Baseline Equilibrium of Enamel Minerals
Enamel is never static — ions leave and return every minute of every day.
- ~6.7–7.2: Resting biofilm pH (healthy plaque fluid)
- 5.5: Critical pH (demin exceeds remin below this)
- Hydroxyapatite: Enamel mineral (Ca₁₀(PO₄)₆(OH)₂)
- Thousands: Ion exchanges per day (per enamel surface)
A dynamic, not a static, surface
Enamel constantly trades ions with saliva.
Two opposing fluxes
Demineralization and remineralization run simultaneously, always.
Why equilibrium matters
Net zero flux means no cavity, no extra mineral gain.
Biofilm Acid Drops Below the Critical Threshold
Sugar-fed bacteria excrete acid, and pH plunges under enamel's tipping point.
- 5.5: Critical pH threshold (below this, enamel dissolves)
- ~2–5 min: Acid onset time (after fermentable sugar exposure)
- Lactic, acetic: Main acids produced (bacterial fermentation)
- 4.0–5.0: Typical challenge pH (peak acidity)
Where the acid comes from
Streptococcus mutans ferments sugars into organic acids.
Crossing the critical pH
Below pH 5.5, hydroxyapatite solubility rises sharply.
The balance tips
Demineralization rate now outruns remineralization rate.
Calcium and Phosphate Exit the Crystal Lattice
Hydrogen ions attack the lattice surface and pull mineral ions into solution.
- Ca²⁺ + PO₄³⁻: Ions dissolved per event (lattice unit breakdown)
- First sign: Subsurface lesion (of early caries)
- Yes: Reversible window (before cavitation)
- H⁺ concentration: Driving gradient (surface vs. lattice)
H⁺ attacks the crystal surface
Hydrogen ions replace calcium at the lattice edge.
Subsurface porosity forms
Repeated attacks widen microscopic channels in enamel.
Still reversible
No cavity yet — mineral can still redeposit later.
Salivary Bicarbonate Neutralizes the Acid
Saliva floods the biofilm with buffer, and pH climbs back toward neutral.
- HCO₃⁻: Main buffer (bicarbonate ion)
- Up to 10×: Saliva flow increase (when stimulated)
- 20–40 min: Recovery time to pH 7 (typical after a sugar exposure)
- Higher when flow is high: Buffer capacity (stimulated > resting saliva)
Bicarbonate mops up acid
HCO₃⁻ combines with H⁺ to form water and CO₂.
Flow rate matters
More saliva flow means faster acid clearance.
pH crosses back above 5.5
Demineralization rate falls as buffering proceeds.
Ions Redeposit and Balance Is Restored
Calcium and phosphate migrate back into the lattice, closing the porosity.
- >5.5: Remineralization pH (favorable above critical pH)
- Fluorapatite: Fluoride benefit (more acid-resistant crystal)
- Hours: Time to net-positive (after buffering completes)
- Many: Full cycles per day (each meal or snack)
Ions return to the lattice
Calcium and phosphate re-bind at exposed crystal sites.
Fluoride upgrades the repair
Fluorapatite forms, more resistant to future acid.
Net balance returns to zero
The tug-of-war resets until the next acid challenge.