Measurement of pH presents a critical quality control step in the production of dairy products, especially yogurt. pH offers an indication of contamination from bacteria or chemicals, while also providing a convenient method to estimate the acid development of a dairy product. As there are a myriad of different sampling methods, electrode care guidelines, and electrode designs, determining best practices for pH measurement can be a challenge. This article will discuss electrode selection, calibration techniques, sensor maintenance, and best practices for measuring the pH of yogurt. While the focus is placed on yogurt, the guidelines reviewed can be readily applied to a much broader range of dairy products and quality assurance procedures.
Role of pH Monitoring in Making Yogurt
Monitoring pH is crucial in producing consistent, quality yogurt. Yogurt is made by the fermentation of milk with live bacterial cultures. Following pasteurization and compositional adjustment, milk is homogenized for a consistent texture, heated to the desired thickness, and cooled before inoculation. Most yogurts are inoculated with a starter culture consisting of Lactobacillus bulgaricus and Streptococcus thermophilus. Once the live culture is added, the mixture of milk and bacteria is incubated, allowing for conversion of lactose to lactic acid. As lactic acid is produced, there is a corresponding drop in pH. Due to the more acidic mixture, the casein protein in milk coagulates and precipitates out, thickening the milk into a yogurt-like texture.
Yogurt producers cease incubation once a specific pH level is reached. Most producers have a set point between pH 4.0 and 4.6 in which fermentation is arrested by rapid cooling. The amount of lactic acid present at this pH level is ideal for yogurt, giving it the characteristic tartness, aiding in thickening, and acting as a preservative against undesirable strains of bacteria. By verifying that fermentation continues to a predetermined pH endpoint, yogurt producers can ensure their products remain consistent in terms of flavor, aroma, and texture. A deviation from the pH set point can lead to a reduced shelf life of the yogurt or a product that is too bitter or tart.
Syneresis is the separation of liquid, in this case whey, from the milk solids; this can occur if fermentation is stopped too early or too late, resulting in yogurt that is respectively too alkaline or too acidic. Consumers expect yogurt to remain texturally consistent, so ensuring fermentation is stopped at the appropriate pH is vital to consumer perception.
Choose the Right Sensor For the Job
Using the correct electrode for your application is one of the first and most important aspects to consider when measuring pH for quality control or analysis. A variety of electrode features can be combined to ensure reliable and repeatable results for specific samples such as yogurt.
Open junction reference. Conventional pH electrodes have a ceramic frit reference junction that allows the internal reference electrolyte to come into contact with the sample. In dairy products, such as yogurt, proteins and other colloidal solids can partially or completely clog this ceramic frit, resulting in slow electrode response or inability to take a reading. For yogurt, it is recommended to use a pH electrode with an open junction rather than the traditional single ceramic junction.
An electrode that has an open junction design utilizes a gel reference electrolyte that comes in direct contact with the sample; because there is no physical junction, potential clogging is no longer an issue. An open junction design offers the additional benefit of a faster response time because of a higher flow rate of electrolyte into the sample. Other types of clog-resistant, high-flow junctions exist, including polytetrafluoroethylene junctions, triple ceramic frit junctions, and ground glass junctions; these designs confer their own advantages, but are typically better suited for other applications.
Conic electrode tip. Conventional pH electrodes have a spherical sensing bulb that provides an increased surface area for the sample to interact with the sensing glass. This bulb shape is ideal for measurement in aqueous solutions. However, other tip designs exist on the market, and each shape offers an advantage in certain applications. For example, conical tipped pH electrodes are pointed so that they may easily penetrate semisolid or emulsified objects, including yogurts.
If measuring the pH of yogurt with an electrode constructed of a spherical bulb and ceramic reference junction, a homogenized slurry of yogurt and deionized water should be prepared. A slurry is necessary because the flow rate of electrolyte into a semisolid yogurt alone is too slow to enable a direct measurement. An electrode utilizing a conical tip shape in combination with an open reference junction allows for direct measurements of thick yogurt samples, thus saving on preparation time and eliminating a potential source of error. For thinner yogurts or other dairy products such as milk or cream, the spherical tip may be suitable due to its wider area of contact that permits a faster stabilization time. Ultimately, the selection of the tip should be based on the nature of the sample matrix.
Make Sure Your Sensor Works
Calibrate your electrode often. Prior to measurement, pH meters must be calibrated. Calibration adjusts how pH values are assigned to incoming mV (millivolt) readings from the electrode. The pH electrodes generate a mV potential based on hydrogen ion activity. This activity is determined by pH glass, which is specially formulated to measure the hydrogen ion. Hydrogen ions (H+) contribute to how acidic a sample is, while hydroxide ions (OH-) contribute to how basic a sample is. The pH scale ranges from 0 to 14, with pH values less than 7 being acidic, pH values greater than 7 being basic, and pH 7 being neutral.
As pH glass breaks down and changes over time due to normal wear and tear, calibration of the meter corrects for changes in the glass. The quality and frequency of calibration procedures will ultimately determine the accuracy of your data. For best results, it is important to calibrate the pH meter at least once per day with standards that bracket the expected pH range of the samples. Because milk and yogurt typically have a pH range from pH 6.7 to 4.0, ideal calibration standards are pH 4.01 and 7.01; a third buffer such as pH 1.68 or 10.01 may be incorporated for higher precision.
Perform periodic slope checks. The theoretical relationship between pH and mV is defined by the Nernst equation. Based on this equation, a theoretical electrode will read 0 mV in pH 7.0 buffer (the value of which is known as the offset), and will have a slope of -59.16 mV per change in pH unit. Calibration corrects for deviations of electrode behavior from this theoretical relationship, but the extent of this correction is finite before the accuracy of the measurement is affected. Many meters will have indications of electrode condition or slope condition, but it is recommended to use the mV mode on a pH meter to periodically check electrode offset and slope.
To perform an electrode offset and slope check, first measure and record the mV value in pH 7.0 buffer; this is the electrode offset. Next, measure the mV value in a second buffer, such as pH 4.0. To determine the electrode slope, calculate the difference in mV between the two buffers and then divide this by the difference of pH units between buffers. To convert this result to electrode slope percentage, divide the electrode slope by the theoretical slope of 59.16, and multiply by 100. An acceptable offset range is ±30 mV and slope percentage is 85 to 105 percent; anything outside of these ranges may result in inaccurate measurements.
Step 1. Measure mV of pH 7.01 buffer and record value
Step 2. Measure mV value of pH 4.01 buffer and record value
Step 3. Calculate the absolute mV difference (pH 4.01 value – pH 7.01 value)
Step 4. Calculate the slope (mV difference/3)/59.16) = Slope)
Examples:
Electrode 1
pH 7.01 = -15 mV
pH 4.01 = +160 mV
Absolute mV difference is +160 mV – (+15 mV) = +175 mV
Slope = (175/3)/59.16=98%
Electrode 2
pH 7.01 = +15 mV
pH 4.01 = +160 mV
Absolute mV difference is +160 mV – (-15 mV) = +145 mV
Slope = (145/3)/59.16 = 82%
Conclusion: Electrode 1 is working properly while electrode 2 has an unacceptable slope. If changing the fill solution, cleaning the electrode, and calibrating does not help, replace the electrode.
Improve Your Measurement Technique
Consistency is key when performing any pH measurement. This is especially true when many different users are expected to perform the same measurement with the same electrode and meter. Even with a reliable, properly prepared sensor, careless measurement practices can have detrimental effects on a critical pH result. This is especially true for yogurt, where the final measurement window is only four tenths of a pH unit.
Maintain the electrolyte solution. Yet another advantage of owning an electrode filled with gel reference electrolyte is that it does not have to be refilled. The gel electrolyte should last for the lifetime of the sensor. On the other hand, if you own a refillable pH electrode without gel electrolyte, the level of electrolyte fill solution should be inspected before performing any calibration. Over time, the solution flows out of the reference junction, which can happen faster particularly if the electrode is not properly stored. Low electrolyte levels may lead to drifty or erratic readings, so it is good practice to ensure that your electrode fill solution level is no less than one half inch from the fill hole.
Also for refillable electrodes, the fill cap should be removed or loosened prior to calibration and measurement. Removing the cap creates positive head pressure in the reference chamber of the electrode, allowing for a greater flow rate of electrolyte through the junction. This is important for a faster and more stable reading, especially given the thicker consistency of yogurt.
Properly submerge and stir. It is crucial that both the pH sensing glass tip and the reference junction be completely immersed in your sample to function correctly. Make sure there is enough yogurt sample present to perform a suitable measurement.
For pH measurements and calibrations, it is important to stir the sample or buffer. While an automatic or magnetic stirrer is preferred, simply stirring the electrode within the sample can help. It is understandable that this can be tricky for thicker samples such as yogurt, but movement of the sample ensures it is well mixed and helps increase the response time of the analysis.
Keep Up With Your Maintenance
Despite choosing an appropriate electrode and calibrating it correctly, poor maintenance and lack of proper care can reverse any diligence previously exercised.
Regularly clean your electrode. When pH is measured in dairy products such as yogurt, electrode fouling is a common challenge. Electrode fouling occurs when fats and proteins obstruct the reference junction or attach themselves to the sensing glass of the electrode. Electrode bodies and tips may also accumulate mineral deposits such as milkstone, a complex composed of organic matter, calcium, and magnesium.
Electrode fouling can be minimized with regular maintenance, cleaning, and proper storage. Buildup on the sensing glass causes inaccurate and sluggish measurements as it directly affects the impedance of the glass. An offset outside of the acceptable range of ±30 mV usually indicates the pH glass bulb is dirty or coated. Cleaning solutions are effective at both disinfecting and removing oil and protein deposits. These solutions typically contain a combination of mild detergents, solvents, and complexing agents designed specifically for samples of interest such as yogurt.
Always condition your electrode. Conditioning and properly storing your electrode goes a long way to increasing its life and performance. For conditioning and storage, it is recommended to use an electrode storage solution. This solution helps minimize junction clogging and ensures a fast electrode response time, keeping the sensing glass and junction clean and hydrated. When solution isn’t available, it is advised to use a pH 4.01 or 7.01 buffer. The electrode should not be stored in deionized water as this can affect the sensing tip and degrade the electrolyte solution inside of the electrode.
Following these guidelines for selection, calibration, and maintenance can help to ensure reliable, repeatable, and responsive electrode for yogurt measurements.
Masulli is an application engineer at Hanna Instruments. Reach him at [email protected].
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