**3. Environmental factors**

*New Advances on Fermentation Processes*

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

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

**Microorganism Substrate Product References**

root

peels

peels

pulp

pulp

root

peel

starch and leaves

Cassava root

Cassava leaves

Cassava flour

Gari [29]

Bread [65]

Animal feed [66]

Poultry feed [67]

Animal feed [68]

Animal feed [43]

Cassava flour [71]

Feed supplements [72]

[69]

[70]

[73]

Lactic acid production

Cellulase production

Lactic acid and ethanol productions

fermentation process on the protein content of cassava leaves.

products for food and feed applications are summarized in **Table 3**.

*Rhizopus oryzae* Cassava

*Panus tigrinus* (M609RQY) Cassava

*Aspergillus niger* and *Panus tigrinus* Cassava

*Rhizopus oryzae* and *Saccharomyces cerevisiae* Cassava

*Rhizopus oryzae* Cassava

*Lactobacillus plantarum* and *Rhizopus oryzae* Cassava

*Rhizopus stolonifer LAU 07* Cassava

*Rhizopus* sp. Cassava

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

*Bacillus* sp.*, Mucor racemosus, R. oryzae, Neurospora sitophila R. stolonifer* and *Geotrichum candidum*

*Rhizopus oryzae* (TISTR 3052), *Rhizopus oryzae* (TISTR 3058), *Rhizopus delemar* (TISTR 3534),

and *Rhizopus delemar* (TISTR 3190)

*Saccharomyces cerevisiae, Aspergillus niger, Rhizomucor miehei,* and *Mucor strictus*

**56**

**Table 3.**

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 be controlled.

#### **3.1 Water activity and moisture content**

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 between 60 and 80% for an efficient SSF system.

#### **3.2 Temperature**

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 was low conduction efficiency [82, 83].

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 fermentation is to combine ventilation and humidity [77].

#### **3.3 Oxygen concentration**

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

assessments to vary. Ghildyal et al. [87] studied the impact of the gas concentration gradient on product yield in a tray solid-state fermentation bioreactor. The results showed that the variations of O2 and CO2 concentration gradients were visible, which severely affected product yield. The yield decreased when gradient increases. Gowthaman et al. [88] also studied the impact of gas concentration gradient on the product in a packing bed bioreactor. The results showed that the gas concentration gradient could be eliminated and the ability of mass transfer can be enhanced by forced ventilation, which increased enzyme activity.

### **3.4 pH value**

In general, if the initial pH value of the medium is adjusted, the variations of pH value during the solid-state fermentation process need to be considered [89]. During the fermentation process, the pH values change drastically. The reason is that organic acids including citric and lactic are secreted during the fermentation process, which decreases the pH [23]. While the increase in pH was rationalized in terms of organic acid decomposition and protein degradation in the raw materials into amino acids and peptide fractions [90]. The pH values are difficult to determine by conventional detection in SSF due to the low water content of the substrate. Nitrogen-containing inorganic salts (such as urea) are often used as sources of nitrogen to offset the pH variation in the fermentation process [91, 92].

In the study conducted by Ezekiel and Aworh [13] to evaluate the effect of pH on protein enrichment and soluble sugars of cassava peel by *Trichoderma viride*, the fungus was grown in a controlled pH medium of 4.– 6.0 with an incubation time of 8 days. The optimal growth condition was observed at pH 6.0. The protein increased in cassava peels from 230 at pH 4.0 to 270 (mg/gm) at pH 6.0. Also, the sugars yield at pH 5.0 and 6.0 was five times higher compared to pH 4.0. According to the study, the growth rate of the fungi at pH below five was affected by high acidity, leading to reduced bio-conversion of sugars into protein.

## **4. Conclusions**

The results discussed in this chapter highlighted the importance of the SSF technique applied to cassava to improve its nutritional value. The solid-state fermentation using microbial protein is beneficial for the reduction of cyanide contents while the content of protein and other nutrients is increased compared to those obtained by the conventional approaches, i.e., soaking, boiling, and drying. Thus, the SSF technique for processing cassava products is better suited for developing societies and rural communities in the African and Asian countries that do not have easy access to available protein sources.

## **Acknowledgements**

The authors are thankful to the Ministry of Research, Technology and Higher Education of the Republic of Indonesia for its financial support to this project through the grant no. 849/PKS/ITS/2018.

**59**

**Author details**

Surabaya, Indonesia

Mohamed Hawashi, Tri Widjaja and Setiyo Gunawan\*

provided the original work is properly cited.

\*Address all correspondence to: gunawan@chem-eng.its.ac.id

Department of Chemical Engineering, Institut Teknologi Sepuluh Nopember (ITS),

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

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

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

### **Conflict of interest**

The authors declare no conflict of interest.

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