Mug Cooling: Initial Results

One of life’s greatest pleasures is a nice cup of tea or coffee.

  • But what temperature makes the drink ‘nice’?
  • And how long after making the beverage should we wait to drink it?
  • And what type of mug is optimal?

To answer these questions I devised a research proposal involving temperature measurements made inside mugs during the cooling process.

I am pleased to tell you that my proposal was fully-funded in its initial stage by the HBRC*, having scored highly on its societal impact.

Experimental Method

The basic experiment consisted of pouring approximately 300 ml of water (pre-stabilised at 90 °C) into a mug sitting on a weighing scale. The weighing allowed low uncertainty assessment of the amount of water added.

The temperature of the water was measured every 10 seconds using four thermocouples held in place by a wooden splint. The readings were generally very similar and so in the graphs below I have just plotted the average of the four readings.

Experiments were conducted for a fancy vacuum-insulated mug (with and without its lid) and a conventional thick-walled ceramic mug. The results for the vacuum-insulated mug without its lid were so surprising that I repeated them.

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Results

The average temperature of the water in the mugs is shown in the two graphs below.

The first graph shows all the data – more than 8 hours for the vacuum insulated mug – , and the second graph shows the initial behaviour.

Also shown are horizontal lines at various temperatures that I determined (in a separate series of experiments) to be the optimal drinking range.

Slide1

The average temperature of the water in the mugs versus time.

Slide2

The first 120 minutes of the cooling curves. The water was poured in at 4 minutes.

Discussion

The most striking feature of the cooling curves is the massive difference between the results for the vacuum insulated mug with, and without, its lid.

As I mentioned at the start, the result was so striking that I repeated the measurements (marked as #1 and #2) on the graphs.

The table below shows how many minutes it took for the water to cool to the three states highlighted on the graphs above:

  • Too hot to drink, but just sippable
  • Mmmm. A nice hot cuppa.
  • I’ll finish this quickly otherwise it’ll be too cold.

Minutes to reach status

  Vacuum-Insulated Mug

Ceramic Mug

 No Lid

 With Lid

Just Sippable

2

10

66

Upper Drinkable Limit 12 24

151

Lower Drinkable Limit

28

53

296

Conclusion

The insulating prowess of the vacuum insulated mug (with lid) is outstanding.

But the purpose of a mug is not simply to prevent cooling. It is to enable drinking! 

So to me this data raises a profound question about the raison d’être for vacuum insulated mugs.

  • Who  makes a cup of coffee and then thinks “Mmm, that’ll be just right to drink in two and a half hours time!”

Admittedly,  the coffee will then stay in the drinkable range for an impressive two hours. But still.

In contrast, the ceramic mug cools the hot liquid initially and allows it to reach the optimal drinking temperature after just a few minutes.

Further work

The review committee rated this research very highly and suggested two further research proposals.

  • The first concerned the explanation for the very large effect of removing the lid from the vacuum insulated mug. That research has already been carried out and will be the result of a further report in this journal.
  • The second concerned the effect of milk addition which could significantly affect the time to reach the optimal drinking temperature. That research proposal is currently being considered by HBRC.

==============================

*HBRC = Hot Beverage Research Council

7 Responses to “Mug Cooling: Initial Results”

  1. James Says:

    I have been conducting my own research on this vitally important matter, particularly regarding cooling time of cups of tea outside.
    1) different mugs of similar size can have very different cooling coefficients. Is this due to the ceramic or glaze or both?
    2) when carrying a normal sized uninsulated mug of tea with no lid out to somebody 5 minutes away outside then generally the tea would be too cold by the time they get it
    3) a vacuum mug or double walled ceramic mug with a lid will be fine in the same situation and also you’ll have spilt less tea.
    4) very large mugs of tea with no lid perform pretty well outside and are still drinkable for quite a while
    5) the number of flies killed by a mug of tea vs time would make an interesting graph. It is low for small and large times with a peak in the middle

    • protonsforbreakfast Says:

      James, great points. Obviously if one were transporting the beverage then a vacuum insulated mug with a lid makes a lot of sense. And of course, if the beverage were pre-cooled to the drinking range (with milk or cool water) then it could remain drinkable for a long time.

      More experimental results will follow soon.

  2. Jonny Berliner Says:

    I feel it is important to include the effect of drinking the tea, and the subsequent loss of mass on the cooling rate. Presumably a half cup loses heat more quickly by having a higher surface area to volume to ratio. You could then advise people on the optimal time to drink their hot beverage in. I’m always amazed by how much faster my tea cools once I’ve started drinking!

  3. Martyn Says:

    One consideration on the usefulness of the vacuum cup: even though it has no handle, you can comfortably hold it even with the liquid at 90C. The ceramic mug would be very difficult to hold at this temperature without the handle. The vacuum cup can also sustain the drink made using high pressure steam in the coffee shop which tends to be around 95C.

    The vacuum cup is also more time efficient. The longer temperature sustain means fewer refill trips for adding hot water for very slow drinkers.

  4. Mug Cooling: Salty fingers | Protons for Breakfast Blog Says:

    […] I began writing these articles (1, 2, 3) I was just curious about the effect of insulation and […]

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