What do pH 4.6 and "acidified" mean, and how do I measure pH?

In federal food law, pH 4.6 is the line between acid foods and low-acid foods: a food with a finished pH above 4.6 and a water activity above 0.85 is low-acid, and a low-acid food brought down to 4.6 or below by adding acid is an "acidified food," which carries extra rules for anyone who sells it. Jams, jellies and preserves are excluded by name. A home cook using tested recipes never needs a reading. A seller needs one for pepper jelly, vegetable or herb jellies and other products with low-acid ingredients, or when a state asks. To measure, use a calibrated pH meter, not paper strips, calibrated at pH 4.0 and 7.0. This page explains the definitions and the method.

What the federal definitions say

The acidified food regulation, 21 CFR Part 114, rests on four definitions in section 114.3 [1]:

Two more lines in 114.3 matter to jam makers [1]:

• The exclusion. "Carbonated beverages, jams, jellies, preserves," acid foods containing small amounts of low-acid foods whose pH does not differ significantly from the main acid food, and foods stored, distributed and retailed under refrigeration are excluded from Part 114.
• Tomatoes. Tomatoes and tomato products with a finished equilibrium pH below 4.7 are not classed as low-acid foods.

"Finished equilibrium pH" means the pH after the acid has spread evenly through every piece of food in the jar. For pickles, Nevada's guidance puts that at typically 12 to 24 hours after processing [7].

Why jams are excluded

A standard fruit jam is an acid food from the start: the fruit itself is below pH 4.6. Penn State Extension puts the best pH for jam and jelly gels at 2.8 to 3.3 [2], well below the line. Jam is also high in sugar, which Penn State notes reduces the water available to support microbial activity [2]. So the risk the acidified food rule exists to control is not there to begin with.

The exclusion does not answer every case. Federal text excludes "jellies" without qualification, yet Michigan's agriculture department names "pepper (or other herb and vegetable) jellies" as acidified foods [3], and Penn State tells commercial makers of products with low-acid ingredients to use a pH meter to confirm the finished product is well below 4.6 [2]. That tension, and how to handle it if you sell, is on pepper jelly.

When you need a pH reading

A pH meter tells you where a batch sits. It does not design a safe process. For anything acidified, the process authority sets the target and the method, and your readings show each batch meets it.

Meters versus paper strips

Use a meter. The federal method for acidified foods is potentiometric: a pH meter with a glass electrode and a reference electrode [6]. Color indicators or test paper may be used instead only when the pH is 4.0 or lower [6], and a jelly near the safety margin is exactly where you need more than that. Nevada's agriculture department is blunter: paper strips "are NOT accurate enough" to measure the acidity of home-processed foods [7].

What to look for in a meter:

• Accuracy of plus or minus 0.02 pH units or better, with 0.01 a good choice [7].
• Two-point calibration at pH 4.0 and 7.0 [7][8].
• Automatic temperature compensation, or the habit of bringing every sample to the same temperature.
• A replaceable electrode, and its replacement cost; a non-clogging tip helps with foods that are hard to blend [7].

Calibrating at 4.0 and 7.0

1. Let the meter warm up and stabilize [6].
2. Bring the buffers and samples to the same temperature. The federal method uses 25°C (77°F) [6].
3. Rinse the electrode with distilled water and blot it dry.
4. Place it in the pH 4.0 buffer and calibrate [6][8].
5. Rinse, then check or calibrate with the pH 7.0 buffer [6][8].
6. Calibrate each day you use the meter, just before you test product [7].

Follow the manufacturer's instructions for cleaning and storing the electrode [8]. Most glass electrodes must be kept moist in storage solution, and a dried-out electrode drifts.

Taking a reading from jam or jelly

The federal method and the extension trainings agree on the basics [6][7][8]:

1. Wait for equilibrium. Test after the product has had time to settle; Oregon State's training waits at least 24 hours after processing for acidified products [8].
2. Bring the sample to room temperature, ideally 25°C [6].
3. Make it fluid. A jelly can usually be tested as is, once warmed slightly. For a jam or pepper jelly with pieces, blend the sample to a smooth paste. A little distilled water may be added to make it fluid enough to read; the federal method allows this within limits [6], and Nevada notes distilled water does not change the product's pH [7].
4. Take more than one reading. The federal method takes two readings on a well-mixed sample, which should agree with each other [6]; Oregon State's training takes three and averages them, rinsing the probe between readings [8].
5. Record the result in your batch log with the date, batch code and meter calibration [7]. A printable version is on the batch log template.
6. Test every batch of an acidified product [8].

If a batch reads above the target your process authority set, do not sell it. Ask the process authority what to do; do not add acid to finished jars and re-read.

pH and the set

The same meter helps with quality as well as safety. Penn State recommends a meter to decide how much acid, if any, a commercial jam needs to reach the 2.8 to 3.3 range where pectin gels best, warning that too much acid breaks down pectin during boiling and too little gives a weak gel [2]. For the cook at home, a tested recipe's lemon juice does that job; acid, lemon juice and the set explains it.

Water activity, the other number in the federal definition, is measured with a laboratory instrument most small makers never own. A process authority or a university lab can measure it if your product depends on it.

Putting a meter to work in a business

A meter, buffers and a log are standard equipment once you make acidified products. When the investment makes sense, and what else belongs on that list, is on equipment that pays for itself. If your first acidified product is pepper jelly, the next step is process authorities and scheduled processes.

References

1. Code of Federal Regulations, 21 CFR 114.3, Definitions (Cornell LII), current. https://www.law.cornell.edu/cfr/text/21/114.3
2. Penn State Extension, LaBorde, Food for Profit: Commercial Production of Jams, Jellies and Fruit Butters, updated May 24, 2024. https://extension.psu.edu/food-for-profit-commercial-production-of-jams-jellies-and-fruit-butters
3. Michigan Department of Agriculture and Rural Development, Low acid foods and acidified foods, undated (viewed October 2026). https://www.michigan.gov/mdard/food-dairy/wholesale-industry-resources/wholesale-manufactured-foods-new-business-guide/food-operations-with-special-requirements/low-acid-foods-and-acidified-foods
4. Oregon Department of Agriculture, Oregon's Cottage Food Exemption, March 2026. https://www.oregon.gov/oda/Documents/Publications/FoodSafety/CottageFoodExemptionENG.pdf
5. Alabama Department of Public Health, Cottage Food, July 9, 2026. https://www.alabamapublichealth.gov/foodsafety/cottage-food.html
6. Code of Federal Regulations, 21 CFR 114.90, Methodology (Cornell LII), current. https://www.law.cornell.edu/cfr/text/21/114.90
7. Nevada Department of Agriculture, Measuring pH (craft foods), undated (viewed October 2026). https://agri.nv.gov/uploadedFiles/agrinvgov/Content/Food/Craft_Foods/Measuring_pH_.pdf
8. Oregon State University Extension Service, Clackamas County, pH meter calibrating and testing for pickled food products, December 19, 2022. https://extension.oregonstate.edu/node/198996