This week we explored dairy products with a mouth-watering introduction talk given by Professor Vanja Klepac-Ceraj, a scholar with personal interest in the microbial communities in cheese. After Vanja left the class with dreamy visions of Gruyere and Munster, we set to work making our own mozzarella cheese.
First, we added 1/8 of a teaspoon of vegetable rennet and ¾ of a teaspoon of citric acid to a half gallon of heated milk. Immediately, curd formation occurred. Curd separation from the whey continued during the 25 minutes of rest.
Citric acid works in much the same way as the traditional rennet containing the enzyme chymosin, obtained from the abomasum of a milk-fed calf. 
This enzyme effectively attacks only one milk protein at a single point, targeting negatively charged kappa-casein that repels individual casein particles from each other. By clipping these pieces, chymosin allows the casein particles to bond and form a continuous solid gel, the curd. Plain acidity would also cause milk to curdle but also causes the casein micelle proteins to disperse, thus most of the calcium is lost in the whey. On the other hand, rennet allows the micelles to remain intact and causes each to bond to several others, forming a firm and elastic curd.
By straining the mixture through a cheesecloth and heating the solid in the microwave, we further separated the curds from the whey. After much kneading, wringing, and heating to extract excess liquid, we achieved the pliable and slightly stringy consistency that we desired. Fresh mozzarella is a relatively simple process!
