Association of aqueous hydrogen concentration with methane production in continuous cultures modulated to vary pH and solids passage rate.

The objective of this study was to evaluate the effects of altering pH and solids passage rate (k) on concentration of aqueous H [H(aq)], CH production, volatile fatty acids (VFA) production, and fiber digestibility in a continuous culture fermentation system. The present study was conducted as a 2 × 2 factorial treatment arrangement in a Latin square design using continuous culture fermentors (n = 4). Our continuous culture system was converted to a closed system to measure CH and H emission while measuring H(aq) concentration and VFA production for complete stoichiometric assessment of fermentation pattern. Treatments were control pH (CpH; ranging from 6.3 to 6.9) or low pH (LpH; 5.8 to 6.4) factorialized with solids k that was adjusted to be either low (Lk; 2.5%/h) or high (Hk; 5.0%/h); liquid dilution was maintained at 7.0%/h. Fermentors were fed once daily (40 g of dry matter; 50:50 concentrate:forage diet). Four periods lasted 10 d each, with 3 d of sample collection. The main effect of LpH increased nonammonia nitrogen flow, and both LpH and Hk increased nonammonia nonbacterial N flow. We observed a tendency for Hk to increase bacterial N flow per unit of nonstructural carbohydrates and neutral detergent fiber degraded. The main effect of LpH decreased H(aq) by 4.33 µM compared with CpH. The main effect of LpH decreased CH production rate from 5 to 9 h postfeeding, and Hk decreased CH production rate from 3 to 9 h postfeeding. We found no effect of LpH on daily CH production or CH produced per gram of neutral detergent fiber degraded, but Hk decreased daily CH production by 33.2%. Both the main effects of LpH and Hk decreased acetate molar percentage compared with CpH and Lk, respectively. The main effect of both LpH and Hk increased propionate molar percentage, decreasing acetate-to-propionate ratio from 2.62 to 2.34. We noted no treatment effects on butyrate molar percentage or total VFA production. The results indicate increasing k and decreasing pH decreased acetate-to-propionate ratio, but only increasing k decreased CH production; lack of differences for LpH might be a result of compensatory methanogenesis during the second half of the day postfeeding.