by Richard Aiken MD PhD (Twitter @rcaiken)
There has been some concern as to the effect of high-speed mechanical blending on the nutrient value of fruits and vegetables. The main concern is that rupture of the cell walls and organelles within the plant cell releases nutrients but vigorously exposes them to atmospheric oxygen with potentially deactivating oxidation reactions.
There is a significant amount of oxidation that occurs during blending bananas. The oxidation reaction is slowed somewhat by blending at slower speeds but even then significant oxidation occurs. Reduction of the temperature, an increase in acidity and particularly the chemical influence of ascorbic acid apparently stops the catalysis of DOPA by PPO and therefore its oxidation.
Although this experiment was specifically performed on a fruit with the major phenolic component DOPA, the results might be extended to other phenolic-containing plants. If so, it is recommended prior to general blending of plants, that a cold water solution containing a fruit with a high ascorbic acid content and low pH (lemons, oranges, limes as examples) be blended at high speed. Subsequently, general blending can be achieved with other additional plant foods at high speed minimizing oxidation.
Blender mechanical properties
There are two primary physical processes that work to mechanically break down the cell wall of plants, releasing nutrients: shear forces and cavitation.
Shear forces are created by the high-speed impact of the food with the blender blades. This includes direct cutting by the blade itself as well as shearing by application of high kinetic energy of the particulate matter moving through surrounding medium and striking other particles and the container.
Cavitation is caused by the Bernoulli effect – the same principle behind air flight – planes and helicopters and why boats can sail faster against the wind than with the wind. The speed of the blades in fluid cause a decrease in pressure above the blades equal to the vapor pressure of the fluid, similar to boiling. Bubbles form on the blades (assuming a fluid component), are flung away and implode causing very powerful shockwaves that further break down even the smallest of remaining particles.
The enzymes in the class PPO appear to reside in the plastids of all plants and are released when the plastid cell membrane is disrupted. PPO is thought to play an important role in the resistance of plants to microbial and viral infections and to adverse climatic conditions.
Phenolic compounds are responsible for the color of many plants and impart taste and flavor. They are important antioxidants.
In the presence of oxygen from air, the enzyme catalyzes the first steps in the biochemical conversion of phenolics to produce quinones, which undergo further polymerization to yield dark, insoluble polymers referred to as melanin. This is the same melanin that determines darkness of human skin and hair. In plants, melanin forms barriers and has antimicrobial properties that prevent the spread of infection in plant tissues.
Note enzymatic browning is considered desirable for the color and taste of tea, coffee and chocolate.
Phenolic substances in plants
There are many phenolic (or polyphenolic) compounds in fruits and vegetables. Epidemiological studies and associated meta-analyses strongly suggest that long term consumption of diets rich in plant polyphenols offer protection against development of cancers, cardiovascular diseases, diabetes, osteoporosis and neurodegenerative diseases.
Polyphenols can be divided into many different subcategories, such as anthocyans and flavonoids. Flavonoids are formed in plants from the aromatic amino acids phenylalanine and tyrosine. Tyrosine also synthesizes DOPA (3,4-dihydroxyphenethylamine) that forms dopamine.
Many plants synthesize dopamine to varying degrees. The highest concentrations have been observed in bananas, levels of 40 to 50 parts per million by weight.
Effect of acidity
The optimum pH for PPO activity has been shown to be 7 (dopamine substrate). However, the enzyme displays high activity between pH 6.5–7.5 and the activity rapidly decreases at more acidic pH values.
Effect of temperature on PPO stability
Heating at 60 degrees for 30 minutes reduces the enzymatic activity by 50%; heating at 90 degrees C completely destroys the enzyme. The optimum temperature for maximum activity is 30 degrees C (86 degrees F).
Chemical Inhibition of PPO
It has been shown that complete inhibition of PPO activity is found with as low as 0.8 mM ascorbic acid. Ascorbic acid, also known as vitamin C, acts as an antioxidant because it reduces the initial quinone formed by the enzyme to the original diphenol.
Citric acid also can inhibit PPO activity, although not as strongly as ascorbic acid. Citric acid exists in much greater than trace amounts in a variety of fruits and vegetables, most notably citrus fruits. Lemons and limes have particularly high concentrations of the acid; it can constitute as much as 8% of the dry weight of these fruits. The concentrations of citric acid in citrus fruits range from 0.005 mol/L for oranges and grapefruits to 0.30 mol/L in lemons and limes.
Reaction catalyzed by PPO in Bananas
Dopamine has been reported as the major natural occurring substrate in banana pulp and the fastest and most important reactant in the production of melanin (darkening).
Measurement of PPO Activity
PPO activity was determined by visualization of browning on a scale 0 – 5, where 5 is darkest noted and 0 is no noted darkening.
Organic bananas (PLU-94011) at ripening stage 5 (yellow peel with green tip) were used for this study.
Direct blending high-speed one minute
The first trial was blending three bananas directly in the Vitamix blender, first at slower speeds, then when mixed, at high speeds for 60 seconds. A significant vortex formed.
The result is shown here.
Note this picture was taken within 15 seconds of the end of the blending. Already a browning is seen. I will assign a darkness scale of 4 to this, where 5 is the darkest of any of the trials at prolonged time scales.
Blending with water shield low-speed short time
The next trial used two bananas with a water shield (room temperature). It was attempted to keep the bananas under water during the blending and the vortex was mechanically disturbed. The mixture was blended for about 30 seconds on an intermediate to low setting.
The result, just after blending, is shown below on the left, compared to the first trial, now after about 15 minutes.
I shall assign a darkening scale of 2 to this mixture.
After about a half hour, the two trials have the following appearance.
The first trial remains at a score of 4 while the second trial has darkened to a 3.
High-speed blending at cold temperature and with lime juice
The juice of a single lime was added to ice cold water. Lime was chosen as the pH of lime juice is quite low (2.0 – 2.4) and the ascorbic acid content is high. Bananas were then introduced. The mixture was then blended at high speed for about 60 seconds. The result, appearing on the far left in the picture below indicates a “0” on the darkness scale.
The first trial is in the middle and has reached a score of “5”, while trial 2 is a “4” after about an hour and a half.
Further high-speed blending with ice water and lime
The last trial was the same as the third except the mixture was further subjected to an additional 90 seconds of high speed blending (for a total of 150 seconds). This trial appears second from the left in the picture below. The third trial has now begun to separate after about a half hour but there is negligible browning.
Taste and flavor
Trial #1’s taste was bland; also a scum formed on the top of the glass. Trial #2 tasted much better initially but lost taste with time.
Trials #3 and #4 were far superior – strong banana taste but the citrus was evident and tangy. This remained the case after several hours.
The browning (oxidation) results are summarized on this table:
|Elapsed time after blending, minutes||0||30||90|
|Trial type||Darkening score|
|1. high-speed blending, 60 sec||4||4||5|
|2. low-speed blending under water, 20 sec||2||3||4|
|3. high-speed cold water blending with lime, 60 sec||0||0||1|
|4. prolonged high-speed blending with lime, 150 sec||0||0||1|
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