Addressing Rennet & Setting Issues in Late-Season Milk

Rennet is to cheese what amylase is to beer…but this is not what I expected to be writing for my first cheese-related blog post. In fact, the issue I am blogging about here has interfered with what was supposed to be my first cheese related post. A post which would have linked brewing to the making of the best beer-adjacent foods – poutine. So instead of telling you all about an easy to make at home recipe for poutine cheese curds, instead I’m describing the solution I had to find for making cheese using late-season milk.

For those not in the cheese making community, one of the first steps in cheese making is setting the curd – that is, solidifying the protein and fat component of the milk, and separating this from much of the liquid portion. This is done by adding rennet to the milk – an enzyme found in the stomachs of young mammals that coagulates milk protein (casein) into a solid mass. Normally, this mass is processed and pressed to make cheese. But cheesemaker error, changes in milk, over-heating milk during pasteurization, and other issues, can prevent the proper setting of the curd. And a failure to set curd equals no cheese.

Some of our goats.

Starting in early November, the milk from out goats began to not set properly. At first, we simply had to wait a bit longer for the curd to set. This transitioned into curd that never set right, and instead of forming a solid mass, instead formed something more akin to cottage cheeses. By late November the milk simply wouldn’t set, no matter how long we let the rennet work, nor if we added two or even five times the normal amount of rennet.

Given that we get 3-4 L (~1 gallon) of milk per day, all of it intended for cheese, this is a huge issue. So I did what I typically do – I did a deep dive into the scientific literature and – at least for out milk – found a solution.

How Rennet Makes Cheese

To understand the issue, you need to understand how milk turns into cheese. I plan on a proper post on this in the near future, but to make a long story short, it’s all about the interplay between the milk protein casein and soluble calcium. Normally, casein forms a “shell” around globules of fat (AKA “cream”), ensuring that the fat is stably suspended in the milk. You can think of casein as the rubber of a latex balloon, but instead of being filled with air, it forms a balloon filled with cream. Casein has a negatively-charged “tail” that keeps these cream “balloons” from sticking together. Much like static electricity can make your hair stand on end (if you’re fortunate enough to still have hair), the charges on the casein push casein globules apart, keeping them from glooping together.

cheese on a cutting board
When rennet (& the cheesemaker) does their job.

This negative charge is concentrated on one end of the casein, essentially forming a “tail” of charge that sticks out into the liquid portion of the milk. The other side of the casein molecule is fat-loving, and sticks to the cream. To go back to the balloon analogy; the inside of the balloon is fat-loving and sticks to the cream, the outside is negatively charged, meaning it loves to bind to water and therefore will be soluble – but doesn’t want to stick to other casein/fat globules.

Rennet coagulates the casein/fat complexes by cutting off that charged tail. The now neutrally charged casein can stick together into larger complexes. But this isn’t enough – losing the “tail” won’t make cheese. You also need to stabilize the complexes together. This is not a simple thing. Negatively charged phosphate groups on the casein (still there, even after the tail is cut off) complex with positively-charged calcium ions. This allows the calcium to form “bridges” between casein proteins. These calcium “bridges” are what give cheese structure. To over-simplify things, the number and density of these bridges help determine how hard or soft a cheese is, how (or if) it melts, and how stretchy it is.

Once these “bridges” form, the cheese will curdle into a large mass with a jello-ish texture. This mass is essentially chains of casein/fat balloons bridged together by calcium and phosphate. To turn this mass into cheese, the cheese maker cuts it into cubes, and stirs the cut curd so that it will expel whey – the liquid portion of milk. Once the desired amount of whey is expelled, the resulting curd (solid part) is pressed to form a cheese. This is a vast over-simplification of the cheese making process, but hopefully it gives you enough information to understand where issues with curd setting can arise.

How Does Curd Formation Fail?

From the above description, its probably obvious that there are a lot of places where making curd can go wrong. Too little rennet and you won’t form curd. Too much rennet an you’ll over-set your cheese, giving it an undesired texture. Too little calcium and the curd won’t have the right texture – or even form at all. Too much calcium and the texture, hardness, and melting prosperities, can be wrong. But, as difficult as that sounds, for most of the year milk is naturally within the range that gives generally good characteristics.

Assuming your animals are well fed and well cared for.

But regardless of how well an animal is cared for, fall and winter can make things difficult. These seasons see a loss of fresh forage (if your animals are pasture raised) and fresh hay. It also sees lower temperatures, decreased light, shorter periods of activity, pregnancy, and a tendency of animals to remain in the barn – all factors that affect the animals caloric consumption, nutrient availability, and stress. It’s probably of no surprise that all of these factors can impact milk quality – essentially, the animals needs more, so their milk gets less.

The changes which can occur to milk in this season are extensive, and I cannot hope to convey the changes that were measured in the dozen-or-so papers I read on this topic. The biggest three are that calcium levels tend to drop, casein levels can drop, & the amount of casein-associated phosphate can drop. And, based on what I wrote above, it should be obvious how each of these would negatively affect the setting of curd by rennet. Less casein = less for the rennet to work on. Less calcium and/or phosphate = fewer “bridges” cab be built. But, luckily for us, adding extra calcium can usually overcome these limitations – with the caveat that your yield of curd (per volume of milk processed) will likely be impacted. Hey, some cheese is better than no cheese!

But How Much Calcium Do We Add to Make the Rennet Work?

So for many cheese makers, adding more calcium should resolve these late-season milk woes. But how much should be added? Sadly, there is no singular answer to this as your animals diet, the amount of calcium in the environment, the weather at the time, presence of other stressors, milk pH, casein content and phosphorylation level, pasteurization, and other factors, all factor in. Most cheese makers already know that they need to add some calcium (in the form of a 30% w/v calcium chloride solution) to pasteurized milk, as pasteurization will precipitate some calcium, making it unavailable for setting curd.

Obviously, one approach would be to toss some calcium in and hope for the best. But don’t forget how important calcium is for cheese texture and consistency – over-doing it could be almost as bad as under-doing it. Which brings me to the actual point of this post: how to figure out with reasonable accuracy how much calcium to add to late-season milk to get a proper set.

The method described below is a simplified version of an assay used by commercial cheese makers to determine the need to add calcium. It is highly simplified, so rather than giving you an absolute value, it’ll simply put you in the ball-park. You will get a set (unless your issues are unrelated to casein levels, calcium, and phosphate), but you may need to tweak slightly to get exactly the result you want.

The Highly-Simplified Calcium Set Test

The principal of this test is simple – we hit milk with an obscene amount of rennet to force a rapid set, and slowly increase the calcium until we get a set. Once we know how much calcium to add to get a set in this test, we simply scale up. This is done by brining the caclium level in the milk intended for cheese to the same dose used in our test. But we don’t scale the rennet as well – use the same amount normally called for in the recipe.


  • 2x or 1x liquid rennet
  • A small amount (1 liter/quart) of milk, as you would use for cheese making (e.g. pasteurized if this is your normal approach).
  • A glass measuring cup, 1/2 cup to 1 cup in volume
  • An accurate thermometer
  • Boiled and cooled water (distilled or RO is even better)
  • 30% solution of calcium chloride
  • Syringe or another way to accurately measure volumes between 1 ml and 10 ml
  • A set of cheesemakers measuring spoons (ideally going down to 1/64th of a teaspoon)


  1. Using your syringe, measure out 24 mL of water and add to this 1.25 mL of 2x rennet, or 2.5 ml of 1x rennet. Then place 10 ml of this solution into your syringe
  2. Measure out 60 ml (1/4 cup) of milk and place in the glass measuring cup. Using a microwave, warm to 33C (92F). My microwave needs 8-12 seconds to do this.
  3. Swirly the milk in the measuring cup and quickly add the 10 ml of diluted rennet. Keep swirling while counting to 20 seconds. Watch the liquid on the sides of the cup – you are looking for minute curds, or a general thickening of the milk, to occur within 20 seconds.
  4. If you see curds form in within 20 seconds, you milk is good-to-go without added calcium. If curds do not form, let the milk sit for an additional 40 seconds (1 minute total). If in this time the milk forms curdles, or thickens significantly, you should be able to set your curd by adding 25% more rennet to the recipe & by setting for 20-50% longer than usual.
  5. If you do not see the indicators in step 4 within a minute your milk likely needs calcium. Repeat steps 2-4 using fresh milk, but adding 1/64th a teaspoon of 30% calcium chloride prior to warming the milk.
  6. If after step 5 you don’t get curd, repeat steps 2-4 using 1/32nd, and then 1/16th, teaspoon of 30% calcium chloride prior to warming the milk. If 1/16th doesn’t give you a set, your issue cannot be resolved by the addition of calcium.

Its probably not necessary to work out to the nearest 1/64th of a teaspoon how much calcium is needed. If you find a higher dose that over-sets in 20 seconds (e.g. largely curdles) and a lower dose that thickens the milk over a minute, you can just split the difference instead of repeating the test with an intermediary volume of calcium chloride.

You want to add roughly the same amount of calcium chloride (per volume) to your milk for cheesemaking. I’ve prepared a table below to help you out. Keep in mind that you only scale the amount of calcium added – use the same amount of rennet normally used in your recipe.

Calcium added to test
(per 60 ml milk)
Calcium1 to add per liter Calcium1 to add per 10 L2Calcium1 to add per US gallon3
1/64 tsp1/4 tsp2.5 tsp1 tsp
1/32 tsp1/2 tsp5 tsp2 tsp
3/64 tsp3/4 tsp7.5 tsp3 tsp
1/16 tsp1 tsp10 tsp4 tsp
More than 1/16thCalcium won’t fix your issue Calcium won’t fix your issue Calcium won’t fix your issue
1 – Assuming a 30% weight/volume solution of Calcium chloride in water. 2 – because most of my cheese recipes use 10 L of milk. 3 – approximated as 4 L/ US gallon (actual is 3.79 L/gallon)

These values are approximate and may need to be adjusted. But they will give you a set, and will result in cheese.

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