When descaling a machine we use either straight white vinegar or 1-2oz of citric acid in 64oz of water.
These compounds will not harm you or your machine if fully flushed from the system.
Water Reservoir or Pour Over Espresso Machines
Fill the reservoir with straight white vinegar or 1-2oz of citric acid. Let the machine fill, then pull about 8 oz through the group and 8oz from the hot water wand. Let the machine sit about 1 hour then repeat every hour until the reservoir is empty. If your machine has drains, let the machine cool before emptying the boilers. Reseal the drain plug. If your machine does not have drains, then turn off the machine and open the hot water wand to allow the steam pressure to push out as much water as possible. Fill the reservoir with fresh water. Allow it to fill and heat. Flushing you will pull about 8 oz through the group then pull about 6-8 from the hot water wand let the machine fill and reheat. This time, we don't have to wait an hour between cycles--only as long as it takes to reheat. Flush until the water runs clear and tastes clean. It is important not to drain more than 8oz from the hot water tap at a time so as to avoid damaging the element."