Bioethanol fermentation from kitchen waste using Saccharomyces cerevisiae

Bioethanol obtained from microbial fermentation can replace conventional fossil fuels to satisfy energy demand. In this respect, a fermenting isolate of , obtained from date juice, was grown in YEPD Saccharomyces cerevisiae medium as a part of a previous published research project. In this study, the isolate was tentatively characterized for alcoholic fermentation in organic kitchen waste medium, prepared from discarded fruit and vegetable peels. Fermentation in shaking condition resulted in the production of 7.3% (v/v) ethanol after 48 h, after which the pH of the medium increased slightly in response. Further research should be conducted to assess the potential of kitchen waste as a raw material in ethanol fermentation.

No competing interests were disclosed.

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 fraction 1 .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 bonds 6 .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 blocks 7 .
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 diversity 8 .Alkaline pretreatments were also found to be successful at degrading hemicellulose 9 .
Lignin provides additional strength and protection to prevent enzymatic activity of fungi and insect attack by linking cellulose and hemicellulose 10 .Substantial moisture and rigidity resistance also added to biomass 11 and known as a cellulase inhibitor 12 .As a result, exogenous proteins, such as bovine serum albumin, and surfactants, such as MgSO 4 , and CaCl 2 13 , 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 decomposition 14 .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 efficiently 17 .

Composition and pretreatment
Yeast samples (Saccharomyces cerevisiae) were isolated and identified from date-juice using previously described methods 15,18 in YEPD medium (10 6 -10 7 cells/ml; 0.3% yeast extract #Y1625, 1% peptone #P7750, 2% dextrose #G8270, 1.5% agar #A1296, pH: 5; Sigma-Aldrich, St Louis, MO, USA).Discarded solid kitchen waste was collected from different households.This included peels from potatoes, pumpkin, papaya, cucumber, okra, green banana, balsan apple, carrot and basil.After chopping, pulverizing and blending with 1 L water, 250 g solid waste was taken as raw medium.Concentrated HCl (2 ml) was added to convert the calcium present (a fermentation inhibitor) to calcium sulfate salt 19 .HCl also regulates the pH of the medium to control for bacterial contamination and facilitate chemical hydrolysis of plant residues, which were boiled for 1.5 h, giving carbohydrate units of cellulose and starch.Monomers of amylose, amylopectin and glucose arose from further degradation.Urea (0.1 g) was also supplemented prior to boiling as a nitrogen source nutrient.The final pH was adjusted to 6.0 by dropwise addition of NaOH or HCl (measured using a pH meter; Mettler Toledo, Switzerland).

Fermentation
The 250 ml fermentation medium was transferred into 500 ml Erlenmeyer flasks and a homogenous suspension of yeast (10 ml YEPD broth) was inoculated in aseptic conditions.The flask was incubated in a rotary incubator (120 rpm) at 30°C for 48 h.Two separate experiments were conducted and ethanol production was recorded at 24 and 48 h intervals, and the average were calculated.The ethanol in this experiment was analyzed using the Conway method 20 .Downstream processing is required before isolation of usable ethanol.

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 researchers 21 .
Most of the kitchen waste was similar feedstock to lignocellulosic raw materials, which is considered to be an excellent substrate 22 .Previous works delineated the pathway of converting plant-based waste biomass to bioethanol, in which enzyme pretreatment was conducted before yeast fermentation [23][24][25][26] .
Gnansounou & Dauriat produced 30.9 g bioethanol and 65.2 L biogas using 1 kg of kitchen waste 27 .Velasquez & Ruiz fermented 346.5-388.7 l/ton bioethanol in a similar study using banana pulp and skin 28 .Industrial waste has also been used in fermentation technology 29 .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.The benefits of publishing with F1000Research: Your article is published within days, with no editorial bias You can publish traditional articles, null/negative results, case reports, data notes and more The peer review process is transparent and collaborative Your article is indexed in PubMed after passing peer review Dedicated customer support at every stage For pre-submission enquiries, contact research@f1000.com MM and Choudhury N. Creative Commons Attribution Licence which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.The author(s) declared that no grants were involved in supporting this work.