Steam explosion boosts methane from banana residues and food waste digestion

Banana farming leaves behind a mountain of waste that most of the world never sees. For every tonne of fruit that reaches a supermarket shelf, the banana plant discards a pseudostem, a rachis, leaves and other residues that are typically left to rot in the fields. In Colombia, one of the planet’s largest banana producers and exporters, this agricultural stream runs into another waste problem altogether: the organic fraction of municipal solid waste piling up in cities. A new study now shows that these two waste streams, when combined and treated with the right technology, can be transformed into a surprisingly potent source of renewable energy, offering a template for circular bioeconomies in banana-producing regions around the world.

Researchers from the National University of Colombia, the University of La Laguna in Spain, the Spanish research centre CIEMAT and the National Open and Distance University of Colombia have published their findings in Biotechnology for Biofuels and Bioproducts. Their work set out to answer a deceptively simple question: can the stubborn, fibrous leftovers of banana cultivation be converted efficiently into methane when digested alongside urban organic waste? The answer, they found, is yes, but only if the banana residues are first subjected to a violent physical transformation known as steam explosion.

The two banana crop residues at the heart of the study were the rachis, the stalk that carries the fruit bunch, and the pseudostem, the thick, false trunk formed by tightly packed leaf sheaths. Both are abundant and both are problematic for anaerobic digestion, the microbial process that breaks down organic matter in the absence of oxygen to yield biogas, a mixture rich in methane. The obstacle is lignocellulose, the tough composite of cellulose, hemicellulose and lignin that gives these materials their structural strength. Lignin in particular acts as a physical barrier, shielding the energy-dense cellulose and hemicellulose from the hydrolytic enzymes that anaerobic microbes depend on. Untreated, lignocellulosic biomass digests slowly and incompletely, releasing only a fraction of its theoretical methane potential.

To unlock that potential, the team turned to steam explosion, a pretreatment that subjects biomass to high-pressure steam at elevated temperatures for a defined residence time and then abruptly releases the pressure. The sudden decompression causes water trapped within the plant tissue to flash-vaporize, physically tearing the material apart. At the same time, the heat and moisture trigger chemical changes: acetyl groups in the hemicellulose are cleaved to release acetic acid, which catalyses further hydrolysis, while lignin is partially redistributed and solubilized. The net effect is a material whose cellulose fibres are exposed, accessible and far more amenable to microbial attack.

Pretreatment conditions matter enormously, however. Too mild, and the lignocellulosic matrix remains intact. Too severe, and sugars begin to degrade into compounds such as furfural and hydroxymethylfurfural, which inhibit the very microbes the process relies on. The researchers therefore explored a severity window for each residue. The rachis was exploded at 180 and 200 degrees Celsius for ten minutes, while the pseudostem, which has a somewhat different composition, was treated at 160 and 180 degrees Celsius for the same duration.

The team first ran biochemical methane potential tests, standard laboratory batch assays that measure how much methane a substrate can ultimately yield under ideal conditions. The results confirmed the challenge. Untreated organic fraction of municipal solid waste delivered 534.5 litres of methane per kilogram of volatile solids, the organic fraction of the material that microbes can in principle consume. Raw pseudostem yielded 363.3 litres per kilogram of volatile solids and raw rachis only 202.2 litres. The gap between the urban waste and the agricultural residues illustrated precisely why lignocellulosic materials have historically played a modest role in biogas production.

Co-digestion, however, changed the picture. Mixing substrates in anaerobic digestion is more than simple arithmetic: complementary feedstocks can balance nutrients, buffer acidity and improve the overall stability of the process. The researchers tested mixtures of the municipal organic waste with pseudostem and rachis at defined proportions on a volatile-solids basis, with the best-performing blend consisting of 70 percent municipal waste, 20 percent pseudostem and 10 percent rachis equivalent proportions explored across the experiments. Steam explosion pretreatment of the banana residues raised biodegradability in these co-digestion assays by 25 percent when the pseudostem and rachis had been exploded at 180 degrees Celsius.

That winning combination, pairing municipal organic waste with pseudostem and rachis both pretreated at 180 degrees Celsius, produced the highest methane yield of the entire study: 457.5 litres of methane per kilogram of volatile solids. The authors translate that figure into practical terms as 201.85 kilowatt-hours of electrical energy per tonne of wet biomass, a number that begins to look meaningful when scaled to the millions of tonnes of banana residue generated each year in tropical producer nations. The yield also demonstrates an important point of principle: steam-exploded agricultural residues can be blended with urban organic waste without dragging down performance, and in the right proportions the mixture performs as a coherent, high-yielding feedstock rather than a dilution of the municipal waste’s potential.

The researchers are careful to frame their results appropriately. The methane yields reported come from laboratory-scale biochemical methane potential assays conducted under optimized batch conditions, which means they represent maximum biomethane potentials rather than predictions of what a full-scale industrial digester would deliver. Real plants operate continuously, face fluctuations in feedstock composition and must manage process stability over long periods. Still, the study establishes a technically viable strategy under controlled conditions and points the way toward pilot-scale validation.

The broader significance lies in what the approach could mean for waste policy and energy planning in the Global South. Colombia’s banana sector, like those of Ecuador, the Philippines and Costa Rica, generates enormous volumes of residues that carry no market value and often pose phytosanitary risks when left in the field. Meanwhile, cities across Latin America struggle with the organic fraction of their municipal waste, which dominates landfills and generates methane emissions as it decomposes uncontrolled. Co-digestion offers a way to address both problems at once: the municipal waste provides moisture, nutrients and buffering capacity that the lignocellulosic residues lack, while the residues add carbon and energy density that improve yields. The digestate left over at the end of the process can be returned to soils as fertilizer, closing the nutrient loop.

Steam explosion itself is a mature technology in other bioenergy contexts, notably second-generation ethanol production, but its application to banana residues within a co-digestion framework remains comparatively unexplored. The finding that a severity window around 180 degrees Celsius for ten minutes suits both the pseudostem and the rachis is operationally useful, suggesting that a single pretreatment regime could serve a mixed residue stream without requiring the two materials to be processed separately. That kind of simplification matters when the goal is a process that could plausibly be deployed at the scale of a regional biogas plant rather than a laboratory bench.

The work also feeds into a growing body of research on banana waste valorization. Recent studies have examined biomethane optimization from food waste and banana stems, anaerobic co-digestion of cow manure with banana waste, and biogas enhancement from banana stem juice with agro-industrial washings. What distinguishes the new study is its systematic comparison of two distinct banana residues, its exploration of pretreatment severity for each, and its demonstration of synergy within a three-substrate mixture anchored by municipal organic waste. The 25 percent boost in biodegradability attributable to steam explosion is a quantified measure of just how much value pretreatment can add when the severity window is properly calibrated.

For the biogas industry, the message is twofold. First, new feedstocks are essential if the sector is to grow beyond the food waste, manure and energy crops that currently dominate. Banana residues represent a vast, geographically concentrated and currently unused resource in precisely the tropical regions where energy demand is rising. Second, pretreatment technology is not optional for lignocellulosic substrates but a necessary investment, and the returns can be substantial when it is matched to the material. The study’s authors, led by corresponding author Juan Luis Ramos-Suárez of the University of La Laguna, alongside Diana Marcela Durán Hernández, Nely Carreras, Zulma Lorena Durán Hernández and Mario Enrique Velásquez Lozano, argue that this integrated approach supports the development of circular bioeconomy systems in regions with high availability of both waste streams.

There remain engineering and economic questions that laboratory assays cannot answer. Steam explosion is energy-intensive, and a full techno-economic analysis would need to weigh the energy cost of generating high-pressure steam against the incremental methane gained. Handling and logistics also matter: banana residues are dispersed across fields and plantations, and their high moisture content makes transport costly. And the inhibitory compounds generated under more severe pretreatment conditions would need monitoring at scale. But the study provides the essential proof of concept, with hard numbers, that banana waste and urban organic waste can be married productively with the help of the right pretreatment chemistry. In a world searching for ways to squeeze value from waste while cutting greenhouse gas emissions, turning banana stems and city garbage into renewable natural gas is an idea whose time may finally have arrived.