pH in Cooking
pH is a scale from 0 to 14 that measures how acidic or alkaline something is: below 7 is acidic, 7 is neutral, and above 7 is alkaline (basic). In the kitchen, pH quietly controls how fast food browns, what colour vegetables turn, how firm they stay, how proteins set, how baked goods rise, and whether preserved foods are safe.
The scale is logarithmic. Each step is ten times stronger, so lemon juice at pH 2 is about 100 times more acidic than tomato at pH 4.
pH of common foods
| Food | Approx. pH |
|---|---|
| Lemon and lime juice | 2.0–2.6 |
| Vinegar | 2.4–3.4 |
| Finished sauerkraut | 3.4–3.6 |
| Yogurt | 4.0–4.6 |
| Tomatoes | 4.2–4.9 |
| Bread dough (yeasted) | 5.0–6.0 |
| Milk | 6.5–6.8 |
| Fresh egg white | 7.6, rising to ~9 with age |
| Baking soda in water | ~8.3 |
Browning
The Maillard reaction speeds up dramatically as pH rises. That's why:
- A pinch of baking soda makes onions caramelize faster.
- Pretzels get their deep mahogany crust from an alkaline dip.
- Acidic marinades brown more slowly on the grill.
Acid also slows enzymatic browning, which is how acidulated water keeps apples pale.
Vegetable colour
- Green vegetables: Acid turns chlorophyll olive drab. Alkaline water keeps them vivid but turns them mushy.
- Red cabbage and blueberries: Their anthocyanin pigments are red in acid and blue, green, or grey in alkaline conditions.
- Cauliflower and onions: Their pale pigments stay white in acid and turn yellowish in alkaline water.
Vegetable and legume texture
Acid strengthens pectin in plant cell walls, keeping vegetables and beans firm. That's why beans simmered in tomato sauce can stay stubbornly hard. Alkaline conditions break pectin down; a little baking soda softens chickpeas for silky hummus.
Proteins
Acid tightens and sets proteins. It curdles milk into paneer, firms fish in ceviche, and stabilizes whipped egg whites, which is the job of cream of tartar. See protein denaturation.
Leavening
Baking soda needs acid to release carbon dioxide. Recipes balance acidic ingredients like buttermilk, yogurt, or natural cocoa with soda; leftover soda raises pH, browning the crumb and leaving a soapy taste.
Food safety
pH 4.6 is the key safety line. At or below it, Clostridium botulinum can't produce its toxin. High-acid foods like fruit jams and pickles can be water-bath canned; low-acid foods such as plain vegetables, meat, and beans must be pressure canned. Fermentation preserves food by driving pH well below that line.
Questions
Why does baking soda make onions brown faster?
Baking soda raises the pH, and the Maillard reaction runs much faster in alkaline conditions. A pinch per large pan of onions speeds browning, though too much makes them mushy and soapy-tasting.
Why do green vegetables turn olive in acid?
Acid replaces the magnesium in chlorophyll with hydrogen, turning bright green into dull olive. Add lemon juice or vinegar to green vegetables just before serving, not during cooking.
What pH is considered safe for canning?
Foods at pH 4.6 or lower are classified as high-acid and can be safely canned in a boiling water bath. Foods above 4.6 need pressure canning to prevent botulism.
Why are old eggs easier to peel?
As eggs age, carbon dioxide escapes through the shell and the white's pH rises from about 7.6 to around 9. The more alkaline white sticks less to the inner membrane, so it peels cleaner.