Vanillin - an overview

25 Jul.,2022

 

synthetic vanillin

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Aromatic Compounds

Vanillin and benzaldehyde are leading aromatic aldehydes in the flavor and fragrance industry. A number of precursors were suggested and studied in pursuit of an economically viable process for these two flavor compounds.

Vanillin is chemically a 4-hydroxy-3-methoxybenzaldehyde or vanillic aldehyde. It is the most common flavor chemical used in a broad range of flavors and fragrances. Market offers synthetic vanillin, natural vanillin, and vanilla extract. Vanilla extract contains about 1–2% of vanillin along some 60–100 other flavor chemicals. In contrast to vanilla extract, vanillin is a single chemical compound which can be synthesized either chemically or biologically. Synthetic vanillin is most widely used due to its low cost. Vanillin from natural sources has become more cost-effective, which has lead to higher demand and more interest in improving yields and understanding the biochemical pathways leading to it. Several biochemical pathways utilizing different precursors were suggested (Figure 6).

Figure 6. Different routes to vanillin.

Phenylpropanoid pathway was suggested as a biosynthetic route to form vanillin starting from phenylalanine. Biotransformation processes using whole microorganisms and precursors such as ferulic acid, vanillic acid, vanillyl alcohol, curcumin, guaiacol, benzoin siam resin, lignin, or eugenol were proposed in a number of studies. Typically, the concentration of these precursors is below 0.1% due to their solubility and toxicity. Microorganisms such as Pseudomonas putida, A. niger, Corynebacterium glutamicum, Streptomyces gloesporium, Rhodotorula glutinis, Serratia, Klebsiella, Pycnoporus, Enterobacter, Amycolatopsis, or Streptomyces setonii are capable of transforming ferulic acid (3-methoxy-4-hydroxycinnamic acid) to vanillin. Pseudomonas species are so far the only microorganism reported to tolerate eugenol up to the concentration of 0.05%. Some strains of Pseudomonas transform eugenol into vanillin and vanillic acid. In some instances, microorganisms can transform one precursor into an array of flavor compounds. This leads to generation of a partial bouquet from one precursor similarly as in plant cells. Cloning and metabolic engineering will improve efficiency of microorganisms and will provide alternative natural sources of vanillin-type flavors, which can replace the synthetic counterpart. Enzymes involved in the vanillin pathway such as feruloyl synthase, feruloyl-CoA ligase, 4-hydroxycinnamoyl-CoA hydratase/lyase, feruloyl CoA thioetherase, and feruloyl esterase have been identified and their role in vanillin formation from ferulic acid was demonstrated in different microorganisms. For example, metabolic engineering of Escherichia coli led to vanillin accumulation in the amount of 1.1   g l−1 by expression of the fcs (feruloyl-CoA-synthetase) and ech (enoyl-CoA hydratase/aldolase) genes from Amycolatopsis sp. This yield was achieved with growth on 0.2% ferulate for 48   h without an additional carbon source.

Benzaldehyde is also known chemically as a benzene carbaldehyde, or phenylmethanal, or benzoic aldehyde. Natural benzaldehyde is typically derived from a cyanogenic glycoside amygdalin found in fruit kernels. However, the toxicity of hydrocyanic acid as by-product presents a safety problem. Therefore, microbial processes have been identified as safe alternative routes to this very popular cherry, almond, and fruity flavor.

Microbial biotransformation of phenylalanine to benzaldehyde offers a natural alternative to the extraction process. Bacterium P. putida, white rot fungus Phanerochaete chrysosporium, and basidiomycetes Ischnoderma benzoinum and Polyporus tuberaster transform phenylalanine through phenylpyruvate and phenylacetaldehyde to phenylacetate, which is then converted into mandelate and benzoylformate. Direct accumulation of benzaldehyde in the culture broth is toxic to the producing organisms. This can be avoided by two-stage process when the second step involves separate decarboxylation by benzoylformate decarboxylase. Additional metabolite, 3-phenylpropanol, a rose-like, flowery flavor, is detected next to benzaldehyde in the broth of I. benzoinum. Also, lactic acid bacteria, in particular cell extract of L. plantarum, forms benzaldehyde from phenylalanine. In this microorganism, aminotransferase plays the initial role in the transformation of phenylalanine into phenylpyruvic acid. The keto acid is subsequently subjected to a chemical reaction, leading to benzaldehyde. The chemical conversion of phenylpyruvic acid was demonstrated under various mild conditions. This reaction most likely takes place in several cheeses where benzaldehyde is part of final bouquet.

White rot fungus Bjerkandera adusta and bacterium C. glutamicum are examples of microorganisms having very versatile enzymatic tool boxes, which are the key for formation of varies aryl metabolites from phenylalanine, where benzaldehyde is one of the intermediates. Other flavor metabolites such as acetophenone, phenyl ethanol, phenyl acetic acid, benzylalcohol, anisaldehyde, anisylalcohol, veratraldehyde, and veratrylalcohol are also formed. Methylotrophic yeasts P. pastoris and bacterium Methylosinus trichosporium oxidize benzylalcohol to benzaldehyde in the presence of methanol. This oxidation of benzylalcohol to its aldehyde requires a strong nonspecific methanol dehydrogenase. On the other hand, benzaldehyde can be reduced by yeast S. cerevisiae to benzyl alcohol, which has a delicate flavor of jasmine and can be used as fragrance. Phenyl ethyl alcohol known as 2-phenylethanol, β-phenyl ethanol, or benzyl carbinol is associated with rose-like odor and is typically present in flower petals and in common beverages like beer. Phenyl ethyl alcohol can be produced through bioconversion of phenylalanine using yeasts S. cerevisiae or Kluyveromyces marxianus. Yields are typically low and need efficient in situ recovery methods. Phenyl ethyl alcohol oxidation leads to phenyl acetaldehyde, which is a typical hyacinth odor.

Phenylacetic acid is another aromatic compound with a valuable honey-like flavor. Among others, Clostridium species are capable of transforming phenylalanine into phenylacetic acid under anaerobic conditions.

Methylanthranilate, the characterizing component of concord grape flavor, can be derived through tryptophane metabolism Aspergillus nidulans or by demethylation of methyl N-methyl anthranilate by Bacillus megaterium.