Determination of Pectic Acid

This procedure is based on that described by Southgate (1991) in which the pectic substances are saponified to remove ester groups, pectic acid is then precipitated and weighed. 

Whole fruit is cut in half, the stone is removed and the halves are finely chopped before extraction. Three hundred g of the chopped sample is mixed with 800 ml of water and heated to the temperature of boiling water for 1 h. Transfer the mixture to a 2-1 flask and make up to volume. Place a 200-ml, filtered portion of the extract in a beaker (600 ml). and add 10 g of sucrose if the solution is low in sucrose. Add 200 ml of ethanol with constant stirring and allow the precipitate to settle. Filter off the precipitate on a qualitative paper and wash with ethanol. 

Return the precipitate to its original beaker with hot water and evaporate to a volume of about 40 ml, then cool the solution to 25 °C. Redissolve any precipitate which forms at this stage by acidifying the solution and warming. Dilute 5 ml of 10% (w/v) NaOH to 50 ml and add to the solution. (The volume of the sodium hydroxide used depends on the bulk of the precipitate.) 

Leave the mixute for 15 min. then add 40 ml of water followed by 10 ml dil. HCl and boil the mixture for 5 min Filter the precipitated pectic acids and wash with hot water. 

Return the precipitate to the beaker and repeat the saponification. Wash the precipitate until the filtrate is only slightly acidic. Transfer the precipitate to a platinum dish and dry at 100 °C, weigh and then ignite and re-weigh. Take the loss in weight as pectic acid. A value of approximately 0.5 g pectin/100 g fresh weight is normally expected for apricots (Southgate 1991). 

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