Dear Sir. Concerning your issue about the measurement of titratable acidity of apple using a pH meter. To measure the pH and titratable acidity,should obtain at least 50 mls of clear juice of apple by one of the following methods: 1. Cut fruit, press with a hand press, and filter through cheesecloth, or 2. Cut fruit into a blender, homogenize, centrifuge slurry, and pour off clear liquid for analysis. Sugar levels often vary within the fruit, being higher at the stem-end and lower at the calyx-end. For this reason, it is important to use longitudinal slices of fruit (from end to end) when sampling. B. Make sure samples are at room temperature before taking measurements. C. Measure the pH of the samples with a pH meter and record the value. D. For each sample, weigh out 6 grams of juice into a 100 ml beaker. E. To each sample, add 50 mls of water. F. Titrate each sample with 0.1 N NaOH to an end point of 8.2 (measured with the pH meter or phenolphthalen indicator) and record the milliliters (mls) of NaOH used. G. Calculate the titratable acidity.  I think the following below links may help you in your analysis:

http://fruitandnuteducation.ucdavis.edu/files/162035.pdf

http://cjoliprsf.ca/Documents/Acidity-pH.pdf

Thanks

     titratable acidity is measured using a well known procedure:mix 5gm of fruit tissue with 100ml of distilled water (boiled distilled water) in a blender.filter and take 10 ml of the filterate, and then titrate with 0.1N NaOH  using phenophthalene as indicator up to the end point .calulate  titratable acidity using the following formula .  Normality of NaOHx0.064xtotal volume of solution/volume of titrated solution xweight of sample x100. please, contact A.O.AC,1980. 

Thank you dear Abbas

If mixing of fruit with distilled water  doesn't change fruit's pH? Because distilled water is a pH=7 liquid and i think that it will affect on amount of used 0.1 NaOH for reaching mixed solution to end point of pH=8.2 and thus, on titratable acidity.

Thank you dear Elgailani 

If mixing of fruit with distilled water  doesn't change fruit's pH? Because distilled water is a pH=7 liquid and i think that it will affect on amount of used 0.1 NaOH for reaching mixed solution to end point of pH=8.2 and thus, on titratable acidity.