Today, we bring you a fun, educational guest post from our friend and colleague, DrRubidium, a chemist researching and teaching on the Pacific Northwest. You can follow her on Twitter at @DrRubidium – DJK.
This summer, I began experimenting with jams and jellies, whipping up sweet and savory spreads in my kitchen laboratory. Blackberry, strawberry, grape, pineapple, tomato herb… rose?
After tasting rose jelly at a local farmer’s market, I decided make it at home.
The selected rose jelly recipe was easy:
- Steep hand-washed rose petals in water
- Filter out and discard solids, retain liquid (rose extract)
- Add sugar to rose extract
- Reach rolling boil
- Add pectin (read about the science of pectin here)
- Return to rolling boil
- Bottle and seal jars.
This recipe doesn’t just yield jelly, it also provides a great example of acid-base chemistry.
The recipe’s author writes…
Put the petals in a stainless steel saucepan with a litre of water, bring to the boil and simmer for 15 minutes. They will completely lose their colour, and the water will turn a murky shade of reddish-brown, deliciously scented. Strain this into a glass bowl and cool.
Next you add the lemon juice, and a magical thing happens – the murky brown suddenly becomes a beautiful and bright pink.
This color change isn’t magic, it’s the acid-base chemistry of anthocyanins. These compounds are the reason roses are red and violets are blue. Particular anthocyanins give us the blue of blueberries and the red of strawberries, along with the color variety that are autumn leaves. Anthocyanins are sensitive to pH, appearing different colors (or colorless) at a low (acidic) pH to high (basic/alkaline) pH. The changing color of anthocyanins over a wide pH range allows these compounds to be used as pH indicators. Anthocyanins have a long history of being used to test the acidity and alkalinity of substances. Back in 1664, famed scientist Robert Boyle discussed anthocyanins (‘Syrup of Violets’) as pH indicators in Experiments and Considerations Touching Colours.
For roses, the anthocyanin of interest is likely anthocyanidin 3,5-O-diglucoside. In water, anthocyanidin 3,5-O-diglucoside will change forms with changes to pH giving solutions of different colors.
Step #1 of the rose jelly recipe (steeping rose petals) likely results in a rose extract containing a bit of this anthocyadin.
At low pH, anthocyanidin 3,5-O-diglucoside is a form that appears red. The picture directly above (right) indicates that after recipe step #1, the rose extract is not at a low enough pH to give a rose-red color. Adding lemon juice fixes that (below left).
Instead of making the rose extract more acidic, what if it was made more alkaline? This can been done by adding the weak base sodium bicarbonate, known in the kitchen as baking soda.
Out of the kitchen and into a laboratory, a wider array of colors could be observed using acids and bases found in any standard chemistry stockroom. For a home or super-safe in-class demonstration of acid-base chemistry, lemon juice and baking soda provide plenty of color.
For a demonstration during jelly making, steep a few rose petals on the side so your rose jelly goes undisturbed. You don’t want to risk your jelly not jelling.
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