|
Non
Food uses of Cassava
Starch
makes a good natural adhesive. There are two types of adhesives
made of starches, modified starches and dextrins: roll-dried
adhesives and liquid adhesives.
The
application of cassava in adhesives continues to be one of
the most important end uses of the product. In the manufacture
of glue the starch is simply gelatinized in hot water or with
the help of chemicals. For conversion into dextrin it is subjected
separately or simultaneously to the disintegrative action
of chemicals, heat and enzymes.
In
gelatinized starch adhesives, quality requirements are such
that the medium-quality flours can be used. In dextrin manufacture,
the demands are much more exacting: only the purest flours
with a low acid factor are acceptable. Cassava dextrin is
preferred in remoistening gums for stamps, envelope flaps
and so on because of its adhesive properties and its agreeable
taste and odour.
Dextrins
were accidentally discovered in 1821 when during a fire in
a Dublin (Ireland) textile mill one of the workmen noticed
that some of the starch had turned brown with the heat and
dissolved easily in water to form a thick adhesive paste.
Three
primary groups of dextrins are now known: British gums, white
dextrins and yellow dextrins.
British
gums are formed by heating the starch alone or in the presence
of small amounts of alkaline buffer salts to a temperature
range of about 180°220°C. The final products range in colour
from light to very dark brown. They give aqueous solutions
with lower viscosities than starch.
White
dextrins are prepared by mild heating of the starch with a
relatively large amount of added catalyst, such as hydrochloric
acid, at a low temperature of 80º-120°C for short periods
of time. The final product is almost white, has very limited
solubility in water and retains to varying degrees the set-back
tendency of the original starch paste.
Yellow
dextrins are formed when lower acid or catalyst levels are
used with higher temperatures of conversion (150°-220°C) for
longer conversion times. They are soluble in water, form solutions
of low viscosity and are light yellow to brown in colour.
The
following are some of the major uses of dextrins in nonfood
industries.
Corrugated
cardboard manufacture. One of the large users of dextrins
is the corrugated cardboard industry for the manufacture of
cartons. boxes and other packing materials. The layers of
board are glued together with a suspension of raw starch in
a solution of the gelatinized form. The board is pressed between
hot rollers, which effects a gelatinization of the raw starch
and results in a very strong bonding. Medium-quality flours
are suitable for this purpose provided the pulp content is
not too high.
Remoistening
gums. These adhesives are coated and dried on surfaces,
such as postage stamps and envelope flaps, for moistening
by the user before application to another surface. Cassava
dextrins in aqueous solution are well suited for this purpose
as they give a high solids solution with clean machining properties.
Wallpaper
and other home uses. Various types of starch-based products
are used as adhesives for wallpaper and other domestic uses.
Foundry.
Starch is used as an adhesive for coating the sand grains
and binding them together in making cores which are placed
in moulds in the manufacture of castings for metals.
Well
drilling. Starches and modified starches mixed with clay
are used to give the correct viscosity and water-holding capacity
in bores for the exploratory drilling of oil wells or water
wells. These starch products are replacing other commercial
products for making the muddy materials which are indispensable
for drilling wells. For this purpose a coldwater-soluble pregelatinized
starch which can be made up to a paste of the required concentration
on the spot is desired.
Paper
industry. In the paper and board industries, starch is
used in large quantities at three points during the process:
(i)
at the end of the wet treatment, when the basic cellulose
fibre is beaten to the desired pulp in order to increase
the strength of the finished paper and to impart body and
resistance to scuffing and folding;
(ii)
at the size press, when the paper sheet or board has been
formed and partially dried, starch (generally oxidized or
modified) is usually added to one or both sides of the paper
sheet or board to improve the finish, appearance, strength
and printing properties;
(iii)
in the coating operation, when a pigment coating is required
for the paper, starch acts as a coating agent and as an
adhesive.
Cassava
starch has been widely used as a tub size and beater size
in the manufacture of paper, in the past mainly on account
of its low price. A high colour (whiteness), low dirt and
fibre content, and, above all, uniformity of lots are needed
in this instance.
An
important new application of starch is in the machine-coating
of magazine paper, formerly done exclusively with caseins.
There are indications that cassava is particularly well suited
to the purpose; however, definite specifications for the starch
still have to be worked out.
Textile
industry. In the textile industry, starches occupy an
important place in such operations as warp sizing, cloth finishing
and printing. Warp sizing is the application of a protective
coating to prevent the single yarns from disintegrating during
weaving. The size consists of an adhesive and a lubricant
and is generally removed after weaving. Cloth finishing alters
the "feel" of the fabric by making it firmer, stiffer
and heavier. Cassava starch is also used for cloth printing
or producing certain designs in various colours on the smooth
surface of a finished fabric. While cassava accounted for
about 20 percent of all starch for these purposes in 1937,
it has been largely replaced by other starches after the Second
World War.
An
exception is the manufacture of felt, where cassava continues
to be used exclusively in the finishing process.
Wood
furniture. Before the Second World War the manufacture
of plywood and veneer relied mainly on cassava as a glue.
The basic material in this case is gelatinized at room temperature
with about double the amount of a solution of sodium hydroxide.
After prolonged kneading of the very stiff paste in order
to give it the required stringy consistency, the glue is applied
to the wood with rollers. As the presence of a certain amount
of the pulp is useful, medium- to low-quality flours are acceptable
or even preferable, although the presence of sand is objectionable.
Since
1945, however, the use of cassava as a glue has declined to
second place owing to the increasing success of water-resistant
plastics.
As
cassava cultivation increases, more stalks will become available
for disposal. The Tropical Products Institute, London, has
been working on the utilization of the cassava plant. Particle
boards could be made from cassava stalks by cutting them into
small sections and mixing them with certain resins. The strength
of the board can be varied by altering the resin content or
the density.
CASSAVA
ALCOHOL
Cassava
is one of the richest fermentable substances for the production
of alcohol. The fresh roots contain about 30 percent starch
and 5 percent sugars, and the dried roots contain about 80
percent fermentable substances which are equivalent to rice
as a source of alcohol.
Ethyl
alcohol is produced from many carbohydrate materials. In Malaysia
and some other countries, many factories are equipped to use
cassava roots, starch or molasses (by-product of the sugar
industry), the type of product depending on the costs of the
raw materials. When cassava is used, the roots are washed,
crushed into a thin pulp and then screened. Saccharification
is carried out by adding sulfuric acid to the pulp in pressure
cookers until total sugars reach 15-17 percent of the contents.
The pH value is adjusted by using sodium carbonate, and then
yeast fermentation is allowed for three to four days at a
suitable temperature for the production of alcohol, carbon
dioxide and small amounts of other substances from sugar.
Alcohol is then separated by heat distillation. The yield
of conversion is about 70-110 litres of absolute alcohol per
ton of cassava roots depending on the variety and method of
manufacture. The crude alcohol of cassava is described as
average in quality. It has a disagreeable odour, but can be
improved if the first and last fractions in the distillation
process are discarded. It is usually utilized for industrial
purposes, as in cosmetics, solvents and pharmaceutical products.
If the production is required for human consumption, special
care should be taken in handling the roots to rid them of
hydrocyanic acid.
|
