The Practical Planter

Getting Starch Out

Of Sweet Potatoes

The sweet potato (Ipomoea batatas ) has a starch content of between eight and 29 per cent. There are hundreds of varieties: those of average starch content are more suitable as a food, while varieties of average to high starch content are

good for starch production.

Sweet potatoes which give a large deld, have a light coloured flesh and ligh starch content are most desired or starch manufacture. Except for nethods used to remove the colour : rom the starch, the process is said o be quite simple and similar to hat for other root starches.

Indeed, the Japanese have been >roducing low-grade starch from iweet potatoes for many years for purposes where colour is not so mportant. It is understood that pro luction in Japan is from a large lumber of small factories, and the juality of the starch varies con liderably.

Success in Japan

According to a report on Japan ;se cottage and small-scale industries, iweet potato starch production is run in well organised lines and its success s due to abundant cheap electric lower, the ability of small enterprises nostly catering for the domestic narket to adapt themselves quickly ;o changes in fashion, a plentiful supply of cheap labour, the close lersonal relationships of employees ind co-operative relations with larger mterprises. The by-products of sweet lotatoes are also used, e.g. the pulp md vines are used for cattle feed.

In Japan, the tubers are processed during the three months from October to December. The process is alkaline throughout, and the yield is from 20-26 per cent, (average 23 per cent.). The starch is stored as wet starch in concrete tanks.

In the US, experimental work on sweet potato starch production was begun during the last part of the nineteenth century. The first sweet

potato starch factory was set up in Louisiana in 1928 by the La Fourche Starch Refining Company. Owing to various factors, including competition with white potato starch, operations were not successful.

The Department of Agriculture pursued studies on sweet potato starch and in 1934 initial trials were started at a factory in Laurel, Mississippi.

The plant could not be run econom ically and had to be abandoned. Then in 1944 the United States Sugar Com pany authorised the erection of a large plant in Clewiston, Florida, but that also failed. The reasons for the failure of the industry in each case were attributed to the following factors: • Low yields of crop per acre. • The use of unsuitable varieties for starch production. • Storage difficulties. • Seasonal production. • Competition from other cheaper starches. • Quality variation and poor colour of the starch. • High production costs.

In view of that experience with sweet potatoes for starch production, the American view was that any future production of starch from sweet potato should be in small units, e.g., 10 tons per day and should be considered only where the raw material could be obtained cheaply, and all the by-products should also be made use of as in Japan. Apart from the proximity of the supply of raw material other factors reported to be important in considering the location of a sweet potato starch factory were: • Abundant good water supply, free from algae and iron salts.

• Good transport facilities. • Facilities for the disposal of effluents. • Cheap labour. • A good market for the final starch product.

It was also recommended that full use should be made of the crop by having the factory associated with a cannery which could us any tubers that were unsuitable for starch pro duction.

American system

A system of producing starch from sweet potatoes, developed in Missi ssippi, worked like this: Sweet potatoes were thoroughly washed, crushed in a hammer mill and with a clear saturated solution of calcium hydroxide to flocculate certain impurities and dissolve the pigment present which would other wise have discoloured the starch. The mass was passed over a series of electrical shaker screens to remove the liberated starch granules.

The screened slurry was re-ground and washed with calcium hydroxide solution. It was then re-sieved giving a suspension with a pH of 8.6 to 9.2, which was the optimum range for all the tabling operations.

The starch milk was again sieved to remove fibres, etc., and its spe

fic gravity adjusted to about 5° Be ;fore being pumped into the tables, he tables sloped 1/32-in. per foot ad were made of concrete, 19 in. ide by 110 ft long. The starch jposited on the tables was re-tabled ith fresh water and the deposit om this was flushed from the tables i give a slurry with specific gravity the range of 10-15°Be and re reened.

To obtain a good coloured product e starch from the second tabling as bleached for two hours with ight excess of sodium hypochlorite a pH value just above 8.3. The sidual chlorine was eliminated by e addition of sulphur dioxide and final adjustment of the pH value as made.

The liquor was then centrifuged at 200 r.p.m. in a basket type of ntrifuge. The resultant starch still •ntained about 35 per cent, moisture id further drying in a batch vacuum ier (4 ft in diameter and 20 ft ng) was necessary to reduce the oisture content to 12 per cent. The led sweet potato starch was then ilverised and screened over bolting k to give the final product.

The wet process was alkaline roughout. Clear water was intro iced at the shaker screens and roughout the whole process the pH as maintained within the range 8.6 9.2. Speed of operation was es ntial because the process was prone bacteriological troubles.

The starch produced by this ocess was said to be valuable for arp sizing and gave good clear •lours and soft handle when used r finished cotton goods. It was so found to be suitable for paper aking and for the manufacture of Ihesives and dextrin.

To prepare sun-dried chips from reet potatoes, the tubers are washed :11 then sliced and scraped with knife to remove the skin. They are en cut into chips of about i in. ickness, spread over a clean cloth on a clean cement floor and dried the sun. When freshly cut, the ces are white in colour and change a creamy yellow after two hours drying.

After about eight hours of drying, e slices generally become dull white colour and crumble easily on essing. Drying is usually complete about 10 to 12 hours.

Sweet potato flour is formed by •unding the sun-dried chips in an »n or stone mortar followed by iving. The flour thus prepared is creamy white colour. —lnformation om the British Government's •opical Products Institute.