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2.3.3 Agro‐wastes as a Source for Biovanillin Production
Agro‐wastes are considered useless and include plant stalks, leaves, manures, and any other vegetable matter that generally produced through farming activities. Accumulation of such huge agro‐wastes poses serious threat to the environment and human health. These wastes are rich in essential organic components like carbon, nitrogen, and many micronutrients, which can be used as ideal substrates for the microbial fermentation and thus agro‐residues are successfully recycled for the production of several metabolites. Agro‐wastes are vital renewable natural resources, and they are cheap and readily available. Employing agro‐wastes rich in ferulic acid esters as the raw material for cleaner production of vanillin have been a well‐supported way to dispose and valorize the waste. Lignin, a complex phenolic polymer rich in biomass, is obtained by sulfite pulping of wood, which was used as a substrate for the production of vanillin (da Silva et al., 2009). However, when using waste from this industry, this method is not recognized as a “green technology,” since vanillin is produced together with additional wastes, which needs to be taken into account as unsustainable and not as environmentally friendly. As a result, there is an instant opportunity for the production of vanillin from other agricultural wastes. Potential agro‐wastes for the transformation of biovanillin are highly dependent on cereal bran, sugar beet pulp, and rice bran oil (Pandey et al., 2000; Das and Singh, 2004). Ferulic acid is found abundance in rice endosperm cell wall, sugar beet pulp, bamboo shoots, Oat flakes, red beet, soybean, etc. (Table 2.2). Cereal bran is a plentiful by‐product of the agricultural process, which contain significant amount of ferulic acid in the form of dimerized or esterified with polysaccharides and proteins (Boz, 2015) and has been delineated as a possible supply of ferulic acid for producing vanillin. Di Gioia et al. (2007) used the hydrolysate of wheat bran for the first time for producing vanillin (0.375 mM/l) through bioconversion of ferulic acid from an agro‐waste with a recombinant E. coli JM109 (pBB1). Pediococcus acidilactici has produced 1.06 g/l of vanillin through bioconversion of the substrate rice bran (Chakraborty et al., 2016). Chattopadhyay et al. (2018) has shown 708 mg/l of vanillin production from wheat bran as a substrate using Streptomyces sannanensis. In a two‐step bioconversion, A. niger CGMCC0774 and P. cinnabarinus CGMCC1115 are shown to produce vanillin, where 4 g/l of ferulic acid is converted to 2.2 g/l of vanillic acid by A. niger, which is further converted to 2.8 g/l of vanillin by P. cinnabarinus (Zheng et al., 2007). Similarly, oil palm is used as a source of ferulic acid in a two‐step bioconversion with A. niger and P. cinnabarinus, which has produced 237 mg/l of vanillin (Lesage‐Meessen et al., 1996). Sugar beet pulp has also been utilized as main precursor for biovanillin production in two‐step bioconversion, where A. niger has transformed 834 mg/l of ferulic acid to 357 mg/l vanillic acid within six days, further vanillic acid is converted to 105 mg/l vanillin using P. cinnabarinus (Lesage‐Meessen et al., 1999). On the other hand, P. chrysosporium ATCC 24725 has produced 0.5 g/l of vanillin from 0.750 g/l of ferulic acid of lemongrass when used as a substrate (Galadima et al., 2020). Similarly, from the pineapple crown leaves and peel, 34 and 109 μmol of ferulic acid is converted to 2.5 and 5 mg/l of vanillin, respectively (Tang and Hassan, 2020).
Table 2.2 Amount of ferulic acid in different known natural sources.
| Source | Ferulic acid (mg/0.1 kg) |
|---|---|
| Refined maize bran | 2610–3300 |
| Soft and hard wheat bran | 1351–1456 |
| Rice endosperm cell wall | 900 |
| Sugar‐beet pulp | 800 |
| Wheat bran | 660 |
| Bamboo shoots | 243.5 |
| Rye bran | 280 |
| Maize, dehulled kernels | 174 |
| Barley grain | 140 |
| Whole‐wheat kernels | 64–127 |
| Whole‐wheat flour | 89 |
| Whole‐grain rye flour | 86 |
| Whole brown rice | 42 |
| Maize flour | 38 |
| Whole oats | 25–35 |
| Oat bran | 33 |
| Red beet | 25 |
| Soybean | 12 |
| Grapefruit | 10.7–11.6 |
| Orange | 9.2–9.9 |
| Peanut | 8.7 |
| Water dropwort | 7.3–34 |
| Eggplant | 7.3–35 |
| Spinach | 7.4 |
| Banana | 5.4 |
| Tomato | 0.29–6 |
| Radish | 4.6 |
| Broccoli | 4.1 |
| Carrot | 1.2–2.8 |
| Avocado | 1.1 |
| Berries | 0.25–2.7 |
2.4 Strain Development for Improved Production of Vanillin
2.4.1 Metabolic and Genetic Engineering
The characteristics property like rapid growth rates and less complexions to molecular genetics, microorganisms are innovative targets for biotechnology research and due to its ability to grow on versatile carbon sources, microorganisms are preferred so far as good candidates for the production of vanillin. Microbes such as bacteria, fungi, and yeast have been used for the small as well as large‐scale production of vanillin from various substrates, such as eugenol, isoeugenol, ferulic acid, and other organic compounds. A drawback of microbial production of vanillin is the excess oxidation and reduction of end product to vanillic acid and vanillyl alcohol, respectively