Does the browning reaction have an effect on the nutritional quality of plant foods?
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The first chemical reaction to which I remember paying any attention was between polyphenols and the enzyme polyphenol oxidase (PPO). That was long before I knew anything about chemistry. My mother’s remedy for any sort of upset stomach was grated apple sprinkled with some sugar. I recall that it didn’t take long for the slivers of apple to turn brown.
I didn’t give this colour change much thought until I started to teach organic chemistry and made a point of emphasizing practical applications.
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Reactions of a substance with oxygen are a common feature in organic chemistry. The browning of an apple or banana is a typical example and is related to polyphenols, a class of compounds that occur in many plants and are believed to contribute to the “healthy” properties of plant foods.
However, if a plant is damaged, polyphenols can react with oxygen and are converted to quinones that then undergo a polymerization reaction to form melanin, a brown pigment. In plants, melanin is believed to protect damaged tissues from further damage by UV light and to deter insects looking for crevices in which to deposit their eggs. Melanin is the same substance that occurs naturally in the human body and is responsible for the colour of brown eyes, nipples, dark skin and hair.
Enzymes are special proteins that act as catalysts, meaning that they speed up chemical reactions without being consumed. PPO is such a catalyst stored in chloroplasts, the organelles in a cell where photosynthesis takes place. Polyphenols, on the other hand, are stored in cell compartments called vacuoles. When a cell is damaged, the enzymes, polyphenols and oxygen from the air come together and the browning reaction begins. Grating an apple, or subjecting a banana to cold temperatures, damages cells.
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A common experiment suggested to children is to treat a cut-up apple with various liquids to see the effect these have on browning. Inevitably they conclude that lemon juice stops the reaction. Why? Enzymes are sensitive to acidity and tend to function only within a specific pH range. Lemon juice is quite acidic, and as a bonus, citric acid in the juice can bind metal ions. PPO contains copper ions that are essential for its functioning and citric acid can pilfer them from the enzyme.
Now we come to an interesting question. Does the browning reaction have an effect on the nutritional quality of plant foods? That is worth exploring, since the loss of polyphenols is not desirable. These compounds can neutralize the reactive oxygen species that are byproducts of normal metabolism and have been linked with conditions ranging from cancer to heart disease.
We can look to an interesting study that focused, of all things, on smoothies.
In a paper published in the journal Food and Function, researchers reported on the effect of a food with a high content of PPO, such as a banana, on the bioavailability of polyphenols. A specific amount of flavanols, a class of polyphenols, was added to a banana-based smoothie and to a berry-based smoothie. Berries were selected for their low PPO content.
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Subjects then consumed the smoothies and had their blood and urine tested for flavanol metabolites to determine how much of the flavanol had been absorbed. With the banana-based smoothie, the flavanol metabolites were decreased by about 80 per cent relative to the berry-based smoothie. Based on this observation, the researchers suggest that if one wants to have the maximum benefit from polyphenols in a smoothie, it is best to avoid including a banana.
Why did they choose flavanols as a representative of polyphenols in this trial? A clue comes from the acknowledgement that the study was funded by Mars, Inc., a chocolate manufacturer. Flavanols are found in cocoa, and Mars has had a long interest in exploring their health benefits based on early studies that demonstrated a reduction in blood pressure. The company also sponsored the double-blind, placebo-controlled Cocoa Supplement and Multivitamin Outcomes Study (COSMOS), which found a reduction in cardiac deaths in subjects taking a cocoa supplement.
It is important to point out that the supplement pills were not made from chocolate, which is an unreliable source of flavanols, but rather from a specially developed cocoa extract. Therefore, this study cannot be used to claim any health benefit for eating chocolate, and indeed the company makes no such claim. But it does have an understandable interest in exploring potential benefits of flavanols and how these may be impacted by other components of a diet, such as polyphenol oxidase.
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I took particular note of the banana smoothie study because of personal interest. My breakfast every day is avocado toast made with sourdough bread and a bowl of berries, mostly blue, topped with some banana slices and a spoonful of Fiber One cereal. I can make a case for this being healthier than a Danish, or even a bagel with cream cheese. Since blueberries are touted for their polyphenol content, you can appreciate why the study about the PPO activity of bananas caught my eye. But that is all it did. I am not expelling bananas from my berry breakfast. I think in the context of an overall diet, whatever reduction in polyphenol intake occurs by letting my slices of banana mingle with my blueberries is inconsequential.
Let me return to my childhood and those grated apples. Could they really have done something to settle my stomach? Apples contain pectin, a carbohydrate that when mixed with sugar forms a gel. That is the chemistry behind making jams. At one time, it was suggested that pectin via such gel formation can bulk up stools and prevent diarrhea. It was even included along with kaolin, a type of clay, in Kaopectate to treat diarrhea. When it was determined that kaolin alone did the job just as well, pectin was removed from the preparation.
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There is still a smidgen of evidence that pectin may be of some benefit for diarrhea associated with irritable bowel syndrome. In this case, it may be acting as a “prebiotic,” serving as food for bifidobacteria, microbes that then multiply and squeeze out bacteria that can cause diarrhea.
I think I can conclude that the grated apples were unlikely to have had any sort of therapeutic effect other than providing small bits of apple that are easy to digest. But nevertheless, likely out of nostalgia, when I have some sort of digestive issue, I’ll go for the grated apple. But I do squeeze lemon juice on it. And thanks to chemistry, I even know why. And now, so do you.
Joe Schwarcz is director of McGill University’s Office for Science & Society (mcgill.ca/oss). He hosts The Dr. Joe Show on CJAD Radio 800 AM every Sunday from 3 to 4 p.m.
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