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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.

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 13% 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.

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

 

Size distribution.

 

 

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 %

 

A: Typical Composition of Starch Raw Materials

 

B. Distribution of Starch by Origin.

2004 World production totalled 60 million t

C. World Starch Production By Region.

Estimated production of 60 million t by 2004

 

D. Starch Usage.

Chemistry. Glucose is formed in plants from carbon dioxide absorbed from the air using sun light as energy source. Part of the glucose is polymerised into long chains of glucose and stored as starch in granules as a reserve. In spring starch is broken down again to support new growth.

This break down of starch can be imitated in a our factories by applying acid or enzymes to cooked starch. The way we do it cause the starch to hydrolyse into a variety of mixtures of glucose and intermediates and the way we characterise these various mixtures is by its DE number. DE means Dextrose Equivalent. The analytical procedure measures reducing end groups and attach a DE of 100 to pure glucose (glucose = dextrose) and a DE of 0 to pure starch.

Only glucose solution of high DE can crystallise easily and yield a product in powder or granular form. A most popular crystallised product is dextrose monohydrate with applications in medicine and used in chewing tablets by people doing sport. Dextrose monohydrate is pure glucose. A less purified product known as Total Sugar is produced by instant crystallising a 97 DE syrup leaving no hydrol (mother liquor) to dispose off.

Standard Acid converted 42 DE Syrup. Lowering the DE, the syrup loose gradually its tendency to crystallise and below approximately 45 DE the syrup can be evaporated into a stable, non-crystallising and auto sterile liquid. These qualities are one of the reasons behind the success and wide spread use of the standard 42 DE syrup. Starch is hydrolysed by acid or enzymes to 40 - 42 DE and evaporated to a viscous liquid with a dry matter of 80% - 84%.

This standard product has a bland sweet taste, stores and ship well in drums or tank lorries. It find applications in canned fruit preserves, ice cream, bakery products, jam, soft drinks, candy and all kinds of confectionery. Large quantities are also used as a booster in the fermentation of alcohol The relative sweetness of 42 DE to sucrose is 40 - 45%.

High quality starch is supplied either as a slurry from a starch factory or a slurry of approximately 21 oBe is prepared from ordinary native dried starch. Acid - preferably hydrochloric acid, HCl is added to the slurry in order to acidify before cooking. The acidified slurry is heated to the desired temperature by injecting steam of 9 bar. The liquefaction temperature is kept for a few minutes. The degree of liquefaction (hydrolysis) is controlled by the temperature in the holding zone. The acid is neutralised and the hydrolysate enters a cyclone - via a back pressure valve - where the hydrolysate is flashed down to atmospherical pressure. The crude hydrolysate is refined by means of activated carbon in order to remove discoloration from the interaction of protein and other starch constituents during hydrolysation. Filter aid is added as body feed and the filtered off on a filter press. The purified hydrolysate passes a check filter and the water clear hydrolysate is evaporated until the dry substance reaches 80 - 84 %. From the evaporator the final product can be drummed off. Dependent on raw material and end product requirements various filtration steps and deionization etc. may be added to the process

Enzymes as catalysts. The acid catalyst allows the manufacture of intermediate conversion products ranging from 35 - 55 DE. Intermediate and higher conversion products for special purposes can also be made by substituting acid with enzymes - typically in a two step process. For the first step, the liquefaction, termostable a-amylase or acid is used. After cooling and pH adjustment a saccharification enzyme like amyloglucosidase is applied. Except for a different holding time the processes are in principles identical regardless of catalyst. However, enzymes and acid breaks down starch differently resulting in different sugar composition for identical DE, but it is possible to work around that problem and even produce the classic 42 DE syrup by an all-enzyme process only. With enzymes it is possible to produce syrups with DE from 28 and up to 98. Glucose syrups may be grouped according to the degree of conversion:

 

Conversion Groups

 

Glucose Composition

A=Acid E=Enzyme A/E=Acid liquefaction plus enzyme saccharification

High DE syrups are intermediates for fructose syrup, sorbitol, and many fermentation products and find uses in beverages, foods etc. Glucose syrup and maltose syrup are referred to as wort syrups in breweries, where they substitute malt improving capacity, adjusting protein, taste, mouthfeel etc.

Sugar confections will either pickup or lose moisture to the atmosphere, depending on the external conditions to which they are subjected. Therefore the water activity of the sweetener is an important property. This value is known as the equilibrium relative humidity (ERH).

Water Activity of Sweeteners

HFSS. High Fructose Starch-based Syrups are produced from refined very high DE glucose syrups. An enzymatic process using isomerase fixated on a resin facilitates the conversion of glucose to fructose. By using more resin columns in parallel the enzyme activity is completely exhausted before a refill.

The isomerase catalyses the formation of 42% fructose in equilibrium with glucose. This syrup may be refined and evaporated as such and it is an excellent all-purpose sweetener.

In order to obtain a more perfect match with sucrose based liquid sugar (cane and beet sugar) the fructose content has to be increased to 55% by enrichment. A stream of HFSS-42 is fractionated. Previous attempts to do this by crystallisation have never gained industrial acceptance. The fractionation is done more elegantly by chromatography. By auto-matically switching the injection point an endless ring column is simulated and the HFSS-42 is fractionated in fructose and glucose. Water or condensate is used to eluate the column. The fructose fraction is backmixed with the HFSS-42 to make up an HFFS-55. In this way a perfect match with traditional sucrose based liquid sugar is obtained. The HFSS-55 finds widespread use as sweetener in soft drinks.

The fructose fraction from the chromatographic column can of course be refined and evaporated to a syrup separately as HFSS-90 finding applications in low calorie foods.

Demineralisation throughout the HFSS-process and precautions against de-cross-linking by oxygen extends the lifetime of the resins. An HFSS-section should preferably run continuously non-stop.

Starch & Sweetener Process Flow Chart in Principle