BS production in liquid media is carried out with aeration and forced agitation. However, this causes problems when BS production starts because a large amount of foam is generated. Moreover, there is a tendency for microorganisms to accumulate within the foam, eliminating cells from the culture medium. Also, the presence of foam increases the risk of cross‐contamination and reduces the efficiency of oxygen transfer between the liquid and gas phase in the bioreactor [97]. The use of antifoams has disadvantages. These can be toxic to microorganisms, have high costs, and are extra‐components that must be separated from the BS during the purification processes [98].
Scientific literature about BS shows a clear tendency in the use of liquid fermentations over SSF, being batch processes at laboratory scale the most used methods. Nonetheless, recent studies, including our group in Mexico, point out that SSF is an advantageous alternative for BS production since oxygen transfers are efficient, and no foaming problems are seen during fermentations, especially when production yields are greater than 0.3 gBS/kgdr.
Our research group has some experience with the use of non‐pathogenic microorganisms for producing SL by SSF both at the laboratory and pilot level. Using glucose, vegetable oils, and solid supports of natural origin, production of BS in our hands has been monitored from respirometry studies using an analytical device patented by our group [99] without disturbing the culture. For example, Figure 2.6 shows the carbon dioxide production rate (CDPR) recorded in real‐time during BS production. This measurement is an invaluable tool for the process because it allows us to make decisions in real‐time. In this example, it is seen an imminent increase of CDPR (ca 1.2) followed by a rapid decrease (ca 0.8) that stabilizes in a plateau to drop off subsequently. This is the common behavior observed in many of our productive fermentation. In Figure 2.7, it is observed that O2 consumption rate is similar to CO2 production rate during the first days of incubation. After that, greater O2 consumption is observed with respect to CO2 production (Figure 2.7). This is reflected in a decrease in respiratory quotient (RQ, Figure 2.8).
Figure 2.6 CO2 formation rate (empty symbols) and O2 uptake rate (full symbols) during the production of SL in SSF.
Figure 2.7 Total CO2 formation (empty symbols) and O2 uptake (full symbols) during sophorolipid production in SSF.
The RQ (Figure 2.8) is less than 1.0 during the entire cultivation time, and the maximum value of the RQ (0.82 mol of CO2/mol of O2) is observed around 36 hours of incubation. These RQ values could be explained by the oxidation of glucose and suggest that, during the first days, fatty acids could be mainly used for synthesizing BS. Literature indicates that fatty acids are hydroxylated and incorporated directly into the synthesis of BS [100], whereas, after glucose depletion, fatty acids can be mainly used as energy source assimilated via β‐oxidation.
Figure 2.8 Respiratory quotient observed during SL production by SSF.
Figure 2.9 Evolution of pH during the production de sophorolipids.
Another parameter that can be easily measured to follow BS production is pH. The change of pH values over time is shown in Figure 2.9. pH values show a significant decrease during the first days of cultivation (from 6 to 3) and then remain. This behavior is similar in liquid fermentation [100]. The addition of alkali has been proposed to control the pH values around 3.5, which are optimal for sophorolipid production [101]. However, for SSF, it is not feasible due to the lack of homogeneity.
Figure 2.10 Kinetics for sophorolipid production (black) and substrates (○, □) uptake in SSF.
Figure 2.10 shows the consumption of hydrophilic and lipophilic substrates over time. Glucose consumption is recorded around the fifth day of cultivation; at this point, the consumption of the lipophilic substrate is around 65%. However, the lipophilic substrate is still consumed until the end of incubation, where consumption of around 90% was observed. It seems that lipophilic substrate remains as the only carbon source available and is used to obtain energy, which is reflected in the low increase in BS production registered from fifth day of incubation.
It is important to mention that the decrease in the production of BS could be correlated with the decrease in CDPR observed by respirometry (Figure 2.10). In this case, CDPR is a variable of the process determining the time of the maximum production of BS according to the physiological state of the yeasts in the SSF.
From the analysis of the experimental data, three balance equations for the substrate’s consumption are proposed based on modified first‐order decay kinetics. These three equations are coupled to the formation of BS through the conversion yields, corresponding consumption of glucose, and oil in the formation of BS. The set of ordinary differential equations (ODE) corresponds to initial value problems and are shown below:
(2.1)
(2.2)
(2.3)
Where Gluc and Gluc0 are the glucose concentration at any time, and the initial glucose concentration expressed in g/kg dry mass; Oil and Oil0 are the oil concentration at any time and the initial oil concentration expressed in g/kgdry mass; SL and SL0 are the sophorolipid concentrations at any time and the initial sophorolipid concentration expressed in g/kgdry mass, respectively. The first‐order reaction constants correspond to k1 and k2 (h−1) for the consumption of glucose and oil, respectively. The constants B
