How long should the Heterotrophic Plate Count (HPC) plates be incubated for?
Incubation times for HPC.
The range of incubation time from the Heterotrophic Plate Count Test Methods application note (5-7 days) is based on data for different controls of heterotrophic bacterium. Different strains of bacterium will take different lengths of incubation time to develop. This would be true for all media, not just HPC. Some strains can develop as early as 3 days, others can take up to 7 days to develop. HPC is most similar to R2A media, so if you are comparing the incubation time for your samples on HPC vs R2A media, the incubation time to get a positive result should be similar, but if comparing HPC to another media, the incubation times can vary.
In short, positive results can be obtained in as little as 72 hours (3 days) but it can take longer depending on what strains of heterotrophic bacterium that are present in the sample. Incubation times compared to R2A agar should be similar, but can vary when compared to other media.
The recommendation would be to take the first couple runs as beta test and incubate for the full 7 days with samples that are positive and compare results day to day to determine what the appropriate incubation time would be for complete development of the strains in your sample. Then moving forward samples should be able to be incubated for that amount of time. This of course would be assuming that the strains present in your sample matrix are consistent.