Colourful Chemistry of Canning

Co-Authored by Iain A. Smellie*, Iain L. J. Patterson*

*University of St Andrews, School of Chemistry, North Haugh, St Andrews KY16 9ST, United Kingdom

Over the last 2 years, we have been interested in the variety colours observed when aqueous solutions of metal ions are added to plant extracts containing anthocyanins. We have focused on using materials that could be sourced and used easily at home. So far, we have found iron, copper and aluminium salts can form colourful complexes with aqueous anthocyanin extracts. Aluminium in particular can give impressive results (figure 1).¹

Figure 1: Example anthocyanin complex with Al³⁺. Blueberry extract (left) and extract containing Al³⁺ (right).

 

Metal-anthocyanin complexes have received a lot of research attention over the last 100 years, much of this interest is due to these compounds playing an important role in the colours of flowers.^2-5 More recently, interesting studies have been directed toward finding anthocyanin derivatives that are suitable for use as food colourings.⁶ The complexes formed between metal ions and anthocyanins can have very complicated structures,^4,5 as a result, a full description of these is beyond the scope of this article. However, for readers who wish to know more, there are excellent reviews available.^4,5,7 For the purposes of this article, a simplified representation of the binding motif between metal ions and anthocyanins (figure 2) is useful.

Figure 2: Simplified binding motif for anthocyanins with metal ions. R = H or carbohydrate substituents, X = H or OH.

 

While surveying the literature, we noted interesting references to the effect of Sn²⁺ ions on the colour of solutions containing anthocyanins,^2,8,9 particularly in the context of food preservation by canning.^8,9 In the early 20^th century, preservation of fruit and vegetables by storage in tin cans was still a relatively new technology and was being continually improved.^10-14 In reality, “tin cans” are not solely made from tin, they are made from tinplate, which is sheet steel with a thin layer of tin metal applied on the surface. In the early days of the canning industry there were documented problems concerning corrosion of the cans and hence spoilage of the contents.^10-14 Significant concerns included: “swelling” where gases released by the contents could lead to the container becoming swollen to the point where it could burst, corrosion of the inside of the can to form metal sulfides^10,12-14 and in some cases, foodstuffs were found to change colour on contact with the tin surface inside the can.⁸ The latter observation was of particular interest to us, since it has been shown that in some fruits and vegetables, Sn²⁺ ions can complex with the anthocyanins present and lead to undesired colouration of the contents of the tin can.

A very useful study we identified was from a paper published in 1927 by C. W. Culpepper.⁸ This article reported the results of a series of canning experiments with various fruits and vegetables. Culpepper provided a detailed account of the behaviour of peaches stored in tin cans. The paper states “Nearly all peaches canned in a tin developed some degree of color, ranging from light pink to deep purple”. It was found that canned ripe peaches, that retained the skin and the peach pits, were particularly susceptible to purple/violet discolouration. In contrast, preservation in glass containers did not lead to significant discolouration. Further investigation revealed that heating aqueous extracts from peach stones with granular tin, or adding tin(II) chloride, “resulted in change of the colour to very dark purple”. The conclusion that Culpepper reached from the experiments was that “it is apparent that the purple discolouration observed in peaches canned in tin is due to a reaction between the tin of the can and the anthocyan pigment present”. Although not shown in the paper, a tin complex that is possibly responsible for the purple colour in solution is illustrated in figure 3.^15,16

Figure 3: Possible binding of Sn²⁺ ions with cyanidin-3-O-glucoside

 

In addition to peaches, Culpepper also studied the interaction between tin metal and raspberries and red cabbage,⁸ we selected these items for our own experiments since they were readily available in local grocery stores. Our aim was to replicate the anthocyanin experiments from the 1927 paper in a way that educators could use these to make a connection between the food we eat and the chemical principles that are employed to ensure that canned foodstuffs can be preserved properly.

 

Initial Experiments with Raspberries

Frozen raspberries were allowed to warm to room temperature, samples of the resulting raspberry extract were removed, and then diluted with water. Addition of SnCl₂ or SnCl₄ to acidified raspberry extracts, resulted in immediate formation of violet/purple-coloured solutions (figure 4).

Figure 4: Initial experiments with raspberry extracts. A = raspberry extract with HCl, B = raspberry extract with SnCl₂, C = raspberry extract with SnCl₂ and HCl, D = raspberry extract with SnCl₄ and HCl.

 

These observations were in line with our expectations, however the tin compounds that were used, require careful manipulation and are only available from specialist chemical suppliers. Tin metal is easier and safer to manipulate and is the same material used for the interior of tinplate cans, so reactions with tin were also attempted. Granular tin metal was added to raspberry extract and extract to which we had added sodium metabisulfite (Na₂S₂O₅). The effect of Na₂S₂O₅ interested us because some preserved fruits contain sulfite as a preservative, and it was anticipated that some tin corrosion to form tin sulfides may occur under these conditions.¹⁰ As can be seen in figure 5, after 24 hours, the raspberry extract sample containing tin metal had changed colour from red to violet, just as had been observed when tin compounds had been added. In order for the colour change to take place, there must be Sn ions available in solution to bind to the anthocyanins present. Raspberry extract contains organic “fruit” acids,¹⁷ these result in the pH value of raspberry extract being typically 3 to 3.5, and these conditions can facilitate reaction with the tin layer on the inside of a tinplate can.⁹

Figure 5: Experiments with raspberry extracts. A = raspberry extract, B = raspberry extract with granular tin, C = raspberry extract with sodium metabisulfite, D = raspberry extract with sodium metabisulfite and granular tin, E = raspberry extract acidified with HCl.

 

The extract samples treated with sodium metabisulfite showed bleaching as expected,¹ however the acidified metabisulfite solution also showed a black/brown material on the surface of the tin granules (figure 6). This observation is consistent with the formation of tin sulfides, reportedly a commonly encountered problem in canned goods in the past. In a detailed early study conducted by Norton in the early 20^th century,¹⁰ it was noted that canned pears could be discoloured with a "heavy brown-black deposit". Investigations of the effect of aqueous sodium sulfite (mixed with acids found in fruit and vegetables) on tin metal and solder led Norton to conclude that hydrogen sulfide was released and that this could react with the metals in the can to form undesirable metal sulfides.

Figure 6: Close-up picture of granular tin covered in sulfide deposits (raspberry extract with sodium metabisulfite and granular tin)

 

Experiments with Raspberries in Food Cans

The results shown in figure 4 encouraged further experiments, the goal being to establish if discarded food cans could be used to induce the red to purple colour change that was observed when granular tin was soaked in raspberry extract. Modern food cans can employ a range of coatings to protect the interior surfaces of the can from corrosion by the contents.^13,14,18 The literature suggested that pineapple, peach and pear tins were likely to have an exposed tin coating on the inside,¹⁸ while many other acidic food products (such as chopped tomatoes) were likely to use lacquer coatings to protect the tin surface. The results of a simple experiment that involved exposing a pineapple can to raspberry extract are shown in figure 7. Raspberry extract obtained from frozen berries was added to the can (the can was washed with water and dried prior to use) and the can was rested on its side for 16 hours, this ensured efficient contact of the extract with the sides of the can. The colour change was quite remarkable, the red raspberry extract had become a rich violet colour and a dark purple residue could be observed on the surface of the tinplate at the edge of the point of contact between the liquid and the metal. The surface of the metal that had been in contact with the raspberry extract was visibly different to the unexposed areas.

Figure 7: Experiment with raspberry extract in a discarded pineapple can. Panels A, B and C all show the effect of raspberry extract on the internal surfaces of the can. Panel D shows a sample of the violet liquid removed from the can (diluted with water) and a comparison sample of the original red raspberry extract.

 

A further experiment was conducted to compare the effect on raspberry extract in a food can that had a protective lacquer. A discarded chopped tomato can was selected, since this had a distinctive white lacquer on the interior surfaces. Figure 8 shows the outcome after 16 hours of allowing raspberry extract to be stored in a lacquered can versus a discarded baked beans can that did not have the same protective coating. The protected can did not show any colour change or damage to the interior surfaces, whereas the inside of the baked beans can showed signs of corrosion, and the raspberry extract had changed from red to violet/purple.

Figure 8: Further experiments with raspberry extracts in discarded cans. A = raspberry extract in a tomato can B = raspberry extract in a baked beans can, C = raspberry extract in a tomato can after 16 hours, D = raspberry extract in a baked beans can after 16 hours. Panels E and F show raspberry extract samples taken from the tomato can (left side) and baked beans can (right side) after 16 hours.

 

Experiments with Red Cabbage

In addition to successful experiments making colourful tin complexes from raspberry extracts, we also examined aqueous extracts of red cabbage. Red cabbage is used frequently by educators for a variety of teaching activities.^1,19 Using it as an anthocyanin-based pH indicator is particularly popular.¹⁹ In this case, cabbage extracts made an interesting comparison with raspberries, since the cabbage extract is significantly less acidic. Figure 9 shows that red cabbage extract exposed to tin metal did not readily form a violet-coloured tin complex at room temperature (even after several days), unless acid was added. Citric acid was found to give a red/violet coloured cabbage extract, whereas addition of 2M HCl led to a more violet/purple solution.

Figure 9: Experiments with red cabbage extracts, all solutions were monitored over the course of 7 days, colour changes started to become apparent within 24 hours. A = red cabbage extract, B = red cabbage extract with granular tin, C = red cabbage extract acidified with 2M HCl, D = red cabbage extract with granular tin and acidified with citric acid, E = red cabbage extract with granular tin and acidified with 2M HCl.

 

Granular tin metal was also added to red cabbage extract containing acidified sodium metabisulfite (Na₂S₂O₅) (figure 10). The results in this case were very similar to raspberries, the anthocyanin was initially bleached, and then a black/brown sulfide deposit was observed on the surface of the tin granules. This result is in line with the experiments conducted by Norton,¹⁰ his original study noted that sulfide deposits formed on pieces of tin and tin solder in the presence or absence of anthocyanins as long as acid and sulfites were present in solution.

Figure 10: Experiments with bleached red cabbage extracts. A = red cabbage extract with sodium metabisulfite, B = red cabbage extract with sodium metabisulfite and 2M HCl.

 

The raspberry experiments in discarded food cans were repeated with red cabbage extracts, although in this case the solutions were acidified by addition of 5-6 drops of 2M HCl (figure 11). After 16 hours, both cans were observed to contain red solutions, this indicated that no reaction with tin ions had taken place. However, on closer inspection, the kidney beans can that had been selected (because it was not lacquered), was found to have a thin coating of polymer material on the tin surface. It was found that the coating was easily damaged by scratching it with a screwdriver. The acidic cabbage extract was then allowed to contact the exposed metal surface for 16 hours, in this case, the expected violet coloured extract was observed. Scratching the inner surface of the can, could also expose some of the steel below the tinplate layer. Previous experiments with iron salts have shown that the resulting complexes formed from cabbage extracts become dark blue, almost black in some cases (this is likely dependent to some extent on the presence of tanins).¹ The UV-visible absorption spectrum of the extract exposed to metal is provided in the supporting information document. (The Supporting Information can be found at the conclusion of this article when the reader is logged into their ChemEd X account.)

Figure 11: Experiments with acidified red cabbage extracts in discarded cans. A = red cabbage extract in a tomato can (after 16 hours) B = red cabbage extract in a scratched kidney beans can (after 16 hours) C = scratch marks made on the inside of the kidney beans can to expose the tinplate surface, D = red cabbage extract samples taken from the tomato can (left side) and kidney beans can (right side) after 16 hours.

 

Detection of Tin Ions in Pear Juice

While collecting food cans for our experiments, we noted a difference between the interior surfaces, of two otherwise identical pear cans. One of the cans was marked with a “best before” date for 2023, the other can was labelled with a date in 2021. The older can showed some signs of minor corrosion (figure 12, panel A), while the other can (figure 12, panel B) did not show many signs of imperfections on the internal surfaces. We reasoned that the contents of the older can would likely have more Sn²⁺ ions in solution, and that these could possibly be detected by adding a source of anthocyanins. Addition of red cabbage extract to pear juice from both cans resulted in a violet/purple colour, as had been observed in our earlier experiments with tin salts. The colour of the pear juice/extract mixture from the older can was noticeably darker than the mixture from the newer can (figure 12, panels E and F).

Figure 12: Experiments with pear juice and cabbage extract. A = tin surface of the older pear can (areas of corrosion highlighted), B = tin surface of the newer pear can, C = Cabbage extract with acetic acid, D = Cabbage extract, E = Pear juice from older can and cabbage extract, F = Pear juice  from newer can and cabbage extract.

Summary

We set out to replicate experiments that had been conducted in the early 20^th century to understand why certain foods became discoloured on prolonged exposure to tinplate cans. These experiments have now been repeated and simplified so that educators can readily make colourful tin complexes from anthocyanins with materials that can be sourced in most grocery stores, without the need to purchase chemicals from specialist suppliers. The source of tin in these experiments is from used food cans, the cans can be rinsed and recycled once the activity is complete. We have selected raspberry and red cabbage extracts for our study, however, we expect that many other anthocyanin sources (from fruit, vegetables or flowers) could also be used. There are also many different types of tin plate food cans in circulation, the main difference between them being whether the interior metal surface has additional protective coating or not. We think that these variables could provide further scope for investigation and make a series of open-ended experiments for educators in classes or science clubs. For example, in addition to testing a selection of food cans, an extension to this activity could examine the effect of commercially available tin solders on anthocyanin-rich extracts.

Hazards and Disposal

Safety glasses are recommended for these activities. Do Not allow any of the substances in use to come into contact with skin or eyes.

All the solutions should be diluted in water and can be disposed of by flushing down the sink with plenty of water. Used food cans, should be rinsed and recycled using locally available facilities.

Acknowledgements

We wish to give very special thanks to Isobel Everest from Bedford Girls’ School. Isobel is a huge inspiration to us through her #Gardenindicators activities, she has been very encouraging to us and has made very helpful suggestions for the experiments described in this article. We would also like to thank all our family members who provided a variety of tin cans for our experiments, these proved to be very useful!

Supporting Information - (Log into your ChemEd X account to access. Don't have an account? Register here for free!) Experimental procedures (and safety information) are provided for experiments conducted in discarded food cans. In addition, UV-visible spectra of the relevant extracts have been provided. 

References

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