The acidity or alkalinity of any substance is quantified using the pH scale, which ranges from 0 (highly acidic) to 14 (highly alkaline), with 7 representing a neutral state. The pH value is a fundamental factor in food science because it influences a food’s flavor, texture, shelf life, and the growth of microorganisms. As a common culinary ingredient, the onion’s specific pH value significantly impacts how it is cooked, preserved, and paired with other foods.
The Measured Acidity Range
Raw onions generally fall within a mildly acidic range, typically measuring between pH 5.3 and 5.8. This places them slightly below neutral, a characteristic shared by most fresh vegetables. The precise pH value is not static and varies depending on the specific cultivar.
Yellow cooking onions usually exhibit a pH range of 5.3 to 5.8. Red onions often measure slightly more acidic, falling between pH 5.2 and 5.6. Sweet varieties, such as Vidalia or Walla Walla, are characterized by a pH closer to 5.8, which contributes to their milder flavor profile.
Chemical Composition Determining pH
The inherent acidity of an onion is a direct result of its cellular chemistry, particularly the presence of sulfur compounds and organic acids. Onions store specialized compounds known as S-alk(en)yl cysteine sulfoxides (ACSOs), the most abundant of which is trans-(+)-S-propenyl-L-cysteine sulfoxide (PeCSO).
When the onion is cut, the physical damage ruptures the cell walls, causing the PeCSO to mix with the enzyme alliinase. This enzymatic reaction rapidly converts the ACSOs into sulfenic acids and other reactive sulfur compounds. The release of these acidic compounds, including the lachrymatory factor that causes eye irritation, contributes to the sharp taste and the initial measured acidity of a freshly cut onion. The concentration of these sulfur compounds, which is higher in pungent varieties, dictates the lower end of the onion’s pH range.
How Preparation and Storage Affect pH
The measured pH of an onion can shift considerably due to both preparation and storage conditions. Applying heat during cooking introduces significant changes. Caramelizing onions involves the Maillard reaction and sugar concentration, which initially proceed more slowly in the mildly acidic environment.
As water evaporates and sugars caramelize, the pH stabilizes or may increase slightly toward neutral. Furthermore, the heat converts the PeCSO compounds into new sulfur derivatives like cycloalliin, changing the flavor complexity. Long-term storage also affects the pH; while curing may cause a slight initial decrease, slower enzymatic activity can lead to a gradual increase in the pH over several months.
Practical Uses of Onion pH in Food Safety
The mildly acidic pH range of 5.3 to 5.8 is significant for food preservation and safety. Foods are categorized as low-acid if their pH is above 4.6, which is the established threshold for inhibiting the growth of Clostridium botulinum. Since raw onions exceed this value, they are considered a low-acid vegetable.
This classification means onions cannot be safely preserved alone using a simple boiling water bath method. To ensure safety for shelf-stable preservation, such as in pickling or canning, the pH must be lowered below 4.6. This acidification is achieved by adding substantial amounts of high-acid ingredients, typically vinegar or lemon juice.