By Vicki Dunaway

Back in 2002, I had the privilege of touring France with Ricki Carroll (of New England Cheesemaking Supply) and a group consisting mostly of amateur and professional cheesemakers from all over the U.S. The guides had arranged for us to take cheesemaking classes and to visit farmstead and cooperative cheesemaking operations all over the country. It was an amazing experience to learn from some of the world’s most accomplished cheesemakers!

And it was on this tour that I began to develop a deeper understanding of the true importance and value of rennet.

For the first couple of days, we attended classes at a cheese school and visited several farmstead goat cheese operations in central France, all of which specialized in lactic curd type cheeses. Though I have to admit that the Pouligny made at the school was one of the most sublime cheeses I had ever eaten, by the third day the lactic cheeses — in all their fantastic forms — were running together in our minds and on our palates, and most of us on the tour were ready for some variety!

It is rennet that gives us that variety. Its gift is the possibility of making cheese that is sweeter than lactic cheese can ever be – and the opening of vast new worlds. Rennet also contributes strongly to the flavor and texture of cheese, an attribute often overlooked by cheesemakers.

Most cheese books speculate about the beginnings of cheese. Unrefrigerated milk would eventually curdle and separate on its own — that’s a no-brainer, and “curds and whey” have been immortalized from way back. But the discovery of renneted cheese presents some interesting possibilities — did someone notice the curds found in a baby’s up-chuck? Or did milk, carried in an animal stomach used as a vessel, agitated as the traveler walked, become cheese en route? (Good thing they didn’t have our current patent laws in those days.)

What is Rennet and What Does it Do?

Lactic coagulation takes place, without assistance, at a very low pH which, by definition, is a strongly acidic state. Basically, if milk is left out long enough it will naturally acidify and curdle (also known as souring). Examples of lactic cheese include chèvre spread, crottins, and cottage cheese. A miniscule amount of rennet may be used in these cheeses to speed up coagulation or ensure a firmer curd, but it’s not really necessary. Milk for lactic curd must set for a long time (usually eight hours or more) to achieve enough acidity to allow curdling. Rennet curdling works in an entirely different manner, taking place at higher temperature and a higher pH. Thus, rennet curd is referred to as “sweet” (less acid) curd.

Rennet is an extract made from the fourth stomach — the abomasum — of a young milk-fed animal. This extract contains chymosin, an enzyme that starts the process of the coagulation of milk by cleaving (splitting) a particular chemical bond on a casein molecule. Casein is an important protein found in milk, the one that gives us most types of cheese.

The strong links of calcium and phosphates in rennet curd give it a great deal more strength and elasticity than are present in lactic curd, which is why lactic curd is often referred to as “fragile.” This is particularly true of goat milk curd, which tends to be quite delicate compared to cow or ewe milk curd. These minerals (calcium and phosphate) link to the casein micelles in rennet curd, whereas they drain away from lactic curds. Intermediate characteristics can be obtained by using only a small amount of rennet; this explains why adding a few drops of diluted rennet to the milk when making chèvre-type cheese results in a faster set, stronger curd, and better yield. For a mixed curd, low temperature (room temperature) favors lactic coagulation, while higher temperatures favor rennet coagulation.[1]

When to Add Rennet

When to add rennet and how much to add are functions of the particular cheese recipe. Some require significant acidification before adding rennet; in others we are instructed to add the rennet almost immediately after stirring in the starter culture. Note that the latter case should probably be adjusted if using direct vat set (DVS) cultures, the ones that have a powder added. These freeze-dried cultures require a bit of “wake-up” time to rehydrate and get started working, and it is usually recommended that the cheesemaker wait at least 30 minutes from the time of adding a DVS culture to the time of renneting, regardless of the recipe’s instructions.

Nowadays we have pH meters and acidometers for measuring the lactic starter activity in our milk. Unfortunately, it often takes a lot of digging to find appropriate values for the various steps in the process of many cheeses. The English cheeses are well-documented — apparently the English are sticklers for using acidity to determine what to do when — but for many other types, particularly the less popular cheeses, we are left to hunting down obscure books or just plain old trial and error. The best book I’ve found for TA or pH markers for a wide variety of cheeses is Cheesemaking Practice, by R. Scott.

In The Book of Cheese, Thom and Fisk say, “After the rennet extract has been added, all control of the acid development is lost,” warning that sufficient acidity must develop before adding the rennet. Too much acidification at the time of adding rennet will result in the acid being too high at each stage of the process, and the cheesemaking will be “hurried” and may result in an acid cheese. If the cheese has gone as far as beginning lactic coagulation, there is no point in adding rennet. Too little acidification, on the other hand, may give one a “sweet” cheese that has a short shelf life or may even be dangerous, allowing pathogens to grow in the cheese.[2]

[1] Larcher Ivan, Cheese Processing (Le Chaffaut, France, 2002), p. 15.

[2] Charles Thom and Walter W. Fisk, The Book of Cheese (New York, 1918), pp. 65-66.

Vicki Dunaway was formerly an artisan cheesemaker and editor of CreamLine and Home Dairy News.

Originally published in the March/April 2014 issue of Dairy Goat Journal.