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.