The manufacture of starch is a very traditional process and can only be sourced from plants. It is a key component of the food supply chain by virtue of how much starch is used as a starting material in the production of dextrins, hydrolysates and other starch derivatives.
Roughly 60 million tonnes of starch are extracted every year. It is used in paper, textiles, bioplastics as well as food.
Industrial Sources Of Starch
Most raw materials come from plants which are commonly grown as sources for a range of feedstocks. The main sources are:-
potatoes (Europe) – 12%
maize (Europe, USA, China) – 70%
wheat (Europe & Australia) – 8%
tapioca (South-East Asia & South America) – 9%
sorghum, rice etc. –( various countries) – about 1%
The production of starch from the wet milling of corn is described elsewhere in greater detail. It is one of the most important manufacturing processes in the food industry and has almost achieved commodity status in its own right.
Starch From Potato
The potato (Solanum tuberosum) is the most common source of starch in Europe. It is a seasonal crop so starch processing is only possible within a particular timeframe.
Potatoes are generally grown for consumption so the main source of starch is from poorer quality tubers such as cull potatoes, from surplus stock and any waste streams that comes from other forms of potato processing. Some cultivars have been developed specifically for starch production but they are not that well known.
A potato tuber contains between 65 and 80% starch on a weight for weight basis. The starch is contained as granules in leucoplasts. These starch grains have to be released by crushing the tuber cells to release them. The starch is washed out and dried to a powder.
Potato starch unlike corn (maize), rice or tapioca starch contains 800ppm phosphate which is bound. The presence of phosphate raises the viscosity. Potato starch is slightly anionic. It has a low gelatinization temperature which is about 60°C (140°F) and with a high swelling power.
Most often at the grower the potatoes which have not been sent for eating are washed with water in a flume to remove most of the dirt and foreign matter.
Potatoes are received at the processing plant and passed into large hoppers where they need to be cleaned of soil and sand. They are sometimes dry cleaned and then further washed with water (Ratnayake and Jackson, 2003). More sophisticated producers use machine vision sorting equipment to remove damaged or diseased potatoes as a means of reducing the poor quality of starch entering the system.
The potatoes are pulped using a saw blade rasp in a process called rasping or with a hammer mill. Raspers are rapidly rotating drums fitted with saw blades on their periphery. This also releases starch granules from the fibrous matrix.
Potato proteins in the tubers are highly soluble. In very large processing facilities the proteins are separated out using decanter centrifuges. In smaller processors, proteins are separated later on and removed in the wastewater flowstream.
Sulphur dioxide usually in the form of sodium bisulphite is added when the potatoes are pulped. This compound helps to prevent browning and acts as a bleaching agent. It also stops oxidation of various compounds.
It is not common for the potatoes to be peeled but the mashed up product is screened to remove skins or peel and any fibre. The fibtrous pulp is dewatered using decanter centrifuges. A lot of fibre will be extracted from the skins. Much of the waste pulp is sold off as livestock feed.
Any water-soluble impurities are separated by further washing. Any other impurities are removed by gravity separation. The bulk of the mashed potatoes are often converted into a slurry for easier movement to other parts of the factory which is then further refined. Screening relies on a screening battery. This separates the starch from any of the potato pulp. This can be reground so that a second starch extraction is performed. The total yield is 12% w/w from the raw potatoes.
Any starch isolated on the screens and sieves is reslurried in water to remove soluble material. Further dewatering occurs in a continuous centrifuge or decanter.
Water is added to the starch component for further washing and then concentrated with a centrifugal hydrocyclone or nozzle centrifuge. With some processors, this is followed by more batch centrifugation or table operation for the final purification of the potato starch.
The starch slurry isolated from such operations is dewatered by vacuum filtration and then flash dried with the inlet temperature just below a maximum of 175ºC.
Any potato starch is dried to 17-18ºC or less moisture which is screened and packaged.
Native starch, modified starch and other form of starch are extracted.
Proteins From Potato
The soluble proteins are valuable materials. These are often recovered after rasping. It involves a two-step process using a decanter centrifuge. The soluble protein is precipitated out using pH and heat adjustment.
Precipitated proteins are separated and dewatered using a decanter centrifuge followed by drying. The non-precipitated proteins which are roughly half the protein load are finally concentrated in an evaporator.
Starch From Tapioca/Cassava
Cassava is often used as a ready source of starch in South American countries.
The roots of cassava are cleaned and then peeled. These are then roughly chopped using a rasper as for potato processing then ground to a pulp to maximise extraction. Much of the insoluble fibre is separated from the pulp. Starch is extracted from the pulp, then dewatered through drying before it is further processed.
Starch From Wheat
Five commercial processes are known. The fractionation of wheat is usually by physical means only. Wheat starch and what is called vital gluten are the products isolated of value.
The main method is similar to wet-milling corn (maize) when the gluten fraction is of little value. For soft wheat with 10% protein, the yield is 73% of which protein is 0.6 to 0.8% according to the grain type (Kempf & Rohrmann, 1989).
Wet-milling of wheat kernels are not scaled-up or commercialised because gluten denatures with processing and usually becomes contaminated with bran. The first two processes are no longer followed but are often referenced in a number of starch processing manuals.
In all cases wheat is first milled to generate wheat flour. This is mixed with water to create a dough and is usually allowed to mature for a while. Various processes are then employed.
[1] Martin Process (Dough-Washing Process).
Wheat flour is the main starting material and was started in Paris 1835 in the Martin Process. It comes however from an earlier method developed in 1745 by Beccari. It was an industrial process into the mid-1970s (Mittleider et al., 1978).
Flour is mixed with water (1 part to 0.6 parts) to form a stiff dough that is rested for about 30 minutes. The dough is further kneaded using a stream of water which is passed through a screening sieve. Starch, fibre and other solubles are extracted leaving the gluten behind which is rubbery. This fraction contains 80% protein.
The starch milk as its known is further sieved to remove fibre leaving a watery mix called ‘throughs’. A centrifuge or decanter is used to separate off the A-starch whilst the overflow. is further centrifuges to separate B-starch from process water which is discarded.
[2] The 1944 batter process.
The batter process was developed in 1944 by Hilbert, Dimler, and Rist in the USA and Shewfelt and Adams in Canada. Here wheat flour is blended with warm water in the proportion of 1 to between 1 and 1.6 of water. A dough is formed in a ribbon blender which easily flows and is best described as a batter.
The ratio of water used depends on the level of protein and type of wheat. The batter is rested for 30 minutes. Water is added again using a cutting pump to form a gluten curd and starch mix. The gluten curd is isolated using a vibrating sieve whilst the starch and gluten mix is then treated as in the Martin process.
[3] Modified Fesca process or ‘thin-mash’ process
The starting proportions of flour and water are similar to the batter process. Blending occurs at 25 C not 50C. the resulting slurry is adjusted to pH7 or 8 and then subjected to high shear and not6 moderate shear. Excessive shear is avoided to prevent early agglomeration of gluten. A starch and gluten particulate suspension is created.
The slurry is departed continuously in a decanter centrifuge producing A-starch of less than 1% protein with a 55% yield and a protein concentrate stream (38% protein) that contains gluten for baking.
[4] The Alfa-Laval/Raisio plant process
A flour and water mix in the proportions of 1.2 to 2 respectively is blended at 25ºC with high shear using a pin mixer. The proportioning level again depends on the quality of the wheat flour. This sheared slurry is decanted and centrifuged. In some cases a series of hydrocyclones are employed with fresh water running countercurrent to the slurry. A-starch leaves in the last stage along with fibre. The gluten leaves in the overflow.
The protein concentrate contains gluten in the form of lumps and threads which are between 1 and 10 cm long. These remains in this state because of the high-shear system operating. The overflow is further refined by resting for 90 minutes at 30ºC to form a ‘mature’ concentrate.
Process water is again added and using a disc disintegrator produces a agglomerated gluten. Further screening with rotating washer systems separates the gluten from the B-starch along with other solubles. This fraction is then centrifuges further to isolate the B-starch whilst the soluble fraction is evaporated.
[5] The Westfalia Separator High-Pressure Disintegrator (HD) Process/Scandinavian process
A flour and water slurry is generated in similar proportions to the Alfa-Laval/Raisio process. A high-shear high-pressure homogeniser is used to create this sheared slurry. A tricanter centrifuge or three-phase decanter centrifuge generates three fractions for further processing called the heavy, middle and light phase.
The first (heavy) phase is mainly the high quality A-starch and fibre, a second phase (middle) containing the B-starch fraction with vital gluten and a 3rd fraction (light phase) of solubles with suspended fines which is mainly very fine starch particles (C starch) and the pentosans.
The A-starch fraction is separated from fibre relatively straightforwardly using screens, membrane systems or centrifugation. A peeler centrifuge is often used for this type of dewatering. The filtrate from the peeler is recycled. The A-starch is concetrated using nozzle centrifuges and then washed in hydrocyclones using fresh water.
The B-starch fraction with gluten is complicated unless the viscous pentosans in the solubles fraction are removed. The middle phase is initially separated using screens which splits the B starch from the glutens. The gluten is washed in washing drums and dried. This protein is then milled and will be sold off as a dough improver in the bread-making industry.
Any B-starch is concentrated using a nozzle centrifuge. the B-starch is mixed with A-starch and converted to glucose syrup. B starch is also dewatered in a decanter centrifuge and dried to be sold as secondary starch.
The light phase contains the pentosane fraction. It is treated with enzymes and concentrated using an evaporator.
Starch Analysis
Starch during its extraction is analysed for total nitrogen, total lipids, moisture and ash contents using the AACC standard methods. These are 46–30, 30–10, 44–19 and 08–01. Reducing sugars are monitored using standard approaches e.g. Miller (1959).
The total protein is calculated using a multiple number of N x 5.7.
Starch content can be measured using an enzymatic colorimetric method (Khabou et al., 1996). Damaged starch can be analyzed iodometrically using a Chopin RFT (Seedburo Equipment Co., Chicago, IL) where calibration curves are provided by the supplier according to AACC standard method 76–31.
The rheological properties are explored using Brabender amylographs, various types of viscometer. Chopin technologies produce a device called the Mixolab for monitoring starch dough performance.
References
AACC (1992). Approved Methods of the American Association of Cereal Chemists. 9th ed. Method 76–31, approved September 1992, reviewed October 1994. St. Paul: The American Association of Cereal Chemists.
AACC (2000). Approved Methods of the American Association of Cereal Chemists, 10th Ed., Methods 46–30, 30–10, 44–19, 08–01, 54–30. St. Paul: The American Association of Cereal Chemists.
Kempf, W. and Rohrmann, C. (1989). Wheat is Unique. (Y. Pomeranz, ed., American Association
of Cereal Chemists, St. Paul, MN., p. 521-540
Khabou, W., Trigui, A., Ghorbel, R., Bejar, S. (1996) L’amidon dans les rameaux d’olivier (Olea europaea) Cv.“Chemlali de sfax”Etude comparative de deux méthodes d’hydrolyse. Olivea. 61 pp. 57–61
, , & (1978). An analysis of the economic feasibility of establishing wheat gluten processing plants in North Dakota. North Dakota State University and U.S. Department of Commerce, Bulletin No. 508.
Ratnayake, W.S., Jackson, D.S. (2003) Starch; Sources and Processing. In: Encyclopedia of Food Sciences and Nutrition (2nd Edt.) (Article)
Leave a Reply