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Transforming potato peel waste: Exploring a biorefinery approach and its economic viability

This article was written by Jorge Luis Alonso G., an information consultant specializing
in the potato crop.

A research team at the University of Coimbra in Portugal conducted a study on the potential valorization of potato peel waste from the processing industry using a biorefinery approach. The study was published recently in the Journal of Environmental Chemical Engineering, titled “Biorefinery perspective for industrial potato peel management: technology readiness level and economic assessment“.

The study by P.V. Almeida, L.M. Gando-Ferreira, and M.J. Quina aimed to explore the valorization of potato peel waste using a biorefinery approach that includes extraction, anaerobic digestion and composting processes. This article provides a summary of its contents.

1. Introduction

Natural resources are becoming increasingly scarce as human populations grow, leading to concerns about depletion and pollution. The agricultural sector generates significant amounts of organic waste, with households and food production being the largest contributors.

This study focuses on the valorization of potato peel waste from processing industries through a this approach, exploring extraction, anaerobic digestion, and composting processes. It examines the industry, technology readiness, recovered products, and economic analysis, addressing a gap in the literature on economic feasibility.

2. Potato industry

Potatoes are traded globally, with 30% being processed into products such as French fries and processings. This is an industry with a significant socio-economic activity in the EU, valued at €9 billion in 2019.

The production of potato processings involves selection, peeling, washing, cutting, frying, flavoring and packaging, all of which generate residues such as potato peels and defective potatoes. Of these, steam and mechanical abrasive peeling are the most common methods, with peels accounting for 10–12% of the potato’s weight.

It should be noted that not only the potato processing industry generates significant amounts of potato peel, but so do other potato industries, with some companies using off-specification potatoes for animal feed or anaerobic digestion.

3. Potato peel composition

Potato peel (PP) has a high moisture content and con sists mainly of carbohydrates, lignin, lipids and ash. It contains minor amounts of glycoalkaloids and phenolic compounds, which have potential applications in the pharmaceutical and healthcare industries due to their antimicrobial, anti-inflammatory and antioxidant properties.

The composition of PP depends on the potato variety, maturation and peeling mechanism. Glycoalkaloids such as α-solanine and α-chaconine are predominantly found in potato peels, while phenolic compounds such as chlorogenic acid, caffeic acid and gallic acid are also present.

Many authors have demonstrated the benefits of these compounds, some focusing on selective extraction methods. However, crude extracts may have synergistic effects and their use should be prioritized to determine the need for purification.

In addition, studies show that potato peel extract powders effectively inhibit the growth of pathogens in plants and exhibit antimicrobial and antioxidant activities in edible films and meat.

4. Potato peel valorization

PP is a residue that is often discarded or used for animal feed or biogas production, but it contains valuable compounds and several processes have been evaluated to extract them or recover energy from PP. Among them, anaerobic digestion, alcoholic fermentation, and pyrolysis are the most important.

Some studies have proposed a biorefinery approach that integrates multiple processes to manage PP and produce different products while minimizing waste. Several routes for PP valorization exist, but their technology readiness levels (TRL) vary. In this context, food processing, anaerobic digestion and composting are well established on an industrial scale, while other processes for managing PP are not well studied.

The authors of this study propose a biorefinery approach involving sequential extraction of starch, phenolic compounds and glycoalkaloids, followed by anaerobic digestion for energy recovery and composting of the digestate. However, the economic viability of this strategy should be further evaluated.

4.1. Recovery of value-added compounds through extraction processes

Commercial interest in PP compounds is focused on starch, phenolic compounds, and glycoalkaloids. Starch extraction involves soaking biomass in water or other solutions, wet milling, decantation, and multiple washes. It should be noted that starch can be found in other sources such as corn, wheat, rice, cassava, quinoa, amaranth, and lentil.

Glycoalkaloids and phenolic compounds can be extracted from PP by solid-liquid extraction (SLE), ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE), and supercritical fluid extraction (SFE). Methanol, ethanol, and water are the most commonly used solvents.

Importantly, comparison of extraction technologies is difficult due to different parameters and units used in reports. An economic analysis would help select the most advantageous process and operating conditions.

4.2. Energy recovery through anaerobic digestion

Anaerobic digestion is a strategy for managing organic residues and recovering energy through the production of methane-rich biogas. PP has an industrially exploitable biochemical methane potential (BMP) ranging from 218 to 320 mL CH4/g VS.

Factors such as substrate-to-inoculum ratio (SIR) and operating conditions significantly affect the performance of anaerobic digestion. Pre-treatment such as grinding and hydrolysis, or co-digestion with other materials such as cow manure, sugar beet leaves, or marine microalgae can improve the BMP of PP. Besides, the integration of several processes, such as alcoholic or lactic fermentation followed by anaerobic digestion, can increase the methane yield.

4.3. Composting

Composting of PP is often done by co-composting because of its high moisture content, which can reduce oxygen permeability. To balance its composition, PP is mixed with other materials such as grass clippings or wheat straw. Mixtures with rice hulls and eggshells can result in higher temperatures. The resulting compost is of high quality, but mixtures with grass clippings take longer to mature.

In a biorefinery approach, digestate from anaerobic digestion can be used as a composting substrate. However, bulking agents are required to increase the free air space (FAS). Mature and sanitized compost can be obtained and used for soil amendment, closing carbon and nutrient cycles. It is important to note that his technology is already used on an industrial scale, with compost valued at 21–28 €/t fw.

5. Techno-economic analysis

The techno-economic analysis is essential to evaluate the industrial viability of agro-waste management routes, but such information is lacking in the literature on potato peel management.

Economic viability depends on feedstock characteristics, operating conditions and product demand. Net Present Value (NPV), Internal Rate of Return (IRR), and Payback Time (PBT) are key parameters for assessing profitability and viability.

To clarify, the integration of multiple processes in biorefineries can improve NPV, IRR and PBT, although it increases investment and production costs. And factors such as product selling price, raw material composition, production capacity, and utility/input costs affect economic viability.

High selling prices for pectin, polyphenol extract and oilseeds can generate significant revenues. However, variations in raw material composition can affect extraction yields and product purity. On the other hand, low processing scales are typically unprofitable, and high production capacity can reduce costs while increasing NPV and reducing PBT. Mass and energy integration can reduce input and operating costs. Capital depreciation and transportation costs are other significant factors in operating costs.

Biorefineries appear to be a good alternative for the valorization of agro-industrial residues from an environmental perspective, and they generally present a positive NPV, PBT within the project life, and IRR lower than the most common interest rate (10%).

6. Conclusions and future perspectives

The potato processing industry in many countries generates massive amounts of potato peel waste. And in order to promote a circular economy, various strategies have been proposed to manage this residue.

Companies mainly use animal feed and anaerobic digestion on an industrial scale, while extraction of value-added compounds and alcoholic fermentation remain on a laboratory scale.

By adopting the biorefinery concept, industries can optimize the valorization of potato peel, recovering bioactive compounds, energy and soil amendments. Although research on this approach for potato peel is still limited, existing literature suggests its feasibility. However, several gaps need to be addressed for more conclusive results.

Researchers should conduct scale-up studies and carefully investigate the integration of multiple processes. For now, techno-economic analysis shows that large-scale biorefineries can be economically viable, with value-added compounds driving profitability due to their high market value. But risk analysis is needed to account for price fluctuations.

Finally, life cycle assessment data for potato peel management processes are currently lacking in the literature, a critical piece of information for selecting best management practices. By filling these gaps, industries can implement well-planned strategies for the treatment and valorization of potato peel waste, reducing costs and promoting a circular economy.

Source: Almeida, P., Gando-Ferreira, L., & Quina, M. (2023). Biorefinery perspective for industrial potato peel management: Technology readiness level and economic assessment. Journal of Environmental Chemical Engineering, 110049.
Photo: Credit Couleur from Pixabay

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