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Cassava Starch

 

Cassava

The Cassava harvest can take place most of the year with less manufacturing and reduced storage costs. These results of tapioca starch give a competitive advantage over the use of potato starch in many applications.

THE OCCURRENCE OF STARCH

Starch makes up the nutritive reserves of many plants. During the growing season, the green leaves collect solar energy. In plants with tuberous roots, this energy is transported as a sugar solution down to the tubers, and it is down there, that the sugar is converted to starch in the form of tiny granules occupying most of the cell interior.

The conversion of sugar to starch takes place by means of enzymes. Then, the following spring, enzymes are also responsible for the re-conversion of starch to sugar, which is transported upwards as energy for the growing plant.

CASSAVA ROOTS.

The cassava plant is cultivated in most equatorial regions and is known by many names, e.g.:

 

 

In Europe and USA cassava is the term usually applied to the roots, and tapioca is the name given to starch and other processed products.

The plant grows to a height of 1 - 3 m and several roots may be found on each plant. Manioc prefers a fertile sandy-clay soil.

There are many varieties of cassava, but they fall into two main categories, namely bitter and sweet cassava (Manihot palmata and Manihot aipi), depending on their content of cyanohydrin. For industrial purposes bitter varieties are most frequently used because of their higher starch content. Sweet cassava is preferred for food because of its taste and dough forming ability. It pounds well.

A typical composition of the root is:

 

Starch content may be as high as 32%. 

QUALITY OF RAW MATERIAL.

The roots are living plants and need some air for respiration and life activity. During storage the roots consume a small amount of their own starch to maintain life functions until spring. This will require some fresh air, and the respiration causes development of heat. If the tubers get warm, respiration increases, raising the temperature further. A lot of starch is used for respiration and the tubers will die of heat.

Unfavourable storage conditions cause starch losses and, in the worst case, dead and smashed raw materials, which are disruptive for the process. Therefore roots are processed in the order they are delivered to the factory, and the roots must be processed within 24 hours of harvest.

Supplies of bad raw materials have to be rejected.

ROOT RECEPTION.

First point of reception is weighing of the lorry on a platform scale. Second point is sampling. A sample of roots is washed on a laboratory washer. The difference of sample weight before and after washing indicates the proportion of dirt in the delivery. The next step is the determination of the starch content. Both figures are used to settle the account with the farmer and encourage the delivery of high quality tubers rich in starch.

The roots are stored in the reception yard on a concrete floor. The yard may be divided in several bunkers alongside a main yard conveyor to accomplish the processing of the oldest tubers first. At this point the tubers are still bulky with an average weight of 600 - 650 kg per m3.

The main yard conveyor may be a long horizontal band positioned 10 - 20 cm below floor level. In large reception yards more bands may be used to feed the main conveyor. A Bobcat may find use as well or instead of a yard conveyor. 

RAW MATERIAL HANDLING.

First of all, any stalks must be removed. This is most easily done during harvest. Stalks will interfere with the peeling, blunt the rasps, and increase the fibre mass with adverse effect on the process.

Loose dirt, sand, and gravel are removed in different ways. Preferably a rotating bar screen is used for a dry cleaning of the roots prior to the washing step.

From a buffer bin the tubers are fed into the bar screen. From the bar screen the roots enter the washing station. Paddle washers are still in use, but rotary washers have proved their efficiency as they have in the potato industry.

Thorough dirt removal will lessen the problems with stones and sand later. The soil also contains considerable quantities of nutrients, which will dissolve in the washing water and contribute to the environmental burden created by the effluent.  

EFFICIENT WASHING MAKES REFINING EASIER.

Soil and dirt not removed in the washing station yield problems later. The washing is therefore very important.

High quality washing improves refining, because many impurities resemble starch in specific weight and size, so washing is the only way to get rid of them. The rubbing in the washing machine is a most important quality factor.

The quantity of impurities adhering to the surface upon delivery depends to a great extent on weather conditions and the soil where the tubers are cultivated.

The paddle wash takes place in two compartments - one with a water level and one without. The rotary wash machine combines flushing with a low water level and continuous removal of dirt and peel.

The wash water may be recycled after filtering off peelings on a rotary screen and settling of sand in basins. Process water from the refining station and crude water replace the loss of wash water.

The washed tubers are conveyed on an inspection belt to the pre-cutter. In order to feed the rasps properly, the roots are chopped into pieces. 

RASPING.

Rasping is the first step in the starch extraction process. The goal is to open all the tuber cells, so that all the starch granules are released. The slurry obtained can be considered as a mixture of pulp (cell walls), fruit juice, and starch. With modern high-speed raspers, rasping is a one-pass operation only. An even feed of the rasps is essential for a steady flow throughout the rest of the plant.

After rasping, the hydrogen cyanide and cyanohydrin are released and go with the juice and process water. The effluent may be disposed of by land spreading.

USE OF SULPHUR.

The cell juice is rich in sugar and protein. When opening the cells, the juice is instantly exposed to air and reacts with the oxygen, forming coloured components, which may adhere to the starch.

Food grade sulphur dioxide gas or sodium-bisulphite-solution therefore has to be added. The great reduction potential of the sulphur compounds prevents discoloration. Sufficient sulphur has to be added to turn the juice and pulp light yellow.

EXTRACTION.

Powerful flushing is needed to release the starch granules from the cells - the cells are torn apart in the rasper and form a filtering mat trying to retain the starch. Water has previously been used for the extraction, but today the extraction takes place in closed systems, allowing the use of the juice itself or process water from the refining step.

The starch that is flushed out leaves the extraction sieves along with the fruit juice. The cell walls (pulp) can be concentrated further on pulp dewatering sieves. In this case the pulp leaves the dewatering sieves wet, but drip-dries to 10 - 15 % dry matter.

The extraction takes place on rotating conical sieves. The high efficiency makes it feasible to utilise high quality sieve plates made of stainless steel, which will withstand abrasion and CIP-chemicals. The sieve plates have long perforations that are only 125 microns across.

The extraction is a counter current process. It is followed by a fine fibre washing on conical sieves also. The washed fibres are combined with the pulp and may be used as cattle feed.

CONCENTRATING THE CRUDE STARCH SLURRY.

As much juice as possible is excreted on a couple of hydrocyclone batteries or on a nozzle centrifuge. The starch leaves the concentrator as pumpable slurry of approx. 21 oBe.

The juice leaves the factory as a by-product. The best way to dispose of the fruit water is to utilise it as a fertiliser on near by agricultural land.

REFINING

It now remains to purify the crude starch slurry (suspension) and remove residual fruit juice and impurities. The way it is done is more or less based on the same principles used when removing soapy water from the laundry - you wring and soak in clean water repeatedly. Everyone doing laundry realises how often it is necessary to wring before the rinsing water is all clear, and that the harder you wring the fewer rinsing steps are required.

In the same way the starch slurry is diluted and concentrated again and again. With hydrocyclones it is feasible to reduce fibre and juice to low levels with a minimum of fresh water. To save rinsing water the wash is done counter currently - i.e. the incoming fresh water is used on the very last step and the overflow is reused for dilution on the previous step, and so on.

Increasing the number of hydrocyclone refining steps may accomplish considerable savings of fresh water. This is one of the advantages of using hydrocyclones. Dual refining lines represent the ultimate design.

In the strong gravitational fields of a hydrocyclone and a centrifuge, the starch settles quickly, while fibres (pulp residuals) just float in the water. Fibres with adhering air bubbles are lighter than water and seek towards the overflow. Fibres with starch granules enclosed are heavier and sink towards the underflow and mix with the starch. The juice is directly diluted in the water and goes with the water phase. Refining is based on the differences in weight density between water, fibres, and starch:  

By creating a water flow moving towards the starch, lots of fibres that are floating in the water may be forced into the overflow. Soil, sand, and many fungi, etc. are of equal density, or heavier than starch and it is not possible to separate these particles from starch by centrifugal force. That is why it is so important to remove as many impurities as possible from the tuber surface in the washing station. If the inlet concentration gets too high, even lighter particles may be retained because of plug flow conditions occurring.

Although some impurities go with the starch in the underflow, there is, by means of a sieve, a last chance to remove the larger particles.

Impurities not removed this way are not removable by any known technique.

HYDROCYCLONES.

The hydrocyclone has no moving parts and the separation is totally dependent on the pressure difference over the cyclone.

For the removal of the juice hydrocyclones are far more efficient than centrifuges due to the large dilution rate of the feasible application of multi-stage units.

Starch is among the most pure of all agricultural products. Actually, purity is the most important parameter for being competitive.

No significant amount of juice is left in the starch. The colour or whiteness may be improved by the use of sulphur in the right place and dosage, and by removing iron and manganese from the process water. Oxides of iron and manganese (e.g. rust) are dark coloured components, which have to be removed in the water treatment plant.

CIP - CLEANING IN PLACE.

Cleaning in Place is done with caustic and hypochlorite as cleaning agents. Caustic is a powerful agent for removal of the protein build-up on the interior walls and the hypochlorite is an efficient germ killer

During CIP it is of the utmost importance to keep the pipes filled up. Tanks are most efficiently CIP'ed with rotating disc nozzles - and covered tanks are required.

DRYING AND SIFTING.

The purified starch milk is dewatered on a continuous rotating vacuum filter or a batch operated peeler centrifuge.

The moist dewatered starch is dried in a flash dryer with hot air. The inlet air temperature is moderate. High temperatures may increase cold-water soluble starch. The moisture of tapioca starch after drying is normally 12-13%.

MODIFICATION

Most starch is used for industrial purposes. Starches are tailor made to meet the requirements of the end-user giving rise to a range of specialty products. Many and sophisticated techniques are applied. A most versatile principle comprises a three step wet modification:

Preparation

v

Reaction

v

Finishing

By applying different reaction conditions - temperature, pH, additives - and strict process control, specialty products with unique properties are made.

These specialty products are called modified starches. They still retain their original granule form and thereby resemble the native (unmodified) starch in appearance, but the modification has introduced improved qualities in the starch when cooked. The paste may have obtained improved clarity, viscosity, film-forming ability, etc.

Starch finds uses in fast food, sweets, sausages, tablets, paper, corrugated board etc. and plays a prominent part in our everyday life.

APPLICATION.