Food Chemistry Practical Notes

Analysis of colorants in beverage products

IntroductionThe objectives of this experiment are to:

Determine the concentration of the colorant additives in a beverage;

Establish whether the product conforms the current regulatory standards;

Provide experience in using a spectrophotometer. This is one of the instruments used most widely in the analysis of food molecules;

Illustrate the Beer-Lambert Law; and

Apply this law to the determination of the concentration of colorants in a food.

Background

When light is shone through a coloured solution some light is absorbed by the solution and some passes straight through. We see the light that passes through. A red solution is red because red light passes through and most other wavelengths are absorbed.

Absorbance and transmittance

The spectrophotometer actually measures the transmittance and then calculates what we are really interested in, which is the absorbance (the internationally accepted abbreviation is A). Older instruments will provide us with the option to record either transmittance or absorbance. The latter is all that we need because it is directly related to how much of a component is present. Note also that the term optical density is an older name and has now been replaced by absorbance.

Beer-Lambert Law

The amount of light absorbed by a solution is proportional to the concentration of the solution.

A = c l where is the extinction coefficient

c the concentration; and

l the length of the light path (cm)

Note that the units of c will depend upon those of the value used.

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Experimental procedures

Prior to carrying out any of the further steps check that the spectrophotometer and weighing balance are turned on to allow these to warm up for at least thirty minutes.

Note that there are spaces on the attached form for recording of laboratory data. This form will become the basis of your report for the practical.

1. The products are provided as powder and so you need to dissolve these. So firstly look at the level, decide how much you will weigh and the volume to be prepared. Some of the aspects you may consider are:

2. The procedure recommended on the product label

3. If the label does not indicate how much to weigh then check whether there is information on serving size, number of servings in the package and the total weight in the pack. These can often tell us the concentration.

4. For the measurements to be made in the spectrophotometer we need only 3.0 mL. Therefore it makes sense to prepare either 50 or 100 mL.

5. The analysis you are doing will be used to establish regulatory compliance. Therefore also ensure that your plan gives you good reliability in your measurements. So for example, you need to be able to trust the accuracy of the weighing and the volumes being prepared. Consider what is the most reliable glassware to use for this purpose.

Measurements using the spectrophotometer

Your demonstrator will explain the Varian Cary IE Spectrophotometer and the associated Varian software. Briefly the procedure involve:

1. Turn on the instrument to allow a period of at least 15 minutes warm-up.

2. Using the Set-up option of the Scan software, select the wavelengths you want to scan.

The instrument is able to cover the whole visible spectrum (350 to 800 nm wavelength) as

well as the ultraviolet (UV) in the range of 200-350nm. As we are interested in visible

colorants, scan the wavelengths for the visible region after entering the highest first as this is

where the instrument commences. Select a scan rate of 200nm/min.

3. Once the software set-up options have been selected we need to balance the light beams

that are passing through the two positions where we place our blank or reference and also our

sample. To zero the instrument, place two cuvettes both containing distilled water into the

positions.

4. To record the spectrum of the beverage sample, place a cuvette of the solution into the

position closest to the front of the Spectrophotometer. After closing the lid start the scan

5. Save the scan as this can also be plotted using Excelsoftware as long as we save it into a

CSV file format.

6. Inspect the spectrum and then record the maximum(peak) wavelength for each major

absorption band.

7. Using the Simple Reads software, record absorbance measurements of the sample at each

wavelength after adjusting Zero in between readings.

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Table 1 Selected literature data for synthetic colorants

Additive Name INS Number

CI designation(Marmion3)

FD and C designation(Marmion3)

max(nm)

(Socaciu2)

Extinction (1%, 1cm)(Socaciu2)

max(nm)

(Marmion3)

Extinction (g per L)

(Marmion3)

Tartrazine 102 19140 Yellow No 5 426 530 428 53

Quinoline yellow 104 47005 D & C Yellow No 10 413 865 413 92

Sunset yellow FCF 110 15985 FD & C Yellow No 6 481 555 484 55

Azorubine (Carmoisine) 122 14720 516 510

Amaranth 123 16185 FD & C Red No 2 520 440

Ponceau 4R 124 16255 506 430

Erythrosine 127 45430 FD & C Red No 3 526 1100 527 110

Red 2G 128 18050 520

Allura red AC 129 16035 FD & C Red No 40 520 - 502 55.6

Patent Blue V 131 42051 638 2050

Indigotine 132 73015 FD &C Blue No2 610 480 610 47.8

Brilliant blue FCF 133 42090 FD &C Blue No1 630 1630 630 164

Green S 142 44090 608

Fast green FCF 143 42053 FD & C Green No 3 625 156

Brilliant black BN 151 28440 570 530

Brown FK 154 NA

Brown HT 155 20285

Notes 1 Not all of these colorants are currently approved for use in Australia, NA – not available2 Socaciu, C. 2008 Food colorants: Chemical and functional properties, CRC Press, Boca Raton, Florida, USA p 540.

Marmion, DM. 1979. Handbook of U.S colorants for foods, drugs, and cosmetics. New York. Wiley.

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Food Chemistry - Analysis of colorants in beverage products

Form for recording of laboratory data

This form should be attached to your final report as an appendix. Remember to include units when you record your results and in the calculations wherever appropriate.

Your name: Student number:

Group: Partner(s) for this exercise:

Name of your food product:

Record here any label information from your product which may be useful when you write your report. You may want to include relevant nutrition panel data and the ingredient list:

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From the spectra determine the wavelength value ( max) for each peak and record these.

Based upon the literature values given (Table 1) determine which colorants you appear to

have found in each of the products.

Sample max values Colorants found

Then for each peak you have identified calculate the concentration of the colorant present. These calculations use the equation

A = c l which you need to rearrange because we want to calculate the concentration.

Remember that the amount you calculate will be expressed in units corresponding to those of the extinction value you select from Table1 .Attach your calculation and complete the following table. Note that in expressing results you need to include units. You have flexibility in choosing units consider using the same units as the reference values that you will compare your data with.

Product Colorant Concentration

Conclusion: (Include another page if required….comment on your results in relation to any relevant aims on page one of the notes).

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