A key part of making cheese is forming milk into curd. Achieving a good set of your milk is one of the most important steps in making cheese.

But it can also be one of the most inconsistent tasks of making cheese.

There are several practical considerations that the cheesemaker needs to be aware of that can affect how well a set can occur in making cheese.

Background to Forming a Cheese Curd

The original rennet was an extract of the ruminant. Ruminants are hoofed herbivorous grazing or browsing mammals.

The abomasum is the fourth and final stomach compartment in ruminants that secretes rennet. The use of rennet can be traced back to around 5,000 years ago when nomads used the abomasum of ruminants such as goats and sheep, as storage bags.

The natural rennet in the abomasum was used by that animal to digest the milk. Warm raw milk with lots of bacteria would have fermented the milk and you then have basic curds and whey and the origins of cheesemaking.

In more recent times there are rennet alternatives to natural rennet. Some of these are :

• Microbial (derived from Rhizomucor miehei)
• Fermentation Produced Chymosin (FPC) (derived from Aspergillus niger or Kluyveromyces marxianus)
• Vegetable (derived from cardoon thistle, artichokes, or nettles)

Rennet also comes in liquid form, powder form and tablet form. All of these rennets come in different strengths. So there are a few variables to consider for the cheesemaker even before they start making cheese.

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 into a solid.

Chymosin is the active enzyme in the rennet that does this. When rennet is added to the milk, the Chymosin cuts (cleaves) off the casein portion of the protein from the other proteins (whey proteins) in the milk and the caseins then bond together into a fibrous structure referred to as a gel (or often referred to as curd).

The curd grows in strength during the set period. This curd now contains most of the milk components: fat, casein, calcium, starter cultures, some water, and some of the chymosin itself.

Cheese curd can also be formed by either the direct addition of an acid or the overnight production of acid in the milk by a starter culture. These are referred to as acid set cheeses.

Yoghurt, Persian feta, Ricotta, and some Mozzarella types are examples of acid set cheeses. Sometimes a very small amount of rennet is also added to the milk when acid set cheeses are made.

The second method used to form a curd is by the addition of rennet and this is called a rennet set curd. Most cheeses that you will make and eat will come from rennet set curds eg. Cheddar, Camembert, Blue Vein, Gouda, Parmesan and so on.

The volume of rennet added to these cheeses is also not constant. You will see when dosages on rennet bottles are listed, a dosage range should be provided.

What are the Practical considerations for the cheesemaker?

The formation of cheese curd is never always the same and the cheesemaker needs to understand why there are differences that determine how well the curd will set.

A cheesemaker requires the curd to be a certain strength, (referred to as being set), before the curd can be cut and then stirred and maybe cooked.

If the set is not strong enough at cutting, known as a weak set, the finished cheese may have inconsistencies and it may turn out as an inferior quality cheese.

A curd that is weak will not drain sufficiently and may make a cheese that is over moist, over acidic with a weak structure. A curd that is overset will produce a cheese that is too firm, too dry and will not ripen adequately.

The issue for the cheesemaker is that milk from different batches does not always set 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 also future batches of cheese.

Some of the reasons why curd sets vary are:

1. The Type of Rennet Being Used:

Rennet can be powdered, liquid or tablet format. Natural rennets can be sourced from sheep, cows, goats, and camels.

Rennet can be produced as FPC or microbial. Each of these rennets can behave slightly differently and can provide different flavours and textures in your cheese.

2. Different Strengths of Rennet

Rennet strength will vary between different manufacturers. Rennet strength is measured as International Milk Clotting Units (IMCU) which is an international standard to measure rennet strength.

If you are using a 200 IMCU rennet and add 1 ml of rennet at that strength. If you then change to a 100 IMCU strength rennet you will need to use 2ml of rennet to achieve a corresponding set.

3. Volume of Rennet Added

Only add the required rennet according to the recipe to make a firm set in the required amount of time.

Generally, rennet volumes can be varied up and down just a little bit. Adding too much additional rennet may achieve a firm set, but the additional rennet may produce a weak and pasty body and off flavours as the cheese matures.

This is because 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 sanitised plastic syringe, a small, graduated measuring cylinder or similar device and stirred into the milk very well.

4. Temperature of the Milk

Rennet works best at 40°C to 43°C, but rennet will work satisfactorily but more slowly at lower temperatures.

For example, if using the same milk and the same volume of rennet you will get a firmer and faster set of the milk at 42°C, by about 5 – 10 minutes than you would if the same milk was held at 32°C.

Some recipes use lower temperatures for setting milk because it is the most suitable temperature for starter growth e.g. Mesophilic starter cultures are usually required for recipes that use temperatures around 34°C or less.

5. Acidity of the Milk

The action of rennet in the milk is assisted by small increases in acidity. Rennet achieves an optimum set if the pH of the milk is at 5.8.

The speed of the set then decreases once the pH moves higher or lower from 5.8. 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 be set before the milk with a lesser volume of starter culture.

But that is not a license to add more starter culture as this may over acidify the cheese.

6. Cold Storage of Milk

When milk is cooled and held at refrigeration temperatures causes some precipitation of the calcium in the milk.

This cold stored milk gradually loses its ability to form a firm curd. Some of this precipitation is reversed, but not all, when the milk is heated for making cheese.

7. Milk from Sick Animals or Animals 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 cows also retards coagulation. Similarly, the imbalance in the minerals and proteins in the milk of cows that are almost ‘dried off’ or at the colostrum stage may also cause inconsistencies in sets.

8. Different Animals and Different Breeds

Cheese has been produced from the milk of many animals over 5,00 years.

The milk from each species will vary and with that, the set of the milk will vary. As a rule, milk with a higher level of casein and calcium will provide a firmer set curd than milk with a lesser volume of these components.

A Jersey or Guernsey cow will have more casein and calcium than a Friesian cow, so you would expect Jersey milk to set faster and stronger than Friesian milk from a Friesian cow.

Even cows within the same herd and the same breed can differ in their ability to set milk. Buffalo and sheep milk have significantly higher levels of both calcium and casein compared to cow and goat milk.

Usually about 30% to 40% less rennet is required to achieve a firm set with sheep and buffalo milk.

9. Different Compositions of Milk at Different Times of the Year

The composition of the milk from an individual animal varies depending upon the influence of factors such as time of year, stage of lactation, availability, type of feed, udder and general animal health of the animal.

Milk is not a constant composition throughout the year, and this will influence your sets.

10. Soluble Calcium Available in the Milk

Coagulation of milk will not occur in the absence of dissolved calcium. 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 addition of calcium in the form of calcium chloride to the milk will 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 so that the Calcium can bond with the phosphorous and proteins in the milk.

11. Heating of the Milk

Excessive heating of the milk precipitates the soluble calcium and denatures some of the proteins in the milk. Extra heating means these components are not fully available when you add the rennet.

Heating is usually carried out during pasteurisation. It’s important not to overheat raw milk in the pasteurisation step. Use a recommended pasteurisation time and temperature combination.

Batch pasteurisation provides a lesser impact on the milk than the high temperature short time (HTST) pasteurisers. A good time temperature for batch pasteurisation is 63°C for 30 minutes or 68°C for 2 minutes.

12. Age of the Milk

Milk that is several days old will not set as well as fresh milk. Some of the naturally occurring soluble calcium in the milk will have precipitated over time.

Also, the proteins may also be affected by the action of unwanted bacteria, (usually psychotropic bacteria), in the milk as it ages.

13. Quality of the Rennet

Rennet needs to be fresh and stored in the fridge. Dispose of rennet when its best before date is reached and get a new batch.

Yes, you want to keep using it, but rennet slowly decreases in activity as it ages. Rennet tablets I personally find do not work as well as liquid rennet.

14. The Process of how You Add the Rennet

• Never use rennet syringes or measuring devices that have been sanitised previously in a chemical sanitiser. Rennet is very susceptible to contact with even small amounts of chemicals. Even a few parts per million (PPM) of chlorine in reticulated water can partially deactivate rennet.
• Sanitise your rennet measuring equipment in boiled hot water just prior to adding the rennet.
• Store rennet in the fridge, never freeze.
• Know the strength of the rennet you have and the exact dosage and strength of the rennet to be added to the milk.
• I personally do not think the practice of diluting rennet in 20 times the volume in distilled or boiled cooled water. This is not necessary for rennet’s that are up to 300IMCU strength. What is important is the process of how you add the rennet. Pour the rennet over the length of the cheese vat, stirring the milk at the same time, and then stir it in very well for up to one minute. The milk should then left undisturbed to set.
• If using circular cheese vats, use your stirrer as a baffle to stop the flow of the milk when the rennet is added.