Food Chemistry Basics – Acid/base reactions

There are a lot of different chemical reactions within food, far too many to all mention on this blog. However, chemical reactions can be organized in different types of reactions. One your understand the general mechanism of such a type of reaction, you’ll be able to understand most reactions of this type. One of these common types of chemical reactions are acid/base reactions.

We’ve already seen several of these on the blog, for example the changing colour of red cabbage or how baking soda and baking powder work. We’ve also cooked using acids, when making ceviche and zoomed in on a very common acid: vitamin C (ascorbic acid). Here though, we’re covering the basics to help you understand all of those better.

What is acidity?

You probably know several examples of acidic foods: lemon & lime juice, vinegar, buttermilk to name just a few. You will taste the acidity, it’s a sensory property of the food. But this sensory property is caused by the presence of certain chemical components, acidity is pure chemistry.

Whether or not something is acidic, or the reverse, alkaline, depends on the presence of so-called protons. These are ions of single hydrogen atoms (H+). These protons can react with other molecules and ions and participate in acid/base reactions. The counterpart of the protons are hydroxide ions (OH). When a solution contains a lot of hydroxide ions it is alkaline instead of acidic.

Water and acidity

Acidity evolves around the presence of at least some water since the protons and hydroxide ions will be formed through water. These form the start of understanding these acid/base reactions.

Water can act as both a base as well as an acid. Water itself is neutral but can split itself in both a proton as well as a hydroxide ion. Notice that protons again tend to react with water to form an hydronium ion (H3O+). (If you are not familiar with writing down chemical reactions, read up on the chemical reaction basics first.)

proton reacting with water to h3o
Protons will react with water (H2O) to form a hydronium ion. Both protons and hydronium ions can be used interchangeably when describing acid/base reactions.

Water forming proton and hydroxide ion equilibrium

When an acid is present in the water, it will make the water more acidic by producing a proton (or hydronium ion):

acid reacting with water
An acid (AH) reacting with water

On the other hand, when an alkaline substance is present, it will form a hydroxide ion, causing the moisture to be more alkaline.

Base reacting with water
A base (B) reacting with water.

pH-value

Since the concentration of protons determines whether a mixture is acidic or alkaline, this concentration is used to develop an acidity scale: the pH-value. The pH-value will indicate whether something is acidic or alkaline through the formula described below.

Since the presence of protons (H+) and hydroxide ions (OH) are so closely related, when you know the concentration of one, you’ll know the concentration of the other. Using the pOH-value you can find the pH-value and vice versa. Most commonly though you’ll only find pH-values, the pOH-value is barely used.

definition of pH and pOH
Note, the concentration of proton and hydroxide ions (indicated in [] brackets) should be in mol/l.
Anything with a pH of 7 is neutral, it is neither acidic nor alkaline. Anything with a pH<7 is acidic and anything with a pH higher than 7 is considered alkaline.

Examples of pH-values in food

Foods have a wide spectrum of pH-values, although most of them are either around a value of 7 (thus neutral) or lower than seven. Since food products, especially fresh foods such as fruits and vegetables and meat, aren’t very constant in their properties, the exact pH-value will differ per individual item. A lot of factors influence the pH-value, amongst others the ripeness of produce. The list below does give some general guidelines (source).

  • Apples: 3,3 – 4,0
  • Avocados: 6,2 – 6,6
  • Eggplant: 4,5 – 5,3
  • Fruit jam: 3,5 – 4,5
  • Honey: 3,9
  • Vinegar: 2,0 – 3,4

What is the pH-value used for in food science?

Besides the different taste an acidic product will have, pH is also important with regards to shelf life. Most micro organisms don’t grow anymore when the pH-value has sunk under a certain value. Apart from shelf life, pH-value can be used for monitoring food processes, to determine whether a reaction has occurred sufficiently (or too much). And the pH-value will influence how a lot of other processes occur and how proteins behave.

Acid/base reactions

This pH-value isn’t just interesting for determining shelf life, etc. it is a strong indicator of the types of chemical reactions that can occur. During acid/base reactions protons are exchanged. An acid will tend to give away a proton (increasing the concentration of protons, thus lowering the pH). An alkaline substance on the other hand will tend to take a proton. As a result, acid/base reactions are essentially exchanges of protons.

Let’s take a look again at the reactions we saw previously. Have a look at how these protons are exchanged.

acid reacting with water

Base reacting with water

Equilibrium reactions

One thing you might notice in these reactions is the double arrows in both of them. These double arrows indicate that it is an equilibrium reaction. In other words, the reaction can occur from left to right and from right to left.

Only very strong acids will give away all their protons. These are, appropriately, named strong acids and bases. In most cases though only a certain percentage of the acids will give away their protons and the same goes for the bases. These are weak acids and bases.

How many of the acid will have given away its proton can be expressed using an equilibrium constant. Let’s take a look again at the acid reaction equation we just saw:

equilibrium constant for a weak acid

The equilibrium constant for this equation can be calculated using the formula below. The [ ] signs indicate that the concentration for a specific component should be filled in. Since the concentration of water is virtually constant this is left out of the equation. Most publications will use the Ka-value, or acid dissociation constant. The larger the Ka-value, the stronger the acid.

This equilibrium constant changes with temperature. Thus an acid might release protons more or less easily at higher or lower temperatures.

For a base the equilibrium equation is very similar and it works in the same way as well:equilibrium constant for a weak base

These are the most basic principlies you should know when it comes to acid/base reactions in food. Have a look again at those posts mentioned at the start (leavening agents, red cabbage) and see whether it’s all a little easier to understand, now that you’ve got that basic understanding!

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