**1. Introduction**

Cassava (*Manihot esculenta Crantz*) is grown in tropical and subtropical countries. It is a vital source of food and feed and it can promote economic development and provide food security [1]. Cassava production has been promoted globally by the International Fund for Agricultural Development (IFAD) and the United Nations Food and Agriculture Organization (FAO) to develop cassava strategies [2]. Reports indicate that production rates will reach 300 million tons per year by 2020 [3]. Due to its high drought tolerance, cassava plant cultivation can take place even under critical environmental conditions, with an ideal high yield of approximately 50% for leaves and 6% for roots at plant maturity [4]. Its peel may make up 10–20% of the roots' wet weight [5]. However, cassava has some disadvantages; its tissues contain anti-nutritional compounds and very low protein content [6, 7].

Among all the antinutrients, hydrogen cyanide (HCN) is of great concern, the concentration of which is in cassava and its by-products are much higher than the World Health Organization (WHO) safe limit for human consumption (10 ppm) [8, 9]. Konzo is an irreversible neurological disease associated with intake of HCN [10]. Therefore, a detoxification process is needed to reduce anti-nutritional levels in order to consume cassava safely. Solid-state fermentation (SSF) has been used as an economical and efficient processing method for enriching and detoxifying cassava and its by-products [11, 12]. Various process parameters such as particle size, moisture content, water activity, pH, the inoculum size, incubation time, concentration of nutrient supplementation, and temperature can affect the microbial growth, enzyme production, and formation of the product during the SSF process [13].

This chapter discusses fermented cassava products through solid-state fermentation for food and feed applications, as well as microorganisms involved in solid-state fermentation and the essential processing variables used to optimize the process.

### **2. Fermentation processes**

Fermentation has been one of the most used technologies to improve the taste and sensory properties of food and continues to be one of the most widely used methods of preserving the food for a length of time [14, 15]. The cassava fermentation process is a strategy to improve nutritional value by enriching protein and detoxifying toxic and anti-nutritional compounds, in particular by reducing toxic cyanogenic glycosides to a safe level of consumption in cassava products as well as reducing post-harvest losses [16–18].

There are two kinds of fermentation, i.e., spontaneous (natural) fermentation and controlled fermentation. For the natural fermentation, the conditions are selected so that to produce the most suitable microorganisms for the production of growth by-products characteristic of a particular type of fermentation [19]. The controlled fermentation is generally used when the natural fermentation is unstable or the bacteria are not able to grow. In this case, specific microbial strains, such as lactic acid bacteria (LAB), yeast, and fungal are isolated, characterized, and preserved for later use as starter cultures [20]. Under optimal growth conditions, these cultures can be used as single or combined starter cultures. As a result, the quality of products and their organoleptic characteristics are well controlled and predictable [20, 21].

However, the fermentation process can be broadly categorized into submerged fermentation (involving soaking in water) and solid-state fermentation (without soaking in water) [22]. The solid-state fermentation (SSF) technique has several advantages over submerged fermentation (SmF). However, the SSF has some constraints. **Table 1** illustrates the advantages and disadvantages of SSF over SmF [23].

#### **2.1 Solid-state fermentation and its application in cassava products**

In recent years, the cassava population has developed numerous processing methods (soaking, boiling, drying, and fermentation) [24–26]. SSF is one of the promising processes of enriching protein and detoxifying of cassava products [27–29].

Fermented cassava products by SSF, such as flour, gari, starch, bread, and biomass contain high protein content that can either be consumed by humans or animals, replacing expensive, conventional protein sources in different parts of Latin America, Africa, and Asia [30]. The major fermented cassava products by SSF can be derived from different parts of the cassava plant, such as roots, peels, and leaves.

#### *2.1.1 Cassava roots*

Cassava is grown in many developing countries for its roots as a primary source of carbohydrates and ranks third in the developing countries as the leading source

**51**

**Table 1.**

*Solid-State Fermentation of Cassava Products for Degradation of Anti-Nutritional Value…*

cellulose, pectins, lignin)

**Parameter Solid-state fermentation Submerged fermentation**

Temperature Difficult temperature control Easy temperature control Water Low water consumption High water consumption pH control Difficult pH control Easy control of pH

designed equipment is needed

Inoculation Spore inoculation, batch process Easy inoculation, continuous

Energy Low consumption of energy High consumption of energy

Pollution (effluents) No volumes of effluents High volumes of effluents

Concentration/products 100/300 g/L 30–80 g/L

Soluble substrates (sugars)

Sterilization of heat and aseptic control

The industrial level is available

Contamination risk of single

High volumes and high equipment costs

process

strain bacteria

of energy in human diets along with rice and wheat [31]. World production of cassava is estimated at 277 million tons of fresh root in 2017 [32]. Cassava root has several advantages compared to other crop roots, including high productivity, resistance to droughts and pests, flexible harvesting age, and it can be kept in the ground until they are needed [33]. However, cassava root also has certain disadvantages; its tissues contain toxic compounds (a cyanogenic glycoside), low protein content

Food processing techniques have been used to convert cassava tubers into flour as an alternative way to preserve the roots after harvesting and then further use it for industrial and traditional purposes [35, 36]. Gari and flour are the most popular fermented food products from cassava roots by SSF. In West Africa, approximately 200 million people consume gari [37, 38]. **Figure 1** shows the production of flour

The purpose of cassava root fermentation is to increase the low protein content from 2% to about 7% or more than the critical crude protein content [39]. To achieve this goal, several solid-state fermentation techniques have been used. Raimbault et al. [40] reported the principle underlying the SSF procedure for the enrichment of cassava flour. This procedure led to the enrichment of crude protein from 1 to 18–20%, which improved between 1700 and 1900% after 30 h of fermentation. Oboh and Elusiyan [41] studied the effect of solid-state fermentation by *R. oryzae* and *S. cerevisiae* on the improvement of nutritional values of cassava flour produced from two different varieties of cassava root. The nutritional contents of cassava flour were assayed before and after 72 h of fermentation. This study has observed that *S. cerevisiae* was more effective than *R. oryzae* in the nutrient enrichment of cassava flour. The results of this study are presented in **Figures 2–5**. Essers et al. [42] investigated the effect of SSF on the degradation of hydrogen cyanide level in cassava root using six fungal strains, namely *Rhizopus stolonife*r, *Rhizopus* 

(1% fresh root weight), and short shelf life of 1–3 days [34].

and gari under the solid-state fermentation [11].

*DOI: http://dx.doi.org/10.5772/intechopen.87160*

Substrates Insoluble substrates (starch,

Industrial level Relatively small scale, newly

Contamination Contamination risk of low-growth fungi

Equipment volumes Low volumes and low equipment costs

*Comparative characteristics of solid-state and submerged fermentations.*

Aseptic techniques Sterilization of steam and non-sterile conditions

*Solid-State Fermentation of Cassava Products for Degradation of Anti-Nutritional Value… DOI: http://dx.doi.org/10.5772/intechopen.87160*


#### **Table 1.**

*New Advances on Fermentation Processes*

**2. Fermentation processes**

reducing post-harvest losses [16–18].

predictable [20, 21].

process.

in order to consume cassava safely. Solid-state fermentation (SSF) has been used as an economical and efficient processing method for enriching and detoxifying cassava and its by-products [11, 12]. Various process parameters such as particle size, moisture content, water activity, pH, the inoculum size, incubation time, concentration of nutrient supplementation, and temperature can affect the microbial growth, enzyme production, and formation of the product during the SSF process [13]. This chapter discusses fermented cassava products through solid-state fermentation for food and feed applications, as well as microorganisms involved in solid-state fermentation and the essential processing variables used to optimize the

Fermentation has been one of the most used technologies to improve the taste and sensory properties of food and continues to be one of the most widely used methods of preserving the food for a length of time [14, 15]. The cassava fermentation process is a strategy to improve nutritional value by enriching protein and detoxifying toxic and anti-nutritional compounds, in particular by reducing toxic cyanogenic glycosides to a safe level of consumption in cassava products as well as

There are two kinds of fermentation, i.e., spontaneous (natural) fermentation and controlled fermentation. For the natural fermentation, the conditions are selected so that to produce the most suitable microorganisms for the production of growth by-products characteristic of a particular type of fermentation [19]. The controlled fermentation is generally used when the natural fermentation is unstable or the bacteria are not able to grow. In this case, specific microbial strains, such as lactic acid bacteria (LAB), yeast, and fungal are isolated, characterized, and preserved for later use as starter cultures [20]. Under optimal growth conditions, these cultures can be used as single or combined starter cultures. As a result, the quality of products and their organoleptic characteristics are well controlled and

However, the fermentation process can be broadly categorized into submerged fermentation (involving soaking in water) and solid-state fermentation (without soaking in water) [22]. The solid-state fermentation (SSF) technique has several advantages over submerged fermentation (SmF). However, the SSF has some constraints. **Table 1** illustrates the advantages and disadvantages of SSF over SmF [23].

In recent years, the cassava population has developed numerous processing methods (soaking, boiling, drying, and fermentation) [24–26]. SSF is one of the promising processes of enriching protein and detoxifying of cassava products [27–29]. Fermented cassava products by SSF, such as flour, gari, starch, bread, and biomass contain high protein content that can either be consumed by humans or animals, replacing expensive, conventional protein sources in different parts of Latin America, Africa, and Asia [30]. The major fermented cassava products by SSF can be derived from different parts of the cassava plant, such as roots, peels, and leaves.

Cassava is grown in many developing countries for its roots as a primary source of carbohydrates and ranks third in the developing countries as the leading source

**2.1 Solid-state fermentation and its application in cassava products**

**50**

*2.1.1 Cassava roots*

*Comparative characteristics of solid-state and submerged fermentations.*

of energy in human diets along with rice and wheat [31]. World production of cassava is estimated at 277 million tons of fresh root in 2017 [32]. Cassava root has several advantages compared to other crop roots, including high productivity, resistance to droughts and pests, flexible harvesting age, and it can be kept in the ground until they are needed [33]. However, cassava root also has certain disadvantages; its tissues contain toxic compounds (a cyanogenic glycoside), low protein content (1% fresh root weight), and short shelf life of 1–3 days [34].

Food processing techniques have been used to convert cassava tubers into flour as an alternative way to preserve the roots after harvesting and then further use it for industrial and traditional purposes [35, 36]. Gari and flour are the most popular fermented food products from cassava roots by SSF. In West Africa, approximately 200 million people consume gari [37, 38]. **Figure 1** shows the production of flour and gari under the solid-state fermentation [11].

The purpose of cassava root fermentation is to increase the low protein content from 2% to about 7% or more than the critical crude protein content [39]. To achieve this goal, several solid-state fermentation techniques have been used. Raimbault et al. [40] reported the principle underlying the SSF procedure for the enrichment of cassava flour. This procedure led to the enrichment of crude protein from 1 to 18–20%, which improved between 1700 and 1900% after 30 h of fermentation. Oboh and Elusiyan [41] studied the effect of solid-state fermentation by *R. oryzae* and *S. cerevisiae* on the improvement of nutritional values of cassava flour produced from two different varieties of cassava root. The nutritional contents of cassava flour were assayed before and after 72 h of fermentation. This study has observed that *S. cerevisiae* was more effective than *R. oryzae* in the nutrient enrichment of cassava flour. The results of this study are presented in **Figures 2–5**. Essers et al. [42] investigated the effect of SSF on the degradation of hydrogen cyanide level in cassava root using six fungal strains, namely *Rhizopus stolonife*r, *Rhizopus* 

**Figure 1.**

*The production chart of cassava products (flour and gari) under SSF.*

**Figure 2.**

*Proximate composition of the cassava flour obtained from cassava varieties of low HCN subjected to SSF.*

*oryzae*, *Mucor racemosus, Bacillus sp*. *Geotrichum candidum*, and *Neurospora sitophila*. The reduction in cyanide content was more than 60% after 72 h of fermentation.

In addition, Oboh and Akindahunsi [11] investigated the effect of solid-state fermentation with *S. cerevisiae* on the nutritional and antinutrient contents of cassava products (flour and gari). After 72 h of fermentation, the results revealed that the content of protein and fats in cassava flour increased by 10.9 and 4.5%, respectively. The protein and fat content of fermented gari also improved by 6.3% and 3.0%. In contrast, the content of cyanide in flour and gari decreased to 9.5 and 9.1 (mg/kg),

**53**

*2.1.2 Cassava peels*

**Figure 4.**

**Figure 3.**

*Solid-State Fermentation of Cassava Products for Degradation of Anti-Nutritional Value…*

respectively. However, the tannin content, crude fiber, and ash content of the cas-

*Mineral contents of the cassava flour obtained from cassava varieties of low HCN subjected to SSF.*

*Proximate composition of the cassava flour obtained from cassava varieties of medium HCN subjected to SSF.*

Cassava wastes, such as peels and leaves and starch residues make up 25% of the total cassava plant [43]. Cassava peel is the leading waste from the cassava plant, but its use is limited due to the high content of cyanide and fiber as well as low protein and therefore disposed of it after cassava processing into food or other industrial products [44, 45]. Many efforts have been made using SSF techniques to enrich the protein content and degrade the cyanide level of cassava peels for animal feed. Bayitse et al. [12] studied protein enrichment of cassava residue using *Trichoderma pseudokoningii* under solid-state fermentation for 12 days, urea, and ammonium sulfate was used as a nitrogen source, and the moisture content ranged from 60 to 70%. The result showed an improvement in crude protein content of 12.5% using urea as a nitrogen source, and a moisture content of 70%, as compared

sava products did not change significantly under SSF.

*DOI: http://dx.doi.org/10.5772/intechopen.87160*

*Solid-State Fermentation of Cassava Products for Degradation of Anti-Nutritional Value… DOI: http://dx.doi.org/10.5772/intechopen.87160*

#### **Figure 3.**

*New Advances on Fermentation Processes*

**52**

**Figure 2.**

**Figure 1.**

*The production chart of cassava products (flour and gari) under SSF.*

*oryzae*, *Mucor racemosus, Bacillus sp*. *Geotrichum candidum*, and *Neurospora sitophila*. The reduction in cyanide content was more than 60% after 72 h of fermentation. In addition, Oboh and Akindahunsi [11] investigated the effect of solid-state fermentation with *S. cerevisiae* on the nutritional and antinutrient contents of cassava products (flour and gari). After 72 h of fermentation, the results revealed that the content of protein and fats in cassava flour increased by 10.9 and 4.5%, respectively. The protein and fat content of fermented gari also improved by 6.3% and 3.0%. In contrast, the content of cyanide in flour and gari decreased to 9.5 and 9.1 (mg/kg),

*Proximate composition of the cassava flour obtained from cassava varieties of low HCN subjected to SSF.*

*Proximate composition of the cassava flour obtained from cassava varieties of medium HCN subjected to SSF.*

#### **Figure 4.**

respectively. However, the tannin content, crude fiber, and ash content of the cassava products did not change significantly under SSF.

#### *2.1.2 Cassava peels*

Cassava wastes, such as peels and leaves and starch residues make up 25% of the total cassava plant [43]. Cassava peel is the leading waste from the cassava plant, but its use is limited due to the high content of cyanide and fiber as well as low protein and therefore disposed of it after cassava processing into food or other industrial products [44, 45]. Many efforts have been made using SSF techniques to enrich the protein content and degrade the cyanide level of cassava peels for animal feed.

Bayitse et al. [12] studied protein enrichment of cassava residue using *Trichoderma pseudokoningii* under solid-state fermentation for 12 days, urea, and ammonium sulfate was used as a nitrogen source, and the moisture content ranged from 60 to 70%. The result showed an improvement in crude protein content of 12.5% using urea as a nitrogen source, and a moisture content of 70%, as compared

*Mineral contents of the cassava flour obtained from cassava varieties of low HCN subjected to SSF.*

#### **Figure 5.**

*Mineral contents of the cassava flour obtained from cassava varieties of medium HCN subjected to SSF.*

to 8.89 and 6.37% improvement observed with ammonium sulfate as a nitrogen source, and without using nitrogen source. The study observed a decrease in cyanide content, but it did not attribute it to the fermentation effect of *Trichoderma pseudokoningii*, rather it stated that the reduction could have been as a result of the pre-processing of cassava peels.

Iyayi and Losel [43] also evaluated protein improvement of cassava peels using different types of microorganisms and fermentation time (*Saccharomyces cerevisiae, Aspergillus niger, Rhizomucor miehei,* and *Mucor strictus*). The solid-state fermentation of cassava peels by *S. cerevisiae* produced the highest protein content from 5.6 to 16.74% for 21 days. Also, they reported the maximum fermentation period for the protein enrichment of cassava peel to be from 12 to 15 days, after which no significant change was observed, which is in line with the work reported by Bayitse et al. [12].

Ezekiel and Aworh [13] evaluated the effectiveness of SSF with *Trichoderma viride* on the reduction of cyanide content and enrichment of the crude protein content of cassava peel by optimizing the fermentation conditions such as moisture content, pH, particle size, nitrogen source, and incubation temperature. The optimum SSF conditions were found at the initial moisture content of 60% (v/w), the particle size of 4.00 mm, a pH of 6.0, 30°C of temperature, and ammonium sulfate (10 g N/kg substrate) as nitrogen sources. After 8 days of fermentation, the cyanide content was reduced by 71% and improved the crude protein content from 4.2 to 10.43% at optimized conditions.

In another study by Ruqayyah et al. [45], the application of response surface methodology was used to optimize SSF conditions (moisture content, inoculum size, and pH) with *P. tigrinus* to enrich the crude protein content of cassava peel. A maximum protein content of 89.58 (mg/g) was obtained at 75% (v/w) moisture content, 7% (v/w) inoculum size, and pH of 5.3 with a fermentation time of 15 days. The optimum level resulted in a significant enrichment of the protein content by 55.16%.

Oboh [46] investigated the effect of solid-state fermentation of cassava peel with a mixture of *Saccharomyces cerevisiae* and two strains of lactic acid bacteria, *Lactobacillus delbrueckii* and *Lactobacillus coryniformis* to improve the nutritional value and detoxification of cassava peel. The chemical composition of cassava peel has been analyzed before and after fermentation. The results showed the effective

**55**

**Table 2.**

*Solid-State Fermentation of Cassava Products for Degradation of Anti-Nutritional Value…*

performance of the SSF technique in removing cyanide by 86% after 7 days of fermentation. On the other hand, the mineral composition of the cassava peel did not change during the fermentation. The results of this study are presented in **Table 2**.

Cassava leaves are an extremely rich source of proteins, vitamins, and minerals that exceed some of the other green vegetables [47, 48]. The production of cassava leaves is estimated at 10 tons of dry leaves per hectare, which has a similar yield with the roots [49]. Cassava leaves are consumed in most Southeast Asian and African societies, such as Indonesia, Malaysia, Congo, Madagascar, and Nigeria [50, 51]. However, cassava leaves contain both nutritive (33.8–37.4% protein content) and anti-nutritional compounds [301.04–192.47 (mg/100 g) HCN content] [52]. Boiling, soaking, steaming, drying the sun, drying the oven, and cooking are the most common methods for processing cassava leaves in African and Asian countries [53]. The origin of HCN in the cassava leaves is a two-step process [54, 55]. First, the linamarin, a cyanogenic glycoside, which represent 93% of cyanogenic glycosides found in cassava (7% is lotaustralin), is hydrolyzed by linamarase (a beta-glycosidase) into glucose and cyanohydrin. Then, in the second step, the cyanohydrin is decomposed, either enzymatically or not, to HCN and acetone. The nonenzymatic pathway depends on pH. At pH > 6, the HCN is liberated, but at an acidic pH (~5), the process is much lower, and the resulting HCN is therefore relatively lower in concentration. However, this approach did not assure full hydrolysis of cyanogens. The partial breakdown of the leaf cells only partially releases linamarase resulting in only a certain proportion of the cyanogenic compounds being converted to HCN. This implies that a proportion of the cyanogens remain present in the leaves after processing and resulting in the release of HCN directly

The conventional methods have been proven to be ineffective for lowering the cyanide content in cassava leaves to the safe limit, at the same time causing a significant loss of protein and essential nutrients, which is highly desired from the cassava leaves [56–60]. Hence, the establishment of a universally acceptable method that produces edible leaves with low cyanide level while maintaining

Crude protein (%) 8.2 ± 0.1 11.1 ± 0.3 21.5 ± 1.2 Crude fiber (%) 11.7 ± 0.5 6.5 ± 0.5 11.7 ± 0.5 Fat (%) 3.1 ± 0.4 3.5 ± 0.2 2.1 ± 0.1 Ash (%) 6.4 ± 0.4 6.0 ± 0.2 7.2 ± 0.2 Carbohydrate (%) 64.6 ± 0.2 67.3 ± 0.4 51.1 ± 0.4 Moisture (%) 5.1 ± 0.3 5.7 ± 0.2 6.4 ± 0.4 Ca (ppm) 0.03 ± 0.00 0.03 ± 0.00 0.03 ± 0.00 Na (ppm) 00.04 ± 0.00 0.04 ± 0.00 0.04 ± 0.00 Zn (ppm) 0.01 ± 0.00 0.01 ± 0.00 0.01 ± 0.00 K (ppm) 0.05 ± 0.00 0.05 ± 0.00 0.05 ± 0.00 HCN (mg/kg) 45 ± 0.3 24 ± 0.2 6.1 ± 0.4

**fermented**

**Fermented with a mixed culture**

*DOI: http://dx.doi.org/10.5772/intechopen.87160*

into the human body upon consumption.

**Composition Fresh Naturally** 

*The effect of fermentation on the chemical composition of cassava peels.*

*2.1.3 Cassava leaves*

performance of the SSF technique in removing cyanide by 86% after 7 days of fermentation. On the other hand, the mineral composition of the cassava peel did not change during the fermentation. The results of this study are presented in **Table 2**.
