Posts Tagged ‘kitchen physics’

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!
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    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.

    Ten tips for practial molecular gastronomy, part 2

    Sunday, February 11th, 2007

    2. Know what temperature you’re cooking at.

    A dip probe thermometer with a digital read out is a cheap way to bring science into your kitchen. It should preferably cover the temperatures from -30 to 300 °C (-22 to 570 °F). It’s a good idea to check how accurate it is. This is easily done using a water/ice mixture and boiling water.

    calibrate-zero.jpg

    Fill a glas with crushed icecubes and top of with cold tap water. Leave if for some minutes for the water to cool and stir every now and then. Make sure the tip of the probe does not come in direct contact with ice. A mixture of water and ice is exactly 0 °C (32 °F). If the reading is off by 2 °C (~4 °F) or more I would take the thermometer back to the shop and claim a refund.

    calibrate-ninetynine.jpg

    Similarly, you can use boiling water as a high temperature reference point. Water boils at 100 °C (212 °F) at sea level and standard barometric pressure. The exact boiling point at your location can be calculated.

    When I bought my first thermometer it turned out that the temperature readings were quite erratic so I had to return it. The one I have now however works fine (1 degree off for the boiling water is OK).

    As an addition to a dip probe thermometer, contact-less thermometers with infrared sensors are becomming more affordable. Suppliers include Raytek, Strathwood, Radiant (here, here or here) and Extech Instruments (links to product pages at Amazon).

    *

    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.

    TGIF: Levitating strawberry

    Thursday, February 8th, 2007

    The video is from the High Field Magnetic Laboratory in Nijmegen. Read more about levitation and check out their other movies (includes a levitating tomatoe!).

    A short explanation of how this works:

    An object does not need to be superconducting to levitate. Normal things, even humans, can do it as well, if placed in a strong magnetic field. Although the majority of ordinary materials, such as wood or plastic, seem to be non-magnetic, they, too, expel a very small portion (0.00001) of an applied magnetic field, i.e. exhibit very weak diamagnetism. The molecular magnetism is very weak (millions times weaker than ferromagnetism) and usually remains unnoticed in everyday life, thereby producing the wrong impression that materials around us are mainly nonmagnetic. But they are all magnetic. It is just that magnetic fields required to levitate all these “nonmagnetic” materials have to be approximately 100 times larger than for the case of, say, superconductors. This experiment was conducted at the Nijmegen High Field Magnet Laboratory.

    (Via food for design)

    Ten tips for practical molecular gastronomy

    Saturday, January 27th, 2007

    In a recent survey 72% of chefs say they may want to experiment with molecular gastronomy in 2007. That’s an impressive number and considering the attention molecular gastronomy gets in media I bet many home cooks would want to experiment in the kitchen as well. Here’s a list of things to consider if you want to make a scientific approach towards cooking:

    1. Use good and fresh raw materials of the best quality available.

    2. Know what temperature you’re cooking at. A dip probe thermometer with a digital read out is a cheap way to bring science into your kitchen.

    3. Get a basic understanding of heat transfer, heat capacity and heat conductance. “Heat” in this context des not imply high temperature since it also applies to the understanding of freezing/thawing.

    4. Learn how to control the texture of food. Some key points: temperature induced changes (freezing, heating), emulsifiers, thickeners, gelling agents, moisture content, pressure/vacuum, osmosis.

    5. Learn how to control taste and flavor. Some key points: flavor pairings, spice synergies/antagonies, influence of temperature (Maillard reaction, caramelization, temperature stability, volatility), taste enhancers, taste suppresants, solubility of flavour compounds in fat/water, extraction.

    6. Remember that prolonged exposure to a flavor causes desenzitation, meaning that your brain thinks the food smells less even though it’s still present in the same amount. Therefore, let different flavours enhance each other. Similarly, variation in taste, texture, temperature and color can open up new dimensions in a dish. This is referred to as “increased sensing by contrast amplification”.

    7. Be critial to recipes and question authority – they do not necessarily represent “the truth”. Nevertheless, you can certainly learn a lot from the experts.

    8. Dare to experiment and try new ingredients and procedures. Do control experiments so you can compare results. When evaluating the outcome, be aware that your own opinions will be biased. Have a friend help you perform a blind test, or even better a triangle test to evaluate the outcome of your experiments.

    9. Keep a written record of what you do! It would be a pity if you couldn’t recreate that perfect concoction you made last week, simply because you forgot how you did it.

    10. Have fun!

    blue_gas_flame.jpg
    Heat causes many changes in food, but few appreciate how important it is to know at what temperature they are cooking and at what temperature the desired change occurs.

    These tips for molecular gastronomy relate to the technical and scientific aspects of food preparation and eating, and I plan to elaborate on each of the points in separate blog posts. However, according to Hervé This’ definition of molecular gastronomy, one should also investigate the social and artistic components of cooking. A good example of this is the “Five Aspects Meal Model” developed at Grythyttan in Sweden (Gustafsson, I.B. et al. Journal of Food Service, 2006, 84.). Although intended for a restaurant setting, the general idea can also be applied for home cooking.

    The meal takes place in a room (room), where the consumer meets waiters and other consumers (meeting), and where dishes and drinks (products) are served. Backstage there are several rules, laws and economic and management resources (management control system) that are needed to make the meal possible and make the experience an entirety as a meal (entirety – expressing an atmosphere).

    Or to put it differently: average food eaten together with good friends while you’re sitting on a terrace with the sun setting in the ocean will taste superior to excellent food served on plastic plates and eaten alone in a room with mess all over the place.

    One last thing: once you’re finished in the kitchen with your culinary alchemy, your gastro physics, your cutting edge science cuisine, your molecular cooking, your hypermodern emotional cooking, your science food or whatever fancy name you attach to it – remember the social and artistic components when you serve the food. Just so people won’t refer to you as a techno chef, a mad scientist or a modern day Willy Wonka. After all, molecular gastronomy is about the science of deliciousness, not technical wizardry.

    Questions and topics for future blog posts are welcome at webmaster [a] khymos.org (substitute @ for [a]) or as a comment below.

    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

    Happy New Year with the Science of Champagne!

    Sunday, December 31st, 2006

    Have you ever though about how far you can shoot a champagne cork? The swedish physicist Hans-Uno Bengtsson has actually done the necessary calculations in the wonderful Swedish book “Kring flaskor och fysik” (which translates to something like “Among bottles and physics”, it was written together with sommelier Mischa Billing). Assuming a bottle pressure of 6 atmospheres, a cork length of 25 mm (the part in contact with the bottle), a radius of 9 mm and a mass of 7.5 g, this gives an initial cork velocity of approximately 20 meters per second or 70 km/h! This translates into a maximum shot length of around 40 m (if we neglect air resistance). In case you prefer not to shoot the cork, you could of coarse turn to a saber or a heavy kitchen knife instead to open the bottle.

    When opening a bottle of champagne, you might have noticed the cloud forming right above the bottle neck (see picture below). This is due to a significant temperature drop, caused by gas expansion when we open the bottle. Assuming an adiabatic expansion (meaning no heat exchange with the surroundings), Hans-Uno Bengtsson has calculated a temperature drop of 112 °C! No wonder the vapor around the bottle neck immediately freezes forming a small cloud.

    cloud at neck of champagne bottle
    (picture by polarunner at flickr.com)

    If this doesn’t satisfy your craving for champagne science, there’s a whole book on the subject: “Uncorked – The Science of Champagne” by Gérard Liger-Belair. He’s an associate professor of physical sciences at the University of Reims Champagne-Ardenne and probably knows more about champagne bubbles than anyone else! In addition to many fascinating pictures of bubbles, the book has many interesting facts. Did you know that:

  • 0.1 liters of champagne (the contets of an average flute) contains approximately 0.7 liters of carbon dioxide which must escape to restore equillibrium – assuming an average bubble size of 500 micrometers in diameter this corresponds to 11 million bubbles!
  • Contrary to popular belief, nucleation sites for bubbles are not found on scratches or irregularities on the glass itself, but on impurites stuck on the glass wall. These impurities are typically fibres from paper or fabrics.
  • From the point when a bubble leaves the nucleation site till it reaches the surface, the volume increases by a factor of 1 million. This is due to diffusion of carbon dioxide from the solution and into the bubble.
  • Surfactant molecules in champagne form a protective shield around the rising bubbles. This stiffens the bubbles and significantly increases the drag on the bubble as it rises (which gives us more time to admire the trail of bubbles!).
  • The surfactant coating of the bubbles helps keeping them in line as they rise. In pure water, the bubbles would jostle around.
  • The bursting bubbles play an imporant role in flavor release as they collect and concentrate surface active molecules which are thrown against your nose once the bubble bursts, creating a cloud of droplets.
  • (these facts should be perfect conversation starters!)

    trail of champagne bubbles
    (photo by Gérard Liger-Belair)

    An interesting article by Gérard Liger-Belair, “Effervescence in a glass of champagne: A bubble story” is available from Europhysics news.

    Happy New Year!