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A pH meter can measure the acidity and alkalinity of a substance.

pH

Also known as measure of Acidity and Alkalinity


What is pH?

pH is a key quality indicator in the food industry, influencing things such as shelf life and quality. It stands for “potential of hydrogen,” and is the degree (intensity) of the acidity or alkalinity of a given product.1

In the baking industry, pH helps food scientists and technologists know how to better:

  • mix different types of dry and wet ingredients;
  • balance flavors;
  • determine the exact final point of a given formulation and/or process

This concept should not be confused with the amount of acid (titratable acidity) present in the food.2

How it works

pH is based on the fact that molecules (acids and bases in aqueous solutions) disperse throughout the liquid, split up (dissociate), and expose their free hydronium [H3O+] and hydroxide ions [OH−].1

It is represented on a scale from 0  to 14. A pH value of 0 corresponds to an extremely acidic product, and a value of 14 corresponds to an extremely alkaline product. The pH of most bakery products ranges from 3.5 to 8.0.

Application

The acidity or pH of foods is important for several reasons:

  1. Certain pathogenic bacteria (those responsible for foodborne illnesses) cannot survive in acidic environments. In such cases, acidity plays a vital role as a preservation or inhibition mechanism, as in the case of sourdoughs.
  2. Low pH values in products (4.0–5.5) inhibit the growth of bacteria and mold. For example, rope (Bacillus subtilis spores) in doughs or breads cannot grow with pH values between 5.4 and 5.5.
  3. Such low pH can be obtained by using pH control agents (acidulants) such as propionic acid (commercially marketed as calcium propionate). A mold inhibitor, calcium propionate is most effective below pH 5.5. At pH lower than 5.5 (excess H+ concentration), the potentially active component, propionic acid, is undissociated, and becomes active in excess of H+ ions.1
  4. pH can also be modified by using vinegar (acetic acid), which is another example of both a pH control agent and a preservative (also acting against rope contamination). It has little intrinsic antimicrobial activity, and so is added to increase the acidity (reduce the pH) and retard the initial growth of the bacteria.3
  5. During fermentation (alcoholic and/or lactic), there is an increase in dough acidity (lowering of pH), which contributes to the shelf life, flavor, color, and rheological properties of the product.
  6. Neutralization of baking powders (the amount in grams of sodium bicarbonate that are needed to neutralize 100 grams of an acid leavener to convert the bicarbonate to carbon dioxide). In this case, pH is an indicator of the extent of such a reaction, and results in a final neutral value (7.0).4
  7. In cases where the acid leavener does not neutralize the sodium bicarbonate (baking soda), then the excess bicarbonate will increase the degree of browning of the sugars during baking. Maillard browning increases at higher pH values (>8.0). To prevent this, additional acid, e.g., cream of tartar (potassium bitartrate) is added to lower the pH and neutralize remaining baking soda.4
  8. In gluten-free applications, where addition of hydrocolloids is a must, the pH of the product can be critical in terms of product stability (gelling). A hydrocolloid molecule held at its isoelectric point, i.e., the pH at which there is zero net charge, will likely come out of solution as it becomes insoluble in water.5

Measurement of pH

The pH value of any product can be measured by placing a pH meter device in direct contact with the product (previously prepared), or can be obtained quantitatively by following a math expression.

A pH meter determines electric potential between glass and reference electrodes, using a commercially calibrated apparatus standardized against known buffer solutions (a method known as potentiometry). The basic principle of potentiometry involves the use of an electrolytic cell composed of two electrodes immersed into a test solution. A voltage develops, which is related to the ionic concentration of the solution.2

pH measurement of a product sample can be performed by following these steps:

  1. Prepare 100 mL of deionized water.
  2. Place 15 g of product sample into a blender with the water.
  3. Thoroughly blend mixture for 10–30 seconds.
  4. Pour it into a beaker to measure the pH with the potentiometer (pH meter).

Calculation of pH2

pH calculation can be done with the following equation:

  • pH = – log10 [H3O+]

[ ] means molar concentration of ions (in moles per liter or Molar).

The pH scale6

pH is represented on a scale from 0  to 14.

Acidic product pH < 7.0 [H3O+] > 1.0 X 10-7 M
Neutral product pH = 7.0 [H3O+] = 1.0 X 10-7 M
Basic, or alkaline, product pH > 7.0 [H3O+] < 1.0 X 10-7 M

 

The neutral point, pH 7.0 (value of pure water), is the midpoint of the pH scale. A pH of 0 indicates  extreme acidity, and a pH of 14 extreme alkalinity.

pH is a logarithmic value. H3O+ molar concentrations progress arithmetically, but each unit increment of pH indicates a tenfold increment in H3O+ concentration.

As a consequence, successive increments of pH do not indicate the same increments of H3O+ concentration. Therefore, from pH 7 to 6 there is a tenfold increase in acidity; from 7 to 5, the increase is 100-fold; and from 7 to 4, it is 1000-fold.6

Approved methods for pH measurement

  • AOAC 981.12 (pH meter – matrices: vegetables/acidified foods)
  • AACC 02-52.01 (Electrometric method – matrices: flour and some bread, crackers, cakes, and pastry products)

pH values of some articles

Food Product pH
Baker’s yeast bread 5.3–5.8
Sourdough bread 3.8–4.6
Wafer sheets 6.8–7.4
Snack crackers 5.5–5.6
Soda crackers 7.2-8.0
Most biscuits 7.0–7.2
Eggs 6.6
Honey 3.7–4.2
Milk (cow) 6.4–6.8

 

Miscellaneous pH
1 M HCl solution 0.0
Vinegar 2.8
Apple juice 3.8
Water (pure) 7.0
Sea water 8.5
Milk of Magnesia 10.5
Ammonia 11.0
Bleach 12.0
1 M NaOH solution (lye) 14.0

 

References

  1. Cauvain, S.P. “Testing Methods.” Baking Problems Solved, 2nd ed., Woodhead Publishing Limited, 2017, pp. 469–470.
  2. Nielsen, S.S. “Standard Solutions and Titratable Acidity.” Food Analysis Laboratory Manual, 3rd ed., Springer International Publishing, 2017, pp. 179–184.
  3. Cauvain, S.P. “Other Ingredients.” Baking Problems Solved, 2nd ed., Woodhead Publishing Limited, 2017, p. 109.
  4. Cauvain, S.P. “Cakes, Sponges and Muffins.” Baking Problems Solved, 2nd ed., Woodhead Publishing Limited, 2017, p. 277.
  5. Edwards, W.P. “Science.” The Science of Bakery Products, The Royal Society of Chemistry Publishing, 2007, p. 13.
  6. Timberlake, K. “Acids and Bases.” Basic Chemistry, 5th ed., Pearson Education Limited, 2017, pp. 490–496.