Cooking is science and tradition

Cooking is all about science. In this blog our chef Peter Coucquyt will give you insights in the chemistry going on in the kitchen...

maandag 21 februari 2011

Raisin-like caviar

In our post on osmosis we explained how the principle works and which changes occur in food. We also briefly mentioned that this phenomenon could take place on alginate caviar. If so what could we obtain?
Osmosis is the movement of water molecules through a selectively permeable membrane. The membrane is usually an intact cell membrane, present in many food products, but it can also be artificially created, e.g. when making alginate caviar. Water will move from an area of low solute concentration to an area of high solute concentration.
This is the reason why alginate caviar should be stored in the encapsulated fluid. In this case there is no difference in solute concentration so there will not be movement of water from or to the alginate capsule. When capsules are stored in pure water, the solute concentration in the capsule will be higher than in the storage liquid. So the capsules will take up water and swell, this will also dilute the concentration of solutes and the flavor in the capsule. This process goes on until equilibrium is achieved.
What if we now put the alginate caviar in salt or sugar?
Pickling products in salt or sugar was once a popular method. Salt and sugar extract moisture from the food, which increases the shelf life. The same will happen with the alginate caviar. The withdrawal of water from the fluid center of the caviar is an osmosis reaction. The result is a raisin-like caviar. Yet the caviar itself will not take up the sugar or salt. So the raisin caviar will not taste like the pickling fluid. Although the content of the caviar will be concentrated.
So now we know that by osmosis water can flow from the center trough the membrane. We just need to use a highly concentrated storage liquid, or even pure salt or sugar. What if we uses some energy like heat, could we than obtain the same results? 

pumpkin caviar in alginate bath

pumpkin caviar before drying

pumpkin caviar after drying

The results of the drying test are similar as pickling in salt or sugar, but faster. Drying till a fruit leather-like texture gives better results by pickling in sugar. Drying gives a tough result.

zondag 20 februari 2011

Brining or pickling

One of the new trends in kitchens is brining or pickling. What is brining and what happens during this process?
Originally this method was used as a preserving method and not so much for altering or enhancing the flavor and texture. This was just an unavoidable side effect. By putting food in a jar with salt, sour, alcoholic beverages or sugar you could preserve it. (For example: pickles, pickled onions, anchovies, herring, cherries in alcohol, ...).
A high content of salt, a high acidity or low humidity inhibits the growth of microorganisms, which extends the shelf life of products. Brining or pickling will result in microbiological stability, but not in chemical stability. In all cases there will be chemical changes in the food products, resulting in new flavors, textures and colors. Osmosis and diffusion are displacement phenomena that take place in a brined or pickled food product. These processes are mainly responsible for the microbiological stability, while the high concentration of salts, acids and/or alcohol will cause the  chemical changes. Yet these substances are also responsible for the displacement phenomena.
Osmosis is the movement of water molecules through a selectively-permeable membrane. The membrane is usually an intact cell membrane, present in many food products, but it can also be artificially created, e.g. when making alginate caviar. Water will move from an area of high solute concentration to an area of low solute concentration. This process does not require any energy so it occurs naturally and goes on until an equilibrium is achieved. This is the essence of osmosis and is life critical in the cells of all living organisms. Yet osmosis can still occur after the death of the organism, if the cells remain intact.
Osmosis can be and is being utilized in the kitchen.  All plants consist of cells, surrounded by a cell wall that gives strength to the plant. The fluid in the cell is under pressure, providing a crisp and fresh texture. When bitten, the cells snap. Older vegetables lose their moisture (carrots, beans, ...). The pressure in the cells decreases and the vegetable loses its crispness. However, this process is reversible with the aid of osmosis. Soak old vegetables (peeled carrot, limp lettuce, ...) in cold, clean water. They will soak up the water like sponge. After an hour or so, they are crisp again. Since the concentration of solutes in the washing water is lower than in the cells, there will be water movement from the washing liquid to the cells, resulting in a recovered pressure in the cells.
When cleaning lettuce in salted water, there will be water movement from any organism, like bugs, to the salted water. This will essentially dehydrate the bugs causing them to die and release the lettuce. But there will also be water movement from the lettuce to the salted water, causing a drop of cell pressure which we will experience as a reduction in crispiness. In most cases this in not desirable. The effect can again be reversed by washing the lettuce afterwards in clean ice water. Now the lettuce will take up water since the concentration of solutes is higher in the cells than in the ice water.
Diffusion is the movement of solutes, like salts or dissolved sugars, from an area of high solute concentration to an area of low solute concentration. This movement does not require an intact membrane, which is critical for osmosis. Yet diffusion can occur through a membrane if the solutes can pass through it. Diffusion is greatly involved in the marination of a food product. The substances, like salts and flavors in the marinade will move in the food product by diffusion.
When brining or pickling. Water will be extracted from the food product by osmosis, resulting in a concentration of solutes in the food product. Meanwhile, diffusion will cause the food product to enrich itself with solutes in the brining solution. It is also possible that substances in the food product leach out to the brining solution. Consider the curing of a piece of meat, say bacon. Over time, the salt concentration of the bacon will be equal to the salt concentration in the bath.  Water is drawn out of the meat by osmosis and diffusion makes the meat saltier, all because the salt concentration of the bath is higher than in the meat. The meat will also lose acidity, acids migrate by diffusion from the meat to the brine, this is because the latter contains no acid.
Brining or pickling will preserve food microbiologically. It will actually make water unavailable for any living organism in the brine or brined food product. Since the concentration of solutes, in this case the concentration of sodium and chloride is far greater outside the cell of any bacteria, or other harmful organisms, than inside. The result is that bacteria will not be able to uptake water, which is essential to remain alive, on the contrary, water will be forced out of the cell, causing the bacteria to dehydrate and eventually  die. The same happens in a pickling solution, with added pH effects.
Brining or pickling will not preserve food chemically. In fact brining or pickling will promote chemical changes in the food product. Today this is one of the main reasons why foodstuffs are pickled in gastronomy. By brining or pickling, new flavours will arise, the texture and color will also be modified. Acid, alcohol and high salt concentrations have a similar effect on proteins as heat does. HervĂ© This established the term for the increase in strength without heat, in 2003. He called it “cocteren”. Acids will strengthen cell walls of plant cells while salts will weaken them. Brining vegetable will lead to a loss of crispiness, because the weakened cell walls can not support the pressure, adding to that, the pressure is also lost due to osmosis. 
By brining and pickling, we can alter following properties of the food.
- The water balance: The water balance of the food product will be altered by osmosis.
- The taste and flavor: Diffusion will cause of movement of odorants and tastants from and to the food    product. The substances in the brining or pickling solution will cause chemical reactions, creating or destroying flavors.
- Texture and color: The substances in the brining or pickling solution will cause chemical reactions, altering color and texture.
- Shelf life: Brining and or pickling creates an environment that is inhospitable for food pathogens. 

donderdag 3 februari 2011

Sous-vide cooking

Low temperature cooking, is there a difference with sous-vide cooking? Why should we cook some products at low temperature?
Can one compare sous-vide cooking with low temperature cooking? No, both terms are often mixed but they have different meanings. Sous-vide cooking does not always happen at low temperatures.
Sous-vide cooking or “under vacuum” cooking is a technique in which food products are put in a vacuum bag. The bag is then put under vacuum and sealed. The product is usually cooked at temperatures between 90 and 100 ° C
Cooking at low temperatures is mostly done in a vacuum bag like sous-vide cooking, but can also just happen in oil, butter, broth, ... without sous-vide. The temperatures used are below 90 ° C. and take more account of the structure, composition of the product. Cooking at the right temperature might be a better description. Low temperature cooking refers to the temperature of the cooking medium, not to how the food product is placed in that medium. It’s a technique where the cooking temperature is at, or close, to the desired internal temperature of the food product
Cooking of food products.
There are 4 specific reasons for cooking food:
  • Cooking promotes the accessibility of some nutrients such as starch in potatoes (starch granules are available for digestive enzymes) and colour pigments. So the food will be more digestible
  • The food products become tender, so it is easier to eat. (e.g. connective tissue in meat)
  • Cooking eliminates potentially harmful microorganisms and leaches out some toxins.
  • Heating food launches a series of chemical reactions, breaking down big, tasteless molecules into small molecules detectable by our taste buds.
When we cook, we usually use heat. The methods are varied and each way has its own impact on the product. The medium in which we heat is also important. (e.g. papilotte, can be cooked in the oven or placed on the barbecue. Inside the food, the reactions are the same; the products will cook in a humid vapor atmosphere).
Apart from stews, most cooking processes occur at high temperatures: 100 ° C for boiling water,   160 ° C or more for frying, 200 ° C in an oven, 250 ° C or more on the plancha, ... All these temperatures are much higher than the temperatures whereupon proteins denature and coagulate with significant effects: dry crust around meat, disintegrated fish tissues,... . As the cooking time proceeds, those effects occur more and more.
The “Roner” and other devices
By scientific studies of the chemical and physical phenomena during cooking of food, we learn to understand the changes food undergoes during cooking: changing of colour and texture, development of flavour and aroma. This knowledge and especially with regard to the behaviour of the proteins, led to the determination of the optimal temperatures that should be achieved in the core of food products. The aim is not only cooking the products, but also retaining its shape, making it soft, tender and give it an excellent flavour.
By technical progress, one is able to make very accurate temperature controlled equipment. With the right equipment, one can achieve cooking circumstances that are consistent and reliable. In the beginning of the nineties, a commercial device was placed into the market allowing us to cook at very controlled temperature, called “Roner”.
The Roner is developed by Joan Roca of the restaurant “El Celler de Can Roca" and Nora Caner of the restaurant "La Fonda Caner", both from the province of Gerona (Cataluña).
The Roner is a water bath with a thermal circulator, ensuring that the water maintains a uniform, controlled temperature. Also, it ensures that we can cook, with precision, sous-vide at lower temperatures (mostly between 38 ° and 85 ° C). Now days there exist a lot more devices than the Roner to cook this way (e.g. Noon pro, Sous Vide Professional of Polyscience,…).
These temperatures are not a coincidence. They depend on the composition of the product (e.g. ratio of muscle tissue and connective tissue, starchy vegetables,...). As said, cooking at the right temperature is a more correct expression After all it refers to the best cooking temperature for the product.
The principle of cooking at low temperature in a vacuum bag is clear. Also important is the accuracy over the entire duration of the cooking process. Water is one of the best media for heat transfers by which the temperature can be adjust with the greatest accuracy to one tenth of a degree. (heats up quickly and the transfer of heat is 23 times faster than in a traditional way e.g. conventional oven), While a steamer is precise, his cooking area does not guarantee total uniformity.
Devices like a Roner are often used in laboratories. These thermostats are immersed in water, therefore called thermostat bath. Elbow of a thermostats are heat pumps providing water circulation. They are equipped with a temperature probe. Thermal bain-maries give a fixed temperature and don’t fluctuate a hundredth of a degree.
The technology of a thermostat bath can also be used without sous-vide. One can cook at low temperature directly in oil or melted butter. These fats do not penetrate into the product because the cooking temperature prevents the release of the cell fluid from the product. The fat sticks on the surface in the form of droplets. During cooking both the product and the fat are in contact with each other, making the fat spoil quickly.
Most food products can be successfully cooked sous-vide at low temperature. Although some ingredients can’t be cooked at low temperature such as: green vegetables, cereals, rice,... . Below a brief overview and their advantages on low temperature, sous-vide cooked products.
  • Tender pieces of meat: exact core temperature, uniform cooking and temperature throughout the whole piece
  • Hard, tougher pieces of meat can be softened over time and still look pink. Traditionally confit is done in a large amount of fat. By confit made sous-vide, less fat is used and obtains the same result. In both ways you can cook the meat until it falls apart or just simmer it so one can cut it into slices.
  • Fish: pure flavour and even cooking. Some textures can’t be obtained on traditional way, such as salmon cooked at low temperature. A less flavoured liquid for a more and better taste can now replace cooking in a broth.
  • Other seafood such as lobster, octopus and squid become quickly tough at high temperatures. Low temperature cooking can soften them.
  • Hard vegetables: good results in terms of texture and colour (even cooking of potatoes with not to far cooked outsides, carrots are brilliant orange)
  • Other vegetables: they are soft without being cooked too far or breaking up
  • Fruits: like vegetables. Fruits sensitive of oxidation such as apples and pears retain their colour and gloss
For sous-vide cooked products or dish, we distinguish two methods
  • Indirect cooking
  • Direct cooking
Direct cooking is applicable in restaurants or at home. It replaces other direct cooking techniques and provides a specific core temperature. For fish we speak of 40 to 50 ° C and 50 to 65 ° C for meat. One can cook at a much higher temperature than the desired core temperature where a core thermometer is essential.
Indirect cooking is a kind of mise-en-place preparation, often for products with a long cooking time, such as collagen rich meat. The indirect cooking is similar to pasteurization.
The students made following dishes, using sous-vide technics
Apple, 9h at 85°C, salted butter caramel, vanilla ice, muscovado sugar foam, crumble of hazelnut
Beef 53°C, broccoli florets, broccoli puree, truffle caramel, triple cooked potato, silver onions, onion croquant
Veal sweetbread, 3h at 62°C, braised chicory, liquorice foam, coffee sauce
Pork loin 58°C, cauliflower (puree, couscous, pickled), stalk of the cauliflower, cream of piccalilli
Cod 53°C, spinach, mussel, mussel juice foam, crispy onion
Pineapple 70°C, vanilla ice, mango cream, bourbon caramel, hazelnut crumble
Pheasant breast 62°C, pumpkin (puree, pickled) truffle, trompette de la mort mushrooms, pecan
Venison 52°C, Jerusalem artichoke (puree, cooked sous-vide in microwave), pear (pickled and fluid gel), chocolate crumble
Filet of hare 51,5°C, parsnip puree, apple (pickled, jelly), black pudding cream
Cod 53°C, chervil root (puree, pickled, cooked sous-vide in microwave) bacon crumble