Analysis of fruit juices –I
Objective
To determine the acidity & content of sodium
benzoate & total solids in samples of
various fruit juices.
Determination of total acidity
The acidity of natural fruit juices is the
result mainly of their content of various
organic acids. For example, most fruits
contain the tricarboxylic acid, citric acid,
whereas grapes are rich in tartaric acid &
peaches, apricots & plums in malic acids.
Both tartaric & malic acids are dicaroxylic
acids.
Tartaric acid MW=150.09
malic acid
MW=134.09
citric acid
MW=192.43
The acidity of fruit juice may be
determined by simple direct titration
with 0.1M sodium hydroxide, using
phenolphthalein (ph.ph) as an
indicator.
Method :
1. Weight 10g of sample in conical flask
& add 50ml distilled water
2. Titrate using 0.1M NaOH &
phenolphthalein (ph.ph) as indicator
3- Calculate the total acidity of your
fruit juice
citric acid= 0.1 x vol.of NaOH(ml) x
10-³ x 192.43/3
Total acidity= wt of acid/wt of sample
*100
Normal range for citric
acid =
0.39-1.1
Determination of sodium
benzoate
Sodium benzoate (C6H5COO Na ,MW=144
) is a commonly used preservative which
is added to fruit juices to prevent the
growth of microorganisms. When added
in high conc., it affects the taste of juice.
Sodium benzoate is usually permitted at a
conc. Of up to 1.3g/l of juice.
The benzoate anion is not soluble in
non-polar solvents because of its
negative charge. However, in acid
solution, benzoic acid is formed.
This is neutral & quite non-polar.
Moreover, it is soluble in non-polar
solvents, into which it may be
extracted at acidic PH, i.e. well
below the pka of the carboxyl
group , which is 4.20 .
In this experiment , benzoic acid is
extracted into chloroform, which is
then removed by evaporation.
After dissolving the residue in
50% (v/v) neutralized ethanol,
the benzoic acid is titrated with
0.05M sodium hydroxide, using
phenolphthalein as an indicator.
Method:
Weight 10g of sample into a graduated .1
flask & add 1ml of 10% NaOH solution & 12
g NaCl. Add sufficient water to bring the
vol. up to about 50ml & let it stand for 30
min. with frequent shaking .
2- Add drops of ph.ph (the color will change) ,
add drops of HCL until the color change (or
disappear), then add excess 3 ml HCL
3. Transfer into a separatory funnel
4. Add 25ml of chloroform
4. let it sand for 30min with frequent shaking.
5. Transfer 12.5ml of the chloroform layer (low
layer) into a conical flask & evaporate off
the chloroform on a steam bath
6. Add 50ml of 50%ethanol solution
7. Titrate with 0.05M NaOH using
phenolphthalein(ph.ph) as indicator
8. Calculate the amount of sodium benzoate in
the sample.
1ml of 0.05M NaOH =0.0072 sodium benzoate.
Ttitre ml of NaOH = x
Wt of sodium benzoate(x) = ml * 0.0072
%of sodium benzoate= wt sodium benzoate/
wt of sample *100
Normal range not exceed 0.13%
Determination of total solids
Remnants of pulp & other solids are
easily determined gravimetrically,
after evaporation of water from a known
weight of sample.
The solid present in juce include sugar
, organic acid & pectins
Method:
1-Weight a dry dish
2-Add 15ml of juice & weight
3-Place the dish & its content on a
boiling water bath & evaporate to
dryness
4-Place the dish in an oven for 2hrs.
5-Put the dish in descicator to cool %
weight.
%total solid=( wt of beaker after
heating
_ wt of empty beaker *100)/
Wt of sample(g)