Why food stores energy
- Carbohydrates, lipids and proteins all hold energy in the chemical bonds between their atoms.
- Your cells release that energy in respiration, in a controlled series of small enzyme-catalysed steps.
- Burning the food releases the same energy in one uncontrolled step, as heat and light.
- That is what makes measurement possible, because heat transferred to water can be worked out from a temperature rise.
- Energy is measured in joules, and food labels usually quote kilojoules per 100 g100\,\text{g}100g.
- Lipids store roughly twice as much energy per gram as carbohydrates or proteins, so crisps and nuts give much larger temperature rises than bread.
Key Idea
Calorimetry measures the energy in a food indirectly, by measuring what the burning food does to the temperature of a known mass of water.
Measuring energy by calorimetry
Definition
Calorimetry
A method of measuring the energy stored in a food by burning it and measuring the temperature rise it causes in a known mass of water.
- The food is burned underneath a measured volume of water, and the rise in water temperature is recorded.
- The mass of the food must be measured as well, so the result can be given per gram and different foods compared fairly.
Practical
Measuring energy in food
- Aim: to compare the energy stored in different dry foods by burning them under a known mass of water.
- Apparatus: boiling tube, clamp and stand, 25 cm325\,\text{cm}^325cm3 measuring cylinder, thermometer reading to 0.5 ∘C0.5\,^{\circ}\text{C}0.5∘C, balance reading to 2 decimal places, mounted needle, Bunsen burner, heatproof mat, dry food samples such as pasta, bread and a crisp, and eye protection.
- Method:
- Measure 20 cm320\,\text{cm}^320cm3 of water into a boiling tube and clamp it at an angle above the mat.
- Record the starting temperature of the water.
- Weigh the food sample and write its mass down to 2 decimal places.
- Push the sample onto a mounted needle.
- Set it alight in a Bunsen flame and move it immediately under the boiling tube, holding it as close as you safely can.
- If the flame goes out, relight it and return it at once, and keep going until the food will not burn any more.
- Stir the water gently with the thermometer and record the highest temperature it reaches.
- Variables: you change the type of food and you measure the temperature rise, while keeping the volume of water, the starting temperature, the distance from the flame to the tube and the type of thermometer the same.
- Results: a crisp burns readily with a bright flame and produces a large temperature rise, while a piece of pasta of the same mass is harder to keep alight and warms the water far less.
- Maths: find the energy transferred with Q=m×c×ΔTQ = m \times c \times \Delta TQ=m×c×ΔT, then divide by the mass of food burned to get the energy per gram.
- Watch out: most of the heat escapes into the air rather than reaching the water, and food that stops burning has not released all its energy, so every school result is an underestimate.
- Safety: wear eye protection, tie long hair back, keep the burning food away from your skin and the bench, let hot apparatus cool before moving it, and check for nut allergies in the room before any nut is burned.
The calorimetry calculation
- The energy transferred to the water is found from the mass of water, its specific heat capacity and the temperature rise.
- Q=m×c×ΔTQ = m \times c \times \Delta TQ=m×c×ΔT
- In that equation QQQ is the energy in joules, mmm is the mass of water in grams, ccc is 4.2 J/g/∘C4.2\,\text{J/g}/^{\circ}\text{C}4.2J/g/∘C and ΔT\Delta TΔT is the temperature rise.
- Water has a density of 1 g/cm31\,\text{g/cm}^31g/cm3, so 20 cm320\,\text{cm}^320cm3 of water has a mass of 20 g20\,\text{g}20g.
- Dividing by the mass of food burned then gives a figure that can be compared between foods.
- energy per gram=Qmass of food burned\text{energy per gram} = \dfrac{Q}{\text{mass of food burned}}energy per gram=mass of food burnedQ
Common Mistake
The mass in the first equation is the mass of the water and the mass in the second is the mass of the food, and swapping them is the single most common error in this calculation.
Example
- A 0.50 g0.50\,\text{g}0.50g crisp is burned under 20 cm320\,\text{cm}^320cm3 of water, and the temperature rises from 21 ∘C21\,^{\circ}\text{C}21∘C to 39 ∘C39\,^{\circ}\text{C}39∘C.
- The temperature rise is ΔT=39−21=18 ∘C\Delta T = 39 - 21 = 18\,^{\circ}\text{C}ΔT=39−21=18∘C, and the mass of water is 20 g20\,\text{g}20g.
- Q=20×4.2×18=1512 JQ = 20 \times 4.2 \times 18 = 1512\,\text{J}Q=20×4.2×18=1512J
- Now divide by the mass of the crisp to get the energy per gram.
- energy per gram=15120.50=3024 J/g\text{energy per gram} = \dfrac{1512}{0.50} = 3024\,\text{J/g}energy per gram=0.501512=3024J/g
- That is about 3.0 kJ/g3.0\,\text{kJ/g}3.0kJ/g, which is well below the figure on a crisp packet and shows how much energy the experiment loses.
Hint
Divide by 100010001000 to turn joules into kilojoules, since packet values are quoted in kilojoules.
Why the answer is too low
- Most of the heat from the flame goes into the surrounding air rather than into the water.
- Heat is also absorbed by the glass of the boiling tube and lost again from its surface.
- The food rarely burns completely, so some of its chemical energy is never released at all.
- Every one of these losses makes the calculated value smaller, which is why the result is always an underestimate.
- Improvements include insulating the tube, using a draught screen, holding the food closer, and stirring the water so the temperature is even.
- A commercial bomb calorimeter burns the food in pure oxygen inside a sealed insulated chamber, which is why manufacturers get much higher values.
Exam technique
- When asked why the value is lower than the packet, name a specific route for the heat loss rather than writing that heat was lost.
- Every improvement you suggest should be paired with the error it fixes, so insulation goes with heat lost through the glass.
- Give energy per gram rather than total energy whenever two foods are compared, because the samples are rarely the same mass.
Definition
Reducing sugar
A sugar such as glucose or maltose that produces a brick-red precipitate when heated with Benedict's solution.
Practical
Testing food for nutrients
- Aim: to test a range of foods for starch, reducing sugars, protein and lipids, and record the colour change each test produces.
- Apparatus: test tubes, rack and bungs, spotting tile, a water bath at 707070 to 80 ∘C80\,^{\circ}\text{C}80∘C, pestle and mortar, distilled water, spatula, measuring cylinder, dropping pipettes, iodine solution, Benedict's solution, biuret solution, ethanol, food samples such as powdered potato, milk powder and glucose powder, and eye protection.
- Method:
- Preparing the food: grind the solid food with distilled water in a pestle and mortar to make a suspension, and filter it if a clear solution is needed.
- Starch test: add 2 or 3 drops of iodine solution to 2 cm32\,\text{cm}^32cm3 of the food solution, and a change from orange-brown to blue-black shows starch is present.
- Reducing sugar test: add 2 cm32\,\text{cm}^32cm3 of Benedict's solution to 2 cm32\,\text{cm}^32cm3 of the food solution and heat in the water bath for about five minutes, and the blue goes green, then yellow, then orange, then brick-red as the concentration rises.
- Protein test: add 2 cm32\,\text{cm}^32cm3 of biuret solution to 2 cm32\,\text{cm}^32cm3 of the food solution and shake gently, and a change from blue to purple or lilac shows protein is present.
- Lipid test: add 2 cm32\,\text{cm}^32cm3 of ethanol to 2 cm32\,\text{cm}^32cm3 of the food and shake to dissolve any lipid, leave it to settle, then pour the ethanol layer into a tube of distilled water, where a cloudy white emulsion shows lipid is present.
- Controls: run each test on distilled water instead of food as a negative control, and it should give no colour change, which is what lets you say the change came from the food.
- Watch out: use a clean pipette and spatula for every reagent and every food because cross-contamination gives convincing false positives, always heat the Benedict's test or it stays blue and you record a false negative, and remember that a sugary food can still fail the test because sucrose is not a reducing sugar.
- Safety: wear eye protection throughout, keep ethanol well away from naked flames because it is highly flammable and use an electric water bath, treat Benedict's and biuret solutions as irritants, and never taste any sample.
Common Mistake
- The Benedict's test detects reducing sugars only, so a negative result does not prove a food is sugar-free.
- Iodine solution is orange-brown to begin with, so describing the change means naming both the starting and the final colour.
- The emulsion test needs the ethanol layer to be added to water, since the cloudiness appears in the water and not in the ethanol.
- Calorimetry gives the energy in the food and not the energy your body obtains from it, because some food passes through undigested.
Self review
- Write the equation used to calculate the energy transferred to the water.
- A 0.4 g0.4\,\text{g}0.4g sample raises 25 g25\,\text{g}25g of water by 10 ∘C10\,^{\circ}\text{C}10∘C, so what is the energy per gram?
- Give two reasons why the calculated energy is lower than the value on the packet.
- Name the reagent and the positive colour for each of the four food tests.
- Why must the Benedict's test be heated?
