Oranges are the world’s most popular citrus fruit, and were the 5th-most harvested fruit in 2016. In fact, orange peel is widely known to be more nutrient-rich than even the flesh! However, eating orange peel is not as common in Singapore, as a common practice for imported oranges is to wax their surfaces to preserve their freshness, making them less attractive to consume. We thus decided to analyze a peel from an off-the-shelf mandarin orange to see if we could detect any traces of foreign compounds on it.
Mandarin oranges are commonly eaten in Singapore, especially during the Chinese New Year season. Mandarin peel is usually thinner than regular orange peel, and is dried and used in TCM (Traditional Chinese Medicine), as well as a seasoning or flavoring in cooking.
Mandarin orange, peeled
D-Limonene is by far the most abundant compound present in mandarin peel, comprising 92% of the sample in the tests we conducted. D-Limonene is a non-toxic natural compound found in citrus fruits, and is largely responsible for their aroma. It is widely used as a fragrance, and as a solvent in cleaning agents.
D-Limonene, the primary component of citrus
Fortunately, we did not detect any hazardous compounds in the other 8% of the peel sample. However, we did identify trace amounts of ‘off-flavor compounds’ in the sample, referring to compounds associated with a loss of flavor in the peel. Examples of these compounds would be carvone and cyclohexanone.
Hexanone and carvone, off-flavor compounds
These findings tie in with previous research that applying wax onto orange skin causes changes in their composition. These changes are ideally suited to be measured via the gas chromatography-mass spectroscopy (GC-MS) method. Even if the wax coating is inert, any changes in composition can still be detected through analysis.
For this experiment, a sample of mandarin orange peel was wrung out and the dripping was collected for headspace GC-MS analysis. ChemoPower’s SmartDalton and MoleculeDB software was able to efficiently isolate and identify trace compounds in the sample, a task much more difficult for traditional methods to accomplish.
Identifying compounds using ChemoPower’s SmartDalton software
Here we can see an example of SmartDalton software at work. SmartDalton is able to detect trace amounts of carvone (the dark red peak in the 3D diagram), even when it is obscured by the orange background noise. Notice the large D-limonene peak in the TIC graph at the top, as well as the positions of some of the trace compounds we identified.
Written by Edmund Ho, Intern (National University of Singapore)