The various cheese styles have fairly widely varying acidity profiles, perceived as sourness, tanginess, and as part of the sharpness of a given cheese. Cheeses with less acidity are referred to as “sweeter” cheeses, although it doesn't directly correspond to actual lactose and sugar content, but rather to having less acidity.
Here are some common cheeses that are sweet by the time they are consumed:
Here are some common cheeses that are more acidic/tangy:
Cheese needs to have a reasonable amount of acidity as part of how the cheese, as a fermented food, is protected against spoilage. More acidity creates an environment where bacteria cannot thrive as well, particularly spoilage bacteria such as those in the clostridia family, and allows the cheese to stabilize and be preserved at refrigerator temperatures for quite a long time. Salt plays a big role here as well, in addition to cultures, but much of the action of the cultures to preserve the cheese occurs during the cheese making session, producing lactic acid from the lactose in the milk. Once enough lactic acid is produced, the cheese cultures slow way down due the acidic environment, impeding not only spoilage bacteria but also impeding themselves.
At some point, if a cheese or its curds are left to acidity at room temperature for a long time, the cheese will reach terminal acidity where all of the bacteria have slowed and no more lactic acid can be produced. Usually measures are taken before this point to slow the cultures without making the cheese that sour, such as cooling the cheese and salting it, so it does not become overly sour or tangy but is still well-protected by the acidity already developed.
Acidity is something we can directly measure, and get a good idea where cheese or its curds are in terms of development of the proper acid for the style of cheese. The most familiar method of measuring acidity is the pH scale, which is a logarithmic measure of the number of free hydrogen ions relative to the number of hydroxide ions or other negatively-charged ions. Hydrogen ions are positively charged, so the more of them there are the lower the pH. The scale is based on 7.0 being the middle, neither acidic nor basic, where the hydrogen ions are equally balanced with the negative ions. Going down to a pH of 6.0, there are 10 times as many hydrogen ions compared to negative ions, and to a pH of 5.0 there are 10 times more, or 100 times compared with a pH of 7.0.
Raw milk typically has a natural pH of 6.6 to 6.7, meaning it is slighly acidic already. During cheese making, the production of lactic acid will lower the pH to somewhere in the range of 4.8 (a tangy feta) to 5.5 (a sweet washed-curd cheese). Getting any higher than 5.5 can dramatically increase the risk of spoilage bacteria taking hold faster, while going lower than 4.8 produces a cheese so tangy that it becomes unpalatable to many people.
A large part of cheese making techniques are devoted to managing the process of acidifying the milk and curds to the right pH, in the right amount of time.
There are essentially three methods for measuring the acidity of cheese and its curds while it is being made, going from the least accurate to the most accurate:
An important part of successful cheese making is to manage the rate at which milk, curds and the final cheese acidify and gain a lower pH. If the pH lowers too fast, there will be too little time to press and form the cheese or it will end up too sour to the taste, and the cheese texture may become crumbly or even chalky. If the pH does not lower fast enough, if the cheese is then cooled and salted, it will not have enough acidity to preserve the cheese and it may spoil more quickly.
Many cheese makers refer to the “acidity curve” when managing acid production. If the pH of the milk/whey/curds were continuously measured over time, it would form a graph with the pH steadily lowering, not necessarily in a straight line. The ideal is to have the pH land at the right spot when the make is finished, the cheese is well consolidated in the press, and where it can be cooling and getting salted to move on to the aging stage without developing significantly more acidity.
pH is usually not measured continuously, so some recipes will specify checkpoints where it's good to measure the pH and see how it is progressing relative to expected pH:
Once a cheese is cooled and salted, the pH is not usually measured after that, as there is very little that can be done to change the pH at that point. It's important to hit the right acidity during the make steps above, as the trajectory of the cheese is fairly well set once moves past that point.
Some cheese styles will see an increase in pH during aging, so it comes back up and becomes a sweeter cheese by the time it is consumed. This usually happens as a result of enzyme activity from molds and yeasts, as is typical of blue cheese molds, white/gray surface molds, and morge bacteria for stinky rind cheeses. These will metabolize the lactic acid in the cheese and thus slowly raise the pH as they do so, producing a final cheese that is sweeter than it was at the end of the main cheese making session.
The most typical problem cheese makers face with acidification is that it happens too quickly. The main factors influencing the rate at which cultures produce lactic acid in milk and curds are:
Each type of culture will have typical recommendations for how much culture to use, often given per gallon, per liter, or per 100 lbs (with 8.6 lbs per gallon a conversion can be easily made to gallons). Cheese making recipes will often have a recommended amount of culture to use for cultures of a given type, be it DVI / freeze-dried cultures, mother cultures and so on.
Managing acidity then often means a cheese maker will need to make the same cheese several times, starting with the recommended dose of culture and then tracking pH to see how close to the expected pH targets the cheese is tracking. The next make session can have the culture adjusted to slow down or speed up acidification, and make it easier to hit the final desired pH when the cheese is pressed and/or consolidated properly.
When making culture changes to tune them for a particular cheese type, it's recommended to try to keep all the other steps the same as much as possible. While the ripening time, stirring time, and heating time may need to be adjusted as the acidity curve changes, try to hold them as close to constant as you can so that the only real change is the amount of culture.
When culture changes are not enough, or for one reason or another they can't be modified so much, the rate of acidification can be slowed by lowering the temperature of the cheese making steps. This is best done with small changes in temperature. However, keep in mind that it's not always a simple relationship, and lower doesn't always mean slower.
Mesophilic cultures will acidify at their fastest rate at temperatures from about 90-100 F. If the temperature goes up to about 102, their action will slow way down, and past that temperature the cultures will begin to die. Below 90 F they will still convert lactose to lactic acid at a fairly high rate, but they begin to slow down below approximately 80 F.
Thermophilic cultures will acidify optimally around 100-110 F depending on the particular culture, but they will slow down quite a lot if the temperature goes above 115 F. They will survive quite well up to 130 F, but they will not be producing much lactic acid until the temperature drops back down to their optimal range. They will continue producing lactic acid down into the 70-80 F range like mesophilic cultures do, but they do so at a much slower rate.
Cheese makers can exploit these temperature ranges to slow acid production while stirring curds in order to have enough time to dry the curds out sufficiently for the style of cheese. For example, in a thermophilic cultured cheese the maker can buy some time to stir curds without producing much lactic acid by increasing the temperature up past 110 F. In a mesophilic cheese, raising the temperature to 101 or 102 F may slow the cultures down without killing them, or conversely lowering the make temperature from 90 F to 86 F may slow the cultures down.
The reverse is also true; if the cheese is acidifying too slowly, moving the temperature to the optimal range for the culture can help speed things up.
Another technique, washing curds, can be used to slow acidification and is often the method of choice for producing a sweeter (higher pH) final cheese. After the curds are cut and have been stirred gently to firm up and release some whey, some of the whey can be removed and replaced with clean unchlorinated water. Removing whey will remove lactose, the fuel for the acidifying cultures, and slow them down. Because some whey has been removed, the final terminal acidity of the cheese is raised because there simply is less lactose available to be turned into lactic acid. If curds are acidifying too quickly while being stirred or heated, replacing some of the whey with water can help.
A larger discussion of salting for controlling acidification is here.