Should I Put Bananas into My Smoothie?

Were you disappointed to hear the results of a recent study that discovered that bananas in a smoothie can result in an 84% loss of the polyphenols from the berries that you were hoping to benefit from?  We certainly were.  Yes, we thought, we could leave the bananas out of our smoothies.  We experimented with doing this, exchanging bananas with mangos (a fruit low in the enzyme that causes the polyphenol loss) or extra soy milk but results were definitely inferior.  Bananas take the cake for producing a super rich and creamy smoothie.  Thus began a quest to see if there was some action we could take during our smoothie-making that might minimize the effect of the bananas on polyphenol levels.

First, some background information is in order.

 

What are Polyphenols and Why are They So Healthy?

Polyphenols are one of the many bioactive compounds which are synthesized by plants to perform functions above and beyond the primary requirements for a plant’s survival. (1)  These compounds often help defend plants from dangers like pathogens or plant-eating animals.

The polyphenol family includes phenolics, stilbenes, flavonoids, tannins and lignins. (2)   There are many other types of bioactive compounds too, all created by plants.  These include phytosterols, biogenic amines, carotenoids and dietary fiber.  (3)

Polyphenols are found in natural foods that are vibrant in colour.  Think of bright red strawberries, dark purple blueberries, emerald green kale and vibrant yellow turmeric.  The main health benefits of polyphenols come from their antioxidant and anti-inflammatory effects. (4)

As antioxidants, polyphenols fight free radicals (unstable molecules) that can damage cells and cause oxidative stress, a state that is associated with a wide range of health conditions including cancer, heart disease and autoimmune diseases. Free radicals are naturally occurring molecules that can be created by our bodies as byproducts of regular chemical reactions or as a response to situations such as aging, stress or inflammation.  Free radicals also enter our bodies through consequences of our environment (cigarette smoke, environmental pollutants, pesticides, drugs, industrial solvents, ultraviolet radiation and the types of foods that we eat).  (4)  Free radicals are unstable because they have lost one or more electrons; antioxidants give up some of their own electrons to neutralize free radicals. (5)

The anti-inflammatory properties of polyphenols include reducing inflammation that can lead to chronic conditions such as autoimmune diseases (lupus, rheumatoid arthritis, psoriasis, ankylosing spondylitis); cardiovascular diseases (heart disease, high blood pressure); certain cancers; gastrointestinal diseases (Crohn’s disease and inflammatory bowel disease); lung diseases (asthma, chronic obstructive pulmonary disease); metabolic diseases (Type-2 diabetes, chronic kidney disease, non-alcoholic fatty liver disease); neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease); and mental health conditions (depression,anxiety).  (4,6)

Sources of polyphenols are fruits, vegetables, herbs, spices, seeds, nuts and whole grains. Some examples of foods especially rich in polyphenols are berries, apples, carrots, broccoli, peppers, onions, spinach, oats and other whole grains, dark chocolate, flax seeds, sesame seeds, ginger, cumin, turmeric and green tea. (4)

The Academy of Nutrition and Dietetics in the United States recently recommended that people consume at least 400 to 600 milligrams daily of flavan-3-ols for cardiometabolic health.  Flavan-3-ol is one of the most common flavonoids eaten by humans. Cardiometabolic disorders include high blood pressure, high cholesterol, insulin resistance and abdominal obesity, all of which are risk factors for cardiovascular disease, type-2 diabetes, non-alcoholic fatty liver disease and chronic kidney disease.  It must be noted that this guideline is food-based and not a recommendation for taking flavonoid supplements as they can cause liver injury and gastrointestinal irritation especially in higher doses or if taken on an empty stomach.

This recommended amount of flavan-3-ols can be achieved by eating and drinking a wide variety of healthy plant-sourced foods every day. (7,8)

 

Examples of foods high in flavan-3-ols (8):

8 ounces of green tea (brewed) contains about 320 mg of flavan-3-ols

8 ounces of black tea (brewed) contains about 375 mg of flavan-3-ols

1 cup of raw blackberries contains about 64 mg of flavan-3-ols

½ cup Craisins contain about 34 mg flavan-3-ols

3 squares (18gm) of dark chocolate (70 to 85% cacao solids) contain about 20 mg of flavan-3-ols

5 ounces of red wine contain about 16 mg of flavan-3-ols

1 small apple contains about 15 mg of flavan-3-ols

1 tablespoon of cocoa powder contains about 13 mg of flavan-3-ols

1 cup of raw blueberries contains about 10 mg of flavan-3-ols

1 cup of raw raspberries contains about 9 mg of flavan-3-ols

1 cup of raw strawberries contains about 7 mg of flavan-3-ols

1 cup of raw green or red grapes contains about 6 mg of flavan-3-ols

 

Why do Bananas Have Such a Large Impact on Polyphenols?  (9)

The most common type of polyphenols present in the healthy foods that are often used as ingredients in smoothies are flavonoids.  Bananas naturally contain high levels of an enzyme called polyphenol oxidase (PPO) which, true to its name, can oxidize polyphenols, including flavonoids like flavan-3-ol, a reaction that breaks them down and destroys their health benefits.

When bananas are damaged physically by cutting, shredding or experiencing severe temperature changes, the PPO in the banana becomes exposed to oxygen and the oxidation of all available polyphenols is triggered.  In a mixture of food such as a blended smoothie, the polyphenols from the banana and from all other polyphenol-containing ingredients like berries and cocoa will be oxidized.  The end products of this reaction are quinones and eventually melanin, the same brown pigment involved in the tanning of our skin.

Perhaps not surprisingly, there is a reason why bananas react in this way.  These and other breakdown products have antimicrobial activity that can protect the fruit from pathogens.

 

What did the recent study on banana smoothies have to tell us?   (7)

Researchers from the University of California Davis conducted a small study, published in August 2023, in which they examined the impact of PPO on dietary flavonoids, specifically flavan-3-ols, in fresh home-prepared smoothies.

In a single-blinded cross-over study, eight healthy men consumed one of the following two drink preparations on one day, then returned at least six days later to consume the other drink;

1  A banana-based smoothie (high in PPO) which also contained a standardized cocoa extract (high in flavan-3-ols)

2  A berry-based smoothie (low in PPO) which also contained a standardized cocoa extract (high in flavan-3-ols)

The blood and urine of participants were analyzed 5 minutes after the consumption of each drink then hourly for the next hours for the presence of flavonoid metabolites.

As a control, a standardized cocoa extract in a capsule (high in flavan-3-ols; no PPO) was also consumed by all participants on a different day.  The blood and urine of the volunteers was analyzed in the same way as after their consumption of the drinks.

The time between all trials was a minimum of 6 days during which the participants followed their usual diet. This eliminated any potential carry over effect from previous drinks.

Results illustrated that drinking smoothies which contained bananas (high in PPO) resulted in 84% lower levels of flavonoid metabolites in the bodies of their consumers compared with results from the consumption of the berry-based smoothie.  Flavonoid metabolite blood levels after drinking the berry-containing smoothie (low-PPO) and after ingesting the flavan-3-ol-containing capsules (with no PPO) produced similarly high levels of flavonoid metabolites in the participants.

In addition, when researchers blended up the same banana-based cocoa smoothie and let it sit for one hour, they noted that flavonoid levels in the drink declined rapidly by 90% over the hour, with the flavonoid levels cut in half every 10 minutes.

The scientists also performed a follow-up trial in which eleven participants consumed one of the following two drink preparations on one day then returned at least six days later to consume the other drink;

1  A banana almond milk smoothie (high PPO and low flavonoid) AND a cocoa flavan-3-ol-containing almond milk smoothie (no bananas so low PPO and high flavonoid) which were each prepared separately.  Both drinks were then consumed simultaneously by alternating sips from each drink.  This means that the flavan-3-ols did not mix with the PPO until they were consumed.

2  A cocoa flavan-3-ol-containing smoothie without any banana .

Results again showed reduced plasma and urine levels of flavonoid metabolites after drinking the banana-containing drink, though the reduction was 37%.  This was a much lower drop than the results of the first trial in which the flavan-3-ols and PPO mixed together during the blending of the smoothie.    Researchers noted that in this case the PPO from the bananas became active in the stomach to negatively affect the bioavailability of the flavonoids to a smaller extent during their initial digestion.

 

Is there a way to influence the activity of the PPO in the bananas before they become part of a smoothie?  (9,10,11)

There are several methods used by the food industry to inhibit the PPO enzyme.

Heat:  Heat treatment such as blanching inactivates the enzymatic activity of PPO because enzymes are made up of proteins which are denatured by heat.  Simply immersing fruits or vegetables briefly in hot water (e.g., 80°C for potatoes) can impede PPO and reduce browning.

Heating fruits or vegetables up to 70° C for 2 minutes in the presence of Vitamin C (ascorbic acid) and citric acid has shown suppression of the activity of PPO by up to 80%.

The drawback of using heat is that it can affect the quality of the polyphenols and the structure of the food.

Acids: PPO has optimal activity at a pH of between 5 and 7 while a pH of below 3 can inactivate PPO.

Lemon juice contains both Vitamin C (ascorbic acid) and citric acid, and both will increase acidity (lower pH).  Use of lemon juice alone to lower pH has been shown to cut PPO enzyme activity by about 13%.

Pure pineapple juice added to bananas shows even more ability to inhibit PPO.  Moreover, when its acidic components, malic acid and citric acid, were separated from the whole juice, banana PPO was suppressed by almost 100%.

Chelating Agents:  PPO is a copper-containing enzyme.  Citric acid also acts as a chelating agent that can suppress PPO activity by attaching to the PPO structure.

Antioxidants:  Antioxidants including ascorbic acid bind to intermediates in the oxidation reaction to obstruct its effects.

Competitors for substrates needed in the reaction:  Plant extracts such as onion solution, pineapple juice, wine and lemon juice can competitively inhibit some of the steps involved in PPO-induced oxidation.

 

Adventures in our Kitchen “Lab”:

For a day or two we experimented with various methods which might affect polyphenol oxidation.

We tried heating bananas in either a small amount of pineapple juice or in water with some lemon juice added to just below the boiling point for 2 minutes.  This resulted in less browning although some did occur and the banana became very soft.  It was also difficult to accurately know what temperature we were at without some type of food thermometer.  On the upside, the resulting smoothie was very tasty, probably due to a bit of extra sweetness from the pineapple juice.

Setting heat aside, we decided to combine a couple of the other promising methods.  We placed a banana in a sandwich bag with about 2 teaspoons of lemon juice and ¼ cup of pineapple juice and made sure that all parts of the banana were covered in the solution.  This was set aside for observation and no browning occurred at all even after two hours of the bananas sitting in the juices.

So, we have made up a few bags containing a banana in the lemon juice/pineapple juice mixture and stored them in the freezer.  When we make our smoothie we take out the bagged banana and add it into the blender. The treated bananas are still creamy white at this point.

This method works for us.  It is not as involved or messy as the heating method, takes less time, and lemon juice and pineapple juice are relatively cheap. If the addition of the two juices alters the taste at all, it is just a slight boost in sweetness.

Admittedly this is not great science.  However, we don’t mind taking these simple steps in the hope that more of the polyphenols in our smoothies will remain available to us.

Additionally, we try not to let the prepared smoothie sit around too long before drinking it.

 

Other thoughts:

The study performed at the University of California Davis study only tested the effect of banana PPO on the flavonoids from cocoa powder.  Berries may present a different outcome but we have no way of knowing that.  More testing needs to be done.

We also don’t know to what extent the oxidation reaction is being inhibited by our lemon juice/pineapple treatment.  We can observe that browning did not occur over the two hours that we let it sit but it might very well start up again during the blending process and continue while in the stomach.  In spite of these unknowns, we still feel that it is worthwhile to take this extra step in an attempt to optimize the amount of flavonoids in our morning smoothie.  It would be a shame to waste them if they are there.

 

Conclusions:

If you don’t mind the consistency of a banana-free smoothie you can optimize your intake of flavonoids from your smoothie by leaving bananas out of the mix.

However, if you love the richness of a banana smoothie, you have a decision to make.  You can choose to add a couple of easily available ingredients to your bananas to deter PPO oxidation at least partially, or you can decide to leave the banana in your smoothie and obtain your polyphenols from other sources.

In our plant-based diet we eat many foods that supply us with flavonoids.  Even our 3 cups of tea a day would bring us well over the recommendation for flavonoid intake from the Academy of Nutrition and Dietetics, not to mention the various berries that we consume every day as well as our frequent use of cocoa powder and dark chocolate.  There is no known upper limit on the intake of polyphenols for health so there’s no harm in going over the recommendation.

For us the decision was easy.  We still enjoy our banana-containing smoothies and we’re happy to feel like we’re taking actions to retain those super healthy polyphenols in our breakfast smoothie as best we can.

   

SOURCES:

1  Chemjong, M., Yadav, N.K., Aarzoo, Sarkate, A., & Yaqoob, M. Bioactive Compounds, Types, Stability and Health Benefits.  2021.  Plant Archives, 21, 1863-1869.

2  Tijjani, H., Zangoma, M.H., Mohammed, Z.S., Obidola, S.M., Egbuna, C., Abdulai, S.I.  Polyphenols: Classifications, Biosynthesis and Bioactivities. In: Egbuna, C., Dable Tupas, G. (eds) Functional Foods and Nutraceuticals.2020.   Springer, Cham. https://doi.org/10.1007/978-3-030-42319-3_19

3  Samtiya M, Aluko RE, Dhewa T, Moreno-Rojas JM. Potential Health Benefits of Plant Food-Derived Bioactive Components: An Overview. Foods. 2021 Apr 12;10(4):839. Doi: 10.3390/foods10040839. PMID: 33921351; PMCID: PMC8068854.

4  https://health.clevelandclinic.org/polyphenols

5  https://med.libretexts.org/Courses/Metropolitan_State_University_of_Denver/Introduction_to_Nutrition_%28Diker%29/08%3A_Nutrients_Important_as_Antioxidants/8.01%3A_Generation_of_Free_Radicals_in_the_Body

6  https://my.clevelandclinic.org/health/symptoms/21660-inflammation

7  Ottaviani, J.I., Ensunsa, J.L., Fong, R.Y., Kimball, J., Medici, V. et al.  Impact of polyphenol oxidase on the bioavailability of flavan-3-ols in fruit smoothies: a controlled, single blinded, cross-over study.  Food Funct., 2023,14, 8217-8228.

8  Crowe-White, K.M., Evans, L.W., Kuhnle, G.G.C., Milenkovic, D., Stote, K. et al.  Flavan-3-ols and Cardiometabolic Health: a Guideline Recommendation by the Academy of Nutrition and Dietetics. Adv. Nutr., 2022. 13. 2070 —2083.

9  Moon, K.M., Kwon, E.B., Lee, B., Kim ,C.Y. Recent Trends in Controlling the Enzymatic Browning of Fruit and Vegetable Products. Molecules. 2020 Jun 15;25(12):2754. Doi:10.3390/molecules25122754. PMID: 32549214; PMCID: PMC7355983.

10  Sui, X., Meng, Z., Dong, T., Fan, X. and Wang, O. Enzymatic browning and polyphenol oxidase control strategies.  2023. Current Opinion in Biotechnology; 81: 102921.  ISSN 0958-1669. https://doi.org/10.1016/j.copbio.2023.102921.

11  Iqbal, A., Murtaza, A., Hu, W., Ahmad, I., Ahmed, A., Xu, X.  Activation and inactivation mechanisms of polyphenol oxidase during thermal and non-thermal methods of food processing.  Food and Bioproducts Processing. 2019.  117:170-182.  ISSN 0960-3085.  https://doi.org/10.1016/j.fbp.2019.07.006.