How pressure changes how food cooks
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The fact that humans cook their food before consumption sets them apart from monkeys and became an extinct species 1.8 million years ago Homo erectus developed. But why do we do this work and not just eat all food raw? Cooking changes the consistency and chemical composition of the food: → Proteins coagulate (denature), starchy food becomes gelatinous, hard food becomes soft. This makes it easier to chew and many nutrients are better absorbed in the intestine after cooking than when consumed raw.
The effect of heat kills bacteria and other microorganisms, which on the one hand increases the shelf life of the food and on the other hand prevents pathogens from surviving. On top of that, foods change their taste when they are cooked, and enjoyment also plays an important role when it comes to eating.
First, the cooking methods are briefly presented. The next page looks into the question of → when should the salt be added to the water when cooking.
When cooking, the food is cooked in a lot of liquid (e.g. → water, broth or → milk) at around 100 ° C. For the cooking process, it is irrelevant whether the pot is heated on a stovetop or in the oven.
Food is cooked for 2-5 minutes and then quenched in ice-cold water to stop the cooking process. The vegetables keep their beautiful color when blanched. The cooking process is ideal for food that is then frozen.
When steaming, the food is cooked in its own juice at temperatures between 80 and 100 ° C in a closed container. A little → water or fat can possibly be added. Steaming is a very gentle cooking process that preserves the nutrients as well as the taste and color of the food. It can be steamed on the stove as well as in the oven.
|Figure 1: Sieve insert for a pot|
In this process, the food is cooked in steam at temperatures between 80 ° C and 100 ° C. Only little → water is required for steaming, because water vapor has many times the volume of water. One liter of water produces around 1700 liters of water vapor. The water vapor can be stored in a pot (Figure 1), Steamer or in Combi steamers are generated, the food lies on a perforated insert and the steam flows around it. In steamers or an oven with a steam system, the temperatures are between 75 ° C and 100 ° C. Steaming is a nutrient-friendly preparation that also preserves the color and taste of the food. However, the cooking time can be a little longer than when cooking (e.g. potatoes). Steaming can also be done under increased pressure, in which case one speaks of steam pressure cooking.
The boiling temperature of water depends on the air pressure. This means that when the air pressure is low, the → water boils sooner, but later when the air pressure is high. This fact is made use of in pressure cooking. Sealing the pressure cooker tightly creates an overpressure that allows the boiling point to be raised to a temperature of 120 ° C. The increased temperature significantly shortens the cooking time and thus also the energy consumption. In order not to build up overpressure, a pressure exchange takes place through an overpressure valve. With many pots, different pressure levels can be set, which also allow cooking at lower temperatures.
When braising, the food is first seared in fat (→ lipids), then some liquid is added and cooked at temperatures between 80 ° C and 100 ° C. Stews can be prepared in a closed pot on the hob or in the oven. Roasting produces roasting substances that give a sauce its color and taste.
Food is cooked in a lot of liquid at temperatures between 75 ° C and 95 ° C. The liquid is first brought to the boil and then the food is placed in it. The temperature drops and should be kept at a temperature of approx. 80 ° C during the further course of the cooking process, so that no excessive currents occur in the liquid. This cooking method is used for foods that would disintegrate when cooked, such as B. Dumplings (dumplings), gnocchi or boiled sausages.
Food absorbs the liquid surrounding it and thereby increases in volume. It is similar to cooking, because here too the liquid is first brought to a boil and then the temperature is reduced. The temperature is below the boiling point between 70 ° C and 95 ° C. The process is suitable for rice, semolina, polenta and pasta.
When frying, the food (such as meat, fish, egg dishes or potatoes) is cooked in a little fat (→ lipids). You can roast in an open vessel such as a pan on the → stove or in an open casserole, roaster or saucepan in the oven at temperatures between 160 ° C and 300 ° C. Roasting occurs during roasting, the food is browned and given a crust (Maillard reaction).
|Figure 2: Meat thermometer|
So that too much water is not removed from the meat before roasting, we recommend adding salt shortly before or after roasting (→ osmosis). In the end it is a matter of taste, because there are also cooks who salt meat an extra 12 hours before preparation . The fact that the meat stays juicier at high frying temperatures is not because a crust is formed that is impermeable to water, but because less liquid is lost because the cooking time is reduced . Instead of roasting the meat completely in the pan, you can also sear it and then (perhaps with a bit of herb butter on top) place it in the oven at 100 ° C for about 20 minutes and cook through there. This means that meat that has been fried for a short time remains significantly juicier. The cooking time depends on the size of the piece of meat.
For the preparation of meat in the oven there are meat thermometers (Figure 2) with which the core temperature of a piece of meat can be measured. This shows the temperatures at which the types of meat such as pork, poultry or beef are cooked through, on point or bloody. There are also digital thermometers for measuring the core temperature.
If food is heated to temperatures of over 120 ° C with low humidity, acrylamide can be formed. Acrylamide is suspected of being carcinogenic, i. H. being a possible cause of cancer. This chemical substance is formed during the reaction between amino acids (→ proteins) and sugars (→ carbohydrates). Therefore, the risk of acrylamide being formed when roasting, baking, deep-frying or roasting, especially starchy foods such as potatoes or bread, is particularly high. To minimize the risk, food should not be cooked too hot. The "Associations for Independent Health Advice (UGB)" recommends not to exceed temperatures of 130-140 ° C when roasting. In the United Kingdom, the “Go for gold” initiative was launched in January 2017. As a rule of thumb, the Food Standards Agency (FSA) recommends only frying, baking, deep-frying or roasting food until it has a gold tone [1, 4]. In order to reduce the acrylamide content in food, requirements for the production and processing of potato products, bread and pastry products, breakfast cereals, baby food, coffee and coffee substitutes have been in force across Europe since April 11, 2018 . For example, potato varieties with a low content of acrylamide precursors such as sugar (fructose and glucose) and asparagine should be used. The sweetness of the potatoes can be limited by suitable storage conditions (→ store food) .
When deep-frying, the food is completely immersed in hot fat (→ lipids) and cooked. The food releases water and absorbs fat, is browned and a crust is created (Maillard reaction). Due to the high fat content, the food is difficult to digest. The consumer advice center recommends keeping temperatures below 175 ° C when deep-frying. The best way to check the temperature is with a fat thermometer. If possible, food should also not be fried for longer than 3.5 minutes . See also → acrylamide.
If food is placed in the fat to be fried, its temperature is reduced. So that the temperature drop is not too great, too large quantities should not be added to the fat at the same time. The consumer advice center recommends a food-to-fat ratio of 1:10 to 1:15. This means that 1.0-1.5 liters of deep-frying fat are required for 100 g of food .
Cooking food in the → oven in dry, hot air, generally between 140 ° C and 300 ° C. Doughs (bread and cakes) are baked, but also casseroles and gratin. The food turns brown and has a crust. See also → acrylamide.
Cooking in foil
A similar process to stewing is cooking in foil, in which the food is also cooked in its own juice. Instead of a closed pot, however, a heat-resistant roasting foil is used, in which the food is placed and which is then closed. The foils can withstand temperatures of up to 220 ° C .
Sous-vide means "under vacuum" from French and describes a method in which food is vacuumed in a special plastic bag and then cooked in a water bath. The cooking temperature is below 100 ° C, which significantly increases the cooking time. The result is tender meat and vegetables that don't lose much of their color. If poultry meat does not reach the required core temperature of 70 ° C, it should then be briefly seared to kill any microorganisms. The resulting roasted aromas also round off the taste [9, 10].
When grilling, the food is not heated by the surrounding air, but by direct heat radiation. The temperatures are significantly higher than when baking and are around 300 ° C. If you are grilling in a closed → oven with convection, the temperatures are slightly lower between 160 and 250 ° C. With the contact grill, the food is in contact with the grill surface, so the heat does not have to overcome an air layer. When grilling, the aroma and nutrients of the food are largely retained. Since very little fat, if any, is used for cooking, the food is easy to digest.
Regulation (EU) 2017/2158 of the Commission of November 20, 2017 laying down minimization measures and guide values for lowering the acrylamide content in food. Accessed on July 3rd, 2020
German Society for Fat Science (DGF) (2012): Optimal Frittieren - Recommendations of the German Society for Fat Science. Accessed on July 3rd, 2020
 Food Standards Agency (FSA): Families urged to 'Go for Gold' to reduce acrylamide consumption. Last updated January 23, 2017.
 Press and Information Office of the Federal Government (2018): New regulations in April 2018. Accessed on 7.4.2018
 Official Journal of the European Union: Legislation. Volume 60, November 21, 2017 (German). Accessed on April 7th, 2018
 Food Standards Agency (FSA): Acrylamides. Accessed on July 3rd, 2020
 Consumer advice center (2018): Acrylamide: Problematic substance in food. Accessed on July 3rd, 2020
UGB Associations for Independent Health Advice in Europe (Ed.) (2009): How are unsaturated fatty acids in vegetable oils changed when heated? Accessed on July 3rd, 2020
Federal Center for Nutrition (Ed.) (2017): Less acrylamide in French fries & Co. Accessed on 7.4.2018
 Chefkoch.de: Gentle cooking in foil. Accessed on July 3rd, 2020
 Federal Center for Nutrition (Ed.): Salt steaks before or after grilling? Accessed on July 31, 2020
 Hervé This-Benckhard: Riddles and secrets of the art of cooking explained scientifically. 2011
 Maria Marnitz: Sous-vide. Deutsche Medien-Manufaktur GmbH & Co. KG. Accessed on October 13, 2020
 Stiftung Warentest (Ed.) (2017): Vacuum cooking in a water bath - that's how it works. Accessed on October 13, 2020
BBC News (1999): Health Cooking vegetables 'improves benefits'. Accessed on July 3rd, 2020
Land of Inventors - The Magazine for Innovations / 2010): Duromatic - the Swiss invention. Accessed on July 3rd, 2020
LUMITOS AG (ed.): Boiling point. Accessed on July 3rd, 2020
Christine Dell'Amore: Cooking Gave Humans Edge Over Apes? National Geographic News, February 13, 2009
Anke Polenz: The new school of food preparation. 2012
HEA (Professional Association for Efficient Energy Use eV) (Ed.): The Blue Cookbook - Electric Cooking. 2011
Margot J. L. Nesso: Rational Food Preparation. 2004
Lexicon: cooking. Meyers Großes Konversations-Lexikon (1905), CD-ROM version p. 104117 (cf. Meyer vol. 11, p. 216)
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