Sunflower makes bread green — but that’s OK
Culprits in harmless chemical reaction were sunflower seeds added to mixture
“What is going on here?”
That’s the text message I got from my daughter, accompanied by pictures of the “healthy” bread she had just baked. Upon slicing into the loaf, she was shocked to see that the inside had turned green! Was this safe to eat, she wondered?
The mystery was solved as soon as I saw the ingredients that had been used. The batter included pumpkin, sunflower, chia and flax seeds, olive oil, honey, vinegar, coconut flour, almond flour and baking soda. The culprits that gave rise to the green colour were the sunflower seeds. I was quite confident of this because I was familiar with the problem faced by the sunflower oil industry in marketing the sunflower meal that is left over after the oil has been expressed from the seeds.
Sunflower oil sells well, mostly because of the high levels of oleic and linoleic acids, which being “unsaturated,” are seen to be healthier than “saturated” fats. The large-scale production of sunflower oil means that an abundance of sunflower meal is available for sale. This is a nutritious commodity containing protein, fibre, minerals and antioxidant polyphenols. The high protein content of the meal would make it useful as an ingredient in human food, but the stumbling block is that under conditions such as baking, the finished product develops a green colour. This is completely harmless, but causes consumer alarm, with the result that sunflower meal is sold mostly as animal feed at a cost that is substantially lower than could be obtained were it to be sold as an ingredient for human food. As expected, much research has focused on the formation of the green colour.
Curiously, the felon in the green colour caper is also a nutritional hero. Chlorogenic acid is the main polyphenol found in the sunflower meal and has antioxidant properties that are generally seen as being desirable. Despite the “chloro” in the name, chlorogenic acid contains no chlorine. The name comes from the Greek for “light green” and a suffix meaning “give rise to.”
In the sunflower meal, chlorogenic acid reacts with itself to form a “dimer,” which in turn reacts with polyphenol oxidase, an enzyme released when the seeds are crushed, to yield a compound that forms a green complex with proteins. Polyphenol oxidase is the same enzyme that is responsible for the brown discoloration of apples. The dimerization and subsequent oxidation of chlorogenic acid is enhanced under alkaline conditions, which now brings us back to the green bread.
Sunflower seeds were present in abundance and baking soda provides the ideal alkaline conditions for formation of the green complex. Still, an experiment was needed to confirm the theory. I asked my daughter to immerse some of the sunflower seeds in a bicarbonate solution and set it aside for a while to see what happens. Sure enough, it wasn’t long before the yellow seeds began to show a green tinge, turning completely green within a few hours. With heat, as during baking, the reaction would be expected to be faster. I think the mystery of the green bread was adequately solved and it was consumed without any anxiety. Even the grandchildren were treated to it.
It is not only with sunflower seeds that chlorogenic acid may lead to discoloration; it can cause potatoes to turn grey. The chemistry is quite different from what happens in the sunflower case.
This time, the effect is due to a chemical reaction between iron (a natural component of the potato) and chlorogenic acid. Iron in food can exist in one of two states, referred to as ferrous or ferric. In freshly cooked potatoes, a colourless chlorogenic acid-ferrous iron complex is formed, but upon exposure to oxygen in the air, it is oxidized to the coloured ferric complex. One way of preventing this reaction is by using chemicals that bind iron and prevent it from reacting with chlorogenic acid. EDTA, gluconic acid, sodium acid pyrophosphate and sodium bisulphite can do this and reduce the darkening. Commercial potato products such as frozen french fries, dehydrated mashed potatoes and chips often are treated with sulphites, presenting a problem for people with a sulphite sensitivity. While this is not a true allergy, it can produce allergy-like symptoms including hives, swelling of the throat, cramps and breathing difficulty. Sulphite sensitivity is rare in the general population, but roughly 10 to 20 per cent of asthmatics react to sulphites.
Interestingly, the greening of baked goods made with sunflower seeds can also be prevented by the addition of sulphites, but that would not seem to be a wise use of additives. Why not just accept the greening and revel in understanding the neat chemistry involved? A great way to make holiday cookies without the need for artificial colouring!