As a result, variations in the quality of cheese
do occur, depending on the type of milk used. For example, milk containing high
total solids (sheep) increases cheese yields, and conversely, milk high in fat
produces softer cheese, but improves the mouth-feel of the product. Thus, the
cheesemaking process has to be modified in relation to the type of milk used.
In nature, milk is produced to feed the
offspring; however, let us consider for a moment what happens when a calf takes
in milk from its mother. The milk has to provide all the essentials for the
body-build-up of the calf during the critical period up to weaning. She also
provides certain compounds which give initial protection from bacterial disease,
until the calf can build up its own immunity. First, the milk drawn from the
teat is warm and sweet, and the milk sugar (lactose) provides both encouragement
to drink more and will provide energy later when needed. Passing into the first
of three stomachs, it is progressively acidified until arrival at the fourth
stomach. Here it comes into contact with two coagulating enzymes (chymosin and
pepsin - previously known as rennin). These enzymes are basically organic
catalysts i.e. substances which promote a particular chemical reaction without
being themselves used up in the process. So these enzymes combine with the
acidified milk and curdle it to form a fine clot. The clot (or curd as it is
better known) then passes forward into the intestine. Having been changed into a
curd, its passage through the intestines is slowed down just long enough to be
digested (protein, fat, minerals, vitamins and lactose), and absorbed through
the intestinal wall and into the bloodstream suitable for future body-building.
Later we shall see the role of these enzymes in cheesemaking.
Making Cheese from Curdled Milk
Cheesemaking capitalises on the curdling of milk.
First, the milk is carefully selected to make sure there are no antibiotics or
harmful agents that could affect the process. The milk is then heated and held
at a given temperature for a short period to destroy any harmful bacteria (i.e.
pasteurisation). Special starter cultures are then added to the warm milk and
change a very small amount of the milk sugar into lactic acid. This acidifies
the milk at a much faster rate and prepares it for the next stage. Rennet
(mainly chymosin) is then added to the milk and within a short time a curd is
produced. The resultant curd is then cut into small cubes, and heat is applied
to start a shrinking process which, with the steady production of lactic acid
from the starter cultures, will change it into small rice-sized grains. At a
carefully chosen point the curd grains are allowed to fall to the bottom of the
cheese vat, the left-over liquid, which consists of water, milk sugar and
albumen (now called whey) is drained off and the curd grains allowed to mat
together to form large slabs of curd. It is pressed, and
subsequently packed in various sized containers for maturing.
The Basic Components of Cheese
Milk
Fat
Fat exists in milk as small globules that can
vary in size depending on the breed of cow. The fat in the milk helps to produce
flavour, aroma and body in mature cheese. Cheese made from skimmed milk is hard
in body and texture, and lacks flavour. However, only a small amount of fat (as
low as 1%) can produce a background flavour, and today's makers exploit this
with their 'low-fat cheese' for which there is a growing demand.
Protein
Protein exists in two forms in milk as a
suspension/colloidal (casein) and in a soluble form (whey proteins). As an
analogy, however, consider the first type of protein as a densely woven mesh
rather like a string vest suspended freely in the aqueous phase of milk. As long
as the milk remains sweet, this structure is unaffected and the milk remains
totally fluid. However, if the milk acidifies (i.e. goes sour) without the
presence of coagulating enzymes the structure changes quite suddenly at the
'iso-electric point', and a fragile curd is formed that collapses with the
slightest agitation into tiny fragments. A typical example is the fine mass we
see when milk sours naturally. By adding rennet, at just the right time before
the milk would go completely sour, the structure of the casein is changed
radically to form a solid curd called para-casein. This can then be cut with
knives and saved to be collected as grains of curd for subsequent processing.
The second fraction of protein is called albumen
(alpha-lactalbumin and beta-lactoglobulin). This as described above passes out
with the whey and is usually lost, though it can be recovered by specialised and
expensive filtration methods. When hot milk is allowed to stand still for any
time, whey proteins appear as a 'skin' on the surface.
Enzymes
In milk different enzymes may arise from the cow
herself, from bacteria present in the teat canals or from organisms that gain
entry to the milk at a later stage. As we shall see shortly these enzymes have a
profound effect on the quality of raw milk, and the ripening of cheese in the
store. For example, lipases, proteases and lactase enzymes hydrolyse the fat,
protein and lactose respectively into different components. In this case, these
enzymes, which occur naturally in the milk or which are sometimes supplied by
the indigenous bacteria in the milk and the added starter culture, can change
the milk fats and proteins in the process of ripening the cheese to produce the
delicate flavours and aromas that make mature cheese so enjoyable. Later we
shall see just how a cheese grader can assess these vital elements.
Vitamins
These are organic substances in milk which help
to promote growth. Milk fat holds the fat soluble vitamins (A, D, E and K) and
the water soluble vitamins are the B complex and C which are in the whey. They
also play an important part in encouraging bacteria to grow in the cheese
ripening process.
Lactose
This is the main sugar in the milk. It provides
the energy source for the starter cultures to produce lactic acid, and so helps
to modify the milk for cheesemaking. About 10% of the lactose is used by the
starter bacteria to make lactic acid, and the rest is drawn off with the whey.
It was used in the past to feed to pigs for fattening up, but with the massive
increase in cheese production this no longer became practical.
In the twenties, a private firm (Whey Products
Ltd.) was set up in England to exploit the use of whey by concentrating it to
about 65% total solids, crystallising the lactose, then washing and refining it
for sale to the pharmaceutical and baking industries. For some years after the
Second World War, the United Creameries Ltd. (UC Ltd.) at Tarff accepted whey
from cheese creameries in Galloway for pre-concentration and transfer south to
Haslington for final refining. Tarff creamery closed down in the early seventies
due to the advent of large whey installations at Galloway and Lockerbie
creameries for whey drying.
Surprisingly enough, whey was generally
considered by practical cheesemakers of the day to be little more than a
confounded nuisance and where sewage facilities were not available large
quantities were simply dumped surreptitiously into ditches, down old quarries,
sprayed over land or piped straight out to sea.
Ash
Those substances are present in milk and consist
of metallic components (sodium, potassium, calcium, magnesium, manganese, iron,
copper) and non-metallic elements such as sulphur, chlorine, phosphorous.
Calcium is probably the most important mineral for the coagulation of milk, and
together with the protein is an excellent source of food, especially for
children who can absorb it quickly into their growth system.
Starter Cultures
Cheese is really a form of fermented milk, and
acid production is carried out by starter cultures. Milk being sourced from a
living animal has bacteria in it when fed to the calf. Some bacteria produce
acid, others help to digest the protein in the milk; some use milk as a base for
their own development which, in the case of disease-producing bacteria, can
infect those who drink it. Tuberculosis, brucellosis and undulant fever are
three examples of diseases that can affect those who may drink unpasteurised
milk.
Happily, the acid producing bacteria can in some
cases directly suppress disease-producing bacteria under normal conditions. This
is why fermented milk products are among the safest foods to take in their
natural state particularly in areas where food hygiene may be suspect. The first
breakthrough came when a French scientist called Louis Pasteur was able to show
their harmful effect in wine and later in milk. Lister in 1873 isolated a mesophilic bacterium which he named
Bacterium lactis and later known as
Streptococcus lactis (the present designation is Lactococcus lactis
subsp. lactis) for use as a cheese starter culture.
The first practical use of bacterial cultures for
the dairy industry was in fact for butter. In 1890, the Danish scientist Storch
developed a selected strain of bacteria which he called Streptococcus
cremoris (the present designation Lactococcus lactis subsp.
cremoris), and this knowledge was soon applied to cheesemaking. Workers on the
continent selected pure bacterial cultures just for making cheese to which the
name was given as starters.
Until the middle of the 19th century,
cheesemakers on croft and farm simply held over a portion of soured milk or whey
in a small jug or churn and used it the following day to make cheese. This
worked perfectly well as long as the amount of cheese being made was relatively
small, but cheesemaking was never consistent and results varied greatly.
Cheesemaking was carried out only in the summer months and at the end of the
season starter had somehow to be kept for the next year. This was in fact done
in many rural areas in Scotland by filling up a clean bottle with starter,
corking it securely and burying it in the back garden. The following Spring it
was dug up and, after one or two sub-cultures, used again for cheesemaking.
Moulds play their part in cheesemaking. The white
mould seen on Camembert helps to hydrolyse the protein in the final cheese by
working from the outside in. Blue moulds can be added with the starter, and help
to breakdown the curd produced from the inside of the cheese outwards.
Sometimes, to help the growth of blue mould, the cheese is pierced with a skewer
which lets in air and helps the mould to spread and carry on the good work of
protein/fat hydrolysis. This explains the blue streaks seen sometimes in Danish
Blue cheese.
Over the last sixty years much work has been done
to develop starters that would work consistently under creamery conditions. In
effect we have moved from the forties where starter was made up fresh each day
in liquid form to the situation now where starter is kept as freeze-dried or in
deep freeze cabinets and added as a powder or granules, respectively, to the vat
before cheesemaking begins. These starter culture systems are known as
direct-to-vat inoculation (DVI).
Coagulants/Rennet
The need to coagulate milk has been well
recognised since Roman times, and this can be achieved by the selective use of
certain plants or by extracting the enzyme rennet (chymosin and pepsin) from the
fourth stomach of the milk-fed calf.
Records for the making of rennet go back to the
16th century. The farmer or small-holder cheesemaker would select and slaughter
a milk-fed calf, remove and wash the fourth stomach carefully. He would then
hang this out to air-dry in which case it would become known as a 'vell'. There
was a regular market for dried vells. However, it is most likely that dried pieces of vells were added
directly to the milk, and at later times vell extracts in salt solution were
used. Basically, sliced or mascerated vells were soaked in salty water to
provide a solution of enzymes. Filtration may have been used for the
purification of the final rennet solution (See:.Preparation
of Rennet Storing the rennet in a salt solution
keeps it in good condition and suppresses any bacteria that might cause a
deterioration in quality. Such rennets are known as 'calf rennets'.
Another form of rennet is called 'vegetable'
rennet which is derived from certain strains of fungi and bacteria. Today, this
type of rennet is very popular, reflecting a move towards organic foods, and the
manufacture of 'vegetarian cheese'. Substantial amounts are now used at
farmhouse and creamery level. Recently, due to world shortage of calf rennet,
recombinant or genetically engineered pure chymosin derived from different
microorganisms is available on the market, and is currently used by many
cheesemakers in different countries.
Salt
By this term we mean sodium chloride, the common
salt used at home for cooking and seasoning food. Four main methods are used
depending on the type of cheese that is being made.
1. Hard-pressed cheese
These are called textured cheese, such as
Cheddar, Cheshire and the English regional cheeses including Caerphilly, which
undergo pressing for a period from 18 hours up to 2-3 days after being put into
the cheese moulds. Throughout the cheesemaking process we have described for
Cheddar, the starter is steadily making acid, its speed in so doing reduced
somewhat in the heating process used in the final stages. To stop further acid
development, and also to provide an element of flavour and help preserve the
final cheese, salt is added after the curd blocks are milled. The amount varies
with the type of cheese made, but is usually around 1.5 - 3% (w/w). Salting
provokes a further small rush of whey, cools the curd slightly and controls
further acid development. In traditional cheese vats, the salt was added by hand
after milling either in the vat or in the 'cooler' (a trolley-like vehicle on
which curd blocks were cheddared and made ready for milling). However, in modern
automated plants, the salt can be blown from a salt-silo directly on to the
milled curd laid out on a moving bed. Mechanical probes assess the curd depth
and adjust the amount of salt needed electronically.
2. Brine-salted cheese
These are also hard- and semi-hard pressed
cheese, but usually salted for a much shorter time and relatively large and
small in size, respectively. A typical example would be the Edam (Dutch) and
Emmental (Swiss). In this case, the cheese are removed from their mould and
tumbled straight into a bath of salt solution strong enough to float the cheese.
By holding these cheese in huge shallow tanks, they start absorbing salt, and
after a period they are floated along to similar tanks with an even stronger
salt solution during which the salt continues to be absorbed. They are then
removed by elevator from the brine bath, allowed to dry out by which time the
degree of salt needed has spread through the cheese.
3. Soft cheese salting
Soft cheese types, which tend to be small, can be
rubbed with salt on the outer surface at least once, and sometimes twice. The
salt can then migrate across the cheese in about 24 hours. This method of
salting assists in the formation of rind on the cheese.
4. Blue-veined cheese salting
Salt is usually applied on the curd before
moulding, sometimes on the curd while in its mould or indeed after the cheese
has been removed from the cheese mould.
Coagulation and Treatment of The
Coagulum
The conditions for
coagulation of the milk , including temperature, acidity and method of
application of the coagulant, must be standardized, as must the firmness of the
coagulum at cutting. Thus, cutting the coagulum to release the whey must
be timed correctly, and the method of cutting with a ladle or slicing with
knives into small pieces, is specific for each type of cheese. The size of curd
particles, in particular, affects the rate of loss of whey, the rate at which
scalding can be done and , therefore, the rate of future acid development.
The temperature to
which the curds and whey are heated and the rate of heating (scalding)
affects the rate of syneresis of whey and the rate of growth of starter
organisms. The final temperature will also affect the survival of the starter
bacteria, so that some scalding temperatures are selected to reduce the numbers
of Lactococcus lactis subsp. lactis while
leaving the population of Lactococcus lactis subsp.
cremoris largely intact. This differential inhibition allows
many of the enzymes of L. lactis spp. to remain in the curd to assist with
ripening, but prevents excess acid production. The dilution of whey
with hot water, a step used in the American washed curd process,
similarly reduces the rate of acid development.
Stirring the curd,
so that the curds are kept as separate particles floating in the whey,
assists with effective whey removal.
Removal of the whey is one of the
most important stages in the process, for by removing the curd from the whey,
the curd particles can being to coalesce together to form a cheese.
In some cases, natural compaction- with or without pressure- is sufficient to
form cheese, but with cheeses like Cheddar, a "developed" texture is brought
about by manipulation the curds after whey removal. By contrast, the curd of
Pasta filata cheeses are kneaded in hot water after the initial compaction, and
the success of all these operation depend upon the initial draining of the whey
being completed satisfactorily. It can be important also that most of the
lactose is lost in the whey, so limiting the substrates for post-production
microbial activity.
During the manufacture of
hard-pressed cheeses, the "textured" curd needs to be broken into small pieces
so that the dry salt can be evenly distributed throughout the entire mass of
curd. This breakdown is achieved by milling, and the size of milled pieces has a
dramatic influence on the texture of the finished cheese.
Salting
The recipe will incluide details as
to the rate and time of application (including values for pH/acidity at the time
of salting), whether dry salting or immersion in brine is most suitable, the
grain size of the dry salt, as well as the temperatures that should be suoght
during salting. This definitions of the conditions for salting or brining the
cheese is essential, for the application of salt (a) inhibits the growth of
lactic organisms and, therefore, slows down the rate of acid formation, (b)
assists with the syneresis of whey during moulding and/or pressing and (c)
imparts essential flavour to the final product.
Moulding and Pressing
The prepared curd is formed
into a shape-often typical of a particular variety- by the use of metal, plastic
or wooden moulds or hoops. In some instances, the cheeses are moulded to shape
by hand.
Mechanical pressing-either with
weights or with screw/hydraulic presses-assists with the final whey removal, as
well as providing texture and shape to the cheese. The initial, intermediate and
final pressuresare important to achieve the correct final quality, as in the
timing of any increases in pressure during the overall stage. The temperature of
the pressing room may be defined as well.
Cheese Ripening
The cheese, placed in the store
immediately out of press, was basically a rubbery and elastic mass of curd,
still warm from the cheesemaking operation, and largely without flavour or
aroma. The curd particles still retained their identity in spite of the
pressing over the previous two days. There may well have been some mechanical
openness and free moisture. For the first few days it needed careful handling.
Eventually the curd cooled and became more solid, and a firm bodied structure
ready for the changes that would turn it into the type of cheese aimed at by the
maker. The actual ripening process - then and now - is brought about through the
agency of enzyme systems produced by bacteria which have grown or are growing in
the curd.
Once the cheese has been
removed from the mould, it will undergo a 'finishing' process that varies with
the variety. In some cases, the cheese may be allowed to stand in a warm
atmosphere to develop a dry rind; with other cheeses, like Edam, a coating of
coloured wax is traditional. Other varieties have to be stored under special
conditions to develop a bacterial smear or fungal coating over the cheese. The
nature of the rinds/coats determines, to some extent, the conditions of storage
although, in the case of Emmental, a pre-maturation period is required for
development of the 'eyeholes' in the curd. The storage conditions in terms of
temperature, humidity and turning of the cheese are defined in the recipe.
However, as the temperature determines the rate of maturation of the curd, this
factor is sometimes dictated by the market demand for a slow-or-quick-ripening
cheese.
In spite of the existence of recipes for specific cheeses, it
is important that cheesemaking and maturation involve natural biological
processes, and it is inevitable that there will be occasions when the procedures
deviate from the normal. Hopefully, the final product will still be accetable
and edible, but where a failure has occurred, it is imperative that the
cheesemaker has a means of determining the cause.
Cheese made from raw milk will always have a
subtler and richer flavour at the end of its ripening period as the raw milk
bacteria and their enzymes are carried forward into the final making process.
Pasteurising the milk can destroy the indigenous bacteria and also the lipolytic
enzymes that both contribute to flavour and aroma. However, the pathogenic (or
'illness-causing') bacteria are destroyed by pasteurisation, and where close
control of the milk cannot be exercised ultimately by the cheesemaker (as it may
arrive in bulk from several farms) pasteurisation is regarded as obligatory for
such supplies.
According to Scott (1986) the ripening process
could be briefly described as follows:- "Although the breakdown of the main
constituents of curd, i.e. protein, fats and sugars, is responsible for the
changes in body, flavour and aroma, they are not necessarily degraded step by
step. The amount of cross-linking of degraded products and the multiplicity of
enzymes in the curd give rise to a multitude of substances which, individually
affect the body, flavour and aroma of cheese. However, it is the
combination of these individual flavours and aromas against the
background of the intact fats and proteins which constitute
those characteristics appreciated by the customer".
