Extraction of peppermint leaves with hot water

Water is a polar molecule, meaning that one end has a small negative charge and the other a small positive charge. Because of this water is a very good solvent for other polar molecules and ions. For instance water is the solvent of choice for substances that provide taste, be it salt, sour, sweet or bitter as these are normally quite polar molecules.

A general rule is that the solubility of molecules and ions increases with the temperature of the water. Extractions are therefore faster if the water is boiling. This is the reason why we use hot water to extract tea leaves or ground coffee beans, even if we want to prepare ice tea or ice coffee. But by lowering the temperature and extending the extraction time we can change the relative proportion of what we extract. It therefore makes perfectly sense that different temperatures are recommended for different types of tea. Using different temperatures for the same kind of tea will of course also influence the flavor profile.

Polar molecules are more easily extracted than non-polar molecules. This is evident if we leave a tea bag for a long time in hot water. The bitter taste is due to the slow extraction of large polyphenol molecules which are less soluble in water. If tea is brewed at a lower temperature, less of the bitter tasting substances will be extracted.

Although water is polar, less polar and even non-polar substances can be extracted with water, especially if the water is boiling hot. You do this every day when prepare coffee. If you take a close look at cup of freshly brewed coffee you can notice small pools of oily substances floating on top of the coffee. The more severe conditions used when extracting coffee to make an espresso ensure that even more oily substances are extracted. Other examples of extraction using water in the kitchen include preparation of stock, soups and gravies.

The principle of extraction is simple, but a number of questions remain largely unexplored with regard to flavor: How do ions affect extraction? What role does pH play? How does temperature influence flavor? There is surprisingly little research on this that includes a sensory evalution.

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Photo by spielzimmer via flickr.com (CC).

Eric Devlin over at Home of BBQ interviewed me via email about BBQ and molecular gastronomy. The topic should be of interest to the readers of Khymos as well, so I post the questions and answers in extenso here for your benefit.

Q. Martin, thank you for taking the time to discuss the science of BBQ. Before we get into ‘low and slow’ cooking, can you tell us a bit about your background and your interest in food?
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I have a PhD in chemistry and currently I’m working as a research scientist. When I first became interested in the connection between food and chemistry in the late 90’s, I searched the Internet without finding much information. I did however find some very interesting books in the faculty library, including Harold McGee’s “On Food and Cooking - The Science and Lore of the Kitchen”. Having found books about the subject, I soon started to give popular science presentations. In 2004 I was invited to attend the “International Workshop on Molecular Gastronomy” in Erice, Sicily. This was a great experience and I enjoyed meeting many of the scientists, writers and chefs involved with molecular gastronomy. The website I’ve put up, Khymos, is in many ways what I would have liked to find at the time I became interested in the subject.

Q. Over the past few years we have been hearing quite a bit about how food cooked over a hot flame can have increased carcinogens. Would food that is cooked for a longer period of time over a lower heat be safer?

The carcinogens are formed when meat gets burnt, so although you’d like to use high heat to get the Maillard reaction going (which gives you both flavor and color) you don’t want to overdo it. But even if the meat gets a little burnt, it is a good thing that for the carcinogens, as for all other substances, the poison is in the dose. So if you eat grilled meat every day you should be concerned about this, but for most people I think overeating poses a much greater risk!

Q. Serious BBQ cooks like to produce a ‘bark’ when preparing pork for their pulled pork dishes. Usually the natural ‘bark’ of the meat is enhanced by the sugar found in the dry rubs that are applied. Is there any other method that could be used to achieve or increase those results? Maybe an egg wash prior to cooking?

There are several processes which contribute to the flavor formation. First you have the sugars which caramelize. As you correctly state, this is enhanced by adding sugar to the rubs. Furthermore you have the Maillard reaction were sugars react with amino acids to form a host of compounds which contribute both flavor and color. Even though the Maillard reaction can take place at low temperature (such as in vintage champagne), things really speed up when temperature rises above 110-120 °C. Obviously to reach this temperature you’ll have to get rid of the water first. So using a dry rub makes sense. Apart from that it’s mostly about being patient. Use fresh spices, and where possible whole spices that you ground prior to use. The heat of the grill will toast the spices, thereby intensifying the flavor even more.

I must admit that I have never made nor tasted meat which was prepared with a “bark”, so I don’t dare to go into further details concerning how to improve it. The best thing would be to cook two pieces of meat in parallel, for instance with and without an egg wash to see which one comes out best.

Q. BBQ sauces vary greatly depending on region. Carolina sauces are often thin, while Kansas City and Texas sauces have greater viscosity. If a cook is making a sauce that comes out too thin, what recommendations would you have to thicken it?

You either have to take out some of the water by letting it boil over low heat in a large, wide pot, or you can add a thickening agent such as corn starch. If you use onions, these will help thicken your sauce if you let it boil for a while.

Q. In competitions, some BBQ pit-masters utilize a flavor enhancer called FAB B, which contains msg. The thought behind this additive is that after a judge has consumed numerous samples of the same category of meat, the additive will stimulate the taste buds and help to separate that entry from the rest. Can you recommend any other method of ‘waking the taste buds’ without detracting from the taste?

The problem with this explanation is that if everyone uses FAB, will there be any effect at all? If the idea is to rinse the mouth you would want something acidic which stimulates saliva production, some tannic compounds to bind proteins and perhaps some alcohol to help solubilize fats. Heston Blumentahl at the Fat Duck made a “Green tea sour mousse” from these guidelines.

But even so adaption and habituation occurs in all tasting. I’ve discussed this extensively in a blog post, and the easy answer is variation. Or more scientifically: increased sensing by contrast amplification. Eat something which is as far from meat as you can come, something which is cold, crisp, fresh and acidic (did someone mention a tasty salad?). This will make the next piece of meat taste much better!

Q. FAB contains the following: Hydrolyzed soy protein, vegetable oil (soybean and or corn, cottonseed), sodium phosphates, mono sodium glutamate, autolyzed yeast extract, disodium inosinate and guanylate, xanthan gum. They claim that it enhances natural meat flavors, makes your BBQ juicier, improves texture for better slicing and taste and increases yields. Would you believe that these claims are accurate? Would you recommend other methods to achieve the same results?

I would like to emphasize that MSG’s bad reputation is somewhat undeserved. MSG is the salt of a naturally occurring amino acids and is found in many foods. Parmesan and tomatoes contain lots of it (ever wondered why the Italians sprinkle so much parmesan on their food?). Protein and yeast are excellent sources for MSG and the related compounds listed, so I absolutely believe the claim that FAB will enhance the meaty flavors. When FAB is used in a marinade, the phosphates enhance juiciness and improve texture (more on this later). This is well documented. But even so, every chef should remember that FAB or other products can only make good meat better. Therefore you should pay close attention to the quality of the meat you use.

Q. What is a smoke ring and how is it created? What is the best method of producing a significant smoke ring?

When wood or coal burns, small amounts of nitrogen dioxide is formed which dissolves in the surface of the meat, thereby creating nitrous acid. The acid diffuses further into the meat, and when converted to nitric oxide it reacts with myoglobin to form a stable pink colored molecule.

Q. Is there a point of delineating returns, where a piece of meat will no longer absorb the flavor of the wood that it is cooked with? Are you wasting your time by adding more wood for flavor after a certain point?

Frankly, I don’t know. I think this question should be answered by a chef!

Q. How effective is brining and marinating such as pork shoulder or brisket? How much penetration can you reasonably expect? As competitors often work with a short time frame, is there a way to speed up the results of a marinade? And if alcohol burns off, what’s the advantage of using wine instead of juice? Does the alcohol “do” something before it burns off?

Marinades penetrate meat very slowly, so it should primarily be regarded as a way of adding taste to the surface of the meat (which it does very well). An exception here is chicken and fish which are more easily penetrated by marinades. To speed up marination, use water based, concentrated marinades and leave the meat at room temperature. Piercing the meat with a jaccard will allow the marinade to work from the “inside” as well.

It is perfectly fine to use wine in a marinade. The alcohol will dissolve some fat which can speed up penetration. Wine also contains organic acids which can have a tenderizing effect. Phenolic compounds (tannins) will react with meat proteins to form insoluble complexes which in turn makes meat more juicy and tender (even though the exact reason for this is not understood). Experiments have shown that red wine works better than white in marinades.

An interesting thing with marinades is that to maximize the water retaining capacity of beef, your marinade should not contain both acids and salt as this will in fact lower the water holding capacity! If you go for acids, you can easily add salt later on.

Brining, which is immersing meat in water with about 5% salt, does make sense as the salt helps untangle protein strands. This allows spices to penetrate the meat more easily, and it renders meat juicier. Furthermore it lowers the temperature at which the proteins become “cooked”.

Q. Would searing a piece of meat help to ‘seal’ the juices and allow for a more moist cut?

No. As Harold McGee pointed out, “searing is not sealing”. The only reason to sear meat is to get the Maillard reaction going.

Q. What recommendations would you give to someone that is cooking over wood in a smoker if they wanted to achieve a crisp skin on chicken?

In a smoker the low heat will only be enough to evaporate the water, but only very slowly turn the tough collagen into tender gelatin. To achieve this you’ll need a higher temperature, preferably temperatures around 80-90 °C. But even in a smoker there are a couple of things you can do to improve the crispiness. Use a chicken which has been dry-processed. Alternatively, let the chicken dry uncovered in the fridge for a day. Oiling the skin will improve the heat transfer. You can also pierce the skin to let the juices evaporate.

Q. Barbecuing is often seen as the art of taking a piece of meat that is tough and/or stringy and producing a tender, mouthwatering meal from it. What is it that occurs that renders a tough cut like brisket into a soft, enjoyable meat? Is there anything that can be done to enhance those efforts?

The muscle fibers themselves are tender, but they are held together by connective tissue of which collagen is most abundant. Collagen is tough, but when heated it slowly dissolves and forms gelatin which is very tender. Collagen in young animals dissolves more easily than that of older animals. Collagen starts do dissolve around 70 °C and at 90 °C it dissolves rapidly. But before the temperature get this high enzymes which are present in the meat will help tenderize it. These enzymes lose their activity between 40 and 50 °C, but when you barbecue at low heat the meat will spend quite some time below 40-50 °C.

Q. Why do some meats, after reaching optimal tenderness, seem to get even more tender the longer you cook, while others tend to get tougher if you cook past ideal time?

Preparing meat is more about temperature than time. If you like your beef medium rare you would aim for the center to be 55 °C. Continued heating will cause more proteins to denature and as the contract, water is expelled leaving the dry and rubbery. Unless you have prepared your meat at a temperature very close to the desired temperature of the center, there will be a temperature gradient. So even if you remove the meat from your heating source when the center reaches the desired temperature, the warmer outside of the meat will continue to cook the center as it rests, bringing it outside your desired temperature range. It takes experience to know exactly when to remove the meat from the heat.

Q. What’s happening to the meat during “resting”? Why is this recommended prior to cutting and serving?

Apart from the leveling out of the temperature gradient discussed in the previous question it is a very good idea let meat rest before serving, as this improves the water holding capacity of the meat. This in turn reduces the amount of juice you loose when you carve or slice the meat.

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Among dedicated whisky/whiskey drinkers it is customary to add a little water as this “helps to unlock and release the esters, or flavours, from the fats”. Another site claims that dilution helps “breaking down the ester chains and freeing the volatile aromatics”. Does this make sense from a chemical perspective?

When Erik posted me a question some months ago about why we add water to whisky and the chemistry that is involved, I started to speculate about possible mechanisms and discussed them with Erik. Perhaps the most obvious effect is that the alcohol concentration is lowered. High alcohol concentrations anaesthetises the nose and sears the tongue (as the site metioned above correctly states). This is especially true for cask strength whisky which can exceed 60% ethanol. We considered the possibility of a temperature effect. The obvious effect could be achieved by adding water with a different temperature to either cool or warm the whisky. The less obvious effect could be due to a possible release of heat when adding water to a concentrated ethanol solution. Having thought about the different possibilities I did a search and found a very fascinating article: “Release of distillate flavour compounds in Scotch malt whisky”. It was published in 1999, but was new to me and gave me some totally new perspectives on whisky and water. When reading the article, it seems to me that the motivation for adding water to whisky is in fact to mask some aromas and release others!

Malt whisky contains high concentrations of esters and alcohols with long hydrocarbon chains. When water is added, the solubility of these esters and alcohols decreases, and a supersaturated solution results. In extreme cases, the decreased solubility of fat-soluble, volatile organic compounds can lead to clouding due to precipitation of small droplets as seen with anise/liquorise liqours such as Pastis, Pernod, Arak, Raki, Sambuca, Ouzo… (I think I’ll post about that later some time). This can also occur with whiskys that haven’t been chill-filtered. But even in whisky that has been filtered at low temperature a form of “invisible” clouding will occur. The excess of esters and alcohols in the diluted whisky form aggregates (or micelles) which can incorporate esters, alcohols and aldehydes with shorter hydrocarbon chains. Once these compounds are trapped in the aggregates, surrounded by longer chain esters and alcohols, they smell much less since they have a harder time escaping from the liquid! Fortunately, some of the compounds that are trapped have less desireable aromas described as oily, soapy and grassy.

The presence of wood extracts originating from the aging in oak barrels also influences aroma release. One effect is that wood extracts displace hydrophobic (fat soluble) compounds from the surface layer of the whisky (this effect is significant at room temperature when smelling the whisky, less so at 37 °C in your mouth). Furthermore the presence of wood extracts increases the incorporation of hydrophobic compounds into the agglomerates mentioned above.

So far I’ve only discussed the aggregates formed by long chain esters. But studies have shown that when an aqueous solution contains more than 20% ethanol, the ethanol molecules aggregate to form micelles, just like the long chain esters do. These micelles can also trap flavour compounds. Unlike the micelles formed by the long chain esters however, the ethanol micelles break up when diluting the whisky, thus releaseing the entrapped flavour compounds. It is interesting to note that ethanol is less “soluble” in water at high temperatures (ie. the solution is no longer monodisperse). As a consequence, serving whisky “on the rocks” will actually promote the release of flavour compounds from the ethanol micelles. As Mirko Junge commented below, this is one of the very few cases where cooling actually enhances flavour! But the wood extracts found in whisky matured in oak casks supports the formation of ethanol micelles, so as Mirko Junge points out, matured whisky needs more dilution and/or cooling since there are more ethanol micelles.

The over-all effect is a fractionation of volatile compounds upon dilution with water: water insoluble compounds are concentrated in the aggregates (or micelles) of long chain esters, water soluble compounds remain in solution and compounds (probably those which are slightly soluble in water) that were originally trapped in ethanol micelles are liberated.

So after all, the popular notion that addition of water “opens up” the aroma of a whisky is true, but who would have thought that the effect is a combination of “masking” (inclusion of some arome compounds in long chain ester micelles) and “demasking” (opening up of ethanol micelles) and that there even is a temperature effect?

Serving whisky “on the rocks” helps break down ethanol micelles due to the combined effect of cooling and dilution. (Photo by Generation X-Ray at flickr.com)

Feel free to share your experiences with whisky dilution in the comments section below!

(Note: The text has been revised and expanded on June 3rd following the discussion below. Special thanks to Mirko Junge for his valuable comments and for pointing out the importance of the ethanol micelles.)

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5. Learn how to control taste and flavor.

When invited over to friends for dinner, even before eating, you judge the food by it’s aroma, handing out compliments such as “It really smells nice”! Thankfully, nature is on the cook’s side, because when we prepare food and heat it, volatile aroma compounds are released which trigger very sensitive receptors in our noses. It is generally said that 80% of “taste” is perceived by our nose (what we refer to as aroma), whereas only 20% is perceived by our tongue. How important smell is becomes clear if you catch a cold - suddenly all food tastes the same. Too illustrate the importance of smell, prepare equally sized pieces of apple and pear. Close your eyes, hold your nose and let a friend give you the pieces without telling which is which. Notice how difficult it is to tell them apart. In fact, with a good nose clip you wouldn’t even be able to tell the difference between an apple and an onion! Then, with a piece of either in your mouth, let go of your nose. Within a second you can tell whether it’s apple or pear!

Taste
Our tongue has approximately 10.000 taste buds and they are replaced every 1 to 3 weeks. Their sensitivity increases roughly in the following order: sweet < salt < sour < bitter. In addition to the four basic tastes there is umami, the savory, fifth taste. This taste is produced by monosodium glutamate (MSG), disodium 5’-inosine monophosphate (IMP) and disodium 5’-guanosine monophosphate (GMP). Pure MSG doesn’t taste of much, but can enhance the taste of other foods. There are also some claims of a sixth taste.

A number of taste synergies/enhancements exist. I’ve also included three examples of how flavours can influence taste:

The only general, over-all trend which can be found is that binary tastes enhance each other at low concentrations and suppress each other at higher concentrations (but there are several exceptions!). Do check out “An overview of binary taste–taste interactions” (DOI:10.1016/S0950-3293(02)00110-6) if you’re interested in more details on binary taste interactions. I’ve tried to visualize taste enhancements (green) and suppresions (red) in the following figure using arrows to indicate the direction. For example, salt suppresses sweetnes at high concentrations.

In addition to taste, our tongue also percieves texture, temperature and astringency. An interesting thing about the temperature receptors is that they can be triggered not only by temperature, but also by certain foods. The cold receptor is triggered by mint, spearmint, menthol and camphor. There is even a patented compound, monomenthyl succinate, that triggers the cold receptor, but without the taste of menthol. It’s marketed under the name Physcool by the flavour company Mane.

Substances such as ethanol and capsaicin trigger the trigeminal nerve, causing a burning sensation. Capsaicin also triggers the high temperature receptors of the tongue, hence the term “hot food” which can refer both to spicy food and food which is very warm. For a general article about taste, check out “Taste Perception: Cracking the Code” (DOI:10.1371/journal.pbio.0020064, free download).

Flavour
Our nose has about 5-10 million receptors capable of detecting volatile compounds. There are about 1000 different smell receptors and they allow us to distinguish more than 10.000 different smells - perhaps as many as 100.000! In order for us to smell something, the molecule needs to enter our nose at a concentration sufficient for us to detect. Aroma compounds are typically small, non-polar molecules. The fact that they are small means they will have low boiling points - they are volatile and spread rapidly throughout a room. They are often referred to as essential oils and are very soluble in fat, oil and alcohol. These aroma compounds generally not soluble in water, but there are also water soluble aroma compounds; just think of a well prepared stock - no fat but lots of taste and aroma!

A challenge with aroma molecules is that they should remain intact during storage and not be released until cooking (or even better, until consumption). A example would be to install a Liebieg condenser over your pot. Dylan Stiles has explored this in his column Bench Monkey by placing a bag of ice on top of the lid. He claims that his roommates prefereed the curry which has been cooked under “reflux conditions”. The study was performed in a double blind manner (which I will come back to in part 8 of this series).

Because aroma compounds are volatile, spices should be obtained whole and stored in tight containers away from light. If possible, fresh herbs should be used. The flavour of herbs and spices can be extracted by chopping or grinding to increase the surface area. To speed up grinding in a mortar you can add a pinch of salt or sugar.

Heat can help extract flavour (just think of how we brew tea or coffee), but will also evaporate volatile compounds, so a general advice would be to add spices at the start and herbs towards the end of the cooking time. Some herbs can even be sprinkeled over the food just before serving. In Southeast Asia (and especially India) it is quite common heat spices in a dry pan or in oil. This matures flavours and allows reactions to occur (possibly Maillard reactions). Coarse spices should be added earlier than finely ground spices.

In addition to adding flavour using spices, herbs and other foods, we can also use heat to create new flavours. When sugar is heated, caramel is formed. And if a reducing sugar is heated in the presence of an amino acid, they react and form a host of new flavour compounds in what is known as the Maillard reaction. Caramelisation and the Maillard reaction are known as non-enzymatic browning. Enzymatic browning on the other hand is detrimental to many fruits (such as apples and bananas), but there are a few exceptions. Enzymatic browning is essential in the production of tea (black, green, oolong), coffe, cocoa and vanilla, although this is rarely attempted in kitchen.

Another source of flavour is fermentation. It refers to a process were sugar is converted to alcohol and carbon dioxide by the action of a yeast. In the process a number of flavour compounds are formed as well which is why this is of great interest also from a molecular gastronomy viewpoint. Some examples of fermented products include wine, beer, cider and bread. Fermentation also refers to the process where some bacteria produce lactic acid. Some examples of foods resulting from lactic acid fermentation are yoghurt, kimchi and pickled cucumbers.

Flavour pairing
Cookbooks and recipes throughout the world are the result of billions of experiments. As a result, some very good combinations of herbs and spices have been discovered. Some of these mixtures have even been given names of their own and it is fascinating how easily one can forget that curry for instance is a mixture of spices. Wikipedia has a wonderful overview of herb and spice mixtures from all over the world. I must admit I only new a fraction of these:

Adjika | Advieh | Berbere | Bouquet garni | Buknu | Cajun King | Chaat masala | Chaunk | Chermoula | Chili powder | Curry powder | Djahe | Fines herbes | Five-spice powder | Garam masala | Garlic salt | Harissa | Herbes de Provence | Khmeli suneli | Lawry’s and Adolph’s | Masala | Masuman | Mixed spice | Niter kibbeh | Old Bay Seasoning | Panch phoron | Quatre épices | Ras el hanout | Recado rojo | Shake ‘N’ Bake | Sharena sol | Shichimi | Spice mix | Tajín | Tandoori masala | Tony Chachere’s | Za’atar

A book which I’ve found to be very useful when combining flavours is “Culinary artistry” by Andrew Dornenburg and Karen Page. It is the most comprehensive book about flavour pairing that I’m aware of, and I would say it is indispensible for someone who likes to cook without a cookbook. It has lists of basic flavors contributed by various foods. For example a sweet taste is contributed by foods such as bananas, beets, carrots, coriander, corn, dates, figs, fruits, grapes, onions, poppy seeds, sesame and vanilla (plus sugars and syrups of course). It has lists of “flavor pals”, a term attributed to Jean-Georges Vongerichten. For example, the flavour pals of ginger are allspice, chiles, chives, cinnamon, cloves ,coriander, cumin, curry, fennel, garlic, mace, nutmeg, black pepper and saffron. By far the most extensive part of the book are listings of food matchings. An illustrative example is pork which combines well with (classic/widely used combinations in bold):

apples, apricots, bay leaves, black beans, beer, brandy, cabbage, Calvados, dried sour cherries, clams, Cognac, coriander, cream, cumin, fennel, fruit, garlic, ginger, hoisin sauce, honey, juniper berries, lemon, lime, marsala, molasses, mustard, onions, orange, parsley, black pepper, pineapple, Chinese plum sauce, plums, prunes, quinces, rosemary, sage, sauerkraut, soy sauce, star anise, tarragon, thyme, vinegar, walnuts, whiskey, white wine

Despite the abundance of combinations, I dare say that little is understood about the science behind these flavour pairings. Why do these combinations of herbs and spices go particularily well together? Is it all about getting used to the combinations, so that we learn to like them? What influence does the complexity of the flavour play? These are easy questions that probably have rather complex answers.

Very recently a different approach to flavour pairing has emerged. If two foods share one or more key odorants, chances are that they will go well together. The first step towards finding new pairings would be to identify key odorants. More info on key odorants can be found in the article “Evaluation of the Key Odorants of Foods by Dilution Experiments, Aroma Models and Omission” (DOI: 10.1093/chemse/26.5.533, free download). I’ve initiated the food blogging event “They go really well together” (TGRWT) to explore new flavour pairings and develop new recipes. There are also several blogposts with interesting comments on about flavour pairing.

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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.

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(Photo by vintage_patrisha at flickr.com)

4. Learn how to control the texture of food

Taste and flavour normally get more attention when food is discussed, but the texture of food is equally important and our tongue is very sensitive, not only to taste and temperature, but also to the texture of food. The texture of food determines it’s mouthfeel and it is related to many physical properties of the food. Wikipedia lists the following aspects of mouthfeel (click to see the full description of each aspect) which can be useful when analyzing food:

Adhesiveness, Bounce/Springiness, Chewiness, Coarseness, Cohesiveness, Denseness, Dryness, Fracturability, Graininess, Gumminess, Hardness, Heaviness, Moisture absorption, Moisture release, Mouthcoating, Roughness, Slipperiness, Smoothness, Uniformity, Uniformity of chew, Uniformity of bite, Viscosity, Wetness

I will barely scratch the surface of how texture can be controlled by highlighting a couple of topics and point you to further resources. Hopefully it will spark your interest and give some new ideas for you to play with in the kitchen. Those interested in a comprehensive review of food texture are referred to the CRC handbooks on food texture (volume 1: semi-solid foods, volume 2: solid foods).

What determines the texture of food?
Put very simple, it’s the relative amounts of air, liquid and solids that determines the texture of food. This is complicated by the fact that liquids have different viscosities. Furthermore the air, liquid and solid ratio is not necessarily constant. A liquid can solidify or evaporate, solids can melt or dissolve, and air bubbles can escape during cooking or storage. An elegant but quite abstract way of describing the complicated mixtures of air, liquids and solids found in food, is to use the CDS formalism (CDS = complex disperse systems), introduced by Hervé This.

(Photo by Subspace at flickr.com)

How can texture be controlled and changed?
Texture can be controlled by temperature, pH, air/liquid/solid ratio, osmosis, hydrocolloids and emulsifiers - to mention a few. Here’s some examples:

(Photo by Trinity at flickr.com)

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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.

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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:

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.

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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!

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

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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:

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.

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:

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:

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!

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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.

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:

(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

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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.

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Dominique & Cindy Duby, chocolatiers based in Canada, have put together two “scientific chocolate tasting kits” (one, two). Some of the science behind is explained in their “tasting notes” (copy the text into a wordprocessor to read it). For a review of the first kit, check out Rob and Rachel’s blogpost over at Hungry in Hogtown.

The kits illustrate the use of various hydrocolloids to produce foams, gels, dispersions, emulsions and pearls. The principle of flavor pairing is illustrated and binary taste interactions are explored. They also include experiments to explore crunchy vs. soft textures. Each kit comes with four different experiments and enough ingredients to make 8 servings. Furthermore they let you serve every experiment at two different tempereatures. This is neat because is allows you to explore the great influence temperature has on texture and aroma. Each kit sells for $125 - expensive yes, but from the presentation it seems like a good bundle.

Science tasting kit no. 1

The following is illustrated in kit no. 1:

Experiment 1: foaming of pectin and gelatin gels, spherification of a fruit juice/chocolate emulsion (there’s no info on this, but I guess the spherification is alginate based)

Experiment 2: explore how temperature influences sweet and bitter tastes, make a chocolate emulsion (with cream, strawberry juice, wine, cocoa butter and oil) and serve it at two different temperatures

Experiment 3: explore the fact that “taste” is 80% smell, illustrate how salt can suppress bitterness, use a special powder made from an aromatic liquid and maltodextrin which is then dried under vacuum with microwaves (sort of like freeze drying, only this uses microwaves in stead)

Experiment 4: Hervé This’ double dispersion chocolate “cake” made with chocolate and egg white foam which is set in a microwave oven (described in his Angewante Chemie article on molecular gastronomy), short lived crunchy texture, flavor pairing is illustrated by combining cumin and coffe with chocolate

Science tasting kit no. 2

Kit no. 2 starts of by exploring culinary “equations” which are remarkably similar to (yet somewhat less comprehensive than) the CDS formalism described by Hervé This elsewhere. The following is illustrated in the second kit:

Experiment no. 1: a “whisky” is constructed from ethanol lignin, aromatic aldehydes, sugars, acetic acid, oak flavor, vanilin, malt etc.

Experiment no. 2: ice cream is made without churning using foamed egg whites to incorporate air (is this what Italians refer to as a frozen parfait?)

Experiment no. 3: Hervé This’ chocolate chantilly

Experiment no. 4: meringues floating on a pool of custard sauce drizzled with caramel

If you’d rather reverse engineer the dishes, my list of hydrocolloid suppliers might come handy. The “tasting notes” also gives you some hints if you want to have a go on your own.

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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.

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.

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).

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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.

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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.

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!

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.

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.

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!

Update: Click for more practical tips on molecular gastronomy

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