Acids and My Mom’s Lemonade -

Acids and My Mom’s Lemonade

By Adrian Dingle

November 28, 2022

Good Times with Food

We all love a little nostalgia. So many of those happy memories are built around family,and in particular the family traditions that involve cooking and food. Everybody has their own favorites of course,and one of mine happens to bea recipe that relies upon some pretty significant chemistry.

Truth be told, all cooking involves lots of chemistry whether it be the complex re-organization of biochemical molecules in bread-making or the fermentation of sugars to alcohol in the beer and wine industry, but sometimes that chemistry can be hidden. If you were to look at my mom’s homemade lemonade recipe then the chemistry would be right there in front of your eyes!

Her old-fashioned lemonade recipe produces an amazing, delicious, and incomparable sweet and sour lemon syrup that one would then dilute with ice-cold water on blistering hot summer days. Sounds kind of romantic, doesn’t it? And it is, but some people are surprised when they first read the ingredients.

Carboxylic Acids – Citric & Tartaric

Among the lemons and sugar are two acids in their pure, unadulterated form: citric acid and tartaric acid. Each acid is a member of a family of compounds known as carboxylic acids.

Carboxylic acids are defined by a small group of atoms bonded together in a specific manner as part of their structure. More specifically, carboxylic acids contain this group of carbon, hydrogen,and oxygen atoms.

In each carboxylic acid, the-COOH groups are part of a larger chemical structure. Citric acid has three such carboxylic acid groups, and tartaric acid has two,with the acid groups linked together with other carbon atoms,and with various other atoms making up the remainder of their structures.

Lots of people will recognize citric acid as being the chemical that gives citrus fruits their moniker, but tartaric acid is perhaps less well-known. What cooks are more familiar with is the potassium salt of tartaric acid, potassium bitartrate, better named potassium hydrogen tartrate, and even more commonly referred to as cream of tartar.

This while powder has many uses in the chemistry of food including helping to stabilize the beaten egg whites that are used to make another sweet favorite, meringue.It’s also a by-product of the wine making process. Anyway, I digress, let’s get back to the acids in mom’s lemonade.

The State of Acids

Most people think of acids as being liquids – you know, the middle state of matter between solids and gases. However, in reality those ‘liquids’ are usually solutions. What’s the difference? A liquid is a pure substance, so ‘liquid sulfuric acid’, H2SO4(l) is 100% sulfuric acid and has the state symbol (l) as part of its written formula to denote liquid. It is essentially sulfuric acid that is above its melting point and below its boiling point. Most of the time that we encounter acids they are not pure liquids but rather are ‘in solution’, i.e., dissolved in water and are not pure. This creates what we call an aqueous (where water is the solvent) solution.In this situation we would say that we have ‘a solution of sulfuric acid’ and would show that by using a different state symbol with the chemical formula,i.e., H₂SO_4(aq) where the (aq) stands for aqueous (water).

Because of our common experience of acids being liquids (or more accurately solutions), it might be surprising to learn that citric and tartaric acids are both white, crystalline solids at room temperature, and that they resemble sugar or salt!In fact, in the case of citric acid there semblance to salt is so strong that solid citric acid is often sold under the name of “sour salt”, the sour of course being a reference to its tart (acidic) taste. When are acids safe to consume? It’s a universally held truism that acids are corrosive, and generally potentially harmful to handle and consume. But that’s a wild generalization that fails to address many different aspects of their chemistry.In short, it’s far more complicated.So,what do we need to consider when thinking about handling, and even eating, acids?

Naturally Occurring

To start with acids are naturally occurring chemicals that we encounter every day and are often perfectly harmless.The citric acid in mom’s lemonade is a great example of any acid that is found in lots of fruits, but there is an apparently never-ending list of other naturally occurring acids. Ascorbic acid (also known as vitamin C!) in tomatoes, lactic acid can be found in milk that’s a few days old, and our old friend tartaric acid in grapes. These acids are everywhere, and completely safe.

A great example of a naturally occurring acid that is a little more harmful is that of formic acid. “Formic” comes from the Latin formica that literally means ant. Why is formic acid called formic acid? Well, that’s because the stinging bite that some ants can deliver to humans contains this acidic compound! Formic acid is the simplest of the carboxylic acids with the now familiar-COOH group being attached to just one other atom, a lone H.

Strong or Weak?

All acids are not created equally in terms of their acidity. Some are said to be strong; others are said to be weak. In acid chemistry the words strong and weak have very specific meanings. Acids are acids because of the hydrogen ions that they release. You may have noticed that all the chemical formula that we have shown for various acids have hydrogen in them. When an acid acts as an acid it releases that hydrogen in the form of hydrogen ions, H+. It’s those hydrogen ions that make an acid, an acid!

Strong acids release large numbers of hydrogen ions, whereas weak acids only give up a few. The more hydrogen ions that are released by the acid, the stronger it is. In most cases the acids that we encounter in the kitchen, in food,and in cooking are the weak variety, and that includes citric and tartaric.

Are you Concentrating?

Another thing to consider is the concentration of the acid. In the non-scientific world the word concentration is often confused with the word strength with the terms often being used interchangeably to describe the same thing. However, in acid chemistry that’s incorrect, and acid concentration is a very different property to acid strength. Concentration refers to the degree of dilution, i.e., how much water is present.

As we said above, most acids are encountered as solutions where the acidic substance is dissolved in water. The ratio of acid to water matters a lot in terms of making acids either absolutely harmless, or potentially dangerous. Acids that have relatively little water present are said to be concentrated; those with lots of water present are said to be dilute.

The difference between strength and concentration means that it’s possible to have strong acids that are concentrated or dilute. Similarly,weak acids can either be concentrated or dilute. The acids that are likely to be most harmful are the concentrated strong ones, and the acids that are likely the safest are the weak, dilute ones. One great example of a dilute, weak acid that we consume all the time is a solution of acetic acid better known simply as vinegar!

Kitchen Chemistry

This is just a tiny window on the vast chemistry of acids, and a just a few examples of some that we encounter daily. As you can see, they form an incredibly important part of the chemistry of food and cooking.

Citric Acid, USP/FCC Grade, Kosher, 100 g