African Crop Science Journal, Vol. 9, No. 1, March 2001, pp. 301-307
K. Mtunda, D. Chilosa1, E. Rwiza2, M. Kilima, H. Kiozya, R. Munisi2, R. Kapinga2
and D. Rees3
Code Number: CS01061
Although sweetpotato is primarily grown for home consumption, marketing is becoming increasingly important, and in this case, short shelf-life of the roots is a major constraint. An assessment of the levels of damage of sweetpotatoes when they arrive at urban markets indicated that between 49% and 93% roots exhibited some form of damage. The major types of damage noted were breakages, cuts, infestation by weevils (Cylas spp.), rotting and superficial scuffing. All forms of damage, except superficial scuffing, lead to a shortened shelf-life due to both increased fresh weight loss and rotting. Superficial scuffing increased the rate of rotting but not of weight loss. It was estimated, based on the six cases considered, that the damage resulted in a reduction of shelf-life of between 23% and 47%.
Key Words: Ipomoea batatus, post-harvest losses, Tanzania
Bien que la patate douce est cultivée principalement pour la consommation familiale, la commercilisation devient de plus en plus importante et pour cela la période après recolte devient une contriante majeure pour les racines tubereuses. Une évaluation du niveau de dégats des patates douces arrivées au marchés urbains a montré que les tubercules exposent quelques formes de damages entre 49 et 93%. Les principaux dégats observés sont: les casses, les entailles et des infestations par des charançons (Cylas spp.), la pourriture et des érafles superificielles. Toutes les formes de dégats, à part les érafles superificielles, conduisent à la réduction de la période après recolte suite à l' augmentation de la perte du poids frais et de la pourriture. Les éraffles superificielles augmentent le taux de pourriture mais pas la perte de poids. Il a été estimé sur base de ces six cas étudiés que les dégats ont causé une réduction de la période après recolte entre 23 et 47%.
Mots Clés: Ipomoea batatus, pertes après recolte, Tanzanie
In Tanzania sweetpotato (Ipomoea batatus L.) production is generally on a small-scale, and the crop is used primarily for home consumption (Kapinga et al., 1995). Nevertheless, surveys indicate the importance of marketing; farmers have identified a need for the distribution of marketable cultivars and consider poor accessibility to markets as a major constraint. At present, information on marketing of the crop in East Africa is less extensive than on production, but collection of pertinent information has been initiated over the last few years (Kapinga et al., 1997; Thomson et al., 1997; Ndunguru et al., 1998; Tomlins et al., 2000). Several areas within Tanzania have been identified where sweetpotato production is focused primarily towards providing roots for marketing in the urban centres such as Dar es Salaam and Mwanza. Thus, in addition to its role as a food security crop, sweetpotato has great potential as an income generating crop. Given its low need for inputs, and the fact that it is usually regarded as a women's crop (Kapinga et al., 1995), it is likely to be especially valuable in helping the less privileged sectors of the community.
Short shelf-life of fresh sweetpotato roots has been identified as a major limitation to marketing (Kapinga et al., 1995), and this appears to be exacerbated by poor handling practices both during transport and during short-term storage in the markets. Consignments often have to be transported long distances, and total shelf-life has been reported as only 7-10 days. It is perceived that an increase in shelf-life could greatly enhance the market potential of sweetpotato.
This paper presents the results of a study which was carried out in three urban centres of Tanzania during 1996 and 1997. The primary objective of the study was to obtain information on the extent and types of sweetpotato damage when they arrive at market and secondly to establish which are the most serious forms of damage with respect to their effects on root shelf-life.
MATERIALS AND METHODS
Three major urban centres of Tanzania were selected for this study: Dar es Salaam, Morogoro and Mwanza. Additional observations were made at Ukerewe Island, which is an important sweetpotato supply area. Studies were carried out during months of peak and low sweetpotato supply.
Assessment of levels of damage. For each sampling, three sacks of roots were bought wholesale as they arrived at the urban market, prior to any form of sorting by traders. The roots of each sack were divided into three categories; undamaged roots, roots with superficial damage (scuffing) only, and roots with more serious forms of damage. Within the third category, roots with serious damage were further classified depending on the major type of damage, i.e., broken, cut, weevil (Cylas spp.) infested and rotting. Although many roots suffered from more than one form of damage, each root was classified depending on the most obvious form of damage. Where there was any doubt as to which was the most obvious damage, a hierarchical system was used with the classifications prioritised in the following order, rotting, Cylas infested, broken and cut. The weight of roots in each class was recorded.
Assessment of shelf-life by damage category. For each damage category, 15 roots (or as many as available) were selected from each sack, and placed into separate sacks (clean polypropylene "fertiliser" bags) for subsequent storage. The sacks were maintained open (rolled down to half height) during storage, and were kept in a well ventilated room. At weekly intervals the extent of root deterioration was assessed in terms of rotting and loss of fresh weight.
Rotting was assessed by dividing the roots into five classes depending on the extent of rotting observed on the external surface, (1= 0%, 2=1-25%, 3=26-50%, 4=51-75% and 5=76-100%). The mean score for each sample was thereafter calculated. Following each assessment, those roots that scored 4 or 5 (i.e. > 50 % rotting) were discarded. For the subsequent weeks previously discarded roots were still considered and given a score of 5 when the overall mean score was calculated.
Fresh weight loss was assessed by marking six randomly chosen roots from each sack at the start of storage, and recording the individual weight of each of these marked roots at weekly intervals, over a period of 3 weeks.
For Dar es Salaam, Mwanza and Morogoro, samples of wholesale sweetpotatoes were collected twice, during the high season and low season of sweetpotato supply. An additional assessment of sweetpotato quality was undertaken at a market located on Ukerewe Island, an area of production in Lake Victoria. Table 1 summarises the background information for the specific markets studied and seasons for each sampling undertaken. For all three urban centres, the area from which sweetpotatoes are supplied vanes by season.
The approximate distance of the supply area from the market is given in each case, together with the normal mode of transport, as these are likely to affect the damage incurred by roots during transport.
Levels of damage observed in the markets. Figure 1 summarises the levels of damage observed for each sampling period for the three urban centres, with the sampling from Ukerewe Island included for comparison. Levels of damage were variable between sampling periods, but were generally high with total percentage of severely damaged roots varying from 44% (Morogoro, High season 2) up to 67% (Dar es Salaam, High season 1), and levels of total damage, including superficial damage, ranging from 49% (Morogoro, High season 1) up to 93% (Mwanza, Low season 2). There was a clear seasonal effect in Morogoro with higher levels of damage, mainly due to an increased level of rotting, observed during the low season, but such clear seasonality was not observed in Dar es Salaam or Mwanza. The roots sampled from the rural market on Ukerewe Island showed the lowest levels of damage.
The different forms of damage can, to some extent, be separated depending on the stage in the handling chain at which they originate. Thus, Cylas infestation is known to originate from the field (Sutherland, 1986), cuts are most commonly formed by hoes during harvesting, while breakages and superficial damage are mostly a result of post-harvest handling. Rotting is more difficult to define, being a secondary effect of damage, as any form of damage makes a root more susceptible to pathogenic attack (Clark, 1992).
The level of breakages observed in this study was 20% - 37% in Mwanza and 35 - 55% in Dar es Salaam. Such damage usually occur when roots are transported long distances by road, often of poor standard, and it is at this stage we suspect that most breakages occur. This is supported by the fact that the lowest level of breakages was observed on Ukerewe Island where roots were transported only short distances. It should be noted, however, that where cuts occur during harvesting, this is likely to increase the rate of breakage during subsequent transport.
In Morogoro, infestation by sweetpotato weevil (Cylas spp.) and rotting were significant. Farmers are usually very careful to remove any roots with signs of Cylas infestation prior to transport to the markets. Despite this, we observed that the percentage of infested roots was greater than 10% for seven of the eleven samples. This underlines the fact that it can be difficult to detect infestation when large volumes of produce are being harvested.
The effect of damage on root shelf-life. Given that short shelf-life of sweetpotato roots is a serious constraint to marketing, the implications of damage on shelf-life was examined. It has been shown that the main forms of deterioration of sweetpotatoes under normal marketing conditions in Tanzania are weight loss and rotting (Rees et al., 1998), although the relative importance varies depending on storage conditions. In this study, roots were stored in open sacks in order to simulate the conditions under which roots would normally be stored during marketing or in the home after sale.
The rate of deterioration differed significantly between locations and season, which is to be expected due to the variations in storage and growth conditions. However, the rates of weight loss were higher than anticipated, ranging from 10 - 17% fresh weight loss over seven days, with 67% loss over three weeks in one case (Morogoro, Low season 2).
Table 2 summarises the data for weight loss after 7 days for all six low season samplings. The proportional increase in rates relative to undamaged roots are also shown. In agreement with an earlier study (Strikeleather and Harrell, 1990), the data show that undamaged roots lose weight more slowly than damaged roots, but indicates no clear differences between the different damage classes. Interestingly, superficial damage is shown to have as significant effect on shelf-life as the other forms of damage. The data for rotting (not shown) is generally consistent with the weight loss data, except that roots with superficial damage did not rot more quickly than undamaged roots.
If a root is considered to be unmarketable after a specified weight loss, then as the rate of weight loss is increased, the shelf-life will be reduced. Considering the data on weight loss after seven days, damage results in an increase in rate of weight loss of 1.6 - 1.9 fold. This indicates a decrease in shelf-life to 53-62% of that of undamaged roots. When this information is considered together with the fact that 35% - 93% roots were found to be damaged in these trials, it indicates that bad handling practices not only reduce the market value of roots but also have a significant effect on root shelf-life. Thus, it was estimated that for the six cases considered, the damage resulted in a reduction of shelf-life of between 23% and 47%.
This study examined the level of damage of sweetpotatoes arriving in Tanzanian
markets. Although not extensive enough to give a definitive level of loss for
the whole of the country, it has provided initial information which is valuable
for focusing future work to improve the sweetpotato post-harvest system. The
high levels of damage observed support observations that little care is taken
during harvesting and handling, consistent with the fact that sweetpotato is
considered to be a low value crop. We have demonstrated that the damage incurred
has a significant effect not only on economic value of the commodity in the
market, but also on the shelf-life of this commodity. The reduction in shelf-life
as a result of bad handling, is not immediately recognised as a problem by the
participants of the sweetpotato marketing system, but results in a considerable
loss of opportunity for marketing, which has great economic implications. It
is particularly notable that superficial damage, which is not recognised in
the markets as causing any reduction in sweetpotato quality, has a very significant
effect on rates of weight loss, and thus shelf-life.
TABLE 1. Summary of information on markets and sampling seasons
Reproduced from Rees et al., 2001
TABLE 2. The effect of damage on the rate of weight loss for sweetpotatoes stored under simulated market conditions
n.d. no data; Reproduced from Rees et al., 2001
Figure 1. Levels and types of damage recorded for marketed sweetpotato in high and low seasons in three urban markets in Tanzania (Reproduced from Rees et al., 2001).
The following images related to this document are available:
Line drawing images[cs01061a.gif]