Why doesn’t milk set the same way each time I make cheese?

Achieving a good set of your milk is one of the most important steps in making cheese if you want to make good cheese. But it can also be one of the most frustrating. There are several practical considerations that the cheesemaker needs to be aware of that can affect how well the rennet will work when it is added to the milk to make cheese.

Background to forming a cheese curd
A key part of making cheese is forming a curd. A cheese curd can be formed by either the addition or production of acid in the milk, this is an acid set cheese. Yoghurt, Persian feta and some Mozzarella types are good examples of these. The second way a curd can be formed is by the addition of rennet and is called a rennet set curd. The vast majority of cheeses that you will make and eat will come from rennet set curds eg. Cheddar, Camembert, Blue Vein, Gouda and so on.

What is happening when the rennet forms a curd?
Rennet is an enzyme that coagulates the milk, basically it turns the milk from a liquid to a solid. Chymosin is the active enzyme in the rennet that does this. When rennet is added to the milk (see foot note) it needs to be stirred in very well, the milk is then left undisturbed for a predetermined time, usually from 30 – 60 minutes depending on the type of cheese and the recipe. During this time the Chymosin cuts (cleaves) off the casein portion of the protein from the other proteins (whey proteins) in the milk and the caseins then all bond together into a fibrous structure referred to as a gel. The gel grows in strength during the set period. This gel now contains most of the milks components: fat, casein, calcium, starter cultures, some water, and some of the chymosin itself.

What are the Practical considerations for the cheesemaker?
The formation of this gel is when a cheesemaker may experience differences in how well the curd will set. A cheesemaker requires the gel to be a certain strength (referred to as being set) before the curd can be cut and then stirred. If the set is not strong enough at the cut stage, the finished cheese may have inconsistencies and it may turn out as an inferior quality cheese. A curd that is too soft will not drain sufficiently and will make a cheese that is over moist, over acid and with a weak structure. A curd that is over set will produce a cheese that is too firm, too dry and will not ripen adequately. The issue for the cheesemaker is that all milks do not set exactly the same so it is important for the cheesemaker to understand that there are several variables that may affect how well a cheese is going to set. Understanding these variables will assist you with dealing with the problem for the current batch and or future batches of cheese.

  1. Volume of rennet added:
    Only add the required rennet according to the recipe and to form a firm set in the required amount of time. Generally, rennet volumes can be varied up and down just a little bit, and I mean only a little. Adding too much additional rennet may achieve a firm set, but the additional rennet activity within the cheese may produce a weak and pasty body and off flavours in the cheese. This is because the rennet is a proteolytic enzyme that breaks down the cheese structure as the cheese matures. The required amount of rennet should be accurately measured in a clean plastic syringe or small graduated measuring cylinder and stirred into the milk very well.
  2. Temperature of the milk:
    Rennet works best at 40°C ‑43°C, but rennet will work satisfactorily but more slowly at 30°C ‑ 32°C. For example, if using the same milk and the same volume of rennet you will get a better set of the milk at 42°C, by about 5 – 10 minutes, then you would if the same milk was at 32°C. Some recipes use lower temperatures for setting milk because it is the most suitable for temperature for starter growth eg Mesophilic starter cultures are usually required for recipes that use temperatures around 34°C or less.
  3. Acidity of the milk:
    The action of rennet in the milk is assisted by small increases in acidity. The acid ripening of the milk after starter addition has the effect of helping to speed up rennet setting times. For example, if using the same milk but different volumes of starters, the milk with the greater volume of starter culture will set before the milk with less starter culture. But that is not a license to add more starter as this may over acidify the cheese.
  4. Milk from sick cows, cows in late lactation and mastitis affected milk:
    The very slight increase in alkalinity in these milks is enough to impair rennet action. The additional albumin and globulin caused by mastitis infection also retards coagulation. Similarly, the imbalance in the minerals and proteins in the milk of cows that are almost ‘dried off’ may also cause inconsistencies in sets.
  5. Different animals and different breeds
    Cheese has been produced from many animals over the years. The cow and goat mostly but sheep and buffalo are becoming more common in Australia. The milk from each of these species will vary and with that the set of the milk will vary. As a general rule a milk with more Casein and Calcium will have a firmer set curd than a milk with less of these components. Buffalo and sheep milk have significant higher levels of both of these components compared to cow and goat milk. Usually a lot less rennet is used with sheep and buffalo milks.
    Then there are the differences within different breeds of animals for example a Jersey or a Guernsey will have more casein and calcium than a Fresian, so you would expect a jersey milk to set faster and stronger than a Fresian milk.
  6. Different composition of milks at different times of the year
    The composition of the milk between individual animals of the same breed varies, depending upon the influence of factors such as time of year, stage of lactation, availability and type of feed, and udder and general animal health of the animal. Milk is not a constant composition throughout the year and this will influence your sets, but there is not usually a significant difference in the composition of the milk due to these factors.
  7. Soluble calcium available in the milk
    If milk contains a shortage of soluble calcium this will lead to a soft weak curd. All milk has natural soluble calcium, but you don’t know how much. You cannot see it, taste it and you don’t have the facilities to test its strength. The additional of calcium in the form of Calcium Chloride to the milk will definitley help strengthen your sets. Ensure that if you are adding Calcium Chloride, you do so at least one hour before the addition of the rennet or the positive effect of the additional calcium will be heavily reduced.
  8. Heating of milk:
    Excessive heating of milk precipitates the soluble calcium so it is not available when you add the rennet. Heating is mostly carried during the pasteurisation. It is important not to overheat raw milk in the pasteurisation step. Stick to the recommended pasteurisation time and temperature combinations. Many commercially available milks are pasteurised in large continuous pasteurisers (known as High Temperature Short Time) and the ‘high temperature’ component is great to increase the shelf life of these milks, unfortunately this higher temperature precipitates some of the soluble calcium. If this is the case the addition of Calcium Chloride is very important.
  9. Age of milk:
    Milk that is several days old will not set as well as fresh milk. Some of the natural occurring soluble calcium in the milk will have precipitated. Plus, the proteins may also be affected by the action of unwanted bacteria in the milk as it ages.
  10. Quality of the rennet
    Rennet needs to be fresh and stored in the fridge. It should not be frozen. Rennet tablets are for quantities around 30 – 50 litres of milk are not accurate when using in smaller volumes plus powdered rennet is not as effective as using liquid rennet.

Factors to consider when handling your rennet:

  • Never use rennet syringes that have been sanitised in a chemical
  • Always sanitise syringes and graduated cylinders in hot water
  • Always store rennet in the fridge, never freeze
  • Dispose of rennet when its best before date is reached and get a new batch
  • Know the strength of the rennet you have and the exact dosage and strength of the rennet to be added to the milk eg 10ml of 100 IMCU strength rennet is the same as 5 ml of 200 IMCU strength rennet (IMCU = International Milk Clotting Units and is the standard measure of rennet strength)

Footnote: Many people use the practice of diluting their rennet in 20 times the volume in distilled or boiled cooled water. This is not necessary for rennet’s that are up to 200IMCU strength. What is important is that the rennet is added over the length of the cheese vat and stirred in very well and the milk is then left undisturbed to set.