There is nothing more disappointing than warming a gallon of milk, adding the rennet, and waiting for a curd that never forms.
If your rennet is old, has been stored improperly, or has simply been sitting around for a while, you may wonder whether it is still strong enough to make cheese.
This simple one-cup rennet test lets you check whether your rennet can still coagulate milk before risking a full gallon. If the milk forms a cohesive curd, your rennet is active. If it remains liquid, the rennet may be weak or inactive, although the milk itself can sometimes be the problem.
If the test works, you can even turn the curd into a tiny wheel of fresh cheese.
If it does not, the milk is not ruined. You can still use it as you normally would for cooking.
Note: This same test is also useful for checking a new brand of pasteurized grocery store milk, but only if you already know your rennet works. If both the milk and rennet are untested and no curd forms, you will not know which one caused the problem.
How Do You Know If Rennet Is Still Good?
Active rennet should cause warm milk to transform into curds and whey. In this small test, you should see the milk begin to firm within several minutes and form a cuttable curd or scoopable within about 10 to 15 minutes.
Some pasteurized milk may take longer, especially if you are not adding a drop of calcium chloride, so allow up to 45 minutes before deciding that the test has failed.
Keep in mind that this test evaluates the combination of your rennet and your milk. Milk that has been processed at very high temperatures may fail to form a strong curd even when the rennet is perfectly active.
Choose the Right Milk for Your Rennet Test
For the clearest results, use a regular pasteurized whole milk that you know works well for cheesemaking.
Do not use milk labeled:
- Ultra-pasteurized
- UHT
- Shelf-stable
These milks have been exposed to very high temperatures. That processing can change the milk proteins and make it difficult for rennet to create a strong, cohesive curd.
Not that lactose-free milk is usually also ultra-pasteurized.
What You Will Need
- 1 cup whole milk
- Your liquid or tablet rennet
- 2 tablespoons cool, non-chlorinated water or cool milk
- A thermometer
- A small bowl or cup
- A spoon
- A knife
- A small saucepan or microwave-safe container
- A fine strainer, small piece of cheesecloth, or large coffee filter
- Salt
How to Test Rennet Before Making Cheese
Step 1: Heat the Milk to 90°F
Place 1 cup of milk in a small saucepan or microwave-safe container.
Heat the milk to 90°F.
Microwave Method
Microwave the milk in 15-second bursts. Stir very gently and check the temperature after each burst. Stop heating when the milk reaches 90°F.
Stovetop Method
Place the milk in a small saucepan over low heat. Warm it slowly, stirring gently and checking the temperature often. Remove it from the heat when it reaches 90°F.
Try not to overheat the milk. Excessive heat can make the test more difficult to interpret.
Step 2: Prepare and Dilute the Rennet
Rennet should be diluted before it is added to milk. This helps distribute it more evenly throughout the milk.
You may dilute your rennet in either:
- Cool, non-chlorinated water
- Cool milk
Cool milk is a convenient option when your tap water is chlorinated. You do not need to purchase bottled water just for this test.
Choose the directions for your type of rennet below.
Liquid Animal Rennet or Liquid Microbial Rennet
- Add 2 drops liquid rennet to 2 tablespoons cool, non-chlorinated water or cool milk.
- Stir to combine.
- You will use all of this diluted solution in Step 3.
Double-Strength Liquid Microbial Rennet
- Add 1 drop double-strength liquid microbial rennet to 2 tablespoons cool, non-chlorinated water or cool milk.
- Stir to combine.
- You will use all of this diluted solution in Step 3.
Tablet Rennet
These directions are for small cheesemaking tablets normally used at a rate of ¼ tablet per gallon of milk.
- Crush ¼ tablet into a fine powder and stir it into 2 tablespoons cool, non-chlorinated water or cool milk. A small amount of tablet residue may not dissolve. That is normal.
- You will use 1 tablespoon of this diluted solution in Step 3.
- Discard the remaining tablespoon after the test, or use it to test another cup of milk.
Important: Other types of rennet, including pastes and some animal rennet tablets, may vary widely in strength. If your package recommends a very different amount from the commonly used ¼ tablet or ¼ teaspoon per gallon, use the manufacturer's dosage as your guide.
This is a quick test of enzymatic activity, not a precise cheesemaking recipe. A little extra rennet in this one-cup test is not a problem. The goal is simply to find out whether the rennet can coagulate the milk.
Step 3: Add the Rennet to the Milk
Add the amount of diluted rennet prepared for your type of rennet to the warm milk.
Stir gently with 4 to 5 strokes, then stop moving the milk.
Cover the container and leave it completely undisturbed.
Step 4: Check the Curd
After about 5 minutes, the surface may begin to firm or develop a slight film.
After 10 to 15 minutes, the milk should form one cohesive curd that can hold a knife cut.
The curd may be more delicate than the curd you would expect from a full gallon of milk. This is a very small test batch, so it does not need to be extremely firm.
If the milk has not set after 15 minutes, leave it undisturbed for up to 45 minutes.
How to Interpret Your Rennet Test
A Cohesive Curd Forms
Your rennet is working.
The curd should hold together when cut, although it may be softer and more delicate than the curd in a full cheesemaking recipe.
Tiny or Fragile Curds Form
If the milk separates into small curds that resemble ricotta or cottage cheese, the rennet is probably active, but the milk may not be forming a strong rennet-set curd.
Try the test again with a different brand or source of regular pasteurized whole milk.
No Curd Forms
If the milk remains completely liquid after 45 minutes, one of two things is likely happening:
- The rennet is weak or inactive.
- The milk is not suitable for rennet-set cheesemaking.
Before replacing the rennet, consider whether the milk was ultra-pasteurized, overheated, or otherwise unsuitable for making cheese.
When possible, repeat the test with a milk that has worked successfully for you before deciding that the rennet is no longer usable.
Can You Use the Test Curd?
Yes. Once you know the rennet is working, you can turn this cup of milk into a tiny fresh cheese.
The yield will be small, but it is enough for a snack, a piece of toast, or a small tortilla.
Turn Your Rennet Test Curd Into Fresh Cheese
Step 1: Cut the Curd
Cut the curd into approximately ½-inch cubes.
Because the container is small, the cubes do not need to be perfect. Make several parallel cuts in one direction, turn the container, and cut in the opposite direction.
Step 2: Warm the Curds to 115°F
Slowly warm the curds and whey to 115°F.
Microwave Method
Microwave the curds and whey in 15-second bursts. Stir very gently and check the temperature after each burst. Stop heating when the mixture reaches 115°F.
Stovetop Method
Place the curds and whey in a small saucepan over low heat. Warm them slowly, stirring very gently, until the mixture reaches 115°F.
Do not allow the milk or whey to boil.
Step 3: Strain the Curds
Pour the curds and whey through a fine strainer, a small piece of cheesecloth, or a large coffee filter.
Allow the whey to drain until the curds are no longer dripping heavily. Should be less than 3 minutes.
Step 4: Salt and Shape the Cheese
Sprinkle the curds (while they are still warm) with a small pinch of salt and mix gently.
Taste the curds and add another tiny pinch of salt if needed. You can also add herbs or spices.
Press the curds together with clean hands, or pack them into a very small cup, ramekin, cookie cutter, or cheese form. Let it set in the fridge for 10 minutes.
Turn the cheese out onto a plate.
How to Store and Eat Your Tiny Fresh Cheese
This is a fresh cheese, so store it in a dry, covered container in the refrigerator and eat it within 5 days.
Try it with:
- A drizzle of honey and fresh fruit
- Herbs and freshly ground black pepper
- Toast or crackers
- A warm tortilla
Rennet Testing and Storage Tips
- Dilute rennet immediately before using it. Do not store solution.
- Use cool milk instead of water if you are concerned that your tap water contains chlorine.
- Do not use hot water or hot milk to dilute rennet.
- Store rennet according to the directions on its package.
- Liquid rennet is generally refrigerated.
- Storage recommendations for tablet rennet vary by manufacturer. Ours can be frozen for 5 years.
- Test older or questionable rennet before using it in a full cheesemaking recipe.
Frequently Asked Questions About Testing Rennet
How long does rennet take to set milk?
In this one-cup test, active rennet may begin firming the milk within about 5 minutes. A cohesive, cuttable curd will often form within 10 to 15 minutes, although some milk may require up to 45 minutes.
Can I test rennet with ultra-pasteurized milk?
Ultra-pasteurized milk is not recommended for this test. It may form a weak curd or fail to coagulate even when the rennet is active, making the results difficult to interpret.
Can I dilute rennet in milk instead of water?
Yes. Cool milk works well for diluting rennet and is especially useful when your tap water is chlorinated. There is no need to purchase bottled water.
Does rennet expire?
Rennet can gradually lose strength over time, especially when it is stored incorrectly or kept well beyond the manufacturer’s recommended date. This small milk test can help you determine whether it still coagulates milk before you begin a larger batch.
Why did my milk form tiny curds instead of one solid curd?
Tiny, fragile curds can indicate that the rennet is active but the milk is not forming a strong rennet-set curd. Try a different brand of regular pasteurized whole milk before replacing the rennet. You could also add a drop of calcium chloride before you add the rennet solution, if you have it.
Should I test new rennet before making cheese?
You generally do not need to test a newly purchased rennet that has been stored correctly and purchased from a reputable source that is storing it correctly.
This test is most useful for older rennet, questionable storage conditions, opened packages, or rennet that did not perform as expected in a previous batch.
Complete this simple test before making cheese with rennet you are unsure about. One cup of milk can give you much more confidence before you commit an entire gallon.
Get more clear, no-nonsense guidance like this in our Cheesecraft Club, DIY Cheese Kits cheesemaking books and here on this blog.