pH Meters and Home Canning

A pH meter is a device used to measure acidity .

In canning, “A low pH, below 4.6, will prevent the growth of potentially deadly spoilage bacteria in canned foods.” ^[1]McGlynn, William. Choosing and using a pH meter for food products. Oklahoma State University Extension. FAPC-117. Accessed March 2015 at http://factsheets.okstate.edu/documents/fapc-117-choosing-and-using-a-ph-meter-for-food-products/. If the pH of a food product is below 4.6, it is classed as acidic, and can be water bath canned. If it is above that, it is classed as non-acidic and must be pressure canned.

How does a pH meter actually work? “A pH meter measures the amount of hydrogen-ion (acid) in solution using a glass electrode immersed in the solution.”  ^[2]Ingham, Barb. Purchasing and Using a pH meter. University of Wisconsin Extension Services. October 2009. Accessed March 2015 at https://www.foodsafety.wisc.edu/assets/pdf_files/what_is_ph.pdf

NOTE: This is an advanced topic. You don’t need to know anything about this. Follow tested recipes from reputable sources and you are covered. This is just some background knowledge for very curious advanced canners.

Should you have a pH meter?

If you are a regular home canner and are following tested recipes from reliable sources, you don’t need a pH meter. In fact, USDA (United States Department of Agriculture) Extension people don’t want you to even own one. They feel perhaps that if people do have them, they may use them to try to create their own home canning recipes, or muck with the approved recipes. There’s so much bad canning practice going around, that they just want to get people to start following tested recipes, not trying to make up their own with the aid of a pH meter.

And they’re right; a home canner following tested recipes from approved, trusted sources neither needs a pH meter nor should even wonder if they need one.

That being said, we’re going to cover the topic anyway for:

• those who are intellectually curious about testing the tested recipes they have made and just want to know;
• those who are selling their products and are required to measure and record the pH for government health agencies.

State health boards require small regular home kitchens making high-acid preserves for sale to use a reliable accurate hand held pH meter to monitor their products (in addition to other protocols such as getting the recipe approved, keeping batch records, etc.)

Generally, you’d only even consider pH testing the acidity of something you’d water-bathed, as by definition those food products need to be acidic to be water-bathed. You’d generally never consider using a pH meter on food products you’d pressure canned, as those are suppose to be low acid, anyway, though an exception might be something like tomatoes or fruit which can also be pressure canned as an equivalency to water-bathing but still need to be acidic.

Intellectual curiosity about pH in home canning

There is nothing wrong with being intellectually curious; there’s no law that says you have to check your brains in the vestibule upon entering the church of safe home canning.

Canning is a science after all, we are constantly told: so then, why forbid people from trying to understand it, if they have the capacity to and want to?

This is no longer the 1940s. A lot of people are staying in school past Grade 8. There’s a lot of educated folks now who want to know reasons for things, and understand, and argue that it’s time to take the hocus pocus out of canning and let smarter people know what the science is; otherwise it starts to seem like gate-keepers on a power trip, and people will rebel.

Some users argue, “I’m a chemistry professor at a university…..  I would still suggest a pH meter to add another level of quality control. It is simply naive to ‘trust’ a proven recipe but not use the technology we have available.”   ^[3] https://forums.gardenweb.com/discussions/1947104/using-a-ph-meter-and-lowest-ph-to-kill-botulism#6987063

That being said, the same people should be intellectually consistent with themselves: their argument about wanting to know the science, so they can follow it, should mean that when scientific data available from the NCHFP/USDA is presented, they allow their reasoning to be informed and guided by it. They are not always consistent in that.

Reasons why you might want to check pH of foods other than safety

Many pectin recipes give general ratios of fruit, pectin, sugar and added acid. Say you have some fruit you bought or grew. Fruit pH can very based on soil, weather, other growing conditions, etc. Knowing the exact pH of it can help you determine how much if any extra help that fruit is going to need in firming up, especially if you are using some of the new “flex-batch” pectins. “pH control is required to insure proper gel formation in jelly making. The correct pH is also needed to achieve successful fermentations in the production of many cheeses, pickles and other foods.”  ^[4]McGlynn, William. Choosing and using a pH meter for food products. (That being said, you might need to be a master jam maker to know what to do as a result of what a pH meter was telling you.)

pH meters are expensive

When purchasing a pH meter, bear in mind that for it to be any use, you’re going to have to buy a bit better than the rock-bottom price ones. And, you are going to be buying that meter plus calibration solution, which is not cheap either: so go into the topic with those expectations.

You are also going to need distilled water, which thankfully is cheap, and you should be able to get at any chemist / pharmaceutical store.

Finally, what actually does the “sensing” is something called an “electrode” at the tip of the meter. The electrode won’t last for ever. On cheaper meters, you have to buy a whole new meter. On better quality meters, you buy a new replacement electrode.

Paper pH Strips

What about paper pH strips? Simpler, easier, and cheaper, right?

They’re generally not considered anywhere near accurate enough for food safety purposes. The only exception would be is that if the food pH is just so low anyway, that the safety of the food pH wise is a given, and so the non-accuracy of the paper strips doesn’t matter:

Depending on the pH of the product, you may be able to use paper pH strips (often referred to as litmus paper), or required to use a pH meter. Paper strips that measure pH rely on a color change in the paper to indicate product pH. Paper strips can be used to measure pH if the product pH is less than 4.0. Paper strips are an inexpensive way to test pH, but can be inaccurate or difficult to read” ^[5]Ingham, Barb. Purchasing and Using a pH meter.

pH Meter Purchase Considerations

Resolution vs accuracy

The resolution of a meter purports to show you the pH of an item to a certain number of decimal places. How accurate is that, though? That’s what the accuracy rating, which is always listed separately in specs about the meter, is for.

A USDA extension agent at Oklahoma State says,

The cheapest meters typically feature a resolution of 0.1 pH units. Federal agencies typically require that pH readings be reported to the nearest tenth (0.1) unit. Most units therefore technically offer sufficient resolution to meet government standards. However, it is important to also consider the accuracy range of the meter. Some of the least expensive meters may have an accuracy of plus or minus 0.2 units. In other words, if the meter reads 4.3, the actual pH of the product could be anywhere from 4.1 to 4.5. This might present a problem if the pH of your product approaches the legal limit of 4.6.”  ^[6]McGlynn, William. Choosing and using a pH meter for food products.

The extension agents don’t agree on how accurate is accurate enough

William McGlynn at Oklahoma State says,

It is generally advisable to invest in a pH meter/electrode combination that offers resolution and accuracy of 0.10 pH units or better.”   ^[7]McGlynn, William. Choosing and using a pH meter for food products.

Barb Ingham at the University of Wisconsin says,

Accuracy is listed as a range of +0.XX pH units. This means that the meter may read so many pH units above or below the actual pH of the product. Purchase a pH meter with an accuracy of +0.02 units or better. For instance, a pH meter with an accuracy of +0.01 is a good choice. A pH meter with an accuracy of +0.10 is not a good choice, it is not accurate enough for all products.”  ^[8]Ingham, Barb. Purchasing and Using a pH meter.

Temperature and pH meters

pH readings are affected by temperature. If you can afford it, it’s nice to have a pH meter that will automatically compensate. One less thing for you to think about, or get wrong, in all this.

In order to get an accurate reading, the pH meter must be calibrated at the same temperature as the samples being tested. More expensive meters will compensate for variations in sample temperature (too warm or too cold). If you take care to calibrate your pH meter just before you monitor product pH, and test the pH of room-temperature samples (after equilibrium pH has been reached), you do not necessarily need to purchase a meter with temperature compensation. If you can afford a meter with this feature, it’s nice to have.”  ^[9]Ingham, Barb. Purchasing and Using a pH meter.

Calibration of pH Meters

The biggest pain about pH meters is that you have to calibrate them before pretty much every session in which you use them. By calibrating a pH meter, you are reminding it what it should read as certain pH rates, such as 4.0 and 7.0. It’s been this way for decades and decades, and it’s unclear why the technology still seems somewhat primitive in this way.

Be that as it may, it still is primitive, and you have to calibrate before you start a session. To be clear, that doesn’t mean before each thing you measure. But rather, say on a given day you are going to use the pH meter. You need to calibrate the pH meter. Then, whether you measure one thing or ten with it, you’re good to go. In a lab, where pH meters are used daily, the rule of thumb is apparently to calibrate at the start of each day (or work-shift, if someone else will be taking over.)

Anyway, given that calibration is a minor annoyance that gets repeated for the rest of your life, it’s nice to reduce the annoying factor in this as much as is possible.

On cheaper models, you have to fiddle with a small screw driver that fits exactly a small screw and turn the screw back and forth to set the calibration to identify 4.0, and then 7.0, etc. And always know where the screw driver is.

On better models, there are push button calibration features which make it just that little bit easier and less fiddly.

How often to calibrate

Calibrating a pH meter is like making sure a set of weigh scales are at 0 before using them (aka “taring.”)

For food, you only need to calibrate to 7 and then 4. Always start with 7.

It’s daunting at first and seems like a huge time sucker, but once you have done it a few times, it goes quite quickly, and you learn how to use the buffer solution judiciously to get 110% accurate calibration results without needing to have a tanker truck of buffer solution out back.

All pH meters must be calibrated (checked against a known standard) to assure accuracy. Standards are colored liquids of known pH. Purchase a meter that uses at least a 2-point calibration; for acidified foods you will calibrate your meter with pH 4.0 and 7.0 buffers.”  ^[10]Ingham, Barb. Purchasing and Using a pH meter.

The standard recommendation is to calibrate each and every day that you are going to use your pH meter on. If you are going to test one thing that day, you need to calibrate first, once. If you are going to test 10 things that day, you just need to calibrate once for that day, at the start.

The pH meter must be calibrated using a 2-point calibration with pH 4.0 and 7.0 buffers. The pH meter must be calibrated each day that you use it.”  ^[11]Ingham, Barb. Purchasing and Using a pH meter.

For the record, you will see that the manufacturer’s advice can differ.

Hanna meters says,

Perform a calibration on day 1. On the following day, simply soak your electrode in the original buffers you performed calibration with and note the readings. If the readings are still within your expectations, keep doing this procedure everyday until you are not happy with the accuracy. Then proceed to calibrate and take note of the number of days that went by since day 1 of calibration. If 5 days went by, proceed to calibrate every 4-5 days. As the pH electrode gets older, proceed to do this test monthly to confirm number of days required between calibrations”  ^[12]Hanna Meters FAQ. Accessed March 2015 at https://www.hannacan.com/FAQ_en.html

Oakton meters says of its pHTestr 10, 20, 30, 10BNC, and Spear models:

Calibration should be done regularly, preferably once a week”  ^[13]Oakton Instruction Manual. 68X068042 Jun 10 Rev 7

It would appear that as the electrode gets older, it may need more frequent calibration. If the electrode needs cleaning because something is gummed onto the surface, it may need calibration. But the only way you know this, is by actually testing it by calibrating it. So for food items where health and safety is concerned, there is a strong argument to calibrate at the start of each usage session.

Perhaps that is why the food people are saying to calibrate every time. “The pH meter MUST be calibrated at least daily, or once per shift, if multiple production shifts are scheduled.”  ^[14]McGlynn, William. Choosing and using a pH meter for food products.

Calibration solution

Calibration also involves solution. There are three pHs of solutions used for calibration: 4.0 pH, 7.0 pH, and 10.0 pH. For home canning purposes, 4.0 and 7.0 are all you care about, so you don’t need the 10.

Calibration solution is always sold separately, and always ridiculously pricey for what it is (especially when you see the extremely low base rates that university labs actually pay.) But again, that’s how it is.

You can buy calibration solution in sachets or bottles. The bottles work out vastly cheaper than the sachets — that is, presuming you are going to be using your pH meter that much.

You can’t re-use calibration solution once it’s used. It’s a one-time shot. So if you have a bottle of it, you will want to use enough to do a proper calibration, but no more than you have to, because you dump what you used out afterwards. It won’t be accurate if re-used, and the whole pH and calibration exercise is definitely one of those things in the category of “if you’re not going to do it completely right, there’s absolutely no point in even starting it.”

It is important to use freshly dispensed buffer for calibration. The pH of buffer solutions exposed to air will eventually change due to evaporation and absorption of carbon dioxide from the air.”^[15]McGlynn, William. Choosing and using a pH meter for food products.

Makers of calibration solution also caution against storing bottles of it for years and years. Vendors say it has a one year shelf life. Some experienced users say that it’s the 10.0 that has the one-year shelf life, and that the 4.0 and 7.0 should be good for two years.  ^[16]https://www.reefscapes.net/articles/breefcase/ph_calibration.html . Accessed June 2015. It may be a good idea to write the month and year of purchase on your bottles.

Lifespan of pH meters, and replacement parts

The most important, and most fragile, part of a pH meter is the electrode tip (aka probe) that actually does the detection work. On cheaper pH meters, it’s permanently affixed and when it dies, the whole meter is then garbage.

Meters may come either with detachable, replaceable probes or they may be an all in one unit with an integral probe. Both types may work equally well. The units with detachable probes typically cost a bit more. The all-in-one units are more convenient and may require less maintenance. The important consideration is that all pH probes have a finite lifespan. The useful life of a probe is strongly influenced by the use, or abuse, it receives. But even in the best case, one may expect a probe to have a useful lifespan of about one to three years. Units with a detachable probe allow a user to replace only the probe as needed. All in one type units will need to be completely replaced.”   ^[17]McGlynn, William. Choosing and using a pH meter for food products.

On better models, the electrode probe is replaceable, so that’s obviously desirable if possible. Still, you will want to be sure to check first the cost of replacements, if only to be fully aware of what you are getting into:  “When considering which pH meter to purchase, consider the cost of replacement electrodes.”   ^[18]Ingham, Barb. Purchasing and Using a pH meter.

When to test your product with a pH meter

Do not take the pH of a product just before or right after canning because it will not be an accurate measure of the equilibrium pH (explained below.)

For a proper pH reading, you should test the pH of the product no sooner than roughly 24 hours after processing, once the jars have cooled to room temperature and stabilized.

Note too, that pH changes over time as the food is stored. For this reason, some people recommend to check the pH after several months.

You are going to test the solids, not just the liquid

It’s pretty clear that if a liquid put inside of a jar of dill pickles made with a tested recipe was pure vinegar, that the pH of that liquid has a pretty good chance of being fine. But what you don’t know is, did enough of that vinegar make it into the centre of the cucumber pieces to make them safe all the way through?

For this reason, we want to test it all, even inside the bits. This is called testing the “equilibrium pH.”

Equilibrium pH is the pH of a food product after the added acid has reached throughout the food; the pH of the acid brine and the food have equilibrated (terrible verb, but that’s what they say.) When you monitor pH as part of process monitoring, it is the equilibrium pH that you are measuring.

The best way to do this is to purée up a slurry of the solids and liquid in the jar.

You can test the pH of one jar after the acid in the liquid has equilibrated with the vegetables or fruit — that is, after the acidity has become consistent throughout the ingredients of the jar. This takes about three weeks. To test the pH of fresh or fermented pickles before they’re stored, suggests Kenneth Hall, professor emeritus of food science at the University of Connecticut, puree the contents of one jar in a blender. For both fresh and fermented pickles, the pH should be no higher than 4.0… I recommend the more accurate pH meters available from merchants who specialize in scientific instruments.”  ^[19]Ziedrich, Linda. The Joy of Pickling. Boston, Massachusetts: The Harvard Common Press. 2009. Page 12.

Actually using your pH meter

So, enough background theory and thoughts. Now, how to actually do it.

How to calibrate a pH meter

First, we know we have to calibrate. Allow about 10 minutes to calibrate your meter.

We will use an Oakton meter as an example.

• You pour out a small amount of 7.0 solution in a glass (it does not need to be a huge beaker as shown in the video below), and a small amount of 4.0 in a glass. If you are pouring from bottles, screw the caps tightly back on the bottles right away.
• Turn the meter on.
• Stand or prop up the meter in the 7.0, and let the meter stand in it till it stabilizes at a reading of close to 7.0, or as close as it is going to get. This could take several minutes, till it stops changing.
• Hit the confirm button.
• Rinse the tip of the meter in distilled water.
• Stand or prop up the meter in the 4.0, and let the meter stand in it till it stabilizes at a reading of close to 4.0, or as close as it is going to get. This could take several minutes, till it stops changing.
• Hit the confirm button.
• Rinse the tip of the meter in distilled water.

After calibration, do a quick check that you’ve got it right, by rinsing the meter then dipping the meter back into the 4.01 solution to make sure it registers the 4.0 correctly. “After calibration, check the accuracy of the meter by testing the pH of a standard buffer solution, such as a pH 4 buffer.”  ^[20]McGlynn, William. Choosing and using a pH meter for food products.

Discard the 7.0 and 4.0 solutions down the drain. Rinse the meter a final time.

You are done and ready to start using the meter.

Here’s a video on how to calibrate. (Note that calibration to 10 is generally said to be not needed with food products).

https://www.youtube.com/watch?v=tM5aHQ8Dfxo

How to test the pH of your product

Have the meter calibrated. You only need to calibrate it once per session, no matter how many items you measure in a session.

Allow about 10 minutes per reading . That allows for the prep work of getting a slurry ready, plus the actual measuring time, plus the cleaning of a blender etc, in between readings if you are doing multiple readings and making multiple slurries in one session.

(Reminder: you don’t test a food product just before you can it. You test it after canning.

1. How much to test?

If you are going to be selling it, the advice is one whole jar from each batch.

“Open one jar and take a representative sample of your food product once it has cooled, usually 12 to 24 hours after processing. You should sample each batch”  ^[21]Ingham, Barb. Purchasing and Using a pH meter.

Other professionals in the field seem to be saying that for home kitchens and personal use, a one-time, representative purée from an opened jar is enough.

2. Prepare the food

“Note that food samples should be at a constant temperature, preferably room temperature, when tested for pH.”  ^[22]McGlynn, William. Choosing and using a pH meter for food products.

(i) Uniformly already-puréed food

This is food products such as apple sauce, barbeque sauce, ketchup, chili sauce, etc.

“If a food is homogeneous, that is of uniform consistency, then the pH of any portion may be considered to be representative of the whole.” ^[23]McGlynn, William. Choosing and using a pH meter for food products.

(Note: very thick sauces should be treated as solids, and have some distilled water mixed into them: “Examples of semi-solid foods include puddings and very thick sauces. These foods should be blended to a uniform paste before testing. If additional liquid is required to blend the samples, up to 20 parts deionized or distilled water may be added per 100 parts food sample.”^[24]McGlynn, William. Choosing and using a pH meter for food products.

(ii) Chunky foods. Salsas, chutneys, relishes, pickles, etc.

You need to purée this in a blender, with distilled water, into a slurry.

Here are the various pieces of advice from the food experts.

pH of the solids

“Many food products, such as chunky salsas and pickled vegetables, consist of a mixture of solid particles in a liquid brine or syrup. In these foods, the solid portion may differ in acidity from the covering liquid.”^[25]McGlynn, William. Choosing and using a pH meter for food products.

“Strain the solids, draining out the liquid (brine) from the jar. Place the strained solids into a blender. Blend the product, adding distilled water if necessary, to produce a slurry. Added distilled water will not change the pH of the product and will allow for effective blending. You can purchase distilled water at many grocery stores or drug stores.”  ^[26]Ingham, Barb. Purchasing and Using a pH meter.

“Separate the liquid and solid components by draining the contents of the container for two minutes on a screen or sieve. Regulations specify a U.S. standard No. 8 sieve (available from scientific supply companies), inclined at a 17 to 20 degree angle. … Rinse the drained solids with deionized or distilled water to remove any remaining covering liquid. Blend the solids to a uniform paste and measure the pH …  If additional liquid is required to blend the samples, up to 20 parts deionized or distilled water may be added per 100 parts food sample.”  ^[27]McGlynn, William. Choosing and using a pH meter for food products.

Equilibrium pH

To determine the ‘Equilibrium pH’, when you drain your solids, have the liquid that drains off go into a bowl or cup. then after you have tested the blended solids, mix the two solutions together and check the pH of that. “To determine the ‘Equilibrium pH’ of the food …. blend fractions of both solid and liquid portions in the same ratio as found in the original container ” ^[28]McGlynn, William. Choosing and using a pH meter for food products.

(iii) Oily Foods

You are rarely going to encounter oily foods in safe home canning, but for special cautions (the oil can interfere with accurate measurement) see here:  ^[29]McGlynn, William. Choosing and using a pH meter for food products.

Doing the actual test

Ensure the meter has been rinsed in water first (either tap or distilled water, based on your manufacturer’s recommendations) after either calibrating or the previous measurement you’ve just finished.

Place meter in one half the sample. Give it a minute to stabilize, record reading rounding it to the nearest 0.05 pH unit (or .1 if that is all your meter allows.) Then rinse meter again, and do a reading in the second half of the sample. The two readings should agree within a very close margin.

Immerse the sensing tip of the probe in the sample and record the pH reading to the nearest 0.05 pH unit (or the nearest 0.1 unit depending on the resolution of the meter). Allow at least one minute for the meter to stabilize.  Rinse the probe, blot dry and repeat step 2 on a fresh portion of sample. The two readings should agree to within the accuracy limits of the meter.”  ^[30]McGlynn, William. Choosing and using a pH meter for food products.

Record

“Record the results in your batch log.” ^[31]Ingham, Barb. Purchasing and Using a pH meter.

“If you are canning acidified foods, accurately monitoring and recording the product pH is key to knowing that you are selling a safe product.” ^[32]Ingham, Barb. Purchasing and Using a pH meter.

Rinse

Always rinse off your pH meter right away after each measurement, using either tap or distilled water, based on your manufacturer’s recommendations. This is to help ensure it has the longest lifespan possible. Never allow sampling solutions to dry on the probe. “DO NOT allow food samples to dry on the pH probe. Keep it clean to extend its useful life.”  ^[33]McGlynn, William. Choosing and using a pH meter for food products.

For advice on how to clean a probe end that got dirty anyway, consult your manufacturer’s instructions, and you can also see here.

pH meter models that appear to be popular with home canners

Listed in order of decreasing accuracy

Oakton pHTestr 20 (Accuracy .01)

Oakton Ero Tester (Accuracy .1)

Checker by HANNA Instruments  98103 (Accuracy .2)

Thermoworks

Some people are using Thermoworks Waterproof pH Meters . There are three different models,  of varying accuracy from a low of .3 to .05.  Manuals are here.

At HealthyCanning.com, we decided on the Oakton pHTester 20, as it had the greater accuracy of all those mentioned above. We use it only to test “already tested recipes” from reliable sources. The purpose was to demonstrate to readers just how safe those tested recipes were. The safety margins proved to be massive, proving that those tested recipes can indeed be followed with complete confidence.

Further reading on pH meters and home canning

Acidity / low pH is what makes water bath canning safe

Acidity of water and vinegar combinations in home canning

The role of pH in home canning

Ingham, Barb. Purchasing and Using a pH meter. University of Wisconsin Extension Services. October 2009.

McGlynn, William. Choosing and using a pH meter for food products. Oklahoma State University Extension.  FAPC-117.

Purchasing pH Meters: Fact Sheets for the Small Scale Food Entrepreneur. The Northeast Center for Food Entrepreneurship at the New York State Food Venture Center, Cornell University. Accessed August 2016.

Unofficial advice on using pH meters with home canning

Bronee, Amy. Testing home canning with the LAQUAtwin pH meter. Blog post. 13 February 2014.

McLaughlin, Marissa. How to Can Creatively and Still Be Safe. Blog entry. 15 December 2010.

Trivedi-Grenier, Leena. Buying a pH meter and testing my onion jam. Blog entry. 25 August 2011. Accessed March 2015 at https://www.leenaeats.com/blog/recipes/leena-cooks/leena-cans-buying-a-ph-meter-and-testing-my-onion-jam/