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.
Nutritional Aspects
Cheese is a good source of protein, calcium, zinc, and vitamin B12. However,
full fat cheese is a major source of saturated fat which can lead to raised
serum cholesterol levels. Also, it contains no carbohydrate or fibre, and
is a very poor source of iron. Vegetarians, particularly new vegetarians,
should be wary of too high a consumption of cheese.
Further Information
Cattle (Information
Sheet)
Vegan
Nutrition (Information
Sheet)
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