Practical methods to reduce impurities in Gram-scale amounts of acidic sophorolipid biosurfactants by Baccile et al.  presents a practical and usable approach to the oft-underestimated aspect of product purification. This commentary is intended to provide the reader with an insight into the specific hurdles which have to be overcome when trying to isolate a compound with emulsifying properties, i.e., a biologically produced surface active agent. In a broader context, the commentary also attempts to elevate the topic of purification beyond its current, though admittedly perceived, status of being somehow less attractive and perhaps spark a measure of debate.
The separation and purification of surface active compounds presents its own set of unique challenges. Not only may their amphiphilic nature interfere with classic separation techniques such as solvent extraction by forming a stable emulsion, said nature also overlaps with the two main categories of impurities which are commonly found in organic chemistry, i.e., hydrophilic and lipophilic impurities. The importance and relevance of sophorolipid research has been sufficiently expanded upon by the authors and here the potential medical applications make research into practical purification methods especially poignant. Additionally, the main function usually associated with surfactants, i.e., cleaning, is another very relevant aspect when it comes to isolating the active compound. The presence of residual lipophilic starting material, whether it be from a chemical or biological synthesis, in most cases will decrease the effectiveness of the surfactant as its emulsifying potential is reduced.
As surfactants have a wide range of application – from bulk chemicals for industrial cleaning purposes to fine human pharmaceuticals – the associated purity that is required differs equally great. It is therefore rather odd to notice that, specifically when regarding the field of biosurfactant research, the emphasis of this research is predominantly found to be on the discovery of new molecules and developing ever increasing yields. Actually producing a certain substance in a more or less pure form and/or relevant concentration seems to be nothing more than an afterthought at times. Indeed, techniques and new developments on purification and isolation are usually relegated to short communications rather than full publications in their own right. For this reason, the article was perceived as a very positive sign and this commentator would express the hope that many more may follow.
With regards to the work presented by the authors, three aspects of particular interest came to mind: the fact that a comprehensive and full work on sophorolipid isolation was presented, the proposal of two novel purification routes which were adequately compared and the total approach to remove both lipophilic and hydrophilic residues.
“The presence of residual lipophilic starting material will in most cases decrease the effectiveness of the surfactant as its emulsifying potential is reduced.”
To date, relatively few publications have addressed the problem of sophorolipid purification, especially when compared to the amount of literature available regarding other sophorolipid-related topics such as the use of alternate substrates, genetic engineering of the production strain, (bio)chemical modification of the product and general ways to increase production levels by using, for example, certain feeding strategies. The chosen type of sophorolipid – the deacetylated acid form – in this study is of additional interest as it is the most hydrophilic naturally occurring variant and as such, the most challenging form to separate from hydrophilic impurities. The wild-type sophorolipid mixture is currently produced at the highest reported yield, but still contains one or two acetyl groups. These groups are quite easily removed by alkaline hydrolysation as indicated by the authors, but will obviously generate a significant amount of acetic acid, or its concurrent salt, which makes the removal thereof a serious hurdle in obtaining a pure product. In removing all esteric bonds from the sophorolipid molecule, the overall chemical stability of the molecule increases and so further underlines the potential attractiveness and relevance of the sophorolipid species that was used.
Because of the several proposed applications for sophorolipids beyond cleaning, having two different approaches to obtain a purified product, i.e., liquid–liquid extraction and solid–liquid extraction, may allow for different grades depending on the final usage. As indicated by the authors, the first method would yield a pure product but requires several days to perform whereas the second method can be performed quickly if hydrophilic impurities are of lesser importance. Both methods show definite potential for refinement and it is expected by this commentator that further work would indeed lead to a decrease in extraction time for the first method and an increase in purity for the second.
As a general comment, the comprehensive approach to remove both lipophilic and hydrophilic impurities has the clear advantage in offering a practical starting point to develop an industrially viable process. The current results as presented in the article, however, do not yet warrant such an up-scaling in this commentator's opinion.
The author has declared no conflict of interest.