are among these bacteria [29, 42, 43]. Also, patents have been filed claiming the production of D‐lactic acid by a genetically modified microorganism. Several different species such as Kluyveromyces and Escherichia coli have been claimed so far [44, 45].
1.2.4.3 Nutrients
The most well‐known lactic‐acid‐producing organisms, such as Lactobacillus and Lactococcus species, are members of the taxonomic order of Lactobacillales, also commonly referred to as lactic acid bacteria. These lactic acid bacteria have their really complex nutrient need in common [29]. Vitamins and peptides need to be added to the medium to enable growth. This can be done by adding peptones, yeast extract, or corn steep liquor, but this is expensive. Nutrients for lactic acid production can also be derived from nutrient‐rich waste streams such as rice bran, fish waste, or vinification lees [46–48].
1.2.4.4 Neutralization
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].
1.2.4.5 Carbohydrates for Lactic Acid Production
In principle, any carbohydrate source containing pentoses (C5 sugars) or hexoses (C6 sugars) can be used for the production of lactic acid, although it is very rare that any particular microorganism is able to use all possible and available C5 and C6 sugars. Pure sucrose from sugarcane or sugar beets and glucose from starch are available in large amounts and readily fermentable. Polysaccharides such as cellulose or starch are more complex and need special pretreatment. When using less pure sources such as raw sugar beet juice, the impurities must be removed somewhere in the total lactic acid production process [53]. This can be done before, during, or after the fermentation. This often leads to special adaptations in the production plant. Last but not least, the local price and availability of the carbohydrate source determine the raw material of choice for industrial fermentation. Another usable disaccharide is lactose present in whey, as was used by Scheele when he discovered lactic acid in 1780 [12].
1.2.4.6 Starch
Starch occurs in discrete granules and is usually a mixture of two homopolymers of glucose, amylopectin, and amylose. Starch can be derived from corn, wheat, potato, or tapioca [54]. Although some microorganisms are able to degrade and ferment starch directly to lactic acid, most lactic‐acid‐producing microorganisms cannot hydrolyze starch themselves. A solution is to hydrolyze the starch to glucose prior to fermentation with the commercially available enzymes, α‐amylase, and glucoamylase. This can be done in a separate process, so no incompatibilities are present between the optimal pH and temperatures of the enzymes on one hand and the optimal pH and temperature of the microbes on the other. However, if the right combination of enzymes, microorganisms, pH, and temperature is carefully chosen, the hydrolysis and fermentation can be carried out in one reactor. This process is generally called SSF (simultaneous saccharification and fermentation) [55]. Prior to SSF, the starch granules usually must be gelatinized at high temperature by cooking. However, even a cooker is optional nowadays as commercial enzymes are becoming available that are able to attack and hydrolyze the granules efficiently and fast enough at relatively low temperatures.
1.2.4.7 Lignocellulose
Sucrose and starch have in common that they are used for food and nowadays, with oil wells drying out and prices rising, also for biofuels. A decrease in the availability of fossil fuels is envisaged for the future, and with increasing population, more food is needed at reasonable prices. Therefore, the ideal raw material for biofuels and bioplastics is carbohydrates that are not edible. Such material is abundantly available around the globe as lignocellulose, like in corn stover or wheat straw. Lignocellulose consists of the glucose homopolymer cellulose, the heteropolymer hemicellulose, and lignin. Hemicellulose consists of hexoses and pentoses. In all, lignocellulose contains roughly 80% fermentable sugars, but this largely depends on the source [54]. The remainder, lignin, is a phenolic polymer that is difficult to degrade and is not directly usable for lactic acid production. It may be used for energy production though, which can be returned to the lactic acid plant.
A purer source of cellulose without lignin is waste paper that can be used for lactic acid production at lab scale [56]. Thus, even this book can eventually be converted into PLA!
Complete utilization of cellulose and hemicellulose requires selection or genetic modification of an organism that is able to ferment pentoses. To obtain monosaccharides from the raw material, several pretreatments and/or separations are required. First, the lignocellulosic material is mechanically treated and then delignified (pulped) by strong alkali or acid treatment. The (hemi)cellulose part becomes more accessible for enzymes at the same time. Subsequent enzymatic treatment mainly yields glucose and xylose and some arabinose. The enzymatic treatment and subsequent fermentation can be done in separate reactors or in one fermenter, in an SSF concept similar to starch SSF [57].
1.2.4.8 Batch versus Continuous Fermentation
A process can be run in batch or continuous mode. In continuous mode, there is a constant flow of fermented sugar out of the reactor that is equal to a continuous flow of fermentation medium into the reactor. During batch fermentation, there can be an inflow of medium, but there is no outflow [58]. Batch fermentation needs to be inoculated with a starter culture every time, whereas this is not needed in a continuous fermentation setup. However, in case of problems, the continuous fermentation needs to be restarted, so an infrastructure for starter cultures is needed anyway. A high volumetric production rate can be achieved when combining continuous fermentation with biomass retention, leading to smaller fermenter size [59]. It must be stated that the lactic acid concentration is lower compared with batch culture [58]. The concentration of lactic acid influences the water balance in the production plant.
