FOOD ANALYSIS : ACIDITY AND PH
How strong the acid is
Hydrogen ions, read on a meter
How much acid is present
Alkali used, read from a burette
A juice can hold a large amount of a weak acid and still show a mild pH.
Figure 1: pH and titratable acidity answer two different questions about the same food. Both numbers are needed.
Introduction
pH and titratable acidity are the two acidity tests every food laboratory runs, and they measure different things. pH measures the strength of the acid, the concentration of hydrogen ions, and it is read on a meter with a glass electrode. Titratable acidity measures the total amount of acid present, and it is found by titration with standard alkali until an indicator changes colour. The two are related but not the same: a juice can hold a large amount of a weak acid and still show a mild pH, while a small amount of a strong acid gives a low pH.
Quick comparison
Table 1: pH compared with titratable acidity
| Aspect | pH | Titratable acidity |
|---|---|---|
| What it measures | Strength of the acid: hydrogen ion concentration | Total amount of acid present |
| How it is measured | Glass electrode and pH meter | Titration with standard alkali, usually sodium hydroxide, NaOH |
| Indicator | None: the meter reads voltage | Phenolphthalein: a faint pink colour that stays marks the end point |
| Unit | pH units, 0 to 14, 7 neutral | Percent of a named acid, such as lactic acid for milk |
| What it tells | Safety: microbes respond to pH | Taste and keeping quality: the tongue responds to total acid |
| Safety line | Below pH 4.6 most harmful bacteria cannot grow | No single safety line: it must be read with the pH |
| Calibration | Buffer solutions of known pH, usually pH 4 and pH 7 | Standard alkali of known strength |
| Temperature | Recorded with the reading, because pH changes with temperature | Not part of the reading |
| Example | Pickles near pH 3.8, milk near 6.6, egg white near 9 | Fresh milk about 0.14 to 0.18 percent as lactic acid |
Scientific explanation
Definition
pH: a scale for the strength of an acid, from 0 to 14, with 7 neutral and lower numbers more acidic, defined as minus the logarithm of the hydrogen ion concentration: pH = minus log₁₀[H⁺]. Titratable acidity: the total acid in a food, measured by titration with standard alkali and reported as a named acid.
Working principle
pH: the glass electrode measures pH through voltage. The voltage changes by about 59.16 millivolts for each pH unit at 25 °C, a value that comes from the Nernst equation for the hydrogen ion. The meter turns that voltage into a pH number. Each whole pH unit is a tenfold change in hydrogen ions: pH 3 holds ten times as many hydrogen ions as pH 4, and a hundred times as many as pH 5. Titratable acidity: a measured amount of sample is placed in a flask, a few drops of phenolphthalein indicator are added, and the alkali is added from a burette, drop by drop, until a faint pink colour stays. The volume of alkali used tells how much acid was present.
Apparatus
pH: glass electrode, reference electrode, pH meter, and buffer solutions of known pH for calibration. Titratable acidity: flask, burette, standard sodium hydroxide solution, and phenolphthalein indicator.
Measurement procedure
pH: calibrate the meter with two buffers, usually pH 4 and pH 7, to fix its zero and its slope. Rinse the electrode between samples, place it in the sample, read the pH, and record the temperature with the reading. Titratable acidity: measure the sample into a flask, add indicator, add standard alkali from the burette until the faint pink colour stays, and note the volume used. Report the result as a named acid, because different foods hold different acids: milk acidity is reported as lactic acid, fruit juice acidity as citric acid, and vinegar acidity as acetic acid.
Factors affecting readings
pH: temperature changes the reading, so the temperature is recorded with it. A meter calibrated with one buffer only can drift, which is why two buffers are used. A dirty or dry electrode reads slowly and wrongly. Titratable acidity: the wrong end point, read too early or too late, changes the volume and the result. The acid must be named, because the same titration volume means a different percentage for each acid. As milk sours, bacteria turn lactose into lactic acid, so a rising acidity is the first chemical sign that milk is souring, before the smell gives it away.
Formulas and calculations
pH = minus log₁₀[H⁺]. Pure water has a hydrogen ion concentration of 0.0000001 moles per litre, so pH = minus (minus 7) = 7, which is neutral. A solution with 0.0001 moles per litre gives pH 4, near the range of tomato juice.
Worked example from the FTN Food Analysis notes, titratable acidity: 10 mL of milk needs 1.8 mL of 0.1N NaOH. Each millilitre of 0.1N NaOH neutralises 0.009 g of lactic acid, so the milk holds 1.8 times 0.009 = 0.0162 g of lactic acid in 10 mL, which is 0.162 percent. Fresh milk is usually near 0.14 to 0.18 percent, so this milk is still fresh.
Industrial applications
- Pickle, sauce, and canned acid food makers measure pH on every batch, because below pH 4.6 the bacteria that cause the most serious food poisoning cannot grow.
- Dairies track milk acidity, because a rising value shows the milk is souring before the smell gives it away.
- Juice makers report acidity as citric acid and vinegar makers as acetic acid, so buyers can compare lots.
- Fermented food makers follow both numbers: pH for safety and titratable acidity for flavour.
Advantages and limitations
pH: it is fast, it needs only a small sample, and it is the safety line for acid foods. Its limit is that it says nothing about how much acid is present, so it cannot predict sour taste on its own.
Titratable acidity: it measures the total acid, which guides taste and keeping quality, and it needs only simple glassware. Its limit is that it is slower, it needs a clear end point, and it cannot guide safety on its own: a food can taste mildly sour and still sit above the pH safety line.
FAQ
Question: A sauce reads pH 4.4 and tastes only mildly sour. Which number guides safety, and which guides taste?
The pH guides safety, because microbes respond to pH. The titratable acidity guides taste, because the tongue responds to the total acid. Both are reported.
Question: Why calibrate the pH meter with two buffers instead of one?
Two buffers fix both the zero and the slope of the meter. One buffer alone cannot do both, so the readings can drift.
Question: Two sauces read pH 3.5 and pH 4.5. How many times more hydrogen ions does the first hold?
Ten times more. Each whole pH unit is a tenfold change, and the two sauces differ by one unit.
Question: Why is milk acidity reported as lactic acid rather than just acid?
Because the acid in souring milk is mainly lactic acid, made by bacteria from lactose. The same titration volume would mean a different percentage for citric acid or acetic acid, so the acid must be named.
Question: A milk sample reads 0.25 percent acidity. What does that suggest?
The milk has started to sour: bacteria have turned lactose into lactic acid and the acidity has risen above the fresh range. It should not be accepted as fresh milk.
References and further reading
- FTN Food Analysis, Unit 7: Acidity, pH, safety testing and analytical data (notes on disk): titratable acidity, pH scale and safety line 4.6, worked examples pH 4 and 0.162 percent lactic acid.
- Manual for Analysis of Milk and Milk Products, FSSAI, 2012 (book extract on disk, milk-analysis-manual.txt): milk acidity test with 0.1N sodium hydroxide, fresh milk 0.14 to 0.18 percent as lactic acid.
- Nielsen, S. S., Food Analysis Laboratory Manual, Springer: standard solutions and titratable acidity (consulted through Google Books).
