31 weeks is still early to collect and store them, but you can, if u want to. I'll start storing them around 36 weeks. I just bought 5ml container(syringe grade) to store them. I'm turning 34 weeks. u can use syringe, which is way beta coz won't have any wastage. but u need to get the tip cap cover too. collect your colostrum, write dates on it, place inside the freezer in a zip lock bag or container.
It should be colostrum . https://sg.theasianparent.com/pregnancy-concerns-leaking-breasts