**2. Bioactive compounds**

In a conventional method for extracting bioactive compounds in legume seeds, the first step is to soak the dry seeds in water, followed by heat treatment such as boiling. It is effective in that it increases the nutritional value of legumes to some extent and diminishes the levels of phytates and tannins, leading to a higher starch digestibility [3]. However, phenolic acid and saponins can be destroyed in boiling water.

The isolation and purification of biomolecules, especially phenolic compounds, different types of solvents, such as methanol, hexane, and ethanol in a liquid–solid extraction method, are used based on the polarity of the solute of interest. In a study carried out by Xu BJ et al. [3, 4], 50% acetone can be used to extract bioactive constituents in chickpeas and soybeans, whereas black beans are treated with 70% acetone.

In recent years, new extraction techniques have been developed to provide a significant reduction in extraction time, solvent needed, and energy consumption, as well as to improve compounds recovery (**Table 1**). Microwave-assisted extraction (MAE) and ultrasound-assisted extraction (UAE) have attracted the attention of researchers for isolating bioactive compounds. MAE is an efficient method used in extracting bioactivities from legume seeds [22]. On the other side, UAE has also been used for the extraction by using ultrasound to disrupt legume seed cell walls. This method is regarded to be one of the simplest extraction techniques because it makes use of common laboratory equipment such as an ultrasonic bath [15].

#### **2.1 Phenolic compounds**

Phenolic compounds can be easily found in legume seeds. This is a vast group of bioactive compounds, chemically containing at least one benzene ring with one or more hydroxyl substituents, and ranges in complexity from simple phenolic


#### *Nutraceutical Properties of Legume Seeds: Phytochemical Compounds DOI: http://dx.doi.org/10.5772/intechopen.100171*


#### **Table 1.**

*Content of bioactive compounds in legume seeds.*

molecules to highly polymerized compounds [23]. The primary phenolic compounds found in legume seeds are phenolic acids, flavonoids, and condensed tannins (**Figure 1**). The distribution of these compounds differs in the cotyledon (mainly containing non-flavonoids, such phenolic acid and hydroxycinnamic) and primarily concentrates on the seed coats (mainly flavonoids) [3, 24, 25]. The phenolic compound may exist in a free, solubilized conjugated form or in an insoluble-bound form. Some free and conjugated phenolic compounds are thought to be absorbed in the small and large intestines; otherwise, the bound forms with associated non-digestive sugars are made bioavailable by the digestion of enzymes or microorganisms present in the intestine lumen [26].

#### *Nutraceutical Properties of Legume Seeds: Phytochemical Compounds DOI: http://dx.doi.org/10.5772/intechopen.100171*

Phenolic compounds found in important legumes, including lentil, pea, bean, and chickpea, are flavonoids, such as glycosides of flavonols, flavones, and isoflavonoids (primarily daidzein and genistein), and some hydroxybenzoic and hydroxycinnamic compounds [25]. Lentils were reported to have the highest amount of total phenolic content (TPC), which had 21.9 mg gallic acid equivalents (GAE)/g, compared with soybean, bean, and peas [5], and a slight decrease in red bean and soybean (18.8 and 18.7 mg GAE/g, respectively) and the lowest in mung bean (17.0 mg GAE/g).

In general, processing of legumes (including thermal processing, soaking, and roasting) usually affects the number of phenolic compounds. The study by Lafarga et al. [26] revealed that boiling methods retained more polyphenol than that of cooking broth. This explained that high temperature and the destruction of tissue structures of the cooking process caused the diffusion of phenolics and their leaching into water. By contrast, the germination process in legumes generally improved the nutritional quality, including phenolic compounds [27].

## *2.1.1 Phenolic acid*

According to chemical structure, phenolic acids in legume seeds can be divided into hydroxybenzoic acids and hydroxycinnamic acids (**Figure 2**). Gallic, *p*-hydroxybenzoic, protocatechuic, vanillic, and syringic acids are the most common hydroxybenzoic acids in common beans and are mainly present in foods as glycosides. In addition, caffeic, ferulic, *p*-coumaric, and sinapic acids are the most frequently occurring hydroxycinnamic acids in legumes. The level of gallic acid in velvet beans was the highest (479.26 μg/g), followed by black, broad, and red kidney beans (28.64, 24.55, and 12.26 μg/g, respectively) [3, 28, 29]. Lopez et al. [20] documented that ferulic acid derivatives contained the highest percentage of TPC in

**Figure 2.**

*Chemical structure of major phenolic acid compounds present in legume seeds: Gallic acid (a), p-hydroxybenzoic acid (b), protocatechuic acid (c), syringic acid (d), vanillic acid (e), ferulic acid (f), p-coumaric acid (g), caffeic acid (h), sinapic acid (i).*

both raw and cooked dark beans (19 and 24%, respectively). The *p-*hydroxybenzoic acid (19.2 to 60.5 mg/kg), syringic acid (45.9 mg/kg), and gentisic acid (8.1 to 26.0 mg/kg) were presented in significant amounts in seeds of six chickpea varieties, while the six field pea seeds were found to contain protocatechuic acid of between 12.1 and 163.5 mg/kg, and *p*-hydroxybenzoic acid, which ranged from 45.5 to 101.7 mg/kg [7]. Yihan Liu et al. [30] reported that the differences among four types of cooking methods (traditional or boiling, pressure, microwave, and slow) and heating solution can affect the percentage of phenolic acids. Gallic acid content increased after processing in soybean (79.81 from 54.96 μg/g dry weight), while on the contrary, it decreased in black beans (36.02 from 67.88 μg/g dry weight) [31].

#### *2.1.2 Flavonoids*

Flavonoids are the main phenolic compound found in legumes, and their presence affects the flavor and color of common beans [7, 20]. They are low-molecularweight compounds (approximately 300 g/mol), and their general structures are formed with two aromatic rings, joined by a three-carbon bridge, usually in the form of a heterocyclic ring C. Flavonoids are divided into two groups: anthocyanins (colored compounds) and anthoxanthins (colorless compounds) (**Figure 3**). According to the study by Amarowicz and Pegg [32], flavonols, flavan-3-ols (flavanols), flavones, and anthocyanins are the main flavonoids present in leguminous seeds. The presence of flavonols and flavones can impact the color of anthocyanins group [33]. Catechins are called flavan-3-ols, which are primarily identified in legumes as having colored seed coats, such as kidney, navy, and pinto beans. Catechins, along with procyanidins, are common in raw lentil coats and represent 69% of TPC (74.48 μg/g) [12], while other flavonoid glycosides, such as quercetin, myricetin, apigenin, and luteolin, are only found in trace amounts. Duenas et al. [20] also showed that

*Nutraceutical Properties of Legume Seeds: Phytochemical Compounds DOI: http://dx.doi.org/10.5772/intechopen.100171*

#### **Figure 3.**

*Chemical structure of major flavonoid compounds: Flavanols (a), catechin (b), flavonols (c), flavones (d), quercetin (e), kaempferol (f).*

kaempferol and quercetin in raw and germination of dark common beans contained approximately 6 and 26% of TPC, respectively.

A study by Teixeira-Guedes et al. [31] showed the effect of cooking methods on flavonoid profiles of different varieties of common beans, such as kidney, pinto, black, and borlotti bean. Pressure cooking increased the levels of catechins for all bean varieties, which ranged approximately from 50 to 300 μg/g, except in black beans, where the levels decreased from 372.2 to 287.87 μg/g. The catechin levels in genistein and daidzein content were also increased by this processing and were detected only in soybeans (go up to about 906 and 988 μg/g, respectively).

#### *2.1.3 Condensed tannins*

Tannins are polyphenols that are high in molecular weight with large numbers of hydroxyl groups in their structure, which have the ability to bind with carbohydrates and protein, but only to a limited extent. They are classified as hydrolyzable or condensed (non-hydrolyzable tannins), with flavonoids present at various levels of condensation [3]. Condensed tannins, also known as proanthocyanidins (PACs), are chemically oligomeric and polymeric flavonoids, which at high temperature release into catechins and anthocyanidins [7]. Mostly in the same class as flavonoids, PACs mainly distribute in common bean seed coats and play a crucial function in plant defenses that are susceptible to oxidative damage by many environmental factors. Lentils, black beans, and red beans were recorded to contain a high concentration of condensed tannins [34]. In the case of common beans varieties, PACs' content of black beans was in the range of 4.09 to 5.73 mg catechin equivalents (CAE)/g, for adzuki bean coats the content was 13.8 mg CAE/g, and for lentil cultivars, 3.73 to 10.20 mg CAE/g [9, 26].

#### **2.2 Saponins**

Saponins are bioactive compounds found in legumes, consisting of a triterpenoid aglycone (sapogenin) linked to one or more oligosaccharide moieties. They

**Figure 4.** *Chemical structure of major saponins: Dehydrosoyasaponin I (a), soyasaponin I (b) [36].*

have the ability to absorb free radicals and activate antioxidant enzymes. The most common saponins include the soyasaponins, which are divided into three groups, A, B, and E saponins, based on the chemical structure of aglycone. Saponins from the B group, which have been studied to be the primary compound in legume seeds [3, 35], contain soyasaponin I (approximately from 630 to 900 mg/kg) and dehydrosoyasaponin I (approximately from 650 to 1300 mg/kg) (**Figure 4**). In contrast to the seed coat or cotyledon part, the hilum portion of the seed has the highest concentration of saponins [37].

Saponins have been investigated in a variety of edible legumes, as well as the effect of solvent during UAE, which has been evaluated from lupins (4.55 g/100 g), lentils (10.63 g/100 g), chickpeas (2.97 g/100 g), soybeans (4.08 g/100 g), fenugreek (12.90 g/100 g), and various beans [14, 15, 17, 38]. The research by Wu et al. also showed that mung beans and adzuki beans contained the highest total saponins content in bean hull (24.29 mg saponins Ba equivalent (SbaE)/g and 73.60 mg SbaE/g, respectively) and whole bean (2.20 mg SbaE/g and 10.82 mg SbaE/g, respectively) [6].
