Do you wash your breastpump parts EVERY single time after pumping session while at work? Or just rinse with hot water? I find washing breastpump parts very time consuming, and there isn't any proper place for me to dry them.

Agreed with the mom above! Clean tap water will do. But no rinsing at all, I think not so good idea. When I used to pump in the office, I just ran water to the pump parts and sort of set it in a small rack I brought in and shoot it in a ziplock after a little bit of drying ( I was paranoid some rubbery parts might get smelly when I don't dry them). When I arrive home, I immediately take it from my bag and wash it with soap and warm water. You can also refer to the instruction manual of your breastpump if there's special notes to take in cleaning your unit's part.

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But I might need to pump 2 times at work. If I don't dry them, will water residue inside promotes growth of bacteria in breastmilk?

Just rinse with water will do. After rinsing, put it inside a ziplock bag. Do not need to dry it.