Determination of food acidity
BCH 445
Lab 1
Food acidity
Food acids are usually organic acids, with citric, malic, lactic, tartaric, and acetic acids being the most
common.
However, inorganic acids such as phosphoric and carbonic acids (arising from carbon dioxide in solution)
often play an important and even predominant role in food acidulation.
Citric acid
Malic acid
The organic acids present in foods influence :
- flavor (i.e.,tartness)
- Color (through their impact on anthocyanin and other pH-influenced pigments)
- prevent/retard the growth of microorganisms or inhibit the germination of spores
- Providing the proper environment for metal ion chelation, an important phenomenon in the
minimization of lipid oxidation
Organic acids may present :
- Naturally,
- By Fermentation
- Added as part of a specific food formulation
The importance of determining food acidity
1. Determine the degree of maturity of fruits and vegetables
The titratable acidity of fruits is used, along with sugar content, as an indicator of maturity, generally the
higher the maturity, the lower the acid content.
e.g. in the ripening process, such as tomatoes from green to mature stage , there is an
increase in sugar content.
2. To determine the freshness of foods
for example in milk, the more the lactic acid levels, means that milk is rotten.
3. Acidity indicators reflect the quality of food
The amount of organic acids in food directly affects the food flavor, color, stability, and the level of quality.
4- Determination of acid on the microbial fermentation process
Such as: fermentation products in soy sauce, vinegar and other acids is an important indicator of quality.
There are two ways to express food acidity:
Titratable acidity Hydrogen concentration pH
• Simple estimate of the total acid content of food • Depend on the strength of acid condition
• Better predictor of acid impact on flavor
Titratable acidity
Titratable acidity is determined by neutralizing the acid present in a known quantity (weight or volume)
of food sample using a standard base.
The endpoint for titration is usually either a target pH or the color change of a pH-sensitive dye, typically
phenolphthalein.
The amount of base needed for neutralization reflect the acid content.
Notes
Titratable acidity provide a simple estimate of acid in food, Routine titration cannot differentiate
between individual acids. Therefore, titratable acidity is usually stated in terms of predominant acid
Note the color at end point:
objective
To determine total acidity of milk, juice, vinegar and oil acid value.
1-Determination of Milk Acidity (Titratable Acidity):
Measuring milk acidity is an important test used to determine milk quality.
The acidity of fresh milk is due to (Natural acidity):
phosphates, casein and whey proteins, citrates and carbon dioxide dissolved during the process of milking.
(Developed acidity) which is due to:
lactic acid produced by the action of bacteria on lactose in milk.
Titratable Acidity: The Acidity that results from accumulation of Natural and Developed
acidity.
Milk Acidity
TA%=0.12% - 0.16% the average 0.14%
If it increased more than 0.16% , is an indication of lactic acid by bacteria.
Acidity is expressed as percentage of lactic acid because lactic acid is the principal acid produced by
fermentation.
Method:
1. Mix the milk sample thoroughly by avoiding incorporation of air.
2. Transfer 10 ml milk to conical flask or beaker .
3. Add equal quantity of distilled water .
4. Add 3-4 drops of phenolphthalein indicator and stir.
5. Rapidly titrate the contents with 0.1 N NaOH solution, continue to add alkali drop by the drop and
stirring the content till first definite change to pink colour .
6. Note down the final burette reading.
Result and Calculation:
Lactic acid %= (0.1M NaOH X vol. of NaOH (in liter)X 90.08) x 100
Weight of the sample
90.08 g/ mol is the molecular weight of Lactate.
Normal range =0.12% - 0.16%
2-Determination of total acidity in juice :
The acidity of natural fruit juices is the result mainly of their content of organic acids.
For example, most fruits contain the tricarboxylic acid (citric acid) whereas grapes are rich in tartaric
acid & peaches, apricots and plums in malic acids.
Both tartaric & malic acids are dicaroxylic acids.
Citric acid
Method:
The acidity of fruit juice may be determined by simple direct titration with 0.1M sodium hydroxide,
using phenolphthalein as an indicator.
1- Weight 10 gm juice in beaker.
2- Add 25 ml of distilled water.
3- Titrate with 0.1M NaOH , using 2 drops of phenolphthalein as an indicator.
Calculations:
Calculate percent acidity of fruit juice ( citric acid):
0.1M NaOH X vol. of NaOH (in liter)X 192.43
1- Wt. of citric acid = 𝟑
*192.43 g/mol is the molecular weight of citric acid
2- % of total acidity = (wt. of acid / wt. of sample) X 100
Normal range for citric acid = 0.39 - 1.1 %
*Why when calculation of the weight of citric acid it divided by 3?
3-Determination of total acidity in vinegars:
- The acidity of vinegars is derived by the fermentation of ethanol by acetic acid bacteria which produce
acitic acid.
- It may be determined titrimetrically using phenolphthalein as an indicator .
- The natural acidity of vinegar is mainly due to the presence of acetic acid (CH3COOH) , which is
volatile .
oxidation oxidation
Method:
Determination of total acidity
1- Weight 1 gm vinegar.
2- Add 10 ml of distilled water.
3- Titrate with 0.1M NaOH , using 2 drops of phenolphthalein as an indicator.
Calculations:
Calculate percent acidity as acetic acid (MW=60.05)
1- Wt. of acetic acid= (0.1M NaOH X volume of NaOH in liter X MW)
2- % of total acidity= (wt. of acid / wt. of sample) X 100
Normal range= 4-6 %
4-Acid value :
The acid value is defined as the number of milligrams of potassium hydroxide required to neutralize the
free fatty acids present in one gram of fat.
It is a relative measure of rancidity as free fatty acids are normally formed during decomposition of oil
glycerides.
The value is also expressed as percent of free fatty acids calculated as oleic acid.
Principle:
- The value is a measure of the amount of fatty acids which have been liberated by hydrolysis from the
glycerides due to the action of moisture, temperature and/or lipolytic enzyme lipase.
- The acid value is determined by directly titrating the oil/fat in an alcoholic medium against standard
potassium hydroxide/sodium hydroxide solution.
Method:
1. Mix the oil or melted fat thoroughly before weighting.
2. Weight accurately about 5 g of cooled oil sample in a 250 ml conical flask.
3. Add 50 ml of freshly neutralized hot ethanol.
4. Add one ml of phenolphthalein indicator solution.
5. Boil the mixture( in water bath) for about 5 minutes and titrate while hot against standard alkali
solution shaking vigorously during the titration.
Calculation:
Acid value = 56.1 X ( V x N) / weight of sample
Where V = Volume in ml of standard potassium hydroxide or sodium hydroxide used
N = Normality of the potassium hydroxide solution or Sodium hydroxide solution.
W = Weight in g of the sample
The maximum level allowed for acid value of edible fats and oils is 0.6 mg NaOH/g
Discussion
Discuses the result you got for each sample and compare it to the normal range
