nutrition science: scientific basics

pH, pH value, de.: pH-Wert, fr.: potentiel hydrogène, it.: pondus hydrogenii, es.: potencial hidrógeno

pH or pH-value is a measure of the acidity or alkalinity of solutions. For foods and the habitat of biological systems aqueous solutions apply. PH states the concentration of H³O₊ or so called hydronium ions in a solution. More specifically: pH is an inverse logarithmic representation of the activity of hydronium ions. For very dilute aqueous solutions this activity is approximately the same as the concentration of hydronium ions, measured in mol/liter. The term pH means potential of hydrogen and is derived from the latin pondus Hydrogenii or potentia Hydrogenii (pondus = weight; potentia = power; hydrogenium = hydrogen). The pH-scale commonly used ranges from 0 to 14. Smaller or larger values are possible. A solution of 10 mol sodium hydroxid per 1 litre water has for example a pH-value of 15. A strong acid has a pH of 0, pH 14 is a strong base, a solution between pH 6.6 and 7.5 is neutral. Distilled water (at 25°C) and blood have a neutral pH.

pH and its significance to foods

The microbial spoilage of foods depends among others strongly on the pH of the product. Most microorganisms thrive when the pH of their surroundings is neutral (pH 6.6 to 7.5). The metabolism of these microorganisms is then highest and they can multiply fast. This is especially the case, when protein-rich and almost neutral foods like meat, fish, eggs or products made from these foods are stored to warm.

Buffer capacity and spoilage of foods

During the spoilage of foods the so-called buffer capacity plays an important role. Buffer capacity is a chemical term. It describes the amount of acids or bases a substance can receive without considerably changing its pH. In foods the available protein is crucial to the buffer capacity. Foods of animal origin are rich in protein and have a high buffer capacity, foods derived from vegetables are low in protein and have little buffer capacity. The more protein a food contains the greater is its buffer capacity. The higher the buffer capacity of a food is, the longer it takes for the pH to change so far that bacteria cannot multiply anymore.

Acids are formed during food maturation (e.g. maturing of meat) and storage of foods, which leads to a slow decline in pH in foods that are low in protein. This makes good living conditions for bacteria and the foods spoil easily.

pH-value of foods

Where the pH is very high or low, metabolism of bacteria and microorganisms is reduced drastically. Most bacteria can survive pH-values as low as 4.4 and as high as 9. Below and above these levels only specialised bacteria survive. Lactic acid bacteria for example multiply at pH 3. Bacteria living in acidic environments do need a specific acid to survive in though. Lactic acid bacteria for example survive lactic acid and acetic acid better than hydrochloric acid, citric acid, tartaric acid or phosphoric acid. Yeast and moulds may survive pH-values as low as 1.5, depending on the acid.

pH table of different foods

The pH-value of the below listed foods is a medium. It is subject to natural fluctuations.

• baking powder pH 8,5
• protein pH 7,8
• milk pH 6,4
• sour milk pH 5
• sauerkraut pH 4
• cola drinks pH 4
• vinegar pH 3
• lemonade pH 3
• apples pH 3,2
• lemon juice pH 2,3

Drinking water should have a pH between 6.5 pH and 9.5 pH. For comparison: Gastric acid has a pH between 1.2 and 3.