Bioethanol fermentation from kitchen waste using Saccharomyces cerevisiae

Introduction

Kitchen waste is a raw material available in large volumes. The term applies to organic solids, which are discarded during food preparation. Kitchen waste is mostly composed of lignocelluloses and starch, and is degradable through microbial infestation. An outstanding resource for biotechnology is present in kitchen waste as carbohydrate polymer fraction¹. The usage of lignocelluloses as feedstock would prompt novel challenges for biotechnology, for example, the product diversification^2,3.

Kitchen waste extracted bioethanol is an alluring and sustainable energy source for vehicle fuel, as a gasoline alternative. Present ethanol production (so-called ‘first generation’), utilizing harvests such as sugar cane and corn, has become conventional method, whereas second-generation ethanol generation uses less expensive, non-sustenance feedstocks, for example, lignocelluloses or municipal solid waste, which could make ethanol more competitive alternative to petroleum^4,5.

Plant cell walls are mostly composed of lignocellulosic biomass. Among its main components is cellulose, a linear polymer of cellobiose consisting of two D-glucose molecules connected by β-1, 4 bonds⁶. The more organized or crystalline cellulose is, the less soluble and less degradable it is. Cellulose degradation techniques include the use of effective enzymes, concentrated acid or alkaline and high temperatures for both nebulous and crystalline cellulose in the transformation procedure. Cellulose would be an ideal carbohydrate source for the fermentation due to the uniform hydrolyzable glucose building blocks⁷.

Hemicellulose is another important hetero-polysaccharide present in the plant cell wall. Hemicellulose differs from cellulose by the organization of several sugar units, by the presence of shorter chains and by a ramified central chain. Hemicellulose removal from the plant cell wall is easier than lignin and cellulose, owing to the bonds between cellulose, hemicellulose and lignin. A wide variety of enzymes, including endoxylanase, exoxylanase, mannanase, arabinosidase, acetylesterase, and glucoronisidase, are essential due to its structural diversity⁸. Alkaline pretreatments were also found to be successful at degrading hemicellulose⁹.

Lignin provides additional strength and protection to prevent enzymatic activity of fungi and insect attack by linking cellulose and hemicellulose¹⁰. Substantial moisture and rigidity resistance also added to biomass¹¹ and known as a cellulase inhibitor¹². As a result, exogenous proteins, such as bovine serum albumin, and surfactants, such as MgSO₄, and CaCl₂¹³, are added before microbial or enzyme loading. Moreover, many pretreatment processes have been established to moderate the lignin hindrance.

Functional groups of lignin, including phenolic hydroxyl, benzyl hydroxyl, methoxyl, carbonyl and a minor amount of terminal aldehyde groups, are factors that influence its decomposition¹⁴. The lignin carbohydrate complexes (syringyl, guaiacyl and p-hydroxyphenyl units) formed by crosslink interactions, are also counted as a fermentation-restricting factor.

Starch (α-D-glucose monomer) degradation is complex than the sugar fermentation process. It initially broke down into glucose, through amylase or diastase and maltase hydrolysis. Then, ethanol and carbon dioxide are fermented from sugars through enzyme activity.

Alongside a mainstream project^15,16, in this study, tentative fermentation was carried out in kitchen waste medium to find out if a wild-type microorganism has the ability to ferment cellulose efficiently¹⁷.

Methods

Results

Previous investigations^15,16,18 indicated that ethanol is produced more readily under shaking than non-shaking conditions. After 48 h of fermentation at room temperature in a rotary incubator (120 rpm), a maximum of 7.3% (v/v) ethanol production was recorded (Table 1). The rate of alcohol production showed a cumulatively increasing trend, which was mirrored by a continued rise in pH throughout incubation, recorded as pH 6.52 at 48 h. The results also indicated that the full potential of kitchen waste fermentation will be revealed through longer durations of fermentation.

Discussion

The kitchen waste medium contained plant organelles and was a rich source of cellulose, starch and glucose monomers. Comparison with previous studies^15,16,18 showed the achieved production efficiency is below the level required for profitable commercial production. In this study, production of 7.3% (v/v) ethanol was recorded (Table 1). Further optimization of the process and co-fermentation (e.g. ethanol–butanol co-fermentation) is among the future goals of researchers²¹.

Most of the kitchen waste was similar feedstock to lignocellulosic raw materials, which is considered to be an excellent substrate²². Previous works delineated the pathway of converting plant-based waste biomass to bioethanol, in which enzyme pretreatment was conducted before yeast fermentation^23–26. Gnansounou & Dauriat produced 30.9 g bioethanol and 65.2 L biogas using 1 kg of kitchen waste²⁷. Velasquez & Ruiz fermented 346.5–388.7 l/ton bioethanol in a similar study using banana pulp and skin²⁸. Industrial waste has also been used in fermentation technology²⁹. A planned facility in East London will produce 16 million gallons of jet fuel per annum from 500,000 tons of waste for British Airways (the Green Sky project). A combination of plasma arc gasification with the Fischer-Tropsch method, known as Solena's Plasma Gasification (SPG) technology, will be used. In China’s Jiangsu province, a new waste-to-energy facility with a processing capacity of 900 metric tons, will be constructed.

Conclusions

Results were derived from limited parameters and only a single isolate of microorganism was employed. Future studies should be directed towards elaborate characterization and compare different criteria of kitchen waste fermentation. Optimization of the media and physicochemical parameters, and longer-duration fermentation will also be performed in future. This study was aimed towards fermentation only. A cheap, efficacious downstream processing method of the ethanol generated in this process also requires development.

Data availability

All data underlying the results are available as part of the article and no additional source data are required.

Author information

When the research was carried out, SSR was a MSc student at Biotechnology program, Department of Mathematics and Natural Sciences, BRAC University and NC was the Coordinator of Biotechnology and Microbiology programmes at the Department of Mathematics and Natural Sciences, BRAC University.