BPrac C - General + food tests

Discuss extent to which results support the hypothesis 

• Support 

  • Mean values show decrease

• Does not support 

  • Non-representative sample (age or gender)

  • Some of 'before' results are similar to 'after' results

  • Only one concentration of caffeine tested 

  • No evidence for any other age group

  • No statistical test carried out (to check significance of results) 

  • Only 8 subjects -> small sample size 

  • No repeats 

State hypothesis (the NaHCO3 → time taken for leaf to float)

As conc of NaHCO3 and hence CO2 increases, rate of photosynthesis increases as indicated by fall in time taken for leaf to float 

Explain what would happen if one were to place the syringe containing floating

leaf discs in the dark for an hour. 

• Photosynthesis stops in the absence of light because light is required for the light- dependent reaction, but respiration continues

• O2 trapped within the leaf discs is used during respiration and is no longer replenished by photosynthesis -> decrease in oxygen volume -> reduced buoyancy -> leaf sink

• Shows that measured photosynthesis is an apparent rate and provides an indication of respiration rate

State safety precaution of Benedicts, acid/alkali, ethanol

Benedict’s solution	Harmful and dangerous to environment
Any acid/alkali/iodine/H2O2/Soda lime/manometer fluid	Corrosive, Harmful, wear gloves and goggles to avoid contact with eyes or skin and do not ingest
Ethanol	Highly flammable, ensure there are no flames nearby when using it

State safety precaution of scapel, DCPIP, lamp

Scapel	Handle scalpel with care. Cut away from fingers to prevent cutting themselves
DCPIP	Wear eye protection and gloves as DCPIP can be corrosive
Lamp	Handle lamps carefully to avoid burns as tungsten lamps get hot, do not touch the bulb

State prep for solids in food tests

• For solids : cut samples into small pieces / grind with mortar and pestle -> increase SATVR of sample / break up any cells to release biomolecules 

  • For Biuret and Benedict's test : dissolve crushed sample in distilled water -> filter -> test 

Describe protein test (prodceudre, +ve test, explanation)

Proteins (Biuret test)

Procedure : Add 2cm^3 of sample into a test tube  Add an equal volume of 5% KOH solution and mix well  Add 1% copper sulfate solution drop by drop and mix well  (Or add 2cm^3 of biuret solution)  Positive test : Solution turns purple  Explanation : N atoms in peptide chain form a purple complex with Cu2+ ions

Describe reducing sugar test (prodceudre, +ve test, explanation)

Procedure : 

1. Add 2cm^3 of sample and 2cm^3 of Benedict’s solution to a test tube 

2. Place in boiling water bath for 2-3min 

Positive test : Initial clear solution turns cloudy , then green -> yellow -> orange -> brick-red mixture 

Explanation : Carbonyl groups in reducing sugars reduce CuSO4 to Copper(i)oxide , which is insoluble and appears as a reddish-brown ppt

Reducing sugar test is semi-quantitative, why?

• Test is semi quantitative : 

  • Intensity of colour and amount of precipitate can be used to quantify the relative concentrations

  • Stronger colour intensities -> amount of precipitate increase 

  • Exact amount of RS cannot be determined 

• Use staggering method : place test tubes in at 30 second interval

Describe NON reducing sugar test (prodceudre, +ve test, explanation)

• Benedict’s test conducted BEFORE non-reducing sugar test

• If Benedict’s test is positive -> stronger positive result will indicate additional NRS 

Procedure : 

1. Add 2cm^2 of sample and 1cm^3 of dilute HCI to a test tube 

2. Mix well and place test tube in boiling water bath for 1m 

3. Cool the mixture by placing it under running water 

4. Neutralise the mixture with sodium hydrogen carbonate (check with pH paper)  

5. Conduct Benedict’s test 

Explanation : Carbonyl groups in NRS are hydrolysed by boiling with HCI to its monosaccharide constituents 

• Use staggering method : place each test tube into boiling water bath at 30 second intervals to ensure equal time of reaction

Describe lipids test (prodceudre, +ve test, explanation) (state +ve and -ve test observation)

Procedure : 

1. Add 2cm^3 of sample with 2cm^3 of ethanol and shake vigorously 

2. Add 2cm^3 of cold water and shake

Positive test : Content changed from a slightly cloudy, colourless mixture to a cloudier, white suspension of lipid droplets is formed 

• Food with high lipid content have a higher layer

• Fats absent: Content remained a slightly cloudy, colourless solution. No white suspension

 

Explanation : Lipids are immiscible with water , addition of water to lipids with alcohols leads to an emulsion

Describe starch test (+ve test)

Starch

Observation: solution turned from a clear light orange-brown solution to a dark blue solution

Draw simple dilution table + formula

• Conc of stock solution x volume of stock solution used = conc of sample to be obtained x total volume of sample to be obtained 
  

Format 

Conc of glucose to be obtained / %	Volume of 25% stock glucose solution used / cm^3	Volume of distilled water used / cm^3