This dataset supports the publication "Carbohydrate concentration and type drive product selectivity to a mixture of volatile fatty acids or lactic acid in thermophilic mixed-culture fermentation" and contains detailed reactor operation and analytical data from the thermophilic mixed-culture fermentation reactors. It includes the results of chemical analysis such as total and volatile suspended solids, chemical oxigen demand, substrate concentration and quantified fermentation products (volatile fatty acids, lactic acid, gas).

METHODOLOGICAL INFORMATION

1. Description of methods used for collection-generation of data:

All methods used for data generation are described in Vulart, L., Izcara, N., Estévez, Á. et al. Carbohydrate concentration and type drive product selectivity to a mixture of volatile fatty acids or lactic acid in thermophilic mixed-culture fermentation. Appl Microbiol Biotechnol (2026). https://doi.org/10.1007/s00253-026-13806-0 Influent samples (medium) were taken once a week to measure the soluble chemical oxygen demand (COD). Effluent and gas samples from each of the three reactors were taken two/three times per week to determine total suspended solids (TSS) and volatile suspended solids (VSS), soluble chemical oxygen demand, and main fermentation products. Effluent samples were centrifuged for 20 min at 7197 rcf, the supernatant was then filtered through a 0.20 μm Chromafil Xtra syringe filters (MACHEREY-NAGEL, Düren, Germany) and stored at -20°C until analysis. Pellet was resuspended appropriately to analyze TSS and VSS. TSS and VSS were quantified based on the weight difference of a sample before and after drying at 105°C and 550°C, respectively, in accordance with Standard Method 2540. Soluble COD was measured using Nanocolor test kits (MACHEREY-NAGEL, Düren, Germany). Lactic and acetic acid were analyzed by ion chromatography (IC) using a 930 Compact IC Flex system (Metrohm, Herisau, Switzerland) with Metrohm CO2 Suppression and an 850 IC conductivity detector, and equipped with a Metrosep Organic Acids 250/7.8 column and a Metrosep Organic Acids Guard/4.6 guard column. C3 to C6 carboxylic acids were determined using gas chromatography (GC) on a Shimadzu GC-2014 system (Kyoto, Japan) equipped with a DB-FFAP 123-3232 column (30 m x 0.32 mm x 0.25 μm) (Agilent, Santa Clara, USA) and a flame ionization detector (Shimadzu, Kyoto, Japan). The carrier gas was N2 at a flow rate of 2.49 mL·min-1. Prior to analysis, samples were treated with sulfuric acid and sodium chloride, followed by extraction with diethyl ether. 2-methyl hexanoic acid was added as an internal standard during extraction. Ethanol, glucose and xylose were quantified by HPLC using a Shimadzu LC-2030 i-Series system (Kyoto, Japan), equipped with a Aminex HPC-87H column (Bio-rad, Hercules, USA), a Micro-Guard Cation H Cartridge guard column (Bio-rad, Hercules, USA), a SPD-40/40V UV/Vis detector (Shimadzu, Kyoto, Japan), and a RID-20A refractive index detector (Shimadzu, Kyoto, Japan). Gas composition in the reactor headspace was analyzed by GC using a Compact GC 4.0 (Global Analyser Solutions, Breda, The Netherlands), equipped with a Molsieve 5A pre column and Porabond Q column (CH4, O2, H2 and N2), and a Rt-Q-Bond pre column and column (CO2). Gas concentrations were determined with a thermal conductivity detector. The carrier gases used were N2 and He.

1. Methods for processing the data: Not applicable

2. Instrument- or software- specific information needed to interpret the data: MS Excel LibreOffice

3. Instruments, calibration and standards information: For the information related to instruments see section "1. Description of methods used for collection-generation of data".

4. Environmental or experimental conditions: Fermentation experiments were conducted in three jacketed reactors with a working volume of 1 L. All reactors were run under anaerobic and thermophilic conditions with an operational temperature of 55°C. The pH was set to 5.3 and maintained using an automatic pH control system that dosed 2 M NaOH as needed. Both HRT and SRT were maintained at 4 days.