How much am I hurting the environment when I have a sushi tuna roll?

September 21, 2009 6:02 PM

What are the thermodynamics of frozen sushi-grade tuna?

I just toured the Tsukiji Fish Market and learned that the tuna there is kept on the boat at -48 Celsius for up to 2 years before it is delivered to the market. How much energy is expended in this process, and how does this information translate into the environmental impact of me eating a tuna roll? Say a bluefin tuna weighs 200kg, and let's assume an average tuna is kept frozen for one year and that the tuna is at 15 degrees Celsius when caught (I couldn't find great data on this; tuna body temperature is higher than the average water temperature, but I couldn't find absolute data on this--please chip in if you have any), and that a tuna roll contains one oz. of tuna.

I don't know how the tuna is frozen or what energy source is used for this, so I'm not sure how to make assumptions about that, but perhaps you do.

posted by underwater to science & nature (7 comments total)

Also, I understand that there are many other factors influencing the environmental impact of a tuna roll, but let's limit it to the issue of getting an oz. of tuna from the ocean to the fish market in a frozen state.
posted by underwater at 6:04 PM on September 21

You will need:

the specific heat of muscle (first link suggests 4kJ/(kg C)) for the initial freezing
the efficiency of ship-sized freezers (actually, this looks appropriate) for holding the cargo at a steady temperature. The estimates in the table and text suggest that even the extravagant process of generating and using liquid nitrogen would cost tenths of pennies per pound of fish.

I could BS a physics answer, but the best estimate will come from the cost of the oil needed to power the freezer. I'm sure you are asking this on a lark, but my bet is that David MacKay at this point would tell you driving to the sushi restaurant was more harmful.
posted by gensubuser at 6:29 PM on September 21

I wouldn't worry about body temperature. The amount of energy you have to move out of a liquid (particularly water, which is the main component of a tuna's body) to lower the temperature TO freezing is pretty low compared to how much you have to move out to change the state to solid AT freezing.

Similarly, let's say the tuna freezes at -5° C. Lowering the temperature to -10° or -48° isn't going to use that much more energy. (Keeping a freezer at the lower temperature WILL use a lot more energy though, but without knowing how much they insulate, what the economies of scale are, what the weather is, etc it's impossible to know how much that costs them in energy.)

But I don't understand why you are fixated on the coldness. Surely that's a pretty minor cost compared with, say, powering the fishing boat and shipping plane/truck.
posted by DU at 6:30 PM on September 21

From this page, the specific heat capacity of tuna is 3.18 kJ/kg/degree C above freezing, and 1.63 kJ/kg/degree C below freezing. Thus you'd have to pump out
3.18 * 15 + 1.63 * 48 = 125.94 kJ/kg ~ 3.6 kJ / oz of meat.

The maximum coefficient of performance of a refrigerator is
QP = T_Cold / (T_Hot - T_Cold) (Temps in Kelvin)
which assuming a T_Hot (outside temp) of around 15 C gives
QP_Max ~ 3.5
Then the energy used by the refrigerator must be greater than
3.6 / 3.5 ~ 1.0 kJ / oz of tuna.

These are essentially thermodynamic limits, real tech will have inefficiencies, etc. If the tuna is stored packed in a very large, well-insulated fridge, I would think that the energy required to maintain the cold interior would be relatively small. So maybe we're talking a couple times this as the max energy used.
posted by Humanzee at 6:35 PM on September 21

Whoops, embarassing. DU is right, the freezing is the big issue!
posted by Humanzee at 6:36 PM on September 21

Latent heat of freezing for meat ~ 249 kJ/kg.
(actually that book looks like it's all over this problem).

That just about triples the result, so > 3.0 kJ / oz tuna.
</physics degree revoked>
posted by Humanzee at 6:41 PM on September 21 [1 favorite]

Uh, something else to consider. If you buy a roast and put it in your freezer, you can't unplug the freezer after said roast gets good and solid and expect it to be OK in a month. To really work this out, you'd need to know how good the freezer was at keeping things cold once it got them there in the first place.
posted by Kid Charlemagne at 7:45 PM on September 21