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The Chemistry of Cooking Chapter 24
[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Learning Outcomes You should be able to:   ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]
[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Reasons for cooking food
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)
[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],The effect of cooking on proteins  Chapter 24: The Chemistry of Cooking
[object Object],[object Object],[object Object],[object Object],The effect of cooking on proteins  Chapter 24: The Chemistry of Cooking
[object Object],[object Object],[object Object],[object Object],Chapter 24: The Chemistry of Cooking
[object Object],[object Object],[object Object],[object Object],The effects of boiling on starch
The effects of boiling on starch  ,[object Object],[object Object],[object Object],[object Object],[object Object],Gelatinisation of starch during cooking
[object Object],[object Object],[object Object],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.
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.
Baking ,[object Object],[object Object],[object Object],[object Object],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.
Baking ,[object Object],[object Object],[object Object],[object Object],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.
Baking ,[object Object],[object Object],[object Object],[object Object],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
Baking ,[object Object],[object Object],[object Object],[object Object],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.
Baking ,[object Object],[object Object],[object Object],[object Object],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.
Baking ,[object Object],[object Object],[object Object],[object Object],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.
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.
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.
[object Object],[object Object],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).
[object Object],[object Object],Cooking: Pressure cooker
[object Object],[object Object],[object Object],[object Object],Cooking: Pressure cooker
[object Object],[object Object],[object Object],Use of chemicals
[object Object],[object Object],[object Object],[object Object],Vitamin C Chapter 24: The Chemistry of Cooking Vitamin C (C 6 H 8 O 6 )
Functions of Vitamin C ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Chapter 24: The Chemistry of Cooking
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
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

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C24 the chemistry of cooking

  • 1. The Chemistry of Cooking Chapter 24
  • 2.
  • 3.
  • 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.
  • 6.
  • 7.
  • 8.
  • 9.
  • 10.
  • 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.
  • 13.
  • 14.
  • 15.
  • 16.
  • 17.
  • 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. 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.
  • 21.
  • 22.
  • 23.
  • 24.
  • 25.
  • 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. 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