Category Archives: Cooking Chemistry

Back to basics

Any scientific investigation into the world of cooking and chemistry must begin with a discussion of acids and bases. Armed with an understanding of this simple chemical principle a home cook can understand a variety of topics including how the use of chemical leaveners serves to create airy baked goods (as well as the difference between baking powder and baking soda – and why they’re not interchangeable!), how to harness science to prevent botulism when canning food at home, and much more. Indeed, a basic understanding of simple chemistry will prove invaluable in the future as we continue to explore the science of everyday life.

So, what are acids and bases?

Simply put, solutions that are acidic have a high concentration of \text{H}^{+} ion and solutions that are basic have a high concentration of \text{OH}^-. However, the salient point for most home chefs is that when an acidic substance (such as vinegar) and a basic substance (such as baking soda) are combined the result is a chemical reaction (in this case, a vigorous bubbling that you may be familiar with). And, if you combine the correct amount of acid with the correct amount of base you can use them to neutralize each other, so that the resulting solution will be neither acidic or basic. This is because when \text{H}^{+} and \text{OH}^- are mixed, the following reaction occurs:

 \text{H}^{+} + \text{OH}^- \rightarrow \text{H}_2 \text{O}

Thus, if I mix the correct amount of acid with the correct amount of base I can create a solution which has a neutral pH.

Now as with any useful scientific principle, we must have a scale with which to measure and quantify, and for acids and bases the scale is called the pH scale (because it is a measure of the proportion of \text{H}^+ ion present in the solution). A neutral solution (one that is neither acidic nor basic, such as water) is given a pH of 7 on a scale from 0-14. Solutions which are acidic have a pH < 7 and those which are basic have a pH > 7.

Applying the science of acids and bases to cooking:

One way in which acids are important to cooking is in home canning projects. Acidic solutions are often relatively safe from microbial growth, it’s the reason why pickling is such an effective method of food preservation. Thus, if you happen to be interested in home canning projects, having an acidic pH in your canned goods is an important safeguard against dangers such as botulism (since Clostridium botulinum, the bacteria responsible for botulism cannot grow at a pH below 4.6).

Second, as I mentioned at the beginning, chemical interactions between acids and bases are used for chemical leavening in many baked goods such as quick breads, muffins, and biscuits. In this reaction a base (usually baking soda, i.e. sodium bicarbonate, \text{NaHCO}_3) reacts with an acid present in the dough (from buttermilk, vinegar, lemon juice, or a chemical acid such as cream of tartar) and produces carbon dioxide (a gas) as a result of this reaction. The resulting gas bubbles are preserved in the cooked dough and are responsible for the texture of your baked goods. Too little chemical leavener, and you’ll find yourself with a very dense bread indeed. For those scientists in the audience, here are the relevant reactions:

\text{NaHCO}_3 +\text{H}^+ \rightarrow \text{Na}^+ + \text{CO}_2 +\text{H}_2 \text{O}

and

\text{2NaHCO}_3 +\text{heat} \rightarrow \text{Na}_2 \text{CO}_3 + \text{CO}_2 +\text{H}_2 \text{O}

And have you ever wondered why you cannot switch out baking soda for baking powder in a recipe? The science of acids and bases strikes again! Baking powder is a mixture of acid and base whereas baking soda is a purely basic compound. Thus, if you substitute baking soda for baking powder in your recipe you may find yourself with no acid present to react with the baking soda and leaving you with an unleavened result.

Another area in which acids interact with cookery is in the science of proteins. If you’ve ever substituted buttermilk in a recipe with a mixture of milk and acid (lemon juice, vinegar, cream of tartar, etc.) you’ll have noticed that introducing the acid into the milk causes it to curdle. This is because milk contains protein, and protein structures can become disrupted in an acidic environment. This disruption causes the proteins to lose their structure (to become denatured), and the result is curdled milk. And if you ever tried to skip the acid and substitute regular milk for buttermilk in a recipe you may have been disappointed because the lactic acid in buttermilk is caused by fermentation (the souring process) and unsoured milk therefore contains less acid.

While protein denaturation isn’t always desirable (unless you’re trying to substitute for buttermilk, curdled milk is rarely the goal in a recipe), it can also be used to great effect. If you’ve ever struggled to poach an egg, you could try cracking the egg into a bowl of white vinegar and letting it stand for a few minutes before pouring the egg into your poaching water. The acid in the vinegar will begin to coagulate the proteins on the outside of the egg and this will help to prevent the egg becoming disrupted when introduced into the poaching water.

The point I’ve hopefully convinced you of is that acids and bases are fundamental to the science of cooking, and as we move into discussions of specific types of recipes and how and why they work, an understanding of acids, bases, and the effects they can have on ingredients will be inescapable. Plus, a basic grasp of the science of cooking will enable you to make recipe substitutions with confidence, and even to create your own recipes from scratch.

References:

  • Edwards, W. P. The Science of Bakery Products. Cambridge: Royal Society of Chemistry. 2007.