BLBS102-c25 BLBS102-Simpson March 21, 2012 13:23 Trim: 276mm X 219mm Printer Name: Yet to Come
25 Biochemistry of Milk Processing 473When approximately 85% of theκ-casein has been hydrolysed,
the rennet-altered micelles aggregate to form a gel in the pres-
ence of a critical concentration of Ca^2 +and at a temperature
about> 18 ◦C.AcidificationThe second characteristic step in cheesemaking is acidification
of the milk and curd from pH approximately 6.7 to a value
in the range 4.6–5.2, depending on the variety. Until relatively
recently, and for some minor artisanal varieties still, acidifi-
cation was due to the production of lactic acid from lactose
by adventitious LAB. Today, cheese milk is inoculated with a
culture (starter) of selected LAB for more controlled and repro-
ducible acidification. If the cheese curds are cooked to< 40 ◦C,
a culture ofLactococcus lactisand/orcremorisis used, but for
high-cooked cheese (curds cooked to 50–55◦C), a culture of
Lactobacillus delbrueckiisubsp.bulgaricus,possibly in combi-
nation withStreptococcus thermophilus,is used. Cheese starters
have been refined progressively over the years, especially with
respect to the rate of acidification, resistance to bacteriophage
and cheese-ripening characteristics. Today, mixtures of highly
selected defined strains of LAB are used widely.
Acidification at the correct rate and time is essential for suc-
cessful cheesemaking; it affects at least the following aspects: Activity and stability of the coagulant during renneting
Strength of the rennet-induced gel
Rate and extent of syneresis of the gel when cut and hence
the composition of the cheese
Retention of rennet in the curd
Dissolution of colloidal calcium phosphate; the concen-
tration of calcium in the cheese has a major effect on the
texture and functionality of cheese
Inhibition of growth of undesirable microorganisms, espe-
cially pathogenic bacteria
Activity of various enzymes in the cheese during ripeningPost-Coagulation OperationsIf left undisturbed, a rennet-coagulated milk gel is quite stable,
but if cut or broken, it synereses (contracts), producing curds and
whey. By controlling the rate and extent of syneresis, the cheese
maker controls the moisture content of cheese and thereby, the
rate and pattern of ripening and the quality and stability of
the cheese.
Traditionally, the point at which the gel was ready to be cut
was judged by the cheese maker based on experience; however,
in some modern cheesemaking systems, this decision may be
made automatically based on sensors in the vat or rheological
measurements (Fagan et al. 2008).
Syneresis is affected by: Concentrations of fat, protein and calcium
Denaturation of whey protein
pH
Size of curd particles
Temperature of cooking Stirring the curds–whey mixture and the curds after whey
drainage
Pressing of the curd
Salting (2 kg H 2 O lost per kg NaCl taken up; NaCl should
not be used to control moisture content)When the desired degree of syneresis has occurred, as
judged subjectively by the cheese maker, the curds and whey
are separated, usually on some form of perforated metal
screen. The curds are subjected to various treatments that
are more or less variety-specific. These include cheddaring,
kneading–stretching, moulding, pressing and salting. Salt is
added to essentially all cheese varieties, but may be added ei-
ther in dry form (e.g., for Cheddar cheese) or, more commonly,
through immersion of the cheese in brine for sufficient time for
salt uptake to the desired final level, followed by enough time
for diffusion throughout the cheese so as to achieve a uniform
concentration. The functions of salt in cheese are in flavour and
control of bacterial growth and metabolism.
The extent of physical force applied to cheese curd during
the later stages of manufacture depends on the target final mois-
ture content, and may range from significant pressures being
applied through piling of curds in the vat followed by pressing
in moulds (Cheddar), to very light drainage under gravity in per-
forated moulds (e.g., Camembert). In addition, variety-specific
techniques such as heating–stretching may be used at the final
stages to achieve specific desirable functional properties (e.g.,
for Mozzarella).RipeningRennet-coagulated cheese curd may be consumed at the end of
the manufacturing process, and a little is, for example Junket
or Burgos cheese, but most is ripened (matured) for a period
ranging from approximately 2 weeks (Mozzarella) to>2 years
(Parmigiano-Reggiano, extra-mature Cheddar) during which the
characteristic flavour, texture and functionality of the cheese
develop. The principal changes in cheese during ripening are
listed in Table 25.4.
Ripening is a very complex biochemical process, catalysed
by: Coagulant: Depending on the coagulant used, the pH of the
curd at whey drainage, the temperature to which the curds
are cooked and the moisture content of the curds, 0–30% of
the rennet added to the milk is retained in the curd.
Indigenous milk enzymes: These are particularly important
in raw-milk cheese but many indigenous enzymes are suf-
ficiently heat stable to withstand HTST pasteurisation. Im-
portant indigenous enzymes include plasmin, xanthine oxi-
dase, acid phosphatase and, in raw-milk cheese, lipoprotein
lipase. Information on the significance of other indigenous
enzymes is lacking.
Starter LAB and their enzymes: These reach maximum
numbers (approximately 10^9 CFU/g) at the end of curd
manufacture (6–12 hours). They then die and lyse at strain-
dependent rates, releasing their intracellular enzymes.