  TBS/AFDC 3(73) P3 
ICS 67.250  
  
 
© TBS 2020   
 
 
 
DRAFT TANZANIA STANDARD  
 
 
 
 
 
 
 
 
Methods for determination of organic preservatives in foodstuffs – 
Part 3: Sorbic acid and its salts  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TANZANIA BUREAU OF STANDARDS  
   TBS/AFDC 3(73) P3 
ICS 67.250  
  
 
© TBS 2020  0. Foreword  
For protecting food from microbial deterioration, a numb er of methods as application of heat or cold, 
dehydration, fermentation , irradiation or addition of certain chemicals are employed. Besides extending 
the periods of use of food a chemical preservative should be safe for human consumption, should not 
impart  undesirable organoleptic changes, be economical in use and be capable of being analyzed. While 
the use of preservative to be safe under conditions of use is governed by law, it is considered necessary 
to prescribe methods for their analysis. The use of th ese methods would not only ensure repeatable and 
reproducible results f or their correct interpretation, but would also facilitate inter -laboratory comparisons  
There are two classes of preservatives, class I and class II. Class I preservatives include comm on salt, 
sugar, dextrose, glucose (syrup), wood smoke, spices, vinegar   honey, etc. class II preservatives include 
inorganic substances such as sulphurous acid including salts thereof, nitrates of sodium or potassium and 
organic substances like benzoic ac id including salts thereof, sorbic acids and including its sodium, 
potassium and calcium salts and sodium and calcium propionate.  
This standard, covering the determination of organic preservatives, is being issued in three parts. This 
part (Part 3) covers  the determination of sorbic acid and its salts in foodstuffs. The part  1 covers benzoic 
acid and its salts and part 2 covers propionic acid and its salts . In reporting the results of a test or 
analysis made in accordance with this standard, if the final v alue, observed or calculated, is to be 
rounded off, it shall be done in  accordance with TZS 4  
 In the preparation of this Tanzania Standard assistance was drawn from  IS 12014 (Part 3):1986, 
Methods for determination of organic pre servatives in foodstuffs – Part 3: sorbic                  
acid and its salts published by Burea u of Indian Standards (BIS)  
1.0   Scope   
This standard prescribe s the methods for determination of sorbic acid and its salts used as preservatives 
in foodstuffs  
 2.0 Normative r eferences  
2.1 The following referenced standards referred to in the text in such a  way that some or all of their 
content constitutes requirements of this standards.  For dated references, only the edition cited applies. 
For undated references, the latest edition of t he referenced document (including any amendments) 
applies   
TZS 59: Wate r for analytical laboratory use – Specification and test method  
TZS 4: Rounding off numerical values  
TZS 672: Automotive gasoline (premium motor spirit ) 
3. Quality of reagents    
3.1 analytical grade chemicals and distilled water conforming to TZS 59 shal l be employed in tests.  
   TBS/AFDC 3(73) P3 
ICS 67.250  
  
 
© TBS 2020  4. General  
4. 1 this standard specifies two methods for determination of sorbic acids and its salt, viz colorimetric and 
spectrophotometric methods. Colorimetr ic method is applicable to cheese and flour confectionary 
products only . Whereas the spectrophotometric method is applicable to fresh dairy products like cheese, 
sour cream, yoghurt and flour confectionary products  
4.2 Principle  
sorbic acids are isolated f rom food by extraction with diethyl ether and successive partitioning  into 
aqueous NaOH  and CH 2Cl2. Acids are converted to trimethylsilyl (TMS) ester and determined by gas 
chromatography ( GC). Caproic acid is  used as internal standards for sorbic acid   
 5. Colorimetric Method    
5.1 Reagents  
5.1.1 Sulphuric Acid - 0.3N and 2N  
5.1.2 Potassium Dichromate Solution - dissolve 147mg potassium dichromate in distilled water and dilute 
to 100 mL  
5.1.3 Thiobarbituric acid solution (0.5 percent) - dissolve 250mg thio barbituric acid in 5 mL of 0.5N sodium 
hydroxide solution in a 50 mL volumetric flask by swirling under hot water. Add 20 mL distilled water, 
neutralize with 3 mL of 1N hydrochloric acid (HCl) and dilute to volume with distilled water. This solution 
should be freshly prepared before  analysis  
5.1.4 Crystalline magnesium sulphate - MgSO 4. 7H 2O 
5.1.5 Standard Sorbic Acid Solution - accurately weigh 134mg potassium sorbate (equivalent to 100mg 
sorbic acid) and dilute to 1 litre with distilled water. 1 mL of solutio n corresponds to 0.1mg of sorbic acid. 
This solution is stable for seve ral days when refrigerated  
5.2 Equipment  and apparatus  
5.2.1 Steam distillation apparatus  
5.2.2 Volumetric flask - 50mL and 1litre capacity  
5.2.3 Pipette - 5mL and 20 mL capacity.  
5.2.4 Test Tubes - 15mL capacity.  
5.2.5 Spectrophotometer with 10mm Cells  
5.2.6 Flask - 250mL.  
5.3 Procedure  
5.3.1 Preparation of the Sample    TBS/AFDC 3(73) P3 
ICS 67.250  
  
 
© TBS 2020  5.3.1.1 Cheese - Cut the cheese very finely and mix thoroughly  
5.3.1.2 Flour confectionary products  
a) All types of bread and cakes not containing fruits - take 1 or half loaf of bread or  cake and cut it 
into slices of 2 -3 mm thickness. Spread the slices on the paper and let them dry in a warm place 
or at room  temperature  until sufficiently crisp and brittle to grind we ll. Grind entire sample to pass 
through 850 -micron sieve, mix well and keep in an air -tight container before proceeding for 
analysis  
b) Bread and cake containing raisins and fruits - proceed as in 5.3.1.2 (a) except comminute by 
passing twice through food cho pper and dry the sample in an oven at 70˚C under pressure of less 
than 0.06666bars  
5.3.2 Test portion - weigh 1.5 to 2.0g prepared sample into a distillation tube containing silicon chips. Add 
10mL of 2N sulphuric acid and 10g magnesium sulphate. Steam distill the contents maintaining 20 -30mL 
volume in distillation tube with small burner. Avoid charring. Collect 100 -125mL distillate in 250 mL volume 
flask within 45 minutes. Rinse condenser with distilled water and dilute the distillate to volume and mix 
thoroughly.  
5.3.3 Determination - Pipette 2 mL of test portion and 2 mL of distilled water (for blank) into separate 15 mL 
test tubes. Add 1 mL of 0.3N sulphuric acid and 1 mL of potassium dichromate solution and heat in a 
boiling water -bath exactly for five minutes . Immerse   tube in icebath and add 2 mL of thiobarbituric acid 
solution. Place it in boiling water -bath and boil it for 10 minutes. Cool and determine optical density of 
solution at 532nm against blank using matched 1 0mm cells.  
5.3.4 Plotting of the calibrating curve - pipette 5, 10, 15, 20, 25mL of sorbic acid standard solution into 
separate 500 mL volumetric flasks. Dilute each to volume and mix thoroughly and proceed as prescribed 
in 5.3.3. Plot the optical density against µg sorbic acid/ mL 
5.3.5 Calculation - calculate the sorbic acid cont ent in the sample after reading the corresponding sorbic 
acid value of the optical density.  
 
Where  
 A = Sorbic acid contents obtained from calibration curve, and   
 M = mass of sample taken    
Percent of sodium sorbate = percent sorbic acid x 134  
6. SPECT ROPHOTOMETRIC METHOD  
6.1 Reagents  
6.1.1 Metaphosphoric Acid Solution – dissolve 5g phosphoric acid in 25 0mL distilled water and dilute to 1 
litre with absolute ethanol    TBS/AFDC 3(73) P3 
ICS 67.250  
  
 
© TBS 2020  6.1.2 Mixed Ethers – petroleum ether and anhydrous diethyl ether (1:1)  
6.1.3 Potassium Permanganate Solution - dissolve 15g potassium permanganate in distilled water, dilute 
to 100 mL and filter through glass wool.  
6.1.4 Sorbic Acid Solution ( stock) - dissolve 200mg of sorbic acid in 200 mL of mixed ether  
6.1.4.1 working solution - dilute 10 mL stock solution to 200 mL with mixed ethers. This solution 
corresponds to 0.05mg/ mL  
6.1.5 Reference solution - shake 100 mL mixed ethers with 10 mL metaphosphoric acid  solution and dry 
supernatant ether fraction with 5g anhydrous granular sodium sulphate  and decant to obtain clear 
reference solution  
6.2 equipment and Apparatus   
6.2.1 High -Speed Blender  
6.2.2 Separating Funnels - 500mL. 
6.2.3 Volumetric Flasks  
6.2.4 Graduated Pipette  
6.2.5 Ultraviolet Spectrophotometer - provide with a 0.5mm monochromator wi th silica cells of thickness 
20mm, fitted with ground lids  
6.3 Procedure  
6.3.1 Preparation of the Sample  
6.3.1.1 cheese – cut the cheese sample very finely and mix thoroughly  
6.3.1.2 Flour confectionary products  
a) All types of bread and cakes not contai ning fruits - take 1 or half loaf of bread or cake and cut it 
into slices of 2 -3 mm thickness. Spread the slices on the paper and let them dry in a warm place 
or at room temperature until sufficiently crisp and brittle to grind well. Grind entire sample to  pass 
through 850 -micron sieve, mix well and keep in an air -tight container before proceeding for 
analysis  
b) Bread and cake containing raisins and fruits - proceed as in 5.3.1.2 (a) except comminute by 
passing twice through food chopper and dry the sample in  an oven at 70˚C under pressure of less 
than 0.06666bars  
6.3.2 Test Portion – Accurately weigh 10g  of the prepared  sample in the high-speed  blender cup. Add 
enough metaphosphoric  acid solution to yield a total of 100 mL liquid in mixture. Blend for 1 minute  and 
immediately filter through Whatman No. 3 paper  or equivalent filter paper with the same pore size . 
Transfer 10 mL filtrate  to 250 mL separating funnel containing 100 mL mixed ethers and shake for 1 
minute. Discard   aqueous layer and dry the ether extrac t with 5g anhydrous sodium sulphate.    TBS/AFDC 3(73) P3 
ICS 67.250  
  
 
© TBS 2020  6.3.3 Determination – place the ether solution in a silica cell with a ground lid of thickness 20mm and 
measure the absorbance of the solution at 250nm with respect to reference solution in a similar silica cell.  
6.3.4  Plotting of the Calibrating Curve – into a series of a four 100 mL volumetric flasks, add 1,2,4 and 
6mL of working standard sorbic acid  solution and dilute to volume with  mixed ether. Determine 
absorbance of the solution at 250nm. Plot the absorbance again st mg of sorbic acid/ mL.  
6.3.5 Calculation  – calculate the sorbic acid content in the sample after reading the corresponding sorbic 
acid value from the calibration curve.  
Percent sorbic in  the sample  
    
                                 
                              
  
 Percent sodium sorbate = percent sorbic acid x 1.34  
  