Berries are red, berries are blue…I’ve got a berry surprise for you!

Blueberries with text: Why blueberries change color

Berries are red, berries are blue…I’ve got a berry surprise for you!

DESCRIPTION: I observe a red to blue color change when I rinse my bowl after eating frozen blueberries. Sounds like an acid-base reaction, doesn’t it? Read on to learn about the blueberry surprise!

INTRODUCTION: I often eat frozen blueberries for breakfast. When I do so, I notice that the blueberries leave a red-purple residue in the bowl. This color of the frozen blueberry residue is due to the presence of anthocyanins and anthocyanidins (Figure 1)1-3, which impart red, violet, and blue colors to many parts of plants: stems, leaves, fruits, and flowers.1-7 (For the rest of this post, both anthocyanins and anthocyanidids will collectively be referred to as AC).

 

Figure 1: General structure of many anthocyanins (R3 = glucose) and anthocyanidins (R3 = OH). R1 and R2 = H, OH, or OCH3 for various compounds. For example, for the particular compound, delphindin (commonly found in blueberries),1-3 R1 = R2 = OH and R3 = glucose.

 

For quite some time now, I have noticed an interesting color change when I rinse my bowl with tap water after eating frozen blueberries. When doing so, the residue left from the frozen blueberries changes from red-violet to blue-violet (Figure 2).

 

Figure 2: Left - residue left in a bowl from frozen blueberries. Right - same bowl after rinsing with tap water.

 

When I first noticed this color change several years ago I reasoned that the blueberry residue changed color because of a change in pH upon addition of tap water. It is well known that AC molecules change color with pH4-7 so I simply assumed a pH change was occurring (Figure 3).

 

Figure 3: Simplified description of changes in structure in anthocyanins and anthocyanidids induced by increases in pH (indicated from left to right). These structures tend to impart (from left to right) red, violet, and blue colors. The full range of reactions involving pH-induced color changes is beyond the scope of this article.

 

Over the years I have become increasingly skeptical of the explanation that a shift in pH causes the color change I see upon rinsing my blueberry-stained bowl. That’s because blueberries tend to be acidic, so a mixture of blueberry residue and tap water is likely to be acidic. But AC molecules tend to be red or violet in solutions that are slightly acidic to acidic (Figure 2). How could blueberry residue turn blue in an acidic environment?

I decided to run several tests to try to figure out how the blueberry residue could be changing blue when rinsed with tap water. You can see the culmination of these investigations in the video below:

Video 1: Blueberries Are Red, Blueberries Are Blue..., Tommy Technetium YouTube Channel, 1/21/2020

 

PERFORMING THE EXPERIMENT WITH HOUSEHOLD ITEMS: You can perform the experiments seen in the video quite easily by using only items obtained around the home or at the grocery store. To do so, frozen blueberries are placed in a clear or white container. The container is shaken to deposit blueberry residue on it. After the blueberries are removed, a small amount of distilled water is added. Next, salts from various common sources (Table 1) are dissolved into the blueberry residue/water mixture. When doing these experiments I noticed that some of the metal ions from household sources induced color changes not observed in the experiments shown in the video.

 

Table 1: Descriptive results of experiments in which salts containing metal ions from various common sources were added to a mixture of frozen blueberry residue and distilled water

Metal ion

Common source of metal ion

Color observed

None

Distilled water

Red-purple

?

Tap water

Grey-blue

Na+

Table salt

Very similar to distilled water

Mg2+

Epsom salts

Blue-purple

Al3+

Alum

Deep violet

K+

No Salt® Salt Substitute

Very similar to distilled water

Ca2+

Prestone® Driveway Heat®

Blue-purple

Cu2+

Powder from corrosion on old pennies

Blue-purple

 

DISCUSSION: In the video it is demonstrated that it is not only pH, but also metal ions that can cause AC molecules to change color. Thus, it could be that metal ions in my tap water bind with AC compounds to cause the blue color. I am not certain exactly which metal ion(s) and AC compounds interact to cause the blue color. Nevertheless, I did observe that copper ions induced a color change in blueberry residue strikingly similar to the color change caused by addition of tap water. I will note that some studies have indicated that when AC molecules bind to Cu2+, a shift to a bluer color appears.8-11 However, the blue shift seems to be more commonly observed upon interactions between AC molecules and Al3+, Mg2+, Ca2+, and/or Fe3+.4,5,12-14

The associations of metal ions with AC molecules often involve the formation of beautifully complex structures that involve as many as 12 individual AC molecules. In addition, molecules that are similar to AC, called flavones, are often involved in these intricate structures. The process of two or more AC molecules interacting is termed copigmentation, which is often accompanied by color change. Interactions leading to copigmentation can be mediated by metal ions and/or intermolecular forces between AC molecules (Figure 4).4-5,7,12-14

Figure 4: Highly simplified representations of some copigmentation structures involving AC molecules and flavones (adapted from references 4, 5, and 14). In the diagram, black bars represent AC molecules, red bars represent flavone molecules, and black spheres represent metal ions.

 

CONCLUSION: Who would have thought that the simple red to blue color change I observe when washing my dishes could involve such intricate chemistry? The beautiful nature of chemistry never ceases to amaze me. I hope you enjoyed learning about this “blueberry surprise” as much as I did. As always, feel free to leave a comment if you and your students try some of these experiments on your own – especially if you learn something new or observe something that makes you think my explanations might be wrong!

REFERENCES:

1. Soumalainen, H.; Keranen, A. J. A. The First Anthocyanins Appearing During the Ripening of Blueberries, Nature, 1961, 191, 498-499.

2. Francis, F. J.; Harborne, J. B.;  Barker, W. G. Anthocyanins in the Lowbush Blueberry, Vaccinium angustifolium, Food Science, 1966, 31, 583-587.

3. Routray, W.; Orsat, V. Blueberries and Their Anthocyanins: Factors Affecting Biosynthesis and Properties, Comprehensive Reviews in Food and Science Safety, 2011, 10, 303-320.

4. Trouillas, P.; Sancho-García, J. C.; De Freitas, V.; Gierschner, J.; Otyepka, M.; Dangles, O. Stabilizing and Modulating Color by Copigmentation: Insights from Theory and Experiment, Chem. Rev. 2016, 116, 4937−4982

5. Kumi Yoshida,K.; Mori, M.; Kondo, T. Blue flower color development by anthocyanins: from chemical structure to cell physiology, Nat. Prod. Rep., 2009, 26, 884–915

6. Alkema, J.; Seager, S. L. , J. Chem. Educ. 1982, 59, 183-186.

7. Khoo, H. E.; Azlan, A.; Tang, S. T.; Lim, S. M. Anthocyanidins and anthocyanins: colored pigments as food, pharmaceutical ingredients, and the potential health benefits Food & Nutrition Research, 2017, 61, 1-21.

8. Vestergaard, M.; Kerman, K.; Tamiya, E. An electrochemical approach for detecting copper-chelating properties of flavonoids using disposable pencil graphite electrodes: Possible implications in copper-mediated illnesses, Analytica Chimica Acta 2005, 538, 273-281.

9. Smyk, B.; Pliszka, B.; Drabent, R. Interaction between Cyanidin 3-glucoside and Cu(II) ions Food chemistry 2008, 107, 1616-1622.

10. Brown, J. E.; Khodr, H.; Hider, R. C.; Rice-Evans, C. A. Structural dependence of flavonoid interactions with Cu2+ ions: implications for their antioxidant properties, Biochem. J. 1998, 330, 1173–1178.

11. Mira, L.; Fernandez, M. T.; Santos, M.; Rocha, R.; Florêncio, M. H.; Jennings, K. R. Interactions of Flavonoids with Iron and Copper Ions: A Mechanism for their Antioxidant Activity, Free Radical Research 2002, 36, 1199–1208.

12. Kosaka Takeda, K. Blue metal complex pigments involved in blue flower color, Proc. Jpn. Acad. 2006, 82, 142-154.

13.Goto, T.; Tadao Kondo, T. Structure and Molecular Stacking of Anthocyanins – Flower Color Variation, Angew. C'hem. Int. Ed. Engl. 1991, 30, 17-33.

14. Shiono, M.; Matsugaki, N.; Takeda, K. Structure of the blue cornflower pigment, Nature 2005, 436, 791.

Safety

General Safety

For Laboratory Work: Please refer to the ACS .  

For Demonstrations: Please refer to the ACS Division of Chemical Education .

Other Safety resources

: Recognize hazards; Assess the risks of hazards; Minimize the risks of hazards; Prepare for emergencies

 

NGSS

Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.

Summary:

Asking questions and defining problems in grades 9–12 builds from grades K–8 experiences and progresses to formulating, refining, and evaluating empirically testable questions and design problems using models and simulations.

questions that challenge the premise(s) of an argument, the interpretation of a data set, or the suitability of a design.

Assessment Boundary:
Clarification:

Scientific questions arise in a variety of ways. They can be driven by curiosity about the world (e.g., Why is the sky blue?). They can be inspired by a model’s or theory’s predictions or by attempts to extend or refine a model or theory (e.g., How does the particle model of matter explain the incompressibility of liquids?). Or they can result from the need to provide better solutions to a problem. For example, the question of why it is impossible to siphon water above a height of 32 feet led Evangelista Torricelli (17th-century inventor of the barometer) to his discoveries about the atmosphere and the identification of a vacuum.

Questions are also important in engineering. Engineers must be able to ask probing questions in order to define an engineering problem. For example, they may ask: What is the need or desire that underlies the problem? What are the criteria (specifications) for a successful solution? What are the constraints? Other questions arise when generating possible solutions: Will this solution meet the design criteria? Can two or more ideas be combined to produce a better solution?

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories.

Summary:

Constructing explanations and designing solutions in 9–12 builds on K–8 experiences and progresses to explanations and designs that are supported by multiple and independent student-generated sources of evidence consistent with scientific ideas, principles, and theories. Construct and revise an explanation based on valid and reliable evidence obtained from a variety of sources (including students’ own investigations, models, theories, simulations, peer review) and the assumption that theories and laws that describe the natural world operate today as they did in the past and will continue to do so in the future.

Assessment Boundary:
Clarification:
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Comments 2

Amiee Modic's picture
Amiee Modic | Wed, 01/29/2020 - 13:45

Hi Tom,

I really enjoyed that discussion. I'm wondering where the copper ions are coming from? Are they leaching from the pipes, leaching from brass fittings?  It would be interesting to check people's water from new houses where all of the plumbing is pex. (sp)  Would that also have copper ions just because of the fixtures, or would the amount go down enough to have no effect on the blueberries?

Tom Kuntzleman's picture
Tom Kuntzleman | Sat, 02/01/2020 - 12:03

Hi Amiee:

Thank you for your comments and questions. I’ve not tested the water from my tap, but I will note that the water report for my township states that the water contains 100 ppb Cu2+. It is also the case that the water pipes in my home are made of copper metal.

Having said all this (and even though my experiments indicate Cu2+) I’m still not entirely sure the effect I’m observing is from Cu2+. First, I have a water softener which should remove a lot of the metal ions (including Cu2+) that could complex the anthocyanins. Also, several metal ions or mixtures of ions commonly found in tap water (Mg2+, Ca2+, Fe3+) can complex with anthocyanins and anthocyanidids. Thus, there are a variety of scenarios that could be causing what I’m seeing. You’ll note that in my experiments I only tested individual ions – but not mixtures of ions – with the blueberry mixture. You’ll also notice that I didn’t carefully control for added metal ion concentrations in my experiments.

So while it’s a fair explanation that Cu2+ is causing the effect, I’m not completely sold on the idea. Hopefully I’ll be able to carry out some more experiments to gain more insight into what is going on. Any suggestions you have for tests are welcomed!