Why Measure Acidity and Moisture?
“All cheesemakers should be monitoring acidity and moisture because they drive how the cheese is going to turn out,” explains Andy Johnson, former outreach specialist/assistant coordinator, CDR Cheese Industry and Applications group.
Dean Sommer, CDR cheese and food technologist, agrees, “Controlling acidity and moisture content—those are two of the really big keys to successful cheesemaking. A lot of cheese defects are from improper moisture content and improper rate and extent of acid development.”
Cheesemaking is ultimately a concentration process that involves monitoring and controlling multiple factors and reactions. “We’re reducing the moisture content, we’re concentrating fat, we’re concentrating protein, and we’re using up a large majority of the lactose retained in the curd to develop a significant amount of lactic acid, which drops the pH,” Sommer said.
Milk has a pH of about 6.6-6.7, but during cheesemaking acid is developed and the pH drops, i.e. Cheddar has a pH of about 5.1. The cheesemaking process also reduces the moisture content—milk typically has a moisture content of about 87% whereas Cheddar has a moisture content of about 38%.
However, cheesemaking isn’t just about reaching a final moisture level or pH, it’s about controlling the loss of moisture and the rate and extent of acid development. As cheesemakers say, “The journey matters.” Therefore, it’s important to measure and control acid development and the process of moisture reduction during cheese manufacture. Acid and moisture impact the flavor, microbiological content, and structure/texture/body of the cheese.
Why Keep Records?
A good cheesemaker is also a good record keeper. This is especially true when it comes to measuring and recording acidity during cheesemaking.
“If you have a defect or run into a problem with your cheese, it’s almost impossible to figure out what happened if you don’t have a record of the pH development,” Johnson said.
Cheesemakers need to be able to go back and look at their records to see the “journey” that their cheese took during the cheesemaking process. Johnson recommends that cheesemakers detail all observations (not just pH), especially if something is abnormal like odor or the color. Record the milk quality measurements, heat-treatment (time/temperature) and starter growth conditions. Keep detailed records during manufacture of the pH, temperature, time, and curd firmness. Ideally, all batches of cheese would be tested for composition and microbiological quality.
Issues with Improper Acid development
Again, a lot of common defects seen in cheese are related to an improper rate and extent of acid development (too little, too much, too fast, too slow). Here are some of the characteristics associated with excessive or inadequate acidity.
- Excessive acidity (low pH) characteristics of cheese: Short/brittle body in dry hard cheese. Pasty body in soft cheeses. Grainy texture and acid (bitter) flavor. Cheese loses serum, which results in sweating. The cheese is more prone to developing calcium lactate crystals and microorganisms at the surface of the cheese (rind rot if packaged). A common underlying cause of these issues is excessive starter activity.
- Inadequate acidity (high pH) characteristics of cheese: There is a food safety concern (pH>5.6). The curds may not knit very well. A short, crumbly cheese body and a tough, curdy texture. Can result in poor functionality/melt/stretch. Underlying cause: excess calcium remaining bound to the casein (more on this later).
Measuring and Recording Acidity
What is pH exactly? pH is a measurement of the concentration of hydrogen ions. A neutral pH is 7.0 (water). A low pH, like 3.0, is very acidic (orange juice). A high pH, like 12, is very basic (caustic soda). As mentioned, milk typically has a pH of 6.6-6.7.
One important aspect of the pH scale is that it is logarithmic. For example, a pH of 5.0 is 10 times more acidic than 6.0. “What this means to a cheesemaker isthat if you’re shooting for a 5.2 pH and you get a 5.4—that’s a big deal,” Johnson said. “And if you’re shooting for a 5.2 and get a 5.5 or 5.6 that’s a huge deal. You’ve basically made a different cheese. Tenths of a pH unit make a difference in cheesemaking.”
Most cheesemakers measure pH by using a pH meter (Potentiometric Method). A pH meter is composed of a pH electrode (probe) connected to a voltmeter (pH meter). How does a pH meter work? Two electrodes (housed in one probe) form a circuit and measure the voltage output from the hydrogen ions. Then the pH meter translates the voltage output into a pH value.
The pros of pH meters are that they are highly accurate if calibrated correctly and easy to operate. They also compensate for temperature, which can impact pH accuracy. The cons of a pH meter are that pH probes do require regular cleaning and calibration otherwise readings will be inaccurate. Also, good meters and probes are expensive, and any needed repairs or replacement parts will also be costly (probes are typically a few hundred dollars). There are less expensive models that cheesemakers can successfully use but these are not ideal to work with and will need to be replaced every year or so. When purchasing a pH meter, you also need to keep in mind that separate probes are needed for milk or whey and cheese.
Make sure to follow the manufacturer’s instructions for probe care and maintenance (proper storage will extend the life of your probe and maximize its performance). One tip is to never store the probe in distilled or deionized water—this will cause ions to leach out of the glass bulb of the electrode and will eventually ruin it. CDR recommends that the probe be stored in a solution of 4M KCl (never store the probe dry).
Taking a pH Reading
The process of taking the pH of milk/whey or curd is relatively simple although it is important to do it correctly. To take a pH reading of a liquid sample (milk or whey), first take the probe out of the storage solution and rinse it with distilled water, blot it dry (with a laboratory wipe like Kimwipes or similar), immerse the probe in the milk or whey and gently stir the milk/whey with the probe and keep agitating it with the probe until the meter reaches a stable reading. Then take the probe out of the milk, rinse the probe and return it to the storage solution.
Taking a pH reading of a curd sample is similar. First you take the probe, (a spear tip probe for solids) out of the storage solution, rinse it and blot it dry. Then get your curd sample and squeeze it to get as much whey out of it as possible. Insert the probe into the cheese and hold the probe in the cheese until the reading stabilizes. Once you have a reading, remove the probe, rinse it, blot it dry and return it to the storage solution. For more details on taking accurate pH reading, you can refer to the book “Standard Methods for the Examination of Dairy
Products” Wehr and Frank, 2004.
One issue cheesemakers face is the question of whether to record pH for both whey and curd. Some cheesemakers measure the pH of the whey, some do the curd, and some, like CDR, record both.
“You probably do not need to measure both whey and curd, but one thing to keep in mind is that you need to be consistent in which you measure because they are different,” Johnson said. “Typically, the curd pH is .1 to .2 lower than the whey.” If a cheesemaker adds water or
rinses the curd, pH measurement of the curd is desired.
The curd is usually lower in pH because the curd is where the starter bacteria are creating the acid and releasing it into the whey. One advantage of measuring both curd and whey is that you can get a good indication of how fast your acidification is going based on the how far apart the pH is of the whey and curd.
Titratable Acidity
Before pH meters, cheesemakers used a titrator or acidometer to measure titratable acidity (TA) of milk or whey. Titratable acidity is related to pH in that the more acid developed, the higher the TA. TA is also a measurement of the buffer capacity of the milk or whey. Of interest to cheesemakers is the calcium phosphates, which have a direct effect on the buffering capacity of the milk. The more casein in milk, the more calcium phosphate. Since the calcium phosphate is the main buffer in milk and TA measures buffer capacity, a higher TA sometimes is not associated with more acid. It is the change in TA over time (more starter culture activity) that the cheesemaker needs to monitor.
Additionally, if you have a seasonal milk source, casein and calcium phosphate will change throughout the season so TA is a good way to monitor these changes in the milk. The actual process of taking a TA reading is more complicated than taking a pH reading.
As a cheesemaker, if you have to pick one (pH or TA) it is recommended that you use pH because pH will give a more consistent reading between cheesemakers and a pH meter is able to measure the pH of liquids (milk/whey) and solids (curd). Some cheesemakers who use high solids milk will use both because TA will indicate starter activity sooner than pH. Milks high in calcium phosphate may show little change in pH but a large change in TA.
Role of Acidification in Cheesemaking
Acid development drives the cheesemaking process. Acid development occurs because the starter bacteria ferment the lactose in the milk and produce lactic acid, which drives the necessary reactions for cheesemaking. One of those reactions is that the lactic acid solubilizes colloidal calcium phosphate and thus helps determine the level of calcium in the cheese curd and the ratio of soluble to colloidal calcium. These factors, in turn, greatly influence cheese texture and functionality. Acidity also impacts the activity of the coagulant during manufacture and ripening. Acidity promotes syneresis (expulsion of moisture from the cheese curd) and therefore has a large impact on cheese composition, particularly the moisture content of the cheese. The lactic acid also influences the activity of enzymes during ripening and, hence, affects cheese flavor and quality. Finally, acidity helps control or prevent the growth of spoilage or pathogenic microorganisms.
Acid takes time to develop during the cheesemaking process—the starter cultures need time to ferment or consume the lactose. So, at the beginning the pH is higher but it drops during cheesemaking. As mentioned, lactose is the fuel for acid development—the more lactose, the more lactic acid that can be produced. Fortifying the cheese milk with extra lactose containing solids (condensed skim, NDM) will add more lactose to the system, allowing for more lactic acid to be produced, resulting in a lower pH cheese. Conversely, cheesemakers can remove some lactose from the milk by using ultrafiltration (UF) and adding back water into the UF milk. This will help remove some lactose from the system, resulting in less of a pH drop in the cheese. CDR has been promoting this method as a strategy to avoid making “acid cheese.”
“One of the single biggest cheese defects we’re seeing is acid cheese,” Sommer said. “We’re doing a lot of work using ultrafiltered cheese milk and diluting it with water. This brings down the ratio of lactose to protein. So, there’s less lactose in the system and you don’t develop acidy cheese.”
Acid Development During Cheesemaking
Now, let’s get into acid development during the cheesemaking process. Again, it is critical to measure and record the acidity during the cheesemaking process (including during aging and of the final cheese). This is where a good make sheet with a lot of data points is crucial. For example, at a minimum, cheesemakers should be recording pH of the initial milk, before adding the coagulant, at cut, during milling/salting and hooping/brining. We also need to continue recording the pH during ripening /aging at day 1 and so on.
It’s important to measure the pH of the initial milk because you want to know the “starting point” of the cheese. The pH of the initial milk will give an indication of the quality of the milk. For instance, if the pH reading of the milk is high (more than 6.8) this is likely a sign of mastitis. If this is the case, you will want to monitor the
somatic cell count of the milk. Recording the pH of the initial milk is helpful in other ways as well. For instance, pasteurization slightly lowers the pH, also the stage of
lactation will impact pH and the type of milk (seasonal milk, sheep or goat milk) will also impact pH. In general, if the pH of the initial milk is off, you will want to investigate why.
Creating Wet Acid
The next point in the cheesemaking process when it is important to know (and record) pH is before adding the coagulant. This is when the cheesemaker is creating “wet acid.” It’s called “wet acid” because you are creating acid before draining the whey. Monitoring acid at this point is important because wet acid development has the greatest impact on calcium solubilization. A lower pH before coagulant addition will allow more calcium to be solubilized (removed) from the casein (this also results in a higher moisture cheese). For example, we can have two cheeses that both have a final pH of 5.2. Even though they have the same final pH, one has a curdy texture and the other has a nicer, less curdy body that allows for an easier shred. The difference is that the curdy cheese had a pH of 6.5 at coagulant addition compared to the nicer cheese, which had a pH of 6.35 at coagulant addition. “That’s the power of creating wet acid where you’re solubilizing calcium,” Johnson says. This also illustrates how a “small” variation in pH (6.5 compared to 6.35) can have a big impact on the final cheese.
This also brings up an important point regarding the development of
“wet” versus “dry” acid. As mentioned, wet acid is developed early in
the cheesemaking process
and up to the point that the whey is
drained. Dry acid is developed after the whey has been drained and the
cheese is moved to hoops or forms (the acid is developed when the cheese
is “dry”). The wet and dry acid dynamic is an important concept for
cheesemakers to understand. For instance, let’s say we are making
Cheddar. Sommer says he wants the pH to be 5.4 when the Cheddar is going
into the hoop because a lower pH reading at this point shows that he
has created wet acid (pH has lowered) before the whey was drained.
However, Sommer said that a lot of cheesemakers rely on dry acid and
will hoop their Cheddar when it has a pH between 5.8-6.0.
“I’d run screaming out of the door if I saw that,” Sommer said. Why? Because, if a cheesemaker develops acid in the milk or curd/whey (wet acid) then a lot of the lactic acid will go in the whey and will be removed when the whey is drained. Conversely, if acid is developed in the hoop (dry acid) where will that lactic acid go? It will stay in the cheese. This results in an acid Cheddar that is more likely to develop a bitter taste, whey taint flavors, and defects like calcium lactate crystals.
Cheesemakers next need to record pH when draining the vat. As
described previously, this pH reading will let the cheesemaker see if
they have developed the right amount of acid, depending on whether they
were trying to develop wet acid or if they are relying on more dry acid
development. When draining the vat, we are taking lactose away from the
bacteria, which will slow down acid development.
If making a milled or stirred curd cheese, you will want to measure
pH before milling and before salt addition, respectively. Knowing acid
development before adding salt will allow you to better target final pHs
in your cheese as some cultures are inhibited by salt.
For a brined cheese, whether you’re pumping the curd into a pre-press or taking it right out of the vat to hoops, you want to measure an acidification point at hooping because at this step you’re basically switching from forming wet to dry acid formation. It’s important to get this acid reading for eyed cheeses as typically you want a lot of unsolubilized calcium in the curd with these styles of cheese.
Buffering—Loss of Insoluble Calcium
Tracking and recording pH doesn’t stop when the cheese is out of the vat. It’s very important to record the pH history during post-manufacture because you want to
find the lowest terminal pH that the cheese reaches. “That [lowest terminal pH] is going to determine a lot of the final characteristics of your cheese,” Johnson said. It’s also important to record the cheese pH after manufacture because this is when the cheese pH is going to “buffer.” In other words, this is when the pH will reach its lowest point and then buffer back up until it reaches its final pH.
Buffering occurs because, as cheese ripens or ages, the hydrogen ions in the cheese displace the calcium ions and “disappear” or are absorbed into the cheese matrix. This causes there to be less free hydrogen ions in the cheese and therefore raises the pH. To track the cheese’s buffering and pH history post-manufacture, typically the pH is taken at 4 hours, 1 day, 1 week, 2 weeks, 4 weeks, and 3 months. Of course, there is a lot of variation considering the type of cheese being made. The cheese pH history post manufacture will help indicate if anything is going wrong with the cheese. For example, if making a Blue cheese, the lowest terminal pH will be around 4.8 and then buffer up to a pH of about 6.0. If the lowest terminal pH drops to 5.0, it’s going to be a different cheese in terms of flavor and texture. It will ripen faster.
Strategies to Control Moisture Content
Like acid, cheesemakers need to monitor and control the moisture content of the cheese. Moisture content impacts the quality and safety of the cheese and even a small change (± 1%) can have a negative impact on the final cheese.
Typically, cheese with too much moisture will have a weak body and pasty texture. There may also be food safety concerns because more moisture results in a higher
water activity (aw), which results in more microbial growth (bad) and an increase in proteolysis (good depending on cheese type).
Cheese with lower moisture levels can result in a hard, firm, corky body and texture. These cheeses have lower aw and higher salt-in-the-moisture-phase resulting in decreased microbial growth (good) but also less proteolytic activity (bad depending on cheese type).
Depending on the milk composition, you may need to adjust the moisture content. “As your milk changes, other parameters change; you’re going to have to change
your moisture content to match that,” Johnson says. For example, if you have higher levels of fat, you will need to adjust moisture level down to compensate for that
otherwise you might get pasty, weak bodied cheese. Of course, different cheeses require different moisture contents. The make for an aged Cheddar is different than
a young Cheddar. For an aged Cheddar, the moisture content needs to be 35-37% (depending on how long the cheese will be aged) to give the cheese the best chance to develop the correct body, texture and flavor.
Strategies to Influence Moisture Content
- Pasteurization: Influencing moisture content can start way back at pasteurization. For instance, pasteurization at <166°F will not significantly denature whey proteins and will produce a lower moisture cheese. However, pasteurization at >166°F will begin to denature whey proteins, which will bind or retain water and produce a higher moisture cheese.
- Culture Selection: Different cultures will also influence the final moisture of the cheese. Culture types and even strains within the same genus and species differ in their ability to retain or drive out moisture. For example, Sommer said when he worked in a dairy plant, he knew that certain cultures would produce cheese with a higher moisture content. So, he’d use cultures that retain more water for higher moisture cheeses like Monterey Jack and cultures that drive out more moisture for lower moisture cheeses like Cheddar.
- Add Water: Another strategy to make a higher moisture cheese is to add water to the milk in the vat. This will reduce whey expulsion, resulting in a higher moisture cheese. This strategy is more common in Europe, but if you’re a small artisan producer who is not concerned about whey quality, this might be a good option.
- Add Calcium Chloride: The addition of calcium chloride to the milk in the vat will improve whey syneresis (moisture expulsion) and result in a lower moisture cheese. However, this is not a good option if making aged Cheddar or Parmesan because if you add calcium chloride, you add less rennet and rennet is important in aged cheese because it contributes to the proteolytic activity that is crucial to the cheese aging process and flavor development. Adding calcium chloride is a better option for higher moisture cheeses like Colby and Monterey Jack because then less rennet is needed and you don’t want excessive amounts of proteolysis in
those varieties.
Cutting the Vat
The firmness of the curd when the vat is cut influences moisture content as well as fat loss. “Not all cheese varieties should be cut at the same curd coagulant firmness,” Sommer says. “This is part of the art and skill of cheesemaking.”
The firmer the coagulated milk is at cut, the higher moisture of the final cheese. Conversely, the softer it is at cut, the lower moisture content. For example, when making a high moisture cheese, like Feta, the curd is cut when it is very firm. “The curd is so hard you can walk across the vat,” Sommer said. “It is like concrete. Why? Because they want to retain that moisture in the curd.” Low moisture cheeses, like Parmesan or aged Cheddar, are cut when the coagulant is very soft because the cheesemaker wants to lose moisture but retain more milkfat in the curd.
Why is this? When coagulated milk is soft, the opening or pores in the curd are very small. Since, the pores in the curd are small, they don’t hold much moisture. And,
when the curd is harder, the pores are much larger and hold more moisture. In addition to impacting moisture, the firmness at cut also impacts fat loss. Cutting the curd when it is firmer, results in more fat loss. This occurs because the structure of the casein is more rigid when the curd is firm so it takes longer for the curd to “heal” after being cut. It can’t close the gap and trap the remaining fat, so some of the fat will leak out into the whey, as compared to softer curd, where the casein is thinner and less rigid, and therefore takes less time to heal so it traps the fat in the cheese.
A couple notes about coagulation. The rate of gel firming is:
- Increased by increasing concentration of casein,
- Increased by increasing amount of rennet used,
- Increased by adding CaCl2 (maximum level 0.02%),
- Increased by increasing the temperature, and
- Increased by decreasing the pH at rennet addition.
The other important factor to keep in mind when cutting is determining the size of the cubes. This is pretty straight forward—cutting into larger cubes will retain more moisture in cheese and smaller cubes will lose more moisture. So, if making aged Cheddar, you would cut smaller cubes and if making Feta or fresh Mozzarella, you will cut larger cubes.
Measuring Moisture
Typically, when measuring moisture content, you want to have a consistent time point—1-2 weeks if possible, (depending on the style) and longer for brined cheeses
because you want to have the cheese reach equilibrium before you sample it. To get accurate, consistent results, you need to take a representative sample and it’s best to sample the cheese when cooled. Also, if possible, it’s best to send a representative cut portion to the lab compared to a plug.
There are a couple different ways you can measure moisture. An inexpensive and easy option is to measure moisture using a forced draft oven (see Standard Methods for the Examination of Dairy Products, procedure 17.054). In this procedure, the first step is weighing the sample then placing it in an oven for several hours or overnight, and then weighing it again when it comes out of the oven to determine the moisture content. The loss of weight after heating is used to calculate moisture content of the sample.
In Summary
- Keeping the moisture content in the proper range is critical to achieve the correct cheese flavor and texture.
- Rate (speed) and extent (range) of acid development determines calcium retention and cheese body, texture and performance.
- Everything is connected—especially acidity and moisture. If you make an adjustment in one step, it will impact other areas and/or characteristics of the cheese.
“Cheesemaking is a holistic process—you change one parameter like moisture and you’re going to change another like acidity, which will result in changing the body,” says Sommer. “That’s where the artisanship of cheesemaking comes into play.”
Resources
Johnson, M. 2015. pH Control in Cheese. Dairy Pipeline: Volume 27, Number 1.
Johnson, M., D. Sommer. 2013. Basic Techniques to Gain More Moisture in Cheese. Dairy Pipeline: Volume 25, Number 3.
Johnson, M., D. Sommer. 2013. Importance of Acid Development on Cheese Milk Prior to Renneting. Dairy Pipeline: Volume 25, Number 3.
Wehr, M. Standard Methods for the Examination of Dairy Products. APHA Press: 18th edition (2024).
This article originally appeared in the Winter 2019 issue of the Dairy Pipeline.