C24 the chemistry of cooking


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C24 the chemistry of cooking

  1. 1. The Chemistry of Cooking Chapter 24
  2. 2. <ul><li>State two methods of tenderising meat; </li></ul><ul><li>Explain how a tenderiser works; </li></ul><ul><li>Explain the principle involved in the functioning of </li></ul><ul><li>the pressure cooker; </li></ul><ul><li>Identify Vitamin C as an important vitamin found </li></ul><ul><li>in fruits and vegetables; </li></ul>Learning Outcomes You should be able to: <ul><li>Discuss the effect of heat on Vitamin C; </li></ul><ul><li>Discuss the effect of sodium hydrogencarbonate </li></ul><ul><li>on foods containing Vitamin C. </li></ul><ul><li>List two reasons for cooking; </li></ul><ul><li>Describe the effects of boiling on </li></ul><ul><li>protein and starch; </li></ul><ul><li>Identify the major constituents in flour; </li></ul><ul><li>Describe the reactions involved in </li></ul><ul><li>baking dough; </li></ul>
  3. 3. <ul><li>To improve the flavour and appearance of food; </li></ul><ul><li>To make food easier to digest and be absorbed by the body; </li></ul><ul><li>To remove natural toxins present in raw food through heat; </li></ul><ul><li>To destroy microorganisms in food that </li></ul><ul><li>causes decay and diseases; </li></ul><ul><li>To preserve food and extend its shelf life. </li></ul>Reasons for cooking food
  4. 4. Methods of cooking food Using Electromagnetic waves (e.g. Microwave cooking ) Using oil or fats (e.g. deep frying) Using moist heat (e.g. boiling, pressure cooking) Using dry heat (e.g. grilling, baking)
  5. 5. <ul><li>When food containing proteins is cooked, </li></ul><ul><li>their structure and properties are altered. </li></ul><ul><li>Proteins are made up of long chains of amino acids. </li></ul><ul><li>Naturally, these long chains of amino acids fold up to form three-dimensional structures. </li></ul><ul><li>Cooking causes denaturing of the proteins. </li></ul><ul><li>Heat, together with chemicals, enzymes and varying pH breaks the weak hydrogen bonds in the proteins. The proteins unfold and their structures and properties are altered. </li></ul>The effect of cooking on proteins Chapter 24: The Chemistry of Cooking
  6. 6. <ul><li>Denaturation is the process which alters the natural state of a protein caused by heat and chemicals such as acids and bases. </li></ul><ul><li>When protein is only lightly coagulated, it traps water and forms a soft gel that is easy to digest. </li></ul><ul><li>However, if more water is added and the temperature continues to rise, the proteins become tough and rubbery. </li></ul><ul><li>This is clearly demonstrated when an egg is fried — it is runny initially, becomes soft and gets harder and harder as it cooks. </li></ul>The effect of cooking on proteins Chapter 24: The Chemistry of Cooking
  7. 7. <ul><li>Coagulation is the process where a liquid changes into a semi-solid or solid mass due to the clumping of protein strands. </li></ul><ul><li>The process of coagulation is useful in making tofu, cakes, soufflés and pie fillings. </li></ul><ul><li>Other protein-containing products include meat, chicken and fish. </li></ul><ul><li>Proteins in meat are denatured when heated to a temperature of 60°C or higher, unlike for eggs. E.g. Steak becomes tender on cooking over quick dry heat. </li></ul>Chapter 24: The Chemistry of Cooking
  8. 8. <ul><li>Starch is a polysaccharide. </li></ul><ul><li>Starch is made up of many glucose molecules linked together. </li></ul><ul><li>Starch is insoluble in water. </li></ul><ul><li>Examples of food containing starch are rice, maize, wheat, cassava, potatoes, bread, cereals and pasta. </li></ul>The effects of boiling on starch
  9. 9. The effects of boiling on starch <ul><li>When starch grains are placed in cold water and the water is put to boil, the starch grains begin to absorb water. </li></ul><ul><li>Swelling in the starch grains increase but this is a reversible process when the temperature of the water reaches between 60°C to 70°C. </li></ul><ul><li>The process becomes irreversible at higher temperatures. </li></ul><ul><li>At high temperatures, gelatinisation occurs when the starch becomes gelatinous and very viscous. </li></ul><ul><li>The process of gelatinisation can be seen when cooking rice or when using cornstarch as a thickening agent in cooking . </li></ul>Gelatinisation of starch during cooking
  10. 10. <ul><li>Besides starch, flour also contains </li></ul><ul><li>protein which when mixed with </li></ul><ul><li>water or moisture, forms gluten. </li></ul>Flour Flour contains a high proportion of starch. Wheat flour is most widely used to make bread. Flour is a powder made from cereal grains such as maize and wheat.
  11. 11. Composition of a wheat grain The endosperm contains the most starch within the wheat grain. The bran contains lots of fibre and small amounts of proteins, fats and vitamins. The germ is the embryo within the kernel that contains high concentrations of proteins and essential nutrients such as vitamin E, folic acid, thiamin, minerals and fatty acids.
  12. 12. Baking <ul><li>Baking involves 3 steps: </li></ul><ul><li>kneading , </li></ul><ul><li>fermentation and proofing, and </li></ul><ul><li>baking </li></ul>1. Kneading Dough is made by mixing flour and water. On kneading, the main proteins in the flour, glutenin and gliadin form an elastic network of strands known as gluten. The dough must be kneaded carefully to ensure even distribution of carbon dioxide produced during the fermentation process.
  13. 13. Baking <ul><li>Baking involves 3 steps: </li></ul><ul><li>kneading, </li></ul><ul><li>fermentation and proofing, and </li></ul><ul><li>baking </li></ul>2. Fermentation and proofing Leavening agents such as yeast are added to the dough to produce bubbles of carbon dioxide gas that makes the bread lighter and puffier. The yeast feeds on the sugar and converts it into glucose : The glucose is then broken down by another enzyme in the yeast to produce ethanol and carbon dioxide.
  14. 14. Baking <ul><li>Baking involves 3 steps: </li></ul><ul><li>kneading, </li></ul><ul><li>fermentation and proofing, and </li></ul><ul><li>baking </li></ul>2. Fermentation and proofing Carbon dioxide produced by fermentation is usually trapped in the dough as bubbles. The dough is then left in a warm place for a few more hours to allow the yeast to produce more carbon dioxide. This process is known as proofing. Bacteria found in yeast and dough react with ethanol to produce lactic acid and acetic acid that contributes to the flavour of the bread. Unrisen dough Risen dough
  15. 15. Baking <ul><li>Baking involves 3 steps: </li></ul><ul><li>kneading, </li></ul><ul><li>fermentation and proofing, and </li></ul><ul><li>baking </li></ul>Other leavening agents Besides yeast, chemical leavening agents such as baking powder and baking soda can be used as well. Baking powder is a dry powder made of baking soda, anhydrous acid salts and starch. When it is mixed with water, the acidic solution formed reacts with sodium hydrogencarbonate (baking soda) in a neutralisation reaction to produce carbon dioxide.
  16. 16. Baking <ul><li>Baking involves 3 steps: </li></ul><ul><li>kneading, </li></ul><ul><li>fermentation and proofing, and </li></ul><ul><li>baking </li></ul>Other leavening agents Baking soda on the other hand, does not produce carbon dioxide in water. However, it will react with acid to give effervescence. Baking powder is used sometimes instead of yeast for dough which has low gluten content and thus lacks the elastic structure to retain carbon dioxide as well. It is also used when the fermentation flavours produced by the yeast is not favourable. Baking powder is commonly used to bake muffins, sweet bread, cookies, etc.
  17. 17. Baking <ul><li>Baking involves 3 steps: </li></ul><ul><li>kneading, </li></ul><ul><li>fermentation and proofing, and </li></ul><ul><li>baking </li></ul>3. Baking After the dough has risen, it is placed in the oven for baking. At first, as the temperature in the oven rises, yeast activity increases. However, once the temperature increases past 50 o C, yeast cells die and fermentation stops. The gluten present in the dough also denatures and hardens . At the same time, water changes to water vapour , and assists in aerating and expanding the dough. At higher temperatures, starch on the surface breaks down to give it a golden crust and the sugar in the bread caramelises and adds flavour to it.
  18. 18. Important ingredients in baking Ingredient Description Flour This provides the bulk of the baked item. For baking bread, wheat flour, which is high in gluten , is usually used. Gluten (protein) is the substance that gives bread its fine texture and supports the rest of the ingredients during rising. Yeast Yeast is a fungi that feeds on starch and sugars, releasing CO 2 , alcohol and sugar. The CO 2 bubbles give the dough a light, airy texture. Fat Fat gives a softer texture and helps prevent the CO 2 bubbles from escaping from the mixture too soon. Sugar Sugar provides a direct food source for the yeast, improving its action.
  19. 19. Important ingredients in baking Ingredients Description Baking soda Releases CO 2 according to the equation: 2NaHCO 3 -> Na 2 CO 3 + CO 2 + H 2 O However, as this also produces a strong base, Na 2 CO 3 , which has to be neutralised, baking powder is usually used instead. Baking powder Baking powder is baking soda with acid added. This neutralises the base and produces more CO 2 according to the following equation: NaHCO 3 + H +  Na + + H 2 O + CO 2 Egg Beaten egg white, like fat, helps to retain gas bubbles, while egg alone acts as a binder. Salt Salt adds flavour, and strengthens soft fat and sugar mixtures.
  20. 20. <ul><li>There are several methods of cooking to ensure or preserve the </li></ul><ul><li>tenderness of meat. These cooking methods include: </li></ul>The chemistry of tenderising meat Hanging : This method allows the muscle fibres in meat to loosen as the meat is hung to age. Cooking : This method makes meat more tender and palatable. Some meats that remain tough even after prolonged normal cooking may be tenderised by cooking them in a pressure cooker. Mechanical action : This method involves beating the meat with a mallet to loosen the muscle fibres. Beside beating, meat can also be tenderised by grinding and cubing them as these methods cause the connective tissues in the meat to break up. Use of chemicals : Protein-cleaving enzymes break down tough fibres by hydrolysing amide linkages of protein in meat (enzymatic action). Meat can be marinated with acids to denature proteins (use of acids).
  21. 21. <ul><li>In a pressure cooker, food is cooked by the high temperatures inside the cooker which is made possible by raising the pressure to a point greater than atmospheric pressure. </li></ul><ul><li>Cooking times are therefore shortened drastically. </li></ul>Cooking: Pressure cooker
  22. 22. <ul><li>Pressure cookers create a closed environment that literally forces steam through food using heat transfer . </li></ul><ul><li>When the tightly sealed cooker is set over high heat, steam pressure builds and the internal temperature rises because the boiling point of water increases with the pressure. </li></ul><ul><li>Under high pressure, the fibre in food is tenderised and flavors mingle quickly. </li></ul><ul><li>Fewer nutrients are lost because cooking is so speedy and nutrient-rich steam condenses in the pot instead of being lost in the air. </li></ul>Cooking: Pressure cooker
  23. 23. <ul><li>Meat is made of proteins which are polymers of amino acids. Amino acids are molecules that have an acid group on one end and an amine group on the other. </li></ul><ul><li>Proteolytic enzymes (e.g. papain in pawpaw, bromelin in pineapple) can soften the meat by catalysing the hydrolysis of the amide linkages of a protein thus breaking it down into smaller units. </li></ul><ul><li>Apart from meat tenderisers, acids such as vinegar (acetic acid), buttermilk (lactic acid) or fruit juice (citric acid), can be used to marinate and tenderise the meat. At low pH, proteins in meat are denatured, and collagen and tough muscle fibres are broken down. </li></ul>Use of chemicals
  24. 24. <ul><li>Fruits and vegetables are good sources of vitamins. </li></ul><ul><li>Vitamin C is ascorbic acid (C 6 H 8 O 6 ), a water soluble vitamin that is required daily since it is not synthesised by humans and cannot be stored by the body. </li></ul><ul><li>Vitamin C is important as it is required for cellular respiration , for enzyme function and as a component in collagen formation . </li></ul><ul><li>Vitamin C is used to increase iron absorption within the body. </li></ul>Vitamin C Chapter 24: The Chemistry of Cooking Vitamin C (C 6 H 8 O 6 )
  25. 25. Functions of Vitamin C <ul><li>Some of the functions of vitamin C are as follows: </li></ul><ul><li>Maintaining healthy gums, teeth and skin; </li></ul><ul><li>Acting as an antioxidant to fight signs of aging; </li></ul><ul><li>Building the body’s immunity against viruses; </li></ul><ul><li>Increasing iron absorption in the body; </li></ul><ul><li>Healing wounds; </li></ul><ul><li>Synthesising collagen; </li></ul><ul><li>Producing red blood cells. </li></ul>Chapter 24: The Chemistry of Cooking
  26. 26. Effect of heat on Vitamin C Vitamin C is highly unstable and sensitive to heat, oxygen, pH, light and traces of copper. It is easily destroyed during cooking, preparation and storage methods. Oxidation of Vitamin C takes place rapidly when food is cooked at high temperatures . The longer the food is cooked, the more the vitamins are lost. Vitamin C is also a water soluble vitamin . Thus, it leaches into the cooking water and will be lost if poured away. Chapter 24: The Chemistry of Cooking
  27. 27. Action of sodium hydrogencarbonate on Vitamin C Sodium hydrogencarbonate is sometimes added to fruits and vegetables to remove pesticides or preserve the green colour. However, as it is amphoteric , it reacts with Vitamin C in a neutralisation reaction to form salt and water. The equation for the neutralisation reaction is as follows: C 6 H 8 O 6 (aq) + NaHCO 3 (aq) -> C 6 H 7 O 6 Na(aq) + H 2 O(l) + CO 2 (g) The ionic equation is therefore: C 6 H 8 O 6 (aq) + HCO 3 - (aq) -> C 6 H 7 O 6 - (aq) + H 2 O(l) + CO 2 (g) Chapter 24: The Chemistry of Cooking