How to do masking automatically for irregular shape?

I have images like rice, fish and spinach. I've attached some images. I'm planning to do masking to the images automatically but I only know how to do masking using imfreehand as they got irregular shapes. I want to use masking because i want to get the ROI only. Background will be removed and set to black colour.

For example, i want to do masking to the rice image, and my ROI is the white part of the rice only. Can someone help how to make the images masked automatically?

Thank you.

[KALYAN ACHARJYA]

Yes, do apply color thresholding or appropriate segmentation methods, it can be done