Читать книгу Poly(lactic acid) - Группа авторов - Страница 24

Lactic acid fermentation inevitably leads to a drop in pH, and without neutralization the microorganism is quickly unable to continue the fermentation, as the environment becomes too acidic. Several bases can be used to neutralize the acidity during fermentation, and the choice of the base will determine the nature of the downstream processing (DSP). Most industrial lactic acid plants use Ca(OH)₂ or CaCO₃, which results in the production of a large amount of gypsum as a by‐product.

A major challenge in lactic acid production is to find or construct an efficient microorganism that can produce at such a low pH that the fermentation does not require neutralization. Lactic acid bacteria are usually able to grow at low pH, but it is difficult to find an organism capable of producing lactic acid in reasonable amounts at pH close to the pK _a of lactic acid [49]. Another solution is to construct a lactic‐acid‐producing yeast, but organisms like this still suffer from low productivities (amount of lactic acid produced per hour) and low final concentrations, leading to the requirement for large fermenter volumes and high amounts of water evaporation [50].

Some basic hurdles have to be overcome to improve the low‐pH fermentation by yeasts. Although yeasts are very resistant to low pH, the export of lactate from the yeast cell to the outside medium costs them as much energy as they get from lactic acid production by fermentation. For this reason, lactic‐acid‐producing yeasts need reasonable amounts of oxygen to generate enough energy to survive [51]. In contrast, traditional lactic acid bacteria use another way to transport lactic acid across the membrane and even gain extra energy by exporting lactic acid to the medium [52].