Sweetpotato processing

Sweetpotato processing

Sweetpotato is mainly consumed as fresh boiled roots. However, in some parts of East Africa sweetpotato is traditionally processed into dried chips and/or flour to preserve the roots for household food security and to a lesser extent for sale in rural markets.

 Besides permitting better preservation, the drying and processing of sweetpotato into dried chips and flours offers other opportunities such as:

• facilitating storage and transport; • reducing bulkiness and losses due to high perishability of fresh roots (if roots are left in the ground further weevil attack can occur, this damage increases the risk of other pathogens getting in and destroying the roots); • increased shelf life; • greater nutritive value due to the fact that as a great part of the water content is removed, the carbohydrates, pectin, proteins, oils and mineral salts are concentrated in the tissues of dried food products;

• creating new income opportunities for farmers such as new markets and new sources of income

• changing some of the negative attitudes about sweetpotato consumption, and enabling them to see sweetpotato as an important commercial crop with a diverse range of uses, and consumers Although sweetpotato flour has great potential for use as an ingredient in many marketable products, traditional processing methods of sweetpotato, which involve exposing peeled, sliced and unwashed slices directly to sun present certain problems, including:

• high dependency on climatic conditions (if its going to rain product has to be gathered up and taken to a dry, safe place)

• high manual labour requirements for peeling, slicing, spreading out to dry, turning the product during drying, guarding product from hungry livestock, moving product if weather changes etc

• difficulties in maintaining hygienic conditions can lead to products becoming contaminated by micro-organisms and dust

• lack of control of enzymatic oxidative browning and other reactions leading to discoloured and/or strong smelling sweetpotato flour

• lack of uniformity in terms of chip size Some of the factors commonly mentioned as affecting the quality of dried chips include: appearance; size of slices; uniformity and intensity of colour; defects such as skin planks; presence of contaminants. Different sweetpotato varieties will have different dried chip and flour properties such as taste and colour, and farmers might like to test several different varieties in order to choose which variety to plant for the different products. Improved processing methods have been developed to help overcome some of the problems associated with traditional methods, in order to produce sweetpotato flour with improved odour, colour and nutritional quality. However the cost of the improved methods is higher than that of the traditional methods, and therefore might be more suited to use by farmer groups working together. 89

The steps to follow to obtain quality dried chips and flour are as follows: Selection of raw material Select only healthy roots for drying Cleaning, peeling and trimming Any soil on the roots must be removed before the root is peeled using a clean kitchen knife. Any damaged parts of the root should be trimmed off and destroyed. Washing A drum washer has been developed to help at this stage. The drum washer consists of a 200 litre oil drum mounted on a horizontal axle. The drum is cut lengthwise to provide a door which can be opened and closed during loading and washing of the roots respectively. Inside the drum are brushes which are fixed on a horizontal axle. A handle is fixed on the axle to rotate the drum and wash the roots. At one end of the drum an opening can be made to drain the dirty water after washing. The drum can be used to wash 40 kg of sweetpotato in 10 minutes using 30 litres of clean water. If drums are not available a large saucepan could be used. Slicing/ chipping After washing, the sweetpotato roots are pre-dried in a clean place in the sun for about 10 minutes to remove the surface water. A manual or mechanical chipper/ slicer is then used to cut the sweetpotato roots into uniform pieces of about 5 mm thick. Soaking Slices are soaked in clean water for 90 minutes. The volume of water used is twice the weight of the slices and is just enough to cover all the slices. Drying The slices are then either sun dried on a raised tray for approximately 4-6 hours if weather conditions are suitable, or in a conventional dryer using firewood or charcoal as fuel. The drying tray should be raised off the ground to prevent dust and dirt contaminating the chips. If the drying process is not thorough, the chips will be prone to mould attack during storage. The drying rate will depend on the thickness of the slices, rate of turning chips as they dry and the amount of sliced chips place on the tray. Grinding The dried sweetpotato chips can now be milled into flour. Sorting The dried sweetpotato chips can be sorted for uniformity before packaging or further processing, if this might affect the quality, intended use or price. Packing and storage Sweetpotato flour can be safely packed and stored in polythene bags as well as baskets and tins. Sweetpotato chips can be stored in clean sisal or polypropylene sacks, granaries, and polythene bags. The use of black packing material (eg two black linings inside a sisal sacks) helps to minimise the loss of vitamin A (which can be degraded through exposure to ultraviolet light) during storage. Care should also be taken to keep the products in a cool, dry, 90

well ventilated location. Wooden pallets/ platforms can be used to stack the packed sweetpotato products on, to prevent moisture being absorbed from the floor. Regular monitoring of the stored products should be undertaken to build up knowledge about the products shelf life under the site specific storage conditions and to prevent large infestations of insects or rodents developing. See sections 5.4 and 5.5 for more information on storage. Packages should be labelled to trace the origin (farmer/ producer) of the roots and the chip processor. If compositional or nutritional information is needed this will require that a sample of the dry sweetpotato chips are analysed to give a representative content per consignment or batch. The following parameters should be monitored to achieve and maintain high quality dried sweetpotato chips, especially when they are being bulked from different production batches: moisture content (preferably less than 10-15%); appearance/ colour; absence of undesirable odours; hygiene standards and cleanliness of chips; absence of contaminants (eg soil, insects; human hair etc); uniformity of shape; nutritive value (eg beta-carotene content)

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