Brix Basics: Where Plant Sugars Come from and Why They Matter

What Is Brix, and How Does a Plant Build It?

Regarding the recent regulations, you may have heard about citrus (orange) juice and how it supports growers, with a breakdown of key points based on science. There is a lot of science that takes place before you enjoy the cup of OJ that you know and love.

While this does not represent any concept of Brix, it is a measurement that does not necessarily indicate quality. It is used for various purposes, including market standards, breeding, and commercial regulations.

The concept of Brix can be confused with juice flavor, but there is much more to consider than just this numerical value.

Growers use Brix to decide when to pick a crop. Winemakers use it to judge how ripe grapes are. Federal regulators use it to define the legal term for orange juice. But what is actually being measured and how does a plant produce it in the first place? This article walks through the science in plain language and explains why Brix has become front-page news in Florida this year.

What Brix Actually Measures

Degrees Brix (written as °Bx) is defined as the measurement of the total dissolved solids in a liquid, based on how much the liquid bends light.

One degree Brix equals about one gram of sucrose dissolved in 100 grams of solution.

In practice, the number is measured with a refractometer. This small optical instrument measures how light refracts as it passes through a juice sample.

In fruit juices overall, sugars typically account for 90-94%. The rest comes from organic acids, minerals, and other dissolved compounds. That is why Brix is not just a sugar unit. In fact, it is best understood as a quick, practical stand-in for sweetness and ripeness.

The following table lists the Brix levels of the most commonly eaten fruits. These are average values, but they can vary depending on several factors, such as source, origin, and variety. This was prepared for educational purposes.

Rank	Fruit	Typical Ripe Brix (°Bx)*
1	Banana	18–23
2	Watermelon	10–13
3	Apple	11–16
4	Grapes	16–25
5	Orange	10–14
6	Mango	14–22
7	Pineapple	12–18
8	Pear	11–16
9	Papaya	10–15
10	Avocado	4–8**

Sources 1, 2 3  4 5 6 7 
**Avocado is commonly tested for its oil content rather than its Brix.

Where the Sugar Comes From: The Leaf

A plant builds its own sugar. It does not come from the soil, and it is not absorbed through the roots. Sugar is manufactured in the leaves through photosynthesis, using sunlight, water, and carbon dioxide. In plant physiology, a leaf that is actively making more sugar than it needs is called a “source.” Fruit, roots, seeds, and other growing tissues that receive sugar are called “sinks” because they are primarily where those sugars are deposited.

A Handheld diagnostic research equipment being utilized on citrus leaves to evaluate physiological parameters — Picture 1. Handheld diagnostic research equipment is being utilized on citrus leaves. Photo taken 02-18-26

This source-to-sink relationship is the foundation of nearly everything that determines a plant’s final Brix reading.

How Sugar Travels: The Phloem

Once sugar is made in a leaf, it needs a delivery system to reach the fruit or root where it will be stored. That delivery system is the phloem, a network of living tubes that runs alongside the plant’s water-carrying tissue. Sugar is actively loaded into the phloem in the leaf, transported throughout the plant, and then unloaded into the sink tissue, where it accumulates and eventually appears as a Brix reading.

Anything that damages or blocks the phloem interrupts this delivery, no matter how much sugar the leaves are producing.

Two Different Ways Brix Goes Up

A rising Brix number can mean two very different things, and it matters which one is happening:

More sugar is arriving. Strong sunlight, warm weather, healthy leaves, and a well-fed root system all increase the amount of sugar a plant produces and successfully delivers to fruit.
Less water dilutes the same amount of sugar. When a plant is mildly water-stressed, fruit tissue loses water faster than it loses sugar, so the same amount of sugar becomes more concentrated. This is sometimes called the concentration effect, and it is the mechanism behind deficit-irrigation strategies that growers use to raise Brix before harvest.

The second path comes with a real trade-off. Pushing water stress too far reduces fruit size, fruit count, and overall yield, and it increases problems such as blossom-end rot caused by poor calcium uptake. Raising Brix and maximizing yield often pull in opposite directions.

The Roots role

Roots do not make sugar, but they set conditions determining how much sugar a plant can produce and move. Roots supply the water that travels up through the plant, and they take up the minerals that leaves need to run photosynthesis and load sugar into the phloem efficiently.

Potassium (K), the most commonly associated with Brix, may support the carbon “loading” into fruits. Nitrogen (N) and phosphorus (P) deficiencies reduce photosynthesis, causing sugar to back up in the leaves rather than move outward.

They also shift the plant’s growth priority toward the roots. Magnesium (Mg) deficiency may play a similar trapping effect: it can leave a leaf sitting on sugar it is physically unable to export. In short, a struggling (As caused by HLB) root system limits a plant’s ceiling for sweetness long before any fruit symptoms.

Environmental and Physiological Factors

The following summarizes various factors that can affect Brix levels in plants.

Factors	Common effect on Brix	Why
Light intensity and duration	Increases	More photosynthesis, more sugar produced
Moderate warmth	Increases	Faster enzyme activity and sugar loading
Mild to moderate water stress	Increases	Concentration effect: less water dilutes the same sugar
Severe water stress	Decreases overall sugar and yield	Photosynthesis itself slows, and growth stalls
Potassium availability	Increases	Supports sugar loading into fruit
Nitrogen or phosphorus deficiency	Traps sugar in leaves	Reduces photosynthesis, redirects carbon to roots
Magnesium deficiency	Traps sugar in leaves	Impairs sugar export from the leaf
Lower fruit load per plant	Increases per fruit	Less competition among sink tissues
Advancing ripeness	Increases	Sugar accumulates through fruit development
Variety and genetics	Sets the baseline	Different varieties load sugar at different rates

Why This Matters Right Now in Florida

Florida’s citrus industry gives a real-world, high-stakes example of everything above. The well-known citrus greening, or HLB, is caused by a bacterium that lives mostly inside the phloem of citrus trees. As the infection progresses, the phloem accumulates callose (a highly dense polysaccharide) and other compounds triggered by the bacteria-host interaction, which clog the phloem, physically blocking the sugar pathway from leaves to fruit and roots.

This disease, combined with hurricane damage, has driven a decline in Florida citrus production and in the average fruit Brix. That decline is directly connected to two regulatory changes made in 2025 and 2026. First, the Florida Citrus Commission issued an emergency rule in October 2025, reducing the minimum Brix level required.for oranges and grapefruit juice, allowing the industry to legally accept fruit with lower sugar (Brix) content than the previous standard required.

Most recently, in July 2026, the FDA (U.S. Food and Drug Administration) changed the federal standard for pasteurized orange juice, lowering the minimum Brix requirement from 10.5 to 10%. Both rule changes recalibrate the legal standard to match what a phloem-damaged crop can produce, rather than forcing blending or importing higher-Brix juice to match the previous target.

Additional information

If you would like more information on this topic, Citrus regulation or technical or technical assistance , please contact me at UF/IFAS Extension Hardee, Desoto or Manatee office.

An Equal Opportunity Institution. UF/IFAS Extension, University of Florida, Institute of Food and Agricultural Sciences. This document is available in alternative formats upon request; contact your local UF/IFAS Extension office for accommodations.