that eventually shift during the “batch cultivation” growth cycle are all constant; however, the experimenter may individually monitor and control them [73–75]. These attributes of the continuous culture method make it a desirable option for research while offering the industrial microbiologist several benefits in terms of more affordable production techniques (Figure 3.3).
Figure 3.3 Diagrammatic presentation of one stage and two stage fermentation process for biosurfactant production.
The continuous culture of microbes is performed in bioreactors called chemostats [76]. Chemostat is a type of bioreactor to which freshly prepared substrate is constantly supplied, while culture liquid comprising remaining nutrients, microbial end products, and microbe culture is continually withdrawn simultaneously at the same rate to maintain a constant culture volume. By adjusting the rate at which freshly prepared culture medium is supplied to the bioreactor, the specific growth rate of microorganisms can be effectively managed under limits [77, 78].
There may be several types of the continuous bioreactor, for example:
1 Auxostat: An auxostat is a method that uses inputs from a microbial growth chamber analysis to monitor the constant media flowrate, keeping the calculation at a constant level [79].
2 Retentostat: Retentostat is a modified form of chemostat in which a filtration assembly is linked to the effluent line and thus recycles the biomass to the bioreactor [19].
3 Turbidostat: A turbidostat is a continuous cultivation system with input on the optical density and dilution rate of the culture vessel [80].
3.5.2 Batch Processes
Batch fermentation is widely used in fermentation industries for the production of various microbial products, including vitamins, hormones, drugs, and secondary metabolites [81]. In the batch process, microorganisms and substrates are supplied into a bioreactor on a batch‐wise basis for product synthesis [82]. The batch approach is a simple way of conducting the fermentation and ensuring controlled environments inside a bioreactor. However, during the fermentation process, competitive changes may occur in microbial biomass, acid concentration, and byproduct concentration. The batch bioreactor consists of a mechanically agitated container with many other fittings, including gas sprayer, insulation jacket for regulating temperature shifts, pH meter, and air spargers [83, 84]. Despite the easiness of the batch process, batch fermentation incurs huge expenses and consumes a large amount of time spent on preliminary and post‐run operations, including bioreactors emptying, filling, and cleaning. In certain situations, such operations can take a much longer time than growing the microbial biomass [85].
3.5.3 Fed‐Batch Process
The batch process is a customized form of batch fermentation. It is the most popular mode of operation of fermentation in the bioprocessing sector. Microorganisms are inoculated and cultivated under the batch system for a period of time after the introduction of nutrients to the fermenter in order to feed them. Fermentation is interrupted only when the fermentation broth volume approaches 75% of the bioreactor volume [86]. In the fed‐batch systems, the steady feed flow of the media substrate enables the target secondary metabolites to achieve very high concentrations/levels. The benefit of this method of culturing is that the fed substrate level can be managed at the target level (very often relatively low) [87]. This will allow the prevention of undesirable changes, like changes responsible for substrate inhibition or changes in cellular metabolism at a high concentration of substrate. Also, fed‐batch systems can be applied when large amounts of biomass are required [88, 89].
3.6 Substrate for Biosurfactant Production
The application of different industrial byproducts/waste from agro‐industrial or industrial processes used for the production of biosurfactant agents is a cost‐effective solution for waste management [33, 84, 90]. Industrial production of biosurfactants, such as the petroleum industry, the sugarcane/syrup industry’s starch, the sugar industry’s byproduct residues, fruit and vegetable processing, distilleries, and slaughterhouse animal fat [91], are shown in Table 3.1.
Table 3.1 Summary of various agricultural and industrial byproducts used for biosurfactant production and respective producing microorganisms.
Source: Modified based on Bhardwaj et al. [92] and Banat et al. [93].
| Microorganism | Byproducts/Carbon sources |
|---|---|
| Pseudomonas sp. | D‐glucose/Molasses/Paneer ‐whey |
| Pseudomonas sp. | Vegetable oil/Rice water/Petroleum product/Milk whey |
| Bacillus subtilis | Glucose/Sunflower oil amended with unrefined petroleum oil |
| Bordetella hinizi strain DAFI | Sucrose/Molasses amended with unrefined petroleum oil |
| Trichosporon asahii | Diesel engine/Motor oil |
| Pseudomonas aeruginosa strain LBI, Acinetobacter calcoaceticus | Soapstock |
| Serratia marcescens | Glycerin |
| Candida sp. strain SY‐16 | Soyabean oil and D‐glucose |
| Pseudomonas aeruginosa strain SP4 | Palm oil |
| Rhodococcus sp. | Sucrose/Petroleum product/hydrocarbons |
| Bacillus subtilis, P. aeruginosa | Edible oil/Refined petroleum product |
| Pseudomonas aeruginosa strain J4 | D‐glucose/Refined petroleum product/Glycerin/Olive oil/Sunflower oil |
| Pseudomonas aeruginosa strain EM1 | D‐glucose/Glycerin/Sucrose/n‐Hexane/Soyabean oil |
| Pseudomonas aeruginosa strain SR17 | Cheese whey |
| Bacillus licheniformis strain KC710973 | Orange‐peel |
| Pseudomonas sp. strain NAF1 | Solid‐waste from dates and condensed fermented corn extractives |
| Pseudomonas cepacia strain CCT6659 | Waste frying rapeseed oil and condensed fermented corn extractives |
| Bacillus subtilis strain LAMI005 | Purified CAJ (cashew‐apple juice) |
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