Background: If I use two or three cups of rice to prepare rice in a pressure cooker, I have to add plenty of water, pressure-cook it until one whistle and wait for around 1.5 hours until I open the pressure cooker. However, for idli or dosa, the raw rice + urad dal is ground to form a paste and allowed to ferment. Then it's either steamed (for idli) for 10 minutes or poured onto a surface or flat pan, and cooked for just a few minutes. The primary questions:  I don't understand how this gives the batter enough time to get cooked fully. Compared to how much time and extra water it took to cook rice grains in the pressure cooker. Also, won't such a process prevent a lot of the bacteria and yeast from getting killed? Does it make more sense to partially or fully cook the majority of the rice grains in a pressure cooker, before mixing it with the raw urad dal and a tiny bit of raw rice ground in a mixie or stone grinder (to bring in the bacteria and yeast)?  Reason for asking: I used a mixie to prepare the batter (so had a slightly coarser texture). Fermentation only began a little, since the ambient temperature here is 21 degree Celsius. The idli and the dosa I prepared with this batter, didn't seem to get cooked fully. It still had a bit of rawness to it. Some initial searching I did revealed:  Bacteria: Leuconostoc mesenteroides, Streptococcus faecalis, Lactobacillus fermentum and Bacillus amyloliquefaciens help in souring and leavening. Yeast: Saccharomyces cerevisiae, Debaryomyces hansenii and Trichosporon beigelli produce flavour, enzymes and helped in the saccharification of starch. source Batter ferments best at 28 degree Celsius and an initial pH of 4.5. source Mixie has a lesser chance of causing starch damage, compared to a stone grinder. source The aerobic bacterial count increases in 16 hours. source  
I'm going to address another part of your question, which is rice cooking time. Normal cooking time for rice via the absorption method is about 20 minutes.  I'm not at all clear on what you're doing with the pressure cooker; presure cooker rice should require only about the same amount of time -- 10 minutes cooking time plus 10 minutes cooling time. So your 90 minute rice cooking time is way outside of the normal range.* So, given that the time required to fully cook whole rice grains is about 20 minutes at a simmer, you can see where cooking finely ground rice at a slightly higher (hot steam) temperature would take less time -- around 10 minutes. (* there are rice techniques that require an hour to cook, but those use very little water and are slow-steaming the rice) 