What's in Cheese?! Read on........
Cheese & Rennet
Introduction -- Manufacture -- Vegetarian Cheeses -- Types of
Cheese -- Nutritional Aspects
Introduction
Cheese is made by coagulating milk to give curds which are then
separated from the liquid, whey, after
which they can be processed and matured to produce a wide
variety of cheeses. Milk is coagulated by the
addition of rennet. The active ingredient of rennet is the
enzyme, chymosin (also known as rennin). The
usual source of rennet is the stomach of slaughtered newly-born
calves. Vegetarian cheeses are
manufactured using rennet from either fungal or bacterial
sources. Advances in genetic engineering
processes means they may now also be made using chymosin
produced by genetically altered
micro-organisms.
Manufacture
The exact processes in the making of cheese varies between
different varieties. However, all cheeses are
made by essentially the same method. Initially, the milk is
usually pasteurised by heating at 72°C for 15
seconds to destroy potentially harmful bacteria. The milk is
then cooled to around 30°C and a starter
culture of lactic acid bacteria is added to help souring. These
convert lactose into lactic acid and help in the
coagulation process. In addition, they also have a beneficial
effect on the eventual quality, taste and
consistency of the cheese. Some cheeses are coagulated entirely
by lactic acid bacteria and are known as
lactic-curd or acid-curd cheeses. However, some cheeses sold as
lactic-curd cheese may have had rennet
added.
The next stage is the addition of rennet, containing the enzyme
chymosin. Rennet is usually sourced from
the abomasum (fourth stomach) of newly-born calves. Here,
chymosin aids the digestion and absorption of
milk. Adult cows do not have this enzyme. Chymosin is extracted
by washing and drying the stomach
lining, which is then cut into small pieces and macerated in a
solution of boric acid or brine at 30°C for 4-5
days. Pepsin may sometimes be used instead of chymosin. This is
usually derived from the abomasum of
grown calves or heifers, or less commonly pigs. Pepsin may be
mixed with calf rennin. Rennet coagulates
the milk, separating it into curds and whey. This is called
curdling.
Chymosin breaks down the milk protein casein to paracasein
which combines with calcium to form calcium
paracaseinate, which separates out. Milk fat and some water
also becomes incorporated into this mass,
forming curds. The remaining liquid is the whey. The strength
of different rennets can vary, though usual
strength varies between 1:10,000 and 1:15,000 i.e. one part
rennin can coagulate 10-15,000 parts milk.
Other substances may also be added during the cheese making
process. Calcium chloride is added to
improve the curdling process, and potassium nitrate is added to
inhibit contaminating bacteria. Dyes (e.g.
annatto, beta-carotene), Penicillium roquefortii mould spores
to promote blue veining, or propionic acid
bacteria to encourage hole formation may be added.
Following curdling, the curds are cut and drained. The size of
the cut and the methods used vary for
different cheese varieties. For soft cheeses, the curds are
sparingly cut and allowed to drain naturally. For
hard cheeses, the curds are heated and more whey is drained
off. The curds are then cut into small pieces,
placed in vats and pressed.
After pressing, the curds may be treated in a number of ways.
They may be moulded into different shapes,
soaked in a saltwater solution, be sprayed with mould forming
spores or bacteria, washed in alcohol, or
covered in herbs.
The final stage is ripening, or maturation. This can vary in
length from 4 weeks to 2-3 years, depending on
the type of cheese. During ripening flavours develop, the
cheese becomes firmer and drier, and special
characteristics such as holes, blue veining and crust formation
occurs.
Vegetarian Cheeses
Vegetarian cheeses are made with rennets of non-animal origin.
In the past, fig leaves, melon, wild thistle
and safflower have all supplied plant rennets for cheese
making. However, most widely available vegetarian
cheeses are made using rennet produced by fermentation of the
fungus Mucor miehei. Vegetarian cheese
may also be made using a rennet from the bacteria Bacillus
subtilis or Bacillus prodigiosum.
Advances in genetic engineering techniques mean that some
vegetarian cheeses may now be made using
chymosin produced by genetically engineered micro-organisms.
The genetic material (DNA) which encodes
for chymosin is introduced into a micro-organism which can then
be cultured to produce commercial
quantities of chymosin. This is done by extracting genetic
material from calf stomach cells which acts as a
template for producing the chymosin encoding DNA. This can then
be introduced into the micro-organism.
Once the genetic material is introduced there is no further
need for calf cells. Alternatively, the chymosin
encoding DNA can be bio-synthesised in the laboratory without
the use of calf cells.
The chymosin produced is identical to that produced by calf
stomach cells. The development of genetically
engineered chymosin has been encouraged by shortages and
fluctuations in cost of rennet from calves. It's
manufacturers claim that genetically engineered chymosin will
end the cheese making industry's reliance
on the slaughter of calves.
Chymosin encoding DNA has been introduced into three different
micro-organisms. These are the yeast
Kluyveromyces lactis, the fungus Aspergillus niger var awamori,
and a strain of the bacteria Escherichia
coli. All of these have now been approved and cleared for use
by the Ministry of Agriculture, Fisheries &
Food. There is no legal requirement for manufacturers to state
whether a genetically engineered rennet has
been used in the cheese making process.
Vegetarian cheeses are widely available in supermarkets and
health food stores. A wide variety of cheeses
are now made with non-animal rennet and labelled as suitable
for vegetarians. No particular type of cheese
is exclusively vegetarian. Soft cheeses are as likely to be
non-vegetarian as hard cheese.
Types of Cheese
The type of cheese produced depends on the milk used and the
cheese making process. The milk used
may be full fat, semi-skimmed or fully skimmed, this affecting
the fat content of the cheese. It may be
pasteurised or unpasteurised. Milk from different animals and
different breeds is important in determining
the final flavour. As well as cow's milk, cheese may be made
from sheep or goat's milk.
Soft cheeses may be fresh or ripened. Fresh cheeses include
quark, cottage cheese and cream cheese.
Ripened soft cheeses include Brie and Camembert. Semi-soft
cheeses include Stilton, Wensleydale and
Gorgonzola. Hard cheeses include Cheddar, Cheshire, and
Gruyere. Parmesan is a strongly pressed, very
hard, dry cheese ripened for 2-3 years and then grated. Whey
cheeses such as Ricotta are made as a
by-product of other cheeses from the whey removed during
pressing. Processed cheeses are either made
with trimmings that are left over from the manufacture of other
cheeses, or from dried milk powder.
Flavourings, colourings and other additives are used.