The paper notes that a 1-in-1000 molar concentration of acid was sufficient to trigger low-temperature melting, indicating that the amount of contamination needed is indeed very, very small. The paper does not discuss exact mechanisms of action, but one of the conjectures is that the acid takes the form of excess hydrogen atoms, which could easily tuck themselves into a crystal structure nearly undetectably.
All it takes is catalyzing a few Z->E transitions when the temperature rises to cause the crystal structure to break down, which would explain the lower melting point despite the small amount of contaminant (catalysts often reduce the energy required to start a reaction, but are not themselves consumed so they can further catalyze other reactions).
All it takes is catalyzing a few Z->E transitions when the temperature rises to cause the crystal structure to break down, which would explain the lower melting point despite the small amount of contaminant (catalysts often reduce the energy required to start a reaction, but are not themselves consumed so they can further catalyze other reactions).