Earl Weak, UC Master Food Preserver Online Program Volunteer
In simple terms, pH is a measure of how acidic or alkaline (basic) a solution is.
- A pH below 7 indicates an acid solution
- A pH above 7 indicates an alkaline (basic) solution
- pH 7 is considered neutral
In the context of food preservation, pH is especially important because acid environments help control the growth of bacteria and other microorganisms that cause spoilage or illness.
The Chemistry of pH
A bit of chemistry is needed to explain what pH means. Hydrochloric acid (HCl) is a strong acid and is produced in our stomachs. When HCl dissolves in water, the molecule breaks apart, forming hydrogen ions (H⁺)—hydrogen atoms that have lost one electron and chloride ions (Cl-)—chlorine atoms that have gained an extra electron. The concentration of hydrogen ions in a solution determines its pH. A higher concentration of H⁺ means a lower pH (more acidic) and vice versa. In contrast, sodium hydroxide (NaOH) is a strong base, or alkaline substance that’s a primary ingredient in drain cleaners. That molecule also breaks apart when dissolved in water, forming sodium ion (Na+) and a hydroxyl ion (OH-). When an acid and a base react—like HCl and NaOH—they neutralize each other, forming water (H₂O) and a dissolved salt (NaCl):
HCl + NaOH → H2O + NaCl
A History of Measuring Acidity: From Colors to Instruments
Before modern analytical tools existed, people used color indicators (pH sensitive pigments) to measure acidity.
- Litmus paper, which uses pigments extracted from lichens, turns pink in acid and blue in neutral or basic solutions (see Fig. 1).1
- The natural pigment in red cabbage also changes color with pH—red in acidic solutions, purple near neutral, and blue, green or even yellow in basic (see Fig. 2).
Figure 1. Blue litmus paper turned pink after being dipped in an acidic solution (Photo caption: Earl Weak, used with permission).
Figure 2. Extracted pigments from red cabbage under acidic, near neutral, and alkaline conditions. Photo caption: Earl Weak, used with permission).
The modern concept of pH was introduced in 1909 by Danish chemist Søren Peter Lauritz Sørensen, who was studying protein solubility in beer. He recognized that the solubility of these proteins depended on the concentration of hydrogen ions in the solution. He introduced the term “pH” as an abbreviation for the “power (Potenz in the original German) of hydrogen” to help his results. Around the same time, American bacteriologist Alice Catherine Evans, working with William Mansfield Clark and colleagues, helped develop improved methods to measure pH—replacing older titration techniques. In the 1930s, Arnold Orville Beckman invented the first electronic pH meter, providing a fast and accurate way to measure acidity3. Today, calibrated pH meters are standard equipment in laboratories and food-processing facilities. A modern, digital, battery-operated, “pen-type” pH meter is shown in Figure 3.
Figure 3. Digital pen-type pH meter flanked by vials of calibration solutions (Photo caption: Henry Shaw, used with permission).
What Does “pH” Mean?
For dilute solutions, the pH can be approximated by the negative logarithm of the hydrogen ion concentration2:
pH = -log10[H+].
For example, if the hydrogen ion concentration of a solution is 10⁻³M (0.001 moles per liter), we know that log10(0.001) = -3, so the solution’s pH is +3.
- Lower pH → higher hydrogen ion concentration → more acidic
- Higher pH → lower hydrogen ion concentration → more basic
In most natural systems, pH values range from about 0 (very acidic) to 14 (very basic). The pHs of common foods lie in a much more restricted range.
Bacteria and pH
Different bacteria grow best at different pH levels (see Fig. 4):
- Acidophiles thrive in acidic conditions. Examples include Lactobacillus species used in making cheese, yogurt, and sauerkraut.
Neutrophiles prefer near-neutral conditions (around pH 7). This group includes many common pathogens such as Clostridium botulinum, Salmonella, and E. coli.
Alkaliphiles grow in basic environments, such as Vibrio cholerae, which causes cholera and grows well at pH 8–11.
Figure 4. Approximate pH ranges for the growth of the different classes of bacteria (Image credit: W. Keenleyside, Microbiology: Canadian Edition figure 9.35 CC BY 4.0).
Why pH Matters in Food Preservation
Because most harmful bacteria cannot grow in highly acidic conditions, controlling pH is a key food safety strategy. Foods with a pH of 4.6 or lower, such as many fruits, are considered “high-acid” and can be safely processed in a boiling-water canner. Foods with a higher pH must be pressure canned at temperatures above 240°F to destroy Clostridium botulinum spores and other pathogens.
In Summary
pH is a measure of the hydrogen ion concentration in a solution. Unintuitively the lower the pH value, the higher the acidity. Because bacteria have specific pH ranges for growth, understanding and controlling pH is essential to safe and successful food preservation.
References:
1 https://en.wikipedia.org/wiki/Litmus
2 https://en.wikipedia.org/wiki/PH
3 https://www.acs.org/education/whatischemistry/landmarks/beckman.html
4 https://ecampusontario.pressbooks.pub/microbio/chapter/the-effects-of-ph-on-microbial-growth/