The continuing trend towards sustainability, naturalness and healthy nutrition is making plant-based food ingredients with biofunctional and technofunctional properties increasingly important. Polyphenols, synthesized by plants as secondary metabolites, possess the molecular characteristics to fulfill many of these requirements. They can also react to form interesting products.
Plant phenols – a diverse group of substances
Fig. 1 Basic structures of selected phenolic compounds. Top row: 1. Hydroxybenzoic acids (C6-C1); 2. Hydroxycinnamic acids (C6-C3); 3. Xanthones (C6-C1-C6); 4. Stilbenes (C6-C2-C6); 5. Flavonoids (C6-C3-C6). The flavonoids, in turn, can be subdivided into: 6. Flavonols, 7. Flavones, 8. Flavanones, 9. Flavan-3-ols, 10. Isoflavones and 11. Anthocyanidins. In plants, phenols are often glycosylated.
Phenolic compounds, also known as polyphenols, are secondary metabolites found ubiquitously in higher plants. Chemically they are an extraordinarily diverse group of substances with numerous subclasses. Figure 1 shows the basic structures of some phenolic compounds. As secondary metabolites, phenolics perform a number of important functions, for example defending the plant against biotic and abiotic stress, but they also play a role in plant-to-plant communication. Therefore, it is hardly surprising that numerous phenols possess antioxidant and antimicrobial properties . This is why they have long been studied as natural alternatives to synthetic food additives. The water-soluble anthocyanins, which belong to the flavonoids, absorb light in the visible range, resulting in colorfulness from orange-red to blue-violet. Anthocyanin-containing extracts are used as natural food colorants.
Fig. 2 Examples of reactions of quinoid systems. As electron-deficient compounds, quinones can react with nucleophiles such as amines, thiols and sulfite. If the resulting adducts are re-oxidized, cross-links of proteins or high molecular weight pigments may result (top). Schiff bases formed from the reaction of quinones with amino acids decarboxylate to a substituted aminophenol, which after hydrolysis releases the Strecker aldehyde (bottom).
While phenolic substances are not very reactive in an unharmed plant, enzymatically induced conversions resulting in reactive intermediates with a quinoid structure can occur after injury to plant tissue, for example when fruits and vegetables are processed. These quinones are electron-deficient compounds that, as Michael acceptors, can react with nucleophiles, especially the thiol and amino groups of proteins. C-C linkages have also been observed. Such reactions lead to the formation of compounds with completely different properties but whose chemical structures are often still poorly understood. A well-known phenomenon is the enzymatic browning that occurs, for example, after cutting up an apple or avocado when phenolic compounds are oxidized by the enzyme polyphenol oxidase in the presence of oxygen . In addition, quinones can react with other substances in a variety of ways. For example, they are reduced when reacting with ascorbic acid. Sulfite can also lead to the regeneration of phenols and, furthermore, form an adduct on the aromatic ring. If the amino and thiol groups in proteins react with quinones in a Michael addition, cross-links are formed upon renewed oxidation . After the formation of a Schiff base from a quinone and an amino acid, the resulting aza-vinylogous β-keto acid can lead to the formation of volatile aldehydes in the course of a Strecker reaction (Fig. 2).
Anthocyanins and pyranoanthocyanins
Fig. 3 Reactions of anthocyanins. The anthocyanins, present in plants usually as glycosides, can be deglycosylated and – if there is an o-dihydroxy structure in the B ring – oxidized. While a reversible addition to C4 of the C ring occurs with sulfite, the analogous reaction with thiols is not observed. Suitable reaction partners, for example p-vinylguajacol, allow stable pyranoanthocyanins to be formed.
A variety of reactions have also been described for anthocyanins, whose structure is strongly pH-dependent. The red flavylium cation exists only in the strongly acidic pH range, with decolorization observed around pH 4.5. At higher pH values that do not typically occur in foods, blue-violet, red and yellow hues are observed as a result of the formation of quinoid bases or chalcones. Deglycosylation of the anthocyanins leads to the unstable anthocyanidins, from which a phenolic aldehyde and a phenolic acid are produced by oxidative C-ring fission. If sulfurous acid is added to anthocyanin-containing matrices, a loss of color is observed, caused by the addition of sulfite to the C4 of the flavylium cation. This adduct is more water soluble than the anthocyanin, which is why the reaction is used technologically to obtain anthocyanins from red grape residues after pressing. The adduct formation is reversible – the color of the anthocyanins can be restored by heating with acid. A corresponding addition of thiols to the flavylium ion has not yet been demonstrated (Fig. 3).
In the presence of suitable reaction partners, such as hydroxycinnamic acids, pyruvic acid, vinylphenols and acetaldehyde, anthocyanins form new derivatives, which are characterized by an additional pyran ring. They are therefore called pyranoanthocyanins. These compounds can be found in red wines, because the anthocyanins’ other reactants are either present there in a native state or formed in the course of alcoholic fermentation . Compared with anthocyanins, most pyranoanthocyanins are more stable and appear with stronger orange tones. Therefore, they are promising food colorants, but their safety is yet to be proved in toxicological studies .
Benzacridines – novel green food colorants?
Fig. 4 Structure of benzacridines after reaction of chlorogenic acid with the α-amino group of an amino acid (A) or with the ε-amino group of lysine (B). Whipping egg white in the presence of chlorogenic acid results in a green foam (C).
Although mixtures of synthetic yellow and blue dyes can achieve green hues in foods, the consumer trend towards additives of natural origin calls for alternative solutions from food manufacturers. Virtually the only natural green colorant worth considering is chlorophyll. However, in its isolated form it is not very stable. Due to the lack of natural blue dyes that can be used in food technology, green colors can hardly be obtained by mixing blue and yellow substances. In the search for suitable alternatives to chlorophyll, studies performed in Japan more than two decades ago are worth noting. These demonstrated that the oxidation of chlorogenic acid esters leads to the formation of green dyes in the presence of amine components. Further mechanistic studies suggested that this green coloration is caused by substituted benzacridines, which are formed after dimerization of caffeic acid esters and subsequent cyclization with insertion of the amine’s nitrogen [6-7].
Fig. 5 The drainage water of the foam is also intensely green in color.
If the a-amino group of a free amino acid, for example lysine, is included in the reaction, only ammonia remains in the benzacridine; if the ε-NH2 group reacts, the complete amino acid is integrated into the benzacridine structure .
These reactions are also the reason why the coffee on top of moss cake turns green. To prepare the cake, beaten egg white is spread on a cake base. Coffee powder is then sifted on top. After storage in the refrigerator overnight, the chlorogenic acid contained in the coffee is oxidized to chlorogenoquinone by reacting with amino groups of the egg white protein’s amino acids to form a green coating, a process that is facilitated by the slightly alkaline environment of the egg white. By means of a simple experiment, the reaction can be demonstrated by whipping egg white in the presence of chlorogenic acid, which creates a green colored egg foam (Fig. 4). The drainage water of the foam is also intensely green in color (Fig. 5). Application studies on various foods suggest the benzacridines are highly stable . However, before benzacridine pigments can be introduced as food colorants, numerous analytical, technological, and regulatory issues remain to be addressed .
While oxidized chlorogenic acid forms green colored benzacridines when reacting with most amino acids, the reaction with tryptophan leads to a red color . Initial studies suggest cyanine-like molecules , but our own studies support the formation of additional structures (unpublished). Such reactions thus hold immense potential to produce colorants for food as well as for other product groups such as cosmetics and pharmaceuticals.
Phenolic compounds can react in a variety of ways, which opens up opportunities to find novel molecules with interesting technological properties. Dyes show the greatest promise, but multifunctional substances are also conceivable if suitable reactants are selected. Efficient preparation methods must be established, structures elucidated and the behavior in different matrices investigated. However, toxicological studies on the safety of these compounds are also essential.
Category: Food Chemistry | Polyphenols
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Header image: Green foam: A. Schieber; iStock.com | Preto_perola, kitsana pankhuanoi
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