Problems with setting milk

One of the most commonly asked questions that I get from people just starting out making cheese is: ‘I had a problem with my milk it took ages to set, or in some cases, it didn’t set at all, what went wrong?’

Setting or coagulating the milk is one of the most important aspects of making good cheese.   So when it does not behave according to plan it can cause some problems for cheesemakers. Milk sets into a curd when the rennet is added to the cultured warm milk. The set transforms the milk gradually from a liquid into a solid.  The aim is to get a firm set before going onto the next step, which is to cut the cheese curd.
Cheesemaking can be really easy when you get a good set in the time indicated in the recipe. On the odd occasion the milk does not set as well as the cheesemaker would have liked they need to adjust the times in the recipe but also understand what happened.  There are two ways to set milk.  One is with acid and examples of acid set cheeses are Persian Feta, Fresh Lactic, Quark, Queso Blanco and Ricotta.  There is not usually a problem setting these styles of cheese.  The second way to set milk is to add rennet to the warm milk and that is what this article refers to.
What is happening when the rennet forms a curd?
When rennet is added to the milk it needs to be stirred in very well, the milk is then left undisturbed for a short period of time that varies depending on the recipe. During this time the milk gradually turns from a liquid into a solid. Throughout this coagulation process the rennet splits off the casein from the other proteins to allow the casein to form a fibrous structure referred to as a gel, a set or a sponge. This gel now contains all of the fat, water, casein, whey proteins, calcium, starter cultures and the rennet itself.
What should the cheesemaker be looking at if the milk is not setting well or has not set at all? 
A. Has the rennet been added and mixed into the milk correctly? Most recipes call for rennet to be diluted in 20 times cooled boiled water. The aim is to get good dispersion of the rennet into the milk. It’s a good idea when you have 500 or 5000 litres of milk. If using diluted water make sure the water is boiled and then cooled and do not use a chemical sanitiser in any part of the process. But if you have only 5 or 10 litres of milk it is usually ok to add the rennet straight into the milk. What is important is that the milk is being stirred before the rennet is added and then the rennet has been well mixed into the milk over a 2 minute period using both an up and down and across stirring motion. Excessive stirring, agitation, frothing and moving of the milk should be avoided.  When the 2 minutes is up use the stirrer to stop the milk from moving and then let the milk stand completely still while the setting takes place
B. Volume of rennet added: Only add the required amount of rennet according to the recipe.  The recipe should also give a time period for when the set should be achieved. If the milk looks like it’s not setting do not move the milk container and do not add any additional rennet. Adding additional rennet and stirring it in will negate any setting of the curd that has already taken place. In the next batch of cheese that you make you may consider adding a small amount of additional rennet to make up for the slow set. Do note that any additional rennet may also produce brittle curds and then weak and pasty bodies and off flavours in the matured cheese. 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.  Check your measuring is correct; I come across cheesemakers that add 1ml when they should have added 10ml. Chemicals will deactivate your rennet. Always sanitise the syringes with clean boiling water and never use a chemical to sanitise rennet containers or syringes.
C. Quality, strength and measuring out of the rennet: This is probably the most common problem that cheesemakers face. Liquid rennets usually have a 12 month use by date.  After the expiry date their ability to set the curd diminishes. Do not overstock your rennet and always throw out rennet when its use by date is reached.  The rennet should be stored in the fridge not the freezer. Check the strength of your rennet. Rennets come in different strengths; 1ml of rennet at 200 IMCU strength is the same as using 2ml of 100 IMCU strength rennet.
D. Temperature of the milk: Rennet acts best at a temperature 40°C 43°C, but it will also work satisfactorily but slower at a temperature of 30°C  32°C. That means that of two milks with the same amount of rennet added but at different temperatures, the milk that is close to a 40°C 43°C will set faster than the milk that is held at 30°C  32°C. Some recipes use lower temperatures for setting milk because the lower temperature is the most suitable for starter growth eg. Mesophile starters and at this stage of cheesemaking it is vital to get the starters growing.
E. Acidity of the milk: The action of rennet in milk is greatly accelerated by small increases in the acidity of the milk. The ripening of milk, which is the time between the addition of the starter and the addition of the rennet. Milk with a longer ripening time will set faster than the same milk with a reduced ripening time.  Also two identical milks at exactly the same temperature, if one has an increased volume of starter culture then it will also set faster
D. Milk quality: Milk from sick cows, cows in late lactation and mastitis infected cows may cause a weak set to occur. The components of the milk including the casein, calcium and phosphorous can become imbalanced and can impair rennet action. The additional albumin and globulin caused by mastitis infection also retards coagulation.
E. Soluble calcium available in the milk: Coagulation will not occur in the absence of soluble calcium. Milk may contain a shortage of this calcium and this will lead to a soft weak curd. The addition of calcium chloride to the milk may rectify this. If adding additional calcium, make sure that you do this at least one hour before the addition of the rennet or the effect of the additional calcium may be minimised.
F. Heating of milk: Excessive heating of milk precipitates the soluble calcium and reduces the setting ability of the milk. For home pasteurisation it is important not to over pasteurise the milk. Milks available commercially are usually pastuerised for general use and not specifically for cheesemaking.  As a result extra heat treatment may be applied to increase the shelf life of the milk.  If the cheesemaker does not know what heat treatment is applied (as in most cases) the addition of Calcium is important.
G. Age of milk: Milk that is several days old will not set as well as fresh milk. This is due to the effect of spoilage enzymes degrading the proteins and solubilisation of the calcium. In addition these unwanted bacteria may also produce some mild off flavours in the milk such as bitterness.
What should you do if the curd has not set?
If you can see that the milk is beginning to set but is still not set sufficiently enough to cut then you have no option but to wait until the curd has properly set. If you end up cutting the curd too early you may end up breaking it during the stirring stage or the curds may not drain sufficiently in the hooping stage and you will have an over moist and over acidic cheese.  But if you delay the cutting due to a soft set you also have to bear in mind that the starter culture will still be producing acidity during that time.
The answer is to shorten the subsequent stages of the recipe from cut to hoop so that you add the curd to the hoop as near as possible to the time indicated on the recipe. An example may be camembert that has 2 stirs 20 minutes apart.  Make each of those stirs 10 minutes apart and that gains you 20 minutes.
If the milk does not set at all, you have no choice but to end the cheesemaking.  Don’t throw the milk out though, it will make a great milk ricotta.

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