Storage and processing characteristics of three Kenyan potato varieties
Kabira, J N
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The studies presented here were conducted to determine the storage and processing characteristics of three established Kenyan varieties of potato (Solanum tuberosum). They included the red-skinned Kerr's Pink and Desiree, and the white-skinned Kenya Baraka varieties, and were grown under similar cultural conditions at the National Potato Research Station (NPRS), Tigoni. Tests were carried out over a period of three growing seasons, starting in 1980/81. Quality aspects of raw potatoes including specific gravity and sugar content were determined at different times of harvest and during storage. In addition, potatoes were processed into crisps and flakes, whereupon, quality of finished products was analysed using standard procedures. Storage characteristics of ware potato tubers of the three varieties were determined during holding at ambient temperature for 16 weeks. The tubers were stored in a chamber which was built inside a naturally ventilated potato seed store and which, too, utilized outside air for ventilation. Treating potato tubers with propham (IPC) at the rate of 2g per kilogramme, significantly reduced weight losses in all varieties compared with the untreated control tubers. Treated and untreated Desiree tubers showed excellent keeping characteristics throughout Pink, while keeping satisfactorily for 12 weeks, had high weight losses after 14 weeks. Those of Kenya Baraka had a short shelf-life of only 8 weeks, after which they lost their marketable and processing value due to excessive dehydration. Potatoes of all varieties had low sugar content during storage. In addition, the specific gravity was found to increase with time in storage. Conversely, levels of reducing sugars and sucrose generally declined with time in storage. Whereas no effect of chemical sprout suppressant was observed on either specific gravity or reducing sugar content, the untreated control tubers of all varieties had slightly higher content of sucrose than the treated samples following a 16 weeks' storage period at ambient temperature. These observations appear to be of significance in relation to short term storage of potatoes for the processing industry in Kenya's high-altitude potato growing areas, where storage IS based mainly on natural ventilation of store houses or buildings. The processing performance of Kerr's Pink, Desiree and Kenya Baraka potato varieties for manufacture of potato crisps was determined at different harvest times (75, 90, 105, 120 and 135 days after planting) during a typical short growing season (1980/81) and in a long wet one (1981) t and after storage at various temperatures (soc, 120C and ambient). Potato crisps were analysed subjectively for colour using the PC/SFA colour chart, and for appearance and flavour by a sensory panel. The data indicated low variability in crisp quality among the varieties tested. High quality crisps were made from potatoes of all varieties and harvest times in the 1980/81 growing season. Lower processing performance was, however, 'observed from potatoes of all varieties at the initial harvest date during the 1981 growing season, following which, crisp quality progressively improved with later harvests. Lower initialcrisp quality was attributed to the cool, wet weather conditions of 1981 growing season which delayed maturation and produced immature tubers having exceptionally high content of reducing sugars. Also, potato tubers of the 1981 growing season had low specific gravity and produced crisps of lower yield than those from the high specific gravity tubers of the relatively drier 1980/81 growing season. Differences in processing performance after storage were pronounced due to the effect of temperature conditions. Although all varieties gave acceptable crisps when processed immediately after harvest or after storage at 12°C, crisp quality rapidly deteriorated upon storage at 5°C for 8 weeks. Reconditioning the tubers at 18°C (controlled) or ambient temperature (14.5 - l6.8oC at night) improved crisp quality. Whereas reconditioning at 18°C produced acceptable crisp c)lour in three weeks, longer periods of up to 8 weeks were required to adequately recondition tubers at ambient temperature. However, such long periods at ambient temperature result in profuse sprouting and high dehydration losses. Therefore, the ambient temperature regimes were considered insufficient for reconditioning purposes. Storage of mature potatoes for periods up to 14 weeks at ambient temperature .gave crisps of satisfactory quality for all varieties. However, potatoes of Kenya Baraka from earlier harvested tubers in 1980/81 tended to accumulate reducing sugars during storage, and produced dark crisps on processing. Also, since ,no chemical sprout suppressants were applied on potatoes used for the crisp tests, many tubers from all varieties were sprouted and dehydrated, particularly after 8 weeks of storage. In a separate processing trial, potatoes of Kerr's Pink, Desiree and Kenya Baraka were used for manufacture of potato flakes. The flakes were prepared using a single drum drier. Quality of finished flakes was tested by sensory and physico-chemical methods. Flakes of acceptable colour, flavour and textural quality were made from all varieties. No variety was rated superior in all quality aspects tested.