On Food and Cooking

times that some plant materials can curdle
milk. Two have been used for centuries to
make a distinctive group of cheeses. In
Portugal and Spain, flowers of the wild
cardoon thistles (Cynara cardunculus and
C. humilis) have long been collected and
dried in the summer, and then soaked in
warm water in the winter to make sheep
and goat cheeses (Portuguese Serra, Serpa,
Azeitão; Spanish Serena, Torta del Casar,
Pedroches). The cardoon rennets are
unsuited to cow’s milk, which they curdle
but also turn bitter. Recent research has
revealed that Iberian shepherds had indeed
found a close biochemical relative of calf
chymosin, which the thistle flower happens
to concentrate in its stigmas.
The Propionibacteria An important bacterium
in Swiss starter cultures is Propionibacter
shermanii, the hole-maker. The
propionibacteria consume the cheese’s lactic
acid during ripening, and convert it to a