Acidic Foods, Tooth Enamel and pH

Tooth enamel is the hardest substance in the human body, harder than bone. It can last a lifetime of chewing. Yet it can be dissolved by something as ordinary as orange juice or a fizzy drink. The reason is simple acid–base chemistry, and understanding it explains most of the standard dental advice you’ve ever heard.

What enamel is made of

Enamel is about 96% mineral by weight. The mineral is hydroxyapatite, a form of calcium phosphate:

Ca₁₀(PO₄)₆(OH)₂

It’s arranged in tightly packed rods of tiny crystals. The remaining few percent is water and protein. Underneath enamel is dentine, which is softer and more easily dissolved.

How acid dissolves enamel

Hydroxyapatite contains two basic ions: phosphate (PO₄³⁻) and hydroxide (OH⁻). In water, a tiny amount of the mineral is always dissolving and re-forming:

Ca₁₀(PO₄)₆(OH)₂(s) ⇌ 10Ca²⁺ + 6PO₄³⁻ + 2OH⁻

Normally, saliva is saturated with calcium and phosphate ions, so this equilibrium sits almost entirely on the left and enamel is stable.

When acid is added, H⁺ ions react with the phosphate and hydroxide ions:

• H⁺ + OH⁻ → H₂O
• H⁺ + PO₄³⁻ → HPO₄²⁻ (and further to H₂PO₄⁻)

This removes those ions from the right-hand side, so by Le Chatelier’s principle the equilibrium shifts right and more enamel dissolves. This process is called demineralisation.

The critical pH: 5.5

Dentists often quote a critical pH of about 5.5 for enamel. Below this pH, saliva and plaque fluid become undersaturated in hydroxyapatite, and enamel starts to dissolve. (The exact value depends on how much calcium and phosphate is in the saliva, so it varies between people, typically from about 5.2 to 5.7.)

Dentine and exposed root surfaces dissolve at a higher pH, around 6.0–6.5, which is why they’re more vulnerable once gums recede.

Two routes to acid attack

1. Tooth decay (caries): acid from bacteria

Bacteria in dental plaque, especially Streptococcus mutans, feed on sugars. They ferment them into acids, mainly lactic acid:

C₆H₁₂O₆ → 2CH₃CH(OH)COOH

After eating sugar, plaque pH can fall from about 7 to below 5 within minutes. It stays below the critical pH for a while, then slowly recovers as saliva neutralises the acid. This up-and-down pattern is called the Stephan curve, after the dental researcher who measured it in the 1940s.

Decay starts beneath the plaque, often as a chalky white spot, and can develop into a cavity if the attacks happen too often.

2. Dental erosion: acid from food and drink

Erosion is caused directly by acids in what we eat and drink, without bacteria involved. It tends to wear away smooth surfaces of teeth broadly, rather than forming cavities.

Food or drink	Typical pH	Main acid
Lemon juice	2.0–2.6	citric acid
Cola	2.5–3.0	phosphoric acid
Sports and energy drinks	2.5–3.5	citric acid
Fruit juices	3.0–4.0	citric, malic acids
Wine	3.0–4.0	tartaric, malic acids
Vinegar-based dressings, pickles	2.5–3.5	ethanoic acid
Sparkling water (plain)	4.5–5.5	carbonic acid
Still water, milk	6.5–7	little or none

Stomach acid is even more erosive (pH about 1.5–2). People with frequent reflux or vomiting can develop significant erosion, especially on the inner surfaces of their teeth.

pH isn’t the whole story. Titratable acidity, how much base it takes to neutralise the drink, matters too, because a well-buffered acidic drink keeps attacking for longer. Citric acid is especially damaging because citrate ions also bind calcium, pulling it out of enamel. Plain sparkling water is only weakly acidic and much less erosive than fruit-flavoured sparkling drinks, which usually contain added citric acid.

Saliva: the body’s repair system

Saliva defends teeth in several ways:

• Buffering. It contains hydrogencarbonate, phosphate and proteins that neutralise acid. Stimulated saliva (for example, when chewing) has much more hydrogencarbonate.
• Supersaturation. It carries calcium and phosphate ions. Once pH rises above the critical value, these ions redeposit on the enamel: remineralisation.
• Washing. It dilutes and clears acids and sugars from the mouth.
• A protective film. Proteins from saliva form a thin coating on teeth (the pellicle) that slows acid attack.

Tooth health depends on the balance between demineralisation and remineralisation over a day. It’s not just what you eat, but how often.

How fluoride helps

Fluoride ions can replace some of the hydroxide ions in hydroxyapatite, forming fluorapatite or fluoridated hydroxyapatite:

Ca₁₀(PO₄)₆F₂

Fluorapatite is less soluble in acid: its critical pH is about 4.5, around a whole pH unit lower than hydroxyapatite. The fluoride ion (F⁻) is a much weaker base than the hydroxide ion it replaces, so it doesn’t react with H⁺ so readily. Fluoride also speeds up remineralisation and interferes with bacterial acid production.

That’s why fluoride toothpaste, fluoridated water in some regions and professional fluoride treatments are so effective at reducing decay.

Chemistry-based tips for protecting teeth

1. Reduce how often, not just how much. Every sugary or acidic snack starts a new acid attack. Keeping sugar to mealtimes reduces total time below pH 5.5.
2. Don’t brush straight after acidic food or drink. Enamel is temporarily softened. Wait about 30–60 minutes to let saliva start remineralising.
3. Rinse with water or eat something neutral (cheese is a classic choice: it’s rich in calcium and phosphate and stimulates saliva) after acidic foods.
4. Use a straw for acidic drinks to reduce contact with the front teeth.
5. Chew sugar-free gum to stimulate saliva flow.
6. Use fluoride toothpaste and spit rather than rinse after brushing, leaving fluoride on the teeth.
7. Sip water, not fruit juice or sports drinks, throughout the day.

An experiment you can do: eggshells as model teeth

Eggshells are mostly calcium carbonate rather than calcium phosphate, but they behave similarly with acids and make a vivid model.

1. Place pieces of eggshell (or whole eggs) in cups of water, vinegar, cola and orange juice.
2. Leave for 24 hours and observe. In vinegar, you’ll see bubbles of carbon dioxide and the shell will soften and thin.
3. For a fluoride comparison, soak one egg in fluoride mouthwash for a few hours first, then place it in vinegar alongside an untreated egg. Many classroom versions show slower dissolving for the treated egg.

This works because CaCO₃ + 2H⁺ → Ca²⁺ + H₂O + CO₂. Wear eye protection and don’t drink the test solutions.

Common misconceptions

• “Sugar itself rots teeth.” Sugar is harmless to enamel directly; bacteria turn it into acid.
• “Diet drinks are safe for teeth.” They contain no sugar, so they don’t feed bacteria, but most are still acidic and can cause erosion.
• “Brushing harder right after eating removes acid.” It can remove softened enamel instead.
• “Enamel damage can always be repaired.” Early demineralisation can reverse; lost enamel does not regrow.

Key takeaways

• Enamel is hydroxyapatite, which dissolves when acid removes its phosphate and hydroxide ions.
• Below about pH 5.5, enamel demineralises; above it, saliva helps remineralise it.
• Decay comes from bacterial acid after sugar; erosion comes from acids in food, drink and reflux.
• Fluoride forms less soluble fluorapatite, lowering the critical pH to about 4.5.
• Frequency of acid exposure matters as much as quantity.
• See how pH varies across the body in pH inside the body.