Wirt-Gast-Komplex β-Cyclodextrin und Phenolphthalein

1. Experiment is contained in the WAKER's Experimental Kit for Schools	yes
2. Experimental procedure has been modified	no
3. Video clip available	no
4. Flash animation available	no

5. Removing the Bitter Principles from Grapefruit Juice

1 Materials, Chemicals, Time Needed

Citrus Press
Glass beaker (100 mL)
Snap-cap vial
Scales
Spatula
Magnetic stirrer
Magnetic stir bar

Grapefruit
β-Cyclodextrin

This experiment takes only a few minutes to prepare. The samples need stirring for 20 minutes to obtain clear observations.

2 Procedure and Observations

Squeeze a grapefruit either by hand or using a citrus press. At least 60 ml of juice is required. When the pulp has settled at the base of the beaker, decant 20 ml of juice into each of the three snap-cap vials. Seal one and set aside as a control sample. To the second sample add 0.5 g β-cyclodextrin, seal and stir vigorously for 20 minutes. The third sample is also a control. Stir this one vigorously for 20 minutes as well, without adding any cyclodextrin. After 20 minutes, compare the smell of each of the three samples.

Initially the grapefruit juice has the characteristic fresh, acidic smell of freshly squeezed citrus fruit. After 20 minutes, the first and third samples cannot be distinguished from one another, and still retain the clearly acidic smell. The acidic smell has disappeared from the sample treated with β-cyclodextrin.

3 Discussion of Results

Naringin and limonene are the substances responsible for the extremely acidic smell of grapefruit and in particular the bitter taste. When the juice is treated with β-cyclodextrin, these two substances are largely encapsulated inside the cyclodextrin molecules. As with masking the smell of garlic, complexes with naringin and limonene are low-odor, so the acidic smell of the grapefruit juice disappears.

Fig. 2.26: The bitter taste of grapefruit juice is removed by the complexation of naringin and limonene

4 Tips and Comments

Since tasting samples is not permitted in chemistry lessons, use your sense of smell to confirm the change produced by treating with cyclodextrins! For a reliable test of the effect of β-cyclodextrin on the grapefruit juice, the sample should be blind tested i.e. students should not know which sample contains the cyclodextrin. The students then describe the smell of the samples and compare their findings. Use freshly squeezed grapefruit juice for this experiment, because the acidic note in shop-bought juice is too weak to show a clear effect of the addition of cyclodextrins.

5 Supplementary Information

The complexation of bitter principles in grapefruit juice is used in food processing to reduce the extremely acidic taste. This is not done using water-soluble cyclodextrins, however, since the cyclodextrin complexes formed would then have to be removed from the juice in a further production stage. To simplify the treatment, the juice is passed through a cyclodextrin polymer.

6 References

Woyke, A.; „Cyclodextrine“ – Molekulare Zuckertüten. Ein Chemie-Praktikum für die 13.Klasse (www.science-forum.de/download/Cyclo-praktikum.pdf)
Szejtli, J.; Osa, T.; Comprehensive Supramolecular Chemistry. Volume 3 Cyclodextrins, Elsevier Science Ltd. Oxford, 1996, S. 489