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Chemistry of roses – Anthocyanins


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Theory:

Anthocyanins are a group of pigments present in most flowers, fruit, stems, leaves, etc. of Magnoliophyta (Flowering plants) providing them with vivid colors ranging from salmon pink to dark blue [1]. They are powerful antioxidants and have perspective to be used in food not only as colorants but also to slow down lipid oxidation for example in yoghurt [2]. Also acidic enviroment of yoghurt makes them more stable – in acidic solution anthocyanins exists in a stable form of protonated  flavylium cation [2,5]. Anthocyanins are made of aglycon (anthocyanidin), sugar(s) bonded with glycosidic bond and usually acyl group(s). Although over 500 anthocyanins have been reported, almost all of them are based on only six anthocyanidins [1]. Furthermore, anthocyanins in roses are mostly based on just three anthocyanidins, that being cyanidin (Cy), pelargonidin (Pg) and peonidin (Pn) [3].

anthocyanidins2

As already remarked, the structure of anthocyanins is highly dependant on the pH value of the solution. With changes of pH, colour of the solution is also changed.  The equilibrium is shown below:

equilibrium

 

Watch this video to see the colour changes in real time:

Bez názvu

We observed that the material degrades faster with rising pH, causing precipitation and formation of oily flakes on the surface of the solution. Anthocyanins belong to a class of organic compounds called flavonoids. While all types of roses contain flavonoids, some of them, like yellow Roses also contains a large variety of carotenoids, contributing to their colors [4]. Therefore, for simplier extraction of anthocyanins, red roses which doesn’t contain carotenoids are prefered.

rosa kordesii [Photo by Juraj Kmotorka]

Rosa kordesii, the species used in the extraction [Photo by Juraj Kmotorka].

Procedure:

Beside procedures in other literature we tried to follow mainly the work of Slovak scientists from University Prešov [5]. 70g of rosa kordesii flower petals were collected and washed in distilled water. Then they were chopped and mashed in a mortar. After that, the petals were moved into a beaker and 100cc of ethanol (w=70%)  were added. The mixture was acidified with  1mL of acetic acid( w=98%). It has been shown that strong mineral acids may degrade some acylated anthocyanins, therefore organic acids like oxalic,acetic,citric, etc. are prefered [5]. it was then  extracted for 1 hour and filtered. then, 2mL of ethyl acetate was added to get rid of polyphenols, as proposed in [5]. The solution was filtrated again. This solution should be evaporated in vacuo but for our experiment evaporating it by heating the solution to 70°C was sufficient. A dark syrupy liquid with a jam like odor was let to cool down and then treated with water and acetone. Precipitate has formed but it was proven impossible to separate. therefore, 1cc of acetic acid was added and it was let to evaporate in air. The evaporation is not yet complete and we will update this article once it´s done. altough we tried to get solid anthocyanin powder, generally it is sufficient to keep the acidified alcoholic solution for uses like food additives, indicators, et cetora.

We have also made a pH paper shown below from the extract by soaking a paper in the alcohol solution and letting it dry out.

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References:

[1] Øyvind M Andersen (2001) Anthocyanins.
[2] Saati E A, Simon B W, Yunianta and Aulanni’am (2011) J Agr. Sci. Tech. A 1 p. 1192-1195.
[3] Yuki Mikanagi,Norio Saito,Masato Yokoi,Fumi Tatsuzawa (2000) Biochemical Systematics and Ecology Volume 28, Issue 9, p. 887-902.
[4] Prof. Dr. Conrad Hans Eugster ,Dipl.‐Chem. Edith Märki‐Fischer (1991) The Chemistry of Rose Pigments.
[5] Ivan Salamon, Daniela Grulova (2015) Optimal Extraction of Pure Anthocyanins from Fruits of Sambucus Nigra, conference paper in Acta Horticulturae.

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