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The dehydration of lactic acid to make the prepolymer should start with an –OH to –COOH ratio of 1 : 1. All other components with –OH and –COOH functionality disrupt the stoichiometric balance and may be incorporated as comonomers during prepolymerization, which limits the final lactide production yield from lactic acid. Little public information is available on the technical and economic relationship between lactic acid quality and lactide synthesis. Only a few patents mention the effect of metal impurities on racemization [68, 69]. Stereochemical purity is one of the key parameters determining lactic acid purity.

Lactic acid purified by crystallization may be taken as the benchmark in lactide manufacture, but the expected unfavorable economics of making crystalline acid in relation to mother liquor processing may prevent its commercial use for lactide/PLA. The next level of quality with the right commercial relevance is heat‐stable lactic acid. Heat stability puts constraints on the content of sugar, and thus on the DSP method used in the process. It is unlikely that suitable acid for making lactide will contain sugar because of the high temperatures involved (see the next section) and the well‐known practical decomposition problems when sugars are cracked. In practice, this means that color, or actually heated color (color after heating of the acid), is an important indicator for the suitability of the acid for lactide/PLA production [6, 70]. The appeal for lactic acid with little or no sugar and the DSP methods mentioned in practice lead to demands for separation methods that are similar for sugar and other heavy components such as proteins, amino acids, and polysaccharides.

It is expected that the desired quality of lactic acid for making lactide/PLA will evolve, with overall process yields and economics as the criteria.