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The year is 2016, your budget is in the tens of billions, and your job is to create the fastest possible toaster: specifically, the transition from regular, cold slice of bread to hot, lightly browned, crisp toast must be as short as possible. Practicality is irrelevant. The toaster cannot burn the bread; it has to be a proper slice of toast.

How fast can one get, and what would such a machine look like? What sort of theoretical limits cap the speed of a toaster?

As a bonus question, how fast could a toaster produced if the only limitations are the laws of physics (and the definitions of bread and toast), thus allowing theoretical technology, planet-sized structures, etc

EDIT: To clarify, the question is not about highest rate of production, but about making the actual toasting process fastest.

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    $\begingroup$ If people want to edit my question, how about some better tags? I couldn't find any more relevant ones than the ones I specified... $\endgroup$
    – user371366
    Feb 18, 2016 at 22:45
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    $\begingroup$ Strap a pair of STS SRBs onto its sides and light them off. Your toaster ought to reach 6 km/s. $\endgroup$
    – Jim2B
    Feb 18, 2016 at 23:15
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    $\begingroup$ I think that current toasting technology is pretty close to as fast as possible. You might be able to reduce the time by 25 or 50% at most, but the main limiting factor is conduction of heat through the bread, which you can't make any faster without changing the laws of physics (or changing your bread). $\endgroup$
    – 2012rcampion
    Feb 18, 2016 at 23:38
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    $\begingroup$ I think the bigger question is why toast is so important to this society. Can they not survive the two minute wait for a piece of delicious, crispy bread? $\endgroup$
    – Xandar The Zenon
    Feb 19, 2016 at 0:37
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    $\begingroup$ @user867 Some people want warm bread and some people want charcoal, that knob is critical or it's no good as a toaster. $\endgroup$
    – Separatrix
    Feb 19, 2016 at 8:11

4 Answers 4

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The point of toasting bread as we all know is the creation of 6-acetyl-1,2,3,4-tetrahydropyridine and to get it crispy enough to hold up in a BLT.

Unfortunately, that molecule of the gods can only be formed in the 285°F/ 140°C and 320°F/ 160°C range. Hotter than that you get bitter flavors and charcoal. This reduces the efficacy of lasers and supernovas.

Your best and mundane bet is to pass it under the salamander and call it a day.

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Simple.

Create a machine that finishes new slice of toast every 1 second (or 0.001 seconds). This is extra simple as multiple slices can be baked in parallel. Then, at any point, you can wait only 1 second (or 0.001 seconds) to pick a freshly made toast. Rest is thrown out as garbage.

This approach can reach any speed you want. It will just require much more parallel system and produce huge amount of uneaten toasts.

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    $\begingroup$ +1 This is the solution that most people ignore, but is exactly correct for this question. $\endgroup$
    – Samuel
    Feb 19, 2016 at 17:15
  • $\begingroup$ "specifically, the transition... must be as short as possible. Practicality is irrelevant." This is not about making a toast available on as short notice as possible, this is about making the actual toasting take as little time as possible (I have now clarified my question to reiterate this) $\endgroup$
    – user371366
    Feb 22, 2016 at 17:55
  • $\begingroup$ @dn3s My answer is still valid. It is still best answer if you are asking for "shortest time between deciding to have a toast to having that toast". $\endgroup$
    – Euphoric
    Feb 22, 2016 at 18:33
  • $\begingroup$ that is the best answer for that question, but that is not my question. I am asking for "shortest time between bread and toast", considering the individual piece of bread. You are getting confused by the fact that it is common for humans to eat toast after deciding they want toast. Im my question, the human is irrelevant as far as toast production is concerned; The focus is on the toast itself. $\endgroup$
    – user371366
    Feb 22, 2016 at 18:36
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26s (perfectly toasted) 11s (good enough)

Regardless of how you heat it, the limiting factor is how fast the heat propagates through the bread without heating the surface to much. From King-Inks answer, the limit is 160° C, and the goal core temperature is 140° C.

I am assuming a 1cm slice of bread, heated from both sides.

Thermal data required to model heat flow.

Together with a typical bread density of 200 g/L, I used a method consisting of slicing the bread into a large amount of individual slices, and calculate the heat flow between them is small time increments.

If you relax the restrictions a little, say allowing a surface temperature of 180°C and only requiring half the bread to reach 140°C, the toasting can go as fast as 11s.

We can not make that so much better, because we are limited by conductive heat flow. A little internal heating, with for example microwaves, may shorten the time a little, but it is not a good idea considering quality. (As anyone who have tried can confirm.)

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  • $\begingroup$ I like this, I think it could serve as a base for other answers. I don't think that the thermal conductivity of bread needs to be the limit; aside from conduction there's convection and ratiation. Another answer mentions lasers/microwaves, and I am imagining some sort of grid of super-thin needles spewing hot air driven through the toast at a controlled speed. $\endgroup$
    – user371366
    Feb 22, 2016 at 18:40
  • $\begingroup$ You could use little microprobes through the side to dump heat into the center directly and reduce the time needed to propogate through the slice. $\endgroup$
    – Oldcat
    Feb 23, 2016 at 1:07
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48 msec

Approach

So what you'll want is a multifrequency laser. Since there is no such thing, then what you'll do is use a MASER (microwave amplification by the stimulated emission of radiation - a microwave laser) plus an IR or optical laser.

The purpose of the microwave is to deposit heat through the depth of the bread.

The purpose of the IR/optical laser is to brown the surface and make it toasty.

Powerful lasers

I know there are IR lasers at least 10x more powerful and probably MIRACL (mid-infrared advanced chemical laser) is 100x more powerful than this one.

Pretty picture of a 15 KW $CO_2$ laser.
Pretty picture of a 15 KW $CO_2$ laser

Energy to toast bread

This site claims that it takes a 1200 W toaster 2 minutes (120 seconds) to toast two pieces of bread.

I know there's a lot of wasted energy in the toaster (hot air escaping the top but I don't know how to scale it down)

$$E = P * t = 1.2 kW \times 120 sec = 144 kJ$$

Since we're interested in a single piece of bread, divide that by 2 to get 72 kJ.

How long for the laser to toast bread

We need our combined lasers to deposit 72 kJ of energy into the bread (this seems way too large).

Our MIRACL laser can deliver 1,500 kW of power so: $$ t = \frac{72 kJ}{1500 kW} = 0.048 sec $$

MIRACL can toast your bread in 48 millisecond.

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    $\begingroup$ 70 kJ is enough to flash about 20 g of water into steam, so I think you might be overdoing it a little. You have got me curious though, I think when I get home I'll try microwaving some bread to see if it makes edible toast. $\endgroup$
    – 2012rcampion
    Feb 18, 2016 at 23:46
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    $\begingroup$ I think just microwaving will make it soggy and then hard (it'll drive out the moisture). I think you'll need both surface heating and heating in depth to get it done right. Without doing the heat transfer equations, I have no feel for what the proportions ought to be. $\endgroup$
    – Jim2B
    Feb 19, 2016 at 0:26
  • $\begingroup$ and I agree. The numbers I got were for how much power a toaster takes to toast bread. I couldn't find anything on how much energy it takes to toast bread. $\endgroup$
    – Jim2B
    Feb 19, 2016 at 0:27
  • $\begingroup$ The energy is almost certainly too high, but I like your approach. $\endgroup$
    – Samuel
    Feb 19, 2016 at 0:50
  • $\begingroup$ @2012rcampion I make cheese sandwiches with the microwave, and I can confirm that microwaving it will only make it warm and soggy. $\endgroup$
    – Xandar The Zenon
    Feb 19, 2016 at 4:35

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