Water-based pigments are susceptible to microbial attack. In general, all organic pigments are utilized by different bacteria as a source of carbon and nitrogen. Among all pigments tested, yellow pigments are more susceptible to bacterial degradation because of their organic origin. In a successional event, bacteria serve as first colonizers, followed by the growth of fungi, leading to spoilage of pigment paste. This spoilage is observed as a foul odor due to the generation of low-molecular-weight degraded compounds and sulphides, and discoloration and viscosity increase due to a precipitation reaction caused by bacteria.
Bacterial degradation occurs in two main stages, which is eventually taken over by fungal growth. In the first stage, waterborne bacteria or pigment-associated bacteria play a major role. This group of bacteria, mainly represented by Pseudomonas, Proteus, Enterobacter and Citrobacter, act as primary degraders. One hypotheses is that these bacteria may secrete exogenous enzymes to degrade pigment and other components like lecithin and surfactant molecules, and carry out a fermentative reaction that causes gas formation, foul odors and pH decrease. In the second phase of pigment spoilage, bacteria such as Staphylococcus utilize the metabolic by-product formed and cause a precipitation reaction by secreting lecithinase enzyme on the lecithin, which causes non-reversible viscosity increase and a pungent odor. To avoid this degradation of pigment paste, the following preventive measures should be in place.