Tapioca Starch Production TM1-10e



The occurrence of starch

Starch makes up the nutritive reser­ves of many plants. During the growing season, the green leaves col­lect solar energy. This energy is transported as a sugar solution down to the roots, 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.


Cassava is cultiva­ted in most equatorial regions and is known by many names, e.g.:
  • Indonesia
    • Ubi kettella
    • Kaspe
  • South America
    • Manioca
    • Yucca
    • Mandioca
    • Aipim
  • Africa
    • Manioc
    • Cassava
  • India
    • Tapioca
  • Thailand
    • Cassava
In Europe and USA cassava is the term usually applied to the plant and its 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. Cassava prefers a fertile sandy-clay soil.

There are many varieties of cassava, but they fall into two main categories - 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 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:
  • Moisture 70%
  • Starch 24%
  • Fiber 2%
  • Protein 1%
  • Other 3%
Starch content may be as high as 30%.

Quality of raw material.


The roots are living plants and need 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 fresh air, and the re­spiration causes devel­op­ment of heat. If the roots get warm, re­spi­ration increas­es raising the tempera­ture further. A lot of starch is used for re­spira­tion and the roots will die of he­at.


Unfavorable 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 after har­vest.

Root Reception.

At the factory gate the lorry is weighed on a platform scale. Roots are sampled and washed. The ratio of dirt is found as the difference in weight before and after washing and the starch content is calculated from the density determined by the under-water weight method. Both figures are used to settle the account with the farmer and encourage the delivery of high quality roots rich in starch.

The roots are stored in the reception yard on a concrete floor in a way to ac­complish the processing of the oldest roots first. At this point the roots are still bul­ky with an average weight of 600 - 650 kg per m3. A yard conveyor below floor level enables the use of a Bobcat for feeding the factory.

Raw material handling.

Stalks must be removed during harvest. Stalks will interfere with the peeling, blunt the rasps, and increase the fiber mass with adverse effect on the process.

Loose soil is removed by a rotating bar screen prior to washing. Thorough dirt removal lessens the problems with stones and sand later.

A regular supply of fresh ro­ots is advan­tageous, but in some regions the weather con­ditions favor cutting and dry­ing the roots for later pro­cessing. The sliced roots (chips) are dried in the sun for 2 - 3 days. The sun drying, however, affects the whiteness of the starch, but starch made from sun dried chips is excellent as raw fruit in breweries.

The graph illustrates rehydration by soaking in water. 


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.

From the bar screen the roots enter the washing station via a stone catcher. Paddle washers are still in use, but rotary washers introduced by International Starch Institute have proved their efficiency.


Efficient washing helps refining.

Soil and dirt not removed in the washing station cause problems later. 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.

The rotary wash machine continuous removes dirt and peel. The wash water may be recycled after filtering off peelings on a rotary screen and settling of sand in basins. Spend process water (fruit water) from the refining station replaces the loss of washwater.

The washed roots 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 (grating) is the first step in the starch ex­traction process. The goal is to open all the root cells and release all starch gra­nu­les. The slurry (rasping) ob­tained can be consi­dered as a mixture of pulp (cell walls), fruit juice, and starch. On modern high-speed machines, rasping is a one-pass opera­tion only. An even feed of rasping is essential for a steady flow throughout the rest of the plant.

The hydrogen cyanide and cyanohydrin are released during rasping and go with the juice and process water.

Use of Sulphite.


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 colored components adhering to the starch.
 
Food grade sulphur dioxide gas or so­dium bisul­phite solu­tion has to be ad­ded. The gre­at reduction po­tential of the sulphite pre­vents discoloration. Suffi­cient sulphite 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 retaining 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 is flushed out and leaves the ex­traction sieves along with the fruit juice. The cell ­walls (pulp) can be concen­trated further on de­watering sieves to a drip-dry pulp with 12 - 15 % dry mat­ter.


The extraction takes place on rotating conical sieves. The high effi­ciency makes it feasible to utilize 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 fiber washing on conical sieves also. The washed fibers are combined with the pulp and may be used as cat­tle feed.

The Split Process.


Excess juice is removed on a couple of hy­dro­cyclone concen­trators. The con­centrated starch - crude starch milk - is washed on the refining line.

The concentrating and refining lines split the crude starch milk in:

  • Starch Underflow
  • Fruit Juice Overflow 1. split
  • Fruit Water Overflow 2. split

Fruit juice is the natural juice initially removed in the concentrators. Fruit water is spent process water mixed up with residual fruit juice removed during refining. The split of the effluent in fruit juice and fruit water is an option of the Millennium process - named after its first introduction.

Refining

The concentrated crude starch milk is washed with fresh process water in order to remove residual fruit juice and impurities.

With hydrocyclones it is feasible to reduce fiber 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.

In the strong gravi­tational fields of a hydro­cyclone and a cen­tri­fuge, the starch sett­les quickly, while fibers (pulp re­sidu­als) just float in the water.

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

Each hydrocyclone is adjusted to force the light fraction of fibers and the smaller starch granules into the overflow and the larger starch granules into the underflow.

Impurities of equal densi­ty as starch cannot be se­parated from starch by cen­tri­fugal force. That is why it is so important to remove as many impurities as possible from the root surface in the washing sta­tion.

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.

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 color or whiteness may be improved by the use of sul­phite 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 colored 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 CIPed 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. Starch may be modified to meet the requirements of the end-user giving rise to a range of specialty products. 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 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.

Application.


Tapioca starch is used in the manufacture of sweeteners, sizing of paper and textile and is in particular an excellent food starch used as a thickener and stabilizer. The pulp is used as cattle feed. Juice and spent process water are valuable fertilizers disposed of by land spreading


Being a pure renewable natural polymer starch has a multitude of applications.

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

Cassava starch represents 10 % of the World production of starch estimated to a total of 60 million t in 2004.



The viscosity (torque) is recorded on a Brabender instrument during controlled heating of starch slurry and subsequent cooling. Temperature is drawn in red.
Native cornstarch develops a characteristic set-back when cooled (green line). Tapioca starch is characterized by its peak viscosity during cook-up and moderate set-back when cooled (blue line).
 
The tapioca starch was tested in 6% suspension and the cornstarch in 8% - illustrating the viscosity difference.

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