Growing the Cane
Sugar cane is a sub-tropical and tropical
crop that prefers lots of sun and lots of water - provided that its
roots are not waterlogged. It typically takes about 12 months to reach
maturity although the time varies widely around the world from as short
as six months in Louisiana to 24 months in some places. Where it differs
from many crops is that it re-grows from the roots so the plant lasts
many cycles [or 'ratoons', a word derived from the Spanish to sprout]
before it is worn out.
Harvesting
Sugar cane is harvested by chopping down the stems
but leaving the roots so that it re-grows in time for the next crop.
Harvest times tend to be during the dry season and the length of the
harvest ranges from as little as 2 ½ months up to 11 months. The cane is
taken to the factory: often by truck or rail wagon but sometimes on a
cart pulled by a bullock or a donkey!
Extraction
The first stage of processing is the extraction of
the cane juice. In many factories the cane is crushed in a series of
large roller mills: similar to a mangle [wringer] which was used to
squeeze the water out of clean washing a century ago. The sweet juice
comes gushing out and the cane fibre is carried away for use in the
boilers. In other factories a diffuser is used as is described for beet
sugar manufacture. Either way the juice is pretty dirty: the soil from
the fields, some small fibres and the green extracts from the plant are
all mixed in with the sugar.
Evaporation
The factory can clean up the juice quite easily
with slaked lime (a relative of chalk) which settles out a lot of the
dirt so that it can be sent back to the fields. Once this is done, the
juice is thickened up into a syrup by boiling off the water using steam
in a process called evaporation. Sometimes the syrup is cleaned up again
but more often it just goes on to the crystal-making step without any
more cleaning. The evaporation is undertaken in order to improve the
energy efficiency of the factory.
Boiling
The syrup is placed into a very large pan for boiling,
the last stage. In the pan even more water is boiled off until
conditions are right for sugar crystals to grow. You may have done
something like this at school but probably not with sugar because it is
difficult to get the crystals to grow well. In the factory the workers
usually have to throw in some sugar dust to initiate crystal formation.
Once the crystals have grown the resulting mixture of crystals and
mother liquor is spun in centrifuges to separate the two, rather like
washing is spin dried. The crystals are then given a final dry with hot
air before being stored ready for despatch.
Storage
The final raw sugar forms a sticky brown mountain in
the store and looks rather like the soft brown sugar found in domestic
kitchens. It could be used like that but usually it gets dirty in
storage and has a distinctive taste which most people don't want. That
is why it is refined when it gets to the country where it will be used.
Additionally, because one cannot get all the sugar out of the juice,
there is a sweet by-product made: molasses. This is usually turned into a
cattle food or is sent to a distillery where alcohol is made.
Power
So what happened to all that fibre from crushing the
sugar cane? It is called "bagasse" in the industry. The factory needs
electricity and steam to run, both of which are generated using this
fibre.
The bagasse is burnt in large furnaces where a lot of
heat is given out which can be used in turn to boil water and make high
pressure steam. The steam is then used to drive a turbine in order to
make electricity and create low pressure steam for the sugar making
process. This is the same process that makes most of our electricity but
there are several important differences.
When a large power station produces electricity it burns a fossil fuel
[once used, a fuel that cannot be replaced] which contaminates the
atmosphere and the station has to dump a lot of low grade heat. All this
contributes to global warming. In the cane sugar factory the bagasse
fuel is renewable and the gases it produces, essentially CO
2,
are more than used up by the new cane growing. Add to that the factory
use of low grade heat [a system called co-generation] and one can see
that a well run cane sugar estate is environmentally friendly.