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Red cabbage juice changes colour when you add a squirt of lime juice or a spoonful of baking soda. Baking powder causes your cake to rise beautifully in the oven. You can ‘cook’ fish by adding lemon and lime juice, this makes a ceviche.
What do all of these examples have in common?
They are all examples of acid/base reactions. These types of reactions are just one of many chemical reactions that happen in food (another one would be browning of bananas). Acid/base reactions all have a lot in common, even though their effect (a colour change, leavened cake or ‘cooked’ fish) may look very different. In this post we’ll discuss the underlying science of these reaction types.
When is a food acidic?
Acidic foods are pretty common. Lemon & lime juice, vinegar, yogurt and buttermilk are all acid foods. If you drink any of these, you taste the acidity.
The opposite of an acid food (or drink) is one that is alkaline. A good example of this is baking soda. A food that is neither acidic or alkaline is called neutral.
Whether or not something is acidic or alkaline is very well defined by chemists. In determining what a food is you use the pH-scale. This scale runs from 0-14. Any food that has a pH value of 7 is neutral. Foods with a pH lower than 7 are acidic, those with a pH higher than 7 are alkaline.
The further away from that neutral value of 7 the more acidic (or alkaline) a food is. For example, lemon juice has a pH-value of 2-3 whereas that of yogurt is around 4-4.5. As such, lemon juice is more acidic than yogurt.
Determining the pH-value
You can measure the pH-value of a food using a pH-meter or a special pH-measurement strip. If you use a pH-meter you can get a specific value for the product you’re measuring, e.g. 2.3. However, since foods are naturally quite diverse, there can be quite a lot of variation between similar foods. One lemon may have a pH-value of 2.1 whereas another one might be 2.4. It is why you often see ranges for specific foods.
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?
The pH-value impacts the flavour of your food, but it can also be very important for the shelf life of your food, how long it stays safe. Food can spoil and go bad because of the growth of undesirable microorganisms. However, a lot of micro organisms don’t grow anymore when the pH-value has sunk under a certain value. As such, lowering the pH-value enough will keep food safe longer.
Apart from shelf life, pH-value can be used for monitoring food processes. For example, making yogurt mostly consists of lowering the pH-value of milk. Because of the lower pH-value, the milk will thicken. Manufacturers will know at which pH-value the process is completed. This way, by measuring this value they can keep track of the process, without having to look or taste the product.
What is a pH-value
So what is it that makes these foods alkaline or acidic? Whereas sugar makes something sweet and salt makes something salty, it is the protons make something acidic.
Protons are single hydrogen atoms (H) that miss their electron. As a result, they are positively charged. Chemists write protons down as: H+.
Protons play a role in a lot of reactions in the human body and in most living organisms for that matter. So where do they come from?
One way is through molecules that are naturally acidic (e.g. citric acid). These hand off one of their protons quite readily. When they do so, the concentration of protons increases. If the concentration of protons increases, the pH-value of the food decreases. More protons, means more acidity. This relationship can be expressed by the following formula:
As you can see in these formulas, there is a balance between protons (H+) and hydroxide ions (OH–). They are one another’s reverse. More protons means a lower pH value, whereas more hydroxide ions results in a higher pH value (or lower pOH value). Most commonly you’ll only find pH-values, the pOH-value is barely used in practice. This relationship is the core of acid/base reactions and has it’s origin in the water molecule.
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Water and acidity
Water itself is neutral but can split itself in both a proton as well as a hydroxide ion.
Instead of seeing the singular proton, a hydronium ion (H3O+) is formed. In the presence of water this will happen which is why in a lot of cases you can use them interchangeable. (If you are not familiar with writing down chemical reactions, read up on the chemical reaction basics first.)
In a neutral environment, there is an equal amount of protons (H+) and hydroxide ions (OH–). Only if the food is acidic is alkaline is this equilibrium off. Acidic molecules or alkaline molecules can either increase the proton or hydroxide content.
When an acid (AH in the example below) is present in the water, it decreases the pH-value by releasing a proton:
On the other hand, when an alkaline substance is present, it will form a hydroxide ion, causing the moisture to be more alkaline.
This pH-value it is a strong indicator of the types of chemical reactions that can occur. Different types of reactions can occur in acidic environments as opposed to alkaline ones.
All acid/base reactions have in common that protons are exchanged. An acid will 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.
Let’s take a look again at the reactions we saw previously. Have a look at how these protons are exchanged.
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. This does not mean it occurs to the same extent in both directions.
Very strong acids will give away (almost) all of their protons. These are, appropriately, named strong acids. In most cases though only a certain percentage of the acids will give away their protons. These are weak acids. The same goes up for their alkaline counterparts, the bases.
How many of the acid will give away its proton can be expressed using an equilibrium constant. For the acid (AH) equation shown above that looks like:
The [ ] indicate a concentration. Since the concentration of water is virtually constant this is often left out of the equation. An acid that gives away almost all of its protons will have a very low concentration of AH and a high concentration of A–. As a result, the Ka-value will be large. The larger the Ka-value, the stronger the acid.
It is important to know that this equilibrium constant changes with temperature. Thus an acid might release protons more or less easily at different temperatures.
For an alkaline substance the equilibrium equation is very similar:
Applying acid/base reactions
In practice you will not discuss Ka and Kb values often. However, you will see their effects all around you. Baking soda for instance is an example of an alkaline food ingredient. Comparing those chemical reactions to these basic formulas will show a lot of similarities.
The same goes for red cabbage. The colour of red cabbage changes with the pH-value of the surrounding liquid. This is caused by acid/base reactions with the molecules that give the red cabbage its colour!
Of course, acid isn’t just about chemical reactions. It is also a lot about flavour and balance of a dish. Some dishes just need some acid (e.g. lemon juice) to work well. Samin Nosrat does a great job explaining that side of acid in her book salt, fat, acid, heat.
Clemson University, pH Values of Common Foods and Ingredients, link ; source for the pH-values of foods mentioned in this article