Practical 1 - Food tests Flashcards
What do we use these food tests for?
Investigating the chemical content of some foods or unknown substances.
How can we use these food tests?
Can use on pure chemicals to demonstrate positive results then use it to demonstrate the composition in different foods
As these tests don’t tell us the exact concentration of different chemicals in the samples, what does that make them?
Qualitative tests
What are the 5 different foods we tested for?
-Reducing sugars
-Non-reducing sugars
-Proteins
-Starch
-Fats and oils
Which test solution gave us our reducing sugar?
Glucose
Which test solution gave us our non-reducing sugar?
Sucrose
Which test solution gave us our proteins?
Albumen solution
Which test solution gave us our starch?
Starch solution
What’s the absolute alcohol that we used? What property does this have that stands out?
Ethanol
Highly flammable
What was the colour change with the reducing sugar?
Blue through to brick red (through the rainbow)
What was the colour change with the non-reducing sugar?
Blue to red
What was the colour change with the proteins?
Blue to violet
What was the colour change with the starch?
Orange to black
What was the colour change with the fats and oils?
Clear to cloudy/milky
Reducing sugars test
-Mix 2cm3 of the test solution (glucose) with an equal volume of Benedict’s reagent
-Heat the mixture in a water bath at between 70-90 degrees for 5 minutes
Non-reducing sugars test
-Mix 2cm3 of the test sample with an equal volume of Benedict’s reagent
-Heat in a water bath at between 70-90 degrees for 5 minutes
-Observe any colour changes
-Add 2 drops of hydrochloric acid to another 2cm3 of the test solution to a boiling tube and heat in a water bath between 70 and 90 degrees for 2 minutes
-Add two drops of Sodium Hydroxide/a small spatula of Sodium Bicarbonate
-Add 2cm3 Benedict’s reagent
-Heat the mixture in a water bath to between 70 and 90 degrees for 5 minutes
Test for Proteins
-Add 2cm3 of the test solution (albumen) to 2cm3 of Biuret reagent in a boiling rube
-Cover the top and invert once
Test for starch
-Add 2cm3 of test solution (starch solution) and 2 drops of iodine in potassium-iodide solution
Test for fats and oils
-Mix the fat or oil with 5cm3 of absolute alcohol (ethanol) in a boiling tube
-Shake the tube
-Pour the mixture into another boiling tube with an equal volume of cold water
How does the reducing sugars test work scientifically speaking?
The reducing sugar donates an electron to reduce copper (II) ions in copper sulphate (blue) to copper (I) ions in copper oxide (red)
Why do we need hydrochloric acid in the non-reducing sugars test?
To break the test solution down into constituent monosaccharides
Why do we add sodium hydroxide/sodium bicarbonate during the non-reducing sugars test?
Benedict’s reagent needs alkaline to work
+ neutralise excess acid
What does Benedict’s reagent need to work?
Alkaline
What do you call the cloudiness created during the fats and oils test?
Emulsion
What type of test is Benedict’s reagent?
Semi quantitative
Which type of food will not change colour with just Benedict’s reagent?
Non-reducing sugars
Why cant we use the Benedict’s Reagent test to estimate the concentration of glucose in a food sample?
There could be more than one reducing sugar in a food- all would produce a positive result in the Benedict’s reagent
What’s the hazard with ethanol?
Highly flammable
What’s semi-quantitative?
Gives an approximation but does not give actual values
Difference between qualitative and quantitative?
Quantitative- numbers based
Qualitative - interpretation based
What factors should be controlled during the food tests experiments?
The volume of test solution and reagents added, the concentration of them and the temperature
What would be a neutral result in the reducing sugars food test?
Blue precipitate
Which problem could arise using very low concentrations of the polymers in these food tests?
Little colour change
What is sucrose?
A non reducing sugar
