One of the most important properties of food products is their pH-value. It gives producers an insight into the shelf life of a product, how stable it might be over time, and how it reacts with other products. It’s what determines the color of red cabbage, preserves your pickles, and more.
Accurately measuring the pH is best done using a pH meter. But, if you don’t have one of those at your disposal, you can get a indicative reading using pH papers, which are a lot cheaper, though less accurate.
What is the pH-value?
The pH-value is a measure of the acidity/alkalinity of a product. It is expressed on a scale from 0-14. Values <7 are acidic. Examples in food as lemon juice, yogurt, apples, vinegar, etc. If your product has a value >7, it’s considered alkaline. Most foods are (slightly) sour, but baking soda is an example of an alkaline product.
We’ve discussed the pH-value and its uses in far greatere detail in a separate article. Read that first if you’re not familiar with it.
Why measure the pH-value?
The pH-value of your food is especially important when determining the shelf life of food. Microorganisms such as bacteria and molds, one of the main causes of food spoilage, can no longer grow at (very) low pH-values. It’s why sour pickles don’t spoil as quickly as a fresh cucumber. Knowing the pH-value will help determine whether a product is shelf-stable or not.
In some cases, however, you do want growth of microorganisms, for instance when brewing beer or making cider. These microorganisms will have a preferred range of pH-values for them to grow at. Within that range, they grow fastest. Knowing the pH-value ensures that the environment you’ve created for them to grow in is ideal!
A lot of cleaning agents used to clean process lines may be very acidic or highly alkaline. At the end of the cleaning process it is important to neutralize these again and to rinse away any remaining cleaning agents. By measuring the pH you can check whether they have been neutralized or removed sufficiently.
An important characteristic of acidic and alkaline ingredients is that they react when mixed together. If you’ve ever used baking soda to make pancakes or a cake, you’ve used this first hand. The baking soda reacts with an acidic ingredient in your batter, e.g. buttermilk, or lemon juice. While reacting, the baking soda makes gas, leavening your pancakes or cake.
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If you’re developing a pancake recipe for a production line, for instance, it’s important to know whether you’re mixes are (sufficiently) acidic to react with the baking soda. A quick test can help confirm that.
How to measure pH-values
There are two ways to measure the pH-value of a material:
- Electrochemical: this method uses electrodes to measure the pH-value. It gives quite a precise value, with at least one decimals behind the comma. As such, you’ll be able to distinguish between a product with a pH-value of 5,6 and 5,9.
- Colorimetric: some colors (e.g. that of red cabbage or turmeric) are sensitive to the surrounding pH value. They will change color when a certain value is passed. You can use these color changes to get an indicative value of the pH of your solution. Manufacturers make special pH-papers, or test strips, that change colors at different pH values giving you a decent indication of the pH of your solution. These papers don’t give a precise number though.
Comparing electrochemical vs. colorimetric
These two methods are both commonly used, but for slightly different applications. That’s because of their individual pro’s and cons, which we list below. Generally speaking, if you need a pH-value for food safety reasons, or are looking for an accurate reading, you should use the electrochemical method. However, if you’re fine with getting a rough indication, or are just looking for a range, the colorimetric method may serve you well.
For the rest of this article, we’ll be focusing on the colorimetric method, using pH test papers/strips.
|Maintenance||needs regular calibration||none, one time use|
|Accuracy||provides a fixed number (e.g. 4.5)||requires comparing the color of a strip with a set of reference colors, can be subjective|
|Medium||there exist a lot of different types of probes for different products (also solids)||often needs a liquid to dip into|
|Impact of color of product to be measured||no influence||may skew readings|
How do pH test papers/strips work?
There are different types of pH test papers, some change color completely, others (such as the ones in the photos in this article) have certain sections that change color. However, they all use the same concept!
A note on definitons: paper strips to measure the pH using using color changes may be called pH test paper, pH papers, pH strips, pH indicator paper
As we quickly mentioned above, some colors change when the pH in their surroundings changes. This color change always happens at the same pH range. Red cabbage will always be pinkish at low pH values and blue/green at high ones. Turmeric is yellow for lower pH-values and turns red at high ones. These changes are also reversible. This means that when you change the pH-value back to the original value, the color will change back as well.
Colors with this property are called pH indicators. They can be used as an indicator for the pH-value. Chemists use indicators in a range of applications and analytical techniques.
Wikipedia has a great chart showcasing several pH indicators and their color transitions at different pH-values.
Mixing & matching
If you’d use just one pH indicator you’ll be able to define whether the pH-value is lower or higher than that one specific transition. In some cases that’s enough. You might just need to know whether the pH is above or below a specific value. However, often you’d want more information. You will want to get a more detailed indication of the pH value of your product.
That is why manufacturers of pH papers have developed ways to combine these different indicators. That way, they can show a range of transitions, and as such pH-values! Sometimes all of these indicators are mixed in one piece of paper, other times different sections of the test paper each contain a separate indicator.
By changing the indicators present in the test paper, manufacturers can make a range of different products. Some papers will be able to cover a wide range of pH-values (e.g. from 1-14). However, those papers don’t tend to be as accurate, often you’ll only be able to distinguish between whole values (e.g. 6 vs 7). You won’t be able to distinguish between 6,5 and 6,8 for instance.
To allow for more precise measurements, manufacturers may also choose to make papers that only work in a specific range, e.g. between pH-values 2-4. They can then tune their indicators such that you will get a more accurate reading, e.g. you might be able to distinguish between 2,5 and 3,0. However, you’ll never be able to get as accurate a reading as you would with an electrochemical method.
- food that you want to measure the pH of
- pH strips of your choosing ; if you're just curious about the regular value, buy universal indicator paper or universal test strips. These can give you a value from 1 to 11-14. If you already approximately know the range you expect, or need a slightly more accurate value, it's best to choose paper with a smaller range. There are dedicated papers on the market for some applications (e.g. wine making or beer brewing).
In order for the test strip to work, you need a liquid environment. You can't just hold a pH strip against a piece of bread to measure its value. As such, the first step to measuring is to make sure you have a suitable liquid to measure. This is also one of the trickiest parts and will influence your results!
To understand this, you need to know that the pH-value depends on the concentration of protons. If you dilute your solution, you will change the concentration of protons and as such the pH-value. Luckily, this happens on a logaritmic scale. This means that in order for the pH-value to change by 1, the concentration of protons needs to change by a factor 10.
This is another factor that will make the use of your test strips less reliable, so keep that in mind! That said, if you prep all your samples in a similar way (or just use liquids) you'll still be able to get a lot of value out of the analysis.
- Dissolve the ingredient you want to learn more about in water, or take your liquid sample and place it in a bowl. You only need a very small amount of liquid. Ensure all the liquids you want to test are at room temperature.
- Follow the instructions of the manufacturer of your strips. In most cases you'll have to dip the strip into the liquid for a couple of seconds.
- Again, follow the instructions of the manufacturer. Most likely, you can immediately compare the color that your strip has turned with the color chart that the manufacturer has supplied with your strips to determine the pH-value.
The pH-value is a measure of the concentration of protons in a water-rich environment. As such, you can't use these strips on ingredients like oil (or expect an inaccurate reading).
Keep in mind that if your product has a strong color (e.g. the coffee we test further down the article) it may make it hard, or even impossible, to read the result of the analysis.
Using pH strips in food – An Example
To demonstrate the concept, we used universal test papers with a range from 0-14. Each individual strip of our paper contained 4 colored squares. Each of these squares would change color, depending on the pH-value. This manufacturer clearly decided not to mix all indicators into one, instead, they split the indicators. This does make it easier to read, especially if you’re measuring a colored liquid (such as the coffee we analyzed).
Some results, the pH values of our liquids
Our simple test showed a few clear (and expected) results!:
- Lemon juice is highly acidic: we tested a value of 0-1 with our strips.
- Granulated sugar has a neutral pH-value. It’s not acidic nor alkaline. This is what we expected, sucrose is not an acid, nor a base.
- Brown sugar: the second square is a little more green than that of granulated sugar, indicating that it’s slightly more acidic
- Syrup: very similar to granulated sugar, so quite neutral. Keep in mind that the syrup (as well as the brown sugar) were both brown in color which may have slightly impacted the color of the yellow squares).
- Coffee: the drak color of the coffee clearly stained te test strips, making the reading inaccurate. A rough guess of the pH value would be around 5.
- Pickle liquid: this liquid was used to pickle pickles, it’s supposed to be slightly acidic and that is indeed the case with a pH value of 5-6.
Testing the pH-value of foods using pH test papers is a fun way to apply food science at home, but it’s also a quick and easy way for food manufacturers to keep track of their processes and products!
Test strips we used: link