Tannins: Extraction from Plants

Dang Xuan Cuong, Nguyen Xuan Hoan, Dinh Huu Dong, Le Thi Minh Thuy, Nguyen Van Thanh, Hoang Thai Ha, Dang Thi Thanh Tuyen and Dang Xuan Chinh

### Abstract

The chapter presents mainly on different extraction methods of tannin. Some technical means required for effective extraction are also presented, for example, collection and treatment of plant and drying and storage of plant. Opportunity and challenges in application of extraction methods are also exhibited in the chapter.

Keywords: extraction, sea plant, tannin, terrestrial plant

#### 1. Introduction

Tannins are high molecular weight phenolic compounds commonly found in plants with molecular weights ranging from 500 to over 3000 Da and up to 20,000 Da. Chemical structure of tannin is very diverse and different. More than 8000 different tannins have been detected. Tannins have been found in both free and bound forms in plant cells. Terrestrial plant tannins can be divided into four major groups: gallotannins, ellagitannins, proanthocyanidins (condensed tannins), and complex tannins. Sea plant tannins have been described as "phlorotannins," including oligomers or polymers of phloroglucinol. Tannin content can range from 0.2 to 25% DW [1], depending on plant species, harvest time, habitat of plants, and extraction method.

Many various bioactivities of tannins have also demonstrated including antioxidant, antibacterial [2], antifungal [3], antitumor, etc. The number of hydroxyl radicals and aromatic rings is important parameter determining in bioactivity of tannins. Ortho-dihydroxyl groups of tannin are important feature for chelate metal ions. High degree of polymerization and molecular weight play an important role for antioxidant activity of tannins. Tannins can bind tightly to protein through hydrogen bonding between the phenolic groups of the tannins and the -NH groups of peptides. These hydrogen bonds cannot be broken down by digestive enzymes or the attack of microorganisms [4].

Currently, tannins have been applied into many different fields including medicine, food, beverage, manufacture of ink and adhesives, dye and tanning industry, plastic resins, water purification, and surface coatings. Their applicability depends on its concentration, as a complexing agent or a precipitating agent [5].

From the above problems, it is necessary to extract and use tannins effectively. Therefore, this chapter focuses on extraction methods of tannins including maceration, microwave, ultrasound, enzyme, decoction, irradiated radiation, and gamma and points out some issues related to extraction efficiency of tannins, for example,

harvesting, handling, and storing materials. Opportunity and challenge of tannin extraction are also presented in the chapter.

4. Extraction methods

Tannins: Extraction from Plants

DOI: http://dx.doi.org/10.5772/intechopen.86040

4.1 Method of maceration

each solute.

(ln Ω).

59

concentration (C <sup>¼</sup> <sup>n</sup>

VÞ.

Gibbs free energy is calculated by following equation:

and Fick's second law of diffusion as follow:

Maceration is one of the techniques used for tannin extraction from medicinal plants. Maceration is the simplest technique of extraction where the plant powder is placed in a closed vessel and soaked with the corresponding amount of solvent for a specified period of time until the tannins are dissolved in the solvent. In the first stage of the maceration, osmose occurs before diffusion occurs, in later stages, osmose occurs simultaneously with diffusion. The first stage is usually very short, and it can be a few seconds or hundredth second. The first-stage time depends on the type of solvent, the extraction material, and the extraction conditions. The movement of solvents in osmotic process from outside the cell into the cell and vice versa complies with van't Hoff's law. Dissolving solutes in osmotic process cause a pressure deficit called osmotic pressure. The osmosis is calculated on the basis of

thermodynamics. Therefore, the osmotic pressure in ideal solutions is

determined by Eq. (1), and the equation includes an osmotic coefficient (φs) for non-ideal solutions (Eq. 2). The osmotic coefficient depends on characteristic of

where T is the temperature; R is the universal gas constant; and Cs is the molar

However, the actual pressure depends on the interaction between the solute and

The osmose stops only when thermodynamic equilibrium occurs. It minimizes Gibbs free energy, ΔG ¼ 0, and temperature and pressure of solvent are fixed.

the cell membrane. Therefore, πobserved requires the reflection coefficient, σs:

where E is the energy; ρ is the pressure; V is the volume; S is the entropy (S ¼ kBln Ω) inside the Boltzmann constant (kB); and number of configurations

In osmotic process, osmotic pressure and hydrostatic pressure appear together playing an important role in fluid flow across the membrane. Characteristic parameters such as the permeability, the hydraulic conductivity, and the reflection coefficient reflect passive material transfer across cell membranes. Osmolarity is a concentration measure including freezing point depression, vapor pressure depression, and boiling point elevation. The diffusion in process of extraction depends on Fick's laws and Maxwell-Stefan diffusion. Therefore, the extraction yield and diffusion of tannin depend on material size, time and temperature of extraction, type of solvent, stirring, and size of tannin. Usually, the material size is optimal from 30 to 40 mesh [15]; the smaller material size creates difficulty during filtration. The material size depends on chemical characters of material and extraction method. In microwave-assisted method, material size larger than 50 meshes is suitable for the extraction and the filtration [16]. A modern mathematical exhibits Fick's first law

π ¼ RT∑Cs (1)

πobserved ¼ RT∑φsCs (2)

πobserved ¼ RT∑σsφsCs (3)

G ¼ E þ ρV � TS (4)

### 2. Harvest and treatment of sample

#### 2.1 Sea plants

Sea plants including seagrass, seaweeds, and mangroves contain a large amount of active tannins. The only way of seagrass collection is to dive into the sea. The harvest of seaweed has two ways: the first way, diving into the sea to collect seaweed; the second way, using machines for the harvest. Mangroves can only use an equipment for the harvest. Marine plant harvest should be selected to avoid the destruction of marine vegetation. Their physiological respiration ability is very strong, and they contain large amount of salts causing quick decay of sea plants immediately after the harvest. Hence, sea plants should be washed with salt water, then with fresh water moving salts. Sea plants are dried until 19% humidity by using freeze drying, infrared-freeze drying [6], microwave-vacuum drying, assisted microwave-vacuum drying [7], or hot air after cleaning seaweed, and infrared-freeze drying is usefully evaluated [8]. Infrared radiation power intensity of 5 kW <sup>m</sup><sup>2</sup> is suitable.

#### 2.2 Terrestrial plants

Terrestrial plants are more diverse than sea plants, so the method of harvesting depends on the type of plant. The structure of terrestrial plants is more stable than marine plants, and they also do not contain salts. They still have a biochemical respiration. Therefore, they should also be dried by the above methods immediately after harvest. After drying, they also need to be grinded like marine plants for study and production. Terrestrial plants and marine plants should be stored in small plastic bags under refrigeration, the thing help longer storage time.

#### 3. Role and position of tannins in plant cells

#### 3.1 Sea plants

Tannins (phlorotannins) in sea plants exist in integral structural of cell walls. They directly participate in the structure of cells and bind to alginate, protein, laminarin, and fucoidan. Tannins also play an important role in the formation of the zygote's cell wall [9]. They have secondary ecological roles such as against UV radiation and grazing [10]. They possess metal sequestration capacity and algicidal effectiveness [11].

#### 3.2 Terrestrial plants

Tannins have an important role in terrestrial plants, for instance, against microbial pathogens, harmful insects, and mammalian herbivores. They help in plant growth via binding to protein. They are involved in the cell structure of plants [12]. In all vacuoles and surface wax of plants, chloroplast-derived organelle and the ta nnosome produce tannins. They often exist in organelles where they are less affected by the protein precipitation. In Japanese persimmon fruits, the accumulation of tannin occurs in the vacuole of the tannin cells [12, 13], and tannin/tanninless vacuoles are found in Mimosa pudica. Tannins are not accumulated in the vacuoles of sensitive plants [14].
