Is it dangerous to microwave formula if I swirl the bottle and test the temperature afterwards?
The overwhelming consensus online is that microwaving a babies bottle can be "dangerous" because microwaving formula can produce hot spots. However, if you simply swirl the bottle you can even out the heat distribution and eliminate said hot spots. The amount of bubbles produced by swirling is minimal and heat redistributes very easily in liquid. That being the case, if I test the formula's heat and its a good temperature, and the heat is redistributed, whats the big deal about microwaving the bottle? Is it just that people can't be trusted to swirl the bottle well and test the temperature?
Edit:
Unfortunately, I don't have access to the full articles, but for what its worth, here are a few articles which discuss the nutritional effects of microwaving formula and/or breastmilk (thanks to @bestander );
www.ncbi.nlm.nih.gov/pubmed/8889628 - Says there are no effects on B1/E/acids if the resulting average heat is not higher than 60 C www.ncbi.nlm.nih.gov/pubmed/11403146 - Reinforcement of first study results, showing no difference in B1/B6 levels if compared to stove top heating m.pediatrics.aappublications.org/content/89/4/667.short - E.coli bacteria grows faster in microwaved milk than control, i.e. antibacterial qualities are reduced pediatrics.aappublications.org/content/90/3/412.abstract - There is no significant loss of either riboflavin or vitamin C. Routine mixing resulted in formula temperatures which could safely be fed to infants. "portions reached a mean temperature of 44.7 C" www.ncbi.nlm.nih.gov/pubmed/2189981 - abstract is missing results
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It's certainly not illegal, it's just ill-advised.
When it comes to your child's safety, is it really worth taking the risk? I believe a better statement would be "During the heating or feeding process, no single part of the bottle or food should ever be hotter than what you would give to your infant".
Swirling the formula around to distribute the heat is explicit acknowledgment that some part of the formula will be hotter than others, and likely hotter than you'd give to your infant. Additionally, the bottle itself might be unevenly heated, and in that case, the heat will not equalize nearly as quickly.
Piggybacking on what Joe mentioned, you're also risking breaking down the nutritive value of the formula by microwaving it.
To counter your point re: microwave sanitization: When you sanitize bottles in a microwave, they're all pointing downward and draining any fluid (and chemical) buildup. When you're microwaving a bottle with formula, there's no such drainage, and any chemical buildup is absorbed by the formula.
Potential Solutions
If you're using powder-mix formula, use warm water to mix instead of cold water.
If you're using full-strength formula, most don't need to be refrigerated, so they can stay at room temperature
If you're using a concentrate to mix pitchers and put the rest in the refrigerator, the best way to heat it is to run the body of the bottle under warm water, swirling the formula around so it all gets heated evenly.
Generally, if you have access to a microwave, you have access to hot water.
I would occasionally microwave a bit of formula, especially to take the chill off if we had placed an unused portion in the fridge. (Yes, I will admit that publically... especially since I don't think my wife is on this forum... but I digress.)
One accommodation I made was this: I placed the bottle in a larger container, like a Pyrex measuring cup. I filled the cup with water a bit higher than the level of formula in the bottle. Then I microwaved the bottle in the surrounding water. I figured this would act to moderate the rate of increase in temperature, and help even out the "hot spots" (if any).
Upon removing from the microwave, I would test the water in the cup (with my fingers) for its temperature. If IT was not too hot, I figure there was a good chance the formula was not, either. I would then "swish" the bottle (as the OP mentions) and test it for temperature, too, by squirting on the wrist, or -- more reliably -- just tasting it.
Depending on the formula that you're using, they might have some probiotics in their composition. For example, Nestlé.
Probiotics have small tolerance to high temperatures, so preparing the formula (or heating it after it was prepared) above some temperature might kill those probiotics, reducing the nutritional value of the formula. For example, this preparation guide advises to use water at 40 degrees Celsius (after it was boiled and let to cool down).
Using the microwave might require you to have a control over this temperature (i.e., you should be sure it won't get higher than 40 degrees). If specific points of the formula over-heat, in that area those probiotics might die, but it doesn't mean that the whole formula lost it value.
Noah & woliveirajr are both correct that there is some risk of reducing nutritional value, I'm sure.
I will offer as counter point my home study (sample size=2) of children who were bottle-fed formula almost exclusively as infants; formula which was on occasion reheated in the microwave. They are now 12 and 9, healthy, and not malnourished. They definitely suffer from one side effect often linked to formula feeding, which is being extraordinarily picky, bland eaters to this day. While the formula feeding may have contributed to that problem, I'm hard pressed to think how occasional microwaving of the formula would have made it worse - does killing nutrients also mean making it blander? Fail to nourish developing taste buds?
It's long enough ago that I don't have a firm recollection of just how often we did this. Certainly not all the time, but probably at least one feeding a day? But I could totally be making that up.
We microwave our infants formula all the time, it's all we use to heat the formula. We always swirl and double-check the temp before feeding. I disagree with the losing nutritional value statement. I don't think that warming milk to 20 degrees celcius is considered "cooking" to anyone. There is a fine line in microwaving time to go from feeding temperature to cooked milk, so you have to be careful and get used to it. But for us now we have the prep times all figured out. 4oz bottle 30 secs from the fridge, 19 secs from room temp for our microwave. Obviously times will vary from different devices. My wife and I had a "milk race" one night to see who could prepare the milk the fastest. She used the microwave, I used the bottle steamer. By the time my bottle was ready she had the baby half ways through the feeding with her bottle. Shame to say I lost. Typical husband I am...
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