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Review of
Food Tests
USE OF INDICATORS
Indicators are substances that detect
the presence, absence or
concentration of certain chemicals
(food substances).
It may show degree of reaction
between two or more substances by
means of a characteristic change,
especially in color.
Chemical Digestion
Food Tests
1. Test for Starch
– Iodine (2 – 3 drops)
– Negative – Iodine remains brown/orange
– Positive – Iodine changes orange to black
Starch Test
Amylose in starch is responsible for
the formation of a deep blue color in
the presence of iodine. The iodine
molecule slips inside of the amylose
coil.
Starch amylopectin does not give the
color, nor does cellulose, nor do
disaccharides such as sucrose in
sugar.
Summary Table
Reagent/Subs
tance
Biuret
reagent
Lugol’s
iodine
Benedict’s
solution
Used to
test
for
Protein
Starch
Glucose
Bile
Fat
Brown paper
Fat
Food Substance
Before
After
Control
Before
After
Chemical Digestion
Food Tests
2. Test for Protein
– Biuret’s solution – 2cm3
– Negative – Biuret’s stays blue
– Positive – Biuret’s changes to lavender
The Biuret Reagent Test
The principle underlying the test can be demonstrated with the chemical
compound biuret which, just as proteins, is able to complex copper (II) ions.
It detects the peptide bond between the urea molecules or between amino
acids. The blue reagent turns violet in the presence of proteins, and
changes to pink when combined with short-chain polypeptides.
Not all biuret tests actually require the Biuret reagent. Rather, the term
"biuret test" is a generic term for the testing of proteins by using
copper (II) sulfate solution in an alkaline environment.
Thus, the process of testing for proteins in a solution by first adding a small
amount of sodium hydroxide, and then adding copper (II) sulfate drop by
drop, is also known as a biuret test.
Summary Table
Reagent/Subs
tance
Biuret
reagent
Lugol’s
iodine
Benedict’s
solution
Used to
test
for
Protein
Starch
Glucose
Bile
Fat
Brown paper
Fat
Food Substance
Before
After
Control
Before
After
Chemical Digestion
Food Tests
3. Test for Fat (a. paper test)
–
–
–
–
Smear oil/fat onto paper
Paper turns translucent
Negative – stain dries (not translucent)
Positive – stays translucent
Summary Table
Reagent/Subs
tance
Biuret
reagent
Lugol’s
iodine
Benedict’s
solution
Used to
test
for
Protein
Starch
Glucose
Bile
Fat
Brown paper
Fat
Food Substance
Before
After
Control
Before
After
Chemical Digestion
Food Tests
4. Test for Fat (b. bile)
–
–
–
–
Add bile to oil
shake
Negative – separation of the bile
Positive – bile emulsifies the fat
Emulsification – breaking up a large blob of fat
into smaller fat bubbles
Summary Table
Reagent/Subs
tance
Biuret
reagent
Lugol’s
iodine
Benedict’s
solution
Used to
test
for
Protein
Starch
Glucose
Bile
Fat
Brown paper
Fat
Food Substance
Before
After
Control
Before
After
Chemical Digestion
Food Tests
5. Test for Glucose
–
–
–
–
Benedicts solution (about 2cm3)
HEAT
Negative – stays blue
Positive – from blue to orange (brick red)
Benedict's reagent
Benedict's reagent is used as a test for the presence of reducing sugars such as
glucose, fructose, galactose, lactose and maltose. Benedict's reagent contains
blue copper(II) sulfate (CuSO4) which is reduced to red copper(I) oxide (Cu2O)
by aldehydes, also oxidizing them to carboxylic acids. The copper(I) oxide is
insoluble in water and so precipitates.
Chemical test
To test for the presence of reducing sugars in food, the food sample is dissolved
in water and about a sample is added to the reagent. The mixture is heated in a
boiling water bath, and any precipitate formed is recorded as a positive result
for the presence of reducing sugars in the food. Sucrose (table sugar) is a nonreducing sugar and thus does not react with Benedict's reagent. Sucrose can
produce positive results with Benedict's reagent if heated with dilute hydrochloric
acid prior to the test. Doing so hydrolyses the glycosidic bond to give the
monosaccharides glucose and fructose.
Summary Table
Reagent/Subs
tance
Biuret
reagent
Lugol’s
iodine
Benedict’s
solution
Used to
test
for
Protein
Starch
Glucose
Bile
Fat
Brown paper
Fat
Food Substance
Before
After
Control
Before
After
Chemical Digestion
Food Tests
6. Test for Liver (Catalase)
– Hydrogen Peroxide (about 1cm3)
– Negative – no bubbles
– Positive – foam / bubbles / gas
Peroxidase speeds up the breakdown of hydrogen peroxide, a
waste product of cell metabolism that can be toxic in high
concentrations. The breakdown that occurs is as follows:
2H 2 O 2 
Hydrogen Peroxide
Oxygen
2H 2 O + O 2
Water
Peroxidase, or catalase, an enzyme found in animals and plants,
and in high concentration in the cells of the liver. Peroxidase has an
active site that fits perfectly, like a lock and key, with hydrogen
peroxide. When peroxidase, found in the liver, is exposed to
hydrogen peroxide, large numbers of oxygen bubbles are produced,
as well as water; therefore, a positive test for liver peroxidase will
produce large numbers of oxygen bubbles when the liver is
exposed to hydrogen peroxide.