Subsurface Injection of Distillation Tail Liquor at the Acidogenesis-to-Esterification Transition and Its Effects on Ester Profiles in Strong-Flavor Baijiu

In strong-flavor Baijiu brewing, surface spraying of recycled distillation tail liquor (TL) leads to volatile aroma loss, uneven substrate distribution and localized fermentation inhibition. A patented telescopic subsurface injector was adopted to deliver 0–30 kg TL per pit at a 50 cm depth on fermentation day 30, the critical transition point between acidogenesis and esterification. At the highest dosage (30 kg), total esters rose 14.8% (from 4.45 to 5.11 g/L), with ethyl hexanoate up 52.2% (to 1.72 g/L) and ethyl lactate up 69.8% (to 3.82 g/L). Fermentation temperature curves stayed unchanged, and grain-derived ethanol yield remained near 38.2% in all groups after correcting for the ethanol already present in the added TL. Subsurface injection of tail liquor-supplying ethanol and organic acid precursors-at the acidogenesis-to-esterification transition enhances ester synthesis without disrupting fermentation or reducing distillate yield. Acid profiles were also stable, suggesting that the added substrates were channeled into ester synthesis rather than acid accumulation. Targeted subsurface TL injection at this metabolic transition thus represents an industrially feasible strategy to boost ester biosynthesis, offering a recyclable TL valorization approach and verifying substrate-limited esterification in solid-state Baijiu fermentation.