Nutrition and Food Tests

• test for sugar:

  • add Benedict's solution to equal volume of food solution in a test tube and heat in a water bath
  • if positive blue to brick red precipitate
  • e.g. apple juice/bread

  
• test for starch:

  • add iodine solution to the food test
  • if positive yellow-brown to blue-black
  • e.g. banana/rice

  
• test for protein:

  • add sodium hydroxide to food solution the add copper sulphate and shake
  • if positive blue to purple
  • e.g. milk/eggs

  
• test for lipids:

  • mix fat with ethanol to dissolve some of it then add to water
  • if positive colourless to white emulsion
  • e.g. oil/butter

  
• carbohydrates contain carbon, hydrogen and oxygen atoms
• simple carbohydrates are the sugars:

  e.g. glucose (single unit of sugar) and sucrose (two sugar molecules joined together)

  they are small, taste sweet and are soluble in water

  provide instant source of energy
• sucrose found in drinks, biscuits and cakes
• glucose and fructose is found in fruit
• sometimes sugar molecules are linked together to form long molecules called complex carbohydrates

  e.g. starch, cellulose and glycogen

  insoluble and used as structural and storage materials
• complex carbohydrates:

  • plants store glucose as starch
  • animals store glucose as glycogen
• any food derived from plants will contain a lot of starch as plants produce glucose by photosynthesis and when they want to store sugars they are linked to form starch
• starch is hundreds of glucoses in an unbranched chain
• starch is broken down into glucose during digestion in the small intestine
• in animals extra sugar is built up into complex molecules of glycogen which is stored in either the liver or muscles, when needed it's broken down into glucose and used in respiration
• glycogen is hundreds of glucoses in a branched chain
• cellulose is the building material of plant cell walls, lots of sugar molecules linked together
• humans cannot digest cellulose therefore we cannot get energy from it however its is our main source of dietary fibre
• cellulose keeps food moving along the gut and helps to prevent constipation and possibly colon cancer
• lipids are made up of carbon, hydrogen, and oxygen atoms
• the two forms of lipids:

  • fats, solid and usually from animals
  • oils, liquid and usually from plants
• fats and oils are good energy store and give twice as much energy as carbohydrates and proteins
• in the body, fats are stored in a layer under the skin acting as an energy store and insulator preventing heat loss
• lipids are made up of a molecule of glycerol with 3 fatty acid molecules attached, by attaching different fatty acids different lipids are produced, triglyceride is most common

  
• proteins are made up of carbon, hydrogen, oxygen and nitrogen
• proteins provide the building blocks for growth and repair of cells but can be used for energy when reserves of lipids and carbohydrates are low
• proteins and built from long chains of smaller molecules called amino acids, there are about 20 different amino acids and thousands of proteins can be made by combining them in different forms
• when proteins we eat are digested, the amino acids are absorbed into the blood, and in our body cells, are reassembled into the different proteins the body needs

  
• proteins are used to form:

  • structures of the body e.g. hair, skin
  • enzymes
  • hormones
  • haemoglobin
  • antibodies
• calculating the energy content of food of the same mass:

  • energy released in joules (J) = mass of water (g) x rise in water temperature (degrees C) x 4.2
• calculating the energy content of food of different mass:

  • energy released (J/g) = energy released by food sample (i) / mass of food sample (g)