Lucidity GC-FID Tastes Sour Candy
Citric acid is a weak, colorless organic acid. The main sources of citric acid are citrus fruits such as limes and lemons, and due to this many people confuse citric acid and vitamin C. Citric acid is an intermediate in the citric acid cycle or Krebs cycle, an important process in the body to get energy from food.
Many foods and candies contain excess citric acid to cause a sour taste, in fact some of my favorite candies are sour candies. So, I decided to find out if I can see citric acid in these sour candies.
First, I had to taste some of the sour candies to make sure that they were indeed sour and still tasted good. The result is that they do in fact taste good and have a bit of sour flavor.
I decided to try and test the citric acid on the GC-FID first. I created two different concentrations of citric acid to ensure that the peak observed was citric acid, using the method below.
| Lucidity GC-FID Conditions | |||
|---|---|---|---|
| Carrier | Hydrogen | ||
| Control | Pressure | ||
| Flow | 1.50 mL/min | ||
| Split ratio | 50:1 | ||
| Column | RTX-624 | 30 m x 0.25 mm, 1.40 um | |
| Injector | 250 ℃ | ||
| FID | 260 ℃ | ||
| Oven Program | |||
| Rate | Temperature | Hold Time | |
| 70 ℃ | 1 min | ||
| 20 ℃/min | 230 ℃ | 5.0 min | |
The citric acid was observed on the chromatogram and the peak was marked in the software for identification in later chromatograms.
Chromatogram showing a citric acid peak
The candy sample had to be put into solution, but I didn’t want to have all of the sugar in the solution as well. The sugar would decompose in the injection port causing contamination. I decided to go with acetone as my solvent, since citric acid is freely soluble in acetone and sucrose is not, so there shouldn’t be any contamination risk from the sugar.
Centrifuge tube with sour candy in acetone
For preparation of the candy, one candy was placed into a centrifuge tube and enough acetone was added to cover the candy. The solution was then shaken vigorously for 5 minutes, then allowed to sit for 30 minutes. After that time, an aliquot was removed and vialed for analysis.
Chromatogram showing 1 piece of sour candy in 10 mL of acetone
The peak of the citric acid was quite small and barely above the noise level of the chromatogram. I did a bit of digging and found that the threshold of tasting citric acid is 3.8 x 10-4g/mL. This means that the concentration of citric acid in my candy can be quite low and still give a sour taste.
Being a fan of sour candy, I know that these sour gummies are on the weak side of the sour chart, and there are some very intense sour candies. I wanted to compare the extreme sour candies qualitatively to see if one contained more citric acid than the other. Each of these candies were hard candies rather than a gummy, so the sample preparation was a bit different. I ground each of the candies using a mortar and pestle to a fine dust, the sample was then placed in a 50 mL centrifuge tube and mixed by hand for one minute. Afterwards, each sample was centrifuged at 3000 rpm for 5 minutes, then allowed to rest for 30 minutes. An aliquot was then put into a sample vial for analysis on the GC-FID.
Chromatogram of candy 1 showing two peaks
Candy number 1 had two organic acids listed on the label, malic acid and citric acid. I know where citric acid is on the chromatogram and I am assuming that the peak at about 6:30 is the malic acid.
Chromatogram of candy number 2
Candy number 2 only had citric acid as the sour component and had much less ingredients on the nutrition label.
After the GC-FID had its taste of the sour candy, I gave some to some coworkers around the office to see what they thought. I’ll let his face tell you if it was sour.
Sawyer’s sour candy face
This was a fun experiment to see if the GC-FID could detect citric acid and compare some candy qualitatively. Stay tuned as we repeat this same experiment on the LC-UV.