Posts Tagged ‘heat capacity’

Staying warm: Cast iron vs. stainless steel

Thursday, March 1st, 2007

Cookware made from cast iron has a reputation for keeping food warm for a long time. Is that really true? Best way to find out is by an experiment. I decided to compare a cast iron pot with one of stainless steel. These are the pots I used:

cast-iron-stainless-steel.jpg

For the first experiment I filled them each with 2,5 L of water, put the lids on and brought both to the boil and let them boil for a minute so the pot itself would be warm throughout. Then both were placed on cork plates and left to cool. The temperature probe was carefully inserted under the lid in order to reduce the heat loss, and removed once the temperature had stabilized. For the second experiment 5 L of water were used. The measured temperatures are shown in the graph.

cooling-curve.jpg

Contrary to what I had expected, the stainless steel pot keeps water warmer! After approximately 1,5 hours there is a 10 °C difference between the two. As expected, when using 5 L of water, it stays warm longer. Physical data for the two pots are given in the following table:

Cast iron Stainless steel
Volume 6 L 6 L
Diameter 27,9 cm 25,0 cm
Height 11,5 cm 14,5 cm
Surface area
(top+sides)
1619 cm2 1629 cm2
Surface area
in contact with 5 L water
1301 cm2 1286 cm2
Weight 6,1 kg 2,3 kg
Wall thickness ~4 mm <1 mm
Heat capacity of pan 2,8 kJ/K 1,2 kJ/K
Thermal conductivity 80 Wm-1K-1 16 Wm-1K-1
Thermal diffusivity 22 x 10-6 m2/s 4.3 x 10-6 m2/s
Emissivity 0.95 0.07

The heat capacity of the cast iron pot is more than double that of the stainless steel pot. But this is negligible compared to the heat capacity of water: 10.5 kJ/K (2,5 L) and 20,9 kJ/K (5,0 L). Also, there is only a small difference in their surface area which cannot explain the large difference in temperature loss observed.

This leaves me with two eplanations:

  • Cast iron is better heat conductor and has a higer thermal diffusivity
  • Cast iron (being nearly black) has a much higher emissivity than a polished stainless steel surface. The reason for this is that absorption and reflection of radiation are related.
  • My guess is that the difference in emissivity is more important (but please correct me if I’m wrong). With an infrared thermometer, one should therefore be able to measure a difference between pots of cast iron and polished stainless steel (even though they’re at the same temperature!) due to the difference in emissivity. Any one who can do the experiment and report back?

    Conclusion: There are many good reasons to use cast iron, but keeping food warm is not one of them!

    Practical molecular gastronomy, part 3

    Monday, February 26th, 2007

    Get a basic understanding of heat transfer, heat capacity and heat conductance.

    Since a lot of cooking involves temperature manipulations, it’s a good idea to get a basic understandning of how heat is transferred and how well it is stored in different materials. “Heat” in this context does not imply high temperature since it also applies to the understanding of freezing/thawing.

    ceramic-stove-top.jpg
    Closeup of ceramic stove top

    Heat transfer

    Conduction: flow of heat through an object or between two objects in contact. Metals are typically good conducters whereas air is a poor heat conductor.

    Convection: heat transfer occurs because particles are moved from a warm region to a colder one. One can say that convection is a combination of conduction and mixing. For example, convection occurs when heating water since its density varies with temperature – warm water is lighter than cold water and will float. This video illustrates convection currents in water as a crystal of potassium permanganate dissolves (this salt is not edible).

    Radiation: in the kitchen we encounter two types of heat transfer by radiation corresponding to two different parts of the electromagnetic spectrum. The heat we feel from hot burning charcoal, a stove top or the sun are all a result of infrared radiation. The other type is microwave radiation. Heat transfer by radiation does not require a material for the heat to pass through (as a consequence, a blowing wind will not have any significant effect when grilling). Microwaves easily penetrate plastic, glass and wood, but not metal. Infrared radiation is blocked by opaque materials.

    Heat capacity and heat conductance

    Heat capacity: the heat requried to raise the temperature of the material. Water has a very high heat capacity, metals (shown in red) generally a low heat capacity.

    Heat conductance: how well heat flows through the material. Some metals (shown in red in the graph) are excellent heat conductors (silver, copper, aluminum), others less so (iron and stainless steel). All other materials (shown in blue) are generellay poor heat conductors.

    The heat capacity (or to be precise, the specific heat capacity – which means heat capacity per weight unit) and the heat conductance of materials encountered in the kitchen are plotted in the the graph below:

    heat-capacity-conductance.jpg
    (for the technically interested, the plot units are Wm-1K-1 for the heat conductance and Jg-1K-1 for the specific heat capacity)

    For a more extensive treatment of heat transfer, heat capacity and heat conductance (+ more on cooking methods and materials) in a gastronomical setting, I recommend the Gourmet Engineering Lecture Notes for a very interesting course given at Tufts University in Medford, MA, USA. Cooking for Engineers also has a nice post on heat transfer and browning of foods and one on common materials of cookware (with comprehensive comparisons of different materials used).

    Examples related to food preparation and handling

  • Convection ovens utilize fans to circulate hot air allowing reduced cooking times and temperatures. Because of efficient convection, two or more trays can be baked simultaneously.
  • In a steam oven water is introduced to increase the humidity (this can also be done by spraying water into the hot oven). Heat transfer is more efficient due to 1) the higher heat capacity of humid air and 2) the energy released when steam condenses onto the surface (it’s the energy it took to boil the water in the first place). For bread, the condesed water prevents the surface from drying out which facilitates the exapansion of the loaf. Furthermore, the hot surface causes starch to gelatinize and subsequently dry into a delicate crust.
  • Water will cool faster than the same volume of a thickened soup because of less resistance to the convection currents in water. The amount of convection decreases in the following order: water > chicken soup > creamy soup > thick onion soup > porridge. In the latter heat is transferred by conduction only from the interior to the exterior (where heat transfer proceeds mainly by radiation and conduction). This will also affect cooling times, which is of importance with regard to microbial safety (food should be cooled rapidly past the window from 30-60 °C where microorganism thrive).
  • For rapid defrosting, place the frozen food in cold water or on a metal object – this will allow an efficient transport of heat to the frozen food. Defrosting in a microwave is not easy because most of the water molecules are locked in rigid structure and even microwaves cannot make them move (they only melt by conduction of heat from melted neighbouring areas).
  • To freeze icecream or a parfait, use a metal container as this will allow a faster dissipation of the heat in the freezer.
  • When whipping cream, it’s essential to keep the temperature low (otherwise the fat will melt). Use a thick glas bowl and cool it in the freezer before whipping.
  • When cooking meat in a pan or on a grill, notice how the surface browns relatively fast compared to the time it takes for the interioir of the meat to heat up. Heat transfer to the surface by radiation or conduction is very efficient compared to conduction of heat through meat itself. Therefore it’s advisable to fry/grill the meat at high temperature first to get a nice browning, then let the meat rest for 5-10 min to allow for heat conduction to the interioir (cover with aluminum foil to reduce radiative heat loss), followed by a second frying/grilling at lower temperature until desired doneness.
  • In an oven, the heating caused by radiation can be increased by moving food closer to the walls or reduced by wrapping the food with reflective aluminum foil. For example, to caramellize sugar on a creme brulee if you don’t have gas burner, place them as high as possible in the oven, preferably using a grill element. Turkey legs stick out and easily get overdone – wrapping them with aluminum foil reduces heat radiation from the oven walls.
  • For a bain marie, always use a metal bowl as this gives you better temperature control. When making egg based sauces such as hollandaise or bernaise, use a thin metal bowl this allows rapid heating and cooling (if temperature gets to high, the metal bowl allows quick cooling which might save the sauce).
  • A pizza baking stone has a higher heat capacity than a metal plate/sheet – this ensures proper rising and gives a crispy crust.
  • Ever burnt your tongue on a pizza? Tomatoes (mostly water) retain heat far better than the crust (many air bubles, low heat capacity) and cheese topping (cools fast due to radiation from surface).
  • The vacuum in a thermos does not conduct heat by conduction or convection, only by radiation. The latter is minimized (in thermoses of glas) by a silver or aluminmum coating, creating a reflective mirror.
  • From the graph it doesn’t seem like cork is a particularly good insulator. This is because the heat conductance is plotted per weight unit. For a porous material such as cork, the effective heat conductance is much lower than for the same volume of other materials.
  • Lastly, just to illustrate how complex heat transfer and convection sometimes can be, take a look at the Mpemba effect: Believe it or not, under certain conditions, hot water freezes faster than cold water!
  • *

    Check out my previous blogpost for an overview of the tips for practical molecular gastronomy. The collection of books (favorite, molecular gastronomy, aroma/taste, reference/technique, food chemistry) and links (webresources, people/chefs/blogs, institutions, articles, audio/video) at khymos.org might also be of interest.

    Perfect steak with DIY “sous vide” cooking

    Sunday, January 21st, 2007

    One important aspect of molecular gastronomy is the application of scientific principles to food preparation in a normal kitchen. This can very well be illustrated by discussing the preparation of a steak. The surface of the meat needs to be heated to > 120 °C (250 F) for the Maillard reaction to take place at a reasonable rate. This gives meat much of it’s characteristic aroma. The interior of the meat however should not be heated to more than 50-65 °C (120-150 F) for a rare or a medium rare appearance. If the heat is provided by a frying pan with a temperature typically in the range 120-160 °C (250-320 F), the different temperature required for the interior and the surface of the meat can actually be quite difficult to achieve. Bringing the meat to room temperature before cooking by taking it out of the fridge 1-2 hours in advance helps. Also, half way through the cooking it’s advisable to let the meat rest on a plate to allow the heat to diffuse into the interior and to let the surface cool down a little.

    There is however an easier way to make a perfect steak! In restaurants the method has been around since the 70’s and is known under the name sous vide (fr. under vacuum, more info on history of sous vide in this NY Times article). The meat is packed in plastic bags, vacuumed and put into thermostated water baths. This equipment is not (yet?) found in the average kitchen. So here is a simple DIY procedure. You just use a normal plastic bag, leave the meat in the water bath for 30 min (or longer) and then quickly fry both sides to generate the products of the Maillard reaction. You do need a thermometer though to control the temperature of the water bath, preferably one with a dip in probe.

    1. Put the meat (I used a rib eye steak for this experiment) in a thick plastic bag. Only put one or two pieces of meat in each plastic bag – this ensures a greater contact surface with the water.

    meat in plastic bag

    2. Add any spices you like (salt and pepper always works well – for the experiment shown I used curry paste, soy sauce and chili sauce in stead), press (or suck) out the air and close the plastic bag tightly by tying a knot (or use a zip-lock bag). You don’t want any water to enter the bag!

    meat in plastic bag

    3. Heat a pot of water to the desired temperature (or use hot tap water) and place the plastic bag with meat in the water. Cover with a lid (not shown in the picture) to reduce heat loss. If you use a large pot of water it’s easier to keep the temperature constant. Also, it’s easier to control the temperature with an induction or gas stove top than with an electric plate since there is no additional heating once you turn them off. Regarding the temperature, start with 60 °C (140 F) and experiment from there (or check this table at Wikipedia for doneness temperatures of meat). You should leave the meat in the water for at least 30 minutes – more for a thicker cut. But the good thing is you can leave it for much longer (several hours) provided the temperature does not come above 60 °C (or whatever temperature you decided on). A convenient way to keep the temperature constant for a long time is to put the pan with water into the oven and use the thermostat of the oven.

    meat in plasticbag, water at 59 C

    4. Heat a frying pan, add a fat of you choice, remove meat from plastic bag and brown both sides of the meat. Since you take the meat directly from the water bath it’s already at about 60 °C. Therefore the browning is very fast.

    meat-in-frying-pan

    5. A temperature of 60 °C (140 F) gives the meat a pink interior. It’s succulent and juicy. The short frying gives it a nice browned crust and the chewing resistance is perfect. All in all a wonderful combination of taste, aroma, texture and mouth feel!

    meat-interior

    Note added January 2009:
    Since I published this procedure the first time I’ve learnt a lot more about sous vide. The procedure above is a rather crude procedure, but it works. If the meat turns out grey you’ll need to turn the temperature somewhat down. If you’re interested in reading more about sous vide, the best discussion I know of which also includes important safety aspects is Douglas Baldwin’s “A Practical Guide to Sous Vide Cooking”.

    Related posts:
    A mathematician cooks sous vide
    Sous vide cooking joy
    Santa came early this year
    Upcoming books on sous vide

    Baking stones / Pizza stones

    Friday, September 15th, 2006

    Every now and then I end up explaning to friends how they can make good pizza at home. One of the main issues is how to obtain the nice, crisp crust. And of course I’m referring to Italian pizza now, not American pan pizza. The simple answer to this is: use a baking stone (pizza stone) (assuming of course that the dough and filling is home made). Or if you like going to extremes: get a wood fired baking oven made from stone. But with a simple baking stone, you can achieve prette good results even with an electric/gas stove. There is a simple explanation: When a pizza dough is baked on a metal plate, the evaporating moisture is not able to escape from underneath the pizza. The result is a soggy, unappetizing crust. Also, the cold dough will let the temperature of the metal plate/sheet drop relatively quickly. As a result, the yeast dough will not rise as much before the gluten network solidifies and prevents further rising of the dough.

    A baking stone is made from a porous ceramic material. It’s heat capacity is good (much higher than that of a metal plate/sheet) and as a result, when the cold dough is placed on the baking stone, it still has enough heat to make the pizza rise immediately. Secondly, the fact that the baking stone is porous lets it absorb moisture from the pizza. This is what gives the nice crisp crust as it transports moisture away from the pizza.

    I also recommend to set your oven at the highest temperature possible – preferebaly in the range 250-300 °C (480-570 F). At this temperature, the pizza will be ready in less than 10 minutes. And remember – don’t use soap when cleaning your baking stone. Remember that it’s porous and you definitely don’t want your next pizza to taste like soap. Just scrape of residues of cheese once they have turned to carbon dust. If necessary, you can wipe it of with a moist paper towel.