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First off, I did see this question about reheating the coffee in a microwave oven, but the answer merely suggested that stale coffee tastes bad irrespective of the microwave.

I make instant coffee a lot, and whenever I use the stove to boil my milk and add coffee, it turns out fine. But if I am lazy and use the microwave to boil the milk and then mix the coffee, more often than not, the coffee tastes burnt. So burnt that I've had to throw it away.

This isn't specific to one particular brand of instant coffee, I have tried 3 brands.

I've noticed that it seems less burnt if I put it in the microwave for lesser time, but I like my coffee piping hot in the morning and it doesn't turn out well.

So what is it about microwaving instant coffee that makes it taste so revoltingly burnt and what can I do to avoid it?

EDIT: I do microwave the milk in short bursts so as to avoid burning the milk. And also, if I just microwave the milk for the same amount of time, the milk doesn't taste burnt.

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Have you tried boiling water instead? Milk has fat solids that could be burning. –  Yamikuronue Jun 29 '12 at 15:47
    
@Yamikuronue: I haven't tried boiling water actually. I like my coffee milky and thick (being south indian :)), so I have always used milk. –  Dharini Chandrasekaran Jun 29 '12 at 15:50
    
Are you adding instant coffee to hot milk? or hot water & milk? –  Cos Callis Jun 29 '12 at 19:15
    
@CosCallis: Hot milk.. –  Dharini Chandrasekaran Jun 29 '12 at 21:17
    
I don't think it's the coffee. I make Black Coffee in the microwave everyday, with instant coffee, and it never tastes burnt. Probably the milk. –  elssar Jan 15 '13 at 3:38
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4 Answers

My suspicion is that the milk is burning. You should microwave it in small bursts, stirring after each one (because microwaves heat unevenly).

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1. I do microwave it in short bursts. 2. When I heat only milk this way, for the same amount of time, the milk doesn't taste any different. It doesn't taste burnt. So why does the addition of instant coffee make it worse? –  Dharini Chandrasekaran Jun 29 '12 at 17:04
    
@DhariniChandrasekaran With that information, I suspect my initial suspicion was wrong. I'm not a coffee drinker, but I have experienced things going wonky when microwaved, so I figured I'd start there. Hope someone else has a better idea! –  Yamikuronue Jun 29 '12 at 17:21
    
@DhariniChandrasekaran: If you left details out of your original question, then you should edit it to be complete. –  Aaronut Jun 30 '12 at 16:13
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It could be that the milk is getting so hot that it burns the coffee granules when you add them. Liquids can boil in the microwave without bubbling, because the inside surface of ceramics are often so smooth as to have few nucleation points. Try leaving the milk to cool for a minute before adding the coffee.

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Excellent suggestion. To put it differently, liquids in the microwave can get above their boiling point whilst remaining liquid (i.e. they don't turn into gas, which is the bubbling) - so you have a hotter liquid than you can get from the stove. It would be interesting to know if leaving it to cool fixes the problem. Note - stirring it will help cool it down, but if it is superheated it can explode out of the cup when you do this which would be very dangerous to your face. –  standgale Jul 5 '12 at 2:56
    
Indeed. I cringe when I see people making tea by boiling water in the microwave then chucking in a teabag - superheated water, meet millions of non-dissolving nucleation points! –  ElendilTheTall Jul 5 '12 at 8:03
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At what power level do you use your microwave? If it is the milk that burns, setting your microwave to, say, 200 Watt and tripling the duration can heat it more evenly. When I heat something like a few slices of cheese in the microwave, I always set it to 100 Watt, because at 900 Watt there will be boiling patches of fat in the cheese within 10 seconds. At 100 Watt for 20 seconds, however, it gets warmed up evenly to room temperature (starting from fridge temperature).

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Milk does not get burnt when microwaved, if the milk is still very fluid. Impossible to get burnt.

It is the coffee beans that can be possibly burnt by microwave.

Food gets heated or cooked by the micro-wave radiation inducing resonant vibration in the molecules of materials. When molecules vibrate they get hot.

The most susceptible radicals in molecules of our food are the -OH radicals.

Water being H2O, i.e. H-O-H, is almost the target of microwave inventor/manufacturers to induce that vibration.

However, oils also have that significant -OH radical. Oils have a significantly higher boiling point than water. Some oils can sustain up to 450 deg F without their -OH radicals breaking down. Hence, microwave oven manufacturers caution you against deep-frying in a microwave oven. You could possibly cause a fire.

There are other radicals that are also susceptible to being vibrated by microwave. For example polymer bonds in melamine-ware.

Coffee beans have oils. Imagine subjecting your coffee beans to 450 deg F. And if the -OH bonds do break down, the oil is burnt. If not the high temperature would be roasting the kernel material.

Even if you placed the beans into the milk to microwave it, the water in the milk will be unable to permeate the beans as deep as the microwave could. Therefore, you could be roasting the internals of the beans at high temperatures without being able to be cooled down by the surrounding water.

Not being a coffee connoisseur, I request that you explore soaking the coffee beans in hot milk or water for a couple of hours to soften the kernel of the bean and allow water to permeate into the bean before microwaving the beans with the milk. Alternative, microwaving the beans+milk at 10% power until the beans softens enough to absorb water.

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'Tis instant coffee per the original question--no whole beans, just the dehydrated coffee granules, I would imagine. –  SAJ14SAJ Jan 15 '13 at 12:48
    
"dehydrated" coffee granules. How far can water permeate into a 2mm coffee granule before the microwave penetrates its core to hot spot its temperature to 400 deg F? Let's say that 20% of the granules could hot-spot - is that sufficient to give the concoction a burnt taste? Hot-spots is a persistent problem in microwave cooking. Water/steam convection is a mitigating agent that distributes heat away from hot-spots. Keywords: dehydrated, oils, hot-spots. –  Blessed Geek Jan 15 '13 at 13:14
    
The standing wave patterns in microwaves are on the scale of 3-4 inches, since that is the wavelength of the particular microwaves used in home style ovens. –  SAJ14SAJ Jan 15 '13 at 14:04
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