From NewCROP™, Center for New Crops & Plant Products, Purdue University
by J. R. Magness, G. M. Markle, and C. C. Compton.
Sugar cane
Gramineae, Poaceae Saccharum officinarum L.
Sugar
cane is the source of sugar in all tropical and subtropical countries
of the world. Estimates for 1966 and 1967 indicate world production of
cane sugar was between 40 and 41 million tons. Production in the United
States, excluding Puerto Rico, averaged 2,550,000 tons during those
years - from 592,000 acres of cane in Hawaii, Florida and Louisiana.
Sugar production in Puerto Rico averaged 850,000 tons for the two years.
Several species of Saccharum
are found in Southeast Asia and neighboring islands, and from these
cultivated cane probably originated. The sweet juice and crystallized
sugar were known in China and India some 2500 years ago. Sugar cane
reached the Mediterranean countries in the eighth century A.D., and
reached the Americas in early colonial times.
The cane plant is
a coarse growing member of the grass family with juice or sap high in
sugar content. It is tender to cold, the tops being killed by
temperatures a little below freezing. In continental United States,
where freezing may occur during the winter, it is mainly planted in
late summer or early fall and harvested a year later. In tropical
countries it may be planted at almost any time of the year since the
plant does not have a rest period.
The season of active growth in
continental United States is 7 to 8 months while in tropical countries
growth is near continuous until harvest. This results in heavier yields
of cane and sugar under tropical conditions. For example, yields of
cane and sugar per acre in Hawaii, where the cane is grown for about 2
years before harvesting, are from 3 to 4 times vields in Louisiana and
Florida from one season's growth.
Sugar cane plants are
propagated by planting sections of the stem. The mature stems may vary
from 4 to 12 feet or more in height, and in commercial varieties are
from 0.75 to 2 inches in diameter. The stem has joints or nodes as in
other grasses. These range from 4 to 10 inches apart along the
above-ground section of the stem. At each node a broad leaf rises which
consists of a sheaf or base and the leaf blade. The sheaf is attached
to the stem at the node and at that point entirely surrounds the stem
with edges overlapping. The sheath from one node encircles the stem up
to the next node above and may overlap the base of the leaf on the next
higher node.
The leaf blade is very long and narrow, varying in width
from 1 to 3 inches and up to 5 feet or more in length. Also, at each
node along the stem is a bud, protected under the leaf sheath. When
stem sections are planted by laying them horizontally and covering with
soil a new stem grows from the bud, and roots grow from the base of the
new stem. The stem branches below ground so several may rise as a clump
from the growth of the bud at a node.
In planting cane fields,
mature cane stalks are cut into sections and laid horizontally in
furrows. In continental United States sections with several nodes are
laid while in tropical countries sections with 2 or 3 nodes are
commonly used - since temperatures for growth are more favorable.
Usually only one node on a stem piece develops. a new plant because of
polarity along the stem piece.
Planting is in rows about 6 feet
apart to make possible cultivation and use of herbicides for early weed
control. As plants become tall lower leaves along the stems are sbaded
and die. These ultimately drop off, so only leaves toward the top
remain green and active. Between the nodes the stems have a hard, thin,
outer tissue or rind and a softer center. The high-sugar-containing
juice is in this center. More than one crop is harvested from a
planting. After the first crop is removed two or more so-called stubble
crops are obtained. These result from growth of new stalks from the
bases of stalks cut near the ground level in harvesting.
Harvesting
Harvesting
of cane in Hawaii and Louisiana is highly mechanized. Machines top the
canes at a uniform height, cut them off at ground level, and deposit
them in rows. In Florida, cane is mainly cut by hand. Leaves and trash
are burned from the cane in the rows by use of flame thrower type
machines. An alternate method is to burn the leaves from the standing
cane, after which it is cut and taken directly to the mill. Delay
between cutting and milling in either case should be as short as
possible since delay results in loss of sugar content. Machines are
under development that will cut, clean and load the cane so it can be
taken directly to the mill.
In continental United States, where
winter freezing is a hazard, cane harvest must start earlier than is
desirable for maximum yields. When plants are killed by freezing sugar
loss occurs rapidly. While such plants are suitable for sugar
extraction if harvested promptly after freezing, this may not be
possible when large acreages are involved. In non-mechanized areas cane
is still cut and the leaves stripped off by using cane knives. This is
arduous and time consuming work.
Sugar Manufacture
Sugar
is obtained from the cane at mills located near centers of production.
The cane first goes through a washer, then is cut into small pieces by
revolving knives. These cut pieces may then be shredded or may move to
crushers directly. The crushers consist of two large grooved rollers
mounted horizontally, one above the other. The crushed, macerated cane
then goes through three or more roller mills which consist of grooved
rollers with heavy hydrolic pressure maintained on the upper roller.
Water, equal to about 20 percent, is added before the mixture is passed
through each set of rollers except the last one. Efficient mills
extract at least 90 percent of the sugar in the cane. The cane residue,
called 'bagasse', can be used as feed.
The mixture of plant sap
and water, with the sugar in solution, collected from the roller mills
is slightly acid in reaction with a pH of 5 to 5.5. It is neutralized
with lime, which precipitates some of the colloids and other nonsugars
and also stops conversion of sucrose to reducing sugars. The limed
juice is then heated to boiling, which results in further formation of
precipitates that settle to the bottom of the tanks. These are drawn
off and filtered to remove more juice. The nearly clear juice is
continuously drawn off from the top of the tank and goes to the
evaporators.
The evaporators are a set of three vacuum pans or
"bodies" arranged in series, with each successive pan maintained under
higher vacuum. The juice enters the first pan at 16 to 180 Brix and
leaves the third at 55 to 750 Brix. It then goes to high-vacuum boiling
pans - about 25 inches of mercury - there it is further concentrated to
900 Brix and contains sugar crystals. It then is centrifuged to remove
most of the liquid or molasses. The remaining raw or brown sugar is
then ready for final refining. Much of the imported sugar enters this
country as raw sugar and is further refined here before being marketed.
The
final refining steps include melting the brown or raw sugar, decoloring
by passing through carbon filters, recrystallizing in vacuum boiling
pans, and drying by centrifuging. A hundred pounds of raw sugar
produces about 96 pounds of refined. A ton of cane yields from less
than 170 to more than 225 pounds of raw sugar, depending on such
factors as variety, maturity when harvested, promptness of milling, and
incidence of diseases on the cane in the field. Average per acre cane
yields in 1966 and 67 were 23.5 tons in Louisiana, 32.1 tons in
Florida, and 95.9 tons in Hawaii.
The molasses obtained in
milling totals around 150 million gallons in the United States and near
60 million in Puerto Rico. It is used as an additive in livestock feed,
in the manufacture of alcohol and alcoholic beverages - as rum and to
some extent in foods. The fibrous plant residue from the roller mills
may be used as fuel at the mill, made into paper or insulating board,
or used as plant mulches or bedding for livestock.
Sugar Cane Syrup
Sugar
cane for syrup is grown over a somewhat wider area in the United States
than cane for sugar. The area extends from eastem Texas east to South
Carolina. The culture is essentially the same as for sugar cane and
some of the varieties are the same. Since most production is in areas
with a shorter growing season than the sugar producing areas, early
maturing varieties are essential. Most of the cane grown for syrup is
in small acreages and the syrup is manufactured on a small scale,
although there are a few sizable factories.
For best yield and
quality of syrup, harvest should be delayed until the cane is mature,
but before it is killed by freezing. Leaves are stripped from the
standing cane either by beating off with a cane stripper, cutting, off
or pulling off by hand. Stems are topped and cut near ground level.
Delay of up to 30 days between cutting and making the syrup does not
impair either yield or quality of the syrup, provided the cane does not
freeze.
In general, mills with three horizontal rollers turned
by motors are used to extract the juice. Larger mills may use rollers
under hydraulic pressure. From 50 to 60 pounds of juice should be
obtained from 100 pounds of cane. Open-type, continuous flow
evaporators are generally used to concentrate the juice. The cold
juice enters the lower end of the evaporator which is heated by fire
beneath or, in larger installations, by steam coils. When the juice is
heated, proteins and some other nonsugar constituents coagulate, float
on the surface, and are skimmed off at the upper end. In manufactured
apparatus a final finishing or evaporating vat may be used.
Proper
density of the finished product is determined by using a hydrometer
(35-360 Baume), or determining the boiling point with a thermometer
(226-228 F.). The finished syrup is then filtered and placed in
containers while hot. Production of cane syrup has fluctuated widely
reaching more than 28 million gallons in 1945 when sugar was scarce
because of World War II. Production in 1966 and 1967 averaged 2,151,000
gallons annually. All is used as food. With good varieties and good
agronomic practices an acre of cane should produce from 500 to 600
gallons of finished syrup. Average yields, however, are only about half
this amount.
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