I have to cook for 60 people, and one of the things I am going to make is (japanese) rice. Normally I cook it in a rice cooker, or in a pan with an amount of water and let it cook until the water has 'dried up'. Can I use the same way of cooking rice for 60 people? (Say I have 6 kilos rice that needs to be cooked) how much longer is it going to take me to cook rice this way? What can go wrong? Can I just leave it in the pan until its done, or do I need to stir it because of the big amount?
Three main factors you need to think about is the water ratio, method of cooking and portion size.
When cooking Japanese rice(short glutinous grains) the optimal ratio is about 1:1 of water and grain. Normally when cooking smaller portions you add additional water to compensate for the water that evaporates and turns to steam while you cook so in most recipes you will see recipes asking for 1 cup and a couple extra tbsp of water for each cup of rice.
However you are cooking a very large numbers of servings so the evaporation will be negligible. You can stick with 1:1 ratio.
To cook the rice you can use either a large stock pot on the stove top or put it into a large pan and bake in the oven.
For the first method, you need a really thick stock pot(a thicker pot results in better heat distribution which results in less burnt bits at the bottom). I would recommend doing 30 servings at a time. Measure out the rice and water and bring to a boil. Then turn the flames down really low and simmer with the lid on for 30-45 minutes. Then turn off the heat and let it sit with the lid on for at least 15 minutes.
To make it in the oven, measure 30 servings of rice and water in a large oven safe pan and bake at 325F for around 30-45 minutes. Turn the oven light on and check if all the water have been absorbed into the rice. Using this method I would increase the amount of water to about 1:1.1 of rice to water.
Rice will generally double in volume when cooked compared to its uncooked version. If you are making 60 1/2cup servings of rice you would need 15 cups of uncooked rice. Or if you are going with 3/4 cups per person, you would need 22.5 cups of uncooked rice. 6 kilo of uncooked rice is around 25 cups so your estimation was pretty close to the mark. I would cook all 6 kilo of rice. Its better to have a bit extra than to not have enough.
I've seen this at some take out:
Do this in smaller batches that can be easily handled.
They prepared all the rice in advance and put individual portions in small food and heat safe plastic bags.
I assume they keep it in the fridge until serving time, and when they setup service, they put the individual bags in a steamer.
When it is time to serve, just pop the bag open and set on plate.
You can't boil any rice for 30-45 mins, let alone let it sit for another 15 mins while it cooks under it's own heat further. At solid boil, rice is done in max. 10 mins. At boil and steam down, max 20 mins. So with a 40 litre pot, you're gonna need to pre-boil the water, then add rice, and remove fast after 10 mins or so, or it'll cook itself into a mush off the fire after 20 mins (and for the next 5 hours after, till water in pot is cool).