key concepts ++ testing foods Flashcards
describe how to test for starch in food
Add iodine solution to the food being tested. Foods containing starch will turn a blue-black colour. (the iodine test can also be used with a microscope to stain starch grains in plant cells)
describe how to test for reducing sugars in food
Reducing sugars are detected using Benedict’s solution.
Reducing sugars include:
- monosaccharides – such as glucose and fructose
- disaccharides – such as maltose.
Benedict’s solution gradually turns from blue to cloudy orange or brick red when heated with a reducing sugar. Sucrose is a non-reducing sugar and does not react with Benedict’s solution.
describe how to test for proteins in food
Proteins are detected using Biuret reagent . This turns a mauve or purple colour when mixed with protein.
describe how to test for lipids in food
Lipids are detected using the emulsion test. This is what happens:
- The test substance is mixed with 2 cm3 of ethanol.
- An equal volume of distilled water is added.
- A milky-white emulsion forms if the test substance contains lipids.
how calorimetry can be used to measure the energy in food
Food calorimetry allows us to determine the number of calories per gram of food. In this activity, a piece of food is burned and the released energy is used to heat a known quantity of water. The temperature change (∆T) of the water is then used to determine the amount of energy in the food.
what’s the calorimetry equation ?
energy transferred (J) = mass of water (g) × 4.2 (J/g°C) × temperature increase (°C)
describe what variables you would keep the same when comparing different foods and their calorimetry ? how can you make the test more accurate ?
- the starting temperature of the water
- the temperature increase
- the distance of the flame from the boiling tube
More reliable results can be obtained by repeating the experiment. The biggest source of error in calorimetry is usually unwanted heat loss to the surroundings.
