Microwaving coffee does not reduce caffeine.
The caffeine in coffee is still in the coffee when reheating it in the microwave, on the stove etc.
To stop coffee from going hard you need to keep moisture and dampness away from the coffee.
So you need to store the coffee in jars, or other sealed containers in a dry location.
To prevent the ground coffee from hardening, you need to store it away from heat, moisture, and air.
The best way to avoid this is using an airtight container.
Mason Jars are good for storing coffee in.
The mason jars are great for storing coffee as the lid seals in the freshness of the coffee and you can also store sugar, flour, baking soda, beans and other foods in the mason jars as well.
It is safe to drink day old coffee.
As long as the day old coffee has no mold in it the day old coffee is safe to drink.
I've been drinking day old coffee for almost 10 years now and nothing has happened to me.
Sometimes I will make a pot of coffee in the middle of the day and have half a pot of coffee left over.
I just simply leave the coffee in the coffee pot overnight and then the next day I add some water to dilute it and then reheat the cup of coffee in the microwave.
The reheated day old coffee is not the best tasting coffee but it's still not that bad and I've gotten used to the taste of the day old coffee.
I don't want to waste the coffee so I drink it.
However if the coffee is a few days old then it may not be any good anymore.
Just look at the coffee and see if there's any mold in it and if so throw it out.