There are a few things you can do to prevent radishes from getting hot to the taste.
When radishes grow slowly, they tend to become hot to the taste. The key to getting them to grow quickly is to keep them evenly watered (consistently evenly moist but not damp/soggy) so that they grow quickly. And given that they are a cool-weather crop, it is best to plant and harvest radishes when the weather is on the cool side.
Secondly, when you harvest radishes affects the hotness as well. Radishes are best picked as soon as they are mature when they are relatively young and small. If they are left in the ground too long they will get hot.
For a plethora of information on radishes, please visit the following link:
https://www.gardeningknowhow.com/edible/vegetables/radish/
