How to Measure Brix in Tomato Ketchup (and Why Your Reading Isn't Sugar)
A QC-lab method for measuring refractometric Brix in ketchup and thick sauces, with sample preparation, dilution maths, salt effects and the most common sources of error.
Brix is the fastest release test a sauce line has. A good reading takes under a minute, but a bad one can send a batch back to the kettle for nothing. Ketchup is one of the harder products to read because it is viscous, pigmented, salty and full of insoluble tomato fibre. This guide covers a method that holds up in a busy QC lab.
What Brix actually measures
One degree Brix (°Bx) is defined as 1 g of sucrose in 100 g of solution. A refractometer does not measure sucrose. It measures the refractive index of the liquid phase and converts it to the sucrose concentration that would give the same refractive index at 20 °C.
In a pure sugar syrup, Brix and sugar content are the same thing. In ketchup they are not, because every dissolved substance bends light:
| Component in ketchup | Effect on refractometer reading |
|---|---|
| Added sugar or glucose syrup | Raises reading (the main contributor) |
| Natural tomato sugars and acids | Raises reading |
| Salt (sodium chloride) | Raises reading, by more than the same mass of sugar would |
| Acetic acid from vinegar | Raises reading slightly |
| Insoluble fibre, skin and seed particles | Does not dissolve, but blurs the shadow line on optical instruments |
So the number on your certificate of analysis is best described as soluble solids (°Brix, refractometric). That is fine for process control as long as everyone, including your customer's specification, means the same thing.
Equipment
- Digital refractometer with automatic temperature compensation (ATC), or a thermostatted Abbe refractometer. A handheld optical refractometer works for line checks, but the shadow line in ketchup is often fuzzy.
- Distilled or deionised water for zeroing.
- A certified sucrose standard close to your target (for example 30 °Bx) to verify the span.
- Analytical balance (0.01 g), beaker, spatula, soft lint-free tissue.
- Optional: a small centrifuge or a coarse filter to obtain a clear serum.
Method
- Zero the instrument with distilled water at 20 °C. It should read 0.0 °Bx (±0.1).
- Verify the span with your sucrose standard. If it reads outside the tolerance on the certificate, stop and recalibrate before testing product.
- Take a representative sample. Stir the sample container. Do not scoop from the top of a hot kettle where water is evaporating fastest.
- Cool the sample to about 20 °C. ATC corrects for small temperature differences, but a sample at 60 °C will still drift as it cools on the prism and will lose water by evaporation.
- Choose direct reading or dilution.
- Direct: place enough sample to cover the prism completely, close the cover, wait for the reading to stabilise and record it.
- Dilution: weigh about 20 g of ketchup and an equal mass of distilled water into a beaker, mix until smooth, then read the diluted mixture.
- Read in duplicate. Clean the prism with water, dry it and repeat with a fresh portion. Duplicates should agree within your method's repeatability (0.2 °Bx is a common working target).
- Clean the prism with warm water. Do not scratch it with paper towels or spatulas.
Dilution calculation
When you dilute by mass, multiply the diluted reading by the dilution factor:
Worked example: 20.00 g ketchup + 20.00 g water reads 15.3 °Bx. Brix = 15.3 × 40.00 ÷ 20.00 = 30.6 °Bx.
Strictly, the Brix–refractive index relationship is not perfectly linear, so a 1:1 dilution introduces a small error. For process control this is normally acceptable. If you need a contractual figure, measure undiluted on a digital instrument and state the method on your specification.
The salt problem
Ketchup recipes often carry 2–3 % salt. Salt raises the refractometer reading, so two batches with identical sugar can show different Brix if salt dosing drifts. Two practical consequences follow:
- A Brix result alone cannot tell you whether sugar or salt is off. If Brix is out of specification, check salt (by chloride titration or a conductivity method) before adding sugar or water.
- Specifications for tomato concentrates are often salt-corrected. In the United States, tomato paste and purée are defined by natural tomato soluble solids (NTSS), which excludes added salt, under 21 CFR 155.191. Ketchup itself is covered by its own standard of identity, 21 CFR 155.194.
Brix vs total solids in grading
USDA grade standards for tomato catsup are built on total solids (determined by drying), not on refractometric Brix. Total solids include insoluble fibre, so they will always be higher than soluble solids in the same sample. If a customer specification says "solids", ask which one they mean. Our moisture content guide explains how total solids are measured.
Troubleshooting
| Symptom | Likely cause | What to do |
|---|---|---|
| Reading drifts upward over 30–60 seconds | Sample was warm and is evaporating on the prism | Cool sample to 20 °C; close the cover quickly |
| Duplicates differ by more than 0.3 °Bx | Sample not mixed, or particles on the prism | Mix thoroughly; dilute or centrifuge to get a cleaner serum |
| All results suddenly 0.5 °Bx high | Instrument zero shifted | Re-zero with water and check with standard |
| Brix in spec but product tastes too salty | Salt overdose masking low sugar | Test salt; review dosing records |
| Line Brix and lab Brix disagree | In-line sensor calibrated on a different product | Run a paired comparison and apply a product-specific offset |
Recording results
Record the instrument ID, temperature, whether the sample was diluted (and the factor), both readings and the mean. Brix records usually support legal quality claims, not food safety. However, if Brix is part of a validated preservation system with pH and water activity, treat it as a controlled parameter under your HACCP plan.
Sources
- U.S. Food and Drug Administration. 21 CFR 155.191, Tomato concentrates and 21 CFR 155.194, Catsup. Electronic Code of Federal Regulations (ecfr.gov).
- USDA Agricultural Marketing Service. United States Standards for Grades of Tomato Catsup.
- AOAC International. Official Methods of Analysis, Method 932.12, Solids (soluble) in fruits and fruit products, refractometer method.
- Nielsen, S. S. (ed.). Food Analysis, 5th edition. Springer, 2017.
This guide is educational. Safety-critical limits (thermal processes, pH limits, allergen claims) must be confirmed with your process authority, certification body or regulator for your product and market. Spotted an error? Tell the editors.