Preventing Diced Vegetables Added to Simmering Liquid From Becoming Mushy I like adding yams, sweet potatoes, butternut squash, and pumpkin, all in 1/2 inch cubes, to soups/beans and other dishes with simmering liquid. 

I generally add them 10 minutes before the end, so that they do not overcook. Even then, however, if the hot soup/beans sit off the heat for 20 minutes, the vegetable cubes begin to lose their structure. 

Roasting the diced vegetables beforehand helps somewhat. What I would like is to have the structure of the roasted diced vegetables (caramelized harder exterior with cooked softer interior) hold up for as long as possible. 

Does anybody have any tips to achieve this?
Once you add the vegetables to the broth, they are going to begin absorbing water and becoming softer and eventually become mushy. 

This effect is slowed but not eliminated when the soup is cooled.

The only robust way to prevent it is to only add vegetables to the portion that you are going serve immediately, and then serve the soup promptly when the vegetables are cooked to your liking.