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Abstract

Objective

Biomass derived from various lignocellulosic feedstocks offers a viable solution for bioethanol production, contributing to reduced greenhouse gas emissions while generating economic value from agricultural waste. The present study explored the production of bioethanol from ripe pear fruit wastes juice using sorghum flour as an additional fermentable sugar.

Methods

Ripe pear fruit waste juice was prepared by blending the fruit residues and filtering the mixture using a clean sieve to obtain a clear extract. The juice was fermented under anaerobic conditions with sorghum flour used as supplement to enhance hydrolysis and fermentation efficiency. During fermentation, microbes converted fermentable sugars in the juice into raw bioethanol which was subsequently purified through distilled and redistillation at 78 °C.

Results

Ripe pear fruit waste juice had the total soluble solids of 8.0 ± 0.03 and 8.5 ± 01 ^oBrix prior and post addition of 300 grams of sorghum in 8 litres of juice, respectively. Fermented ripe pear fruit waste juice with sorghum achieved the percentage alcohol by volume of 6.56% versus 3.94% for the one without sorghum. The results reveal that fermented ripe pear fruit waste juice broth with sorghum produced bioethanol with higher concentrations of 20%, 16%, and 12% for the first, second, and third aliquots of 100 mL each, respectively. However, the fermented ripe pear fruit waste juice broth without sorghum produced bioethanol with concentrations of 15%, 14%, and 11%, respectively. Redistillation of bioethanol with concentration between 11–20% improved its quality to 49%, 47%, and 39% for the first, second, and third aliquots of 100 mL, respectively.

Conclusion

The present study highlights the potential of ripe pear fruit waste as feedstock for bioethanol production in minimizing postharvest losses and protecting the environment. The integration of fruit growers into bioethanol supply chain presents a unique opportunity to improve their socio-economic conditions while contributing to renewable energy production and supporting circular economy.

Keywords

Bioethanol fermentation pear fruit ripe waste

Introduction

Studies indicate that many Tanzanian farmers rarely benefit from the full feats of their labour as a significant portion of their produce goes to waste, an indication of labour underutilization and economic exploitation.^1–3 A report indicates that the major portion of the total fruits produced, around 20–50% is spoiled as wastes due to their highly perishable nature, lack of reliable transportation, and post-harvest processing technologies in the remote hilly areas.^3,4 It is well known that the use of food-based crops as a substrate for bioenergy generation entails economic and ethical challenges.⁵ Biomass resources such as organic waste, forestry waste, and agricultural residues provide sustainable substitute raw materials for bioenergy production while mitigating the environmental effects of fossil fuels exploitation.⁶ Pear fruit (Pyrus communis) is among the most delicious fruits grown in many countries of the world including Tanzania and is rich in vitamins and minerals though highly perishable.^4,7 Since these fruit wastes contain a lot of sugars, carbohydrates, and organic acids as part of their constituents can finally produce volatile and phenolic compounds as byproduct.^8,9 A study shows that about 60% of carbohydrates in pear fruit is in sugar form with the average sugar content of 10.4 grams/100 grams for which the most predominant sugar is fructose.^10,11 In fact, value-added products can be recovered from these wastes^8,12 such as bioethanol which can be obtained from bioresources (biomass) by hydrolysis and sugar fermentation processes.^13,14 Studies indicate that about 60% of the global bioethanol is produced from starchy grains while the remaining 40% is produced from sugar crops.^15,16 Bioethanol has been described as one of the most exotic synthetic oxygen-containing organic chemicals because of its unique combination of properties as a solvent, a germicide, an antifreeze, a fuel, a depressant and especially of its versatility as a chemical intermediate for other organic chemicals.^17,18 It is easier and cheaper to use free sugar containing juice as feedstock for bioethanol production than starch or lignocellulosic biomass due to the non-requirement of costly steps such as hydrolysis to obtain fermentable sugars.^19,20 The reducing sugars which are present in the fruits waste are interesting feedstocks for production of second-generation bioethanol as reported earlier.^14,18

Therefore, fruit wastes can be transformed into second-generation bioethanol leading not only to wastes reduction but also converting them into useful raw materials thus, offering the integration of farmers’ income.^18,21,22

Pear fruit is more prone to spoilage due to its nature and composition, particularly during harvesting, transportation, storage, marketing, and processing resulting into a lot of postharvest loses.^23,24 However, the postharvest loses can be minimised by utilising ripe pear fruit waste, a potential feedstock for bioethanol production because it is rich in fermentable sugars. To achieve the transformation of ripe pear fruit waste into bioethanol, fermentation must be conducted under appropriate conditions. For the fermentation to occur, there must be enzymes which can degrade the matter.^25,26 Ripe fruits provide juice suitable for fermentation, as a portion of starch is hydrolysed into simple sugars through enzymatic activity.^27–29 Most of the work published on the production of bioethanol from fruit residues uses Saccharomyces cerevisiae as the fermenting microorganism.^25,30,31 However, some research successfully achieved fermentation without addition of Saccharomyces cerevisiae since the presence of sugar in the juice is sufficient to attract microbes in the environment which feed on sugars during fermentation.^6,31,32 Enzymes from natural sources are effective in the sugar hydrolysis and fermentation process. Use of fruits at the verge stage which contain several hydrolytic enzymes including amylase, maltase, invertase, and cellulase have proven to be efficient in the fermentation process.^33–35 Pectinases, amyloglucosidase, and alpha amylase enzymes are effective in fermentation over a wide range of temperature and pH.^6,36 Addition of sorghum flour as an additional fermentable sugar which have several enzymes enhances the fermentation process thus, increasing bioethanol yields.³² Sorghum grains contain enzymes including pancreatic lipase, α-amylase, xanthine oxidase (XO), α-glucosidase, and angiotensin converting enzyme (ACE) which facilitate fermentation by converting the complex sugar into simpler sugar.³⁷ The hydrolysed simple sugars from complex sugars can be efficiently fermented to produce bioethanol. The bioethanol production results in effective management of these fruit wastes and helps in preserving the environment from the effect of discarded wastes.^18,32

The current study explored the innovative approach of utilizing concentrated ripe pear fruit waste juice as a primary substrate for bioethanol production which has not been extensively studied. By incorporating sorghum flour as additional fermentable sugar, the study aims to enhance the fermentation efficiency and overall bioethanol yield. The novelty lies in the synergistic effect of adding sorghum flour into the pear fruit waste juice for effective fermentation leading to a more sustainable and cost-effective bioethanol production process. The ripe pear fruits are mostly preferred rather than fresh pear fruit as it contributes to the valorisation of fruit waste, availability, promoting environmental sustainability, avoiding competition with food supply while addressing the currently global increase in bioethanol demands.^38,39

Results and Discussion

Pre-Treatment and the Effect of sorghum Flour on Ripe Pear Fruit Waste Juice

The weighed sample of ripe pear fruit wastes (20 kg) produced 8.0 L of clear juice for fermentation. The parameters including TSS, pH, temperature (°C), SG, and %ABV were measured as presented in Table 1. Measurements indicate that the TSS of pear fruit waste juice increased from 8.00 ± 0.03 without sorghum to 8.50 ± 01 after addition of sorghum. Besides, the pH increased from 3.74 ± 0.01 without sorghum to 3.97 ± 0.01 with sorghum. This observation sheds light on how the addition of sorghum flour influences the TSS and pH levels of ripe pear fruit waste juice, providing insights on fermentation process and better yield of bioethanol. However, at this stage %ABV maintained constant value between the two fermenting broths because fermentation had not started. These results are in contrast with that reported by Hussain et al (2013)⁴⁰ and Borsai et al (2022)⁴¹ who reported the TSS of pears from different cultivars ranging from 11.03 to 14.42 and 10.53 ± 0.46 to 12.87 ± 0.64 ˚Brix and pH levels between 4.12 ± 0.01 to 5.24 ± 0.01 and 3.80 ± 0.02 to 4.47 ± 0.01, respectively. However, our results show significant differences compared to a study that utilized cashew apple fruit juice for bioethanol production, reporting the TSS level of 11.8 ˚Brix and a pH of 4.4.⁴² Factors like environmental conditions, fruit harvesting timing, genotype, and post-harvest handling may be responsible for the observed differences.⁴¹

Table 1.

Pre-Treatment of Ripe Pear Fruit Waste Juice Before and After Addition of Sorghum Flour.

Parameter	Parameters for the ripe pear fruit waste juice
Parameter	Concentrated juice only	Concentrated juice + sorghum
TSS (˚Brix)	8.00 ± 0.03	8.50 ± 01
SG	1.05 ± 0.01	1.06 ± 0.02
pH	3.74 ± 0.01	3.97 ± 0.01
%ABV	0.00 ± 0.00	0 ± 0.00
Temperature (°C)	22.2 ± 0.04	22.5 ± 0.02

Changes in Sugar Content During Fermentation of Ripe Pear Fruit Waste Juice Broth

The trend of TSS provides the information on the production of bioethanol for the fermenting pear fruit wastes juice broth. Results show that the TSS for pear fruit waste juice broth sample with and without sorghum declined gradually before retaining the constant values along the fermentation process as presented in Figure 1. For the fermenting broth without sorghum, TSS reveals the 0.2 ˚Brix decrease after 24 h with a larger decline of TSS afterwards up to the fifth day before retaining a constant value of 4.0 ˚Brix at the end of fermentation period. Besides, for the broth with sorghum flour, a decrease of 0.5 ˚Brix in TSS was observed after 24 h, experiencing further decline up to the fourth day. However, the TSS retained a constant value of 5.0 ˚Brix up to the end of the fermentation process. The variation in the decline in TSS between the sample with and without sorghum during fermentation is due the difference in the microbial growth in the two fermenting broths. Findings indicate that the addition of sorghum into pear fruit waste juice elevates the TSS. This is a crucial stage in the production of bioethanol because it provides the necessary substrate for microbial fermentation thus, enhancing bioethanol yield. Fermentable sugars found in sorghum provide microorganisms with extra supply of nutrients during fermentation, hence increasing the amount of ethanol produced.

Figure 1.

Trends on Changes of Total Soluble Solids of Ripe Pear Fruit Waste Juice with and Without sorghum Flour During Fermentation.

Changes in Specific Gravity and Percentage Alcohol by Volume of Ripe Pear Fruit Waste Juice Broth During Fermentation

The trend in the changes of SG and %ABV along the fermentation process were studied. Results show that the SG trend along the process mirrors that exhibited by TSS. As the fermentation process progressed, SG declined before retaining a constant value. The SG declined from 1.06 to 1.01 for the fermenting broth with sorghum after 96 h while it was from 1.05 to 1.02 after 72 h for the one without sorghum. The decrease in SG is a clear indication that fermentation was taking place resulting in bioethanol production. However, the constant value of SG after incubation period is an indication of the end of fermentation as presented in Figure 2. Residual sugars after the maximum fermentation period reflect complex carbohydrates that could not be hydrolysed into fermentable forms. Furthermore, results show that the decline in SG in ripe pear fruit waste juice with sorghum was significantly greater than in ripe pear fruit waste juice without sorghum. This is attributed to the increased rate of sugar consumption by the additional microorganisms found in the pear fruit waste juice supplemented with sorghum as previous reported.⁶ At the end of the fermentation period, the fermented ripe pear fruit waste juice with sorghum achieved the %ABV of 6.56 while that without sorghum had the %ABV of 3.94.

Figure 2.

Specific Gravity Trends of Fermenting Ripe Pear Fruit Waste Juice Broth with and Without sorghum Flour During Fermentation.

Changes in pH During Fermentation of Pear Fruit Waste Juice Broth

The pH of the fermenting broth for the ripe pear fruit waste juice with and without sorghum declined gradually from day 0 to 7 as indicated in Figure 3. The decrease in pH means that fermenting broths were becoming more acidic as the fermentation process proceeded. This is attributed to the conversion of sugars into bioethanol, organic acids, phenolic derivatives, and CO₂ during fermentation, as also reported in the literature.⁴³ Findings indicate that the pH recorded was much lower in the broth with sorghum than that without sorghum. This is because organic acids produced in the broth with sorghum were much higher than in the broth without sorghum due to the presence of additional fermentable sugars from sorghum.

Figure 3.

Trends of pH Changes During Fermentation of Ripe Pear Fruit Waste Juice Broth with and Without sorghum.

Distillation of Fermented Ripe Pear Fruit Waste Juice Broths

The fermented ripe pear fruit waste juice was filtered and distilled at controlled temperature to obtain bioethanol of different concentrations. Results from the fermented broth with and without sorghum are presented in Table 2 for the first, second, and third aliquots.

Table 2.

Alcohol Concentration After Distillation and re-Distillation Processes.

	First distillation			Re-distillation
Fermented samples	Aliquot	Volume of distillates (mL)	Concentration (%)	Aliquot	Volume of distillates (mL)	Concentration (%)
Ripe pear fruit waste juice broth with sorghum	first	100	20	first	100	49
	second	100	16	second	100	47
	third	100	12	third	100	39
Ripe pear fruit waste juice broth without sorghum	first	100	15	first	100	36
	second	100	14	second	100	33
	third	100	11	third	100	25

The distillation process achieved bioethanol with alcohol content between 12% to 20% from fermented ripe pear fruits waste juice broth with sorghum and 11% to 15% from the broth without sorghum. Results reveal that as the distillate collection time increase, the concentration of bioethanol decreases as more water is brought into the later aliquots. However, statistical analysis showed no significant difference between the alcohol content (%) produced with a p-value of 0.08 (p > 0.05). The content of alcohol (20%) obtained from the first aliquot of 100 mL in this study was comparable with the one obtained from mango fruit waste juice (20%) augmented with millet as a source of additional fermentable sugar.²² However, the alcohol contents were less than the ones obtained from pineapple fruit waste juice (43%) and mixed fruits juice (25%), and higher than the alcohol content produced from pawpaw (15%), and watermelon (7%) enhanced with millet.²² A separate study using dilute fruit waste juice from pineapple, mango, pawpaw, watermelon, and mixed fruits, supplemented with sorghum as an additional fermentable sugar source, reported bioethanol production with alcohol contents of 25%, 15%, 10%, 4%, and 11%, respectively.⁶ The variations in alcohol content among different fruits may be attributed to differences in sugar content as TSS typically provides more initial sugar availability for yeast fermentation as observed in the case of pineapple. However, this trend was not consistently observed, suggesting that other factors including the type of sugars present, inhibitory compounds, and fermentation parameters such as pH, temperature, and microorganism efficiency may also influence the process.³⁹ Bioethanol with alcohol concentrations between 11–20% were re-distilled at 78 °C to reduce the amount of water thus, improving its quality. The re-distillation process yielded bioethanol with concentrations between 39% to 49% for the fermented ripe pear fruit waste juice broth with sorghum and 25% to 36% for the fermented pear fruits waste juice broth without sorghum, indicating its potential for upgrading into various economically valuable products. Statistical analysis revealed that there is a significant difference in %alcohol content produced between the two broths with p-value of 2.97 × 10⁻⁴, less than 0.05. These findings demonstrate that pear fruit waste juice is a promising feedstock for bioethanol production and the development of value-added products such as disinfectants, hand sanitizers, industrial solvents, and biofuels.

Potential Limitations on Bioethanol Production from Fermented Ripe Pear Fruit Waste Juice

Future improvements in bioethanol production from pear fruit waste juice with sorghum flour supplementation should consider some limitations. Seasonal variations and differences in sampling regions can influence the amount sugar content of the feedstock, thereby affecting reproducibility in terms of bioethanol yield. Besides, the concentration of fermentable sugars in pear fruit waste and sorghum flour may vary between batches, leading to fluctuations in overall fermentation efficiency and bioethanol yield. However, the results of this study are based on laboratory-scale experiments and may not directly translate to industrial-scale production due to factors such as differences in mixing dynamics and microbial performance. Furthermore, the inadequate management of residual biomass poses potential environmental challenges. Moreover, methodological limitations may affect the accuracy of measurements, including sugar content, fermentation efficiency, and overall bioethanol yield.

Conclusion and Recommendation

The present study shows that ripe pear fruits waste is a viable feedstock for bioethanol production through physical pre-treatment, microbial fermentation and distillation and the inclusion of sorghum flour enhanced fermentation efficiency. Fermented ripe pear fruit waste juice broths achieved the %ABV of 6.56 and 3.94 for the one with and without sorghum, respectively. The distillation of fermented broths produced bioethanol with the alcohol content between 11% to 25%. However, re-distillation of bioethanol with the concentration between 11%-25% improved its quality up to 49% with the potential to be transformed into other value-added products. The promise of process scalability and reproducibility make it a viable solution for waste management and renewable bioethanol production. Utilizing fruit wastes for bioethanol generation addresses the environmental pollution and global warming challenge and promotes circular economy. Future research should focus on optimizing fermentation conditions such as temperature, pH, retention time, and exploring other grains other than sorghum to enhance bioethanol production. It is recommended to consider scaling up this study to an industrial level to enhance socio-economic well-being of fruit growers across regions.

Materials and Methods

Study Area and Sample Collection

All experimental procedures were conducted in the Chemistry Laboratory at Mkwawa University College of Education (MUCE) in Tanzania. Ripe pear fruit (Pyrus communis) waste samples were collected between February and May 2024 from Mashine Tatu and Mlandege markets in Iringa Municipality, Tanzania.

Pre-Treatment of Pear Fruit Waste

The ripe pear fruit waste was weighed to obtain specific weight (20 kg) that could produce the defined volume of clear pear fruit waste juice for the fermentation process and subsequent bioethanol generation through distillation. The fruit waste samples were washed with water to remove dirt and impurities, then chopped into small pieces suitable for grinding using a blender (Kenwood-BLPA-10). The mixture of fruit residues and juice was filtered through a clean sieve to obtain clear juice, which was then collected in sanitized buckets and prepared for fermentation.

Fermentation

While the fermentation approach is well-established, this study incorporates modifications by adding sorghum flour to enhance the hydrolysis of complex sugars and improve fermentation efficiency, distinguishing it from previous reports that relied on commercial enzymes.^44,45 The clear concentrated extracted juice from the ripe pear fruit waste samples (8 L) was mixed with 300 g of sorghum flour as an additional fermentable sugar and conditioned at room temperature. The conversion of fermentable sugar into bioethanol was done by the action of fermenting microbes in an anaerobic condition. Additionally, enzymes particularly those from sorghum flour play a pivotal role in breaking down complex carbohydrates into simpler sugars for the microbes to ferment into bioethanol. The broth was analysed before the onset of fermentation and then after every 24 h of fermentation for seven (7) days. The sample was withdrawn from the fermentation bucket for measurement of pH using the pH meter (Model HI98129, HANNA Instruments), specific gravity (SG) using hydrometer (Saccharometer), total soluble solids (TSS) using refractometer (ERMA TOKYO), and the alcohol content using alcoholmeter (Alcoholmeter Gay Lussac).

Determination of Percentage Alcohol by Volume

The determination of percentage alcohol by volume (%ABV) during fermentation was performed using the formula relying on the difference in SG before and after the fermentation period. The formula is presented in equation 1 as reported earlier.^46–48

% ABV = (Initial SG - Final SG) \times 131.25

(1)

In this formula, the initial SG refers to the SG at the start of the fermentation process, which reflects the sugar content in the fermentation medium while the final SG represents the SG after fermentation, indicating the remaining unfermented sugars.⁴⁹ As defined, SG is a measure of the liquid density relative to water, typically determined using instruments such as hydrometer and refractometer. The difference between these two values represents the quantity of sugar metabolized into bioethanol by the fermenting microbes. The constant 131.25 serves as a conversion factor that translates this difference into %ABV, based on the density of bioethanol and its energy content.⁵⁰

Distillation

The bioethanol was separated from water by conditioning the distillation and re-distillation at temperatures between 77–86 °C. This range of temperature from higher to lower value than normal boiling point of bioethanol was caused by the presence of high amount of the matrix and azeotrope mixture, respectively. The obtained bioethanol was analysed to determine its quality. The quantity was determined by measuring the volume of distillates using a measuring cylinder (AXIOM GERMANY), and the quality was determined as the percentage by volume (v/v) of the distillates using alcoholmeter (Alcoholmeter Gay Lussac).

Data Analysis

All experiments were performed in triplicate to enhance the reliability and consistency of the findings. Data was processed using Microsoft Excel (2016), while statistical evaluation was conducted through one-way ANOVA. A probability threshold of P = 0.05 was applied to determine statistically significant differences among the groups.

Footnotes

Acknowledgements

For material support, the authors are appreciative of Mkwawa University College of Education.

ORCID iD

Lewis Mtashobya

Ethical Approval

Ethical approval was not required for this study as it did not involve human participants, animals, or sensitive personal data.

Author Contribution Statement

The conception, design, funding, method development, supervision, data analysis, review, editing, and revision of the text for key intellectual content were all done by LAM and JKE. JAK and FJN took part in the sample collection, drafting of the manuscript, data analysis, and interpretation.

Funding

This study received no external funding.

Data Availability Statement

This study did not involve the generation or use of any datasets, and therefore, no data is available.

Declaration of Interests

The authors state that they have no conflicting interests related to this study.

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Bioethanol Production from Concentrated Ripe Pear Fruit Waste Juice Using Sorghum as an Additional Fermentable Sugar

Abstract

Objective

Methods

Results

Conclusion

Keywords

Introduction

Results and Discussion

Pre-Treatment and the Effect of sorghum Flour on Ripe Pear Fruit Waste Juice

Changes in Sugar Content During Fermentation of Ripe Pear Fruit Waste Juice Broth

Changes in Specific Gravity and Percentage Alcohol by Volume of Ripe Pear Fruit Waste Juice Broth During Fermentation

Changes in pH During Fermentation of Pear Fruit Waste Juice Broth

Distillation of Fermented Ripe Pear Fruit Waste Juice Broths

Potential Limitations on Bioethanol Production from Fermented Ripe Pear Fruit Waste Juice

Conclusion and Recommendation

Materials and Methods

Study Area and Sample Collection

Pre-Treatment of Pear Fruit Waste

Fermentation

Determination of Percentage Alcohol by Volume

Distillation

Data Analysis

Footnotes

Acknowledgements

ORCID iD

Ethical Approval

Author Contribution Statement

Funding

Data Availability Statement

Declaration of Interests

References