Is mustard acidic? Easy Home pH Experiment with Purple Cabbage
Surely you have tasted mustard, but… have you ever wondered if mustard is acidic? The answer is yes, and in this post I’m going to tell you why mustard is acidic, how its spicy flavor works and what acidity has to do with it. And the best way to find out is with a very easy homemade experiment that you can do with things you might already have at home. We’re going to use a purple cabbage pH indicator to tell if a substance is acidic or alkaline, depending on the color of the cabbage juice, so you can see for yourself if the mustard is really acidic.
The origins of mustard and its culinary use date back millennia, as evidenced by archaeological evidence of its use in human food since prehistoric times. A study published in 2013 analyzed Neolithic pottery remains found in northern Germany and Denmark, where researchers found phytoliths, small mineral structures that remain as microscopic evidence of some plants when they decompose.
These phytoliths were identified as belonging to ancient versions of wild mustard seeds(Brassica sp.) and date from approximately 6100 to 5750 years ago, representing one of the oldest known evidence of the use of this plant as a condiment in Neolithic European cuisine (Saul et al., 2013). This finding reveals how these prehistoric communities were already employing mustard seeds to season their food, one of the oldest evidence that humans intentionally transform the taste and experience of their meals.
The three best known varieties of mustard used in cooking are black mustard(Brassica nigra), brown mustard(Brassica juncea) and white or yellow mustard(Sinapis alba). All of these varieties contain natural compounds called glucosinolates, which are also found in other vegetables belonging to the cruciferous family, such as broccoli, cabbage, cauliflower and kale. According to the chapter on glucosinolates in the Handbook of Herbs and Spices, these compounds not only offer health benefits, helping to reduce the risk of cardiovascular disease and certain types of cancer, but are also responsible for the spicy, strong and pungent flavor that characterizes mustard.
But what gives mustard its spicy flavor?
It all starts in the seeds, the black and brown mustard seeds contain a compound called sinigrin and the white or yellow ones, another called sinalbina. These compounds, by themselves, do not have much flavor, but when we break or grind the seed, enzymes called myrosinases are released. This enzyme, when in contact with an aqueous substance (i.e. any liquid containing water), is transformed into a new substance called isothiocyanate and this is what causes the intense spiciness and the intense aroma we know from mustard.
When mustard comes into contact with heat, its aroma and flavor change. The heat decomposes the isothiocyanates and little by little that spiciness fades, leaving a milder taste. That is why mustard has so many uses in cooking, its flavor can vary depending on how it is prepared; whether in curries, pickles, classic Dijon or American mustard, beer-fermented versions, vinaigrettes and marinades. The flavor of mustard can be intensified or softened depending on how it is cooked… but there is another secret that also influences that special flavor.
Why is mustard acidic? The role of acid in mustard preparation
As we have mentioned, water is the key to the release of the mustard’s sharpness, but at the same time that sharpness will gradually fade over time and this is where the role of acidic substances in mustard making comes in. Acids, such as vinegar or lemon juice, help to preserve the intensity of the mustard’s flavor. If you put more acid, the spicy flavor lasts longer; if you use less, the spiciness is milder and fades quickly and that is how the balance in recipes is adjusted. A milder mustard has less acid, and if you are looking for a strong, sharp taste, you have to increase the amount of acid.
In summary, It all starts when the seed breaks; if you mix it with water, the spiciness is released and gradually fades, but if you add vinegar, you control it and make it last. And if you cook it, the flavor mellows. This is how the natural chemistry of mustard works.
Now, in simple terms, when we say that a food is acidic, we mean that it has a low pH, is characterized by a sour or strong taste and also donates protons, a tiny particle with a positive charge. On the contrary, if a food is alkaline (or basic), its pH is higher, it accepts protons and hasthe ability to neutralize acids, which can soften intense flavors, for example. Understanding this difference is fundamental to understanding not only the chemistry behind mustard making, but also many other culinary processes in which acidity and alkalinity determine the taste, preservation and properties of what we eat.
To better understand these concepts of acidity and alkalinity, and to be able to observe in a more tangible way how to recognize whether a substance is acidic or alkaline, I propose the following experiment for children, simple, practical and that you can do at home with accessible materials.
In this activity we are going to experiment with various substances to find out if they are acidic, alkaline or neutral, using a homemade pH indicator made from purple cabbage juice. This vegetable contains a component called anthocyanin, a natural pigment that changes color depending on the pH of each substance with which it comes into contact. We could go into detail about how anthocyanins work and the science behind this amazing color change reaction, but we will leave that topic for another post 🙂 
If you want to learn more about acids and bases and how they work, I recommend this material from Utah State University. It is a simple guide that explains basic concepts clearly and practically, ideal if you want to go a little deeper: Acids and Bases.
Here is a step-by-step activity for you to try at home.
This is a fun sensory activity that allows you to explore how ingredients affect texture – watch the video and try it at home!
Activity: pH indicator with purple cabbage and acidity in mustard.
Purple cabbage juice will change color depending on the acidity of the substance being tested.
- Red in acids(pH < 7)
- Purple or purple in neutral solutions(pH ~ 7)
- Blue, green to yellow in bases(pH > 7)
Materials:
- A piece of purple cabbage (between 1/8 and 1/4 of a head, depending on how much juice you want to prepare).
- 1 to 2 cups of hot or boiling water (ideal if boiled by an adult before starting)
- 4 clear jars (I used small 125 ml glass jars, such as baby food jars, but you can also use clear disposable cups or recycle plastic bottles cut in half).
- 1 teaspoon of baking soda
- 1 teaspoon mustard
- 1 teaspoon liquid soap (can be dishwashing or hand soap)
- 1 teaspoon vinegar
- 1 small or medium saucepan (optional, if you prefer to boil the cabbage)
- 1 strainer (if you use a pot)
- Optional: glass with filter for tea or infusions
Procedure:
Step 1. Blend purple cabbage juice (with adult supervision).
There are two ways to prepare purple cabbage juice. Here I share with you the two options so you can choose the one that is more practical for you. Both work well, although we did it with the hot water method in the glass.
Option 1: With boiling water and rest (safe for children with adult supervision)
- An adult should boil the water beforehand in a kettle.
- In a large glass or pitcher, place the purple cabbage cut into strips.
- Add the boiling water over the purple cabbage.
- If you have a glass with a filter for tea or infusions, use it to facilitate the filtering afterwards. If not, you can simply strain the liquid afterwards.
- Let stand for at least 10 minutes. The water will turn a dark purple color.
- When it cools slightly, strain the liquid into a clean container. This will be your purple cabbage juice, which works as a natural pH indicator.
Option 2: Boil in a pot (recommended if you do not have a filtered glass and prefer to do it as a classic infusion).
- Cut the purple cabbage into thick strips.
- Place the strips in a small or medium saucepan.
- Add enough water to cover the cabbage (about 2 cups).
- Bring to a boil.
- Reduce the temperature and simmer for about 10 minutes, or until the water is dark purple in color.
6. Allow to cool, strain the liquid and store in a jar or container.
Step 2. Prepare the flasks for the experiment
Fill each jar (preferably clear, so you can see the color change better) with about 90 ml of the purple cabbage juice. If your jars are small, half or a quarter will be fine, you don’t need much.
Step 3. Add the substances to be tested
In the first jar, put 1 teaspoon of baking soda; in the second, add 1 teaspoon of vinegar; in the third, add 1 teaspoon of liquid soap and finally, in the fourth jar, put 1 teaspoon of mustard.
Step 4. Watch what happens
Stir each mixture well with a clean spoon (very important! clean the spoon between one jar and the other so you don’t mix the ingredients unintentionally). See how the color of the cabbage juice changes in each one. The color will tell you if that substance is acidic, basic or neutral.
Questions for discussion
- What substance changed the purple cabbage juice to a pink or reddish color?
- What substance caused the color to turn blue?
- What would happen if we added more vinegar to the mustard?
- Why do you think bicarbonate changes color to blue? What does that indicate about its acidity or alkalinity?
- What other foods in your kitchen do you think are acidic? And which would be alkaline?
Curious facts
- Yellow mustard has an acid pH thanks to the vinegar used in its preparation. That is why, when you mix it with purple cabbage juice, the liquid changes color to red or pink.
- Baking soda is a base, and when mixed with purple cabbage indicator, it changes color to blue, indicating that it is alkaline.
What do we learn from this pH experiment?
Mustard is an acidic food.
Thanks to the vinegar it contains (or wine, in some recipes), yellow mustard has a low pH, that is, it is an acidic substance. This not only gives it that characteristic flavor, but also helps it to keep longer without refrigeration. When you do the experiment, you will see that the purple cabbage juice changes to a reddish hue when it detects that acidity.
Acidity influences the flavor and pungency of mustard.
The compounds responsible for the pungency in mustard, the isothiocyanates, are pH sensitive. When mustard is very acidic, its flavor tends to be milder and less pungent, but if it is less acidic, the spiciness intensifies and feels much stronger and more pungent. This explains why the amount of vinegar you add in a recipe can change the result so much.
We introduce children (and adults too!) to the world of pH.
This experiment is a simple and fun way to explain basic concepts such as acid, neutral and base. We use purple cabbage juice as a natural pH indicator because it contains anthocyanins, a pigment that reacts to the acidity of food by changing color. In addition to learning science, the children are surprised and enjoy seeing how the color of the liquid changes from red to green depending on what you mix.
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