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Microbial transformation of phenolic compounds into vanillin is the most attractive alternate for natural vanillin. Many microbes are capable of producing vanillin and vanillic acid by utilizing phenolic substrates such as ferulic acid, eugenol, and isoeugenol (Figure 2.2). Among them, ferulic acid has been widely used as a sole carbon source for vanillin production. Pseudomonas fluorescens is one of the most predominantly used organisms for such biotransformation. Under optimized environmental conditions, P. fluorescence BF13 is capable converting 95% of ferulic acid to vanillic acid in five hours (Barghini et al., 1998). Likewise, a mutant of P. fluorescens FE2 is shown to produce 95% of vanillic acid from ferulic acid in 24 hours (Andreoni et al., 1995). Next to Pseudomonas sp., Lactobacillus sp. is considered to be effective producer of vanillin as it has phenolic transformation properties. A group of Lactobacillus strains are found to produce significant amount of vanillin from ferulic acid supplemented rice bran medium (Kaur et al., 2013). Streptomyces setonii and Amycolatopsis sp. have been reported to produce notable amount of vanillin under optimized pH (Rabenhorst and Hopp, 2000; Gunnarsson and Palmqvist, 2006). Likewise, Delftia acidivorans also has been reported as potent bacteria to transform ferulic acid to vanillin efficiently (Plaggenborg et al., 2001). The second most commonly used carbon source in biotransformation of vanillin is eugenol and isoeugenol, which are essential oil components extracted from clove. The degradation pathway of eugenol in certain bacteria like Corynebacterium sp. (Tadasa, 1977), Arthrobacter globiformis (Rabenhorst, 1996), Pseudomonas sp. (Tadasas and Kayahara, 1983) has resulted in producing vanillin, vanillic acid, and other phenolic compounds. Arthrobacter sp. TA13, Pseudomonas putida JYR‐1, and Bacillus subtilis B2 are reported to produce vanillin from isoeugenol (Shimoni et al., 2000, 2003; Ryu et al., 2005). Bacillus fusiformis has been reported to produce 8 g/l of vanillin from 50 g/l of isoeugenol with the addition of HD‐8 resin to prevent inhibition by product (Zhao et al., 2006). Further, Bacillus pumilus S‐1 is shown to transform isoeugenol to vanillin with higher efficiency with the yield of 3.75 g/l of vanillin from 10 g/l of substrate in 150 hours (Hua et al., 2007). Bacillus licheniformis SHL‐1 has been reported to produce 494 mg/l of vanillin from 1 g/l of ferulic acid within 45 hours (Ashengroph et al., 2012). Other than bacteria, Aspergillus niger is also most frequently used to transform ferulic acid to vanillic acid, which is further converted to vanillin by Pycnoporus cinnabarinus or Phanerochaete chrysosporium. The yield of vanillin by this two‐step process is reported to be 1.1 g/l after 54 hours of fermentation (Stentelaire et al., 2000). Sporotrichum thermophile, a thermophilic fungus has also been reported to produce 4798 mg/l within 20 hours of fermentation using ferulic acid as a substrate (Topakas et al., 2003). On the other hand, vanillin production by fungal bioconversion has many disadvantages, such as lysis of mycelium, extremely viscous broths, uncontrolled fragmentation, and unfavorable pellet formation, which hinder conventional production processes and increase the price of the downstream process. Apart from these, a list of microbes involved in biotransformation of vanillin is listed in Table 2.1. Besides having good yield, biotransformation process has several disadvantages like formation of undesired by‐products, expensive downstream processing, unproductive metabolic flow, cytotoxicity of precursors used (Gallage and Møller, 2015), and the difficulty in process optimization of the microbes used. In order to overcome these problems and to meet the market demand, biotechnological approaches including metabolic and genetic engineering are being employed and these methods have gained much importance nowadays because of higher yield of vanillin.

Figure 2.2 Biotransformation pathways of vanillin in microbes using various substrates.

Table 2.1 List of microorganisms capable of producing vanillin from various substrates.

Organism	Substrate	References
Gram‐positive bacteria
Amycolatopsis sp. HR167	Eugenol	Overhage et al. (2006)
Rhodococcus opacus PD630	Eugenol	Plaggenborg et al. (2006)
Bacillus subtilis B7‐S	Ferulic acid	Bomgardner (2016)
Bacillus licheniformis SHL1	Ferulic acid	Ashengroph et al. (2011)
Bacillus aryabhattai BA03	Ferulic acid	Paz et al. (2016)
Streptomyces halstedii GE107678	Ferulic acid	Brunati et al. (2004)
Streptomyces setonii ATCC 39116	Ferulic acid	Achterholt et al. (2000)
Lactic acid bacteria	Ferulic acid	Bloem et al. (2007)
Pediococcus acidilactici	Ferulic acid	Kaur et al. (2013)
Amycolatopsissp.sp. HR167	Ferulic acid	Overhage et al. (2006)
Streptomyces sp. V‐1	Ferulic acid	Huaet al. (2007)
Amycolatopsis sp. ATCC 39116	Ferulic acid	Fleige et al. (2013)
Bacillus subtilis MTCC 1427	Ferulic acid, Eugenol and Isoeugenol	Rana et al. (2013)
Bacillus subtilis	Isoeugenol	Shimoni et al. (2000)
Arthrobacter sp. TA13	Isoeugenol	Shimoni et al. (2003)
Bacillus fusiformis SW‐B9	Isoeugenol	Zhao et al. (2005)
Bacillus subtilis HS8	Isoeugenol	Zhang et al. (2006)
Bacillus fusiformis CGMCC134	Isoeugenol	Zhao et al. (2005)
Bacillus pumilus S‐1	Isoeugenol	Hua et al. (2007)
Bacillus coagulans BK07	Ferulic acid	Karmakar et al. (2000)
Psychrobacter sp. CSW4	Isoeugenol	Ashengroph et al. (2012)
Gram‐negative bacteria
Pseudomonas putida KT2440	Ferulic acid	Plaggenborg et al. (2003)
Pseudomonas sp.	Ferulic acid	Agrawal et al. (2003)
Enterobacter sp. Px6‐4	Ferulic acid	Li et al. (2008)
Pseudomonas putida I58	Isoeugenol	Furukawa et al. (2003)
Pseudomonas chlororaphis CDAE5	Isoeugenol	Kasana et al. (2007)
Pseudomonas putida IE27	Isoeugenol	Yamada et al. (2007)
Pseudomonas sp. KOB10	Isoeugenol	Ashengroph et al. (2010)
Pseudomonas aeruginosa ISPC2	Isoeugenol	Ashengroph et al. (2011)
Pseudomonas nitroreducens	Isoeugenol	Unno et al. (2007)
Pycnoporus cinnabarinus MUCL39533	Ferulic acid	Alvarado et al. (2001)
Pseudomonas fluorescens AN103	Ferulic acid	Del Carmen Martínez‐Cuesta et al. (2005)
Escherichia coli JM109 (pBB1)	Ferulic acid	Barghini et al. (2007)
Pseudomonas fluorescens	Ferulic acid	Dal Bello (2013)
Pseudomonas sp. HR199	Eugenol	Overhage et al. (2000)
Pseudomonas resinovorans SPR1	Eugenol	Ashengroph et al. (2011)
Fungal strains
Sporotrichum thermophile LWT	Ferulic acid	Topakas et al. (2003)
Aspergillus niger I‐1472	Ferulic acid	Tang et al. (2020)
Aspergillus niger CGMCC0774, Pycnoporus cinnabarinus CGMCC1115	Ferulic acid	Zheng et al. (2007)
Pycnoporus cinnabarinus	Ferulic acid	Tilay et al. (2010)
Aspergillus niger K8 and P. crysosporium ATCC 24725	Ferulic acid	Motedayen et al. (2013)
Streptomyces sp. V‐1	Ferulic acid	Yang et al. (2013)
Aspergillus niger I‐1472	Isoeugenol	Tan et al. (2015)
Colletotrichum acutatum and Lasiodioplodia theobromae	Ferulic acid	Numpaque et al. (2016)
Aspergillus niger	Eugenol	Srivastava et al. (2010)
Phanerochaete chrysosporium NCIM 1197	Ferulic acid	Karode et al. (2013)