a review on the use of cyclodextrins in foods

a review on the use of cyclodextrins in foods

(Parte 1 de 5)

Review A review on the use of cyclodextrins in foods

G. Astray a, C. Gonzalez-Barreiro b, J.C. Mejuto a, R. Rial-Otero b, J. Simal-Gandara b,*Physical Chemistry Department, University of Vigo, Ourense Campus, E-32004 Ourense, SpainNutrition and Bromatology Group, Analytical and Food Chemistry Department, University of Vigo, Ourense Campus, E-32004 Ourense, Spain art i cle i nfo

Article history: Received 3 November 2008 Accepted 2 January 2009

Keywords: Cyclodextrins Food flavours Food additives abstract

Cyclodextrins (CDs) are cyclic oligomers widely used in the food industry as food additives, for stabilization of flavours, for elimination of undesired tastes or other undesired compounds such as cholesterol and to avoid microbiological contaminations and browning reactions. In this review the characteristics of the most important CDs at industrial level (a-CD, b-CD and g-CD) and their main properties from a technological point of view, such as solubility and their capability to form inclusion complexes are described. In addition, the present state-of-the-art on the use of these compounds in the food industry was reviewed. 2009 Elsevier Ltd. All rights reserved.

1. Introduction1631
2. Use and regulatory of cyclodextrins1633
3. Solubility of cyclodextrins1633
4. Formation of inclusion complexes1633
4.1. Energy and inclusion mechanisms1634
4.2. Effect of cyclodextrin structure on the inclusion1635
4.3. Effect of guest properties1635
4.4. Properties of cyclodextrin-included guests in solution1635
5. Applications in the food industry1635
5.1. Encapsulation of flavours using cyclodextrins1635
5.2. Protection against oxidative degradation, heat-induced changes and light-induced decomposition1636
5.3. Taste modifications and elimination of bitter and disgusting tastes and odours of foods1637
5.4. Cholesterol sequestrant1637
5.5. Food preservation1638
6. Final remarks1638
Acknowledgements1638
References1638

Contents

1. Introduction

Cyclodextrins (CDs) are cyclic oligomers of a-D-glucopyranose that can be produced due to the transformation of starch by certain bacterias such as Bacillus macerans (Jeang, Lin, & Hsieh, 2005; Qi, Mokhtar, & Zimmermann, 2007; Qi & Zimmermann, 2005;

Rimphanitchayakit, Tonuzuka, & Sakano, 2005). After their discovery CDs were considered poisonous substances and its capacity for complexes formation was only considered a scientific curiosity. Later on, research on CDs proved that they are not only non-toxic but they can be helpful for protecting flavours, vitamins and natural colours. The preparation process of CDs consists of four principal phases: (i) culturing of the microorganism that produces the cyclodextrin glucosyl transferase enzyme (CGT-ase); (i) separation, concentration and purification of the enzyme from the fermentationmedium;(i)enzymaticalconversionofprehydrolyzed * Corresponding author. Tel.: þ34 988 387060; fax: þ34 988 387001.

E-mail address: jsimal@uvigo.es (J. Simal-Gandara).

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starch in mixture of cyclic and acyclic dextrins; and (iv) separation of CDs from the mixture, their purification and crystallization. CGT-ase enzymes degrade the starch and produce intramolecular reactions without the water participation. In the process, cyclic and acyclic dextrins are originated, which are oligosaccharides of intermediate size. The cyclic resulting products or CDs are formed by the link among units of glucopyranose which constitute the dextrins. The union is made through glycosidic oxygen bridges by a(1,4) bonds (Fig. 1).

In the last 30 years there was a progressive increase in the number of publications and patents related to the production of CDs (Kitahata, Okada, & Fukui, 1978; Matsuzawa, Kawano, Nakamura, & Horikoshi, 1975; US Federal Register, 1980; Yamamoto, Horikoshi, & Wako-shi, 1981) of which b-CDs (Horikoshi, 1979; Nakamura & Horikoshi, 1977), a-CDs (Flaschel, Landert, Spiesser, & Renken,1984; Kobayashi & Kainuma,1981; Kobayashi, Kainuma, & French, 1983; Yamamoto et al., 1981) and g-CDs (Bender, 1983)a re the most used at technological level. The purification of a- and g- CDs increases considerably the costof production, sothat 97% of the CDs used in the market are b-CDs. The three mayor CDs are crystalline, homogeneous, and non-hygroscopic substances, which are torus-like macro-rings built up from glucopyranose units. The a-CD comprises six glucopyranose units, b-CD comprises seven such units, and g-CD comprises eight such units (Fig. 2). The ring that constitutes the CDs, in reality, is a cylinder, or better said a conical cylinder, which is frequently characterized as a doughnut or wreath-shaped truncated cone. The cavity is lined by the hydrogen atoms and the glycosidic oxygen bridges. The nonbonding electron pairs of the glycosidic oxygen bridges are directed toward the inside of the cavity producing a high electron density there and lending to it some Lewis base characteristics (Saenger, 1983).

The C-2-OH group of one glucopyranose unit can form a hydrogen bond with the C-3-OH group of the adjacent glucopyranose unit (Bender & Komiyama, 1978). In the CD molecule, a complete secondary belt is formed by these H-bonds; therefore the b-CD has a rather rigid structure. This intramolecular hydrogen bond formation is probably the explanation for the observation that b-CD has the lowest water solubility of all CDs. The H-bond belt is incomplete in the a-CD molecule, because one glucopyranose unit is in a distorted position. Consequently, instead of the six possible H-bonds, only four can be established fully. The g- CD is a non-coplanar, more flexible structure; therefore, it is the most soluble of the three CDs.

On the side where the secondary hydroxyl groups are situated, the diameter of the cavity is larger than on the side with the primary hydroxyls, since free rotation of the latter reduces the effective diameter of the cavity. The approximate dimensions of CDs are shown schematically in Fig. 3.

Nuclear magnetic resonance (NMR), infrared (IR) and optical rotary dispersion (ORD) spectroscopy studies have demonstrated that D-glucopyranose units have the same conformation in both dimethyl sulfoxide (DMSO) and heavy water (D2O). The spectroscopic studies on CDs in aqueous solution suggest that the conformation of CDs in solution is almost identical to their conformation in the crystalline state (Bender & Komiyama, 1978). b-CDs have perfect symmetry, while a- and g-CDs rings are slightly distorted. The planar structure of a-CDs in solution deviates by about 100 pm from that in the crystal structure. The twist form observed experimentally in the glucose unit number five out of the plane of the five other glucose units disappears in solution (Koehler, Saenger, & van Gunsteren, 1988).

During the past decade, series of the larger CDs have been isolated and studied. For example, the nine-membered d-CD was isolated from the commercially available CD conversion mixture by chromatography. The rings of the higher CDs have to be highly flexible. As the CD cavity diameter increases, this non-polar cavity can accommodate an increasing number of water molecules, and in aqueous solution these ‘‘complexed’’ water molecules will differ energetically less and less from the bulk of the solvent. As a consequence, complex formation in such a system does not result in a significant gain in energy. The d-CD had greater aqueous solubility than the b-CD, but less than thatof a- and g-CD. It was the least stable among the CDs known at this time; their hydrolysis rate increases in the order of a-CD<b-CD<g-CD<d-CD. The larger CDs are not regular cylinder shaped structures. They are collapsed, and their real cavity is even smaller than in the g-CD. The driving force of the complex formation, the substitution of the high

Fig. 1. Glycosidic oxygen bridge a (1,4) between two molecules of glucopyranose.

Fig. 2. Chemical structures of a-CDs, b-CDs and g-CDs.

enthalpy water molecules in the CD cavity, is weaker in the case of larger CDs.

2. Use and regulatory of cyclodextrins

The use of CDs has increased annually around 20–30%, of which 80–90% was in food products. The widespread utilization of CDs is reflected in pharmaceutical, food, chemical and other industrial areas (Szejtli, 1997). In the pharmaceutical industry, CDs and their derivatives have been used in drugs either for complexation or as auxiliary additives such as solubilizers, diluents, or tablet ingredients to improve the physical and chemical properties, or to enhance the bioavailability of poorly soluble drugs (Fromming & Szejtli, 1994). In the chemical industry, CDs and their derivatives are used as catalysts to improve the selectivity of reactions, as well as for the separation and purification of industrial-scale products (Hedges, 1998). In the food, cosmetics, toiletry, and tobacco industries, CDs have been widely used either for stabilization of flavours and fragrances or for the elimination of undesired tastes, microbiological contaminations, and other undesired compounds (Martın del Valle, 2004; Singh, Sharma, & Banerjee, 2002).

However, the regulatory status of CDs in foods differs between countries. In the USA a-, b-, and g-CDs have obtained the GRAS list (FDA list of food additives that are ‘generally recognized as safe’) status and can be commercialized as such. In Japan a-, b-a nd g-CDs are recognized as natural products and their commercialization in the food sector is restricted only by considerations of purity. In Australia and New Zealand a-a nd g-CDs are classified as Novel Foods from 2004 and 2003, respectively (Cravotto, Binello, Baranelli, Carraro, & Trotta, 2006).

The recommendation of Joint FAO/WHO Expert Committee on

Food Additives (JECFA) for a maximum level of b-CDs in foods is 5 mg/kg per day. For a- and g-CDs no Acceptable Daily Intake (ADI) was defined because of their favourable toxicological profiles. Moreover, in July 2005 the U.S. Environmental Protection Agency (EPA) did away with the need to establish a maximum permissible level for residues of a-, b- and g-CDs in various food commodities (US Federal Register, 2005).

3. Solubility of cyclodextrins

The water solubility of CDs is unusual. b-CD is at least nine times less soluble (1.85 g/100 mL at room temperature) than the others CDs (14.5 g/100 mL and 23.2 g/100 mL for a- and g-CDs, respectively). The solubility of CDs depends strongly on the temperature as it is shown graphically in Fig. 4 and mathematically in equation (1) for a-CDs, equation (2) for b-CDs and equation (3) for g-CDs, where c is the concentration of CD in mg/mL and T is the temperature in K.

Relative to the solubilities of acyclic saccharides, the low solubilities of CDs are a consequence of the relatively unfavourable enthalpies of solution (more positive), partially offset by the more favourable entropies of solution (more negative).

The thermodynamic properties of a- and g-CDs are similar. The decreased solubility of b-CD in water appears to be due to the marked structure of water arising from water-b-CD interactions, causing a compensation of the favourable enthalpy by the unfavourable entropy of solution (Linert, Margl, & Renz, 1992).

4. Formation of inclusion complexes

CDs can be considered as empty capsules of a certain molecular size that can include a great variety of molecules in this cavity. In this case, a complex called ‘‘inclusion complex’’ is formed. Inclusion complexes are entities comprising two or more molecules. One of the molecule, the ‘‘host’’, includes, totally or partly, the ‘‘guest’’ molecules by physical forces. Therefore, CDs are considered typical host molecules (Brewster & Loftsson, 2007; Cabaleiro-Lago, Garcıa- Rıo, Herves, Mejuto, & Perez-Juste, 2006a; Dorrego et al., 2000; Garcıa-Rıo et al., 2005; Garcıa-Rıo et al., 2006; Szente, Mikuni,

Fig. 3. Approximate geometric dimensions of: a) a-CDs, b) b-CDs, c) g-CDs.

S/ mg g -1

Fig. 4. Solubility (mg CD/g H O) of (,) a-CDs, (C) b-CDs and (B) g-CDs in water as a function of temperature (K).

Hasimoto, & Szejtli, 1998). The type of bond established between the guest and the host is no covalent. Fig. 5 illustrates the approximate capsule volumes of each capsule (a-, b-, g-CDs).

Complex formation in solution is a dynamic equilibrium process which can be illustrated by the equation (4), where CD is the cyclodextrin, G is the guest molecule, and CD-G is the inclusion complex. The stability of the inclusion complex can be described in termsofrecombinationconstant(kR)oradissociationconstant(kD):

CDþG% k

The larger the guest molecule, the slower the formation and decomposition of the complex. Ionization decreases the rate of complex formation and decomposition. This recombination-dissociation equilibrium is one of the most important characteristics of this association.

4.1. Energy and inclusion mechanisms

The inclusion of a guest in a CD cavity consists basically of a substitution of the included water molecules by the less polar guest (Fig. 6). The process is energetically favoured by the interactions of the guest molecule with the solvated hydrophobic cavity of the host. In this process entropy and enthalpy changes have an important role.

In spite of the fact that the ‘‘driving force’’ of complexation is not yet completely understood, it seems that it is the result of various effects:

a. Substitution of the energetically unfavoured polar–apolar interactions (between the included water and the CD cavity on theonehand,andbetweenwaterandtheguestontheother)by the more favoured apolar–apolar interaction (between the guest and the cavity), and the polar–polar interaction (between bulk water and the released cavity-water molecules). b. CD-ring strain release on complexation. c. Van der Waals interactions and hydrogen bonds between host and guest.

CDs are hydrophobic molecules, since their solubility improves whenasmall amountofethanolisaddedtowater.Water molecules in the CD cavity cannot satiate their hydrogen bonding capacity as occurs with those in the bulk of the solvent. These water molecules have enhanced energy or enthalpy. The decrease in the energy of the system is caused by the reduction of the solvent-guest molecule and solvent-cavity interactions.

The energy of covalent chemical bonding is 400 kJ/mol. The energy of hydrogen bond is about 40 kJ/mol, and the Van der Waals forces represent only about 4 kJ/mol of bond energy. In the case of inclusion complexes, the species may achieve a stability which is proportional to covalent bonding due to the spatial arrangement produced.

Van der Waals forces, hydrophobic interactions and hydrogen bonds hold the CD and its guest together.

The energy of Van der Waals forces is proportional to molecular polarizability and molecular refraction. For structurally analogous compounds, a linear correlation exists between the molecular refractions (refraction index nD) and the dissociation constants of their CD complexes (Gelb et al., 1981).

The other dominating stabilizing force is the hydrophobic or solvophobic. Hydration of the CD complex is energetically favoured when compared with the separate hydration of the components. The role of the hydrogen bonding is not universal because stable

In 1 mol104 mL157 mL256 mL In 1 gram0.10 mL0.14 mL0.20 mL

Fig. 5. Approximate cavity volumes of a-, b- and g-CDs.

(Parte 1 de 5)

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