BIOACTIVITIES & PERSISTENCE OF HERBICIDES

BIOACTIVITIES & PERSISTENCE OF HERBICIDES
BY
AYODELE OLATUNDE PHILIP
CRP\98\0188
A Course Seminar Submitted to the Department of Crop, Soil & Pest
Management, School of Agriculture & Agricultural Technology,
Federal University of Technology Akure
July, 2006

INTRODUCTION

Herbicides perform a vital role in the management of weeds. As the name
indicates, herbicides are chemicals that kill or control weeds 7. Although the ultimate
effect of most herbicides is the same (usually weed death), the way they control weeds
is vastly different 1. Physiologists use the term mode of action to describe the way
herbicides affect weeds. It includes the entire sequence of events that occur from the
time the weed absorbs the herbicide to the final plant response (usually death). Thus,
mode of action includes absorption, translocation to an active site, inhibition of a
specific biochemical reaction, degradation or breakdown of the herbicide in the plant
and the effect of the herbicide on plant growth and physiology.
Large differences exist in the length of time for which specific herbicides
provide acceptable levels of control. Persistence refers to the length of time that an
herbicide remains active in the soil 2. Depending on the herbicide, persistence can vary
from a matter of days to a few years. Long persistence is desirable in terms of weed
control but may be undesirable if new plantings are scheduled for a site or if the
herbicide poses a risk of contamination to ground or surface water.
Herbicide resistance which is the inherited ability of a weed or crop biotype to
survive an herbicide application to which the original population was susceptible is a
problem that could be circumvented by the use of herbicides with different sites or
mode of action in rotation 3. Therefore, the knowledge of mechanism displayed by
herbicides to disadvantage weed is crucial in weed management as it may affect the
efficacy of the herbicides when herbicides of same mode of action is continually used.

BIOACTIVITIES OF HERBICIDE

Herbicides kill plants in different ways. An herbicide must meet several
requirements to be effective. It must come in contact with the target weed, be
absorbed, move to the site of action in the weed, and accumulate sufficient levels at the
site of action to kill or suppress the target plant.
1.1 HERBICIDAL MODE OF ACTION
All herbicide interactions with a plant, from application to final effect, are
considered the mode of action. The mode of action involves the absorption into the
plant, translocation or movement in the plant metabolism of the herbicide, and the
physiological plant response.
1.1.1 Herbicide Translocation
Systemic herbicides are translocated in plants, while contact herbicides are not
translocated. To be effective, contact herbicides must be applied to the site of action.
Most foliar-applied contact herbicides work by disrupting cell membranes. Thorough
spray coverage of a plant is essential with foliar-applied contact herbicides to kill the
entire plant. Systemic herbicides can be translocated to other parts of the plant either in
the xylem or the phloem 1. The xylem is nonliving tissue through which water and
nutrients move from the roots to the shoots and leaves of plants. Translocation in the
xylem is only from the roots to the leaves. Phloem is a living, conducting system in
which materials can move both upward and downward. The phloem transports the food
that is produced in the leaves to the roots and to areas of new growth.
Herbicides can be translocated in the xylem, the phloem, or both. Translocation
depends on the chemical and the plant species. Herbicides translocated only in the
xylem are most effective as soil-applied or early post emergence treatments because
translocation is only upward. Atrazine is a good example of an herbicide that is
translocated only in the xylem. Phloem translocated herbicides that move downward
and suppress root and rhizome growth, as well as top growth, provide the best
perennial weed control. 2, 4-D, and Roundup are examples of systemic herbicides that
will translocate in the phloem and provide good, long-term control of certain perennial
weeds 1.
1.1.2 Herbicide Site of Action
Normal plant growth and development involves complex biochemical
reactions. Herbicides adversely affect some of these reactions in susceptible plants. The
following are biochemical influence of herbicide on plant physiology.
* Photosynthesis.
In the presence of light, green plants produce sugar (C6H12O6) from carbon
dioxide (CO2) and water (H2O) in a process called photosynthesis. Photosynthesis is a
two-phase process that occurs in the leaf chloroplasts. During the light dependent
phase of photosynthesis, the plant transforms light energy from the sun into biological
energy in the form of ATP (adenosine triphosphate) and NADPH2 (nicotinamide
adenine dinucleotide phosphate).
In the light independent phase of photosynthesis, ATP and NADPH2 supply
energy for the conversion of CO2 into sugars. Plants subsequently convert sugars into
longer chain carbohydrates, which represent the major stored portion of biological
energy in the plant. Herbicides that directly inhibit photosynthesis interfere with or
block electron transport and prevent ATP and NADPH2 production. This leads to
decreased sugar or food formation. However, the visual injury symptoms (chlorosis,
desiccation or browning of plant tissue) occur too rapidly to be the result of starvation
of the plant.
Alternatively, chlorosis of leaf tissue may be due to the photo-destruction
(damage from excessive light) of chlorophyll and other plant pigments. When
herbicides block electron transport, chlorophyll continues to absorb light energy but
cannot pass this energy on to make ATP and NADPH2. Thus, chlorophyll either selfdestructs
from the energy it is absorbing or passes this absorbed energy on to
oxygen. This forms radical oxygen, which is highly destructive to cell membranes
and other cell structures. Cell-membrane destruction causes leakage of the cellular
contents and results in the desiccation of plant tissue.
Herbicides that inhibit photosynthesis include simazine and bentazon.
Additionally, the modes of action of oxyfluorfen, diquat, oxadiazon and glufosinate
relate to the photosynthetic process 8.
* Amino-Acid and Protein Synthesis.
Plants use proteins in functional, storage and structural roles. Functional
proteins are called enzymes. Enzymes catalyze thousands of chemical reactions
necessary for plant growth and development. Storage proteins commonly occur in
seeds and supply essential amino acids to young, developing seedlings. Both enzymes
and seed proteins consist of long chains of interconnected amino acids. Commonly,
just l7 to 20 different amino acids occur in plants. However, the amino-acid
composition between different plant proteins varies greatly. In the absence of aminoacid
and protein synthesis, plants cannot complete the chemical reactions necessary for
growth. Imazaquin, halosulfuron and glyphosate inhibit the synthesis of specific
amino acids. Without them, protein synthesis decreases, certain metabolic reactions
cease and the plant gradually dies over a period of one to several weeks.
Protein synthesis is under the direct control of DNA (deoxyribose nucleic
acid) and RNA (ribonucleic acid). DNA is located in the nuclei and chloroplasts of
plant cells and contains the genetic information that determines the sequence of amino
acids in the various plant proteins. RNA (messenger RNA) transports the genetic
information that is contained in DNA and is involved in the assembly (transfer RNA)
of amino acids into proteins. Metolachlor and napropamide interfere with nucleic-acid
synthesis, which in turn decreases protein synthesis 8.

* Cell Division
Plant growth includes the process of cell division, or mitosis, which the
nucleus initiates and regulates. During cell division, a mother cell divides into two
identical daughter cells. Herbicides that interfere with cell division are called mitotic
poisons. When these products block cell division, new cell production decreases and
eventually growth stops. Herbicides that inhibit cell division are the dinitroanilines,
pronamide and dithiopyr 8.
* Cell Membranes
A cell wall and a membrane (the plasma membrane) enclose plant cells.
Contact herbicides cause a breakdown of the cell membrane and leakage of the cellular
contents. The plant then undergoes rapid wilting and desiccation, often within hours
of the herbicide application. Plant tissues appear burned.
However, the mode of action of most contact herbicides is not due to actual
burning or caustic action of the herbicide; contact herbicides affect specific
physiological processes. For example, diquat intercepts electrons during the lightdependent
phase of photosynthesis, creating free radicals. These pass electrons to
other compounds that form superoxide radicals and hydrogen peroxide, which are
toxic and break down the cell membrane. Oxyfluorfen and oxadiazon are other
herbicides that cause massive membrane disruption through the process of free-radical
formation 8.
* Fatty Acid Synthesis
Fatty acids are critical components of cell membranes. If fatty-acid synthesis is
blocked or inhibited, plants are unable to form the cell membranes necessary for
normal growth. The post-emergence grass herbicides sethoxydim, fluazifop,
fenoxaprop and clethodim inhibit fatty-acid synthesis in susceptible grassy weeds.
Tolerant plants (that is, broadleaf) have a different structure of the enzyme that these
herbicides do not affect 8.

* Cell-Wall Biosynthesis
As mentioned, a cell wall (in addition to a membrane) composed of cellulose,
hemicelluloses, pectin and other compounds encloses all plant cells. Cell-wall
biosynthesis begins during the process of cell division and continues during the
growth of the cell. The primary purpose of the cell wall is to impart rigidity and
structure to the plant. The mode of action of isoxaben is inhibition of cell-wall
biosynthesis 8.
* Pigment Synthesis
Carotenoids (yellow in color) and chlorophyll (green in color) are plant
pigments located in the chloroplasts of leaf cells. Both carotenoids and chlorophyll
absorb light during photosynthesis. An additional function of carotenoids is that they
protect chlorophyll molecules from photo-oxidation (damage from excessive light).
Norflurazon inhibits carotenoid synthesis.
In the absence of the protective carotenoids, chlorophyll breaks down in
sunlight and susceptible plants become bleached or white in color due to photooxidation.
Plant death occurs slowly due to the eventual depletion of stored food
reserves and the inability of the plant to manufacture new sugars 8.
* Growth Regulation
Auxins are natural plant hormones that regulate plant growth and are under
direct metabolic control by the plant. At low concentrations, auxins promote normal
growth and development. However, at abnormally high concentrations, auxins inhibit
plant growth. Several herbicides, such as 2,4-D and related compounds, mimic the
activity of heavy doses of auxins and thereby cause abnormal plant growth.
The exact mode of action of auxin-type herbicides is unknown. The first
apparent symptom after application is a downward twisting or curvature of the leaves
and stems of susceptible plants, often within hours of application. Although other
symptoms are slower to develop, plants also undergo rapid, uncontrolled cell division
8
and enlargement. Vascular tissues responsible for the transport of food materials and
water become plugged or broken, and the plant slowly dies over a 2- to 4-week period.
Recent evidence indicates that auxin-type herbicides stimulate the production
of excessive amounts of ribonucleic acids (DNA and RNA). This induces uncontrolled
cell enlargement and division and results in the abnormal growth of susceptible plants.
Herbicides that have auxin-type activity include 2,4-D and related phenoxy herbicides,
and dicamba 8.
* Nitrogen Metabolism
Glufosinate inhibits an essential enzyme involved in nitrogen metabolism.
This enzyme helps convert inorganic nitrogen, in the form of ammonia, into amino
acids. Glufosinate interferes with the activity of this enzyme, which causes toxic levels
of ammonia to accumulate in plant cells. This, in turn, directly inhibits photosynthesis.
The result is rapid tissue necrosis and death of the treated plants 8.
However, it is worth to note that some herbicides are capable of trigging off
their effects in more than one physiological process.
1.1.3 Herbicide Selectivity and Metabolism
The herbicide's ability to kill certain plants without injuring others is called
selectivity. The potency of an herbicide to adversely affect or kill all plants varies.
Base on this, herbicides are classified into selective and non- selective herbicide 7.
Herbicide that kill or suppress the growth of most plant species are relatively non
selective. Herbicide selectivity is relative and depends on several factors, including
environment, herbicide application rate, application timing, and application technique.
Plants tolerant to herbicide often metabolize the chemical into non-toxic
substances. Differential metabolism is often the basis for herbicide selectivity 2.

1.2 THE NON TARGET EFFECT OF HERBICIDE
It is established that the soil enzymatic reactions are influenced by herbicide,
so important ecological functions of the soil- those associated with organic matter
decomposition, mineralization of nutrients, and synthesis of humic substances need to
be reviewed in relation to herbicide application. Ismail, B. S. et al., 1997 discovered that
Metsulfuron-methyl at 5· 0 μg/g caused a reduction in amylase and urease activities in
soil throughout 28 days of study. Also, it was reported that herbicides inhibited the
activities of soil enzymes in the early stage of treatment but increase the activities of
urease, L-glutaminase and protease later 5.
In addition, there is evidence to indicate that herbicides applied in field
cultivations of leguminous plant may influence nodulation, dinitrogen fixation and
development of growing plant 6

2.0 HERBICIDE PERSISTENCE
Herbicide persistence or residual life is the length of time an herbicide remains
active in a soil. The soil persistence of an herbicide is often stated as”half-life,” or
‘T1/2”, which is the amount of time it takes to decompose 50 percent of the applied
chemical to an herbicidal inactive form 2. The ideal soil-applied herbicide is one that
controls weeds for a desired period and then rapidly degrades or breaks down in the
soil to non-phytotoxic levels.
Understanding the residual life, or soil persistence, of an herbicide is
extremely important. It not only determines the length of weed control that is expected
but also influences the plant selection of succeeding plantings. Physical, chemical and
microbial processes affect the persistence of herbicides in the soil. Volatility, leaching
and soil erosion by wind and water are physical processes that also affect herbicide
persistence by determining how their movement from the application site.
* Volatility
This is the process by which an herbicide changes from a liquid or solid state
to the gaseous (vapor) state. Once in the vapor state, the herbicide rapidly leaves the
area of application, and poor weed control or injury to non-target plants can occur if
enough of the herbicide volatilizes. Chemical characteristics, soil moisture,
temperature and adsorption of the herbicide to soil colloids all affect herbicide
volatility. For example, under hot, dry conditions, pronamide volatility is high. The
dinitroaniline herbicides (oryzalin, trifluralin, prodiamine, benefin and pendimethalin)
vary in their volatility characteristics. Oryzalin and prodiamine are perhaps
the least volatile, followed by pendimethalin, benefin and trifluralin 8.
Mechanical incorporation, rainfall or irrigation within 1 to 2 days of
application will prevent or dramatically reduce the volatility losses of dinitroaniline
(and other) herbicides, resulting in better control as well as reduced risk of non-target
effects.

* Leaching.
The movement of herbicides in soil by water is called leaching. Leaching of
herbicides can occur in any direction in the soil, but the most common direction is
downward. Soil texture, the adsorption of the herbicide to soil colloids, the water
solubility of the herbicide and the amount of water movement through the soil all
affect the amount of herbicide lost to leaching.
The movement of an herbicide by leaching is important to weed control
effectiveness, herbicide carryover, and the potential for environmental problems.
When an herbicide is leached downward, the concentration of herbicide near the soil
surface is reduced, lessening the chances for herbicide carryover problems 2.
Herbicides, such as the salt forms of 2, 4-D, have a low tendency to adsorb to soil
colloids and readily leach in fine-sand or silt-loam soils. In contrast, the dinitroaniline
herbicides and most other pre-emergence herbicides readily adsorb to soil colloids and
resist leaching 8.
* Adsorption.
Adsorption is the attraction of ions or molecules to the surface of a solid. After
application, many herbicides adsorb (bind) to the clay and organic-matter fractions of
soils. However, herbicides adsorb poorly to the sand and silt fractions of soil.
Therefore, the extent of herbicide adsorption increases as the percentage of organic
matter and clay increases. The dinitroaniline herbicides, dithiopyr, oxadiazon and
most other pre-emergence herbicides readily bind to soils 8. Weed control is inversely
proportional to how much herbicide is adsorbed to the soil.
In general, small increases in the organic matter content of a soil greatly
increase its ability to adsorb herbicides. A soil high in organic matter content will
generally require a higher herbicide rate than a soil with less organic matter. Adsorbed
herbicide molecules are unavailable for biological, physical, and chemical processes
until released from the soil into the soil solution or vapor phase. Herbicides generally
are more tightly adsorbed in dry soils than in wet soils. Water molecules compete and
displace herbicide molecules from adsorption sites, making the herbicides available for
plant uptake 2.
* Photo-decomposition.
Herbicides break down or degrade in sunlight. Specifically, the ultraviolet
(UV) portion of sunlight is responsible for photo-decomposition. Several herbicides,
such as most dinitroanilines herbicides, are photo-degradable. Therefore, they require
incorporation into soil with tillage, rainfall or irrigation to retain their herbicidal
activity 2.
* Microbial processes.
Microbial decomposition is one of the most important processes by which
herbicides break down in the soil. Microorganisms use many organic herbicides as a
food source. Thus, soil temperature, aeration, pH, organic matter and moisture levels
that favor microbial growth also promote rapid herbicide breakdown. Herbicides that
microbes can affect include the dinitroanilines, metolachlor, napropamide, pronamide,
bentazon, dithiopyr, glufosinate, glyphosate and isoxaben 2.

REFERENCES
1. Anonymous
Herbicide Mode of Action. Kansas State University Extension Publication
C-715.
2. Daniel L. Devlin, Dallas E. Peterson and David L. Regehr, 1992
Residual Herbicides, Degradation, and Recropping Intervals.
Kansas State University Agricultural Experiment Station and
Cooperative Extension Service C-707 April 1992
3. Gunsolus, J.L, 1999
Herbicide Resistant Weed. North Central Regional Extension
Publication 468. University of Minnesota Extension Service
4. Ismail, B. S., Yapp, K. F. and Omar, O., 1997
Effects of Metsulfuron-methyl on Amylase, Urease, and
Protease Activities in two Soils. Australian Journal of Soil Research
36(3) 449 - 456
5. Kim, J.E and Hong, J.U., 1988
Effects of Herbicides on enzyme activities in soil environment.
J. Korean Agric. Chem. Soc., 31:79-85
6. Niewiadomska, A. and Sawicka, A., 2002
Effect of Carbendazim, Imazetapir and Thiram on Nitrogenase
Activity, Number of Microorganisms in Soil and Yield of Hybrid
Lucerne (Medicago media). Polish Journal of Environmental Studies
Vol. 11, No. 6 (2002), 737-744
7. Phillips, T.A., 1977
An Agricultural Notebook. Lowe & Brydone Printers Limited,
Thetford, Britain. pp 312
8. Tim Murphy, 1998
Understand the Mode of Action and Persistence of Ornamental
Herbicides. www.grounds-mag.com

Storage of Roots & Tubers

Crop Storage & Preservation (CSP 806)
Storage of Roots & Tubers
By
Ayodele Olatunde Philip
(CRP 98/0188)

Introduction

Roots and tubers such as cassava, yam, and cocoyam ranks next to cereals in provision of the major part of daily energy need of people in the tropics. Together with cereals, they are classed as staple food because they provide the main item of diet for many people. They however differ from grain in possessing high water content, a major drawback in the utilization potential of the crops. This drawback may however be converted to a plus by developing utilization approaches specific to tubers and roots rather than attempting to apply cereals and legume technologies to them. Because roots and tubers are highly perishable, they experience tremendous losses after harvesting resulting from a number of factors.
Mechanical injury: During harvesting and post harvesting handlings, a common occurrence is the sustenance of physical injuries such as bruises to the skin of the tuber, outright breakage of the tuber tips, cuts resulting from sharp harvesting tools and abrasion following the frictional contact with the soil structure. All these injuries create entry points for deteriorating organism and thus aggravate storage losses
Metabolic losses: This is due principally to respiration which produces CO2, H2O and heat. The H2O and heat produced encourage the growth of micro-organism pest, as well as rots. Storage environment particularly temperature and relative humidity are controlling factors.
Spouting: This terminology is used to describe the resumption of active growth in roots and tuber and renders the produce generally less marketable, as well as accelerated respiration. Sprouting occurs at the end of dormancy period which varies from specie to specie.
Exposure to extreme temperature: This is particularly important for yams which are subjected to chilling at temperature below 10˚c resulting to internal discoloration and tissue breakdown followed by rapid decay and loss of quality. Exposure to very high temperature can also lead to physiological decomposition.
Vascular Streaking: This essentially results in internal discoloration and very common in cassava and following physiological changes resulting in blue-black discoloration along the vascular bundles.
Microbial attack: When microorganisms attack tuber & roots, their activities rapidly result in massive tissue breakdown and many times the food reserves is rapidly used up in the metabolic activities of the microorganism. In some instances toxic substances are synthesized by the infecting microorganism such that the tuber has not only loss food reserves but has also become poisonous to human and animal consumers.
Insect and nematode attack: Important insect pests of root and tuber include scale insect, yam beetle, and several nematodes including Scuttellonema bradys, Scuttellonema pratylenchus.
Attack by rodent and other mammals: The magnitude of post harvest losses caused by the factor of rodents and other mammals can be as high as 10% and often the injuries encourages secondary infestation by insect and other microorganisms. About 25% of roots and tubers are lost to the above factors on a global basis during post harvest storage.

Storage Methods
Subsistence farmers by experience have known that most roots and tubers deteriorate rapidly after they are harvested. They have learnt to counteract this by using several cultural techniques to ensure that the culinary qualities are preserved during storage. Basic storage requirements differ crop to crop and from region to region depending on several factors. Even for the same crop, storage method can differ depending on purpose of storage and intend use. The variations are often related to climate but local resources and custom also influence the choice of storage method used. Storage methods can generally be classified into the following:
Traditional Storage Methods
The most common traditional methods for storing roots and tubers after harvesting are: storage in pit, storing in the house, storing on a platform in the open, leaving the crop in the ground where it is grown until it is needed, clamp storage and barn storage for yam in particular.
Improved or Modern Storage Methods
These include the use of chemical sprout inhibitor, ionizing radiations, controlled atmosphere storage and cold storage. These methods are modified in various root and tuber crops.



Yam
A. Yam Barn
The yam barn is the most common method applied for storage in Africa, especially in the humid forest areas of West Africa. It consists of vertical wooden frameworks to which yam tubers are tied individually by the means of strings or local cordage materials such as raffia. In some cases the barns are covered with thatched roofs to protect the tubers from rain and the heat of the sun. Tubers stored are exposed to attack by pest and microorganism and are subject to substantial weight losses due to respiration. Also, placing yam on shelves requires less time and labour than tying. Shelving also has the advantage that insect infested and rotting yams can be removed easily. The disadvantage with shelving lies in the fact that it is easier for rat to hide amongst the yams. Losses due to decay in some cases can amount to 40 – 50 per cent (Olorunda and Adesuyi 1973). Losses can be reduced by protecting the barn from rat by fixing a barrier made of iron sheeting around the barn.

Yam tubers tied to a vertical wooden framework in the shade for storage
B. Burying (Clamp) or Pit Method
This involves the digging of holes or pits about 1m deep and 1m diameter in the ground and lining them with dry leaves and grasses. After selecting good quality yam tubers, they are placed in the pit, alternating each layer of good quality yam tubers with dried grasses. They are finally covered with a layer of dried grasses and a top layer of soil. This method is not very effective due to lack of sufficient ventilation, and difficulty of inspecting the tubers.
C. Trobriand Yam House and Crib for Yam Storage
Outside the forest zone, for example in savanna region of West Africa, yams can be stored either on the field or in the farm compound. Yams stored in the farm compound are stacked in crib like those of used for maize. The cribs are well raised well off the ground with rat guards fitted to the legs. In Papua New Guinea, the so-called Trobriand yam house is widely used. The typical Trobriand yam house consist of platforms or ‘shelves’ of wicker (bamboo) or of light poles laid together, supported on vertical wooden poles a meter or two above the ground level. The vertical poles, which support the shelves, also carry a pitched roof of thatch, which protects the yam tuber from the sun and, to a certain extent, from the rain. The tubers are placed on top of each other on the open shelves. Since individual shelves would normally contain only a few dozen tubers, adequate ventilation is ensured. However, if the structure is not raised adequately above the ground level, the tubers may not be sufficiently protected from rodent attack or from danger of flooding. The Trobriand yam house is especially suitable for storage of firmer or harder-fleshed yam varieties, which do not bruise easily.

Crib for yam storage (left) and Typical Trobriand yam house (Right)




D. Field Storage
Leaving the yam in the field is the simplest method of storage. Tubers are stacked in pile and covered with grass, sorghum or millet stalks. A tent made of sorghum stalk is also used as a yam store.
E. Cold Storage of Yam Tubers
The application of low temperature storage to yam is limiting by the fact that yam are susceptible to low temperature injuries at temperature of 10-12˚C or less. Temperature of 16-17˚C have been successfully employed to prolong the storage time of yams. At 16˚C the dormancy and hence the storage life of D. alata tuber may be extended by as much as four months, particularly if the tuber is cured prior to storage so as to reduce infection by wound pathogens. The storage of yam tuber at lowered temperature has the advantage of reducing the major sources of storage loss (respiration, sprouting and rotting). However, the widespread application of cold storage for yam is not yet economically feasible due to the relative high cost of the technology involved.

In the recent past however, several practices aimed at improving the storage of yam have been attempted and they include techniques that can be applied by farmers for small scale storage of yams and those that can be applied by farmers for large scale storage.
Small Scale Storage Method of Yam
Manual removal of sprout at the time of first development: This reduces weight loss and conserves moisture and starch food content, improves palatability especially when stored for more than 6 months where good tubers are stored initially. The method is simple, requires no special training, and has no input cost except that of time. Another advantage is that it requires no electricity which may b e limiting for other storage methods in rural areas. The only disadvantage is that it can not be applied to large quantities of tubers and that the first sprout itself is associated with loss.
Application chemical Sprout inhibitors and fungicides: Maleic hydrazide can be used as a pre-harvest foliar spray and as a dip for harvested tuber before storage to suppress sprouting, Tetracholoronitrobenezene, methyl ester of 1- naphthylacetic acid, naphthalene and acetic acid have been found effective at high concentration. Benlate and thiabendazol are effective fungicides for yam storage. Curing at 25˚c and 30-60% relative humidity for 5 days prevents storage rot.
Large Scale Storage Method of Yam
Controlled atmosphere: Storage at low temperature particularly 15˚C suppresses sprouting in yams. This same temperature reduces weight loss, moisture loss, respiration rate, and keeps the tuber palatable
2. Irradiation treatment: The use of gamma ray at 7.5- 15.0 k rad applied 4 weeks after harvesting inhibits sprouting for up to 8 months without affecting acceptability.

Cassava (Manioc)
Most farmers leave the roots of their cassava in the ground until it is needed but this leads to reduced starch quality beyond optimum time. This method is cheap and easy to apply but exposes the tuber to pre-harvest pests. Harvesting may also become difficult if a hard pan forms due to dry spell. Flooding may also lead to decay during subsequent storage. Deterioration in cassava can be primary which include vascular streaking or secondary resulting from pathogenic rot, fermentation, or soften of roots. Curing of the roots prevent the onset of primary deterioration by healing the wounds on cassava roots. It involves keeping the roots at 80-85% relative humidity 25-40˚c for 4-9 days.
The use of field clamp boxes containing saw dust have also been proved effective for up to 2 months. The basic design of these clamps consist of circular bed of straw on which freshly harvested tubers are heaped in a conical style then covered with another layer of straw overlaid with soil which has been dug to form a drainage ditch round the clamp. The temperature is kept below 40˚c by varying the thickness of the straw and soil layers and by inserting ventilators made from straw or hollow bamboo. Storage in moist saw dust requires that the saw dust must be moist enough to maintain high and keep the root fresh but not damp as to make them wet. This can work up to 1 month and it is useful in markets and transportation.
Conclusion
Generally, development of better harvesting, handling and storage technique appear to be critical to reducing losses in roots and tubers. This is achievable by breeding varieties that are less branched, thicker- skinned and uniform size and shape

Processing and Utilization of Roots and Tubers

Crop Processing & Utilization (CSP 808)
Processing and Utilization of Roots and Tubers
By
Ayodele Olatunde Philip
(CRP 98\0188)

Introduction

The tropical root and tuber crops are comprised of crops covering several genera. The principal root and tuber crops of the tropics are cassava (Manihot esculenta Crantz), yam (Dioscorea spp.), sweet potato (Ipomoea batatas L.), potato (Solanum spp.) and edible aroids (Colocasia spp. and Xanthosoma sagittifolium). Root and tuber crops are second only in importance to cereals as a global source of carbohydrates. These carbohydrates are mostly starches found in storage organs, which may be enlarged roots, corms, rhizomes, or tubers. They also provide some minerals and essential vitamins. The high moisture content characteristic of roots and tubers makes them difficult to store for a long length of time. Also, they are bulky and difficult to handle and transport to distant markets. With cassava these problems are increased by compounds of cyanide in the leaves and roots which have to be removed before they can be consumed. Over many years, traditional processes have evolved which yield a more durable product and in many instances a more convenient product for domestic use. In many village communities root crops remain a staple and hence are often the main part of the meal. Village scale processing of root crops is therefore an important part of women's daily work.
Processing of Cassava Tubers
After harvesting, cassava roots are susceptible to spoilage, and without any preservation measures can only be stored for about 48 hours before they begin to deteriorate. Post-harvest deterioration of cassava is related to two separate processes: physiological changes and microbial changes. Physiological deterioration often sets in within 24 hours after harvest, while microbial deterioration usually begins within a week (Cock, 1985). Therefore the roots must be processed as soon as possible after harvest to arrest the physiological process and the subsequent deterioration.
Other factors favouring the processing of cassava are that the processed products are easier to store than raw cassava, they need less storage space and they can be stored for longer periods. For example gari, a dried cassava product, is less affected by biochemical changes during storage and in transit, and therefore does not lose its nutritional value as fast as fresh cassava does in storage. Processing is therefore undertaken primarily to detoxify the cassava product, to improve its palatability and to convert it to a storable form.
Toxicity and Detoxification of Cyanide in Cassava
It is known that cassava contains two major cyanogenic glycosides: linamarin and lotaustralin. Both glycosides are hydrolysed to produce hydrocyanic or prussic acid (HCN), a poison, when they come in contact with the enzyme linamarase, which is released when the cells of cassava roots are ruptured. The equation below illustrates the degradation of linamarin and the subsequent production of cyanide. Cyanide toxicity in humans and animals on cassava diets is a well-recognized problem (Oke, 1968; Osuntokun et al., 1969; Coursey, 1973; Erman et al., 1980).

Enzymatic degradation of linamarin
There is a great variation in toxicity between cultivars. A distinction is usually made between "sweet" cultivars with relatively low contents of cyanogenic glycosides (below 10mg/100g of fresh weight), and "bitter" cultivars with high cyanogenic glycoside content (above 20mg/100g fresh weight), although many intermediate forms exist. Traditionally, the sweet cultivars were considered non-toxic while the bitter ones were considered toxic. Although the sweet cultivars are generally less toxic there is no direct correlation between toxicity and taste (Coursey and Haynes, 1970). Cyanide levels in the range 6 to 370 mg/kg have been found depending on the particular cultivar, growing conditions, (i.e. soil type, humidity, temperature) and the age of the plant. The highest proportion of HCN is found in the peels and the cortex layer immediately beneath the peels (Hahn, 1984; Onwueme, 1978). It is for this reason that cassava root is always peeled before being processed or consumed. Peeling removes the cortex and the outer periderm layer adhering to it. The presence of cyanogenic glycosides in most cassava cultivars necessitates a certain degree of detoxification before the roots can be consumed. The prussic acid in particular is lethal if more than about 0.1 g of it is contained in the food eaten by an individual at any one time (Onwueme, 1978). In general, three methods of detoxification are employed: (a) microbial detoxification through fermentation; (b) decomposition of the glycosides by heating them above 150°C and (c) rupture of the roots to allow intimate interaction between linamarase and the glycoside, then expressing or volatilizing the resultant products of hydrolysis.
All forms of cassava processing therefore only decrease the levels of cyanogenic glycosides and prussic acid in the final product. For example, boiling is said to destroy the enzyme linamarase and eliminate the prussic acid. However, the linamarin itself is not destroyed by boiling, and its long-term ingestion may lead to cyanide toxicity in humans whose diets lack sufficient protein and iodine (Cock, 1985).

Gari
Gari can be considered to be the most popular form in which cassava is consumed in West Africa. Cassava is processed into gari in the following way. The harvested roots are peeled, washed and then grated. The resulting pulp is put into a cloth bag and subjected to pressure by heaping stones on the bag. After pressing, the bag is left for 2—4 days, during which time the pulp ferments. Most of the juice from the cassava pulp is expressed from the bag during this period. The fermented pulp is then removed from the bag, sieved and roasted or fried in wide, shallow metal pans, until they are dried.
The processing of cassava into gari involves several unit operations: peeling, washing, grating, pressing and fermenting, sieving, roasting and drying. Traditional gari production is laborious and time-consuming; on average it takes about 90 hours to process 100kg of gari. About 65 per cent of the total time is spent on peeling and 25 per cent on roasting (Williams, 1979). Gari occupies an important place in the diets of the people of West Africa. It is customarily consumed in the form of meal, which is prepared by soaking the gari in water to swell the starch, and by making the swollen meal into dough. The dough is then made into a ball with the fingers and is dipped into a stew containing ingredients such as palm oil, vegetables, meat or fish. Gari may also be eaten without a stew or soup by soaking it in cold water and adding sugar or milk
Peeling of cassava tubers
Peeling of cassava tubers presents a considerable problem in cassava processing. Traditionally, peeling is accomplished by hand; the roots are cut longitudinally and transversely to a depth corresponding to the thickness of the peel, which can then easily be removed. The structure of the root; the irregular shape and size does not permit easy mechanical peeling. However, in larger factories, the so-called washer-peeler, as described by Edwards (1974) is used. This equipment is divided into two sections; in the first section the tubers are cleaned of any adherent dirt and sand, while second section they are peeled. A machine of this type consists principal tank with a concrete base and a wooden casing. Along its length runs a rotating shaft about 15cm in diameter. Iron spikes or wooden paddles project from either side of the shaft with successive pairs set at right angles to each other; these cause the tubers to turn. Jets along the shaft spray water countercurrent to the flow of tubers; the water is pumped along the shaft centrifugal pump. In the peeling section, the tubers are moved continuously along under the spray of water and are peeled by the friction and the action of the rotating spikes.
Rasping or pulping
The peeled tubers are ground to a homogeneous mixture by means of a mechanical action. This is carried out by slicing the tubers and then rasping, grating or crushing them into a fine pulp. At the village level the tubers are rasped by hand on bamboo mats. A simple but effective grater is obtained by perforating a sheet of galvanized iron with a nail and then clamping it around a wooden wheel with the sharp protruding rims of the nail openings turned outwards. The wheel may be driven by hand or by foot like a bicycle, while the worker presses the tubers from above onto the rasping surface.

Fermentation and pressing of cassava pulp
The grating or rasping of the cassava root enhances contact between the enzyme linamarase and the cyanogenic glycosides, so that most of the glycoside can be hydrolysed to prussic acid. The pressing serves to remove the juice containing prussic acid, while the toasting—drying stage is aimed at vaporizing most of the remaining prussic acid.
The micro-organisms responsible for cassava fermentation are indigenous to the roots. Collard and Levi (1959) identified Corynebacterium manihot and Geotricum candida as the micro-organisms involved in a process similar to that of gari manufacture. More recent studies have questioned the validity of Levi and Collard's results. Ngaba and Lee (1977) isolated and identified Lactobacillus sp. and Streptococcus sp. as responsible for cassava fermentation and the subsequent acidity and flavour of gari. Ejiofor and Okafor (1981) observed, among others, high numbers of Candida and Geotricum spp. in pressed cassava pulp during the later stages of fermentation.
According to Meuser and Smolnik (1980) *fermentation of cassava pulp at 37°C is complete within a maximum of 5 days. The water binding capacity of cassava mash is sufficiently changed only after about 3 days, after which the water present can be pressed out to a reasonable degree. For this reason, an accelerated fermentation would be of limited use. During fermentation, there is decomposition of soluble carbohydrate (sugar) to lactic and acetic acids and ethanol. Also, the concentrations of fructose, sucrose and glucose decrease with days of fermentation. The fermentation rate of sucrose is highest as it is completely hydrolyzed within 18 hours. Fructose increases in the first 18 hours and fall steadily after 72 hours of fermentation. Unlike sucrose and fructose, a certain amount of glucose still remains after a fermentation period of 5 days.
Roasting and drying during gari production
The sieved cassava meal is roasted over fire in the traditional method. Constant stirring is carried out over during the roasting to circumvent burning and to prevent the meal from forming lumps. In the roasting process, some of the starch granules are gelatinized and aromatic compounds are formed from saccharide and soluble nitrogenous substances, so the fermented product has a characteristic taste and aroma. Following roasting, gari is dried so that it can be stored for long period and to further reduce the hydrocyanic acid content of the product.
Flowchart of gari Processing
Cassava Root Washing-- Peeling-- Grating pulp-- Pressing and Fermenting Roasting and drying Gari

*Fermentation is the breakdown of complex organic substances into simpler ones through the action of catalysis (enzymes)

Processing of other Root and Tuber Crops
In contrast to cassava of which a large proportion is consumed in a processed form, other roots and tubers (yams, potatoes, sweet potatoes and cocoyams) are normally stored and consumed as fresh produce. But on some occasions they also are processed into dried chips with the aim of prolonging the storage life, especially for that portion of the crop which has been affected by injuries or diseases.
Traditional methods
Many simple methods have been devised based on local resources to extend the storage life of roots and tubers. At family and village levels, the fresh or precooked roots or tubers are simply peeled, cut into chips and sun-dried by being spread out in the open, on a mat or any clean surface. Drying may take up to one week or more, depending on the weather. The dried chips are ground into flour before being incorporated into the traditional foods. Yams in Nigeria, potatoes in Peru, sweet potatoes in Cameroon and Tanzania are traditionally processed in this way, the smaller tubers or those damaged or partially decayed being particularly selected for the purpose.

Improved methods of production of dehydrated chips
In order to satisfy the demand by urban consumers for a high quality product the same considerations apply to the production of high quality dried chips or flakes as in the natural drying of cassava chips. The improved methods for drying cassava chips have been applied to the drying of yams, potatoes, sweet potatoes and cocoyams. But dried chips of these roots and tubers can be affected by discolouring compounds which do not occur in cassava. The discoloration of the dried chips occurs in three ways (Straw and Booth, undated):
Enzymatic darkening
After-cooking darkening
Browning during drying and storage.
Enzymatic darkening is caused by the oxidation of phenolic compounds resulting in a brown to blue-black discoloration which affects the quality and appearance of the final product. Cooking before peeling and slicing will destroy the enzyme and prevent this type of discoloration. If it is not possible to cook the chips immediately after slicing immersing the chips in water helps to slow down the enzymatic reaction, adding salt (3% w/w) to the water will further slow down the reaction. When the freshly cut chips have to be kept for prolonged period a preferred treatment is dipping for five minutes in a 0.1-0.2% sodium bisulphite solution, or 10 minutes in a 0.5% sodium metabisulphite solution.
After-cooking darkening is the result of the oxidation of ferrous iron present in the tuber to ferric iron. Roots and tubers chips which are still warm after being boiled, parboiled or blanched should not be left exposed to air but should be cooled as quickly as possible, for example by immersion cold water. Lowering the pH also prevents the oxidation taking place; in practice this is done by dipping the chip in a solution of 0.4% citric acid.
Browning usually takes place during the drying and storage of the chips. This is the result of the reducing sugar combining with free amino acids. The reaction occurs more rapidly at temperatures above 55°C. It is, therefore, important to keep the drying temperature as low as possible
Flowchart for Processing Tuber Flour
Tubers Slicing or Cutting-- Peeling and washing --Drying
Chips --Grinding--- Sieving Flour
The flowchart for processing of cassava flour is a bit different from the above as pulping and dewatering is an integral part.
Conclusion
Processing of crop is a widely acknowledged alternatives approach which can be used as a significant means of reducing post-harvest losses. In Africa and in Central and South America the preparation of storable cassava product through fermentation is a widespread tradition. The reduction of post harvest losses and production of convenient, safe and wholesome food through processing depend on proper use of existing traditional methods and recent developments derived from a broad spectrum of scientific disciplines.





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