The pH of milk is often of concern to everyone. In the laboratory, the pH of milk can be obtained by measuring and converting the acidity.
Fresh milk has a certain acidity, which is mainly composed of acidic substances such as protein, citrate, phosphate and carbon dioxide in milk, which is called natural acidity.
During the storage process after milk is squeezed out, due to the activity of microorganisms, lactose is decomposed to produce lactic acid, which increases the acidity of milk. This acidity increased by fermentation is called fermentation acidity.
The sum of natural acidity and fermentation acidity is called total acidity, and the acidity of milk is usually called total acidity.
How to measure the acidity of milk experimentally?
As a dairy product, the method of determining food acidity can be used. There are three main types of specific experimental acidity measurements:
1. Phenolphthalein indicator method
The phenolphthalein indicator method is mainly applicable to the acidity measurement of milk powder, pasteurized milk, sterilized milk, raw milk, fermented milk, cream, condensed milk, and casein.
2. pH meter method
The pH meter method is mainly applicable to the acidity measurement of milk powder.
3. Potentiometric titrator method
The potentiometric titrator method is mainly applicable to pasteurized milk, sterilized milk, raw milk, fermented milk, cream and condensed milk, and casein.
1. How does the phenolphthalein indicator method test the acidity of milk?
The principle is that after the sample is treated, phenolphthalein is used as an indicator and titrated to neutrality with 0.1000mol/L sodium hydroxide standard solution. The volume of the sodium hydroxide solution consumed is calculated to determine the acidity of the sample.
Weigh 10g of the mixed sample, place it in a 150mL conical flask, add 20mL of freshly boiled water cooled to room temperature, mix well, add 2.0mL of phenolphthalein indicator solution, mix well, and titrate with sodium hydroxide standard solution. Rotate the flask while adding until the color is similar to that of the reference solution and does not fade within 5s. The entire titration process should be completed within 45s. During the titration process, blow nitrogen into the conical flask to prevent the solution from absorbing carbon dioxide in the air. Record the milliliters of sodium hydroxide solution used (V1) to an accuracy of 0.05mL, and substitute it into the formula for calculation. For blank titration, use 96mL of water for a blank experiment and read the milliliters of sodium hydroxide standard solution consumed (V0). The volume of sodium hydroxide consumed in the blank should not be less than zero, otherwise distilled water that meets the requirements should be prepared and used again.
X-the acidity of the sample, in degrees (°T) [measured in milliliters of 0.1 mol/L sodium hydroxide consumed by 100g of sample, in milliliters per 100 grams (mL/100 g)];
C-the molar concentration of the sodium hydroxide standard solution, in moles per liter (mol/L);
V1-the volume of the sodium hydroxide standard solution consumed during titration, in milliliters (ml):
V0-the volume of the sodium hydroxide standard solution consumed in the blank experiment, in milliliters (mL);
100-100g of sample;
M-the mass of the sample, in grams (g);
0.1-the molar concentration of sodium hydroxide defined by the acidity theory, in moles per liter (mol/1).
2. How does the pH meter method test the acidity of milk powder?
The principle is to calculate the acidity of the sample solution by the volume of 0.1000mol/L sodium hydroxide consumed to neutralize the sample solution to pH 8.30.
Weigh 4g of milk powder sample into a 250mL conical flask. Use a measuring cylinder to measure 96mL of water at about 20℃ to re-dissolve the sample, stir, and then let it stand for 20min. Use a burette to add sodium hydroxide standard solution to the conical flask until the pH stabilizes at 8.30±0.01 for 4s~5s. During the titration process, always stir with a magnetic stirrer and blow nitrogen into the conical flask to prevent the solution from absorbing carbon dioxide in the air. The entire titration process should be completed within 1min. Record the milliliters of sodium hydroxide solution used (V1) to an accuracy of 0.05mL and substitute it into the formula for calculation. Blank titration Use 100mL of distilled water for a blank experiment and read the milliliters of sodium hydroxide standard solution consumed (V0).
X-acidity of the sample, in degrees (°T):
C-concentration of sodium hydroxide standard solution, in moles per liter (mol/1.);
V1-volume of sodium hydroxide standard solution consumed during titration, in milliliters (mL);
V0-volume of sodium hydroxide standard solution consumed in blank experiment, in milliliters (mL);
12-12g milk powder is equivalent to 100 ml reconstituted milk (skim milk powder should be 9, skim whey powder should be 7);
M-weight of the sample, in grams (g);
ω-mass fraction of water in the sample, in grams per hundred grams (g/100g);
(1-ω-mass fraction of milk powder in the sample, in grams per hundred grams (g/100 g):
0.1-acidity theory defines the molar concentration of sodium hydroxide, in moles per liter (mol/L).
3. How does the potentiometric titrator method test the acidity of milk?
The principle is to neutralize 100g of sample to pH 8.3 with the volume of 0.1000mol/L sodium hydroxide consumed, and determine its acidity by calculation.
Weigh 10g of the mixed sample, place it in a 150mL conical flask, add 20mL of freshly boiled water cooled to room temperature, mix well, and titrate with sodium hydroxide standard solution potentiometrically to pH 8.3 as the end point. During the titration, blow nitrogen into the conical flask to prevent the solution from absorbing carbon dioxide in the air. Record the milliliters of sodium hydroxide standard titration solution consumed (V1, substitute it into the formula for calculation. Use the corresponding volume of distilled water for blank experiment and read the milliliters of sodium hydroxide standard solution consumed (V0).
X--the acidity of the sample, in degrees (°T);
C-the molar concentration of the sodium hydroxide standard solution, in moles per liter (mol/L);
V1-the volume of the sodium hydroxide standard solution consumed during titration, in milliliters (mL);
V0-the volume of the sodium hydroxide standard solution consumed in the blank experiment, in milliliters (mL);
100-100g sample:
M-the mass of the sample, in grams (g);
0.1-the molar concentration of sodium hydroxide defined by the acidity theory, in moles per liter (mol/l).
