Laboratory in the kitchen. milk

Color reactions to proteins

The presence of proteins in biological objects or solutions can be determined using color reactions, the occurrence of which is determined by the presence of specific groups and peptide bonds in the protein.

Reagents: aqueous solution of egg white (white of one chicken egg separated from the yolk, dissolved in 15–20 times the volume of distilled water, then the solution is filtered through gauze folded in 3–4 layers and stored in the refrigerator; 10% sodium hydroxide solution; 30% sodium hydroxide solution; 1% solution of copper sulfate; 1% solution of lead acetate; concentrated nitric acid; 0.5% ninhydrin solution.

Equipment : test tubes; water bath or alcohol lamp.

Task 1. Biuret reaction.

In an alkaline environment, proteins, as well as their hydrolysis products - peptides, give a violet or red-violet color with copper salts. The reaction is due to the presence of peptide bonds in proteins:

The intensity of the color depends on the length of the polypeptide.

Work progress

1. Pour 5 drops of egg white solution into a test tube, then 10 drops of a 10% alkali solution.
2. Add 1-2 drops of copper sulfate solution and stir the mixture. A red-violet color appears.

Task 2. Xanthoprotein reaction.

The reaction is typical for some aromatic amino acids (phenylalanine, tyrosine, tryptophan), as well as for peptides containing them. When exposed to nitric acid, a nitro compound is formed yellow. Next, nitro derivatives can react with alkali to form a sodium salt, which has a yellow-orange color:

Work progress

This work must be carried out in a fume hood, taking special care!

1. Pour 5 drops of egg white solution into the test tube and CAREFULLY add 3-4 drops of concentrated nitric acid along the wall.
2. Heat the mixture carefully. A precipitate forms that turns yellow.
3. After cooling, CAREFULLY pour 10 drops of a 30% NaOH solution into the test tube along the wall; the yellow color turns orange.

Task 3. Reaction to sulfur-containing amino acids (Fol's reaction).

In the residues of sulfur-containing amino acids cysteine ​​and cystine, sulfur is cleaved off during alkaline hydrolysis, forming sulfides. Sulfides, interacting with lead acetate, form a precipitate of lead sulfide of black or brown-black color.

Work progress

1. In a test tube, mix 5 drops of egg white solution, 5 drops of 30% alkali solution and 2 drops of lead acetate solution.
2. Heat the mixture carefully on an alcohol lamp to a boil and boil. After some time, a brownish-black or black color appears.

Task 4. Ninhydrin reaction.

The reaction is characteristic of amino groups in the α-position and is due to the presence of α-amino acids in the protein molecule. When heating the protein with aqueous solution Ninhydrin amino acids are oxidized and decomposed to form carbon dioxide, ammonia and the corresponding aldehyde. Reduced ninhydrin condenses with ammonia and an oxidized ninhydrin molecule, forming a violet-blue compound:

Work progress

Add 5 drops of a 1% egg white solution to a test tube, add 3 drops of a 0.5% ninhydrin solution and heat to a boil. After 2–3 minutes, pink, red, and then blue-violet color appears.

Registration of results

Present your research in the form of a table.

Work progress:

Fill out your experiment in a notebook (in a square):

Experiment “Determination of protein in a cell.”

Goal: learn…..(continue).

Equipment: a glass of water, flour in gauze, scissors.

Work progress:

1. Dip the flour wrapped in cheesecloth into a glass of water for half a minute. Take it out, cut the thread on the gauze, unfold it and touch the resulting dough with your finger. Write at this point

What are you observing? (“If you touch wet flour, it will feel like ...?...., ...?.....”)

2. Find in the textbook: what substance has these qualities? Draw a conclusion: what organic substance did you find? (“Conclusion: this substance is ….?.... (…….).”)

3. Find in your textbook and write down the value of your organic substance in the cell. (" Meaning: …….. .")

Fill out your experiment in a notebook (in a square):

Experiment “Determination of protein in a cell.”

Goal: learn…..(continue).

Equipment: a glass of water, flour in gauze, scissors.

Work progress:

1. Dip the flour wrapped in cheesecloth into a glass of water for half a minute. Take it out, cut the thread on the gauze, unfold it and touch the resulting dough with your finger. Write at this point

What are you observing? (“If you touch wet flour, it will feel like ...?...., ...?.....”)

2. Find in the textbook: what substance has these qualities? Draw a conclusion: what organic substance did you find? (“Conclusion: this substance is ….?.... (…….).”)

3. Find in your textbook and write down the value of your organic substance in the cell. (" Meaning: …….. .")

On the topic: methodological developments, presentations and notes

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Target: study the properties of proteins.

Equipment and reagents:- protein solution;

Copper sulfate solution;

Lead acetate solution;

Test tubes

Work progress:

1. Protein dissolution

Many proteins dissolve in water, which is due to the presence of free hydrophilic groups on the surface of the protein molecule. The solubility of protein in water depends on the structure of the protein, the reaction of the environment, and the presence of electrolytes. Proteins with acidic properties dissolve better in an acidic environment, and proteins with basic properties dissolve better in an alkaline environment.

Albumins are highly soluble in distilled water, while globulins are soluble in water only in the presence of electrolytes.

Proteins of supporting tissues (collagen, keratin, elastin, etc.) do not dissolve in water.

Equipment and reagents:- egg white;

Distilled water;

Potassium chloride solution;

Keratin (wool or hair).

Work progress:

Add 1 ml of distilled water to 2 drops of undiluted egg white and mix. In this case, egg albumin dissolves, and egg globulin precipitates as a small precipitate.

The solubility of keratin protein contained in wool and hair is checked in water and a 5% solution of potassium chloride.

Present the results of the work in the form of a table:

1. Denaturation of protein with alcohol.

Equipment and reagents: protein solution; ethanol, test tubes

1. Precipitation of proteins upon heating.

Proteins are thermolabile compounds and denaturation occurs when heated above 50-60°C. The essence of thermal denaturation is the unfolding of the specific structure of the polypeptide chain and the destruction of the hydration shell of protein molecules, which is manifested by a noticeable decrease in their solubility. The most complete and rapid deposition occurs at the isoelectric point, i.e. at a pH value of the environment when the total charge of the protein molecule is zero, since in this case the protein particles are the least stable. Proteins with acidic properties precipitate in a slightly acidic environment, and proteins with basic properties precipitate in a slightly alkaline environment. In strongly acidic or strongly alkaline solutions, the protein denatured when heated does not precipitate, since its particles are recharged and carry a positive charge in the first case and a negative charge in the second, which increases their stability in solution.

Equipment and reagents: - 1% egg white solution;

1% solution acetic acid;

10% acetic acid solution;

10% sodium hydroxide solution;

4 test tubes, holder, alcohol lamp.

Draw conclusions.______________________________

Conditions for completing the task

1. Place (time) of task completion : the assignment is completed during class time

2. Maximum task completion time: ____ 90 _______ min.

3. You can use textbook, lecture notes

Scale for assessing educational achievements:

Evaluation criteria: Completion of work more than 90% – rating “5”,

70-90% - rating “4”,

50 -70% - rating “3”,

Less than 50% - rating “2”.

Practical work No. 1

Preparation of a solution of a given concentration.

Target:

• prepare solutions of salts of a certain concentration.
• learn to prepare a solution of a given concentration using scales and measuring cups.

Equipment:

• glass spatula;
• glass with a volume of 50 ml;
• glass rod with rubber tip;
• graduated cylinder;
• scales;
• cold boiled water.
• salt;

Theoretical part

Solution-This is a homogeneous system consisting of a solvent, solutes and products of their interaction. A solvent is most often a substance that pure form has the same state of aggregation as the solution, or is present in excess.

Based on their state of aggregation, solutions are distinguished: liquid, solid, gaseous. According to the ratio of solvent and solute: dilute, concentrated, saturated, unsaturated, supersaturated. The composition of a solution is usually expressed by the content of the soluble substance in it in the form of mass fraction, percentage concentration and molarity.

• Mass fraction ( dimensionless quantity) is the ratio of the mass of the dissolved
  substances to the mass of the entire solution:

W ppm = m rast. substances/m solution.

• Percentage concentration (%) is a value indicating how many grams of dissolved substance are contained in 100 g. solution :

W% = m rast. substances 100%/m solution

(textbook O.S. Gabrielyan, I.G. Ostroumov Chemistry, M. “Academy” 2013, p. 57)

• Molar concentration, or molarity (mol/liter) is a value indicating how many moles of a soluble substance are contained in 1 liter of solution:

cm = m height thing/Mr(solvent)V solution .

(textbook O.S. Gabrielyan, I.G. Ostroumov Chemistry, M. “Academy” 2013, p. 57)

Lesson - practical work

Topic: Quality Analysis food products. Detection of protein in the test sample.”

prepared by a chemistry teacher at MAOU Lyceum No. 28 named after N.A. Ryabov

Popova S.I.

Goal: “To determine the quality of food products. Examine the proposed samples for protein content.”

Tasks:

1. Educational:

to develop the ability to solve experimental problems of an applied nature to determine the protein content in food products;

establish interdisciplinary connections of the subject being studied theoretical basis with close, “home” problems.

2. Educational:

continue to develop a responsible, creative attitude to completing tasks, accuracy, observation, perseverance;

continue local history education;

fostering a positive attitude towards healthy lifestyle; education of tolerance, cooperation, independence.

3.Developing:

develop the ability to transfer knowledge to new conditions;

develop the ability to conduct, observe and describe a chemical experiment;

develop mental operations (analysis, synthesis, establishing cause-and-effect relationships, putting forward hypotheses, classification, drawing analogies, generalization, ability to prove, highlighting the main thing);

improve the communication skills of students in joint activities (the ability to conduct a dialogue, listen to an opponent, and substantiate their point of view).

Skills:

1. General laboratory – carry out heating; filtration.

2. Organizational - maintain accuracy and precision in work, safety regulations, work according to instructions, maintain the cleanliness of the workplace, make records, exercise self-control.

(teacher's introductory speech)

Proteins in food play the role of the main building material for the body, without which its vital activity, growth and restoration of cells are impossible.

As you probably already know, any product consists of proteins, fats and carbohydrates, and our main task is to skillfully combine them in our daily diet.

One ofmain principles healthy eating just says that food should be balanced and contain optimal quantity proteins, fats and carbohydrates.

Protein food is one of the most important components of any person’s diet. If there is a lack of protein, the process of breaking down fats is impossible, which in turn makes you nourishing. protein product allows you to increase the time it takes for carbohydrates to be absorbed, helping to maintain stable blood sugar levels and suppress hunger.

What is the structure of protein? (Working with the “Visible School” complex - protein structures)

Discuss qualitative reactions to protein

(Move to new topic)

Assessing the quality of food products and determining microbiological contamination are very important and labor-intensive processes. Each sample is approached strictly individually.

There are several types of food quality research:

organoleptic – smell, taste, color, turbidity, temperature, foreign films, sediments;

physico-chemical – qualitative composition of the declared sample, presence of impurities (both physical and chemical);

microbiological – determination of the presence of foreign microorganisms and infections in the test sample (a particularly important type of research recommended by Analytic Company for baby food);

comprehensive – examination of the product’s compliance with certain GOST, GOST R, TU, SaNPin and other standards establishing safety criteria for humans.

The guys are divided into 4 groups and conduct 2 experiments according to previously given instructions. . All students then complete a lab notebook followed by a discussion of the results.

BEFORE practicing, repeat the safety rules !

Experiment No. 1. Determination of the quality of milk pasteurization.

Pasteurization involves the destruction of pathogenic microorganisms that deteriorate the quality of milk. The protein content should not decrease.

1.Add 3 ml of distilled water to 3 ml of milk.

2.Add 0.1 N solution drop by dropH 2 SO 4 casein flakes. Filter.

3. Filtrate. Heat to a boil.

KEY. Casein flakes reappear in raw milk .

4.Suggest the degree of pasteurization of milk in the samples under study.

5.Suggest the reasons for falsification of various samples.

6. Add a few drops of concentrated nitric acid to the resulting sample. Heat up. Yellow coloration indicates the presence of aromatic rings in the protein (xanthoprotein reaction).

Experience No. 2.

Determining the freshness of milk

To 3 ml of a 1% phenol solution add 3 ml of a 1% ferric chloride solution (III), note the purple color, add 3 ml of milk.

KEY. Sour milk gives a yellow-green color.

Establish the degree of freshness and the reasons for falsification of different samples.

Experience No. 3.

Determination of impurities in sour cream.

Pour 10 ml of hot water into a beaker. IN hot water place a spoonful of sour cream.

KEY. If there is an impurity (cottage cheese, kefir, starch), the fat will float, and the casein will settle to the bottom.

Knowing that sour cream should normally not have sediment, suggest the reasons for the falsification of different samples.

Experience No. 4

Determining the freshness of meat.

1. Prepare meat broth. (4 ml of water + a piece of meat to a boil).

2. Filter into a flask using a funnel.

3.Add 5 drops%H 2 SO 4 and after a few minutes note the result.

KEY. IN fresh broth the solution is transparent; if the freshness is questionable, the solution becomes cloudy, and meat that is definitely stale has a jelly-like sediment with flakes.

Determine the degree of falsification of the sample. Compare several different samples. Suggest reasons for different results.

Experience No. 5.

Determination of protein in the proposed products.

1 Pour 4 ml of water into 3 test tubes. Add 0.5 to 1 bouillon cube, in the 2nd piece of meat, in the 3rd piece of sausage. Boil.

2.Filter.

3.Add concentratedHNO 3 drop by drop and heat a little.

KEY. In a solution in which protein is present there will be a yellow color.

Make a conclusion about the quality of the samples.