Posts Tagged ‘kitchen physics’

TGIF: Food related “Periodic videos”

Friday, August 20th, 2010

I believe most chemists are familiar with the “periodic videos” from the University of Nottingham, covering all the known chemical elements. The series features professor Martyn Poliakoff who’s grey hair is really worthy of a professor! They have now covered the complete periodic table of elements, and have even started to update some of their previously posted videos. There are also thematic videos as well as videos covering specific molecules appearing now. As a chemist I think the videos are great fun to watch since they show a number of exotic experiments I’ve never seen before combined with plenty of nice-to-know facts. I certainly recommend all these videos (for an overview, check out their website), but the reason I chose to blog about this is that I was delighted to find a number of more or less food related videos! These are definitely not going to make you a better cook. But some of them are quite amusing to watch, and you may even learn some chemistry as you go. But most of the food related videos are really just for fun :)

Cheeseburger in hydrochloric acid

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The Flemish Primitives: Glowing lollipops (part 4)

Wednesday, February 11th, 2009

Bruce Bryan demonstrated a glowing cocktail drink (top left), and tempted us with fluorescent cake frosting (top right). The chocolate surprise boxes included a lollipop (bottom left) and I was quite busy sucking the lollipop, listening to the translation of the Belgian/French/Spanish contributions, taking notes and photographing at the same time (bottom right).

The chocolate surprise box was one of the highlights at The Flemish Primitives that I’ve blogged about three times already. As I promised you in the last post I’d come back to the lollipop that was included in the box. Between chocolates number 2 and 3 Bruce Bryan entered the stage. The lights went off, we were instructed to suck intensely on the lollipos and then – when I took the lollipop out of my mouth it was glowing! (more…)

Soda fountain science explained

Thursday, June 19th, 2008


Picture by Michael Murphy (CC-BY-SA)

The soda fountain (Diet Coke + Mentos) has been around the net for quite a while with some spectacular videos available, and it has even made it into a news paper cartoon. People go crazy about this and the largest number of simultaneous fountains is steadily increasing.

Despite the interest, only now did a scientific paper appear on the subject. Many have speculated about what causes the reaction between Mentos and Diet Coke, and some have focused on possible acid-base reactions taking place. Mythbusters investigated this in 2006 (watch episode) and came up with the following factors that contribute to the bubble formation:

Diet coke

  • carbon dioxide is what makes the bubbles form in the first place
  • in synthetic mixtures aspartam, caffeine and potassium benzoate where shown give better fountains

Mentos

  • the most important property is the rough surface which provides plenty of nucleation sites for bubble formation
  • the density makes them sink which is ideal as the bubbles formed at the bottom of the bottle help expel much more soda
  • mentos contains gelatin and gum arabic which could also reduce surface tension

In the paper “Diet Coke and Mentos: What is really behind this physical reaction?” by Tonya Shea Coffey the findings of the Mythbuster teams are largely confirmed.

By measuring contact angles it was shown that aspartame and potassium benzoate reduce the surface tension of water. Aspartame is a winner, and as an extra benefit clean up is much easier with Diet Coke than sugared Coke. The amount of caffeine however is too low to have any effect. The roughness of the Mentos surface was studied with special microscopes (see picture below). Fruit Mentos have smooth patches, but the coating is not uniform and contrary to the Mythbuster experiment normal Mentos and Fruit Mentos performed equally well with regards to foam formation. The roughness of the Mentos surface was inbetween that of rock salt and the Life savers which suggests that roughness is not a single factor determining the reaction. The Mentos surface is covered with gum arabic which reduces surface tension, and experiments showed that even without Mentos, gum arabic could cause a reaction to occur. It is the combined effects of reduced surface tension (due to ingredients in Diet Coke and Mentos) and the rough surface of Mentos which is the key to understand the reaction.

As expected, the article also confirms that the reaction is more vigours at higher temperatures (i.e. solubility of carbon dioxide deacreases with increasing temperature). It was also shown that Mentos sink faster to the bottom of a 2 L bottle compared with rock salt, Wint-O-Green Life savers and sand (this is a function of size and density, not only density). When bubbles are formed at the bottom of the bottle the bubble has more time to grow as it rises. This causes a more explosive reaction and more soda is expelled from the bottle.


The picture shows scanning electron microscopy images of Mint Mentos (a) and (c) and Fruit Mentos with a candy coating (b) and (d). The scale bars in each image represent the lengths (a) 200 μm, (b) 100 μm, (c) 20 μm, and (d) 20 μm. Fruit Mentos has smooth patches, but the coating is not uniform. (Reprinted with permission from Coffey, T. S, American Journal of Physics, Vol. 76, Issue 6, pp. 551-557, 2008. Copyright 2008, American Association of Physics Teachers)

The question which lingers on my mind is whether Diet Coke and Mentos represent the optimal combination of ingredients to create a soda fountain. With regard to convenience, I guess the answer is yes. But perhaps it’s possible to create an even more powerful reaction? Since lowering the surface tension of water is important, I’m wondering if it would be possible to find a surfactant that could be added without setting the reaction off? Mentos would of course still be needed for the rough surface to provide nucleation sites. In the above mentioned study addition of diluted dish washing liquid was enough to give a pretty good reaction, so this is not an option. But perhaps a couple of drops right on the Mentos surface would work? I definitely need to try this some time.

Hydrocolloid recipe collection v.2

Wednesday, May 21st, 2008

Texture – A hydrocolloid recipe collection
It’s a pleasure for me to announce that an updated version of the hydrocolloid recipe collection is available for free download as a pdf file (73 pages, 1.8 Mb).

What’s new?
Several new recipes have been added (now counting more than 220 in total), including recipes with cornstarch, guar gum, gum arabic, konjac and locust bean gum. All in all 14 different hydrocolloids are included (plus lecithin which technically isn’t a hydrocolloid). In each section recipes are now sorted according to the amount of hydrocolloid used. The appendix has been updated with tables for comparison of hydrocolloid properties, hydrocolloid densities and synergies. The perhaps biggest change is that all recipes have been indexed according both to the texture/appearance of the resulting dish and according to the hydrocolloid used. Let’s say you want to make spheres, this index will show you which hydrocolloids can be used (that’s right – there are other possiblities than sodium alginate) and list the example recipes.

Foreword
A hydrocolloid can simply be defined as a substance that forms a gel in contact with water. Such substances include both polysaccharides and proteins which are capable of one or more of the following: thickening and gelling aqueous solutions, stabilizing foams, emulsions and dispersions and preventing crystallization of saturated water or sugar solutions.

In the recent years there has been a tremendous interest in molecular gastronomy. Part of this interest has been directed towards the “new” hydrocolloids. The term “new” includes hydrocolloids such as gellan and xanthan which are a result of relatively recent research, but also hydrocolloids such as agar which has been unknown in western cooking, but used in Asia for decades. One fortunate consequence of the increased interest in molecular gastronomy and hydrocolloids is that hydrocolloids that were previously only available to the food industry have become available in small quantities at a reasonable price. A less fortunate consequence however is that many have come to regard molecular gastronomy as synonymous with the use of hydrocolloids to prepare foams and spheres. I should therefore emphasize that molecular gastronomy is not limited to the use of hydrocolloids and that it is not the intention of this collection of recipes to define molecular gastronomy.

Along with the increased interest in hydrocolloids for texture modification there is a growing scepticism to using “chemicals” in the kitchen. Many have come to view hydrocolloids as unnatural and even unhealthy ingredients. It should therefore be stressed that the hydrocolloids described in this collection are all of biological origin. All have been purified, some have been processed, but nevertheless the raw material used is of either marine, plant, animal or microbial origin. Furthermore hydrocolloids can contribute significantly to the public health as they allow the reduction of fat and/or sugar content without loosing the desired mouth feel. The hydrocolloids themselves have a low calorific value and are generally used at very low concentrations.

One major challenge (at least for an amateur cook) is to find recipes and directions to utilize the “new” hydrocolloids. When purchasing hydrocolloids, typically only a few recipes are included. Personally I like to browse several recipes to get an idea of the different possibilities when cooking. Therefore I have collected a number of recipes which utilize hydrocolloids ranging from agar to xanthan. In addition to these some recipes with lecithin (not technically a hydrocolloid) have been included. Recipes for foams that do not call for addition of hydrocolloids have also been included for completeness. Some cornstarch recipes have been included to illustrate it’s properties at different consentrations. Recipes where flour is the only hydrocolloid do not fall within the scope of this collection as these are sufficiently covered by other cook books.

All recipes have been changed to SI units which are the ones preferred by the scientific community (and hopefully soon by the cooks as well). In doing so there is always uncertainty related to the conversion of volume to weight, especially powders. As far as possible, brand names have been replaced by generic names. Almost all recipes have been edited and some have been shortened significantly. To allow easy comparison of recipes the amount of hydrocolloid used is also shown as mass percentages and the recipes are ranked in an ascending order. In some recipes, obvious mistakes have been corrected. But unfortunately, the recipes have not been tested, so there is no guarantee that they actually work as intended and that the directions are complete, accurate and correct. It appears as if some of the recipes are not optimized with regard to proper dispersion and hydration of the hydrocolloids which again will influence the amount of hydrocolloid used. It is therefore advisable to always consult other similar recipes or the table with the hydrocolloid properties. The recipes have been collected from various printed and electronic sources and every attempt has been made to give the source of the recipes.

Since recipes can neither be patented nor copyrighted, every reader should feel free to download, print, use, modify, and further develop the recipes contained in this compilation. The latest version will be available for download from the static Khymos site and will also be announced here. I would like to thank readers for giving me feedback and suggestions on how to improve the collection. Feedback, comments, corrections and new recipes are always welcome at webmaster (a t) khymos ( dot ) org.

10 elements of basic kitchen knowledge

Tuesday, May 13th, 2008


Salt in oil. According to Pierre Gagnaire, this is Hervé This’ main discovery. It allows him to sprinkle salt on dishes without the salt dissolving in water from the dish. Thereby the “crunch” of the salt is retained.

Rob Mifsud, perhaps best know for his Hungry in Hogtown blog has interviewed Hervé This. At the end of the interview Hervé lists 10 elements of basic kitchen knowledge. Some may seem obvious, but they are not, according to Hervé. Here’s the list so you can judge by yourselves:

  1. Salt dissolves in water.
  2. Salt does not dissolve in oil.
  3. Oil does not dissolve in water.
  4. Water boils at 100 °C (212 °F).
  5. Generally foods contain mostly water (or another fluid).
  6. Foods without water or fluid are tough.
  7. Some proteins (in eggs, meat, fish) coagulate.
  8. Collagen dissolves in water at temperatures higher than 55 °C (131 °F).
  9. Dishes are dispersed systems (combinations of gas, liquid or solid ingredients transformed by cooking).
  10. Some chemical processes – such as the Maillard Reaction (browning or caramelizing) – generate new flavours.

Ice cubes and air bubbles

Sunday, April 13th, 2008

Ice cubes are used both to cool drinks, but also to significantly impact the flavour of certain drinks. No matter your motivation, you should never use “old” ice cubes which have been sitting in your freezer for a while. Why? Melt some “old” ice cubes and taste the water. You’ll smell why! The reason is that volatile compounds in your freezer slowly find their way into the ice cubes which for some reason mostly are made in trays without a cover. But as I surfed around, researching this post I discovered that oxo and other producers now sell ice cube trays with lids. That’s a small step forward!

Another thing about ice cubes is that they look nice. I admit that air bubbles can sometimes be quite beautiful (and even artistic when pictured with a macro lens as above), but there are times when I whish I could make perfectly clear ice cubes. At room temperature a certain amount of air is dissolved in water. When you cool water, the solubility of air increases (!), but only until the water starts freezing. At this point the water can no longer keep the air dissolved and a bubble is formed. Vice versa – when you boil water the solubility of air decreases and the dissolved gases escape.

When making ice cubes, the bubbles that are formed can easily escape as long as there is no ice blocking their way. This is sort of a catch 22 situation since the air expulsion is directly related to the ice formation. When making ice cubes in a normal freezer, the ice cubes are cooled from the outside, causing the air to get trapped throughout the ice cube.

Many people have thought about smart ways to achieve this (as a quick patent search shows). There are two strategies to obtain clear ice cubes. Let the gas escape while the water freezes or degas and filter the water before freezing. Icicles are a good example that when running water freezes, it normally produces very clear ice. This is utilized in commercial ice cube makers. Here a “cold finger” is exposed to water that moves. This way bubbles are carried away before they can get trapped. These ice cubes typically are ring or cup shaped. The second method is suggested many places on the net. I’ve listed them here together with some thoughts and discussion.

Degassing
Degas the water (i.e. remove the dissolved air). This is easily done by boiling the water for a couple of minutes and letting it cool again. Some webpages suggest that the process should be repeated for best results.

Slow cooling
If the water is cooled too quickly, the ice will not be able to push the impurities ahead of the freezing interface. But if an ice cube freezes from all sides it doesn’t really help as the bubbles get trapped in the middle. A drawback with slow cooling is that the solubility of gas will increase when the water is cooled and so it will allow more gas to dissolve before the water freezes. So slow cooling should probably be combined with some kind of gas tight cover.

Directional cooling
I’ve been pondering about making trays with insulated sides and cover and a metal base, thereby utilizing the fact that metals are superb heat conductors compared to plastic, wood or glass. The metal would then serve to conduct away heat from the water. Bubbles would form on the ice front, but they would probably escape, rather than become encapsuled into the ice. I’ve tried to illustrate it here:

Turns out that someone has actually patented something similar where metal “fingers” are used to conduct away heat from the center, giving ring shaped ice cubes. Does anyone know if these were ever made for sale? Perhaps an ice cube tray in aluminum would work if one insulates the top so that the cubes freeze from the bottom and up, keeping the water on top free flowing so bubbles can escape.

Layer-by-layer method
There might be a simple (but time consuming) way of achieving directional cooling: By building up the ice cubes layer by layer. Once the first layer is frozen this will help freeze the next layer from the bottom up and so on. I guess layers of 1-5 mm would work, but this needs more testing. My experiments so far have not been very promising. Plenty of bubbles, even with a layer of only 2 mm.

Filtering
Particles can act as nucleation sites for air bubbles. To avoid this filter the water and make sure that all the equipment is clean. Also, don’t use a towel to try your equipment as this will probably leave small fibers behind.

Remove salts
Both tap water and bottled water contain trace amounts of salts. When water freezes these minerals are not incorporated into the ice structure. As a consequence the soluble salts will concentrate in the water that’s not yet frozen. In the end there is so little water left that the concentration of the salts becomes sufficiently high so that the freezing point of this remaining water is lower than the temperature in the freezer (meaning that this water won’t freeze). Other salts, especially calcium salts such as calcium carbonate will precipitate. And these particles can act as nucleation sites. If after boiling water there are particles present, these should be filtered away before freezing. The easiest way to get rid of salts is to use distilled water.

I’ve done a couple of experiments and it seems there is no quick fix. The water in the ice cubes pictured above was boiled for several minutes before freezing, but plenty of bubbles formed as you can see. I also tried the layer-by-layer method, but even in a thin layer of only 2-3 mm I could detect many bubbles. So clearly I need to do more experiments.

What are your experiences with making clear ice cubes?

Carbonated fruit the iSi way

Monday, April 9th, 2007

I blogged about carbonated strawberries some while ago. Those were made using dry ice which unfortunately is not always easy to get hold of. Last week however I bought a iSi Gourmet Whipper – one of those Ferran Adria uses to make foams/espumas. I plan to experiment with that as well, but the first thing I decided to prepare was carbonated fruit. In fact this is a safe way (the only?) to make carbonated fruit at home using a pressurized container.

isi-whipper.jpg

The instruction booklet which comes with the iSi Gourmet Whipper only mentions cream chargers (filled with N2O, dinitrogen oxide), whereas soda chargers (filled with CO2, carbon dioxide) are not mentioned (I guess the opposite is true for the iSi Siphons?). This is quite amazing actually! Luckily however the cream and soda chargers are exactly the same size and both hold 8 g of gas. So it should be possible to make carbonated fruit with any of the iSi whippers (cream, easy, gourmet, dessert, thermo) or siphons available.

Here’s how you proceed:

  1. Fill you iSi whipper (or siphon) with fruit, preferably fruit which has a cut, wet surface to allow the carbon dioxide to dissolve in the water/juice.
  2. Screw on top securly
  3. Charge with one soda charger (two if you have the 1 L whipper)
  4. Leave in fridge over night
  5. Release pressure with valve (Important!)
  6. Unscrew top and serve immediately!
  7. Enjoy!

carbonated-grapes.jpg
This is what carbonated grapes look like. As you see, I decided to cut the grapes in to halves.

carbonated-grapes-closeup.jpg
Notice how they sizzle!

A quick recap of the chemistry: cold water dissolves more CO2 than tempered water, that’s why we leave it in the fridge. Also, remember that it takes some time for the carbon dioxide to dissolve in water, therefore it’s better not to be in a hurry. A quick calculation of the pressures gives the following: Both gases have molecular weights of 44 g/mol, so 8 g of gas corresponds to 0.1818 moles or 4.1 L at 25 °C and 1 atm pressure. The volume of the chargers is 0.01 L which gives an initial pressure in the chargers of impressive 445 atm! With an approximate volume of 0.7 L this gives a pressure (in an empty whipper) of nearly 6 atm – the same as in a bottle of champagne. However once you add water, the equilibriums will change and the pressure in the head space will drop. Anyone who remembers how to calculate the head space pressure at equilibrium if the container is filled with 0.5 L of water and cooled to 4 °C?

I’ve done some googling and there is also some mention of making carbonated fruit with an iSi whipper over at Ideas in food.

(The word play in the title works better for those with a mother tongue where iSi would be pronounced just like “easy”!)

Simple temperature calculations

Thursday, March 8th, 2007

Although I recommend the use of a thermometer, sometimes it’s convenient to know how you can also manage without. If you mix water at two different (but known) temperatures, you can easily calculate the temperature after mixing. Just multiply the temperature of each part with the relative amount. For example, if you have 3 dL at 100 °C and 7 dL at 10 °C (which happens to be the approximate temperature of my tap water), this gives (3 dL x 100 °C + 7 dL x 10 °C) / 10 dL = 37 °C which is just perfect for dissolving fresh yeast when making bread.

You can also do it the other way around. Let’s say you have boiling water and you know that your tap water is approximately 10 °C. If you want water at approximately 37 °C, you can do as follows:

temperature-calculation.jpg

Start by writing what you have to the left (100 °C and 10 °C) and what you want in the middle (37 °C). Subtract: (100-37) = 63 and (37-10) = 27. And voilá – you need 27 parts water at 100 °C and 63 parts at 10 °C (and 27:63 simplifies to 3:7 which is what we found above). Now of course if you really wanted water at 37 °C, you would simply put your finger in to see if it’s at body temperature…

Are there any practical applications of this? Yes – a simple, but elegant way to prepare fish would be to drop a fish of known weight and temperature (fridge @ 4 °C or freezer @ -18 °C) into water that has been brought to boil. Cover pot and turn off heat. The amount of water would be calculated based on the desired temperature of the fish. We are assuming here that there is no heat loss to the surroundings, which of course isn’t quite true. How fast pot of water will cool depends on how much water you use and on the pot. This can be corrected for, and luckily someone has already done it. More on this in my post on how to cook fish in cooling water.

We can apply the temperature calculation from above to figure out roughly what the temperature will with this cooking method. 800 g of fish from the fridge (4 °C) and 2,4 L of boiling water gives a temperature of (0,8 x 4 °C + 2,4 x 100 °C) / 3,2 = 76 °C. The cooling curves for a pot with 2,5 L of water suggest a temperature loss of 15-20 °C in 30 min which would bring us down to 55-60 °C which – considering that no thermometer is used – is quite good.

Cooking fish in cooling water

Thursday, March 8th, 2007

I have previously written about how you can cook a perfect steak with a simple DIY sous vide technique. Of course low temperature cooking applies equally well to fish with the only difference that the temperature can be turned down even lower.

A slightly different approach for cooking fish was presented by Haqvin Gyllensköld in the Swedish book “Koka, steka, blanda” from 1977, which I became aware of through Östen Dahlgren’s book “Laga mat – hur man gör och varför”. In stead of keeping the fish at a constant temperature (which requires quite some attention unless you have a thermostated waterbath), in this method, as the hot water cools, the temperature of the fish increases until they’re at the same temperature.

This is how you do it:

  1. Weigh the fish
  2. Boil the triple amount of water
  3. Add some salt to the water (15 g / L)
  4. Put the fish in the water and remove the pot from the stove
  5. Check the graph below for how long the fish should be left in the cooling water
  6. Serve!

cooking-fish-in-cooling-water.jpg

Need help on fish names in different languages? Yeah, me too!

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!