Technical Memorandum on Potato Starch
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Potato starch
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PotatoesPotato starch has been produced in Denmark on an industrial scale since 1900.

75% of the potato crop is grown for industrial processing and the Danes produce per capita more starch than any other nation.


OCCURRENCE OF STARCH

Starch constitutes the nutritive reserves of many plants. During the growing season, the green leaves collect energy from the sun. In potatoes 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 next spring, enzymes are also responsible for the re-conversion of starch to sugar - transported upwards as energy for the growing plant.

THE BASIS FOR STARCH QUALITY IS LAID IN THE POTATO CLAMP.

In the field or stored in clamps during winter, the tubers stay alive and need some air for respiration and life activity.

Potatoes consume a small amount of their own starch during winter to maintain life functions until spring. This requires fresh air and the respiration causes generation of heat.

If the surrounding temperature falls with a risk of frost, the tubers try to save their skin by extensive conversion of starch to sugar in order to lower the freezing point in the cell juice. If this does not suffice, the tubers die. Potatoes therefore must be adequately covered when stored.

If the potatoes get warm, respiration increases, raising the temperature further. A lot of starch is used for the respiration and the tubers will die of heat.

Unfavourable storage conditions cause starch losses and, in the worst case, dead and smashed potatoes, which are disruptive for the process.

Supplies of bad potatoes have to be rejected.

Damage during transport also causes quality problems. Every single blow damages cells, with starch losses and a dead spot on the tuber as a result. It is therefore of utmost importance to handle the potatoes during transport as carefully as possible with the techniques and equipment available.

REFINING BEGINS ALREADY DURING RAW MATERIAL INTAKE.


Drop damper for initial filling of empty store.
During unloading at the factory, damage can be reduced by covering buffer silos with rubber and minimising drop impact with rubber curtains. Smashed potatoes loose a lot of juice, causing foam and unnecessary problems in the washing station.

Loose dirt, sand and gravel are removed on a rotating screen before the potatoes are deposited in the store - the better the dirt removal, the lesser the problems with stones and sand in the fluming channels later. The soil also contains considerable quantities of nutrients, which will dissolve in the washing water and contribute to the environmental effect caused by the effluent.

The potato store is a necessity to secure the supply of potatoes overnight. Supplies for the weekend may also be required because of restrictions on heavy road transport outside ordinary working hours.

The ideal situation is to reach the bottom of the potato store every morning, because the potatoes suffer during long storage in thick layers without adequate ventilation.

EFFICIENT WASHING MAKES REFINING EASIER.

Soil and dirt not removed in the washing station give problems later. The washing is therefore very important. The washing is a counter current process, with fresh water added through pressure nozzles in the final step.

The potatoes are flumed by water in channels - passing a stone trap - to the washing station. The stone trap utilises the difference in specific weights between stones and potatoes - an upstream water flow carries the potatoes over the stone trap, while the heavier stones are trapped and collected on a stone conveyor.

The water level in the washing drum has to be kept low so that the potatoes do not float. The drum is not merely a conveyor, but also ensures that the potatoes rub vigorously against each other. The rubbing is essential for the removal of fungi, rotten spots, skin and dirt from the surface. The floating water may be recycled after settling of sand in pools.

A high standard of washing improves refining because many impurities resemble starch in specific density and size, so washing the potatoes is the only way to get rid of them.

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

The quantity of water used for fluming and washing is identical with the quantity of clean water applied in the final high-pressure spray.

RASPING.

Rasping is the first step in the starch extraction. The goal is to open the tuber cells and release the starch granules. 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.

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.

Sulphur dioxide gas or sodium-bisulphite-solution therefore has to be added. A considerable reduction potential of the sulphur compounds prevents discoloration. Sufficient sulphur has to be added to maintain the juice and pulp light yellow. (Re.: TM 23)


EXTRACTION.

Powerful washing is needed to flush the starch granules out from the cells - the cells are torn apart in the rasper and form a filtering mat that tries to retain the starch. Water has previously been used for the extraction, but today extraction takes place in closed systems allowing the use of the potato juice itself. It has the advantage that the juice can later be recovered in concentrated and undiluted form, reducing transport costs for its use as a fertiliser.

The flushed-out starch discharges from the extraction sieves along with the fruit juice, and the cell walls (pulp) are pumped to the pulp dewatering sieves. The pulp leaves the dewatering sieves as drip- dry - i.e. approximately 8% dry matter.

The extraction takes place on rotating conical sieves, where centrifugal power increases the capacity per unit of area. 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 only 125 microns across.


Operating Principle of a Starch Extractor.
The extraction is a counter current process in which the pulp-dewatering screen is actually the last step. If the pulp is required in almost dry form, the number of spray nozzles with washing water is reduced. Instead continuous back spraying is maintained to ensure that the dry pulp will slide down the screen.

CONCENTRATING THE CRUDE STARCH SLURRY.

On hydrocyclone unit as much juice is excreted as possible. The starch leaves the concentrator as pumpable slurry of approximately 19 oBe.

The concentrating stage typically consists of a unit with hydrocyclone blocks for defoaming, concentrating and starch recovering arranged in series.

REFINING

It now remains to purify the crude starch milk (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 soap water from the laundry - you wring and soak in clean water again and again. Everyone doing laundry realises how often it is necessary to wring before the rinsing water is completely 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. 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 recycled for dilution on the previous step and so on.

HYDROCYCLONES.

Refining is based on the difference in specific density of water, fibres and starch:

  Specific density g/ml
Starch    1,55
Cell walls (fibres)    1,05
Water    1,00
Soil, sand    above 2

In the strong gravitational fields of a hydrocyclone and a centrifuge, starch settle quickly, while fibres (pulp residuals) just float in the water. The juice is directly diluted in the water and goes with the water phase.


By creating a water flow moving towards the starch, lots of fibres just 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 potato 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 - that is particles larger than 125 microns. The particles are not spherical. On the contrary, they are of irregular shape and may be forced through refining sieve, if the spray pressure is too high.

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

In the recovery steps all starch has to be retained in the underflow so only very little is wasted in the effluent (fruit water).

COOLING

The lower the water consumption, the more pumps are involved in the process and the more heat is generated. To retard bacterial growth refrigerator temperatures are ideal.

In the effluent of concentrated fruit juice, cooling during extraction is a must because in hot juice microbes that break down protein and a bad smell may take control.

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. (Re.: TM 23 CIP)

DRYING AND SIFTING.

The moist starch from the rotating vacuum filters is dried in a flash dryer with moderate hot air. The air is indirectly heated.

Before delivery the starch is sifted on a fine sieve in order to remove any scale formed in screw conveyors etc.

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


Modification. Most starch is used for industrial purposes. Starch is tailor made to meet the requirements of the end-user giving rise to a range of speciality 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 speciality products with unique properties are made.

These speciality products are named modified starches, because 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.

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

Composition of potato starch

Constituents Typical analysis
Starch, dry substance 80%
Water 20%
Ash 0.3%
Sand 0.02%
Protein 0.09%
Phosphor, P 0.07%
Calcium, Ca 0.03%
Iron, Fe 3 ppm
Cold water soluble 0.1%

Size distribution.

Interval, microns %
87-140 3
53-87 24
38-53 34
28-38 17
22-28 9.1
17-22 7.7
13-17 3.3
10-13 0.9
8-10 0.5
6-8 0.2
0-6 2.0

Properties of starch.

Surface of starch granules app. 30 ha/g
Specific density app. 1.55 g/ml
Specific heat 1.22 J/g
Bulk weight of starch 80% DS app. 0.7 g/ml
DS of moist centrifugate app. 0.6 g/ml
Brightness (MgO2 = 100%) app. 95 %

Determination of starch content in potatoes.

Starch content versus density

The starch content of fresh potatoes is correlated with potato density. A sample of 5050 g potato in a net basket is weighed above water and then again immersed in clean water of maximum 18 oC.

Wo = weight of the potato sample
Wu = weight of sample under water
Density dpotato = Wo / (Wo - Wu) g/ml
Starch dry matter = (dpotato -1,015059)/ 0,0046051 %

The calculated figure in per cent deviate less than 0.05 from values read out of the EU-table enforced by the European Commission covering potatoes with 8% to 23% starch dry matter. A short version of the table - as valid per 1. July 1996 - is as follows:

Wo 1

g

Wu 2

g

Density d 3

g/ml

Starch dry matter of potatoes 4

%

Potatoes to produce 1 t commercial starch 5

kg

EU-minimum price 6

ECU/t potato

EU-subsidy I to the factory 7

ECU/t potato

EU-subsidy II to the factory 8

ECU/t potato

5050 352 1.075 13 6,533 32.110 3.405 13.31
5050 372 1.080 14 6,065 34.590 3.671 14.33
5050 392 1.084 15 5,664 37.040 3.924 15.35
5050 412 1.089 16 5,308 39.520 4.190 16.38
5050 430 1.093 17 5,000 41.960 4.456 17.39
5050 450 1.098 18 4,720 44.440 4.709 18.42
5050 470 1.103 19 4,467 46.960 4.987 19.46
5050 488 1.107 20 4,299 48.800 5.180 20.22
5050 508 1.112 21 4,234 49.550 5.253 20.53
5050 527 1.117 22 4,140 50.670 5.373 21.00
5050 545 1.121 23 4,056 51.720 5.482 21.43

1) Weight of potato sample above water. 2) Weight of sample immersed in water. 3) Calculated density = (Wo / (Wo -Wu)). 4) Starch content according to the official table. 6) The minimum price to be paid by the manufacturer directly to the farmer in order to qualify for a subsidy. 7) An EU-subsidy I- a production premium - is paid to the manufacturer, if he pays the EU-minimum price or better. 8) A subsidy II- an equalisation payment - is paid directly to the factory, if he pays the EU-minimum price or better. The exchange rate 1. July 1996 was 1 ECU = 1,24 US$.

This empirical method is used to settle accounts with suppliers of potatoes to starch factories. Similar method is used for cassava. Unfavourable storage conditions however may cause enzymatically conversion of starch to glucose affecting starch yield without changing the under-water-weight and density of potatoes. Therefore the method only apply to fresh potatoes. The method is by no means scientifically correct. Reliable methods for the determination of carbohydrates in potatoes exist, but the soluble non-starch fraction and the fraction of particular starch grains are indistinguishable from one another by practical accurate means.