*2.1.3 Cassava leaves*

*New Advances on Fermentation Processes*

pre-processing of cassava peels.

4.2 to 10.43% at optimized conditions.

et al. [12].

**Figure 5.**

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

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

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

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

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

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

**54**

content by 55.16%.

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 into the human body upon consumption.

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


#### **Table 2.**

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

maximum nutritional content is challenging and still far away from being established. Among the efforts made so far, Morales et al. [61] proposed a solid-state fermentation of cassava leaves, reducing the cyanide content while improving the nutritional value of the processed leaves. SSF was performed using *Rhizopus oligosporus*, and babassu mesocarp flour was the substrate used, supplemented by cassava leaf flour. The solid-state fermentation decreased the total cyanide content of the cassava leaves by 94.18%, also SSF increased the quantity and quality of crude protein content by 15%, resulting in the relative nutritional value of 98.18% for food, which is equivalent to casein (100%). Furthermore, Kobawila et al. [62] investigated the effect of alkaline fermentation on the reduction of cyanide level in cassava leaves to produce ntoba mbod. The dominant microflora in the fermentation of the cassava leaves was *Bacillus subtilis, Bacillus macerans,* and *Bacillus pumilus*. These bacteria can utilize cyanide acid for their nutrition [63]. Thus, they are responsible for the reduction of the cyanide content in the medium of fermentation (~70% removal). However, the report did not provide the effect of the fermentation process on the protein content of cassava leaves.

One of the essential criteria for the solid-state fermentation is the selection of an appropriate microorganism [64]. Several research works have explored different types of microorganisms mainly fungi, yeasts, and bacteria, as well as different substrates to favor the metabolism of the microorganisms in SSF of cassava products. Examples of microorganisms associated with solid-state fermentation of cassava products for food and feed applications are summarized in **Table 3**.


**57**

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

The process control of the solid-state fermentation parameters is closely related to the metabolic regulation of microorganisms [74]. Based on the metabolic needs of the fermentation microorganisms, the control of water activity, oxygen content, temperature, and pH are the main solid-state fermentation parameters [23]. In the solid-state fermentation process, the water, gas, and heat caused by the growth microbes are the dominant factors that determine the environmental changes. The environmental factors can affect the microbial growth and formation of the product during the SSF process [75, 13]. Therefore, the physical-chemical parameters must

The unique feature of solid-state fermentation is that there is almost no free water in the substrate [76]. However, microorganisms can grow depending upon the water activity of the substrate [64, 75]. The growth of fungi and some yeast usually requires a water activity value between 0.6 and 0.7 [77]. In addition to meet the microbial physiological requirements, the water content level plays a decisive role in the variation of the three-phase structure relating to water retention, permeability, and thermal conductivity. The degree of swelling in the SSF system was low at a lower moisture level and hence increased water stress reduces nutrient solubility. On the contrary, the higher level of humidity results in changes in substrates that reduce porosity, thus contributing to stickiness and reduced gas exchange [78, 79]. According to Grover et al. [80], the required moisture content should range

The fermentation temperature affects microbial growth, spore germination, and the formation of product [81]. Heat generation in solid-state fermentation system is more problematic than in liquid fermentation. Due to poor heat conductivity and accumulation of metabolic heat in the material combined with substrate shrinkage and decreased porosity, gas convection is severely impeded. Previous studies showed that the significant resistance to heat transfer in solid-state fermentation

Therefore, moisturizing is a common measure of temperature control. In addition, routine operations (e.g., forced ventilation and jacket cooling) all can solve these problems [84]. The evaporative cooling is one of the main solid-state fermentation temperature control measures [85, 86]. In general, the aeration could reduce the temperature gradient of the medium [23]. The forced ventilation can take away more than 80% of the heat generated from the substrate [84]. From the current investigation, it is difficult to maintain the temperature at an ideal range in SSF system. To reach this aim, the main strategy used in large-scale solid-state

The gas environment is a critical factor that significantly affects the relative levels of biomass and the production of an enzyme [23]. Oxygen uptake rate (OUR) and carbon dioxide production (CDPR) can be used to assess the state of the solid-state fermentation process. However, different microorganisms cause these

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

**3.1 Water activity and moisture content**

between 60 and 80% for an efficient SSF system.

was low conduction efficiency [82, 83].

**3.3 Oxygen concentration**

fermentation is to combine ventilation and humidity [77].

**3. Environmental factors**

be controlled.

**3.2 Temperature**

#### **Table 3.**

*Examples of microorganisms associated with the SSF of cassava products.*

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