Acidity of milk determination of factors of influence technical significance. Raw milk quality indicators

The acidity of the milk of individual animals can vary over a fairly wide range. It depends on the state of metabolism in the body of animals, which is determined by feed rations, breed, age, physiological state, individual characteristics animal, etc. The acidity of milk changes especially strongly during the lactation period and when animals become ill.

So, in the first days after calving, the acidity of milk is increased due to great content proteins and salts, then, after a certain time (40-45 days), it decreases to the physiological norm. Milk before the end of lactation of cows has a low acidity.

When animals are sick, the acidity of milk, as a rule, decreases. It changes especially sharply in animals with mastitis.

Although titratable acidity is a criterion for assessing the freshness and naturalness of milk, it should be remembered that milk can have increased (up to 26°T) or low (less than 16°T) acidity, but nevertheless it cannot be considered poor quality or adulterated, since it is heat resistant and withstands boiling or gives a negative reaction to the presence of soda, ammonia and impurities of inhibitory substances. The deviation of the natural (native) acidity of milk from the physiological norm in this case is associated with a violation of the feeding rations. Such milk is accepted as varietal based on the testimony of a stall sample, confirming its naturalness. More precisely, the acidity of milk can be controlled using the pH method.

The observed increase (up to 23-26°T) in the acidity of milk obtained from individual animals and even the whole herd is the result of a serious violation of mineral metabolism in the animal body. It is caused, as a rule, by an insufficient amount of calcium salts in the feed. Such cases occur when feeding animals large quantities acidic feed (green mass of cereals, corn, corn silage, beet pulp, stillage) poor in calcium salts. Fresh milk with increased natural acidity is suitable for production fermented milk products, cheese and butter.

The decrease in the acidity of milk is mainly due to high content urea, which can be caused excess consumption proteins with green fodder, the use of significant amounts of nitrogen supplements in the diet of animals or nitrogen fertilizers in pastures. Milk with low acidity is inappropriate to be processed into cheeses - it coagulates slowly rennet, and the resulting clot is poorly processed.

Active acidity (pH).

Active acidity is expressed by the pH value. It characterizes the concentration of free hydrogen ions (activity) in milk and is numerically equal to the negative decimal logarithm of the concentration of hydrogen ions (H +), expressed in moles per 1 liter.

pH value whole milk averages 6.7-6.5 and ranges from 6.3 to 6.9, which indicates a slightly acid reaction of milk.

Since in current GOSTs And technological instructions acidity is expressed in units of titratable acidity, to compare with them the pH readings for milk and basic fermented milk products, there are established average ratios. For example, for harvested milk, these ratios are as follows:

There is no complete correspondence between active and titratable acidity, since titratable acidity does not indicate the content of any alkalis in milk, but a shift in pH from 6.3 to 8.2-8.5. This is established by the appearance of a red color of phenolphthalein introduced into milk. Freshly milked milk may have a high titratable acidity, but a low active one, and vice versa. With an increase in titratable acidity as a result of the formation of acid during the development of microorganisms, the pH does not change for some time due to the buffer properties of milk, characterized by the presence of proteins, phosphates, and nitrites in it. If, instead of acid, a certain amount of alkali is added to milk, then the pH will not change, and the titratable acidity will change. Only when the acid and amide groups of the amino acids of proteins are neutralized does a sharp change in active acidity occur.

The pH index is of great importance, since it determines the stability of the polydisperse system of milk, the conditions for the growth of microflora and its influence on the processes of maturation of cheese, the speed of formation of components that determine the taste and smell of dairy products, the thermal stability of milk proteins, and the activity of enzymes. The pH value evaluates the quality raw milk and dairy products.

Acid dissociation of proteins is negligible, so the concentration of hydrogen ions remains constant, while titratable acidity increases, since both active and bound hydrogen ions react with alkali when it is determined.

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Active acidity is the concentration of hydrogen ions (H +), its value is expressed by the pH value. Hydrogen indicator pH - the decimal logarithm of the concentration of hydrogen ions H +, taken with the opposite sign: pH - lgaH.
At pH 7 the reaction is neutral, at pH below 7 it is acidic and at pH above 7 it is alkaline. For clean water at a temperature of 22 ° C pH 7. Therefore, the higher the active acidity, the lower the pH. The higher the alkalinity of the medium, the higher the pH value (limiting pH value 14). The value of pH in biochemistry is enormous.
There is a relationship between pH and titratable acidity of milk and some dairy products. It has been established that the active acidity of milk (pH) changes much more slowly than the titratable acidity. The property of milk to resist pH changes is called buffering. It depends on the presence of calcium caseinate, phosphate and citrate salts in milk.
The concentration of hydrogen ions in fresh normal milk is characterized by a pH of 6.67-6.68. Active acidity fresh milk, as we see, is small and close to a neutral reaction. Such acidity is favorable for the stability of the colloidal system of milk and the development of bacteria in it.
Milk and other dairy products have a certain buffer capacity - this is the number of milliliters of 0.1 N. acid or alkali per 100 ml of solution, which shifts the pH by one. The buffer capacity of milk for acid is 2.4-2.6, for alkali 1.2-1.4 ml. Different samples of milk differ in the degree of buffering properties.
The color transition of the phenolphthalein indicator from colorless to red occurs at pH 8.2-8.3. When titrating fresh milk, pH 6.67-6.68 shifts when 0.1 N is added. alkali solution to pH 8.2-8.3 (the appearance of a pink color of the indicator). In the case of an increased buffer capacity, more alkali will be required for such a shift and the titratable acidity will be higher. The buffer capacity of milk increases with an increase in the content of proteins, citrate and phosphate salts. This is one of the reasons why the titratable acidity of different samples of fresh milk fluctuates.
A slow change in the active acidity of milk favors the reproduction of lactic acid bacteria in it, the development of which, as you know, slows down and then stops with a significant decrease in pH. The buffer properties of milk and cheese are of great importance in cheese making.
Currently, the industry uses the HM-68 device, developed by VNIMI, to automatically control the acidity of milk using the pH method in individual samples. This device can control the acidity of raw, pasteurized, baked milk, as well as cream.

The acidity of milk is determined in Turner degrees. For 1 degree of acidity, take 1 ml of 0.1 N sodium hydroxide solution used to neutralize acids in 100 ml of milk or 100 g of a dairy product .

normal fresh milk has an acidity of 16-19 degrees; fairly fresh milk has 20-22 degrees, stale milk- 23 degrees or more. Milk diluted with water or mixed with soda has an acidity below 16 degrees.

Progress:

To determine the acidity, pour 10 ml of milk into a flask, add 20 ml of distilled water and 3-4 drops of 1% alcohol solution phenolphthalein, mix well and titrate with 0.1 N sodium hydroxide solution until a slightly pink color does not disappear within 2 minutes.

The number of ml of 0.1 N sodium hydroxide solution consumed, multiplied by 10 (for conversion to 100 ml of milk), will show the number of degrees of milk acidity.

Example: Let's say that 2 ml of a 0.1 N alkali solution was used to titrate 10 ml of milk, then the acidity of milk is (2 times 10) 20 degrees.

Determination of fat content in milk.

8 ml of a 10% soda solution, 10 ml of test milk and 3-3.5 ml of an alcohol mixture consisting of 1 part of amyl alcohol, 6 parts of ethyl alcohol and a few drops of phenolphthalein solution. After that, the butyrometer is closed with a stopper and the contents are shaken well until the gelatinous lumps formed at the beginning are completely dissolved and the mass turns into a uniform liquid. Next, put the butyrometer with the stopper down for 4-5 minutes in water bath at 65-70 0 C, after which it is transferred to a centrifuge and rotated for 4 minutes. At the end of centrifugation, the butyrometer is carefully removed, holding the stopper down so as not to mix the contents, as a result of centrifugation, the butyrometer is divided into 2 layers: the upper one is transparent, amber color͵ the lower one is red (for better peeling, the butyrometer is again placed in the bath for 3-4 minutes). Upper layer is fat, and the calculation of its amount is carried out in the same way as in original way Gerbera (butyrometer), one large division of the butyrometer corresponds to 1% fat.

Note: after filling the butyrometer, it is extremely important to check that its contents, with the stopper down, reach the first or second division of the scale. If the liquid level is lower, which should be the result of fluctuations in the volume of the butyrometer, then you need to add the missing amount of soda solution.

The fat content of milk is normally 3.2-3.6%.

Definition of adulteration of milk.

Determination of soda in milk: Approximately the same amount of 0.2% solution of rosolic acid in 96% alcohol is added to 1/3 test tubes of the milk under study and shaken. In the presence of soda, the mixture turns pink color. Soda is added to milk to delay its sourness. From a hygienic point of view, the addition of soda to milk, according to current sanitary legislation, is not allowed.

The acidity of milk and dairy products (except butter) is expressed in Turner degrees.

The Turner degree indicates the number of milliliters of 0.1 N. sodium hydroxide solution (or potassium hydroxide) required to neutralize 100 ml or 100 g of the product. The true acidity of milk is pH 6.5-6.8, the total acidity is 15.99-20.99°T. If the milk value drops below pH 6.5, this may indicate that the animal is infected. If it drops to pH 4.4, the animal is seriously ill.

Table of the ratio of acidity in degrees Turner and pH

Milk purchased by processors must be collected from healthy cows on farms that are free from infectious diseases and in accordance with the rules of veterinary legislation.

In terms of quality, milk must meet the requirements of the standard; it must be filtered and cooled after milking; its storage with manufacturers must comply with the requirements of "sanitary and veterinary regulations for dairy farms of collective farms, state farms and subsidiary farms”, approved in the prescribed manner.

The shelf life of milk before sale should not exceed 24 hours at a temperature not exceeding 4 ° C; 18 hours - at a temperature not higher than 6 ° C; 12 hours - at a temperature not higher than 8°C.

Milk conductivity measurement

Conductivity (or electrolytic conductivity) is defined as the ability of a substance to conduct an electric current. It is the inverse of the resistance value.

*These values ​​depend on the geographical area, breed and other factors.

The conductivity of milk varies depending on the concentration of ions in it, in the following relationship:

The addition of water, sugar, proteins, insoluble salts - reduces the concentration of ions and therefore reduces the conductivity of milk.

Addition of salts - increases the concentration of ions and therefore increases the conductivity of milk.

Exceptionally high readings (6.5 - 13.00mS/cm (18°C) - indicate the presence of mastitis. The infection has penetrated the tissues of the udder. This allows sodium and chloride ions in the blood to penetrate into the milk. The concentration of ions in the milk increases and it is easier to conduct electric current, therefore its conductivity increases

Mastitis is a disease of the mammary gland and is most often provoked by a bacterial infection of the tissues of the udder. Mastitis leads to changes in the electrical conductivity of milk, mainly due to changes in the concentration of sodium, potassium and chloride ions. Therefore, conductivity measurement can help

If, using its conductivity measurement option, you get exceptionally high conductivity readings (6.5 - 13.00 mS/cm (18°C)), this is an indicator of the development of mastitis.

Determination of milk density

The density of milk varies between 1.030 - 1.034, which depends on the composition nutrients in him. Density skimmed milk rises and can reach 1.037. Milk diluted with water has a low density (1.018), since the percentage of solids in it decreases.

If the temperature of the milk at the time of measurement was above or below 20°C, the results of the reading must be checked against the table.

In the absence of tables, the calculation method is used. It has been established that a change in temperature by 1 degree changes the density of milk by 0.2 divisions of the lactodensimeter, or by 0.0002 units of density.

If the temperature of the milk is above 20 ° C, then its density will be less than at a temperature of 20 ° C, therefore, 0.0002 must be added to the density value found for each degree of temperature.

If the temperature of the milk under study is below 20 ° C, then its density is higher than at a temperature of 20 ° C, i.e., 0.0002 must be subtracted from the density found for each degree of temperature.

Determination of the acidity of milk by the titrimetric method

Determination of milk acidity was carried out according to GOST 3624-92 “Milk and dairy products. Titrimetric methods for determining acidity.

The acidity determines the freshness of milk. The acidity of milk is expressed in Turner degrees. The acidity of fresh milk is due to the presence of proteins, phosphate and citrate salts in it, a small amount dissolved carbon dioxide and organic acids. During the storage of milk as a result of the development of microorganisms that ferment milk sugar, lactic acid accumulates and the acidity of milk increases.

Method order. Pipette 10 ml of well-mixed milk into a 100 ml conical flask, add 20 ml of distilled water and 2-3 drops of phenolphthalein. The mixture is thoroughly mixed and titrated with a 0.1 N burette. alkali solution with continuous shaking. First, about 1 ml of alkali is immediately poured, and then drop by drop until a faint pink color appears, which does not disappear within 1 minute.

Titration should be carried out at the same speed, as a fast titration results in underestimated results compared to a slow one.

acidity of milk X in Turner degrees is determined by the formula:

where v - amount 0.1 n. sodium hydroxide solution used for titration 10 ml of milk, ml;

10 - coefficient for conversion to 100 ml of milk.

The discrepancy between parallel determinations should not exceed 2.6°T.

Determination of the mass fraction of fat in drinking milk

The determination was carried out according to GOST 5867-90 “Milk and dairy products. Methods for determining fat. The essence of the method: Fat is isolated in the form of a continuous layer, the volume of which is measured in special device- butyrometer. Fat in milk is in the form of fat globules, surrounded by a lipoprotein shell, which prevents their fusion and determines the high stability of the fat emulsion in milk. Therefore, to release fat, the protein shell is destroyed by the action of concentrated sulfuric acid, which converts the casein-calcium complex of milk into a double soluble compound of casein with sulfuric acid:

NH2R(COO)6Ca3 + 3H2SO4 >NH2--R--(COOH)6 + 3CaSO4

caseincalcium complex casein

NH2-- R--(COOH)6 + H2SO4 >H2SO4 NH2R(COOH)6

For faster release of fat, in addition to acid, isoamyl alcohol is introduced, which reduces surface tension fat globules and promotes their fusion.

Method order. In a clean dry butyrometer, trying not to wet the neck, 10 ml of sulfuric acid are measured with an automatic pipette and, carefully so that the liquids do not mix, add 10.77 ml of milk with a pipette, placing the tip of the pipette against the wall of the butyrometer neck at an angle. In this case, the level of milk in the pipette is set at the lower point of the meniscus. The milk should flow out of the pipette slowly. After emptying the pipette, take it away from the neck of the butyrometer no earlier than after 3 s. The pipette tip must not touch the sulfuric acid.

Blowing out the remaining drop of milk from the pipette is not allowed. Then 1 ml of isoamyl alcohol is measured into the butyrometer with an automatic pipette. While filling the butyrometer, the neck of the butyrometer must remain dry and clean. To neutralize the acid in case it gets on the neck of the butyrometer, the surface of the rubber stopper is treated with chalk and only after that the butyrometer is closed.

The cork is inserted into the neck in a helical motion slightly more than half of its length. While holding the cork with a finger, the butyrometer is shaken until the protein substances are completely dissolved, turning it over 5 times so that the liquids in it are completely mixed. Butyrometers may not have the same volume, as a result of which, when the same amount measured reagents in different butyrometers, the column of released fat can take a different position.

In order to measure the volume of released fat at the end of the analysis, its column after centrifugation must be in the graduated part of the butyrometer, and before centrifugation, the upper liquid level in the device must be within nine to ten divisions of the scale. This limit is determined by holding the capped butyrometer cap down. If the upper limit of the liquid is at the bottom of the scale, sulfuric acid is added to the butyrometer. The addition of sulfuric acid does not affect the result of the determination. After checking the fullness of the fat meter with liquid, it is placed with the stopper down for 5 minutes in a water bath with a temperature of 65±2°C. At this temperature milk fat is in a molten state, which facilitates its separation during centrifugation. After removing from the bath, butyrometers are inserted into the centrifuge cartridges with the working part towards the center, placing them symmetrically one against the other. If there is an odd number of butyrometers, a butyrometer filled with water is added.

After inserting the butyrometers into the cartridges, the centrifuge is closed with a lid and centrifuged for 5 minutes at a speed of at least 1000 rpm. At the end of centrifugation, each butyrometer is removed from the cartridge and the movement of the rubber stopper adjusts the column of fat in the butyrometer so that it is in the graduated part of the device. Then the butyrometers are immersed with the stoppers down in a water bath, the water level in which should be slightly higher than the level of fat in the butyrometer. After 5 minutes, the butyrometers are removed from the water bath and the fat is quickly counted. When counting, the butyrometer is held vertically, the border of fat should be at eye level. By moving the plug up and down, the lower limit of the fat column is set on the whole division of the butyrometer scale and the number of divisions is counted from it to the lower point of the meniscus of the fat column. The interface between fat and acid should be clear, and the column of fat should be transparent. Cloudy or dark colored fat indicates misidentification