DEAR DR. BLONZ: I was reading about the acid-base balance in the body and was surprised to learn that the pH of lemon juice is lower than that of vinegar, which means it's higher in acid. This makes me wonder how I can tolerate drinking lemon juice straight, but lower-acid vinegar takes my breath away. Does pH not equal the acidity we perceive with our tastebuds? I had always thought that if I substituted lemon juice for vinegar in a dressing or recipe, I needed to use slightly more because it has less acid. -- G.G., San Diego, California
DEAR G.G.: Degrees of acidity or alkalinity (acid or base) are expressed in terms of a measure known as pH, which is based on the concentration of free hydrogen ions. pH ranges from 0 to 14, with 7.0 being neutral. Distilled water has a pH of 7.0. If the pH is above 7.0, the substance is a base; if it is lower, it's an acid.
The greater the numerical distance from 7.0, the stronger the acid or base. Also, pH is a logarithmic scale, where every integer change represents a tenfold difference in acidity or alkalinity, so a substance with a pH of 1 will be 10 times stronger than an acid with a pH of 2. A typical cup of coffee has a pH of 5.0; compare this to the acid in our stomach, with a pH between 1.5 and 3.5, or battery acid, with a pH of 0.8.
In the body, the physical act of digestion begins the moment food enters the mouth, where it is acted upon by mechanical mixing with saliva and its enzymes, then with acids in the stomach, then digestive juices with their enzymes in the intestines. The process is to break complex foods into parts suitable for absorption. A food's pH can change after being metabolized (broken down), so lemons, while more acidic before the swallow, actually have an alkalizing effect on the overall metabolism after digestion and absorption.
The pH of a healthy human body is slightly alkaline and within a very narrow range, between 7.35 and 7.45. Chemical reactions in the body rely on specific pH levels, so overlapping systems are designed to maintain the correct pH to make things happen as needed. Different pH levels are used in areas responsible for particular types of processing during digestion. For more on the pH in different body parts, see b.link/a6ev7x5s.
Now, for the specifics of your question: pH does not necessarily correlate with smell or taste. The vinegar used in foods comes from acetic acid, a substance that is produced when foods decompose. The association with spoilage explains why a sniff of vinegar is such an unpleasant experience for many. Our senses are designed to assess things close to the body, especially anything being considered for ingestion. Vinegar's scent can trigger caution designed to keep us away from risks; for some, the smell of vinegar is so unpleasant that it can cause nausea.
Contrast that with a lemon scent, which is not associated with anything negative. Indeed, the pleasantness of the lemon scent makes it a popular fragrance additive, especially to household products.
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