The ins and outs of coagulating the curd.
According to Paul Kindstedt, author of “Cheese and Culture: A History of Cheese and its Place in Western Civilization”, around 7000 BC, members of herding communities in southwest Asia would have observed that if they left milk out, it coagulated.
The human gut, at that time in history as well as today in much of the world, tended to become increasingly lactose intolerant after infancy, so discovering that clotted milk solids were a digestible form of protein proved groundbreaking.
First, these clots, or elementary curds, represented a nutritional leap forward. And one could argue that taking notice of this natural clotting phenomena not only launched cheesemaking but also contributed to a shift from migratory herding to farming.
Some people assert that only animal-derived coagulant is correctly termed “rennet,” and that all other clotting agents are best simply labeled “coagulants.” But the word rennet derives from the Middle English rennen, meaning to cause to coagulate, from Old English gerennan, to run together, and Old German, gerinnen, to curdle. So etymology pushes back against an inflexible source-based classification.
Call non-animal agents rennet or not, as you wish. But what exactly is coagulation in the cheese world? It’s the process of separating the solids (curds), made of milk protein (casein) and fats, from the liquid (whey), which contains lactose and salts. Curd formation is a necessary step in cheesemaking, and there are several ways to achieve it.
Meanwhile, we can assume that our Neolithic ancestors were not content with the sour milk clots resulting from this coagulation-in the-wild. Over time, they would learn to decrease unpleasant tastes by speeding up the process and applying heat. They found that clotting could be facilitated by adding various plant-derived acids or enzymes to milk. Also, the slaughter and butchering of suckling animals revealed curdled milk in their stomachs, leading to the discovery of stomach lining-derived rennet as a coagulant.
To add context, the rise of pottery making should be considered an essential parallel development that enabled milk to be mixed or heated in containers as coagulants were added. Archeologists have found perforated pottery that served as sieves to drain off and separate liquid whey from the curds. The curds could be consumed as fresh, and eventually brined, cheeses. Processing refinements and aging emerged as cheesemaking evolved over the Millenia. However, even today around some rural parts of the world, the basic methods and resulting cheeses would be recognizable to the distant ancestors of contemporary cheesemakers.
Technology leads, as well as follows. Let’s start with our Neolithic pals. Before they created pottery, they had pouches made of animal parts, such as stomachs or bladders, crudely carved wooden bowls, and woven mats or baskets. With such basic objects, they could produce curds and strain away the liquid whey. They could eat the freshly-collected curds, shape them or let them dry in the sun. They could wrap cheese in leaves, coat them in bees wax and perhaps hide them in a cool, dark cave. As barter and trade developed, pottery facilitated more than the making of cheese; it enabled storage and transporting.
The primary coagulants used thousands of years ago included acids and plant-derived enzymes. Lemon and vinegar were commonly used acids, but the quality of the curds produced is best suited for soft cheeses, such as those resembling ricotta and queso blanco. Not all acids deliver the same result; for example, in the case of some contemporary cheeses, citric acid is fine for mozzarella while tartaric is the preferred acid for mascarpone.
Plant-derived enzymes were produced from fig sap, safflower, nettle, thistle and a variety of wild flowers. Stinging nettle leaves (urtica dioica) boiled, steeped and strained, produces a liquid with enzymatic properties. Various types of thistles also have been used.
The Cynara cardunculus or Cardoon, also known as the artichoke thistle, grew abundantly around the Mediterranean and was well exploited by the Romans and on the Iberian Peninsula. Portuguese Serra da Estrela and Spanish Queso de La Serena are examples of this thistle coagulant product. But there are also often overlooked weeds, such as the Bull or Spear Thistle (Cirsium vulgare) that can be found elsewhere taking root in backyards. Once native to most of Europe, Western Asia and North Africa, they were introduced to North America and Australia. Cirsium vulgare thistle bears flowers and, once dried, can be harvested for its stamens. These can be ground, steeped and strained to produce a enzymic liquid particularly suited for sheep or goat milk cheeses. Yellow-flowered Ladies’ Bedstraw or Curdwort (Galium verum), a medicinal plant, is widespread across most of Europe, Africa, the Middle East and beyond. In Britain, it was called “cheese rennet.” Its yellow flowers gave Cheshire cheese its distinct color until replaced by annatto. There is one caveat with many plant-derived coagulants; they produce cheeses that don’t age well and can introduce off-flavors beyond two months.
While each type of coagulant is best suited for particular cheeses, given the relatively short shelf life for plant-based coagulants, it is easy to see why animal rennet emerged the preferred type for aged cheese.
Animal rennet was originally prepared by the extraction of the abomasum, the fourth stomach of calves, kids or lambs. Camels, by the way, are also rennet-producing ruminants. I cannot speak to the history of camel rennet, but traditionally, the stomach lining of the other ruminants was dried, then strips were soaked in the milk.
Today, rennet is processed in labs for uniformity of quality and strength. Animal rennet, particularly from calves, is still the gold standard for many traditional, imported cheeses, from Parmigiano Reggiano and pecorinos to brie. Many cheeses that are Protected Designation of Origin (PDO) require animal rennet. Some traditional English and Spanish cheeses that originally used plant-derived coagulants now add, or have switched entirely, to animal rennet or microbial enzyme.
Cheesemakers often use more than one type of coagulant these days. For example, someone making an acid-based coagulant might add a bit of rennet to achieve a better quality result. All the traditional coagulant sources remained popular over the centuries, adding modifications in how they were prepared or preserved as needed. Today, coagulants can be found in liquid, dry and powdered forms. In addition to these traditionals, we now have lab-produced microbial enzymes.
There is currently great demand for vegetarian sources, adding to long-standing religious dietary needs, so increasingly lab-produced microbial enzymes appear in the lists of ingredients on domestic cheese labels. Microbials were first introduced in the 1960’s. The source is usually the fungus Rhizomucor miehei. Fermentation Produced Chymosin or FPC, first introduced in the 1990’s, became popular when Mad Cow disease caused a panic in the meat and dairy industry in 2000-2002. As a result, today more than 90% of American-made cheeses use it. Chymosin (FPC) is produced by fermentation of a genetically-modified organism (GMO). The main FPC, which has become the standard, is bovine chymosin -B. However, ironically, FPC camelus has been found to be more efficient for cow’s milk, resulting in higher yield without bitterness.
How do coagulants actually work? There are two basic models: the acid or lactic, and the enzymatic or chymosin. In the acid/lactic model, acidity added to the milk interacts with lactic ferments that are naturally present in unpasteurized milk. Ferments are various types of bacteria, such as Lactobacillus, Lactococcus and all the organisms prized in “cultured” dairy products, such as buttermilk, yogurt, kefir and some butters.
Culturing ferments convert lactose into lactic acid, producing a low pH milk that resist spoilage. But raw milk can contain detrimental bacteria, yeasts and molds, such as salmonella, campylobacter, staphylococcus, Listeria, E. coli, and others that thrive in higher pH milk and give rise to 90% of all dairy-related diseases.
Given the necessary controls needed to keep milk on a healthy track, pasteurization is often a convenient fix. And when milk is pasteurized, a “starter” must be added. Starters are a collection of the good bacteria or culture that presents a desired profile of properties. Some cheesemakers order their own designer starter, a proprietary recipe of these cultures, specifically selected to achieve their unique brand.
But back to coagulation and the difference between the acid/lactic and the enzymatic/chymosin models. In the acid/lactic case, milk must reach a target pH value in order for micelles (aggregates of molecules) to link up and form a gelatinous clot containing the fat globules. On the other hand, in the enzymatic or chymosin model (rennet or microbial) chymosin cuts the casein protein chain, destabilizing the micelles. At approximately 86-98 degrees F, the micelles join together into a gel.
Before the 20th Century, traditional cheesemakers may not have understood the chemistry of why these coagulation methods worked, yet they nevertheless applied centuries of accumulated wisdom and practical know-how. But the industrial age brought with us efficiency and uniformity in food production. It’s easy to get lost in all the chemistry; but here’s a question more to the point: Does the type of rennet affect taste? Usually, it takes rennet going wrong to detect it. When too much is used, the result can taste bitter. Otherwise, as it turns out, most of the rennet is lost in the whey run-off. The little that remains is easily obscured by many other flavor components. Still, as tasting is my passion, I’d love to have the opportunity to definitively answer the question by conducting a controlled tasting experiment with cheeses made by the same recipe, varying only with respect to the type of rennet. Any cheesemakers out there care to try?