a molecule, such as: (a) hemiterpenes, (b) mono- terpenes, (c) sesquiterpenes, (d) diteipenes, (e) sesterterpenes, (f) triterpenes, (g) sesquiteipenes, (h) tetraterpenes, (i) polyterpenes, and (j) norisoprenoids.

Among all plant secondary metabolites, terpenes are the most diverse [19]. All living organisms including plants produce terpenes as a product of certain essential physiological function. It has been proven that selected teipenes have antitumor activity with slight or no side effects [84]. Furthermore, teipenoids can be utilized as pesticides and fungicides in agriculture and horticulture due to their strong antimicrobial and insecticidal properties [71].

9.2.2 ALKALOIDS

The word ‘alkaloids’ is derived from the Latin word ‘alkali.’ Alkaloids are natural organic compounds that contain mostly basic nitrogen atoms. In addition, they may also contain carbon, hydrogen, sulfur, oxygen, and rarely other elements (such as: bromine, chlorine, and phosphorus). Alkaloids are classified as: (a) true alkaloids, originated from amino acids and consisted nitrogen in the heterocycle, (b) protoalkaloids, originated from amino acids and contained nitrogen, but not the nitrogen heterocycle, (c) polyamine alkaloids, derivatives of putrescine, spermidine, and spermine, (d) peptide and cyclopeptide alkaloids, and (e) pseudo alkaloids (not originated from amino acids but alkaloid-like compounds). Alkaloids are useful for the development of pharmaceuticals, related to the nervous system and as a painkiller [6, 8, 9]. In addition, some alkaloids have been used as insecticides in agriculture.

9.2.3 POLYPHENOLS

The word ‘polyphenol’ is derived from the Latin word ‘polus,’ which also signifies ‘many’ or ‘much.’ Polyphenols are most abundant antioxidants that can be classified based on the hydroxyl groups in benzene rings and the number of phenol rings. In polyphenols, more than one hydroxyl group is attached to benzene rings [86, 103]. The main classes of polyphenols are phenolic acids and flavonoids. Furthermore, phenolic acids are classified as hydroxybenzoic and hydroxycinnamic acids. Flavonoids include anthocyani- dins, aurones, catechins (including proanthocyanidins), dihydrochalcones, isoflavones, flavanones, flavans, flavonols, and leukoanthocyanidins [81]. These are produced as a by-product along with metabolic abnormality in the plant. According to research studies, these molecules have antioxidant activity and destructive behavior of reactive oxygen and nitrogen species [56, 118]. Epidemiologic reports indicate a significant role of polyphenols to protect against the development of diseases including cardiovascular, cancer, both Type I- and Type II-diabetes, inflammation, aging, asthma, etc., [56, 120].

9.2.4 CAROTENOIDS

Carotenoids are pigments soluble in fat having antioxidant properties [58] and additional physiological role in immunostimulantion [72]. Different classes of carotenoids are: (a) carotenes (hydrocarbons without oxygen, such as (3-carotene, a-carotene and lycopene) and (b) xanthophylls (oxygen- containing compounds, such as lutein and zeaxanthin). In the plant, carotenoids take part in photosynthesis, absorb heat and protect the plant. Four different types of carotenoids (including a-carotene, (3-carotene and y-carotene, and (3-cryptoxanthin) are considered as precursors of vitamin A in our body. Reaction of carotenoids with radical species in a biological system reduces the stress [58]. Research has shown the possibility of lowering chronic illness by consuming diets, rich or fortified with carotenoids [29].

9.2.5 GLUCOSINOLATES

Glucosinolates are plant secondary metabolites, contain sulfur and nitrogen, found in several vegetables (such as: cabbage, broccoli, mustard, capers, watercress, horseradish, and radishes). These molecules are derived from amino acid and glucose [95] and are well known for their insecticide, antimicrobial, antioxidant, anti-inflammatory, and cytoprotective activities [51].

In some cases, based on the chemical composition, plant secondary metabolites are classified as (a) alkaloids, (b) alkamides, (c) amines (d) amino acids, (e) cyanogenic glycosides, (f) glucosinolates, (g) lectins, peptides, and polypeptides, (h) steroids and saponins, (i) terpenes, (j) flavonoids and tannins, (k) phenylpropanoids, lignins, coumarins, and lignans, (1) polyacetylenes, waxes, and fatty acids, (m) polyketides and (n) carbohydrates and organic acids. The sources and biological activities of different plant secondary metabolites are shown in Table 9.1.

TAB LE 9.1 Sources and Beneficial Activities of Plant Secondary Metabolites

Plant Secondary Metabolites

Sources

Beneficial Activities

References

Carotenoids

Plastids of roots, stem, flowers, fruits, and seeds

Antioxidant and prooxidant effects, provitamin activities, therapeutic effects

[75]

Glucosinolates

Cruciferous families Brassicaceae, Capparaceae, and Caricaceae

Antimicrobial, antioxidant, anti-inflammatory, and cytoprotective attributes

[51]

Polyphenols

Fruits, vegetables, cereals, and beverages; Tea, red wine, coffee, vegetables, leguminous plants, and cereals.

Antioxidants, protection against the development of chronic diseases. Antioxidant activity, development of functional foods

[32, 56]

Terpenes

Lamiaceae, coniferae, compositae, leguminoseae, taxaceae

Antitumor, anti-inflammatory, immunomodulatory, cardiac, and antithrombotic, endocrine

[3]

ISOLATION AND RECOVERY OF PLANT SECONDARY METABOLITES

Among all types of unit operations, extraction is widely used as a unit operation in food and biotechnology industries for the production and purification of several medicinal or functional compounds from the crude matrix. The target plant secondary metabolites are generally found inter- or intra-cellular positions with a complex microstructure. Thus, increase in the permeability of cell-wall and membrane is required to improve the extraction rate and yield of secondary metabolites from the plant tissue. Extraction has been identified as an essential step to recover the desired chemical components from the plant materials for further separation [6, 8, 9, 104]. In general, the extraction technologies of secondary metabolites can be divided into two subclasses: (a) conventional technology and (b) emerging or novel technology. These technologies (both conventional and novel) for isolation and purification of secondary metabolites from the plant matrix are shown in Table 9.2.

TABLE 9.2 General Processes and Operations for the Recovery of Plant Secondary Metabolites

Steps

Processes

Operations

First

Extraction

Conventional organic solvent-based extraction: Soxhlet. Maceration, EA. PEFA. HVEDA. HPA. NPCA, HPHA. MA, UA. MEFA

Second

Separation/Fractionation

Membrane-based separation: nanofiltration, ultra- filtration, microfiltration.

Chromatographic process: ion-exchange chromatography, membrane chromatography, silica-gel column chromatography

Third

Concentration

Distillation, evaporation, freeze-drying, membrane processes with reverse osmosis, forward osmosis.

Source: Self-developed with information from Refs. [6, 8, 9,104].

CONVENTIONAL TECHNOLOGIES

SOL VENT-BASED EXTRACTION

Conventional extraction of plant secondary metabolites consists of a solid-liquid and liquid-liquid extraction process. Soxhlet and maceration are commonly used technologies on a small laboratory-scale extraction process. Soxhlet extraction is performed with organic solvents. For any target plant secondary metabolite, the extraction yield significantly depends on the chemical nature of the solvent and characteristics of the targeted compound [10]. Hence, the selection of solvent and extraction technique are chosen according to the specific nature of plant secondary metabolite [62].

Criteria for the selection of a suitable solvent for a particular purpose include: less toxicity, volatility, and high distribution coefficient. Accordingly, hexane, methanol, chloroform, ethanol, water, ether, and acetone are commonly used solvents during the extraction of plant secondary metabolites. Properties of the solvent during extraction processes can affect further downstream processing. Although conventional solvent-based extraction technology is effective for the extraction of plant secondary metabolites, yet in some cases, it may cause the degradation of heat-sensitive compounds and offer a chance to contaminate with a toxic solvent. The method requires longer processing times, higher power, and the amount of sample and solvent [99]. Some examples of solvent-based extraction of plant secondary metabolites are shown in Table 9.3.

TABLE 9.3 Extraction of Plant Secondary Metabolites by Using Different Solvents

Solvent

Plant Secondary Metabolite

Acetone

Flavonoid

Chloroform

Flavonoid, terpenoid

Dichloromethanol

Flavonoid

Ethanol

Tannin, alkaloid, polyphenol, flavanol, terpenoid

Ether

Alkaloid, terpenoids

Methanol

Anthocyanin, flavones, terpenoid, saponins, tannins, polyphenols

Water

Anthocyanin. terpenoids, tannins, saponins

Source: Self-developed using information from Refs. [74,104].

 
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