ISI 25-2e Preparation of Starch from Tubers

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1. Scope	The method is applicable to potatoes. The method can be adapted to other tubers.	Ref.: LT   March 1994 Rev. LT  Oct. 1998
2. Principle	The tubers are cleaned and grated. The grating (rasping) is screened and the crude starch milk is washed and screened.
3. Apparatus	3.1 Balance weighing to the nearest 1 g. 3.2 Extraction screen 250 microns openings 3.3 Refining screens with 125 microns and 75 micron openings 3.4 Imhoff precipitation cone 1000 ml, graduated 3.5 Grater with sharp cutting edges 3.6 Büchner funnel with suction and filter paper.
4. Reagents	4.1 Water of high sanitary quality free of suspended particles and odor. pH: 6.5 – 8.0; Iron: max. 0.05 ppm Fe; Manganese: max. 0.02 ppm Mn; German hardness: max. 2 DH; Temperature:  max. 30 oC. 4.2 Sodium bisulphite solution, 1%	DH = German Hardness degree is calculated as 0.14*Ca + 0.23*Mg where Ca and Mg is expressed in ppm.
5. Procedure	Weigh ½ kg fresh tuber.
Cleaning	Remove immature tubers and any tuber damaged by frost, heat or sprouting. Cut away any damaged part of the tubers. Rub or brush the tubers in water until free of adhering dirt and infected skin.	Careful cleaning is essential - peel properly with knife  if necessary.
Grating / rasping	The clean tubers are disintegrated with a grater.	The finer the better.
Extraction	Collect the rasping on a 250 micron screen and flush starch through the screen with water until the starch stream ebbs away. Discoloring of the juice can be hampered by immediately adding 20 ml of bisulphite (4.2) to the rasping or to the crude starch milk.	A  "juice extractor" can do the rasping and the extraction in one operation.
Concentration	Collect the crude starch milk in an Imhoff cone (3.4) and let rest until precipitation of starch has ceased. The supernatant is decanted leaving only the precipitated starch in the cone. Keep and mark supernatant as A/O0	It is essential to remove particles floating at the boundary between fluid and starch cake.
Refining Stage I	Add water equivalent to three times the volume of the sediment and stir until all starch is suspended. Let rest until starch precipitation has ceased. The supernatant is decanted only leaving the precipitated starch in the cone. Keep and mark supernatant as A/OI	Half volume of sediment being fluid the dilution factorI is seven or higher
Refining Stage II	Add water to the cone with the sediment equivalent to the volume of the sediment and stir until all starch is brought in suspension.	The calculated dilution factorA-II is min. 49
Filtration	Pour the starch slurry on a 125 micron screen and flush with water. Collect the filtrate in the Imhoff cone. Let rest for 3-5 minutes. The supernatant is decanted leaving 5-10% of the starch in the cone. The supernatant is filtered through a 75 micron screen.	The coarse filtration removes substantial amount of pulp (fibers). The fine filtration removes very little.
	Combine the filtrate with the residual sediment in the cone. Let rest until starch precipitation has ceased. The supernatant is decanted and the cone is turned upside down for a fraction of a second only leaving the precipitated starch in the cone. Keep and mark supernatant as A/OII.	When the upper third volume of the liquid phase appears clear and transparent it is time for decanting.
Refining Stage III-VI	Add water to the cone with the sediment equivalent to the volume of the sediment and stir until all starch is suspended. The supernatant is decanted and the cone is turned upside down for a fraction of a second only leaving the firm precipitated starch cake in the cone. Repeat the procedure and keep and mark supernatant as A/OIII, A/OIV, A/OV and A/OVI.	The calculated dilution factorA-VI is min. 117,000
Quality control	Quality can be controlled at this point before dewatering. A teaspoon sample  is scraped from top of sediment. Add water up to 15 ml and let settle in a test tube or even better centrifuged. (1) The boundary between liquid and sediment is examined carefully by the naked eye. None or only insignificant colored or non-starch particles should be visible. If not acceptable add more refining stages before dewatering and improve the decanting technique. (2) A tiny sample is drawn from the boundary between liquid and sediment for examination under microscope. None or only insignificant particles originating from the surface of the potato skin should be observed. If not acceptable improve the washing of the potatoes or peel properly by the use of a paring knife.	If necessary carry out a Determination of Fiber in Starch as Fiber Number - method ISI 08. The fiber number should be 1 or better.
Dewatering	Re-slurry sediment with equal quantity of water. Pour on a Büchner funnel. Apply vacuum and flush beaker/cone with equal quantity of water.
Drying	The starch cake is distributed in a thin layer and dried at room temperature to minimum 80% dry matter.
Screening	The dried starch is screened on a 125 micron screen. The screened starch is considered as the end product.
A second cycle B	With a large sample the cycle described above can be repeated.
Water saving	Water can be saved in the second cycle by using available supernatant  from a subsequent stage in a previous cycle as replacement of water. Introduce an extra stage before dewatering and use water in this last refining stage. Keep and mark supernatant as B/O0  through B/OVII	Supernatants marked /O0 are not used.Supernatants marked /OI are used for extraction  only.
Additional cycles C, D, E ... X	The cycle can be repeated and each time an additional stage using water for dilution is introduced before the dewatering step. A total of twelve stages are adequate.	The extra stages compensate for the smaller dilution factor.
6. Notes	No more than a total of two liters water is used for the extraction, refining and dewatering during each cycle. Water savings can be achieved by reducing the above dilution factor from six to five, four or minimum three simultaneously increasing the number of stages to seven, eight and nine within cycle A and with correspondingly more stages in subsequent cycles. The calculated dilution in last stage before dewatering shall be minimum 1:60,000.	Combined sediments recovered from prolonged resting of supernatants can be entered as a sample in one and just one separate cycle equivalent to cycle A in case of heavy-handed decanting
	If excess water is used for refining of potato starch the extra water must be demineralized or the water hardness will depress peak viscosity.	Potato starch is an ion exchanger
	If the refined starch milk for whatever reason is slightly acid (as is the case using demineralized water throughout) it must be neutralized to pH 7 before drying.	Even slight acidity reduces shelf life (viscosity break down)
	The preparation can be adapted to other tubers including cassava by extending the time for starch precipitation. The supernatant may alternatively be removed with suction in cases the starch does not form a firm sediment remaining in the beaker, when turned upside down. Dependent on the distribution of starch granule size refining screens (3.3) with smaller openings can be used. For cassava and possible other tubers the water hardness specification can be relaxed.
	In stead of increasing number of cycles the above quantities can be upscaled in case of a large sample.
7. Reference	ISI Technical Memorandum TM 5 on Potato Starch