**3. What are them and why are so important?**

DES were described by Abbot et al. [33] when combined choline chloride and urea (1:2 molar ratio), observing a decrease in the melting point of the mixture compared to that of each individual component (302 and 133°C, respectively). The eutectic mixture obtained was liquid at room temperature (melting point 12°C). **Figure 1** depicts the eutectic phenomena of two compounds. Components involved in the mixture interact via intermolecular forces mainly hydrogen bonding (except covalent or ionic bonds) [34, 35].

DES consists of a mix of halide salt or another hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD) [22]. The combination and molar ratio of HBA-HBD to form a DES led to specific thermophysical and extraction characteristics [36]. Commonly preparation methods are i) *heating and stirring* (component plus heat over 60°C and magnetically stirring), ii) *evaporation* (predissolved components in water plus evaporation at 50°C in a rotatory evaporator), iii) *freeze*-*drying* (aqueous solution of both components submitted to lyophilization) [22, 23]. No additional solvent or purification step is necessary to synthesize DES [23]. The main HBA compound is choline chloride, which is mixed with other inexpensive and safe HBD like urea, ethylene glycol, and glycerol. Other examples, can be natural components present in vegetables such as sugars, polyalcohols, amino acids, organic acids and organic bases [22, 37, 38] (other examples are shown in **Figure 2**). The use of these natural

#### **Figure 1.**

*Schematic representation of phase diagram of two components (A, B) of deep eutectic solvents.*

**Figure 2.** *Some HBD and HBA that can be used to form DES (NADES).*

compounds introduces the concept of natural deep eutectic solvents (NADES), which are a classification of DES. NADES have the advantage of mimics the biological function of compounds mixtures in nature, playing specific roles in organisms [37]. For example, Vanda et al. [37] have mentioned that the transport of compounds, like flavonoids from certain tissues or cells, to another cell could be possible to fluids with similar composition to DES (and NADES).

Since eutectic phenomena were described, many authors have been used it for several purposes; DES has been used in fields such as chemistry (solvent for synthesis reactions), materials science (separation of azeotropes), physics, biology (in the isolation and fractionation of compounds), energy fuels (purification and processing in biodiesel production), agricultural (contaminant removal) and pharmacology [21, 22]. Furthermore, DES have the advantage of being green or ecologically friendly solvents [39]. DES are durable, biodegradable, have low toxicity, high solubility, and have relatively low cost [35].

Thus, when DES are used for the extraction, they have the ability to increase yield, selectivity, solubility, viscosity, and other important physicochemical properties [23, 40]. In addition, the use of DES in anthocyanin extraction could prevent thermal degradation that is common in extraction techniques. Besides, DES can improve the thermostability of anthocyanins once extracted [40]. Finally, DES can be easily eliminated, because during its formation/synthetization no chemical reaction occurs; then, the interactions of hydrogen bonding can be broken under appropriate

*Deep Eutectic Solvents: A Promising Technique to Anthocyanin Extraction for Food Coloring… DOI: http://dx.doi.org/10.5772/intechopen.105162*

conditions [38] or by using microporous resins [20]. Furthermore, DES made with natural compounds can be considered safe for further application without further purification [41].
