Nutrient Density – Can it be Measured With a Brix Refractometer?

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Robert Pavlis

Everybody wants to grow nutritious food and it has become common to measure nutrient density with a Brix refractometer. High Brix is equated to highly nutritious food, but is this really true?

Brix readings have been used in agriculture for many years but is there a reason for gardeners to bother making this measurement? What does it tell you about the quality of food? I’ll answer these and other questions in this post as I take you down the yellow brix road.

Nutrient Density - Can it be Measured With a Brix Refractometer?
Nutrient Density – Can it be Measured With a Brix Refractometer?

What is Brix?

Brix is a unit of measure for the total soluble solids in a sample, expressed as a % weight. The value can be measured by a device called a refractometer. You take a drop of juice from your plant, put it on the refractometer, and take a reading. The device is simple, relatively inexpensive, and anyone can learn to use it in a matter of minutes.

You can measure the Brix value of any type of plant. For example, you can use Brix readings to compare different kinds of apples or carrots. Apple A might have a higher Brix reading than apple B, which means that apple A contains a higher concentration of solids.

Soil Science for Gardeners book by Robert Pavlis

The term solids may be a bit confusing, but basically all chemicals in a liquid are considered to be solids.

What Does a Refractometer Measure?

refractive index in water

A refractometer measures how light is bent as it travels through a liquid. If you look at a pencil in a glass of water you will notice that the part of the pencil in water does not join up with the part of the pencil in air. This is obviously an optical illusion. The water has bent the light coming from the submersed part of the pencil to make it look as if it is in a slightly different position than it really is.

The degree of bending depends on the amount of chemicals in the sample. If we add a small amount of chemical to water, we get a small amount of bending. If we add more chemical, we get more bending. A refractometer measures the amount of light bending which is directly related to the amount of chemicals in the water. Stated another way, it measures the concentration of chemicals in water.

A really important point here is that all chemicals in the sample bend light.

Scientists have standardized on a scale, or number system, that is called Brix. We measure temperature using a centigrade or Fahrenheit scale and we measure total solids with a Brix scale.

Brix and Nutrient Density of Food

The concept of nutrient dense food has become very prevalent with the increased concerns over non-organic agriculture, hydroponics and GMO foods. People are concerned about eating food that has little or no nutrition in it. This has lead to a whole movement that wants to eat and grow only nutritious food, which they call nutrient dense food.

Food is nutrient dense when it contains high amounts of nutrients. One way to measure nutrient density is to measure the amount of solids in food by taking a Brix reading. In some circles, Brix is now equated to nutrition.

The Brix Fallacy

The reading on a refractometer is the sum of all the solids in a sample. This includes sugar, carbohydrates, proteins, vitamins, minerals and everything else you can think of. However, the relative impact of any one chemical on the final reading depends very much on the amount of that chemical in the sample.

Vitamins, for example, occur in very small amounts, compared to say carbohydrates. What this means is that you could change your growing conditions to double the amount of vitamins, and you would not see a significant change in the Brix reading. The same is true for most minerals.

Many of the chemicals important to our health exist in low concentrations, so changes in their concentration have little or no impact on the Brix reading.

Consider this analogy. You have a penny and a silver dollar in your pocket. You measure how much you have and come up with a reading of 1.01. If you double the amount of pennies, the reading stays about the same, 1.02.

So what does affect Brix? The main chemical that changes the Brix reading is sugar. A plant that makes more sugar will have a higher Brix reading. Other carbohydrates and proteins also affect Brix because they exist in high concentrations.

You find statements like this on the internet; “Any brix reading above 15 means the plant is going to be high in nutrients, thus making higher quality food.” But I can mix some water with sugar to make a 15 Brix solution – does that make it nutritious?

A high Brix may indicate a more nutritious food, but it might not. Statements like the above quote indicate the author does not understand Brix.

The one chemical that probably has the biggest impact on Brix, is water. Remember that Brix measures concentration. The solids in a sample will be diluted if the sample contains more water. A grape has a much lower Brix reading compared to the same fruit once it is dried into a raisin. The total amount of solids has not changed, but the concentration of solids has.

Even a tomato sitting on the counter, slowly loses moisture, and therefore has a higher brix. If you believe the Brix stories on the internet, a tomato is more nutritious tomorrow than it was yesterday.

Water affects Brix readings much more than any of the chemicals we consider nutritious. Stop watering your tomatoes before harvest and they will have a higher Brix reading. Water more and Brix drops.

Because the Brix reading is mostly affected by sugar and water, it is not very useful for measuring nutrition in food. A high Brix reading tells you nothing about nutrition, unless you are looking to consume more sugar.

Accuracy of Brix

The accuracy of refractometers is quite low compared to other types of laboratory equipment. Handheld refractometers have an accuracy of 0.2.

Orange juice contains 50 mg of vitamin C per 100 g of juice. Lets say we could grow orange trees to double this level. The extra vitamin C is significant compared to our daily requirement, but the Brix reading of our new orange would only go up by about 0.05. Due to the low accuracy of Brix, this is not enough to significantly change the final Brix reading of the juice.

Brix is not a good way to measure the amount of minor chemicals in our food.

Brix and Reduction of Pests

Numerous claims have been made about high Brix plants having lower pest and disease issues. The science on this was reviewed in 2010 and no correlation was found between Brix, and pests and diseases.

Brix and Measurement of Minerals

How does Brix relate to mineral content in plants? Here are the results of one interesting study.

Nineteen cultivars of lettuce, both modern and heritage varieties, where tested for calcium content. Brix readings were also taken. Testing was done with conventional and organic fertilizer, which included the addition of various amounts of calcium. The researchers concluded that, “Interactions were nonsignificant or presented no responses to suggest that any of these interactions were important factors in affecting °Brix in lettuce. Polynomial regression analysis showed no significant relationship of percent Ca and °Brix.”

A similar study looking at tomatoes found the same thing. A Brix reading is not related to mineral nutrient content.

Translation, Brix is not a good way to measure mineral content.

Does High Brix Equate to Nutritious Food?

The answer is no.

Brix readings can be useful for measuring things like sugar content in grapes, where it has been used for years and it has other uses in agriculture, but in the home garden there is no good reason for measuring Brix.

More importantly, don’t be fooled by any advertising that promises nutritious food based on Brix readings.

References:

  1. Image of refractometer by Fernando G;  https://commons.wikimedia.org/wiki/File:Refractometer.jpg
  2. Image of pencil in water; https://www.maxpixel.net/Pencil-Refract-Pencil-In-Water-Bent-Pencil-2403662

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

29 thoughts on “Nutrient Density – Can it be Measured With a Brix Refractometer?”

  1. Responding to: “Photosynthesis produces sugar – it does not produce the other nutrients. So, no other nutrients don’t come with it.“

    Correct, however the majority of this sugary product is used to feed the rhizosphere microbiome, which is populated by organisms who use their enzymes and fine filaments to mine minerals for the plant.

    Reply
  2. I did a trial, growing a flat of lettuces in lifeless dirt from my garden path, vs another flat of the same lettuce variety in soil heavily amended with earthworm castings. All other factors were equal. BRIX readings were consistently much higher in the latter soil than in the former. Was this due to more sugar in the leaves? If so, what would make a plant produce more sugar (photosynthate) other than having more of the micronutrients needed to do so?

    Reply
  3. More myth-busting at its finest!

    I’ve read quite a bit about the correlation between Brix levels and pest resistance but nothing that is peer reviewed. The link in the article is broken. Surpsising that it was hosted on an organic farming website! Don’t suppose you have another source or can say what it found?

    Reply
  4. If the sugar content is the thing best reflected by Brix testing, then does that not act as a proxy measure of efficiency of the photosynthetic action of the plant since that is the process that produces the sugars? A large proportion of the sugars and proteins made by plants are pushed out into the soil via the roots as exudate which feeds the soil microbiota which, in turn, provides the micro-nutrients that the plant cannot produce itself. They gather around the root zone to feed and are then consumed by larger organisms, leaving behind their digested remains in a form that plants can then absorb. The more and higher quality sugars and proteins that the plant produces, the better that exchange with the soil life will be. Therefore, albeit a crude measure, there is a connection between the Brix value, the dissolved sugars and the overall nutritional status of the plant, especially if you combine it with a soil analysis of microbiota – bacteria, fungi, nematodes, micro-arthropods etc. If you’re not satisfied with Brix, how DO you propose that small-scale growers can best gauge the nutrient quality of what they are growing?

    Reply
    • The process you describe is correct. But that does not mean high sugar levels in plants translates to higher nutrition. It could mean that the plant stored extra sugar instead of using it as exudates. For example, some beets are sweeter than others, they have a higher brix. That does not mean they have higher values of other plant nutrients or plant produced chemicals. It just means they contain more sugar.

      The only way to know nutrition levels in plants is measure the nutrients of interest, in a lab.

      Reply

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