pH in Foods

Introduction:

• pH measures the amount of hydrogen ions in the equilibrium condition.
• A pH of 7 is neutral; it is neither an acid nor a base.
• A low reading (below 7) is an acid.
• A high reading (above 7) is a base.

The importance of pH in foods:

• The natural pH of foods is significant to determine the type of organisms that can grow and survive during food storage.
• pH in foods can be lowered by adding acidulants (acetic, citric, ascorbic, and lactic acids.)
• Useful in many fermented foods (dairy, meat, and alcoholic beverages). The antimicrobial effect that results are mostly due to the decrease of pH below the microbial growth range through production of lactic or acetic acids.
• Many foods and beverages are preserved by adding low concentrations of antimicrobials that are weak acids and most effective at low pH.

During the canning process, pH plays an important role. The lower the pH, the lower the degree of heat required for sterilization. Thus, foods that have a high acidity are quite easy to sterilize.

Citric and acetic acid are commonly added to moderately acidic fruits and vegetables to lower the pH to below 4.5. This permits the sterilization under less heat. Thus, reducing the ablity for microorganisms to grow.

The optimum pH for bacterial growth is 5.5 to 7.0. Below 4.5 bacteria does not usually grow. However, yeasts and molds may grow very well in this condition. Some species grow at a pH of 2.0 or less.

Acids in foods and buffering capacity :

• Buffering is the tendency of a solution or substance to resist changes in pH upon the addition of acid or base.
• Plant tissues contain weak organic acids and salts. When placed in acidified water or acidified dilute salt brines and heated, they create a buffering action to maintain their original pH.

Classification of Foods according to pH values:

Alkaline Foods	pH above 7	Eggs, soda crackers
Low Acid Foods	pH of 5.0-7.0	Meat, fish, poultry, dairy
Medium Acid Foods	pH 4.5 - 5.0	Soups, pasta, pimento
Acid Foods	pH 3.7 - 4.5	Peaches, oranges, tomatoes
High Acid Foods	pH 2.3 - 3.7	Lemons, pickled products

Methods of pH Measurements:

1. Colorimetric Method
2. pH meter

Procedures:

Titration -

Reagents:
1. 0.1 N NaOH
   1. Prepare a 50% solution by dissolving 50g NaOH pellets per 100ml of DEIONIZED water.
   2. Dissolve 4 ml of 50% NaOH solution per liter of DEIONIZED water to make a 0.1 N NaOH solution.
2. Phenolphthalein indicator
3. Sample preparation:
   1. Juices: shake juice and filter with filter paper
   2. Fresh fruits: blend or crush, filter with cheese cloth until the pulp is dry.

Standardization of NaOH:

1. Dissolve 0.5 g KHP(Potassium Acid Phthalate) in 25 ml of distilled water.
2. Add 2-3 drops of phenolphthalein indicator
3. Titrate with previously prepared NaOH until a faint pink color persists for 15 seconds.
4. Calculate the Normality of NaOH used.
   N NaOH = g KHP
   N NaOH __________
   N NaOH (ml NaOH)(Meq wt)

   N NaOH = Normality of NaOH

   ml NaOH = the volume of NaOH used in the titration until the end pink color persists for 15 seconds.

   Meq wt = milli equivalent weight of NaOH = 0.20422

Titration with standardized 0.1 N NaOH:

1. Prepare sample and take a known volume of sample.
2. Add 2-3 drops of phenolphthalein indicator.
3. Titrate with previously prepared NaOH until a faint pink color persists for 15 seconds
4. Calculate % acidity of sample:
   % acid = 100(ml)(N)(C)
   % acid = ____________
   % acid =   ml sample

   ml = volume of NaOH used (ml)

   N = the exact Normality of NaOH (from standardization step)

   C = constant (0.064 or equivalent weight of acid/1000)

pH meter:

1. Plug in the pH meter.
2. Set the function key to "standby"
3. Wash the electrode with distilled water using a squirt bottle and blot dry. Place in the pH 4 buffer and read.
4. By turning the 'calibration' knob, adjust the pH to 4.00 when necessary.
5. Repeat steps 3 and 4 until two reading are very close.
6. Repeat steps 3-5 for pH 7 buffer.
7. Do not touch the 'calibration' knob again!
8. Repeat procedure for the sample to be analyzed and read pH directly from the digital readout.
9. It is ideal to read pH of a sample at room temperature.
10. Remember to always wash and blot dry the electrode in between samples and before putting the electrode back in its keeping solution.
11. Always do 2 readings per sample to account for sample variability.

Lab Data Tables:

Sample	pH meter at Room Temperature.	Acid class
Soft drink
Lemon juice
Milk
Tomato sauce

Sample	ml sample	ml NaOH	% Titratable acidity
Pineapple juice
Milk

Define:

Acid

Alkaline

Normality

Molarity

Buffer

Questions:

1. What is the function of pH in food preservation?
   
   
   
   
   
2. Give two examples where buffering is important in food systems. Explain how that works.
   
   
   
   
   

Reference:

Burns, E.E. "Laboratory Manual for FSTC/HORT 311 - Principles in Food Processing." Texas A&M University. College Station, TX.

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