Co-Authored by Iain A. Smellie* and Iain L. J. Patterson*
*University of St Andrews, School of Chemistry, North Haugh, St Andrews KY16 9ST, United Kingdom
In previous work, we have explored reactions of anthocyanins with aqueous solutions of sodium metabisulfite.1,2 Under these conditions, anthocyanin-containing extracts can be decolourised, the process is often referred to as “anthocyanin bleaching”. It has been shown that in aqueous solution, bisulfite ions can react with anthocyanins to form colourless flavene-4-sulfonates (often called a “bisulfite adducts”, see scheme 1).1-3 This process is also reversible, addition of an oxidant or excess of acid will shift the equilibrium from the colourless form to the red flavylium ion form of the anthocyanin.1,2 Video 1 and 2 are examples that show anthocyanin bleaching in solution by bisulfite and with sulfur dioxide.
Video 1 - Anthocyanin Bleaching: Red Radish, Gardenindicators YouTube Channel, January 2024.
Video 2 - Bleaching Anthocyanins with Sulfur Dioxide, Gardenindicators YouTube Channel, April 2024.
Scheme 1 – Equilibrium between anthocyanin flavylium forms and flavene-4-sulfonate “bisulfite adducts” in the presence of bisulfite ion (HSO3-). R1 = glycosyl substituent, R2 = H or glycosyl substituent.
This type of bleaching process is well studied in the context of preservation of commercial food and drink products.4-6 Research in this area is important, since sulfites are used to control undesirable growth of yeasts, moulds and bacteria in various foodstuffs. Sulfites have other familiar uses in food preservation, a colourful and distinctive example is the production of maraschino cherries.7 A simplified overview of the process for making maraschino cherries is shown in figure 1.
Figure 1 – Simplified overview of the process to make maraschino cherries.7
A key step in the process involves decolourisation of the fruit anthocyanins using a solution containing sodium bisulfite. The fresh cherries are pitted, washed and then soaked in a “brine” solution. The brine often contains sodium metabisulfite, citric acid and calcium chloride, although other additives may be also present. Citric acid is used to maintain the pH of the solution below 4 and calcium chloride is added to maintain the fruit texture. The bleached cherries are soaked in a sugar syrup, then a dye is added to the mixture to give the fruit a distinctive red colour. Over the years, several red dyes have been used, typically these have been azo dyes, but other red dyes are available.7
Figure 2 – Example dyes used to make maraschino cherries.
During the late summer, our local area is usually a good location to pick wild blackberries (colloquially known as “brambles”). We speculated whether it was possible to devise a maraschino-inspired method to bleach and then dye blackberries. If successful, this approach could be used in teaching as a model of for a commercial application of anthocyanin chemistry.
Experiments with bleaching blackberries
In previous experiments, we have found that sodium metabisulfite is a convenient reagent for bleaching anthocyanins, so it was selected for our initial tests. Sodium metabisulfite is cheap and readily available from chemical suppliers, this material is also available in the UK in tablet form for domestic use (these are colloquially known as “Campden tablets”).6 Figure 3 shows the effect of soaking blackberries in 20% sodium metabisulfite solution. The bleaching process required 24 hours to go to completion, although after a few hours, the colour of the berries visibly started to fade.
Figure 3 – Blackberries soaked for 24 hours in 20% solution of sodium metabisulfite.
Once the bleaching process was complete, the berries were filtered through a sieve and then rinsed with cold water. As shown in scheme 1, the bleaching process can be reversed in the presence of acid or peroxide. Attempts to reverse the bleaching reaction within the fruit tissue were successful, the berries became bright red when they were immersed in 1 M hydrochloric acid or 3% hydrogen peroxide (see figure 4, reactions 1 and 2). An attempt was also made to change the berry colour to violet/purple by soaking the berries in a solution containing Al3+ ions. The berries did not change colour when soaked in saturated potassium alum solution, even when immersed for several days (figure 4, reaction 3). Under the experimental conditions the formation of violet/purple anthocyanin with Al3+ appears to be disfavoured.
Figure 4 – Bleached blackberries soaked in 1) 1 M HCl 2) 3% hydrogen peroxide and 3) saturated KAl(SO4)2
Dyeing experiments with bleached blackberries
The final stage of process for making maraschino cherries involves soaking bleached cherries in a sugar syrup that contains a red dye. In our experiments, we soaked bleached blackberries in a 40% glucose solution containing a few milligrams of a selection of dyes (see figures 5 and 6). The berries were immersed in the dyed glucose solutions for 48-96 hours and then the berries were removed and rinsed in a beaker of water.
Figure 5 – Dyes used in experiments with blackberries.
Figure 6 – Bleached blackberries being soaked in a mixture of 40% glucose and a selection of dyes.
Soaking bleached fruit in methyl red, methyl orange, methylene blue or erythrosine did not work very well, colour transfer was found to be limited, although small patches of dye were visible on some fruits. In contrast, bleached blackberries soaked in solutions of crystal violet and brilliant blue FCF were successful, crystal violet in gave a particularly vivid purple colour (see figure 7, example B).
Figure 7 – Berry A has been bleached with bisulfite. Berry B has been bleached and dyed with crystal violet. Berry C has been bleached and dyed with brilliant blue FCF.
In summary, we have investigated the effect on the colour of whole blackberries when they are treated in an aqueous solution of sodium metabisulfite. We have found that after an overnight soak, all of the anthocyanin-derived colour in whole fruits was readily bleached. The colour of the berries could be changed to bright red by adding acid or dilute hydrogen peroxide. Furthermore, soaking bleached blackberries in a glucose/dye mixture allows bleached berries to be dyed different colours. In this short study we have used foraged fruit that was seasonally available, however, we hope readers will try other berries and fruits and share their results!
Supporting Information - See the Supporting Information for experimental procedures and safety information for experiments described. Supporting Information can be viewed when you are logged into your ChemEd X account. Not a member? Register for FREE!
References
- Iain Smellie and Iain Patterson, “The Disappearing Rainbow Demonstration - A colourful variant using red cabbage extracts” ChemEd X, February 2021. (accessed 8th October 2024).
- Keiller, K. A.; Patterson, I. L. J.; Stewart, D. M.; Smellie, I. A. A Simple and Inexpensive Ink System Based on Red Cabbage. J. Chem. Educ., 2023, 100, 1398-1403.
- Berké, B.; Chèze, C.; Vercauteren, J.; Deffieux, G. Bisulfite addition to anthocyanins: revisited structures of colourless adducts. Tetrahedron Lett.,1998, 39, 5771-5774.
- McClure, D. W. The Chemistry of Winemaking and Brewing. J. Chem. Educ., 1976, 53, 70–73.
- Andrea-Silva, J.; Cosme, F.; Ribiero, L. F.; Moreiira, A. S. P.; Malheiro, A. C.; Coimbra, M. A.; Domingues, M. R. M.; Nunes, F. M. Origin of the Pinking Phenomenon of White Wines. J. Agric. Food Chem., 2014, 62, 5651-5659.
- Money, R. The Chemical Preservation, Colouring and Flavouring of Foodstuffs with Special Reference to Fruit and Vegetables. Br. J. Nutr., 1947, 1, 253-258.
- Wrolstad, R. E. Maraschino Cherry: A Laboratory-lecture Unit. J. Food. Sci. Educ., 2009, 8, 20–28.