lecture 3


Analysis of fruit juices –I


To determine the acidity & content of sodium benzoate & total solids in samples of various fruit juices.

Introduction & principle 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 malic acid citric acid MW=192.43 MW=150.09 MW=134.09 The acidity of fruit juice may be determined by simple direct titration with 0.1M sodium hydroxide, using phenolphthalein as an indicator.

Materials * Determination of total acidity:

0.1M NaOH 1% phenolphthalein indicator

Method :

* Determination of total acidity


1. Weight 10g of sample in conical flask & add 50ml distilled water 2. Titrate using 0.1M NaOH & phenolphthalein as indicator 3 Calculate the total acidity of your fruit juice

Calculate the total acidity:

eq. OH (moles) = 0.1 x vol.of NaOH(ml) x 10-³ g. citric acid = 0.1 x vol.of NaOH(ml) x 10-³ x 192.43/3 g. tartaric acid = 0.1 x vol.of NaOH(ml) x 10-³ x 150.09/2 g. malic acid = 0.1 x vol.of NaOH(ml) x 10-³ x 134.09/2 % 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.

* Determination of sodium benzoate:

Electric ste am bath Conical flasks Graduated flasks of 500ml capcity Separating funnels of 250ml capcity Water bath , balance NaCl crystals of analytical grade 10% NaOH

0.05M NaOH 20%( v/v) HCl Neutralized 50% (v/v) ethanol Chlorophorm of analytical grade

* Determination of sodium benzoate:

1. Weight 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 1 hr. with frequent shaking . 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 2. Add 25ml of chloroform 3. Transfer in to a separatory funnel 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 as indicator 8. Calculate the amount of sodium benzoate in the sample 1ml of 0.05M NaOH =0.0072 sodium benzoate.Wt of sodium benzoate = 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.

* Determination of total solids:

Nickel or stainless steel dishes Measuring cylinder.

* Determination of total so lids:

Weight a dry dish Add 15ml of juice & weight Place the dish & its content on a boiling water bath & evaporate to dryness Place the dish in an oven for 2hrs.

Put the dish in descicator to cool % weight


1. Lees, R."Food Analysis", (1975), Leonard Hill Books.

2. Saudi Standard "benzoic acid, sodium benzoate & potassium benzoate used in preservation of foodstuffs", (1977),SSA 37/1397H 3. Kramer, A. & twing, B. A. (1970), "fundamentals of quality control for the food industry", 3rd.edn. Vol. 1, AVI Publishing Co., Westport, Conn., USA