The juice of many plants can clot the milk. Extract of the following plants can curdle the milk: fig (Ficus carica), dried caper leaves (Capparis spinosa), nettles (Urtica dioica), sodom apple (Calotropis procera), thistles (Carlina spp., Cardus spp., Cynara spp.), mallow (Malva spp.), ground ivy (Glechoma ederacea), Galium sativum, pineapple (Ananas comosus), silk tree (Albizzia julibrissin), dandelion (Taraxacum officinale), and others. In the past, some of these extracts were used to prepare specific types of cheese, but many of them are too proteolytic and release bitter peptides, causing bad taste of the cheese. Nowadays, Cynara cardunculus is used in small artisanal dairies in Portugal and Spain and elsewhere. In Nigeria and in Benin, extracts of Calotropis procera (Sodom apple) are used for traditional cheese making.
In Portugal, Spain, and some other Mediterranean countries, the flowers of Cynara cardunculus are harvested and dried. The proteolytic enzymes are concentrated in stylets and stigmae and are called cynarase or cardosins. They are thermostable at least until 70°C and show their optimal activity at 35°C. They have not yet been conclusively investigated. Their proteolytic activity is weaker on sheep milk than on cow milk. This fact explains why sheep milk cheeses that obtained curdling by cardon are only a little bitter, while the cow milk cheeses show frequently bitter off flavors.
Their cost is higher than the other coagulants due to extensive labor in collecting flowers and to the small quantity of enzymes in the flowers. The yield of transformation of the milk to cheese is lower than using animal rennet because the vegetable enzymes are more proteolytic, so higher protein degradation products are lost into the whey. For these reasons, their use is limited to the production of only a few types of sheep and goat cheese in small artisanal dairies.
For cheese making, a fresh extract is prepared every day, soaking the dried flowers in water at room temperature for a variable time. The extract is then filtered to eliminate the flowers and the purple-brown solution is added to the milk; coagulation occurs in 30 to 60 minutes. Coagulation takes place at temperature between 27°C and 30°C. The curd is cut, partially drained, and then kneaded slowly by hand to aid drainage; dry salting is used.
Cheeses produced in Portugal from sheep milk curdled by Cynara cardunculus rennet are: Serra de Estrela, Serpa, Azeitao, Nisa, Castelo Branco, Evora. In Spain, the cheeses are Casar de Caceres (sheep), Torta del Casar (sheep), La Serena (sheep), Los Pedroches (sheep), Los Ibores (goat milk), Flor de Guia (sheep and goat). In Portugal, vegetable rennet is usually mixed with animal rennet.