Talk: McDonald's coffee case

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This is terrible. It was her fault for spilling it.

It needs to be added that the coffee was served extra hot because water expands when it is heated up, and thus people think they are getting more coffee than they actually are. This is a good example of companies doing something which will detract from the product for marketing reasons. There was no practical reason for the coffee to be so hot. Another example would be feeding cows salt, which makes them retain more water so they weigh more, even though they have no more actual meat. This renders their manure, which could otherwise make good fertilizer, useless.

Answers to common arguments in McDonald's Coffee Case

According to the The Physics Factbook, the best temperature at which to brew coffee is between 85 C and 93 C (176 F to 200 F). These temperature ranges also correspond to just about all home coffee makers sold today. However, the longer the coffee is exposed to those high temperatures the faster the organic matter breaks down. This produces the biterness so often found in coffee.

The question of expansion is mute since the average temperature at which McDonalds keeps their coffee is approximately the same as coffee makers found in homes today. But, for the sake of argument let's assume the following:

• Diameter of an 8 oz McDonalds coffee cup (near the top) is 3 in (7.68 cm).
• Cylinder height = h
• Formula for a cylinder's volume is Vc = h x pi x r^2
• 8 oz (US liquid) of coffee is approximately 2.366 x 10^-4 m^3
• Average McDonalds coffee temperature (Tmc) = 89 C
• Hypothetical average coffee temperature for sensitive mouths (Ts) = 60 C
• Volume Coeffecient of thermal expansion of water (B) = 210 x 10^-6 C^-1
• Change in Volume = Vd (delta)
• Change in Temperature = Td (delta)
• Formula for the change in volume is Vd = B x Vo x Td

Plugging the numbers into the formula give us:

Vd = (210 x 10^-6)(2.366 x 10^-4)(89 C - 60 C)
Vd = 1.44x10^-6m^3 or 1.44 cm^3

The change in the level of the coffe in the cup (when full) is solved by finding h.

h = Vc / (pi x r^2)
h = (1.44 cm^3) / (3.14.15 x 3.84^2)
h = 0.031 cm or 0.31 mm

The change in volume between these two temperatures is therefore negligible.

--
Larry Snider
eSnider.net

Actual Data Point

Someone in my family was working at McDonald's about that time. She said many of the stores were switching from the usual brew-pot setups to single-serving equipment that used forced steam. These machines would yield a liquid at nearly the boiling point, if not actually superheated. She herself was burned by the equiment many times.

Whether or not you think it's negligent to put a liquid at a temperature that causes third-degree burns within seconds into flimsy cups and hand them out a window to motorists (who in 1992 typically had nowhere to put them but between their legs), no evidence has been given to support your premises that McDonald's brewing procedures are based on "the best temperature at which to brew coffee" or that the "average temperature at which McDonalds keeps their coffee is approximately the same as coffee makers found in homes today".