Symmetric Evaporation and Entropy Dilution of the Baryon Excess between 50 and 30 MeV

We quantify how thermal energy injection with no net baryon-number source changes a homogeneous expanding plasma between 50 and 30 MeV. A hypothetical population of nonrotating black holes is coupled to a thermal bath and an effective baryon-antibaryon production and annihilation equation. For an initial black-hole mass of 10^8 g and initial black-hole-to-plasma energy-density ratios f = 0, 0.01, 0.1, and 1, the reduced bath yields comoving plasma-entropy growth factors of 1.0000, 1.0109, 1.1093, and 2.0798. Net baryon number per entropy decreases inversely; the reduction is 51.92% for f = 1. With a common initial excess, the crossing n_antiB / (n_B − n_antiB) = 1 shifts from 38.205 to 37.974 MeV. With the initial excess adjusted to a common final yield, it instead shifts from 38.204 to 39.263 MeV. The complete PDG2025 hadron resonance gas gives 39.269 MeV for the latter f = 1 case. A local mass scan identifies a reheating trajectory with three crossings. Separate equilibrium calculations impose charge and strangeness neutrality with vanishing net lepton numbers; these constraints are not evolved in a coupled kinetic network. A repeated execution reproduced all 36 compared CSV files. The study quantifies the consequences of specified energy deposition and boundary conditions; the final baryon yield is an input constraint, and convergence tests do not determine total physical uncertainty.