Mozzarella is a traditional Italian cheese made with only a few ingredients — milk, rennet, salt, whey (optional), and critic acid.
To make the mozzarella taste good and stretchy, you need all of them.
Interestingly, some of you are asking about the use of critic acid and rennet in the cheesemaking, or even possible to go without one of them. Like in this article here about using lamb rennet in feta.
The straight answer is yes. Many homemade mozzarella recipes teach people to use alternatives to citric acid, which including lactic acid, vinegar, lemon juice, acetic acid, apple cider, and many others.
These acids are fine to turn milk into the curd. But the problem is the added ingredients will usually give it a strong or sometimes weird aftertaste, apple cider for example.
So, it would be better to go with plain white vinegar.
What do acid do in mozzarella
First, you have to understand the composition of milk.
Milk contains roughly about 85% of water and 15% of solid content. Out of these solid, it has about 22% casein — a protein in milk, 38% lactose — a sugar, 30% fat, 5% ash, and 5% whey.
Citric acid and rennet are both chelating agents to coagulate or “glue” the casein together.
They worked to form curd from milk but in a slightly different approach. Citric acid is also much cheaper than rennet.
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Why do we need citric acid?
Most of the food-grade commercial citric acids, like this powder one from Amazon, they were largely made from fungus not the citric fruit.
For that, some people may find these citric acids have an unpleasant aftertaste, so people sometimes use other ingredient as a replacement such as the lemon juice or plain vinegar.
For some of the cheeses like parmesan or Parmigiano Reggiano, no acid is added but they will use whey starter from the batch one day before.
That is because of the presence of thermophilic lactic acid bacteria in the starter whey, where the bacteria will eat up the lactose and make some lactic acid naturally.
However, the acid volume wasn’t as much as citric acid used in making mozzarella, therefore, Parmigiano Reggiano curd settlement is more of a solid mass and not as soft, as stretchy as mozzarella’s.
Similarly in cottage cheese and quark, starter is added for the bacteria to work.
The function and purpose of citric acid
The main function of citric acid in mozzarella is to adjust the pH — raising the acidity.
The outer layer of casein consists of a negatively charged substance called kappa casein, and because of that, all the negatively charged casein molecules repelling and bounce off each other just like the same pole magnets under normal condition — and therefore milky.
By adding citric acid,
it neutralizes the pH by providing more H+ ion (positive charges) to remove the negative charges on the casein— and therefore coagulate.
Citric acid also removes/dissolves the calcium ion from the cheese (later drain out through the whey).
The calcium ion is one key player binding the casein molecules together to form curd after the rennet was added, I will explain this part further in below.
Just like glue, too much calcium can hold the mozzarella too tight and less stretchy. So, it can be regulated by acidity.
High-fat milk can also make the cheese softer by creating spaces in between the caseins — thus spongy cheese. Heating the milk will destroy the structure of whey protein.
Then, the disrupted whey protein has both stick ends which form bonds in between the casein micelles under an acidic environment.
That’s why heating will help in curd aggregation as well.
Take 1 min, read this article I prepared last week soft cheese Mozzarella vs hard cheese Gouda.
Problem without citric acid
If you’re planning to completely skip the citric acid and use rennet only, it will work but you may find your mozzarella has a bit of texture problem and the cheese could hardly coagulate well to the point you looking for.
That’s because of the calcium regulation as explained above.
Think about citric acid as an ingredient to make the mozzarella curd shrink and soft. Ideally, the cheesemakers will add a suitable volume of citric acid and rennet to get the best consistency.
Why do we need rennet
Rennet is derived naturally from the mammal stomach.
It contains a set of protein-cutting enzymes. For vegetarian, you can use gluten-free vegetable rennet like this one. The rennet contains enzymes like chymosin (learn chemistry here) and lipase to break down casein.
With that, rennet acted like a knife to cut off the Casein Macro-Peptide (CMP) tails which located all over the surface of casein miselles. See the image above. It results in the release of peptides — which is contributing to the bitterness umami taste in mozzarella.
Without the tails, casein micelles are then free to bind to each other with the help of calcium ion and disrupted whey protein, and thus the curd formation.