Friday, 8 March 2013

Basics of Heat Transfer

Cooking is the act of applying heat to foods to prepare them for eating.  When foods are cooked, changes in flavor, texture, aroma, color, and nutritional content occur during the process.

There are three ways that heat is transferred to foods.  Conduction is the direct transfer of hear between adjacent molecules.  An example of conduction is cooking on a flattop range.  Heat is transferred from the molecules on the hot range surface to the molecules of the adjacent pan bottom, then from the pan bottom to the pan side and the food contained in the pan.  the pan must be in direct contact with the range for conduction to occur.

Some materials are better conductors of heat than others.  Generally, most metals are good conductors, while gases (air), liquids, and non-metallic solids (glass, ceramic) are not.  Because it relies on direct contact, conduction is a relatively slow method of heat transfer, but the slow, direct transfer of heat between adjacent molecules is what allows a food to be cooked from the outside in, resulting in a completely cooked exterior with a moist and juicy interior.

Convection is the transfer of heat through gases or liquids.  When either o these substances is heated, the portions of the gas or liquid closest to the heat source warm first and become less dense causing them to rise and be replaced by cooler, denser portions of the gas or liquid.  Convection, therefore, is a combination of conduction and mixing.

Convection occurs both naturally and through mechanical means.  Natural convection is at work in a pot of water placed on the stove to boil.  Conduction transfers heat from the stove to the pot to the water molecules in contact with the interior of the pot.  As these water molecules heat up, convection causes them to move away and be replaced by cooler molecules.  This continual movement results in convection currents within the water.  If a potato is added to the water, the convection currents transfer heat to the surface of the potato, at which point conduction takes over to transfer heat to the interior of the potato.

Mechanical convection occurs when stirring or a fan is used to speed and equalize heat distribution.  When you stir a thick sauce to heat it faster and keep it from scorching on the bottom of the pan, you are creating mechanical convection.  Convection ovens use fans to rapidly circulate hot air, allowing them to cook foods more quickly and evenly than conventional ovens.  (Natural convection occurs in conventional ovens as air in contact with the heating element circulates, but the majority of heat transfer in a conventional oven is the result of infrared radiation.)

Radiation is the transfer of energy through waves of electromagnetic energy that travel rapidly through space.  Radiation does not require direct contact between the energy source and the food.  When the waves traveling through space strike matter and are absorbed, they cause molecules in the matter to vibrate more rapidly increasing the temperature.  Two types of radiation are important in the kitchen: infrared and microwave.

Sources of infrared radiation include the glowing coals of a charcoal grill or the glowing coals of an electric toaster, broiler, or oven.  Waves of radiant energy travel in all directions from these heat sources.  Foods and cookware that absorb the energy waves are heated.  Dark, dull, or rough surfaces absorb radiant energy better than light-colored, smooth, or polished surfaces.  Transparent glass permits the transfer of radiant energy, so conventional oven temperatures should be lowered by approximately 25 F/5 C to offset the additional energy transfer that occurs when using glass baking dishes.

Microwave radiation, produced by microwave ovens, transfers energy through short, high-frequency waves.  When these microwaves are absorbed by foods, they cause food molecules to vibrate faster, creating heat.  Microwave radiation cooks foods much faster than infrared radiation because it penetrates foods several inches deep, whereas infrared is mainly absorbed at the surface.  Depending on their composition, foods react differently to microwaves.  Foods with high moisture, sugar, or fat contents absorb microwaves best and heat up more quickly.

Microwave cooking has a few drawbacks, however.  It is best suited to cooking small batches of food.  Meats cooked in a microwave oven lost greater amounts of moisture and easily become dry.  Microwave ovens also cannot brown food, and metal cannot be used in them because it reflects the microwaves, which can cause fires and damage the over.  There is also some conversation on the nutritional effect of cooking with microwaves.  Generally speaking, microwave radiation will alter nutrients faster than any other kind of heat transfer.

Thursday, 7 March 2013

Recipe -- Strawberry, Black Pepper and Balsamic Vinegar Truffles

Strawberry Flakes
500 grams strawberries
1 teaspoon cracked black pepper
300 grams white chocolate, finely chopped
150 grams dark chocolate (60-70 percent cocoa solids), finely chopped
4 teaspoons balsamic vinegar
2 tablespoons strawberry puree
1 teaspoon fresh cracked black pepper
500 grams tempered white chocolate

Strawberry Flakes
1- Preheat the oven to 225 F/110 C.  Blitz the strawberries to a puree in a blender until very smooth.  Do not sieve the puree.  Set aside two tablespoons for the truffles.
2- Line two baking trays with silicon baking mats and spread the puree out as thinly and evenly as possible, using an offset palette knife.  
3- Place the trays in the oven and leave the puree to dry and crisp, about 1 1/2 hours.  As the puree dries it will change from light red to a darker shade.  Be careful that it does not brown.  If you observe this happening, lower the temperature slightly.
4- When the puree is done, remove the trays and let cool completely.  Once it is cold it should be very firm and crisp.  Remove from baking sheets and flake.
1- Place the finely chopped white and dark chocolate in a bowl with the balsamic vinegar and reserved strawberry puree.  Heat the cream until it is just about to boil and pour it over the chocolate.  
2- Wait a moment and whisk gently with a hand whisk into a smooth, shiny ganache.  Mix in the cracked black pepper.  
3- Cover the bowl with cling wrap and let is cool completely at room temperature.
4- Lining a baking tray with parchment paper and fit a piping bag with a 1 cm plain nozzle.  When the ganache is cold, whisk it by hand for a moment until it thickens slightly, then transfer quickly to the piping bag and pipe straight lines about 2 cms apart down the length of the baking tray.
5- Place the tray in the fridge to chill for 2 hours, until the ganache is firm to the touch.
6- Spread the strawberry flakes and the teaspoon to cracked black pepper onto a parchment lined tray so it forms and thin, even layer.
7- Take the piped ganache from the fridge and cut using a bread knife into 4 cm sticks.  Warm the knife in a jug of hot water and dry it well before making each cut.
8- Take ten pieces of ganache at a time, keeping the remaining ganache cool and dip them one at time into the tempered white chocolate, using a dipping fork, then drop them on to the tray of strawberry flakes.  Before the chocolate sets, sprinkle a little of the strawberry flakes on top.
9- Repeat the process with all the ganache.  Once the truffles have set, you can lift them off of the tray and store in the fridge in an airtight container.

(The recipe is modified from a truffle recipe by Claire Clark.)

The Science of Chocolate: Ganache

Simply speaking, ganache is a mixture of chocolate and heavy cream.  The addition of the cream to the chocolate changes the consistency that it will be at varying temperatures.  Depending on the consistency of the finished ganache it has many uses, including filling cakes, making the centers of truffles, glazing pastries or it can be whipped into a mousse.  Light gaanache is sometimes used a chocolate sauce.

Depending on the kind of chocolate used, and for what purpose the ganache is intended, the ratio of the chocolate to cream is varied to obtain the desired consistency.  For truffles, the ratio is roughly 1 part cream to 3 parts chocolate, for filling cakes or making a base for a mousse, the ratio is roughly 1 part cream to 2 parts chocolate.  For glazing pastries, the ratio is roughly 1 part cream to 1 part chocolate.  These ratios are for using dark chocolate (approx 70 percent cocoa solids).  As a general rule, as the percent of cocoa solids decreases, so should the amount of cream added relative to the amount of chocolate. 

You should always use the best quality of chocolate available to you to ensure a smooth, richly flavored ganache.  Here are the general steps for making a ganache at home:

1- Begin by measuring out the proper ratio of cream to chocolate.  This should be done by weight, not by volume.

2- Chop the chocolate as small as possible, to facilitate even melting.  One of the most efficient ways to do this is to use a sharp, serrated knife; the serration causes the chocolate to break into small shards as it is cut.  Place the chocolate in a heatproof (preferably metal) bowl.

3- Heat the cream in a saucepan until almost boiling.  You do not want to boil the cream or it will be too hot and the texture of the finished ganache will not be as fine.  Also, you should avoid using a microwave to heat the cream, as it will not retain the heat as well as it would being heated over the stovetop.

4- Pour the cream over the chopped chocolate.  Allow the mixture to stand, undisturbed, for a minute or two.

5- Stir the ganache gently until the cream is fully incorporated and the mixture is completely smooth.  At this point, add any desired flavoring (flavored liqueurs, extracts, or purees).  Depending on how you intend to use your ganache, it may be ready to use, or at this point be chilled.

Ganache should be intensely flavored, with the chocolate flavor enriched and smoothed by the addition of the cream.  The texture should be smooth and dense.  The more chocolate in the ganache the thicker it will be.  When ganache is warm is should be very glossy and when cooled and whipped it becomes more opaque and matte.

The Science of Chocolate: Tempering

Chocolate contains two distinct types of fat, which melt at different temperatures.  Tempering is necessary to bring chocolate back to the correct crystalline form once it has been melted.  It is essential whenever you are using chocolate to make decorations or for finishing purposes, such as coating cookies and petits fours, or dipping chocolate truffles.

When the crystals in the chocolate are stable, it will be firm and easy to work with, whereas if it contains too many unstable crystals it will be uneven and streaky.  Tempering chocolate encourages the  formation of the right kind of crystals.  Successfully tempered chocolate has the following desirable characteristics:
  • high gloss
  • resistance to warmth
  • a pleasant aroma
  • a smooth mouth-feel
  • a longer shelf life
  • the chocolate will be crisp and snaps when broken
The undesirable characteristics of untempered chocolate are:
  • a white/grey colour or white streaks
  • vulnerability to warmth
  • dull appearance
  • a soft, flexibly consistency 
Before chocolate can be tempered, it needs to be melted.  Never let it come into direct contact with the heat source: it will burn.  The best way to melt it at home is with a bowl placed over a pan of gently simmering water, just make sure no moisture gets into the chocolate.  Here are the correct melting temperatures for chocolate:

Dark chocolate: Melt until it reaches 104-113 F/40-45 C.  Cool to 80-82 F/27-28 C.  Reheat to working temperature of 88-89 F/31-32 C.

Milk chocolate: Melt until it reaches 90 F/32.5 C.  Cool to 80-82 F/27-28 C.  Reheat to a working temperature of no more than 86 F/30 C.

White chocolate: Melt until it reaches 87 F/30.5 C.  Cool to 80 F/27 C.  Reheat to a working temperature of 82 F/28 C.

You will need a chocolate thermometer in order to get accurate readings of the temperatures.  Domestic chocolate tempering machines are available.  They are not cheap but they do work very well, and are worth considering if you plan to do large amounts of chocolate work regularly at home.

Although many recipes only need a little tempered chocolate for decoration, it is not practical to temper less than 300 grams at a time.  Decorations made from chocolate will keep for up to three months in a sealed container in the fridge.  Any leftover tempered chocolate can be poured on to a piece of baking parchment and then chopped up ready for cooking in recipes.

Here is a step-by-step guide to tempering chocolate at home:

1- Working in a cool, draught-free room, chop the chocolate as finely as you can with a large, sharp knife.  Place a little over two-thirds of the chocolate into a clean dry bowl (metal or glass).

2- Place the bowl over a pan of simmering water, making sure the water does not touch the base of the bowl.  there should be no water or steam coming up around the sides of the bowl (if any steam or drops of water came into contact with the chocolate, it would seize and be unworkable).  The water should be simmering gently, not boiling.

3- Melt the chocolate to the temperature specified about, using a chocolate thermometer to check it.  Stir very gentle with a spatula as it melts and do not leave it attended at any time.  When the chocolate is nearly two-thirds melted, remove the bowl from the pan of water and place it on a folded dish towel.  This prevents the bowl from sitting directly on the work surface, which would cool it too quickly and also keeps the bottom of the bowl dry.

4- Continue to stir gently; the heat of the chocolate and of the bowl will help to melt the remaining pieces of chocolate.  Add a tablespoon of the remaining pieces of chopped chocolate and stir until it has melted.  This process is called seeding.  Keep adding the chopped chocolate a tablespoon at a time and stirring gently.  The temperature of the chocolate will be reduced.  Be careful not to add so much chocolate that it no longer melts.  The aim is to reduce the temperature of the melted chocolate be adding small, room temperature, crystalline pieces of chocolate.

5- When the pieces of chocolate no longer melt, stop adding them.  The precrystallising state has now started.  The chocolate is beginning to come down in temperature and the crystals are starting to form a stable structure.  The chocolate now needs to cool to a temperature of 80-82 F/27-28 C.  If you leave the chocolate in a cool place and stir it from time to time, it will come down in temperature by itself.  The amount of time it will take to do this depends on the quantity of chocolate and the working environment. Use the chocolate thermometer to check on the temperature.

6- Once the chocolate has reached the correct temperature, it is at a stable level and fully tempered, but it is not at the best temperature to easily work with.  So place the bowl back on the pan of simmering water and bring it up to the working temperature given above.  As this is only a few degrees higher and you will be tempering a relatively small amount of chocolate, extreme caution should be taken to avoid bring the chocolate above the ideal temperature. (If this does happen, simply restart the cooling process.)  I suggest you place the bowl back on the simmering water only for a few seconds, as it will heat up very quickly and retain much of the heat. After five seconds, remove the bowl and stir gently, then check the temperature.  Repeat until the ideal temperature is reached.

7- You can test the chocolate to see if it has all the desirable qualities by dipping the tip of a knife into it and putting the knife in a cool place.  It should set in an even manner, be free of white streaks and have a high shine and gloss.  If it is not right, simply start the tempering process again

8- Keep the bowl of tempered chocolate resting on the folded kitchen towel while you work with it.  If it begins to cool down, you can warm it again so long as it does not go past the working temperature.

While this may some like a lot of work, let me assure you it completely worth it.  This process, though it may seem tedious will give you products a profession look, feel, and help give it a longer shelf life.

Denaturing and Coagulating Proteins

At a molecular level, natural proteins are shaped like coils or springs.  When natural proteins are exposed to heat, salt, or acid, they denature -- that is, their coils unwind.  When proteins denature, they tend to bond together, or coagulate, and form solid clumps.  An example of this is a cooked egg white, which changes from a transparent fluid to an opaque solid.  

As proteins coagulate, they lose some of their capacity to hold water, which is why protein-rich foods give off moisture as they cook, even if that are steamed or poached.  Fortunately, some heat-induced denaturation is reversible through cooling.  This is why roasted meats should be allowed to rest before carving; as the temperature falls, so of the water or "juice"that was forced into the spaces between the proteins are reabsorbed and the food becomes more moist.  Denatured proteins are easier to digest than natural proteins.

Wednesday, 6 March 2013

Recipe -- Frozen Chocolate Air

1 cup water
85 grams good quality dark chocolate, finely chopped
2 grams soy lecithin

1- In a small saucepan over medium-low heat mix the water and chopped chocolate.  Stir and bring to a boil.
2- Pour the mixture into a wide container and refrigerate the mixture until quite cold.
3- Dissolve the soy lecithin into the mixture and mix with a hand blender from top to bottom to incorporate maximum air.
4- When maximum foam is achieved, freeze the mixture as quickly as possible.
5- To serve either spoon or slice the foam and serve immediately.

(Recipe based on a Chocolate Wind recipe from molecule-r recipes.)

Recipe -- Seared Tuna with Wasabi Air

Wasabi Air
1 cup water
2 tablespoons rice vinegar
1 1/2 teaspoons good quality wasabi powder
2 grams soy lecithin

Seared Tuna
4 5oz fresh ahi tuna steaks
2 tablespoons ponzu
1/2 teaspoon sesame oil
freshly ground pepper

finely sliced chives
sesame seeds

Wasabi Air
1- In a wide container, mix a small amount of water with the wasabi powder to make a paste.
2- Add the rest of the water, the rice vinegar and the soy lecithin and refrigerate until very cold.
3- Remove from the fridge and mix with a hand blender from top to bottom until the maximum amount of foam is achieved.
4- Let the mixture rest for a minute to let the extra moisture settle out of the foam and remove with a slotted spoon.

Seared Tuna
1- In a small bowl mix the ponzu and sesame oil.  Brush onto the tuna steaks and season them with pepper.  Press gently to help the pepper adhere to the tuna.
2- Heat a properly seasoned cast iron skillet on medium-high heat until very hot.  Sear the tuna steaks, approximately one minute per side until medium rare.

1- Slice the tuna and arrange on serving dishes.
2- Top with a spoonful of the wasabi air.
2- Sprinkle each serving with a bit of chive and sesame, if desire.