Encapsulation of Flavor Compounds as Helical Inclusion Complexes

Chapter 14

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Encapsulation of Flavor Compounds as Helical Inclusion Complexes of Starch K. Kasemwong1 and T. Itthisoponkul*,2 1Nano

Delivery System Laboratory, National Nanotechnology Center, National Science and Technology Development Agency, Klong-Luang, Pathumthani 12120, Thailand 2Faculty of Agricultural Product Innovation and Technology, Srinakharinwirot University, Bangkok 10110, Thailand *E-mail: [email protected].

Inclusion complexes between starch and flavor compounds are of great interest in food science as they influence the retention and release of flavor in food systems. Starch is a mixture of the glucose polymers, amylose and amylopectin. Amylose, the linear chain is mainly responsible for the complex formation, while amylopectin, the highly branched component of starch, can also form complexes with certain types of guest molecules. In the presence of flavor compounds, amylose changes from a double helix to a single helix, forming a helical structure that has a hydrophobic cavity and a hydrophilic exterior enabling it to form inclusion complexes. The flavor molecules are included within the cavity, in between the helices, or in both locations, depending on the structure of the molecules. It has been suggested that amylose inclusion complexes can be used in the food industry to prevent the loss of volatile or labile flavoring materials during processing and storage because the complexes are markedly resistant to high temperature and oxidation. Furthermore, the release of the complexes can be controlled by α-amylase enzyme hydrolysis and changes in the moisture content and temperature.

© 2013 American Chemical Society In Advances in Applied Nanotechnology for Agriculture; Park, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Introduction Encapsulation is widely used in the food industry for providing the stability and controlled release of flavor ingredients. The advantages of flavor encapsulation include protecting compounds against loss through evaporation, chemical degradation, or reaction with other components during processing and storage. The controlled release of flavor during the consumption of foods is also achieved by using this technology. It is well known that cyclodextrins, the cyclic oligosaccharides, are playing the important role of being a host to stabilize a number of flavor compounds. Cyclodextrins have a hydrophobic cavity and a hydrophilic exterior which enables them to accommodate a range of guest molecule shapes and sizes (1). Starch can be used as an alternative host for molecular encapsulation because it offers a unique advantage in this regard and the material cost would be less than for cyclodextrin. Starch, particularly amylose, can also form a helical structure with a hydrophobic core that can accommodate hydrophobic flavor molecules. The starch-flavor inclusion complexes are of interest in connection with flavor retention and release in foods as well as the texture of foods (2–4). The linear amylose fraction of starch has the ability to form molecular inclusion complexes, termed Vh-amylose, with small molecules. The molecular dimensions of the inclusion complexes are varying with external diameter of 1.35-1.62 nm and inner diameter of 5.4-8.5 Å (5, 6), and thus, Vh-amylose is considered as a possible platform for encapsulation.

Figure 1. Left-handed single helix of Vh-amylose. (a) side view, (b) front view. Amylose is a linear polymer consisting of (1→4)-α-D-glucose units with very minimal α-(1→6) linked branching (7). Native amylose usually occurs as a double helical assembly. However, a simpler single helix is favored during the inclusion complexation with guest molecules. In such complexes, the hydrophobic parts of the ligand are entrapped in the central helical cavity of amylose. This type of complex, a so-called Vh arrangement (8, 9), normally features six glucose residues 236 In Advances in Applied Nanotechnology for Agriculture; Park, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


per turn in a left-handed single helix (Figure 1a). Vh-amylose has an overall helix diameter of approximately 13.5 Å, a channel of 5.4 Å width, and an axial pitch of 8.1 Å per turn (9) (Figure 1b). It is most often observed with small guest molecules or those of linear shape such as iodine, monoglycerides, fatty acids and long-chain alcohols (9, 10). Most flavor compounds are small molecules with short carbon chains and generally present a ring structure. It seems that lipids and most flavor compounds have a common property in term of the hydrophobic attribute and flavor compounds generally have poor water dispersibility. For this reason, it is possible to encapsulate flavor compounds by forming the inclusion complex with amylose. Formation of Starch Inclusion Complexes The important factors in forming starch inclusion complexes include starch or amylose, guest compounds and condition during preparation. The amylose form double helices in water through intramolecular hydrogen bonding. However, in the presence of suitable guest molecules, amylose undergoes conformational rearrangement to form a single helix. The other main component of starch, amylopectin, also has the ability to form inclusion complex but only amongst its longer, external branches (2). Inclusion complexes have been shown to be stabilized by a combination of hydrogen bonds, hydrophobic, dipolar and charge interactions (11). Typically, compounds that can form complexes with amylose posses both polar and nonpolar parts in their molecular structure (12). Complex formation of pure amylose in aqueous system requires temperatures above 100 °C to avoid the spontaneous crystallization of amylose in the double helices form which impedes the formation of complexes due to the absence of a central channel (2). To obtain amylose from native starch it is first necessary to break up the starch granular structure by heating it in excess water. The method of preparing starch-flavor inclusion complexes can be explained as follows (3). First, the native starch or lipid free starch need to be suspended with water to make a proper concentration. The starch suspension is then placed in a seal containers and heating to 85°C with gently stirred. Second, just before cooking, the excess amount of flavor compound is added to starch paste. However, a concentration in the final products of 0.4 mmol of flavor compound per kg of starch paste is suggested. The starch-flavor mixtures are held at about 85°C for 15 min, then cool down to room temperature and held at room temperature over night to allow precipitation of complexes. The starch inclusion complexes are collected by centrifugation and rinse with ethanol to eliminate free flavor compounds. The formation of complexes can be indicated by the turbidity of solution and the formation of a precipitate during the cooling of starch mixtures (13). In the solid form, the single helix structure and its crystalline packing is revealed by wide-angle X-ray diffraction (9, 10, 14). Electron microscopy and electron diffraction has also be used to investigate the inclusion complexes of amylose (15). The quantitative determination of inclusion complexes can be carried out by measuring the iodine binding capacity (12). Moreover, calorimetric methods are appropriate to follow the formation, melting and characterization of complexes (16). 237 In Advances in Applied Nanotechnology for Agriculture; Park, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Molecular Structure of Amylose Inclusion Complexes The nature of the guest molecules has a considerable influence on the helical inclusion complexes of amylose. Many different types of Vh-amylose complex are known, featuring a wide variety of guest molecules as shown in Table I.

Table I. Characteristic of amylose-flavor inclusion complexes


Type of guest compounds Decanal Hexanal

Type of crystal Vh amylose or V6I

Characteristic of complex

Ref.

Sixfold left-handed amylose helices in which the guest is included in the cavity.

(9)

Lactones

(17) (18)

Butanol

V6II

Six monomers of D-glycosyl per turn.

(9)

Isopropanol

V6III

V6II , V6III : ligand could also be entrapped between helices

(19)

Menthone

II, III represent varying volume between helices in the crystalline stacking which are larger than Vh

Fenchone Geraniol

(15) (15, 20) (15)

Thymol

(15)

Linalool

(3)

α–Naphthol

V8

Amylose complex with largest helix diameter. The ligand is included in the helix and between the helices.

(9, 15)

Amylose complexes can be classified by the size of helix, which corresponds to the number of glucose monomers per helical turn, or by the packing of Vh-amylose in the crystalline structure. Linear flavor compounds such as decanal, hexanal and lactones induce the formation of amylose complexes with six glucose units per turn, and yield a Vh amylose or V6I crystalline form (9, 17, 18). Crystalline amylose complexes features a larger size of cavity than Vh amylose have been observed with compounds such as menthone, fenchone, geraniol, thymol and linalool (3, 15, 20). It has been suggested that the ligands of V6III complexes are entrapped between the helices in the crystal. For bulky ligands such as α–naphthol, a larger helical diameter involving eight glucose units per helical turn (V8-amylose) has been reported (9, 15). Flavor compounds are postulated to be accommodated within amylose helices (Vh amylose or V6I) or may be entrapped outside the crystalline amylose helices (V6II, V6III), depending on the size of the ligands (Figure 2). In addition, as the exposed surface of the helices is more hydrophilic than the interior, the hydrophobic flavor compounds preferred to stay inside the helices while the hydrophilic flavor compounds preferentially lie entrapped between the helices (21). 238 In Advances in Applied Nanotechnology for Agriculture; Park, B., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Figure 2. Schematic showing modes of complexation between amylose and ligand flavor compounds.

Inclusion Complexes between Starch and Flavor Different starches exhibit different tendencies to form complexes, largely on account of their differing amylose content. As expected, amylose-rich starch tends to bind greater amount of compounds (4, 6). Furthermore, use of commercial native starch appeals because of its low cost compared to that of pure amylose. Native starch that contains little or no internal lipid is preferred because internal lipids can compete with flavor compounds and interfere with amylose complex formation (22). In the system that contains native lipid and low solubility flavor compounds, the structure may consist of starch-lipids and starch-lipid-flavor compounds. While in the presence of high solubility flavor compounds, the system may consist of starch-flavor compounds, starch-lipids and starch-lipid-flavor compounds (21). The mixed helical structure of complexes would result in compromised stability and an altered release profile of flavor compounds. For this reason, potato starch (internal lipid (

Encapsulation of Flavor Compounds as Helical Inclusion Complexes

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