Food Analysis – Protein Content of Foods
Students will:
Laboratory report due:
There are many proteases produced by the gastrointestinal tract. Each of the various proteases acts on a specific site on the protein. The mouth does not possess digestive enzymes that initiate any digestion of protein. But in the stomach, hydrochloric acid denatures protein and provides a substrate for the enzyme pepsin to initiate protein digestion in the stomach. Once food reaches the small intestine many more proteases and peptidases are produced by both the pancreas and the small intestine to cleave peptides into their smallest units, individual amino acids.
This lab involves quantitatively determining the amount of protein in a liquid food. A protein in solution can be quantitatively assayed by several spectrophotometric methods. Most spectrophotmetric methods for determining the protein content of a food are based on the binding of a reagent to either specific amino acids or specific bonds (peptide) in the protein. The amount of color produced from this binding reaction can be detected at some wavelength of visible light and analyzed with a spectrophotometer.
Spectrophotometry
A spectrophotometer is an instrument employed to measure the amount of light absorbed by a sample. The instrument operates by passing a beam of light through a sample in a cuvette and measuring the intensity of light reaching a detector.
The beam of light consists of a stream of photons. When a photon encounters a molecule in the solution being studied, there is a chance the molecule will absorb the photon. This absorption reduces the number of photons in the beam of light leaving the solution, thereby reducing the intensity of the light beam that reaches the detector on the far side. The more molecules there are in the cuvette to absorb photons, the greater the absorbanceof the solution (and the fewer photons thatmake it through the sample to the detector).
Color exists on an absorbance spectrum that is dependant on its interaction with light energy. Different colors absorb different wavelengths of light depending upon the amount of energy they possess. For example, red light has slightly less energy than blue light. This difference in energy results in a different absorption and thus a different color perceived by the eye.
The amount of light absorbed by a sample is directly related to its color. When a sample absorbs light of a certain wavelength, what we see is the sum of the remaining colors. For example, if a sample absorbs all wavelengths no color reaches our eyes and the sample appears black. On the other hand, if it does not absorb any light, it would appear colorless or white. Likewise, a dye that absorbed all of the colors except blue, would give a blue appearance.
Protein Assay
This lab measures the protein content of a solution using the Biuret assay. Determination of the protein content of an unknown solution requires comparison with a known or “standard” protein solution.
In the Biuret assay, absorbance measured by the spectrophotometer is a function of protein concentration. By first analyzing samples with known amounts of protein and plotting their absorbance on a graph, one can determine the protein concentrations of the unknown solutions. This graph is called the standard curve. The absorbance of the unknown solution is then measured and then plotted on the standard curve. The standard curve is used to convert the absorbance readings for the experimental samples into a protein concentration.
In this lab you will develop the protein standard curve using solutions of the protein called bovine serum albumin (BSA) that are of known concentration. The known solutions of protein are mixed with an alkaline Biuret reagent. Under alkaline conditions, substances containing two or more peptide bonds form a purple complex with the copper salts in the Biuret reagent producing a colored solution. The more protein in the sample the more color there is in the solution and the greater the absorbance indicated by the spectrophotometer.
Summary
This lab experiment is designed to acquaint you with standard lab procedures for using a spectrophotometer and for determining the amount of protein present in a solution. In this lab, you will create your own protein standard curve and then using your standard curve, estimate the relative amounts of protein in several solutions that contain protein.
Spectrophotometer Stock solution of Bovine Serum Albumin(BSA)
Biuret reagent Protein solutions:
13 cuvettes a. skim milk (M)
Cuvette holder b. soy milk (shake gently) (S)
Wax pencil c. chicken broth (B)
Kim wipes d. protein shake (shake gently) (P)
Distilled water
1000 ml micropipet
Pipet tips
Wooden stir sticks
A). Set Up
|
B). Sample Preparation for determination of standard curve using BSA of known concentrations:
| Cuvette # [BSA] | BSA (ml) | H2O (ml) |
| 1 [0.0 mg/ml] | 0 | 1000 |
| 2 [1.0 mg/ml] | 100 | 900 |
| 3 [2.0 mg/ml] | 200 | 800 |
| 4 [3.0 mg/ml] | 300 | 700 |
| 5 [4.0 mg/ml] | 400 | 600 |
| 6 [5.0 mg/ml] | 500 | 500 |
C). Unknown Sample Preparation:
(Before you pipette your protein samples, be sure to mix each gently by repeatedly tipping the tube upside down. In particular, make sure the soy milk and protein shake are well mixed.)
| Cuvette | Protein Solution (ml) | H2O (ml) |
| M (skim milk) | 100 | 900 |
| S (soy milk) | 100 | 900 |
| B (chicken broth) | 100 | 900 |
| P (protein shake) | 100 | 900 |
D). Determination of the Standard Curve
*Click the link below to watch: How to Use a Spectrophotometer:
https://www.youtube.com/watch?v=QcCrRDYiejg
(*Source of website: ISU Gen Chem Lab Tutorials-How to use a spectrophotometer (Spectronic 200E))
E). Determination of the Protein Content of Unknown Solutions.
F). Drawing the Standard Curve
G). Estimating the Protein Content of Foods Using your Standard Curve
Laboratory Report for Food Analysis – Protein Content of Foods (Sections E & F Below)
Student Name:____________________________ Date:______________
TA Name:_________________________________
________________________________________________________________
E and F. Data Record and Lab Write-up (10 points)
Table 1: BSA protein solutions of known concentration.
| Tube # | Protein concentration | ABS 1 | ABS 2 | ABS (avg) |
| 1 | 0.0 mg/ml | 0.075 | 0.075 | |
| 2 | 1.0 mg/ml | 0.302 | 0.299 | |
| 3 | 2.0 mg/ml | 0.356 | 0.359 | |
| 4 | 3.0 mg/ml | 0.412 | 0.412 | |
| 5 | 4.0 mg/ml | 0.470 | 0.469 | |
| 6 | 5.0 mg/ml | 0.516 | 0.516 |
Table 2: Protein solutions of unknown concentration.
| Tube # | Food sample | ABS 1 | ABS 2 | ABS (avg) |
| M | skim milk | 0.262 | 0.262 | |
| S | soy milk | 0.321 | 0.320 | |
| B | chicken broth | 0.463 | 0.462 | |
| P | protein shake | 0.541 | 0.543 |
Table 3: Determine the concentration of protein solutions by plotting the absorbance of each on the standard curve and reading the value on the X-axis.
| Protein Solution | Protein Concentration (mg/ml) |
| Skim Milk | |
| Soy milk | |
| Chicken broth | |
| Protein Shake |
***Please make sure that you turn in your standard curve (section G) with the data (from table 1) plotted to show how you calculated the protein concentrations in table 3. Note: Make sure you plot known BSA concentration in mg/ml on the x-axis and corresponding av
Food Analysis – Protein Content of Foods
Students will:
Laboratory report due:
There are many proteases produced by the gastrointestinal tract. Each of the various proteases acts on a specific site on the protein. The mouth does not possess digestive enzymes that initiate any digestion of protein. But in the stomach, hydrochloric acid denatures protein and provides a substrate for the enzyme pepsin to initiate protein digestion in the stomach. Once food reaches the small intestine many more proteases and peptidases are produced by both the pancreas and the small intestine to cleave peptides into their smallest units, individual amino acids.
This lab involves quantitatively determining the amount of protein in a liquid food. A protein in solution can be quantitatively assayed by several spectrophotometric methods. Most spectrophotmetric methods for determining the protein content of a food are based on the binding of a reagent to either specific amino acids or specific bonds (peptide) in the protein. The amount of color produced from this binding reaction can be detected at some wavelength of visible light and analyzed with a spectrophotometer.
Spectrophotometry
A spectrophotometer is an instrument employed to measure the amount of light absorbed by a sample. The instrument operates by passing a beam of light through a sample in a cuvette and measuring the intensity of light reaching a detector.
The beam of light consists of a stream of photons. When a photon encounters a molecule in the solution being studied, there is a chance the molecule will absorb the photon. This absorption reduces the number of photons in the beam of light leaving the solution, thereby reducing the intensity of the light beam that reaches the detector on the far side. The more molecules there are in the cuvette to absorb photons, the greater the absorbanceof the solution (and the fewer photons thatmake it through the sample to the detector).
Color exists on an absorbance spectrum that is dependant on its interaction with light energy. Different colors absorb different wavelengths of light depending upon the amount of energy they possess. For example, red light has slightly less energy than blue light. This difference in energy results in a different absorption and thus a different color perceived by the eye.
The amount of light absorbed by a sample is directly related to its color. When a sample absorbs light of a certain wavelength, what we see is the sum of the remaining colors. For example, if a sample absorbs all wavelengths no color reaches our eyes and the sample appears black. On the other hand, if it does not absorb any light, it would appear colorless or white. Likewise, a dye that absorbed all of the colors except blue, would give a blue appearance.
Protein Assay
This lab measures the protein content of a solution using the Biuret assay. Determination of the protein content of an unknown solution requires comparison with a known or “standard” protein solution.
In the Biuret assay, absorbance measured by the spectrophotometer is a function of protein concentration. By first analyzing samples with known amounts of protein and plotting their absorbance on a graph, one can determine the protein concentrations of the unknown solutions. This graph is called the standard curve. The absorbance of the unknown solution is then measured and then plotted on the standard curve. The standard curve is used to convert the absorbance readings for the experimental samples into a protein concentration.
In this lab you will develop the protein standard curve using solutions of the protein called bovine serum albumin (BSA) that are of known concentration. The known solutions of protein are mixed with an alkaline Biuret reagent. Under alkaline conditions, substances containing two or more peptide bonds form a purple complex with the copper salts in the Biuret reagent producing a colored solution. The more protein in the sample the more color there is in the solution and the greater the absorbance indicated by the spectrophotometer.
Summary
This lab experiment is designed to acquaint you with standard lab procedures for using a spectrophotometer and for determining the amount of protein present in a solution. In this lab, you will create your own protein standard curve and then using your standard curve, estimate the relative amounts of protein in several solutions that contain protein.
Spectrophotometer Stock solution of Bovine Serum Albumin(BSA)
Biuret reagent Protein solutions:
13 cuvettes a. skim milk (M)
Cuvette holder b. soy milk (shake gently) (S)
Wax pencil c. chicken broth (B)
Kim wipes d. protein shake (shake gently) (P)
Distilled water
1000 ml micropipet
Pipet tips
Wooden stir sticks
A). Set Up
|
B). Sample Preparation for determination of standard curve using BSA of known concentrations:
| Cuvette # [BSA] | BSA (ml) | H2O (ml) |
| 1 [0.0 mg/ml] | 0 | 1000 |
| 2 [1.0 mg/ml] | 100 | 900 |
| 3 [2.0 mg/ml] | 200 | 800 |
| 4 [3.0 mg/ml] | 300 | 700 |
| 5 [4.0 mg/ml] | 400 | 600 |
| 6 [5.0 mg/ml] | 500 | 500 |
C). Unknown Sample Preparation:
(Before you pipette your protein samples, be sure to mix each gently by repeatedly tipping the tube upside down. In particular, make sure the soy milk and protein shake are well mixed.)
| Cuvette | Protein Solution (ml) | H2O (ml) |
| M (skim milk) | 100 | 900 |
| S (soy milk) | 100 | 900 |
| B (chicken broth) | 100 | 900 |
| P (protein shake) | 100 | 900 |
D). Determination of the Standard Curve
*Click the link below to watch: How to Use a Spectrophotometer:
https://www.youtube.com/watch?v=QcCrRDYiejg
(*Source of website: ISU Gen Chem Lab Tutorials-How to use a spectrophotometer (Spectronic 200E))
E). Determination of the Protein Content of Unknown Solutions.
F). Drawing the Standard Curve
G). Estimating the Protein Content of Foods Using your Standard Curve
Laboratory Report for Food Analysis – Protein Content of Foods (Sections E & F Below)
Student Name:____________________________ Date:______________
TA Name:_________________________________
________________________________________________________________
E and F. Data Record and Lab Write-up (10 points)
Table 1: BSA protein solutions of known concentration.
| Tube # | Protein concentration | ABS 1 | ABS 2 | ABS (avg) |
| 1 | 0.0 mg/ml | 0.075 | 0.075 | |
| 2 | 1.0 mg/ml | 0.302 | 0.299 | |
| 3 | 2.0 mg/ml | 0.356 | 0.359 | |
| 4 | 3.0 mg/ml | 0.412 | 0.412 | |
| 5 | 4.0 mg/ml | 0.470 | 0.469 | |
| 6 | 5.0 mg/ml | 0.516 | 0.516 |
Table 2: Protein solutions of unknown concentration.
| Tube # | Food sample | ABS 1 | ABS 2 | ABS (avg) |
| M | skim milk | 0.262 | 0.262 | |
| S | soy milk | 0.321 | 0.320 | |
| B | chicken broth | 0.463 | 0.462 | |
| P | protein shake | 0.541 | 0.543 |
Table 3: Determine the concentration of protein solutions by plotting the absorbance of each on the standard curve and reading the value on the X-axis.
| Protein Solution | Protein Concentration (mg/ml) |
| Skim Milk | |
| Soy milk | |
| Chicken broth | |
| Protein Shake |
***Please make sure that you turn in your standard curve (section G) with the data (from table 1) plotted to show how you calculated the protein concentrations in table 3. Note: Make sure you plot known BSA concentration in mg/ml on the x-axis and corresponding av
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