**3.1. Biological synthesis of cyclodextrins**

The CDs are produced by degradation of the prehydrolyzed starch and their subsequent cyclization-mediated cyclodextrin glucosyltransferase enzyme (CGTase, EC 2.4.1.19) produced by bacteria that belong to the genus *Bacillus.* Due to the helical structure of the starch molecules, the primary cleavage product undergoes an intermolecular reaction forming cyclic products joined by α-1,4 linkages, generally designated by cyclodextrins. To distinguish them, Greek letters are used to specify the number of D-glucose units (in brackets): α (6) β (7) γ (8) δ (9) ε (10) ξ (12) η (13).

**3.2. The structure of cyclodextrins**

of glucose units (**Figure 2 (a)** and (**c)**).

overall structure (see **Figure 3**) [3, 4].

for use in liquid products [5].

ture was mainly determined by the following factors:

red are represented statistical locations of cavity water molecules [3, 4].

**1.** The nature and size of the cavity included in the molecule;

The native cyclodextrin molecules (α-CD, β-CD, and γ-CD) have the shape of a short truncated cone with a cavity inside, i.e., a toroidal shape. The length is determined by the height of the glucose unit (7.9 Å = 0.79 nm), and the diameter of the cavity is determined by the number

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The glucose rings linked together by α-1.4 linkages as in amylose. They are oriented in the same direction, and thus, the narrow end of the torus is formed by the primary hydroxyl groups (O (6) H), while the wider edge of the truncated cone is occupied by the secondary hydroxyl (O (2) H, O (3) H) groups. These peripheral hydroxyl groups confer hydrophilic properties to the CD surface. Moreover, the internal cavity has mainly hydrophobic characteristics due to the methine group (CH) and the oxygen atoms of the ether type (O (4) and (5)).

The CDs may crystallize in the form of hydrate or inclusion compound, and the crystal struc-

**2.** Hydrogen bonding between the included molecules and between CD and CD drives.

The interstices between the CD units are occupied by water molecules incorporated in the

The CDs cavity in the center, with predominantly hydrophobic character, is large enough to hold, accommodate, or include other molecules. When this occurs, there is the formation of an inclusion compound. These compounds, or complexes, may be described as a molecular-level nanoencapsulation. Food ingredients formulated with cyclodextrins become stable to heat and oxidation processes and are not affected by dispersion forces and are readily dispersed

**Figure 3.** The crystalline hydrate of β-CD. The blue are represented statistical locations of the interstices water molecules;

The shapes α, β, and γ are the natural cyclodextrins and most commonly used (**Figure 2 (c)**). Higher numbers of counterparts of glucose units also exist but are difficult to purify, with weaker inclusion properties. Cyclodextrins with a number of glucose units less than 6 do not exist, probably due to steric hindrance.

The preparation of cyclodextrins can be subdivided into the following main stages:


In industrial production of cyclodextrins, the most frequently used source of enzyme is *Bacillus macerans*, renamed as *Paenobacillus macerans*. Other enzymatic sources used are *Klebsiella pneumonia* and *Alkalophilic bacterium* 38–2. The forms α, β, and γ are dependent from the source of CTGase enzyme. The *Bacillus macerans* and *Klebsiella pneumonia* CTGase mainly produce the α form. *Alkalophilic bacterium* 38-2 mainly produces β-cyclodextrin. However, the relationship between the CD formed also depends on the incubation time of the enzyme in starch medium culture because most CTGases initially produce the α form, while the synthesis of other forms is slower [3].

**Figure 2.** Structure of a cyclic oligosaccharide, cyclodextrin, CD (a); The "donut" Molecular (b); Paragraph equal to six, seven or eight rings of D-glucopyranose, joined by glycosidic linkages of the type α-1.4, representing α-CD, β-CD and γ-CD, respectively (c); white crystalline powder β-CD (d).
