Adequacy of Traditional Methods for Drying Sweet Potato in Uganda.

Indigenous methods for processing sweetpotato in North Eastern Uganda consist in crushing the storage roots, and sun drying them on rocky surfaces or courtyards smeared with cow dung. To improve the quality and sanitation of the product, the International Potato Center (CIP) has been evaluating, at the village level of Soroti, Uganda, the use of a hand-operated slicer and drying of the sweetpotato slicers on raised stands. Sweetpotato roots from orange-fleshed variety Naspot 5 were processed, and the dried slices from CIP’s sites compared to those produced using local traditional methods and laboratory controlled methods from the Department of Food Science, Makerere University, for the nutrient content, microbial quality, and foreign substance contents. Microbial analysis revealed that the total plate count of micro-organisms exceeded the safety level of 5.3 log cfu/g for the samples traditionally processed. This was also true for the coliform contents (2.8 log MPN/g), while the counts for yeast and moulds (2.7 log cfu/g) remained far below the recommended safety levels. The samples processed using traditional methods had high levels of foreign substances ranging from 0.2 to 1%, and showed considerable browning. The difference in nutrient contents was, however, not significant among different samples. Introduction Sweetpotato is an important crop in Uganda and much of the densely populated midelevation region surrounding Lake Victoria in eastern Africa (Bashaasha et al., 1995, Hall et al., 1998, Scott et. al., 1999). Millions of farmers produce the crop primarily for home consumption and to a lesser extent for commercial sales in local or urban markets. The increasing importance of sweetpotato in the Lake Victoria region and 1 To whom correspondence should be addressed (v.hagenimana@cgiar.org) elsewhere in sub-Saharan Africa is largely attributable to the crop’s relatively high productivity across a range of environments, and its short cropping season (Hagenimana and Low, 2000). Periodic droughts and the rising costs of grain production have also contributed to the increasing importance of sweetpotato both for food security and income generation. In Uganda, sweetpotato is mainly marketed in the fresh form, and is eaten boiled, steamed or roasted. Fresh sweetpotato is bulky with moisture content over 65% (Hagenimana et al., 1998) and quite perishable due to water loss and rots (Gatumbi et al., 1994). However, in some drier areas, such as north-eastern Uganda and the Lake Zone of Tanzania where sweetpotato is a major crop, slicing or crushing with a wooden implement or stone and sun-drying of sweetpotato on flat rocky surfaces or courtyards smeared with cow dung, are widely practised by local people to produce a dried food product that can be stored for some months. This indigenous product (inginyo) is ground into flour and mixed with sorghum, millet or maize to produce staple dishes. Although dried sweetpotato products are very important in local diets where they are produced, there is little commercial trade in these products. The quality of sweetpotato flour produced at the household level is generally very low, due to low levels of food sanitation used in the processing and handling. CIP’s effort on sweetpotato research and development in sub-Saharan Africa are aimed at enhancing the role of sweetpotato for both food security and income generation for small poor farmers in the region. The effort is interdisciplinary, involving crop production (plant breeding, plant health) postharvest and socioeconomic research. In Uganda, this work is concentrated in Soroti and surrounding Districts, where sweetpotato production is important and the potential for expanded production and processing are high. New varieties, including orange-fleshed varieties with high dry matter are being introduced in the area (Hagenimana and Low, 2000). To improve the quality and technology of producing affordable dried sweetpotato products at the village level, production of dried sweetpotato was technically evaluated in three villages of Soroti District considering the steps of washing, use of the sweetpotato slicer, adjustment of drying, milling and storage to local conditions, and quality control of the dried product. The product was compared for the nutrient content, microbial quality, and foreign substance contents to that produced using local traditional methods and laboratory controlled methods. Materials and Methods Dried sweet potato samples processed using traditional methods were obtained from 23 farmers in Gweri sub-county, Soroti District in January 2000. They were mainly processed by crushing scrapped storage roots with a wooden implement or stone and sun-dried on flat rocky surfaces or courtyards smeared with cow dung. Sweetpotato processing usually takes place at the beginning of the dry season. Samples from CIP’s sites were processed using the flow diagram for producing dried sweetpotato chips and flour as described by Hagenimana et al. (1999). Roots were cleaned and trimmed, washed, sliced using an improved hand-operated slicer, and then sun-dried on raised stands for 48h. Besides, fresh roots were collected from 23 farmers in Soroti and processed in the laboratory of the Faculty of Agriculture, Makerere University. Total protein, fat, fibre and s-carotene contents were determined by methods of AOAC (1990). Starch and sugar contents were determined using the Anthrone method as described by Clegg (1956). Microbial quality, yeasts and moulds were estimated by the total plate count (TPC), total coliforms were determined by the most probable number (MPN) as described by Bainbridge et al. (1997). Foreign materials were determined by physical separation, while the colour was estimated by Lovibond tintometre. 3. Results and Discussions 3.1. Chemical Analysis In accordance with previous studies, this investigation revealed that sweet potato is a good source of nutrients (table 1), such as carbohydrates and proteins (Woolfe, 1992) and that some varieties can supply appreciable amounts of the s-carotene (Ameny and Wilson, 1997). In processing sweet potato, indigenous methods and CIP technology showed no significant differences with respect to the nutrient content and s-carotene content, but minimal variations on the aspects of moisture, ash and sugar. The fact that the s-carotene content in all samples turned out to be low (< 3 mg / 100 g) compared to fresh sweet potato tubers (4 mg / 100 g) (Diop, 1998) implies that the challenge to preserve s-carotene would still remain common to all technologies, thereby taking into consideration that this important constituent degrades when exposed to sunlight. Table 1: Results of the Chemical Analysis