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

0.39-1.1Normal range for citric acid=

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:

.1Weight 10g of sample into a graduated 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 solution7 .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

1-Weight a dry dish2-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.

Method:

%total solid=( wt of beaker after heating _wt of empty beaker *100/)

Wt of sample(g)