When fat is oxidized, a large amount of peroxide compounds and atomic oxygen is released. These substances are stronger oxidizing agents than iodine. Oxygen displaces iodine from potassium iodide. The presence of free iodine is determined using starch. To determine the amount of free iodine, determine the amount of sodium sulphate used to neutralize it.

The peroxide value is the number of grams of iodine isolated from potassium iodide by peroxides contained in 100 g of fat.

Preparing the material for the reaction. The fatty tissue of the bird is crushed with scissors, rendered and filtered.

Reaction setup: A sample of the melted fat under study (weighing 1 g) is weighed in a conical flask with an error of no more than 0.0002 g and dissolved in 20 ml of a mixture of glacial acetic acid and chloroform (1:1). Add 0.5 ml of freshly prepared saturated potassium iodide solution to the solution and keep in a dark place for 3 minutes. Then 100 ml of distilled water is added to the solution, to which 1 ml of 1% starch solution is added in advance. The released iodine is titrated with 0.01 N. solution of sodium sulphate until the blue color disappears. In parallel, under the same conditions, a control determination is carried out, in which the same amounts of reagents are taken, but without fat.

Fat peroxide value X(%) is calculated using the formula:

where K is the correction to the titer 0.01 n. sodium sulfate solution;

V - quantity 0.01 n. sodium sulphate solution used for titration of the test solution, ml;

V 1 quantity 0.01 n. sodium sulphate solution used for titration of the control solution, ml;

0.00127 - the amount of iodine corresponding to 1 ml of 0.01 N. sodium sulfate solution, g;

m - fat mass, g.

Accounting for the reaction: Fat from chilled and frozen carcasses of all types of poultry is considered fresh: if the peroxide value does not exceed 0.01 g of iodine; chicken fat from chilled carcasses with a peroxide value of 0.01-0.04 g of iodine, goose, duck, turkey - 0.01-0.1 g of iodine, fat from frozen carcasses of all types of poultry with a peroxide value of 0.01-0, 03 g of iodine is considered to be of questionable freshness; if the specified values ​​are exceeded, poultry meat is considered stale.

Edible rendered fat obtained from slaughtered cattle, depending on the peroxide value, is considered: fresh - up to 0.03; fresh, but not subject to storage - from 0.03 to 0.06; questionable freshness - from 0.06 to 0.1; stale - above 0.1.

Reaction with neutral red.

The hydrolysis of fats produces a large amount of free fatty acids, and the products of fat oxidation can be volatile fatty acids. The accumulation of these products in fat leads to an increase in its acidity. Neutral red oxidizes in an acidic environment, acquiring a red color. In addition, neutral red can be oxidized under the influence of peroxide compounds, atomic oxygen and a number of other oxidizing agents formed during the oxidation of fats.

Setting up a reaction. 1 g of the test fat is placed in a porcelain mortar, then 1 ml of a working (0.01%) aqueous solution of neutral red is added there. After this, the contents of the mortar are intensively ground with a pestle for 1 minute. An aqueous solution of neutral red does not mix with fat, so the remaining paint must be drained off.

Accounting for reactions. Fresh fat turns yellow or beige; pork and lamb fat may have a greenish tint. Fat of questionable freshness turns brown to pink. Spoiled fat turns bright pink to red.

Qualitative reaction to aldehydes.

Aldehydes are one of the main products of fat oxidation, so their presence in fat indicates its spoilage.

The essence of a qualitative reaction to aldehydes lies in their ability to form a colored compound with polyhydric phenol in an acidic environment.

Setting up a reaction. 2 ml of the test fat, previously melted in a water bath, is placed in a test tube, 2 ml of hydrochloric acid with a density of 1190 kg/m 3 and 2 ml of a saturated solution of resorcinol in benzene are added. Then the test tube is closed with a rubber stopper and its contents are mixed.

Accounting for reactions. If there are aldehydes in the fat being tested, the contents of the test tube turn lilac-red. If the color of the contents of the tube has not changed, then the reaction to aldehydes is considered negative.

Determination of hydrogen sulfide.

The reaction is based on the interaction of lead acetate with hydrogen sulfide gas, which results in the formation of a salt of hydrogen sulfide acid - dark-colored lead sulfide:

H 2 S + (CH 3 COO) 2 Pb = PbS + 2CH 8 COOH.

Determination of hydrogen sulfide does not give good results for all types of meat spoilage. A positive result is usually obtained when meat decomposes under anaerobic conditions (in the skin). When meat rots under normal conditions, hydrogen sulfide may not be detected by this reaction.

Progress of the reaction. 25-30 pieces of meat are placed in a short test tube or vial with a wide neck. A strip of filter paper moistened with an alkaline 10% solution of lead acetate is fixed near the cork, and the paper should not touch either the meat or the walls of the glass below the cork.

The reaction is read after 15 minutes. If there is no hydrogen sulfide in the meat, then the piece of paper remains white. Hydrogen sulfide turns the paper brown or dark brown. If there is little hydrogen sulfide in the meat, then only the edge of the paper darkens, and if there is a large amount of hydrogen sulfide, the coating on the paper acquires a metallic sheen.

The reagent for hydrogen sulfide is prepared as follows: 10% sodium hydroxide is added to a 10% aqueous solution of lead acetate until a precipitate forms. The solution is stored in a tightly closed bottle.

Determination of pH.

For the determination method, see the topic: determination of meat from sick animals. As meat decomposes, alkaline products accumulate in it, as a result of which the concentration of hydrogen ions decreases.

To assess the freshness of meat, the pH value is of relative importance, since it depends not only on the degree of freshness of the meat, but also on the condition of the animal before slaughter. In filtered extracts from fresh meat, the pH is 7-6.2, and in defrosted extracts it is 6.0-6.5; in extracts of meat of suspicious freshness - 6.3-6.6 (defrosted - 6.6); in extracts of stale meat -6.7 and higher.

Luminescent analysis.

It is known that meat of different degrees of freshness fluoresces differently under the influence of ultraviolet radiation.

Rice. 2. Luminoscope “Filin”

Setting up a reaction. For luminescent analysis of meat freshness, the Filin luminoscope is used (Fig. 2). The device is connected to the network. A sample of the meat being tested or a meat extract of 1:4 is placed in the working compartment of the device and viewed under ultraviolet light.

Accounting for reactions. Fresh cattle meat fluoresces in a red-velvet color, lamb - dark brown.

pork - light brown. When meat decomposes, a glow is observed in the form of yellow dots on a dirty dark background.

Meat extract from fresh meat fluoresces pink-violet; from meat of questionable freshness - pink-violet with a greenish tint; from stale meat - green-bluish color.

The peroxide number (I p) is the number of peroxides, expressed in milliequivalents of active oxygen, contained in 1000 g of test material.

my substance. The peroxide value can be determined by two methods.

Tests are carried out by protecting solutions from exposure to ultraviolet radiation.

of the world.

Method 1. About 5 g of the test substance (exactly weighed) is placed in a conical flask with a ground stopper with a capacity of 250 ml. Add

30 ml of a mixture of glacial acetic acid and chloroform (3:2), shake until the test substance dissolves, add 0.5 ml of a saturated solution

Ra potassium iodide and close the flask with a stopper. Shake for exactly 1 minute, when

add 30 ml of water and titrate with 0.01 M sodium thiosulfate solution,

adding the titrant slowly with constant vigorous shaking until light

yellow color of the solution. Add 5 ml of starch solution and continue

Continue the titration with vigorous shaking until the blue color completely disappears. A control experiment is carried out under the same conditions. If the quantity

The content of the titrant in the control experiment exceeds 0.1 ml, the determination is carried out

with freshly prepared saturated potassium iodide solution.

where: V is the volume of 0.01 M sodium thiosulfate solution used for titration in the main experiment, in milliliters;

V 0 – volume of 0.01 M sodium thiosulfate solution consumed in the control experiment, in milliliters;

a is the weight of the test substance, in grams;

c is the molar concentration of sodium thiosulfate solution.

Note. Preparation of starch solution. 1.0 g of soluble starch is ground with 5 ml of water and the mixture is poured into 100 ml of boiling water containing 10 mg of mercury(II) iodide.

Method 2. An accurate portion of the test substance, depending on the expected peroxide value (Table 28.1), is placed in a 250 ml conical flask with a ground stopper. Add 50 ml of trime-

tilpentane and glacial acetic acid (2:3).

Table 28.1

Weight of the test substance depending on the expected peroxide value

Expected peroxide value	Weight of test substance, g

Shake the flask until the test substance dissolves, add

Add 0.5 ml of saturated potassium iodide solution and stopper the flask. Ras-

The solution is kept for exactly 1 minute, shaking constantly, then 30 ml of water is added and titrated with 0.01 M sodium thiosulfate solution, shaking vigorously until the solution turns light yellow. Then add about 0.5 ml of 0.5%

starch solution and continue titration with constant shaking until the solution becomes discolored.

In the case of a peroxide number of 70 or higher, after adding each por-

After titration, the solution is kept for 15–30 s with stirring or a small amount (0.5–1.0% (w/m)) of an emulsifier (for example, polysorbate 60) is added.

For peroxide values ​​above 150, it is recommended to use

add 0.1 M sodium thiosulfate solution. A control experiment is carried out under the same conditions. If the amount of titrant in the control experiment exceeds 0.1 ml,

determination is carried out with a freshly prepared saturated solution of

ly iodide.

The peroxide value is calculated using the formula given in method 1.

29. SAPTONIZATION NUMBER (OFS 42-0124-09)

The saponification number (I s ) is the amount of potassium hydroxide, expressed in milligrams, required to neutralize the free acids and saponify the esters contained in 1.0 g of the test substance.

Method. An exact weighed portion of the test substance, depending on the expected saponification number (Table 29.1), is placed in a reflux flask with a capacity of 250 ml. Add 25.0 ml of a 0.5 M alcohol solution of potassium hydroxide and several glass beads, heat at boiling in a water bath for 30 minutes or the time specified in the private pharmacopoeial monograph, until a clear solution is obtained.

Add 1 ml of a 1% phenolphthalein solution and immediately, while the solution is hot, titrate the excess potassium hydroxide with a 0.5 M solution of hydrochloric acid.

Table 29.1

Weight of the test substance depending on the expected saponification number

Expected number	Test subject's sample
saponification	substances, g

The saponification number is calculated using the formula:
		I s =	28.05 × (V 2 −V 1)
where: V 1	– volume 0.5	M hydrochloric acid solution,
spent on titration in the main experiment, in milliliters;
	– volume 0.5		hydrochloric acid solution,

consumed in the control experiment, in milliliters; a is the weight of the test substance, in grams;

28.05 – the number of milligrams of potassium hydroxide contained in

1 ml of 0.5 M alcohol solution of potassium hydroxide.

In the case of difficultly saponified substances, add 5–10 ml of xylene and

heated for a longer time (heating time is indicated in the private pharmacopoeial monograph).

When analyzing colored oils, the titration end point is set potentiometrically.

Liliya Suleymanova (Omsk, Russia)

The regulatory documents by which the most common vegetable oils can be identified and their quality assessed are the following: mustard oil - GOST 8807-94; corn oil - GOST 8808-2000; rapeseed oil - GOST 8988-2002; soybean oil - GOST 7825-96; sunflower oil - GOST R 52465-2005 and GOST 1129-93 (lost in force in the Russian Federation) Other types of oils (cedar, camelina, linseed, etc.) are assessed according to technical specifications or standards of organizations, the texts of which are the property of manufacturers (developers) ), in connection with which there are difficulties for conducting the examination. The quality of vegetable oils is assessed based on organoleptic and physicochemical indicators.

Among the organoleptic indicators for most types of oils, transparency, as well as smell and taste are standardized. In particular, all refined deodorized oils must be transparent, without sediment, and be impersonal in taste and smell. All unrefined oils must have the taste and smell characteristic of the corresponding type of raw material, without any foreign odors or tastes. Due to the presence of phospholipids in these oils, turbidity and the so-called “net” are observed; during long-term storage, a sediment forms at the bottom of the container.

The set of physical and chemical indicators provided for by the regulatory documents in force in the Russian Federation includes for most vegetable oils: color number, acid number, peroxide number, mass fraction of phosphorus-containing substances, mass fraction of moisture and volatile substances, mass fraction of non-fat impurities.

The values ​​of these indicators are differentiated taking into account the type of oil, cleaning method, and commercial grade. For all vegetable oils that have undergone refining (alkaline neutralization), current regulatory documents provide for the absence of soap (based on qualitative reaction).

The acid number values ​​​​normalized by standards are inversely related to the depth of refining and oil purification. This indicator is increased by the presence of native lipase, which catalyzes the hydrolysis of triglycerides, as well as some metals and their oxides of calcium, magnesium, zinc, and iron.

The value of the peroxide number is limited for any type of edible oil sold and should not exceed 10 millimoles of active oxygen per kilogram of product. Peroxides are the primary products of fat oxidation with oxygen; they are extremely unstable and easily enter into secondary reactions, the products of which are aldehydes, ketones and low molecular weight fatty acids. The initial stage of fat peroxidation does not lead to changes in organoleptic characteristics. However, peroxide compounds are toxic to humans, and hydroperoxides are initiators of further oxidation. Currently, acid and peroxide numbers for all types of vegetable oils are also safety indicators, as they are standardized in the Technical Regulations (they are indicators of oxidative spoilage). The purpose of our research was to determine the acid and peroxide numbers in experimental samples of vegetable oils at the time of their arrival at the commercial enterprise and to establish the dynamics of these indicators depending on storage conditions.

The objects of the study were the following samples of vegetable oils from homogeneous batches received at the supermarket of Holiday Company LLC in Omsk:

refined deodorized sunflower oil “First grade” TM “Yuzhnoe Solntse”, manufacturer “Labinsky MEZ” LLC - a branch of “Yug Rusi” LLC, packaging date 10.17.12, (GOST R 52465);

unrefined sunflower oil “First grade” TM “Yantarka”, manufacturer “Sigma” LLC, Chelyabinsk region, packaging date 02.11.12, (GOST R 52465);

refined deodorized corn oil TM “ALTERO beauty”, manufacturer OJSC “EFKO” Belgorod region, packaging date 10.22.12, (GOST 8808-2000);

natural extra virgin olive oil TM "Vitaland", manufacturer EXOLIVA, S.A., Plasencia (Caceres), Spain, packaging date 07/11/12, (EU standard).

The first three samples above were packaged in colorless polyethylene terephthalate (PET) containers, olive oil in a dark glass bottle. The acid number in samples of vegetable oils was determined according to GOST R 52110-2003 by the titrimetric method with visual indication. The acid number is a physical quantity equal to the mass of potassium hydroxide (mg) required to neutralize free fatty acids and other alkali-neutralized related substances contained in 1 g of oil. The acid number is expressed in mg KOH/g. The essence of the method was to dissolve a certain mass of an oil sample in a mixture of solvents, followed by titration of the available free fatty acids with an alcohol solution of potassium hydroxide with a concentration of 0.1 mol/dm 3 . The peroxide value was determined in accordance with the requirements of GOST 26593-85. The method is based on the oxidation of potassium iodide with peroxides and hydroperoxides contained in the oil. The iodine released in this case was titrated with a solution of sodium thiosulfate (Na 2 S 2 O 3).

In order to study the prediction of the dynamics of acid and peroxide numbers during storage, these indicators were determined after 15 days for 2 months. The samples were stored at room temperature, but under different conditions (in the light, away from light sources and in the dark).

According to the results of the organoleptic assessment, all experimental samples of vegetable oils, except for the unrefined sunflower oil “First Grade” TM “Yantarka”, met the requirements of regulatory documents. Despite the fact that no more than 3 weeks passed from the date of packaging (02.11.12) until the batch of TM “Yantarka” oil arrived at the supermarket, the oil already had pronounced signs of spoilage. It can be assumed that the oil was in storage for a long time (some time passed from the date of manufacture to the date of packaging) or the storage and transportation conditions were inadequate. The oil may have been exposed to high temperatures and/or direct sunlight.

For olive oil produced in Spain, when assessing quality indicators, instead of studying the acid number, preference is given to the “acidity” indicator. Acidity is determined by the percentage of free fatty acids based on any predominant acid. For all olive oils, this acid is oleic. To determine acidity, you should multiply the acid number by the coefficient of the predominant acid in the oil (for oleic acid the coefficient is 0.5). In accordance with clause 3.1 of CodexStan. 33-1981, the content of free fatty acids in Extra virgin olive oils should not exceed 0.8 g per 100 g of oil, i.e. acidity should be no more than 0.8%.

More than 4 months passed from the date of packaging (07/11/2012) until this batch of olive oil arrived at the supermarket. The declared shelf life is 1.5 years from the date of manufacture. But already from the study of acidity it becomes clear that the actual value of this indicator is only 0.08% less than the maximum level.

The results confirmed the organoleptic evaluation data. In particular, the feeling of “scratching” in the throat when swallowing Yantarka TM oil may be due to the increased content of peroxides. It is noteworthy that in the sample of refined sunflower oil TM “Southern Solntse” the peroxide value was significantly higher than in other samples. This fact may indicate the activation of the lipoxygenase enzyme in this batch of oil, which stimulates the formation of peroxide compounds.

When stored in the dark, the values ​​of acid and peroxide numbers in the oil sample increased extremely slightly - after 2 months of storage by 0.04 and 0.3 units, respectively. When stored under the influence of direct sunlight and artificial light, the peroxide number increased to the greatest extent - after 1 month of storage by 1.6 units, and after 2 months - by 3.5 units and amounted to 11.1 mmol of active oxygen / kg of oil, which exceeds permissible value according to the Technical Regulations and Standard and makes the product unsafe. When stored away from light sources, after 2 months of storage, the acid number was at a stable level - 0.32 mg KOH/g of oil, and the peroxide number, which was already quite high at the beginning of storage, increased by only 0.3 units.

In oil TM “Southern Solntse”, in unrefined oil TM “Yantarka”, oxidative processes occur most intensively in the light. After 2 months of storage in the light, the acid number increased by 2.37 units, and the peroxide number by 4.1 units. Under this storage regime, after opening the package, the oil had a pronounced unpleasant odor, and there were also signs of discoloration of the oil, probably associated with the oxidation of carotenoids. When stored in the dark, the acid number increased by 0.77 units after 2 months, and the peroxide number increased by 0.6 units.

The greatest increase in the values ​​of both acid and peroxide numbers in the ALTERO beauty TM corn oil sample was recorded during storage in the light. After the end of the observation period (60 days), the acid number value under this storage mode increased by 0.1 units. and amounted to 0.29 mg KOH/g oil. The growth rate of the peroxide number turned out to be more significant - over the above period, the acid number increased from 1.8 to 4.2 mmol of active oxygen, i.e. 2.3 times. When storing samples in the dark, the acid and peroxide numbers remained consistently low.

In a sample of olive oil, over 2 months of storage, the value of the acid number when stored in the dark increased by 0.38 units, away from light sources - by 0.75 units, in the light - by 1.27 units. Calculations showed that after 15 days of storage away from light sources and in the light, the acidity exceeded the permissible values ​​​​according to Codex Stan. 33-1981. In our opinion, the shelf life of Extra Virgin olive oil (1.5 years) is unreasonably high.

Unlike other samples, different storage conditions did not significantly affect the dynamics of the peroxide value in the olive oil sample. The difference in values ​​after 2 months of storage in the dark and in the light was only 0.2 mmol of active oxygen/kg of oil. The reason for the stability of the peroxide value is that the olive oil was packaged in a dark glass bottle, which protects the oil from the negative effects of light.

Thus, research results show that exposure to light is most responsible for the increase in peroxide value values. This applies, first of all, to oil samples packaged in transparent, colorless polymer containers.

Manufacturers of vegetable oils are recommended to package the product in containers made of dark-colored materials. First of all, this note applies to unrefined oils. In a supermarket sales area, the most acceptable condition is to place shelves with vegetable oils away from light sources. Oil stocks should be stored in the dark.

Literature:

Technical regulations for fat and oil products: Federal Law No. 90-FZ of June 24, 2008.

GOST R 52110-2003. Vegetable oils. Methods for determining acid number. – M.: IPK Standards Publishing House, 2003. – 10 p.

GOST 26593-85. Vegetable oils. Methods for determining peroxide value. – M.: Publishing house of standards, 1987. – 8 p.

Food chemistry / A.P. Nechaev, S.E. Traubenberg, A.A. Kochetkova and others; edited by A.P. Nechaeva. – St. Petersburg: GIORD, 2004. – 540 p.

Examination of oils, fats and their products. Quality and safety / E.P. Cornena, S.A. Kalmanovich, E.V. Martovshchuk, L.V. Tereshchuk et al.; edited by Doctor of Biological Sciences, Prof. V.M. Poznyakovsky. – Novosibirsk. Sib. Univ. publishing house, 2009. – 272 p.

Scientific adviser:

Ph.D. tech. Sciences, Associate Professor Tabatorovich Alexander Nikolaevich