You made a few errors which you can easily correct the next time around.
First off, there is no such thing as "industrial strength rennet". Unless you are using Junkett, all normal rennets have specific strengths. 1:15,000 is single rennet. (In Europe it's 1:10,000). 1:30,000 is double strength (In Europe 1:20,000). The dry rennets (tablets or powder) can be 1:60,000 and 1:100,000 which is super strong. the most common type out there is the double strength.
4 drops of double-strength rennet would not be enough to coagulate 2 liters of milk. You would need about 1/8 of a teaspoon (about 10 drops). If you use a bit more it's okay too, because Mozarella is a milk un-aged cheese and using a couple of more drops of rennet would not effect its flavor.
Secondly, there is a matter of technique. You did two things which you should never do once you add rennet.
- you continued to heat the milk while waiting for it to coagulate. This is bad because the heat accelerates the milk and causes it to stir, move and mix. This acts against your coagulation. Also, when the milk begin to coagulate it doesn't mix well anymore so only the part of the milk that is close to your heat source is getting scalded while the rest of the milk is cold.
- you added rennet to the milk after already waiting for coagulation. The minute you do that, you have mixed the milk and broke apart any coagulation. Also, because you have chunks in your milk now, the new rennet will not mix evenly into the material. When you mix it and break up the curd, whey begins to move from your solids to the water part of the milk. You will not be able to coagulate and gel the milk. If you have a problem, it is better to wait longer than to add more rennet.
Rule of thumb: NEVER, EVER disturb the milk once you put the rennet in
. No heating, no stirring. Rennet (diluted in water) should be added to the milk, the milk should be stirred well (but gently) for about 30 seconds to 1 minute. At that point you leave the pot until it is time to cut the curd. You don't heat up the pot, you don't add anything to it. You don't mix it, you don't scoop it or touch it. You don't move the pot. You don't touch anything. Rennet coagulation is a very delicate process that should never be disturbed.
The next thing has to do with the milk quality. You didn't say where your milk was from, but if it is an ultra pasteurized and/or homogenized milk, you can expect that it will be too "dead" to coagulate properly and you may experience milk texture issues or very low yield. Sometimes you need to try several brands of milk until you find one that works. While they may all seem similar when you drink them in a glass, put them in your coffee or use them for breakfast cereals, in cheese they all act very differently. Often, merely switching brands can fix your Mozzarella!
The pasteurization and homogenization of commercial milk is a harsh process that strips the milk off much of its calcium. Calcium is the "glue" which binds solids, (proteins, minerals, fats) and water together. If you are working with supermarket milk (even if it says that Calcium was added to it), chances are that your milk is not strong enough to create a good solid gelled curd. The best thing to do is add a little bit of calcium chloride (for 2 liters, you need 1/16th of a teaspoon diluted in 1/4 cup of water) just before you add the rennet. It will make a HUGE DIFFERENCE in the results. Sometimes you will get as much as 50% more cheese out of it and it will improve the texture and acidity control.So in conclusion:
- Curdle and coagulate/gel the milk at 88°F. Do not go to 105°F
- Add 1/16 teaspoon calcium chloride diluted in 1/4 cup of water (for 2 liters) just before renneting. Mix thoroughly but gently for 45 seconds. Turn off the heat
- Add 1/8 teaspoon rennet, diluted in 1/4 cup of water (for 2 liters). Mix thoroughly but gently for 45 seconds
- Leave the milk alone. Do not touch the pot.
This should also get your curd to the right acidity level so when you knead it, it will stretch beautifully and make a shiny, smooth Mozzarella. This should also result in a slightly larger cheese.