Making Sense of Waste: Fermentative Production of Flavour and Aroma Compounds from Agroindustrial By-products

Abstract

The global flavour and fragrance industry is expected to increase in value from $27 billion US to $37 billion US by 2021. This is largely due to consumer preferences for sustainable and ethical natural flavours, and fragrances. This study investigated a biotechnological approach to identify novel and alternative sources of natural flavours and fragrances. This thesis explored the potential fermentation to produce flavour and fragrance compounds from an ethical and abundant substrate – agro-industrial by-products. Nine agroindustrial by-products were investigated and fermented with five food-grade microorganisms. Hundreds of potentially valuable, volatile compounds were produced and identified through a preliminary screen using HS-SPME coupled to GC-MS from 39 fermentations. Several were extracted and quantified including a rose-scented compound − phenylethyl alcohol. This was produced by the yeast Bretannomyces bruxellensis on carrot pomace at a yield of 50 mg/kg (w/w) and was selected for further optimisation efforts. Based on two potential microbial pathways to produce phenylethyl alcohol 1) de novo from glycolysis and the Shikimate pathway, and 2) bioconversion of phenylalanine in the Ehrlich pathway, five additional yeasts were selected for comparison of production on carrot pomace. An enhanced extraction process allowed yields of 123 mg/kg using B. bruxellensis, while extending the fermentation duration resulted in 509 mg/kg (w/w) yield. Candida spp. however, were more efficient producers of phenylethyl alcohol, yielding over 1000 mg/kg. Given carrot pomace was deficient in essential nutrients, the food-grade yeast Candida utilis was selected for further optimisation trials supplementing carrot pomace with sucrose, yeast-available nitrogen and phosphate. Using factorial design, the key factors affecting phenylethyl alcohol yield were determined. Consequently, high sucrose (10 g/L) gave the highest yield of phenylethyl alcohol, while increasing levels of nitrogen and phosphate were detrimental to production. In conclusion, C. utilis is an efficient producer of phenylethyl alcohol on carrot pomace and after optimisation a yield of 11.14 g/kg (w/w) was achieved. This finding provides a promising method for the de novo production of natural phenylethyl alcohol using an abundant and ethical substrate. Future work involves determining optimal sucrose and nitrogen concentrations for fermentation, followed by scale-up efforts to a pilot plant level to examine the commercial feasibility of this novel production method.