As you may have seen on the postharvest.co blog, an exchange with Denis Twinamatsiko, currently working in Liberia, has sparked the birth of this forum. The first few posts presented here catch up on some of our historical exchange:
as with many supply systems, the key to success is doing the simple things right . This means dealing with the principal challenge: that the simple things are not necessarily easy!
You have already put your finger on some of the key questions – what, then, can be done to provide effective answers?
*The cassava tubers start deteriorating 48 hours after harvesting.*
Time is indeed one of the critical elements that needs to be managed carefully. If deterioration becomes visible within 48 h, you can bet that the processes that lead to that have started within moments of harvest.
Under conditions typical of your system, what is the time that 50% of the tubers have become unuseable?
*Because of our limited processing capability we can not take in all the cassava that is brought by the Farmers …..*
This matter of matching supply and processing capacity is absolutely critical, since:
– under supply means that productivity of your processing facility is linearly affected by gaps in inputs
– over supply erodes “packout” efficiency due to both direct losses in product and through indirect impact of slower processing of deteriorated product.
What steps could be taken to improve communication between processing facility and growers so that harvest may be better timed to minimise both gaps in inputs and delays in processing?
One of the keys to implementing this would be the flexibility in timing of harvest for the crop – presumably, cassava harvest can be delayed without excessive penalty for product quality? If so, developing a / refining the scheduling system is likely one of the most powerful tools you have available for reducing losses.
Managing temperature is the other critical dimension for maintaining product quality. Achieving continuous shade for the product from the moment of harvest through to entering intake at the processing facility will reduce major increases in product temperature that would otherwise greatly exacerbate losses.
Given that you are dealing with a peeled and processed product, there may be scope for some chemical control of rots. However, there are major logistical, safety, ethical and marketing issues associated with implementing this approach and so I think the first round of attack should be on time and temperature.
Let me know what you think!
Denis: On keeping the cassava in the field for sometime, we have a problem with quality issues, actually when the cassava tubers are not harvested on time they tend to be too fibrous and sometime start rotting!
Nigel: OK – so we don’t have complete flexibility to use the field as an indefinite buffer store. At the same time, there is probably greater flexibility (less risk of a large decline in quality) to hold the tubers in the field for four days than to hold them at the processing facility for the same period.
So let’s say the first approach might be to try to arrange arrival of the crops to minimise losses.
How is the order of priority of processing a given grower’s product currently determined? By arrival time at the facility? Or is it set at some time before that?
What I am leading towards is: How much scope would there be to prioritise processing times for individual batches on a wait list system, perhaps a week in advance?
If that is politically feasible then perhaps we can shift the block to another zone: how can we achieve communication with growers on timing of harvest (which in turn would need to be informed by a maturity monitoring process)
Anyway, one step at a time ….
Denis: Many thanks for your kind reply. I do greatly appreciate your input.
I realize the biggest challenge that I have is that I do not control the procurement of cassava.
From your discussion I do realize one has to be in control of the raw-material acquisition and for that matter communication is paramount!
Since this is a food assistance project, It is why these salient issues are not being looked at. The Farmers are after producing much so as to earn the much needed money!
In light of our exchanges, I now greatly appreciate the importance of; Time, Temperature, Processing Capacity and Communication as far as post-harvest handling is concerned.
Nigel: Do not despair, Denis – as I ventured in an earlier message, doing simple things is not always (= is rarely) easy
That difficulty should not deter you, though – the challenge we have identified is just one of a hundred that will be involved in developing your system to take it closer to the perfection of full optimisation. Twelve years ago, I devised a prototype rewards system for encouraging growers to produce better tasting kiwifruit. At the time, most participants in the system were skeptical and many of the growers themselves were angry – how could we place a significant part of their potential income on an attribute that we couldn’t tell them how to grow. The secret was in developing a simple metric by which potential taste could be measured and growers of better tasting lines could be rewarded. The rewards in 2013 are thirty times what they were in 2001 and the growers are delighted – the system has provided a way to grow demand for their product in their most lucrative market, The majority of developments in the system have been made by participants themselves: 3000 growers have put forward an enormous diversity of ways to address the challenge of growing great tasting fruit; testing labs have put forward improvements to the measurement system, the marketing organisation has ensured that the better tasting product secures more returns from the market – the list is long and I am long gone. My contribution was just the germ of an idea on how it might work. That is your role now: what are the key features of the system that can really make a difference? Then let the resources, the politics and the energies of the system run for a decade and watch it evolve, perhaps from a distance – that will be your reward.
Communication is a critical feature of your system. Perhaps you could work with other participants to devise a way of feeding information to growers and getting them to book processing times with you. Initially it might look something like this:
– you communicate out to all known growers / grower organisations your intention to implement a booking system for processing times
– product that arrives without booking gets processed but after lines that are booked in
– you continually publish an updated list of available processing capacities / times (website? phone app? response to phone calls?)
– growers can book preferential processing for a estimated volume on their chosen day
– lines of product slip down the list by (?) two days (?) if they are late by (?) one day (?)
– accurate estimates of volume are rewarded by …. ?
There will be a host of reasons why it needs to be slightly / very different from this – all it really needs to be is practicable and fair. Then publish your intent to make changes to the process until it responds perfectly to the whole system.
Let me know your thoughts – then we can start to address product temperature
Denis: Very many many thanks!!!!!!! for your encouragement. I was really
touched by your success story of the kiwifruit Farmers. I strongly
agree with you that the secret is always in doing the simple things
which is not always easy.
I will share the useful information you have given me with my team.
All your information is spot on!
I am looking forward to the discussion about product temperature!
Nigel: You might be amused to know that cassava made it into a poem I wrote to go into my chapter in the upcoming revision of Postharvest Handling – A Systems Approach (written whilst waiting a final couple of days for a flight back from Thailand to New Zealand). The line about cassava was:
Because cassava is the staple food of fungi at the factory gate
You can clearly see that the issues you have described to me have reached a central part of my being / thinking.
Denis: Currently, we have been able to match our production capacity with the supply of cassava from the Farmers (We strengthened our communication channels) and I no longer have heaps of cassava at the processing facility.
We are launching up another processing facility in one of the counties come January, 2014 so as cut down on the delivery time and also
increase our processing capacity.
Nigel: here is a chunk of material from FAO on postharvest handling practice for cassava (to be found at: http://www.fao.org/docrep/x5415e/x5415e04.htm )
Please could you skim through this and tell me how close to (or different from) this is to current practice by your growers? At this point, I am not at all suggesting what might be an optimum process – just seeking to understand a bit more about how things are currently being done.
If there are procedures that your growers use that you think are better than what is here (or perhaps just different from what is here), please could you outline briefly why you think they are currently being done as they are – this will help me to identify both constraints and opportunities in the current system.
Looking forward to exploring more with you and your team
Cassava roots are much more perishable than other major root and tuber crops. This has been attributed to the fact that the root of cassava, the storage organ, has no dormancy, has no function in propagation and possesses no bud primordia from which regrowth can occur (Onwueme 1978, Cooke et al. 1988a, Wickam 1988). Cassava roots generally deteriorate very quickly soon after harvest and this occurs in two separate phases:
· Physiological, or primary deterioration. This often begins within 24 hours after harvest and is characterised by blue or brown discolouration of the vascular bundles of the roots, called “vascular streaking”. There are indications that this primary deterioration is due to wound responses comparable to those observed in other plant storage organs.
· Microbial or secondary deterioration. Usually occurs 5-7 days after harvest and involves a wide spectrum of fungi and bacteria which develop in the flesh, causing a variety of wet and dry rots.
This rapid post-harvest deterioration of Cassava roots places serious constraints on their distribution and use, especially where there are delays in marketing, and on the holding of buffer stocks for large-scale processing. As a result the selection of a market for which production is intended is influenced by its location and the time scale for delivery rather than its simple proximity to the production area
4.1.1 Preparing fresh cassava roots for storage
Cassava roots must be harvested and handled with extreme care if they are to be kept for more than a week. There are a number of different practices which, if used in combination, will assist in delaying deterioration.
· Cutting off the stems, leaving only a short part above ground. This operation should be done about three weeks prior to harvesting.
· As cassava roots usually start rotting from the neck, (i.e. the point of attachment of the root to the parent plant), harvesting the roots with part of the stem (2-5 cm) still attached may prevent a rapid spread of decay into the root.
· Minimize damage at lifting by harvesting while the soil is wet, for example after a rain.
· Retain only those roots that do not show signs of injury. Roots that are to be kept for more that one week or more should be carefully selected since curing will not be effective on roots with extensive damage.
· Establish curing of the roots after harvest as a routine operation with, as far as possible, the minimum of handling.
4.1.2 Traditional storage methods for fresh
The problems of storing fresh cassava roots has led to the traditional practice amongst subsistence farmers of leaving the roots in the ground until they are needed and, once harvested, to consume or process the roots immediately into a more durable form. This practice does have a number of disadvantages. The most important is that large areas of land are occupied by a crop that is already mature and is not available for further cropping. The longer roots remain in the ground the more they become fibrous and woody and their starch content decreases. It is also considered that their susceptibility to loss is increased while the mature roots are still in the ground.
Other traditional methods of storage of fresh cassava include;
· re-burying the roots in trenches covered with plant material and soil;
· piling the roots in heaps and keeping them moist by watering them daily;
· applying a thick coating of soft clay or mud and,
· keeping small quantities of cassava in water.
All these practices are simply attempts to limit moisture loss from the tubers but at best they can extend the storage life of the tubers by only few days.
4.1.3 Improved low-cost storage methods for fresh cassava
During the last twenty years there have been some developments in improving storage methods capable of extending the shelf life of fresh cassava roots by at least two weeks. These, amongst other advantages, make it possible to market the crop further afield and give an increased margin to the opportunity of holding stocks of fresh cassava, even for few days, at a processing plant. A joint project between the Natural Resources Institute, and Centro Internacional de Agricultura Tropical studied alternative storage methods to the traditional re-burial procedures (Crentsil et al. 1995). These included storage in pits, in field clamps and in boxes with moist sawdust. All the storage methods investigated favoured curing conditions in a high humidity and high temperature environment in order to slow down the rates of physiological and microbiological deterioration. However, to be successful they all require careful harvesting and selection of the roots prior to storage, since curing is not effective if root damage is extensive.
184.108.40.206 The field clamp
Storage was successful in field clamps, similar to those used for potato storage, for up to eight weeks. The clamp consists of a layer of straw laid on a dry floor covered by a heap of 300-500 kg of roots followed by a layer of straw and a final layer of soil. Openings were left at the bottom of the heap to provide some ventilation. This storage method was found to be difficult to manage where seasonal variations in climate made it difficult to either limit or increase ventilation and, during a wet season, to ensure that the floor of the clamp remained dry. It also had a high labour requirement.
220.127.116.11 Storage in boxes lined with moist sawdust or wood shavings
The method involves putting alternate layers of sawdust and cassava roots, starting and finishing with a layer of sawdust. As an alternative to sawdust, wood shavings, peat or any other suitable packing material can be used. However, the packing material must be moist but not wet. Physiological deterioration occurred if the material was too dry and microbial decay accelerated when it was too wet. In Uganda this storage method is being tested in combination with the lining of the box with plastic (Nahdy and Odong, 1995).
The study indicated that 75% of the roots remained acceptable after four weeks in store, provided the roots were packed immediately on the day of harvest. With a delay of only one day only 50% of the roots were rated as acceptable. This technique has been used for some export markets but the higher transport cost involved because of the box containers has precluded its use for domestic market.
18.104.22.168 Storage in plastic bags or plastic film wraps
This appears to be the most practical and promising method of storing cassava roots intended for the urban markets. A number of studies have shown that cassava roots treated with an appropriate fungicide and kept in an airtight plastic bag or a plastic film wrap can be stored for two to three weeks. At present thiabendazole, which is widely used to treat potatoes, is the safest and most appropriate fungicide to be used. Most of the chemical remains in the thick peel of the root. Only 1 mg/kg of residue was found in the parenchymal tissue, which is substantially below the limit of 5 mg/kg set for potato (Cooke et al. 1988a).
The method was developed by CIAT. It involves the dipping of cassava roots in a 0.4% w/w solution of thiabendazole for 10 seconds and storing in plastic bags. It has been successfully tested in Ghana (Crentsil et al. 1995) where it was found that household bleach (0.95% active chlorine) was as effective as thiabendazole if sound cassava roots were not stored for much longer that seven days. In addition, transport of the produce over rough roads seemed not to be detrimental to the keeping quality of the roots, suggesting that the technology could be effective for commercial operations.
4.1.4 Improved higher cost techniques
Some modern methods, such as refrigeration, deep freezing, waxing, controlled atmosphere and chemical treatments, have been suggested for the storage of fresh cassava. Freezing and waxing have been used primarily for export markets in Europe and America, where the customers of African and Latin American origin are prepared to pay high prices. These techniques require specialized equipment and skills and are very capital intensive.
Denis: Many thanks for your mail. The information is very useful.
Currently, the Farmers here are still using the traditional practice of leaving the roots in the ground until they are needed.
The fact that this is a post-conflict country, the Farmers are trying to get back to normal production. The production capacity has been greatly improved since we opened up another facility and so at the moment whatever they produce, we are able to take it on.
Off course in the near future preservation techniques will be needed so as to regulate supply since production is set to increase substantially.
Nigel: thankyou for this background – you are right: now is the time to act and prepare for the future
I attach a system diagram for lettuce. Could you use this as a starting point for laying out the steps in the current handling of cassava? That will give me a basis for thinking about where the opportunities for improvement may lie
Don’t worry about producing tidy layout – I am interested in content only. And if you can, please provide as much detail as you can about the steps between the act of lifting the roots and taking in at the factory gate. Do the roots sit in the sun at any point? What time of day is harvest made? How long between harvest and processing? How far are they transported to the factory? What containers are the carried in? (all the good standard postharvest stuff)
Now you’re up to date with the conversation between Denis and myself.
My next task is to do some thinking about temperature in Denis’s system and how that can be managed to secure better outcomes. You may have thoughts on this too. Equally, you may have a host of other suggestions you might like to provide for Denis, his cassava growers and the Super-Gari processing facility. This forum has been established precisely for this purpose – please: join in
Now we have some catching up to do – we left Denis’s cassava travelling on its journey from the field to the processing facility over a period of between 12 and 48 hours. Temperature would be most critical for product at the greatest distance in time and kilometres from the processing facility. For product just 12 h away, there would be a strong likelihood that the cassava would make it into the processing facility without too much risk of physiological or pathological deterioration. On the other hand, a journey that takes four times as long might leave the product at significantly greater risk – a risk that would be further compounded by every degree it is above optimum handling temperature.
So perhaps our first step is to alert growers to their varying levels of risk according to distance:
Growers in Zone A would implement a baseline level of temperature management, whereas those in Zones B and C would take progressively more focused measures for keeping the product close to the optimum handling temperature:
What are the real world handling issues that may constrain ability to implement these approaches? What complementary strategies might be developed / adapted to optimise transportation and arrival temperatures of the harvested cassava?
You must be logged in to reply to this topic.