After about 45 min rennet is added. The time of renneting and the
amount added are other important variables in cheesemaking which
differ with cheese type. Rennet is a preparation from the fourth stomach
or abomasom of suckling calves, lambs or goats. Its most important
component is the proteolytic enzyme rennin or chymosin which cleaves
k-casein, the protein responsible for the stability of the casein micelle,
between phenylalanine 105 and methionine 106. This releases a 64 amino
acid macropeptide into the whey leaving the hydrophobicpara-k-casein
attached to the micelle. Loss of the macro-peptide leads to the formation
of cross-links between the micelles to form a network entrapping mois-
ture and fat globules.
Authentic chymosin is produced as a slaughterhouse by-product but
microbial rennets are available, produced from fungi such as Mucor
miehei,Mucor pusillusandEndothia parasitica.These lack the specificity
of animal rennet and have been associated with the production of bitter
peptides in the cheese. Now however the genes for chymosin have been
cloned into a number of organisms and nature-identical chymosin is
available commercially, produced using the bacteriumE. coliand yeasts.
After 30–45 min, coagulation of the milk is complete and the process
of whey expulsion is started by cutting the curds into approximately 1 cm
cubes. Whey expulsion is further assisted by the process known as
scalding when the curds, heated to 38–42 1 C, shrink and become firmer.
The starter organisms are not inhibited by such temperatures and
continue to produce acid which aids curd shrinkage. Cheeses produced
using thermophilic starters can be scalded at higher temperatures with-
out arresting acid development. When the acidity has reached the desired
level (generally of the order of 0.25%), the whey is run off from the
cheese vat.
It is at this stage that the process known as cheddaring occurs. The
curd is formed into blocks which are piled up to compress and fuse the
curds, expelling more whey. Nowadays, the traditional manual process is
mechanized in a cheddaring tower.
At the end of cheddaring, the curd has a characteristic fibrous
appearance resembling cooked chicken breast. The blocks of curd are
then milled into small chips. This facilitates the even distribution of salt
which, in Cheddar, is added at a level of between 1.5 and 2% w/w. The
salted curd is formed into blocks which are then pressed to expel trapped
air and whey.
Finally the cheese is ripened or matured at 10 1 C to allow flavour
development. During this stage, which can last up to five months to
produce a mild Cheddar, the microflora is dominated by non-starter
lactobacilli and a complex combination of bacterial and enzymic reac-
tions give the cheese its characteristic flavour. In particular, proteases
and peptidases from the starter culture continue to act, even though the
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