Congratulations 🌷
It’s always a bit of a contentious subject on MN, it can get a bit heated! Don’t let that put you off asking any other questions, there’s always good advice in amongst the bunfight!
There’s a lot of anecdotal advice that suggests it really doesn’t matter very much and most of the time it probably doesn't, but if there is bacteria in the milk powder and your baby gets sick, they can get VERY sick and sadly sometimes it’s fatal, that’s very rare though
The most risk is in any pre existing bacteria in the milk powder. That needs to be mixed with water at or above 70 deg to kill it off. (Our water supply is very safe in the UK so anyone saying they use cool boiled water is missing the point of the hot water).
However, proteins in the milk start to denature (break down, become less nutritious) above 60deg, so you’re better not to go hotter than 70deg.
Anything other than making it fresh carries a higher degree of risk and I personally wouldn’t risk it any more often than is unavoidable and I would rather risk the denature by making it too hot than not hot enough when out and about. You can reduce the risk by using a thermometer, but you’ll probably feel like a proper Charlie.
At home you can buy a kettle that heats to 70deg (I test them to check they’re accurate). Once heated pour the water into the bottle (take note of how much you’ve put in), you need to have plenty so all of the powder gets thoroughly mixed into the hot water then measure out the cold water to the total water needed in another bottle/jug and add that. It sounds like a faff, but it’s really not and you can make a bottle that can be given to the baby straight away - test it obviously!
When they’re tiny it’s really important to get the water measure accurate so that they’re not dehydrated or taking too much water and not enough milk. But when they’re bigger it’s no so critical and you’ll know by then after having made a million bottles already!
Ready made milk has its uses for sure, but at home it’s just as easy to make it fresh really - especially if the little darling will only drink it really warm!
I’m having iPad/MN issues so I’ve probably cross posted with loads saying the same 😖