Resting Plaque pH — The Neutral Starting Point
Between meals, plaque fluid sits close to neutral pH.
- 6.7–7.2: Resting plaque pH (mouth at rest)
- 0.3–0.4: Unstimulated saliva flow (mL per minute)
- 10⁸/mg: Plaque bacteria density (biofilm dry weight)
- 5.5: Critical pH threshold (enamel dissolves below)
A biofilm in equilibrium
Resting plaque balances acid production with salivary buffering.
Saliva as the baseline buffer
Bicarbonate and phosphate hold plaque fluid near neutral.
Why the baseline matters
Every Stephan curve drop starts and ends near this resting value.
Sugar Intake Triggers Rapid Acid Production
Fermentable sugars feed bacteria that produce acid within minutes.
- <2 min: Acid onset delay (after sugar contact)
- Lactic acid: Dominant acid (from Streptococcus mutans)
- ~1 pH/min: Max drop rate (high-sugar exposure)
- Sucrose, glucose: Fastest triggers (readily fermentable sugars)
Glycolysis in the biofilm
Cariogenic bacteria ferment sugar into organic acids almost immediately.
Sugar type changes the curve
More fermentable sugar produces a steeper, deeper pH drop.
Frequency over quantity
Frequent small exposures matter more than one large one.
The pH Nadir — Plaque Acidity Peaks
Plaque pH plunges sharply below the critical threshold.
- 4.5–5.0: Typical nadir pH (high-sugar snack)
- 5–20 min: Time to nadir (after exposure)
- Sugar frequency: Nadir depth driver (not just total amount)
- Immediate: Recovery onset (once nadir is reached)
Curve shape defined by Stephan
Robert Stephan first plotted this rapid-drop, slow-recovery pattern in 1943.
Depth depends on the person
Buffering capacity and bacterial load shift the nadir up or down.
The turning point
Past the nadir, dilution and buffering start pulling pH back up.
Critical Zone — Enamel Demineralization Window
Below pH 5.5, enamel loses more mineral than it gains.
- pH 5.5: Demineralization threshold (hydroxyapatite dissolves)
- 20–40 min: Typical critical duration (per sugar exposure)
- >40 min/day: Daily critical minutes (raises caries risk)
- Calcium, phosphate: Ions lost from enamel (dissolved into plaque fluid)
Below the critical pH
Hydroxyapatite crystals dissolve faster than they reform.
Cumulative exposure adds up
Repeated critical windows across a day compound enamel loss.
A reversible window
Short critical periods can still be offset by later remineralization.
Recovery — Saliva Restores Baseline pH
Salivary buffering gradually lifts plaque pH back to normal.
- 30–60 min: Full recovery time (back to baseline pH)
- Bicarbonate, phosphate: Key buffering agents (delivered by saliva)
- Fluoride-assisted: Remineralization boost (reverses early lesions)
- Each bite resets curve: Snacking risk (exposures compound risk)
Buffering restores balance
Saliva neutralizes acid and clears sugars from the biofilm.
Remineralization follows
Calcium and phosphate redeposit into weakened enamel.
Why timing between snacks matters
Spacing exposures lets each Stephan curve fully recover.