cassava starch processing wastes: potential pollution and

117

[Japanese Journal of Water Treatment Biology Vol.38 No.3117-135 2002]

Cassava Starch Processing Wastes: Potential

Pollution and Role of Microorganisms

TRUONG QUY TUNG1, NAOYUKI MIYATA2, and KEISUKE IWAHORI2*

1Graduate School of Nutritional and Environmental Sciences, University of Shizuoka

2Institute for Environmental Sciences, University of Shizuoka

/52-1,Yada, Shizuoka 422-8526, Japan

Abstract

As a result of agriculture, urban and industrial activities, wastes and residues are increasing and are causing environmental pollution. Current cassava starch industry

processing, which is discharging voluminous amounts of solid waste and wastewater, is reappeared in this review. Potential pollutions of cassava starch processing(CSP)wastes including two major problems: high loading of organic compound and cyanide content in the exceeding limit are also indicated. The important role of microorganism reflected

in the improvement of natural deficient properties of cassava products shows capacity of microorganisms in detoxification of cyanide as well as utilization of raw cassava starch

as a substrate for growth and conversion. The information is collected as an evidence to confirm the feasibility of microorganism, particularly filamentous fungi, in the development of microbial treatment system for the reclaimation of the CSP wastes.

Key words: cassava starch, microorganisms, hydrogen cyanide, solid waste,

wastewater

INTRODUCTION

The cassava (Manihot esculenta Crantz)

plant is one of the most important root crops in about 88 countries of humid tropics1). A

variety of food and industrial products is derived from cassava tubers in which starch is a major used portion. Cassava starch

industrial processing requires a large

quantity of water and it also generates a huge quantity of wastes including waste-water and solid waste.

Waste pollution problems related to cassava starch industry are posing a serious threat to the environment and quality of life2).

The solid wastes, which consists of cassava

peels and bagasse containing considerably residual starch spoiled rapidly in humid environment, are used somewhat as animalfeed but generally discarded in the

environment as landfill without anytreatment3). Characteristic of cassava starch

processing (CSP) wastewater, as well as in general starch industry, is acidic and highly organic. However, one of the most important

problems related to the CSP wastewater is the presence of cyanic compounds of which

toxicity has been warned for humans and animals. The pressing requirement needs to be promoted for the minimization of the

organic content and cyanide in wastewater.A number of investigations for treatment of

cassava starch effluent have been reported which almost focused on either physico-

chemical or biological processes. Neverthe-less, in recognition of potential advantages of

microbial treatment such as application to

biorefractory compound; operation at high and low concentration;operation over a wide range of pH, temperature and salinity; the

ease and simplicity of controlling the process, and the reduction in sludge volume;etc.,

recent researches have focused on thedevelopment of microbial processes for the

*Corresponding Author

118 Japanese J. Wat. Treat. Biol. Vol.38 No.3

treatment of wastewaters, solid wastes.

hazardous wastes and soil4).Enzymes and microbial systems, which

removed cyanide for detoxification5) as well as

digested raw starch6) has been reported. Many microorganisms are capable to hydrolyze starch, but generally its efficient

hydrolysis requires previous solubilization. Some recent reviews concern the hydrolysis of raw starch as it occurs naturally7). Most of

the previous works have been done sufficiently, however, with wheat, potato or

maize starch but deficiently with cassavastarch.The subjective of this review is to provide

current status of effluent from cassava starch industry processing, applications of

microorganisms in the area of cyanide removal and utilizing cassava starch as a substrate; and to estimate the potential of

microorganisms for construction of future CSP wastes treatment solution.

CASSAVA PLANT

The cassava plant is known under various names, such as cassava (English), yucca

(Spanish), and mandioca (Portuguese)8). Furthermore, many other names are also known in the various countries or regions, e.g. as manioca, aipim, castelinha, and macaxeira in South America, as ubiketela, and kaspe in Indonesia, as manioc in Africa, and as tapioca in Indian, Thailand and in other part of the Southeast Asian9-11).Sometimes the term cassava is usually applied to the tubers, and tapioca is the name

given to starch and other processed products. Cassavas belong to the family Euphorbiaceae. The scientific name for the commercial species is Manihot esculenta Grantz, but the name Manihot utilissima Phol, was widely used in the earlier literature8). Cassava

(Manihot esculenta)is a root crop commonly divided into two groups, bitter and sweet cassava depending on their content ofcyanohydrin. Sweet cassava is grown for food use, while the bitter varieties are most frequently used for industrial purposes due to their higher starch content10).

The cassava plant is a perennial shrub that

grows 1-3 meters in height(Fig.1). The stems of mature plants are woody,

prominently nodded, pithy, and fairly brittle. The leaves are simple but palmate with 3-10

lo bes. Each plant has 5-20 tuberous roots radiating from the base of the stem and each tuber attains a length of 20-80cm and

diameter of 5-10cm. The fresh weight of each tuber varies between a few hundred

grams and 5kg. The crop is propagated vegetatively from stem cuttings. Simply, cassava is planted, using 7-30cm portions of the mature stem as propagules. In period of

food shortage, the farmer is required to savea part of the edible cassava crop for replanting of the fields to obtain next year's

crop.Recent estimates suggest that cassava

storage tuber provide 8% or more of the minimum calorie requirement for more 750 million people3). The importance of cassava

plant is increasing for number of reasons12). Cassava is one of the most efficient

carbohydrate-producing crops. It is tolerant of low soil fertility and unfavorable climates

(drought resistance) and has the ability to recover from the damage caused by most

pests and diseases. Further, the roots can be left in the ground for long periods as a food

reserve. Accordingly, growth of cassava

provides an important reserve carbohydrate source to prevent or relieve famine during

periods of adverse climate conditions and offers the possibility to obtain a harvest from eroded and otherwise abandoned fields.

Fig.1 Cassava field

Microorganisms and Cassava Starch Processing Wastes 119

PRODUCTION

Cassava is considered an important source of food and dietary calories for a large

population in tropical countries in Asia, Africa and Latin America. During nearly the last four decades, the trend is increasing of the cassava crops. Table 1 shows the fresh cassava tuber output of the major producing regions and in the world. Total world

production has increased from about 87 million tones in 1968 to an estimated approximately 167 million tones in 1999. Production in Africa and Asia continues to increase, while that in Latin America has remained relatively level. Although cassava is cultivated in about 88 countries, but only five countries are the largest producers of cassava including Nigeria, Brazil, Thailand, Zaire, and Indonesia.The cassava production of these five countries is over 50% of the world conduction.

CASSAVA TUBER COMPOSITION

Cassava is one of the richest sources of starch15). The starch content of the tubers varies according to such factors as age, variety, soil, and climate. The chemical composition of cassava tubers is shown in Table 2. The cassava tubers can contain up to 40% of starch in fresh weight base and to 90% of that in dry matter, while they are low of the protein(only 1% in fresh and averageof 3.6% in dry matter). With such starch

potential, it has given highest yield of starch per hectare of any crop16). However, the inherent defect of cassava is the low protein content in the tubers. In addition, vitamin and mineral components are also unfavorable in cassava.

A major problem with cassava is its

cyanide content. Cassava is famous for the

presence of free and bound cyanogenic

glucosides. It contains two cyanogenic glucosides, linamarin and lotaustralin, in all parts of the plant. They are converted to hydrogen cyanide in the presence of a naturally occurring enzyme known as

linamarase. Upon tissue disruption, linamarase catalyzes degradation of the

cyanogenic glucosides releasing hydrogen cyanide and ketones. Therefore, the use of

Table 1 World cassava production(in million tones)8,13,14)

Table 2 Chemical composition of cassava tubers

(100g basis)3,17-19)

cassava products, as a staple food requires

careful processing to remove the cyanide. The

problems around cyanide will be mentioned further in a latter another part.

APPLICATIONS OF CASSAVA STARCH

Cassava tubers can be processed into different forms for a wide variety of end uses,


and much of this processing can be carried

out locally, providing jobs and income in

rural areas. It can be made into food

products, used as animal feed, and processed

into starch8). In many countries of Africa and

Latin America, dried cassava products are

processed at home or at village level to

produce toasted flour(farinhal, garri), or to

make flat bread(casabe). The products from

cassava starch play an important role in the

applied potential of cassava crops. Cassava

starch is used directly in different ways or as

a raw material for further processing. The

main classes of starch-based products are

unmodified or native starch, modified

(physical, chemical, biological)starches for

industrial purposes; and sweeteners,

including high fructose syrup, glucose,

dextrin, monosodium glutamate, pharma-

ceuticals, etc. Modified cassava starch can

compete with other starches for the

production of alcohol, starch for sizing paper

and textiles, glues, sweeteners, bio-

degradable products, butanol and acetone,

manufacture of explosives, and coagulation of

rubber latex etc.19,20) More than two thirds of

the total production of the world is used as

staple food,30% is used for the animal food

industry, and remaining 5-7% is used as

industry raw material3,15).

CASSAVA CYANOGENIC GLUCOSIDES

At less 8000 species of higher plants of 70

to 80 families, including important agri-

cultural species such as cassava, flax,

sorghum, alfalfa, peaches, almonds, and

beans, are known to be cyanogenic21). Cyanide

occurs in cassava in the form of cyanide

glucosides, i.e. linamarin (93%) and

lotaustralin(7%)22).

Mechanism of degradation of linamarin by

linamarase (β-glucosidase) has been ob-

viously confirmed1,21,23-25). The hydrolysis of

linamarin is the two-step reaction, involving

the formation of an intermediate, acetone

cyanohydrin, which then is broken down

spontaneously or by hydronitrile lyase action

to form acetone and hydrogen cyanide as

illustrated in Fig.2. Linamarase, an enzyme

naturally occurs in cassava acts on the

glucosides when the cells are ruptured. It

means that, when cassava tuber tissues are

damaged, mainly by mechanical action(e.g.,

during processing or preparation for consumption)or by microbial action(e.g.,during a fermentation process or

deterioration owing to poor post-harvest storage), linamarase comes into contact with the linamarin, resulting in its hydrolysis and

the consequently release of hydrogen cyanide12,27,28).

Cyanohydric acid is extremely toxic to a wide spectrum of organisms, due to its ability

of linking with metals that are functional

groups of many enzymes, inhibiting cytochrome respiratory chain reactions,

electron transport in the photosynthesis, and the activity of enzymes like catalase and

oxidase24). Linamarin is not toxic in itself and is an unlikely source of cyanide exposure in human. Nonetheless, it has been shown that

some linamarin may be absorbed by the body and nay be broken down to yield hydrogen

cyanide if suitable glucosidase are provided by microflora present in the gut. Common symptoms of chromic cyanide poisoning in

humans are headache, vertigo, nausea, vomiting and tremors25). An evidence for toxic

potential of cassava cyanogenic glucosides is cited by Knowles(1976)21).

Cyanogenic glucosides are present in all

parts of cassava plant with the exception of seed. There are, however, some differences in the distributions in the plant, with leaves

having the highest cyanide concentrations and with the tubers in which the peel has a higher cyanide concentration than the

interior. This is shown in Table 3. In addition, it is also noticeable that the cyanide

content varies in the tubers of different cassava cultivars. Almost cassava cultivars contain cyanogenic glucosides. The typical

content is around 100mg/kg of fresh weight, but there are many bitter ones that contain up to 500mg/kg1,29,30). However, the tuber

cyanide content can be in extremely high level of range between 75-1000mg/kg and sometimes exceeding 2600mg/kg31).

Traditional processing techniques devised, including to roasting, boiling, ensiling, sun

drying, retting etc., for detoxification of cassava products have been investigated and

applied16,32-39). Requirement on good quality of the cassava products results in the more


Fig.2 Enzymatic degradation of Iinamarin1,26)

Table 3 Cyanide content(mg/kg)in cassava products9)

thoroughly performed processes, and it is not

able to avoid the large loading of cyanide into environment. Particularly, in the industrial

processes of cassava starch from cassava tubers, the large amounts of released

cyanogenic glucosides that decayed rapidly to hydrogen cyanide are evident.

CASSAVA STARCH INDUSTRIAL

PROCESSING

Amajor portion of cassava tubers is used for the extraction of starch. The granules are

locked in cells together with all the other constituents of the protoplasm (protein, soluble carbohydrates, fats etc.), which can

only be removed by a purification process in the aqueous phase. The detail of the stages of

the cassava starch industrial processing has described in many recent articles15,40-42)

. Depending on the processing scale, the

technological degree, and starch-grade etc.,

there are some differences in the concrete operating steps of the CSP factories. Generally, most of them are, however,

relatively identical and include basis stagesshown in Fig.3.

After harvesting, cassava tubers should be

processed during the day to void degradation

of the starch. In small and medium-size

factories, the peel including skin(corky) and cortex is removed and only the central part of the tuber is used. Whereas, in the larger factories, whole tubers are processed since

the inner part of the peel still has starch

Fig.3 Basis operation and water usage for cassava starch industrial processing3,15,20,40,41)


recoverable in modern process. The washing, here, serves to remove the outer skin and the adhering dirt. The rasping stage is necessary to rupture all cell walls in order to release the starch granules. Water is run into the hopper during rasping, in order to facilitate crushing and removal of pulp. After rasping, the hydrogen cyanide in the tuber is set free and dissolved in wastewater.

The disintegrated pulp is washed on screens where starch is extracted and fiber remaining on the screen is discharged. The resulting cassava starch in term of crude starch milk is purified by the series of settling tanks including the whole series of operations in order to separate the pure starch from soluble contaminants. During this process, the flow of starch milk is conducted through the successive tanks. Usually, the settling process is overnight, thus the flour can be kept in settling tanks up to 20 hours or more and the action of microorganisms may also has progressed. Furthermore, at a later stage the fruit water being rather rich in sugars and other nutrients microorganisms start to developand eventually lead to a vigorous fermentation. Some chemicals have been added to improve further properties of the starch such as sulfuric acid, which is added as an aid to sedimentation, results in a

product of enhanced whiteness. The addition of sulfur dioxide is essential to keep microbial action within bounds. The fresh water displaces the impure water that is often transferred to the fiber washer and tuber washing section.

After the purification process, cassava starch is as concentrated slurry at 38-42% solids and is dewatered in a continuous vacuum filter or batch basket-type centrifuge. The final drying of the dewatered starch is always performed by evaporation, either in open air or in ovens. The dried starch(12-14% moisture)is separated from the moist air in cyclones and then ground and sifted. In the whole cassava starch industrial

processing, two types of wastes are generated: solid and liquid. Solid wastes include peels and bagasse. Liquid wastes include wastewaters as combined wastewater.

WATER REQUIREMENT AND

WASTEWATER GENERATION

A large quantity of water was utilized in

production of cassava starch industry43-45). Thetotal water consumption in the starch industry depends on availability of water and it has been observed that the water

consumption per tone of product varies from one industry to another. On other hand, by

recycling it has been possible to reduce the water consumption. Generally, the quantity of water consumption in the starch industry

varies between 20-25m3 per tone of product for small-scale units15). The water require-

ment in starch industry for the extraction of starch from cassava tubers is majority for two unit operations, washing and screening. In other sections, the water requirement is also

concerned such as during operating of the rasper, but in limited quantity.

The combined wastewater from cassava

starch production is released throughout almost sections of the manufacturing process. However, the tuber washing wastewater and

ether the supernatant from the settling tanks, which is known as separator wastewater, constitutes mainly the effluent

from the cassava starch industry. Since the

process involved in the manufacture of starch is commonly of batch type, the effluent

discharge from the industry is intermittent, and on an average, the wastewater

generation is in the range of 16-22m3 per tone of starch15). Generally, the wastewater emanating from tuber washing is less than 10% of the total flow accounts while 90% is

contributed from the starch sedimentation tanks. It is also observed that the wastewater

generation on an average ranges between 80-88% of the water consumption. A recent

report showed as is evident for enormous

wastewater discharged from a CSP factory having small or medium-scale that

processing of 250-300 ton of cassava tuber

(during a day) results in liquid waste about 2655m3 including wastewater with about 1%

sohd3).

WASTEWATER CHARACTERISTICS

Along with the large quantities of the water consumption, cassava starch industry


involves a considerable load of the pollution

compounds in the effluent. Table 4 shows the

physical and chemical characteristics of the combined wastewaters determined by various authors. The wide variation in chemical

composition of the CSP wastewater is also observed. Generally, the combined waste-

water is acidic by nature, its pH ranging from 3.4 to 5.6. The release of hydrocyanic acid during the extraction process of starch from

cassava tubers has been reported as the cause of the acidic nature of effluent. In addition, due to a longer detention period,

fermentation takes place because of the rapid chemical changes resulting in the production

of alcohol and organic acids. Some processing techniques use sulfuric acid in the starch

purifying purpose as above mentioned, and this is also a reason causing the decrease in effluent pH level.

The CSP wastewaters are highly organic but have relatively low nitrogen and

phosphorus concentrations. The COD concentration commonly ranges between 4,000-10,000mg/l. However, COD value is much over 20,000mg/l in few cases. Similarly, the effluent contains the high value of BOD5 concentration being typically in the range of 3,000 to 7,000mg/l, sometimes as high as BOD5 of over 13,000mg/l. The ratio of soluble BOD5 to soluble COD in the cassava starch wastewater estimated is 0.6-0.8, indicating that the wastewater from CSP factories has, although, a high organic but it is easily biologically degradable. It is likely that the biological treatment methods will be most economical for these organic wastes. The total solids in the wastewater are about 5,000mg/l of which 60-80% are total dissolved solids and 20-40% are the total suspended solids. The presence of colloidal starch particles may be resulting in the increasing in total solids content.

During processing operation such as

Table 4 Physical and chemical characteristics of the cassava starch processing wastewater

(a)Large-scale;(b)Small-scale;(c)Values were determined by dichromate method


storage, rasping, screen, soaking etc., hydrocyanic acid liberated from hydrolysis of

cyanide glucosides by enzyme linamarase

present in plant: a process known as cyanogenesis. Because a large quantity of

water is used in the starch extraction

process, hydocyanic acid, which is highly soluble in water, enters the wastewater. The cyanide concentration present in cassava

starch wastewater is in the range of 3.2-6.7mg/l(Table 4). The last few years, however, there have been reports indicating the

seriousness of cyanide content in the CSP wastewater. Results given from this reports

showed that the concentration of cyanide at

point of discharge to environment from the cassava starch factories ranged between 10.4 to 27.4mg/l2). Concentration of acetone cyanohydrin and free cyanide levels in most cases were low when compared with the total

cyanide. This indicates that the bound cyanide is the major cyanide fraction in the wastewater. The large amounts of

wastewater containing the high cyanide concentration and organic loading will be a menace to the quality of life in rural areas

where the factories are located. An assessment for cyanide accumulation in

environment has been carried out and its result showed that the total cyanide concentration in the samples of ground water

sources near cassava starch factories ranged between 1.2mg/l to 1.6mg/l,2) which is much

higher than the acceptable level.

TREATMENT PROCESSES OF THE

CSP WASTEWATER

The researches for treating the CSP

wastewater have just seemed to be concerned in the last few years. A number of treatment

processes of CSP wastewater are listed in Table 5. The treatment processes have been

proposed including the physicochemical and biological processes, individually or in a combination. Physicochemical treatment

process was used to remove suspended solidand organic content from wastewater in some

extent. Two options have been considered including simple settling and physico-

chemical treatment. The wastewater was subjected to simple settling, in partly, suspended solid and BOD was removed. The

Table 5 Physicochemical and biological processes involved in the treatment of cassava starch processing wastewater

(a)HRT:Hydraulic retention time(b)Unit:g/m2.day

(c)Feedstock: CSP wastewater containing 6,2% total solid was amended urea to give a C:N ratio approximately 30:1(w/w)(d)AFFFBR:Anaerobic fixed film fixed bed reactor

(e)UASB:Up-flow anaerobic sludge blanket


physicochemical treatability studies were carried out using lime, alum, ferric chloride and ferrous sulphate. It is proved that the use of lime is favourable for removing suspended solid and BOD by coagulating and neutralizing the wastewater15). However, due to its low efficient, therefore physicochemical treatment process often plays the primary treatment role for reusing the tuber wash water or before subjecting the effluent to biological treatment.

Various types of the biotreatment processesand their improvements have been reported.

An aerobic treatment process in term of modified rotating biological reactor(RBC), where the discs were attached porous sheets to enhance biofilm area, has been used for the treatability studies of the CSP waste-waters. The COD removal was in the range of 97.4% to 68% depending on the influent COD concentration44). This process allowed hydraulic retention time (HRT) short in therange of 4.25-1.43h. A bench scale aerobic reactors (activated sludge process)has been designed for treating wastewater of cassava meal processing industry. Resulting in removal percentage of COD was more than 90% and ofthat of CN- was in the range of 97-99% with the initial concentration of the

COD and tie CN- up to 2000mg/l and 14.4mg/l, respectively31,51).In the anaerobic digestion process, the

possibilities of utilizing cassava starch factory effluent by biomethanation were investigated in batch digester and semi-continuous digesters of which efficiencies in the COD removal were of 63% and 50% with HRT of 60 days and 33.3 days, respectively54). The interest for cyanide treatment has also occurred in the investigations of the single-step methane reactor52) and fixed bed

methanogenic reactor53). Removal efficiency of cyanide has been achieved up to 99% and with fixed bed methanogenic reactor removal efficiency of COD has shown at 90%. The treatment of the CSP wastewater has also suggested in a fixed film fixed bed (AFFFB) reactor15). This study indicated the reduction of organic content(BOD)from 3100mg/l to less 250mg/l. Its two stage systems have

given an overall BOD reduction of around 92% with an HRT of around 12h. However,

since the CSP wastewater is deficient in

nitrogen and phosphorous, thus urea and super phosphate have been added in requisite amount to supplement nutrients. The upflow

anaerobic sludge blanket(UASB)processes for treatment of the CSP wastewater have been much attended. In these processes,

cyanide removal greater than 95% has achieved for influent cyanide concentration up to 25mg/l with synthetic CSP samples49).

The UASB process has also yielded COD removal of 90% to 95% with the initial COD

concentration and the COD loading rate up to 24,000mg/l48) and 40kg/m3.day,40) respectively.

In recent years, the minimization of

pollution compound in the CSP wastewater has also been carried out in the respects of by-

product of yeast and ethanol production. The free sugars and the sugars formed from amylolytic organisms in the CSP factory

effluent can be used as substrates for the

generation of microbial biomass rich in protein(single cell protein). A yeast, Candida tropicalis47) or a mixed culture of Candida utilis and Endomycopsis fibuliger,45) efficiently and rapidly utilizing both starch

and free sugars has been chosen for treatment study of cassava starch factory effluent. The results of mixed cultivation of

the yeasts (C. utilis and E. fibuliger)have suggested that about 28h was needed for

efficient conversion of carbohydrate into microbial biomass and during this period around 79% starch was hydrolyzed,22%

(w/w) of the protein biomass content was formed, and the CSP wastewater treatment has removed 94% of the COD and 91% of the

BOD45). The conversion of waste into single cell protein, a valuable product, not only reduces the pollution load but also the

treatment costs.

SOLID WASTES

During the cassava starch industrial

production, a huge quantity of solid waste including peel and bagasse (pulp) is derived.

It has proved that the total quantity of peels and bagasse generated are in the range of 2-3% and 15-20%, respectively, of the tubers

processed15). The peels contain high levels of nutrients, which could be important for

animal feed production. The composition of

126 Japanese J. Wat. Treat. Biol. Vol.38 Na.3

cassava peels has shown, including in the range of 28-38% of starch,8-11% of crude fibers, and 0.9-1.1% of protein. Compared to other feedstuffs, however, cassava peelings are not too impressive, particularly for

poultry. This is primarily because of the high toxicity of the peel. It has been reported that the peels contain extremely high levels of cyanogenic glucosides (Table 3). In theextraction process, a large amount of bagasse is generated as a solid residue containing fibrous material of the tubers. The chemical composition of cassava bagasse is shown in Table 6. The collected results were analyzed by various authors. The composition shows a variation being may due to the difference in the crop varieties used. The cassava bagasse also contains residual starch, which is estimated about 60% of starch on a dry weight basis that was not extracted. In this, starch is determined as carbohydrates. The cassava bagasse is poor in the protein content. This makes it unattractive as an animal feed3). However, it has a numerous advantage such as the low ash content, the ease in attack of microorganism, in comparison to other raw agro-industrial matter. It has been considered as a rich solar energy reservoir due to its easy regeneration capacity.

In the production of cassava starch, a large amount of solid waste is generated simultaneously. The peels and bagasse possessed the high moisture. In addition due, in part, to the processing practice, these factors combine to create a difficult drying

process that is bolt inefficient and expensive. Poorly dried or fresh pulp, which spoils rapidly in the humid warm tropical areas as

microorganisms quickly multiply on the substrate high in nutrients, is a serious concern to the environment20). The recovery

and modification of wastes are becoming increasing important. In recent years, there

has been an increasing trend towards more efficient utilization of agro-industrial residues such as cassava peels and bagasse.

Several bioprocesses have been developed that utilize these as raw materials for the

production of bulk, chemicals, biogas and value-added fine products such as ethanol,

organic acids, amino acids, enzymes, single cell protein, mushroom, etc.3,57-59) The aim of the application in bioprocesses, mainly in the

area of enzyme and fermentation technology, on the hand utilizes more completely the raw material on the other hand minimizes the

pollution.

MICROORGANISMS INVOLVED IN

DETOXIFICATION OF CASSAVA

PRODUCTS

Several methods are available for cyanide

removal or detoxification, such as natural degradation, hydrogen peroxide process, sulphur dioxide process, alkaline chlorination

process, biological oxidation, and ozonationetc16). Natural degradation, alkaline chlori-nation, and oxidation with hydrogen peroxide

are the most often used methods in full-scale operations. However, there are technical and economical concerns, related to these

methods that make biological process viable in some cases. There have been several reports on microorganisms able to use

cyanide as nitrogen source,21) for instance, Pseudomonas fluorecens. The ability of microorganism to grow on cassava cultivation

Table 6 Chemical composition of the cassava bagasse(Expressed by g/100g dry weight)


in the presence of cyanide has been studied, and feasibility of some bacteria, yeasts, and fungi to detoxify cyanide was identified. Table 7 cites some microorganism is capable

of producing linamarase for detoxification of cassava products.

One of the bacterial strains(Lactobacillus

plantarum), which have been encountered in fermentation process of cassava

tubers,11,36,60-63) isolated from cassava retted and MRS medium glucose was replaced by

2% cellobiose, was demonstrated the ability of certain strains of lactic acid bacteria to breakdown a large amount of cassava

linamarin32). It was also shown that the linamarin has converted into lactic acid and

acetone cyanohydrin by L. plantarum A6. To ameliorate garri quality through micro-biological means i.e. to further reduce any

remaining linamarin in the mash, and simultaneously introduce lysine, an essential amino acid serious lacking in starch foods,

while retaining the usual flavor of garri, the used microorganisms were L. delbruckii, L.

corymeformis, that were associated with the fermentation of cassava, including flavor

production. These strains were found among 214 isolated from cassava processing

environments to be the highest producer. A report for another strain of bacteria showed that the nitrile hydratase activity of

Breuibacterium sp. R312 could be useful for the detoxification of cassava during fermen-

tation of cassava pulp. Fermentation of cassava with Brevibacterium sp. R312 reduced the cyanide content by 70-80%10)

In the culture conditions, sTach as glucose-

yeast extract-mineral salts broth containing cassava mash similar to bacteria L.

delbruckii and L. corymeformis, the yeast strain Sacchromyces sp. has been shown to

produce highly linamarase, amylase, and lysine64).Avariety of filamentous fungi were shown

as predominant as the ability in the detoxification of cassava. Sources of fungal linamarases were released from strains such

as Aspergillus sp., Penicillium sp., and

Table 7 Microorganisms involved in detoxification of cassava products

(*)MRS:Man Rogosa Sharpe-amedium for the cultivation of lactobacilli.


Fusarium sp., when were grown on a liquid medium containing glucose and cassava-tuber extract. In the cultures of F.oxysporum, the linamarase having highest affinity to linamarin have been found65).

Microbial linamarase was also found to be

produced in some microorganisms isolated from the soil in a cassava farm. Aspergillus oryzae strains including SA1 are one of these cases. It was proved that A. oryzae SA1

possesses potential capacity in the detoxifi-cation23). Further, it has been confirmed that the Aspergillus strains are negative to aflatoxin production. Thus, cassava products detoxified by this fungus can be used safely as food and feed. These Aspergillus cells grow during the process may contribute to the protein enhancement of cassava. Another report used a strain of Aspergillus niger, B-1, cultured in medium containing bran. It was devised that A. niger B-1 caused a large degrease in cyanide content (95%) in a controlled fermentation of cassava66). Simi-larly, in here fermentation of cassava by A. niger B-1 increasing its protein content was also obtained. Therefore, not only did the nutritional value of cassava in increase, but the ability of body to detoxify cyanide originating from the cassava was also improve.

Most of the fungal pathogens of cyanogenic

plants are tolerant to HCN. Rhizopus oryzae, amucoraceous fungus associated with the

postharvest spoilage of cassava tubers was found to effectively metabolize cyanide. The study indicates that R. oryzae has potential use in the processing of cassava feed and food

preparations as well as in the effective disposal of industrial wastes that contain high levels of cyanide70). Among micro-organisms(filamentous fungi)were isolated from traditionally heap-fermented cassava as R. oryzae, R. stolonifer, and Neurospora sitophila,67) N. sitophila reduced cyanogenic

glucoside levels most effectively in cultivation of matt extract agar, followed by R. oryzae and R. stolonifer. Whereas, Rhizopus spp. was predominant for reducing cyanogenic

glucoside levels when it were cultivated in medium of yeast nitrogen base adding 1% agar and 5% glucose68, 69)

MICROORGANISMS INVOLVED IN RAW

CASSAVA STARCH CONVERSION

Most agriculture biomass containing starch

can be used as potential substrate for the

production of gaseous of liquid fuels, feed protein, and chemicals by microbial process71). Many microorganisms are capable of using cassava starch as a substrate for growth and

conversion into high-value products insubmerged or solid state fermentation

processes. Table 8 shows microorganisms involved in the capacity of raw cassava starch digestion.

In recent years, fermentative production of ethanol from renewable resource has received

attention due to increasing petroleum shortage. Ethanol production by bacteria Zymomonas mobilis has been studied extensively. It is capable of metabolizing few

types of sugars such as glucose, fructose and sucrose. A high ethanol concentration was observed using co-immobilized cells of

amylolitic yeast Saccharomyces diastaticus and Z. mobilis from liquefied cassava starch3). Ethanol production from raw cassava starch

by a nonconventional fermentation method has been reported on an industrial scale by the use of fungal saccharifying agents in a

cost-effective process. Recently, studies shown that the enzymatic hydrolysate of raw cassava starch and cassava bagasse can be

used as the sole carbon source to produce ethanol by submerged fermentation using

several Rhizopus strains. The Rhizopus strains were found with high efficiency of ethanol such as, R. arrhizus MULL 16179, R.

circicans NRRL 1475, R. delemar MULL 28168,R. oryzae NRRL 395, Rhizopus sp. NRRL 25975, Rhizopus sp. MB4672). Rizopus

sp. MB46 has been selected as a producer of raw cassava starch-digestive glucoamylase in the liquid culture73). The alcohol fermentation

of raw cassava starch has also done without cooking by using only wheat bran koji from

Rhizopus strains90).Flavor and aroma compound synthesis by

biotechnological processes nowadays plays an

increasing role in the food, feed, cosmetic, chemical, and pharmaceutical industry. The

cassava bagasse substrate was shown to be adequate substrate for the growth and aroma


(a)SmF: Submerge fermentation, SSF: Solid substrate fermentation

(b)SCP: Single cell protein


production by the fungus Ceratocystis fimbriata3). A strain of the yeast Kluyveromyces marxianus was also used for the production of a fruity aroma in solid state fermentation using cassava bagasse as

substrate74).Enzymatic hydrolysis of starch is usually

carried out in a two-step procedure including liquefaction and saccharification. A study on

the use of Bacillus licheniformis and Aspergillus niger in the conversion of cassava starch showed the high efficiency of the

formation dextrose with pure cassava starch and glucose syrup with cassava meal19,75). From Aspergillus sp. N2, some studies have

been undertaken to develop a process for

production of glucoamylase, which had high activity of raw starch-digestion at low acidic

pH and high temperature, and the activity was thermo-tolerant and able to digest a high

concentration of raw cassava starch76). An Aspergillus niger strain has been reported

previously that raw starch digesting amylolytic enzymes are produced during the

growth on low grade cassava starch91).Among the various products produced

through microbial cultivation on cassava bagasse, organic acids are important ones. Among these, citric acid production has been

well studied and reported. With a culture of Candida lipolytica3), cassava bagasse hydrolysate was used for the production of

citric acid. More significantly production is obtained with Aspergillus niger.It has been

recognized that cassava bagasse best supported the mould's growth, giving the highest yield of citric acid among the

substrates3). Another important organic acid

produced using cassava bagasse hydrolysate is fumaric acid. The recent studies showed the media containing cassava bagasse could be used as sole carbon source for Rhizopus to

produce fumaric acid72,77).Yeast was among the microorganisms

considered as potetial protein sources using cassava starch. Schwanniomyces castellii

grew fast and produced great biomass and high protein when it was grown on cassava starch as sole carbon source78). The use of

microorganisms for the production of dietary

protein from carbohydrates has been extensively reviewed. Among the fungi,

Rhizopus, Aspergillus, and Neurospora have been found capable of increasing the protein

of cassava, particularly in solid state fermentation from raw cassava starch79-81). Rhizopus spp. are edible filamentous fungi,

employed for thousands years for preparing fermented food. It have been indicated that

Rhizopus fermentation results in protein enrichment and improved digestibility of foods without the production of toxic

products, such as flatoxins77). Several studies have been carried out for using cassava peel

in animal feeds and in production of single cell protein, by solid state Rhizopus sp. fermentation82,83).

SUMMARY

The starch processing of cassava generated a large quantity of solid waste and wastewater. Solid waste includes peels and

pulp(bagasse). The peels contain high levels of nutrients, which could be important for animal feed production. The cassava bagasse also contains residual starch, which is estimated about 60% of starch on dry basis. Efforts should be made for improving further cassava bagasse utilizations. The application of microbial treatment for cassava bagasse may open new ways for converting to high value products and may expand the range of agricultural wastes that can be used as substrates for microbial processes.

Tong with the large quantities of the water consumption, cassava starch industry involves a considerable load of the pollution compounds in the effluent. The CSP wastewater generated mainly from the tuber washing stage and the separator sections. The COD and BOD concentrations of effluent are typically in the range of 4,000-10,000mg/l and 3,000-7,000mg/l, respectively. The combined wastewater is highly acidic. One of the most important problems related to cassava starch industry is its high cyanide content. It has been estimated to be in the range of 10.4-27.4mg/l. Effluent from CSP factories reliably exposed potential pollution

problems.The BOD/COD ratio indicates that the CSP

wastewater can be classified as amenable to biological degradation. Several solutions focusing mainly on physicochemical and


biological processes have been suggested to treat wastewater of cassava starch

processing. However, microbial treatment processes possessing the potential advan-tages have been not much attended.Avariety of microorganisms have been

reported to play an important role in an array of the use of raw starch residues and cyanogenic compound as the substrate for the

growth and conversion. Filamentous fungi seem, however, to be abundant and pre-dominant in capacity of detoxifying cyanide and digesting raw cassava starch.

Sensible development and suitable option of efficient microbial strains, mainly fungal cultures, for bioconversion and utilization of cassava starch industry effluent is still a largely unexplored area.

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