There are many reasons why this behaviour may begin and only by using gentle techniques to change the behaviour will you be able to train your dog to be comfortable around food.
You must not take the food away to make the dog see that you are in control. Food guarding starts usual because they are worried you are going to do exactly that.
Food guarders also get very anxious about treats that take longer to consume and may try to bolt them down too quickly so be mindful about the types of treats and games you play. Always seek professional help for problems like this.