Why is my Coffee Hotter - Page 1
Forum section: electronics / general

author: dimlow
author_rank: Frequent Contributor
timestamp: March 03, 2011, 02:30:15 pm
content: Ok, now i know this is not the place to post this here, but this puzzled me. I like a nice cup of milky coffee. I make it with a Senseo machine. First i normally fill half a mug with milk, pop it in the microwave for 130 seconds then take it to the Senseo machine and top it up with strong coffee. This makes a nice temperature for drinking right away.Now today, i forgot to put the milk in the microwave, so i ended up with a cold cup of coffee after i had filled it from the Senseo machine. So i popped it in the microwave for 130 Seconds. I took a gulp and burnt the roof of my mouth! WTF?? technically its had the same amount of energy added to it, so why is it hotterAn Electronics forum may not be the place to post this. But there must be some bright physics chaps here that knows about these things.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 03:12:52 pm
content: is your microwave setting to fixed temperature setting for the whole 130secs? whats the temp setting (if fixed) or the temp graph/ramp (if not)?

author: Excavatoree
author_rank: Frequent Contributor
timestamp: March 03, 2011, 03:20:25 pm
content: The only theory I have is that the microwave somehow adds more energy to the full cup (1/2 coffee and 1/2 milk) than it does the half cup of milk only.Microwaves are at the resonant frequency of water molecules, so perhaps having a greater number of them (after you add the coffee) causes the oven to be more efficient.There are items you could put in a microwave that wouldn't get warmer at all - milkfat may be more like these items.

author: JohnS_AZ
author_rank: Frequent Contributor
timestamp: March 03, 2011, 04:30:20 pm
content: Quote from: Excavatoree on March 03, 2011, 03:20:25 pmThat would be my bet.But technically, it would be that the full cup ABSORBS more energy, the microwave still creates the same amount.Also, the microwave doesn't bring the coffee to a specific temperature, it raises the temperature based on how long it's in there. So if you start with a liquid that is hotter than usual, it'll come out hotter an you expect.

author: Chasm
author_rank: Regular Contributor
timestamp: March 03, 2011, 04:50:34 pm
content: Do not forget the thermal mass of the cup itself.

author: Simon
author_rank: Unknown
timestamp: March 03, 2011, 06:14:49 pm
content: 130 seconds for half a cup of milk ? I put mine in for 90 seconds and it nears boilig point

author: Zero999
author_rank: Super Contributor
timestamp: March 03, 2011, 06:44:33 pm
content: Quote from: JohnS_AZ on March 03, 2011, 04:30:20 pmYes, that makes sense to me, the larger the mass, the more energy it will absorb.Have you ever tried to zap a small insect in a microwave? It's very difficult because of the very small size of the animal compared to the wavelength of the radiation.

author: Russel
author_rank: Regular Contributor
timestamp: March 03, 2011, 06:59:11 pm
content: The space inside of an operating microwave oven is being exposed to microwave radiation. So, with more mass and surface area there is more energy absorbed. Similar to a larger solar panel collecting more energy than a smaller one.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 03, 2011, 07:30:09 pm
content: Quote from: Excavatoree on March 03, 2011, 03:20:25 pmthis is a correct and general postulate proven by dimlow himself.i was about to propose my theory that mixing hot+cold will result in medium temp drinkbut i changed my mind since i assume the microwave is operating at constant energy or wave radiation per unit time.so, it got something to do with "heat absorption rate of a material"...water+milk have lower absorption rate, adding the coffee will increase it, making it more efficient at absorbing microwave energy.its like aluminium or normal metal (or non metal) heatsink, or resistant in a circuit, less resistance will allow more current coming in.so high heat absorption rate mean less resistant to energy transfer (wave to the drink).

author: PetrosA
author_rank: Frequent Contributor
timestamp: March 03, 2011, 08:08:05 pm
content: Could it also be that the warmer a liquid is, the more excited the molecules are to begin with and the easier it is for the microwaves to excite them further?

author: Wim_L
author_rank: Regular Contributor
timestamp: March 03, 2011, 11:44:03 pm
content: Quote from: PetrosA on March 03, 2011, 08:08:05 pmSort of... While it's easier to excite them at higher temperatures, the molecules also experience less drag, so loss is lower overall at high temperatures in pure water. Hot water doesn't absorb the radiation as well as cold water. However, coffee isn't exactly pure water, and dissolved salt will increase absorption significantly at higher temperatures. See:http://www.lsbu.ac.uk/water/microwave.htmlAnd no, I didn't find measurements on coffee specifically.

author: Chasm
author_rank: Regular Contributor
timestamp: March 04, 2011, 12:21:15 am
content: Quote from: Wim_L on March 03, 2011, 11:44:03 pmI doubt that there never has been any research on coffee, must have been a cover up operation of enormous proportions.

author: PetrosA
author_rank: Frequent Contributor
timestamp: March 04, 2011, 01:04:21 am
content: There has been no research on coffee in microwaves because (and I say this as an ex coffee house owner) no self-respecting coffee drinker drinks nuclear brew.

author: Psi
author_rank: Super Contributor
timestamp: March 04, 2011, 01:49:36 am
content: I would have said it's just because microwaves heat up water molecules, more water means more heat can be absorbed.

author: Mechatrommer
author_rank: Super Contributor
timestamp: March 04, 2011, 02:22:34 am
content: it goes back to "heat absorption rate of a material" generally as the analogy of "R resistance". discussing molecules is like discussing the behaviour of electrons in electrically excited circuit. no problem at all, if you really want to get into deep science. i need to re-open my thermodynamics book to explain it in more detail.

author: apex
author_rank: Regular Contributor
timestamp: March 04, 2011, 08:24:27 am
content: Here is my explanation (pure speculation):If a thing is hot, it means it has entropy.Entropy is heat energy.Entropy is also described as chaos.If something is hot, it's molecules vibrate.The larger the amplitude, the hotter the matter.A microwave stimulates the molecules so they start to vibrate.If they already vibrate (if there is already an amount of entropy in the object),they sync with the phase and amplify the amplitude.This has an influence on the amount of entropy: it grows.Since the volume is nearly constant (nearly due to the thermal expansion coefficient) and the entropy has become more,the temperature has to rise, because temperature is entropy divided by volume.So:If your cup and it's content is already hot, if you put it into the microwave, the microwave works faster.apex

author: dimlow
author_rank: Frequent Contributor
timestamp: March 04, 2011, 08:55:25 am
content: Wow, that got you lot thinking, I'am surprised nobody asked if I took sugar in my coffee. I thought that more sugar, and fats for that matter in a material , the quicker the thing heats in the microwave. I'am pretty sure I have heard that somewhere. Anyway I don't have sugar ! so that counts that out. This morning I made my coffee the normal way, at totally drinkable temperature.Oh my microwave has no fancy temperature settings, just a timer, in fact it was a very cheep microwave. this one in facthttp://direct.tesco.com/q/R.204-5670.aspx

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2011, 06:00:58 pm
content: Adding sugar should increase the temperature because it increases the density and microwave absorption.Fats will heat up but not to the same extent as water.

author: Russel
author_rank: Regular Contributor
timestamp: March 04, 2011, 07:31:17 pm
content: The way in which microwaves heat water molecules is really interesting. Water molecules are polar having two hydrogen and one oxygen atom in a V configuration. Because of the V shape and the covalent bond between the hydrogen and oxygen atoms water molecules are polar with the oxygen side of the molecule (the bottom of the V) being negative and the hydrogen side (the top of the V) being positive. Microwaves cause water molecules to spin, as the wave changes the water molecule changes with it, basically spinning. Basically, the water molecules become hot due to friction as they spin.I remember when microwave ovens first became popular. I don't know how many times that I heard that they cook from the inside out. Well that isn't really correct. Microwave energy heats the outside of the food more than the inside because the energy is absorbed as it travels deeper into the food. This is no big secret, I'm sure everyone that cooks with a microwave oven has seen this.Simply put, I think that with more surface area absorbing microwave energy and heating more mass causes a full cup to absorb more energy than a half cup.

author: Zero999
author_rank: Super Contributor
timestamp: March 04, 2011, 07:48:58 pm
content: Also note that heating has nothing to do with resonance of the water molecules. Liquid water has no single strong resonant because the resonances are damped by the hydrogen bonds which exist in liquid water. The lowest resonant frequency of water molecules (this only applies to steam) is around the 22GHz mark which is nine times the frequency of a microwave.You'll also find it very difficult to heat ice. This is because the water molecules are each firmly hydrogen bonded to each other so they can't rotate. What normally happens when you put frozen food in the microwave is it only starts to absorb microwaves and head once it's defrosted slightly due to room temperature. If you tried to use your microwave to defrost something outside on a cold winter's day, when the temperature is below freezing, it won't work.

author: Chasm
author_rank: Regular Contributor
timestamp: March 04, 2011, 10:45:49 pm
content: Quote from: PetrosA on March 04, 2011, 01:04:21 amCoffee reheated in a microwave is not the best, that's true.It's still better than the usual kept warm variants. May they be Thermos jug, usb heater or the large coffee urns.The important part is to let the coffee cool down right after it comes out of the coffee maker. - After siphoning of the first cups of course.I think getting the right coffee powder has more influence, some stuff is just yucky. But you should know that better than I do. *g*

author: PetrosA
author_rank: Frequent Contributor
timestamp: March 05, 2011, 03:37:55 pm
content: Quote from: Chasm on March 04, 2011, 10:45:49 pmOh! The horror!I mostly drink espresso coffee (and its variations like cappuccino). We're lucky enough to live in a house with an "off-peak" electric meter, which means that 220V loads are cheaper. I keep my commercial machine on 24 hours a day and it only costs me about $5 per month - about the same as two cups of coffee from Starbucks

author: Chasm
author_rank: Regular Contributor
timestamp: March 05, 2011, 11:48:59 pm
content: Quote from: PetrosA on March 05, 2011, 03:37:55 pmOf course! I'm way to lazy to grind my own coffee.Even if I actually have a couple of coffee grinders.(Old hand mills and an old electric that sounds more like a buzz saw.)Quote from: PetrosA on March 05, 2011, 03:37:55 pmUnfortunately the nice machines are everything but cheap.Most "home" and also several commercial machines are a real pain to clean. That is also annoying, I don't want to spend half an hour cleaning the thing - but then I really don't want mold in it.So it is back to the normal filtered coffee. Or the moka pot, or the French press.I really should get another small espresso machine. That thing was cool. For some reason my brother remembered to get it back after a while... *g*

author: tyblu
author_rank: Frequent Contributor
timestamp: March 06, 2011, 12:16:28 am
content: Milk and coffee likely have similar ~2.4GHz absorption characteristics. Microwave puts out the same power regardless of contents. More ~2cm-deep surface area (volume, roughly) results in greater percentage power absorption, hence more energy over equal periods of time. The absorption fraction versus effective surface area relationship is not linear, but can be regarded as such for small changes in volume.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: March 06, 2011, 12:15:21 pm
content: Heating up 150ml of milk in my microwave results in a much lower temperature liquid than heating 150ml of water (empirical evidence, using a liquid temp probe and a Fluke meter). I have no reasoning as to why this is, microwaves are complex.So if you heat the milk alone, it will not get as hot as the mixture of water (coffee) and milk.What power is your microwave? A mug of water hits boiling point in 60s in mine.

author: PetrosA
author_rank: Frequent Contributor
timestamp: March 06, 2011, 03:07:00 pm
content: Quote from: Chasm on March 05, 2011, 11:48:59 pmTrue, they're not cheap. If you're seriously thinking of a good machine, you may want to try looking for a local coffee roaster/seller who leases machines to their customers or a company that does service. Oftentimes they'll have used commercial machines that they take out of circulation for cosmetic reasons but they still work fine. They sell some of them pretty cheap, too. I've seen $5000 machines selling used for $500-$800, so not much more than an entry-level snobby home machine. It's true they get moldy, but only if you're not using them regularly - and never if you keep them turned on.

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2011, 04:16:48 pm
content: Quote from: cybergibbons on March 06, 2011, 12:15:21 pmIt's simple, milk has a higher fat content than water and also contains other chemicals and water absorbs microwaves more than fat so will heat more.Milk is mostly water though so I wouldn't expect it to be significant. Did you make sure the milk and water were exactly the same temperature before heating them and use exactly the same containers?

author: cybergibbons
author_rank: Frequent Contributor
timestamp: March 06, 2011, 04:44:47 pm
content: Quote from: Hero999 on March 06, 2011, 04:16:48 pmI don't think it is simple at all... water does absorb more energy that fats, but fats have a much lower specific heat capacity. But adding salt to water in a microwave changes how quickly the water boils due to increased conductivity of the liquid. Also, adding certain chemicals to water reduces the depth the microwaves penetrate the fluid.Both samples were at room temperature and both were heated in two mugs of the same design at the same time...

author: tyblu
author_rank: Frequent Contributor
timestamp: March 06, 2011, 05:08:51 pm
content: Quote from: cybergibbons on March 06, 2011, 04:44:47 pmEr, "...at the same time"? Microwaves have hot-spots (and cold-spots). Do you mean one right after the other?

author: Chasm
author_rank: Regular Contributor
timestamp: March 06, 2011, 09:40:05 pm
content: Quote from: PetrosA on March 06, 2011, 03:07:00 pmYeah, once I have the money to blow.There are some nice ones out there. I was playing a bit with one of the Schaerer Coffee Art machines as IT admin for a smallish coffee shop -very easy to clean- but currently 3k used without add-ons like the mandatory water filter. The Schaerer Coffee Celebration line is also very nice and even easier to clean, but at the price of a nice car that should better be the case.So it is back for a simple espresso machine without extras. But no pads or capsules, thank you.Quote from: PetrosA on March 06, 2011, 03:07:00 pmFor some of the better commercial ones maybe, I simply don't have enough experience with them.The expensive home versions are often simply impossible to take anything apart to clean, all you can do is is emptying the waste container and run the self clean function. That is not always good if you have internal milk containers and whatnot.Sometimes I wonder when they will make you send it in to empty the waste container.

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2011, 10:10:50 pm
content: Quote from: cybergibbons on March 06, 2011, 04:44:47 pmI don't see why that's any surprise but note that it's not all about conductivity, fats do absorb microwaves despite being insulators in which case it's dielectric heating not eddy currents as is parly the case with salt water.QuoteSalt will do that, not that it makes any difference in a glass of water.

author: PetrosA
author_rank: Frequent Contributor
timestamp: March 07, 2011, 03:05:01 pm
content: Quote from: Chasm on March 06, 2011, 09:40:05 pmWhen you start to look for a machine seriously, I'd recommend something along the lines of one of these in the semi professional category:Isomac Millenium IIIBezzera BZ07 Semi AutomaticPasquini Livia 90 Semi AutomaticRocket Cellini Premium II Semi-Commercialla Valentina Semi-AutomaticVibiemme Domobar Super Manual (needs to be plumbed in)They are more expensive than some, but they have certain advantages:- They all use a single boiler with heat exchange. This means that there is a tube running through the boiler that the water for coffee is made from. You're not using stale water that way. The boiler produces both steam and hot water for tea.- They all have an on/off switch for the pump. Unless you're using a machine commercially, automatic dosing systems aren't worth the extra cost. You can replace the switch yourself if needed with (usually) locally available parts.- They're all designed to be left on for extended periods. Because they have boilers, once the water temp gets up, they only need to cycle on/off to maintain the temperature.- They all have solenoid backflow valves which release the water feeding the group head after making coffee. This helps flush out buildup.- They all use standard, commercial size portafilters and accessories. The two things you'd need to replace on a regular basis (every 6-12 months or so) will be the rubber portafilter gasket and the group head screen.- They are all made to commercial specs with brass, copper and stainless parts. Most have commercial grade internal controls for water temp, pump pressure etc.- Because of the simplicity of mechanical design (little or no automation) they are extremely dependable.Quote from: Chasm on March 06, 2011, 09:40:05 pmThe milk container for the models I named is called a refrigerator. How often you clean it is entirely up to youThe only cleaning on this type of machine is the group head, which is very easy to clean and the steam wand and drip tray which are even easier to clean.These are the closest thing you'll get to a true commercial machine found in real cafes (not starbucks). Places like starbucks have to rely on automated machines because of the class of employee they have to deal with and the volume of coffee they have to make in such a short time because they sell take-out. In good Italian and French cafes (still what I consider the best) they generally have full time professionals working the coffee machines and there is much less automation unless they have huge seating capacity. A large cafe in Italy with two or three machines might use up to 25 kg of espresso coffee per day (55 lbs for the metrically challenged) and at about 9 grams per shot, that's over 2,750 cups of espresso/cappuccino per day. At that point automation comes into play for the additional purpose of reducing workplace injuries. Professional 58mm porta filters weigh a lot and throwing them around all day to make that many coffees will quickly lead to some nasty carpal tunnel issues and very sore joints.Ok, enough of this. I'm off to make another cappuccino

author: cybergibbons
author_rank: Frequent Contributor
timestamp: March 07, 2011, 10:04:07 pm
content: Quote from: tyblu on March 06, 2011, 05:08:51 pmDoesn't your microwave have a turntable to stop this being an issue?

author: cybergibbons
author_rank: Frequent Contributor
timestamp: March 07, 2011, 10:06:57 pm
content: Quote from: Hero999 on March 06, 2011, 10:10:50 pmIt all goes towards the fact that it isn't "simple" to judge if a liquid or solid will heat slower or faster than pure water...

author: Zero999
author_rank: Super Contributor
timestamp: March 07, 2011, 10:54:28 pm
content: Quote from: cybergibbons on March 07, 2011, 10:06:57 pmNo one said that was the case.Quote from: cybergibbons on March 07, 2011, 10:04:07 pmThe mugs would have to be placed an equal distance apart opposite the centre of the turntable to ensure they receive the same dose of radiation.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: March 08, 2011, 09:08:15 am
content: Quote from: Hero999 on March 07, 2011, 10:54:28 pmQuote from: Hero999 on March 06, 2011, 04:16:48 pmHmm.

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2011, 06:38:59 pm
content: What's your point? You obviously don't understand why some substances absorb microwaves and I do. I don't think it's complicated at all, you obviously do.

author: cybergibbons
author_rank: Frequent Contributor
timestamp: March 08, 2011, 07:24:10 pm
content: You've contradicted yourself by saying "it's simple" why one heats up quicker than another, and then say that no one said it was simple to judge if a liquid or solid will heat slower of faster than pure water...More to the point, I read up about heating of milk in microwaves and it sounded like my experiment produced the wrong results. So I've tried it again with a larger volume of liquid, longer times, and running the test one after another at full power. This is semi-skimmed filtered milk that is on the turn.Seems the milk heats up quicker than water.

author: Time
author_rank: Frequent Contributor
timestamp: March 10, 2011, 11:33:42 pm
content: Altering the viscosity of a liquid will effect how quickly it heats. If the water dipole can rotate more freely in a highly viscous environment than the electric dipole fields will be less out of phase with the e-field in the microwaves than in a less viscous solution. The difference in phase in the shifting of these dipoles and u-wave e-field determines the amount of energy lost to heating.Fats and sugars carry hydroxyl groups which can exhibit similar properties. Fat also has a lower specific heat but this would probably be less of a factor in a solution type situation as opposed to solid.

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2011, 10:51:50 pm
content: Quote from: cybergibbons on March 08, 2011, 07:24:10 pmYes, I suppose my language was confusing. I suppose you're right, I thought it was more straightforward than it is.Time,I wouln't expect viscosity to make much of a difference, for example syrup will heat faster than pure water due to the sugar content.

author: Time
author_rank: Frequent Contributor
timestamp: March 11, 2011, 10:59:50 pm
content: Yes, but thats due to specific heat. Water has a very high specific heat so it takes a lot of thermal energy generation to change the temperature. Syrup will generate less thermal energy but it will heat up faster because it takes less thermal energy to change the temperature of the substance.