Frozen vegetables are typically flash-frozen when they are at their peak. This helps retain all the health benefits. Once they defrost they will lose some of the vitamins and minerals as moisture is released from them. u003cbr/u003eu003cbr/u003eDefrosting also can change the color and texture of frozen vegetables. It can also be pretty messy if the bag, box, or ziplock is not sealed properly. So for all these reasons, I do not recommend defrosting before steaming.