Abstract
The notion of a “biobased economy” in the context of a developing country such as South Africa (SA) necessitates the development of technologies that utilize sustainable feedstocks, have simple and robust operations, are feasible at small scale and produce a variety of valuable bioproducts, thus fitting the biorefinery concept. This case study focuses on the microbial production of higher-value products from selected organic waste streams abundant in the South African agricultural sector using microbes adapted to utilize different parts of biomass waste streams. A ruminant-based carboxylate platform based on mixed or undefined anaerobic co-cultures of rumen microorganisms can convert the carbohydrate polymers in the lignocellulosic part of organic waste streams to carboxylic acids that can be upgraded to biofuels or green chemicals. Furthermore, yeast and fungi can convert the simpler carbohydrates (such as the sugars and malic acid in grape and apple pomace) to ethanol and high-value carboxylic acids, such as lactic, fumaric, succinic and citric acid. This review will discuss the combinational use of the ruminal carboxylate platform and native or recombinant yeasts to valorize biomass waste streams through the production of higher-value organic acids with various applications.
Highlights
The world has been reliant on crude oil and coal as the primary source of energy and materials, driven by a linear economy that primarily focuses on continual growth, consuming resources and discarding waste for more than a millennium
Jiang et al [78] showed that Volatile fatty acids (VFAs) concentration may increase as the organic loading rate increases, but may not result in increased VFAs yield for loadings higher than 16 g/Total Solids (TS)/L × d, with higher VFA yields observed for organic loading rates of 5 to 11 g.TS/L × d
This study showed that both citrus (77% Neutral Detergent Fiber (NDF) degradability,136 mM VFA concentration after 96 h fermentations and 12.16 mmol alkyl/g) and apple pomace (75% NDF degradability, 126 mM VFA concentration and 11.31 mmol alkyl/g) can be valorized to produce significant amounts of VFAs that can serve as substrates for hydrocarbon production
Summary
The world has been reliant on crude oil and coal as the primary source of energy and materials, driven by a linear economy that primarily focuses on continual growth, consuming resources and discarding waste for more than a millennium. Volatile fatty acids (VFAs) are short-chain aliphatic mono-carboxylate compounds with two to six carbon atoms, which include both linear acids such as acetic, propionic, butyric, valeric and hexanoic/caproic acid, and branched-chain acids such as isobutyric and isovaleric acids This alternative technology could be of particular importance in developing countries in Africa (including South Africa) where the huge capital costs of large-scale cellulosic ethanol plants present financial constraints in addition to the technological challenges. Whereas the fiber content in fruit waste could be readily converted to VFAs in the carboxylate platform, the production of high-value lactic or dicarboxylic acids (organic compounds containing two carboxylic acid (–COOH) functional groups) from the remaining sugars and organic acids in fruit wastes could potentially increase the economic viability of biorefineries aimed at providing bioethanol as an alternative to current petroleum-based fuels. Given the pivotal metabolic role and natural abundance of malic acid, our discussion on the latter will focus on the production of high-value organic acids that can be derived from L-malic acid in fruit pomace, with special emphasis on yeast strains as potential cell factories
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