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Know Your Meter: pH Meters are one of the most important tools in a Cultured Products Plant

In today’s production plant environment, it is critical to measure and record pHs accurately. After all, the controlled production of lactic acid is what we are measuring when we check the pH during fermentation of our cultured product. Having an accurate and reliable pH meter is imperative for achieving the correct break pHs and cutting pH (in the case of cottage cheese). All too often pH meters are not taken care of, or even standardized regularly. Many employees do not understand them or how to take care of them.  In my opinion, a pH meter is a plant’s best friend and should be treated as such. I will address what pH is, which pH meters might be best for measuring pH in fermented dairy foods, how to take care of that pH meter, and which probes might be the best choice for use near a cheese room.

What is pH?

A pH meter is an instrument that displays a value that is converted to pH units from an attached electrode that is sensitive to H+ (hydrogen ions) in the solution you are measuring. As you know, pH is a measurement in a numerical value from 0-14 that tells how acidic (low pH from 0-7) or basic (high pH from 7-14) a solution is. The electrode is usually made of pH sensitive glass; although there are now gel electrodes and even ion specific field effect transistors (ISFET) that have a chip instead of the typical glass or gel electrode to measure pH. Therefore, pH is a way to compare the relative acidity or alkalinity of a solution (or product) at any given temperature. At pH 7, the pH is considered neutral (neither acidic nor alkaline) since the solution (or product) would contain the same amounts of hydrogen ions (H+) and hydroxide (OH-) ions, which in effect neutralize one another. Sour cream and yogurt have pHs near 4.0 - 4.5, so it is important to have a pH meter that measures acidic conditions accurately.

A change of 1 tenth of a pH unit; say from a pH of 4.6 to a pH of 4.5 is a dramatic change in the amount of acid in your cultured product. Missing a cottage cheese cut pH of say, 4.7 and cutting cottage cheese coagulum at pH of 4.6 rather than at pH of 4.70 equals 25.9% more acid in your cultured product as when the pH was at 4.70. You know what acid does to cottage cheese curd…. (it makes it softer if cooked to the same cookout temperature) so it is important to accurately measure pH!

Choosing a pH meter:

Most pH meters in today’s marketplace are accurate to +/- 0.01 (one hundredth) pH unit and some up to 0.002 (two thousandths) pH units. Those old needle type pH meters with a line scale that were graduated to 0.10 (a tenth) of a pH unit are (or should be) a thing of the past. If you are still using one of those old needle pH meters, I suggest you upgrade and come into the modern digital age! I do not promote or endorse any kind of pH meter, but see many of the Orion brand of meters in use out in the plants.

If I were to choose an accurate ATC (automatic temperature control) pH meter, I might consider Thermo Scientific Orion Star A211 for a lower cost meter, or an Orion Dual Star Mutiparameter meter as a better choice, or even the Orion Versa Star Electrochemistry meter for auto recording into a database as maybe the best choice if you like to review data and have a record of the pH recordings.

Which Probes or pH Meters Might Be Best for Measuring Fermented Dairy Foods?

Selecting the right pH meter with the right electrode is imperative. An electrode compares the pH reading in a sample with that of known buffers (usually pH 7.0 buffer and pH 4.0 buffer, when measuring acidic solutions). The most common cause for error in pH measurement is temperature of the sample! Temperature affects the slope and this can change with variations in temperature. It is critical to get a pH meter that adjusts the pH reading to the temperature of the sample (one with an automatic temperature compensation or ATC probe). Selecting a pH meter that is accurate to a hundredth or two hundreths of a pH unit should be accurate enough.

There are so many pH probes to choose from, I can’t possibly cover them all here. Several types would be more conducive to a production environment rather than in a lab, but don’t sacrifice accuracy for the need to keep the pH meter in a protected cabinet (or even out in another room) to save the cheesemakers a few steps. A flat serviced tip is a probe that I see a lot out in the plants and even one with a “rugged” glass bulb might be better for using in a cheese room. Epoxy bulbs are more durable than glass bulb electrodes.

Electrode fill types

  1. Refillable electrodes require periodic filling and draining. They have a long expected life span and are accurate to +/- .01 to .02 pH units. They have the best response time (fastest pH reading).
  2. Polymer filled electrodes are low maintenance and no filling solution is needed. They are sealed, accurate to +/- .02 pH units and have a good response time.
  3. Gel filled electrodes are easy to use, but are less accurate from +/- .05 to 0.10 pH units and don’t have as good a response time as the others.

I guess if I were to choose one for a cottage cheese room, I would pick a pH meter accurate to +/- .02 pH units with automatic temperature compensation with a flat surfaced tip, perhaps an epoxy body rather than glass, with an open junction for less clogging, and be refillable or even polymer sealed. I would stay away from ISFET meters, as I would think they would not have the accuracy necessary for measurement of cut pH in a cottage cheese plant.

Taking Care of the pH Meter (the electrode):

    Taking care of the pH meter involves just a few simple things. Foremost, it should be standardized at least daily by the same person at the same time, or preferably standardized perhaps 3 times daily at the beginning of each shift, so that the cheesemakers will have confidence in the readings. If each cheesemaker standardizes the meter themselves, then they will know it is accurate. That means making sure all operators have training in how to standardize the pH meter, what solution to soak the electrode in between measurements, and how to take an accurate reading (rinsing off the soaking solution and sample after each measurement). After training is accomplished and everyone feels comfortable, taking care of the pH meter is about taking care of the electrode. This entails inspecting it at the end of the week of use for cracks, salt crystal buildup, and membrane/junction deposits (the little measurement area on the tip). If it is cracked replace it ASAP. If it has salt crystal buildup rinse or soak it in distilled water. If it has deposits on the tip soak the electrode in 0.1 molar HCl or 0.1 molar HNO3 solution for 15 minutes.

    If a refillable electrode is used, make sure the liquid level is ¾ to 9/10 full up to the filling hole at the top of the electrode. Keep your electrode problem free for accurate and consistent pH readings from week to week. Electrodes are not good forever! Did you know they have a “shelf-life” on them, if you will! Most of them have a shelf-life or recommended use life of 6-12 months. After that, buy a new one to maintain accuracy.

    Also, be careful of mixing rinse water into the storage solutions and buffers. It is best to dab the tip of the probe off with a non-abrasive Kimwipe towelette before storing the probe. And if your buffer has expired (there is a good till date printed right on the bottle), or it gets contaminated during the day, change it before your next standardization to keep the reference buffers reading accurately.