Dissolve citric acid in the remaining half a cup of water in a second small bowl.
1.5 tsp citric acid
Pour milk into a large pot set over medium heat. Add in citric acid solution.
4.5 litres milk
Heat milk, stirring occasionally, until it reaches 32°C (or 90°F) on an instant-read thermometer. This should take about 5 minutes.
Remove milk from heat and stir in rennet mixture in a figure-8 motion for 30 seconds. Stir anti-clockwise for 30 seconds to steady the milk.
Rest, covered, until curds are formed. This should take 5 to 10 minutes. Check firmness with the back of a spoon.
Slice vertically into the pot to cut the curd into 2 cm (3/4 inch) cubes using the palette knife. Stir curds gently and be careful not to break the cubes.
Return pot to the heat and cook over medium heat. Gently stir the curds, until temperature reaches 43°C (or 109°F). This should take about 5 minutes. Remove from heat.
Using the ladle, transfer the curds carefully into a colander set over a large bowl. Gently press the curds to release some liquid whey. Pour the whey back into the pot.
Heat whey to 85°C (or 185°F), about 5 minutes. Wearing gloves, tear off a piece of curd and place it in the slotted spoon. Dip into hot whey for about 10 seconds.
Stretch the curd and gently knead it. Repeat until the Mozzarella is smooth and elastic. Add in salt and shape the cheese into a ball. Repeat this step with the remaining curd.
Allow Mozzarella to cool and store in whey solution in an airtight container
Notes
Do not use ultra-high temperature processed (UHT) milk. Regular raw or pasteurised milk are best.Speed up the cooling process by placing mozzarella balls in an ice-water bath.