Lactobacillus pentosus
2.3 Biosynthesis of BS by Yeasts and Molds
Yeasts and molds produce BS, mainly glycolipids, as SL and mannosylerythritol‐lipids. Table 2.3 shows the most important genera of BS producers, particularly, Candida, Pseudozyma, and Starmerella (ex Candida). Compared to bacteria, yeasts and molds have been less explored as a source of BS, with the exception of SL for oil and cosmetic applications. Candida genus are capable to synthetize both types (SL and MEL) of glycolipids [18]. Starmerella bombicola is the most important yeast in SL production and mold Ustilago maydis in other glycolipids. As bacteria, yeasts and molds have a highly regulated metabolic pathway for the BS synthesis, since these compounds are secondary metabolites. The carbon source, hydrophobic substrate, and nitrogen limitation are prevailing factors in SL and mannosylerythritol lipids biosynthesis, while for cellobiose lipids nitrogen source is also important [18].
Table 2.3 Yeasts and molds as biosurfactants producers.
| Biosurfactant | Microorganism | Carbon source |
|---|---|---|
| Mannosylerythritol lipids | Pseudozyma antarctica | Glycerol [63] |
| Pseudozyma rugulosa | Soybean oil [64] | |
| Pseudozyma aphidis | Soybean oil, glucose [64] | |
| Pseudozyma tsukubaensis | Olive oil, arabitol [65] | |
| Pseudozyma hubiensis | Soybean oil [66] | |
| Pseudozyma shanxiensis | Soybean oil [66] | |
| Pseudozyma crassa | Glucose and oleic acid [67] | |
| Cybersan | Cyberlindnera saturnus | Glucose [68] |
| Sophorolipids | Candida floricola | Glucose and olive oil [69] |
| Candida batistae | Glucose and olive oil [70] | |
| Candida apicola | Glucose and olive oil [71] | |
| Candida stellata | Glucose and olive oil [71] | |
| Candida riodocensis | Glucose and olive oil [71] | |
| Candida kuoi | Glucose and olive oil [72] | |
| Meyerozyma guilliermondii | Olive oil [73] | |
| Starmerella bombicola | Biomass hydrolysate [74], palm oil [75], food waste [76] | |
| Wickerhamiella domercqiae | Glucose, repeseed oil [77] | |
| Lachancea thermotolerans | Canola oil [78] | |
| Glycolipids | Wickerhamomyces anomalus | Glucose and olive oil [79] |
| Cellobiose lipids | Cryptococcus humicola | Glucose [80] |
| Glycoprotein | Curvularia lunata | PL‐2 media [81] |
| Glycolipoprotein | Aspergillus ustus | [82] |
| Glycolipid | Ustilago maydis | [82] |
2.4 Screening for BS Producers
As shown above, there are diverse microorganisms able to produce BS, but few of them have the kinetic characteristics for large‐scale production since bio‐based product market is appealing [83] for new or better‐producing microorganisms and is a challenge for biotechnology. Microbial screening has been performed on different environments, such as hydrocarbon and oil‐contaminated soil, coastal and offshore [84–86], gas platforms [87], seawater biofilm [88], marine environment [89], mangrove sediments [90], food materials [91], and Amazon rainforest [92]. Many of the isolates are bacteria related to Pseudomona and Bacillus genera. In the case of yeasts, Candida, Starmerella (ex Candida), and Pseudozyma are the most used genera; but more microorganisms are emerging nowadays as BS producers.
Accurate methodologies for isolation and characterization of microorganisms are important to discover specific metabolic capabilities. By traditional methods, microorganisms are isolated from environmental samples and tested for BS production by qualitatively, e.g. CTAB agar assay or hemolysis test as a primary approach. A different strategy limits the screening to oil or hydrocarbon‐contaminated sites, resulting in an increment of isolates related to BS production [93]. In both cases, it is necessary to employ quantitative or semi‐quantitative methods for BS evaluation: drop collapse test, emulsion index, TLC and superficial tension, or techniques such as HPLC/UPLC, LC‐MS, or MS. Nevertheless, this methodology is only for cultivable microorganisms, so there is an undetermined number of potential producers in different environments. At this moment, the metagenomic approach is not a common practice for the screening of BS producers [94].
2.5 A Case Study: SL by Solid‐State Fermentation (SSF), Kinetics, and Reactor Size Estimation
SL are BS that are produced commonly by liquid fermentation (LF) using glucose as the hydrophilic carbon source and oleic acid (C18:1) as the hydrophobic source [95]. Glucose can be replaced by sugar cane or sugar beet juices, which have high contents of sugars and nitrogen and avoid the use of the expensive yeast extract and urea for the fermentation media [96]. Some potential hydrophobic sources are solid at fermentation temperature (e.g. stearic acid, m.p. = 69.3 °C), complicating their use as substrates in these liquid processes. In such a case, solid‐state fermentation (SSF) is a potentially useful alternative. Furthermore, the SSF avoids the problems that generally occur during the production of SL by liquid fermentation, such as foaming
