Brewer's spent grain (BSG) is an undervalorized organic feedstock residue composed of fermentable macromolecules, such as proteins, starch, and residual soluble carbohydrates. It also contains at least 50% (as dry weight) of lignocellulose. Methane-arrested anaerobic digestion is one of the promising microbial technologies to valorize such complex organic feedstock into value-added metabolic intermediates, such as ethanol, H2, and short-chain carboxylates (SCC). Under specific fermentation conditions, these intermediates can be microbially transformed into medium-chain carboxylates through a chain elongation pathway. Medium-chain carboxylates are of great interest as they can be used as bio-based pesticides, food additives, or components of drug formulations. They can also be easily upgraded by classical organic chemistry into bio-based fuels and chemicals. This study investigates the production potential of medium-chain carboxylates driven by a mixed microbial culture in the presence of BSG as an organic substrate. Because the conversion of complex organic feedstock to medium-chain carboxylates is limited by the electron donor content, we assessed the supplementation of H2 in the headspace to improve the chain elongation yield and increase the production of medium-chain carboxylates. The supply of CO2 as a carbon source was tested as well. The additions of H2 alone, CO2 alone, and both H2 and CO2 were compared. The exogenous supply of H2 alone allowed CO2 produced during acidogenesis to be consumed and nearly doubled the medium-chain carboxylate production yield. The exogenous supply of CO2 alone inhibited the whole fermentation. The supplementation of both H2 and CO2 allowed a second elongation phase when the organic feedstock was exhausted, which increased the medium-chain carboxylate production by 285% compared to the N2 reference condition. Carbon- and electron-equivalent balances, and the stoichiometric ratio of 3 observed for the consumed H2/CO2, suggest an H2- and CO2-driven second elongation phase, converting SCC to medium-chain carboxylates without an organic electron donor. The thermodynamic assessment confirmed the feasibility of such elongation.

Residual banana bulbs (RBB) were characterized and assessed as a potential starch and cellulose-based feedstock for bioethanol production. To facilitate the enzymatic digestibility, hydrothermal pretreatment was performed on RBB prior to simultaneous saccharification and fermentation (SSF) with Saccharomyces cerevisiae . Composition of RBB was similar to traditional starch and cellulose-based feedstocks with high glucan (60 g/100 gDM) and relatively low lignin content (7 g/100 gDM). Both amylase and cellulase were needed to efficiently hydrolyze RBB. The highest ethanol yield (310 kg EtOH/ton_DM_RBB, 93% of theoretical production based on total available glucose) was obtained with non-pretreated RBB. SSF can be carried out at lower RBB concentrations. Hydrothermal pretreatment affected negatively the bioethanol potential due to the loss of fermentable carbohydrates. In a case study of an African leading producer of bananas and plantains (Cameroon), the energy derived from bioethanol was 80 GWh ethanol/year and corresponded to 1.6% of the annual transportation requirement. This study shows that RBB is a promising alternative feedstock for bioethanol production.

Spent coffee grounds mixed with wheat straw or miscanthus straws were used as substrates for oyster mushroom (Pleurotus ostreatus) production. The biomethane potential of spent mushroom composts was determined, and the influence of fungal digestion (FD) on the overall conversion balance of the substrates to valuable products, i.e. edible mushroom carpophores and biomethane (CH4) was investigated. Only 3–7% of the chemical oxygen demand (COD) of the initial substrates was recovered as harvested carpophores, while about 50% was recovered as CH4 by anaerobic digestion (AD) of the fungal compost. Anaerobic digestibility of the fungal compost after FD was very close to the anaerobic digestibility of the initial substrates, but the amount of matter was reduced by 17–22% as dry matter, 20% as volatile solids and 12–27% as COD, most probably by fungal respiration. As a consequence, the CH4 production per amount of initial substrate was lower. However, owing to the much higher economic value of edible carpophores as compared to CH4, a rough assessment of the economic balance was largely in favour of FD prior to AD. As compared to wheat straw, miscanthus straw presented both a lower final conversion to biomethane and a lower amount of harvested carpophores. Valorization through FD before AD is to be preferred as compared to direct biomethanation of the initial substrates.

Banana crop produces large quantities of post-harvest biomass wastes. Some of them are a potential resource of raw materials such as natural fibers, which can be used as reinforcement for composite materials. The recovery of fibers, after bioconversion of the more digestible soft tissues to biogas was assessed for peduncles of three banana varieties (Grande Naine (GN), Pelipita (PPT) and CRBP969). Fibers were sieved out from the digestate. Biogas was monitored manometrically and with gas chromatography. PPT peduncle produced both the highest fibers recovery (0.2g_DM_fiber/g_DM_initial_substrate) and methane production (260ml_CH4/g_COD_initial_substrate) after 74days of anaerobic digestion. This variety was the most suitable candidate to combine both fiber recovery and biomethane production through anaerobic digestion. GN peduncle fibers degraded in less than 20days. This variety was more convenient for biomethane production (around 210ml_CH4/g_COD_initial_substrate). The amount and the quality of recovered fibers strongly depended both on the duration of anaerobic digestion and the banana variety. This work showed that anaerobic digestion was an effective bioprocess alternative to mechanical decortication and biological retting processes for fiber extraction from banana peduncles biomass.

The biochemical methane potential (BMP) of peduncles, bulbs, and peels of three banana varieties (Grande Naine (GN; export dessert banana), Pelipita (PPTA; locally used plantain), and CRBP969 (phytopathogen resistant hybrid-plantain)) was investigated as an assessment of the bioconversion potential of these residues to renewable energy or biorefined chemicals. Biogas production was monitored manometrically for 132 days and its composition was analyzed using gas chromatography. The BMP ranged from 162 to 257 ml_CH4/g_DM for peduncles, from 228 to 304 ml_CH4/g_DM for bulbs, and from 208 to 303 ml_CH4/g_DM for green peels, with methane content of the biogas in the range 56 to 60 %. Bulbs and green peels showed bioconversion yields of 95 % of the chemical oxygen demand (COD). The GN variety was generally more biodigestible than PPTA, which appeared richer in lignocellulosic fibres. The peels biodigestibility reduced with maturation and was already limited to 56 % of the COD at the yellow stage. The energy resource available in the residues of banana production is very significant, increasing by 91 % the energy resource offered by banana crop, which is generally limited to the nutritional value of the fruit pulp. In the study case of the African leading producer of bananas and plantains (Cameroon), the amount of available residues from the sole export variety GN could feed about 4 % of the annual electricity consumed by the country, i.e., a supply of electricity to an additional 9 × 105 people. Such valorization of the residual banana biomass could help banana-producing countries to become less dependent on fossil fuels and less prone to energy shortages.

Background: The by-products of banana and plantain shrubs, especially banana blossom (banana male bud) are usually thrown away by producers in plantations, and produce important quantities of post harvest waste.The nutritional composition of three varieties of banana and plantain blossoms grown in Cameroon; dessert banana (Musa AAA), plantain (Musa AAB) and cooking banana (Musa ABB), was assessed for their potential applications.Methods: The contents in water, ash, lipids, protids, carbohydrates and crude fibres were determined using standard A.O.A.C methods.The sugar levels were assayed using colorimetric methods, dietary fibres by enzymatic digestion, amino acids by HPLC, fatty acids by gas chromatography and minerals by atomic absorption spectrophotometry.Results: The results showed that the water content varies from 92.29 (AAA) to 93.73 % F.W. (AAB).The ash content varies from 9.88 (AAB) to 12.25 % D.W. (AAA).The lipid content varies from 4.95 (AAA) to 15.69 % D.W. (ABB) and contains polyunsaturated fatty acids (33.80 -41.50 g/100g FM), particularly linoleic and gamma linolenic acids.The total protein content varies from 8.89 (AAB) to 10.35 % D.W. (AAA).Leucine, phenylalanine + tyrosine, valine, lysine and threonine are the main essential amino acids (2.75 -3.30 %).The total carbohydrate content varies from 22.36 (AAA) to 62.19 % D.W. (AAB) and glucose, fructose and sucrose are dominant in the AAB genotype with values of (8.15; 3.04 and 1.35 % respectively).The AAA genotype is rich in total dietary fibres (50.09 %).These banana and plantain flowers are rich in macrominerals.Potassium, calcium, magnesium and phosphorus levels are high in the AAA genotype, with values of (6480; 687; 273; 211 mg/100g D.W., respectively).Conclusions: These blossoms could be considered as a source of dietary fibres for the control of obesity and diabetes.Further investigations on the composition and the physiological functions (using animal-feeding experiments) of these dietary fibres are to be considered.