Microwaving broccoli sprouts may not affect phenolic levels

Three papers investigated microwaving plant material for phenolic compounds. The first was a 2020 review that compared industry techniques:

“Phenolic compounds are widely used as natural antioxidants and antimicrobial agents. They also exhibit antiallergenic, antiatherogenic, and anti-inflammatory activities.

Although the nature and properties of raw materials strongly influence extraction, all extraction processes share major parameters:

  • Solubility of phenolic compounds is higher in polar solvents such as water and ethanol or their mixtures;
  • Diffusion of compounds and mass transfer rates are enhanced by increased temperature; and
  • Longer extraction times allow for a more intimate and effective contact between solvent and matrix.

MAE [microwave-assisted extraction] is an efficient technique due to its ability to heat a matrix internally and externally without a thermal gradient. Phenolic compounds and ionic solutions strongly absorb microwave energy. Microwaves cause internal superheating of water molecules of a sample, promoting cellular disruption and enhancing the recovery of target compounds from the matrix.

MAE produces the highest total phenolic content, 227.63 mg GAE [gallic acid equivalent] /g dry basis.”

https://link.springer.com/article/10.1007%2Fs13197-020-04433-2 “Recent advances and comparisons of conventional and alternative extraction techniques of phenolic compounds” (not freely available)

I didn’t see any studies in the first paper that I could directly use. Either temperature was too high, or microwave power too low, or was something I’m not going to do, like substitute ethanol for water.

A second 2020 paper compared three industry techniques to extract phenolics from olive and wine post-processing plant material:

“In the case of olive pomace, TPC [total polyphenol content] improved by increasing the ethanol concentration from 20 to 50%, and temperature from 60 to 90°C. No significant improvement was observed when increasing time from 5 to 15 min.

The best extraction efficiency for olive pomace residues was provided by MAE.”

https://www.mdpi.com/2076-3921/9/11/1074/htm “Olive Mill and Winery Wastes as Viable Sources of Bioactive Compounds: A Study on Polyphenols Recovery”

The second study demonstrated that polyphenols weren’t harmed by temperatures up to 90°C. Microwave power was 500W per correspondence with the authors.

Microwave broccoli to increase sulforaphane levels demonstrated significant differences for 450W (LL) and 900W (HL) power settings in glucoraphanin and sulforaphane amounts when microwaved to the same temperatures. Compare sulforaphane amounts for LL60 and HL60 (60°C), annotated as E and F:

A third study from 2017 measured total phenolic contents, but primarily used indole-3-carbinol to probe food preparation methods:

“This study evaluated the effects of grinding and chopping with/without microwaving on the health-beneficial components, and antioxidant, anti-inflammation and anti-proliferation capacities of commercial kale and broccoli samples. The availability of indole-3-carbinol (I3C) and total phenolic contents [TPC] were evaluated.

Fifty grams and 100 g of commercial fresh kale and broccoli (including stem and leaves) samples were ground for 15 s with 200 mL of water in 5 different commercial available blenders. The ground vegetables were divided into two parts, and one part was microwaved at 700 watts for 30 s.

Availability of I3C was significantly enhanced with grinding as compared to chopping (below the limit of detection), suggesting the effect of particle size on food factor release. Particle sizes of ground kale and broccoli were 10 times smaller than chopped counterparts. Grinding [in broccoli] not only resulted in difference of particle size, but might also break plant cell walls and release enzymes such as myrosinase, which might have hydrolyzed glucobrassicin and caused a greater releasable I3C level.

Significant difference was observed in I3C availability with or without microwaving. Microwaving significantly elevated the extractable amount of I3C from broccoli regardless of the blenders used. For instance, availability of I3C in broccoli was increased by 3.1, 9.1 and 1.9 folds respectively using blenders 1, 2 and 5 with microwaving as compared to their unmicrowaved counterparts.

TPC from blended broccoli samples ranged from 0.28-0.47 mg gallic acid g-1 of fresh weight. This range was lower than a reported mean of 0.99 mg gallic acid g-1 of fresh broccoli [in another study], suggesting different cultivars, locations (USA versus France) and extraction methods (water extraction versus 70% acetone extraction) might affect releasable TPC from broccoli.”

https://www.sciencedirect.com/science/article/abs/pii/S0308814617318484 “Home food preparation techniques impacted the availability of natural antioxidants and bioactivities in kale and broccoli” (not freely available)

1. Funny that I found this third paper in a PubMed “microwave phenolic broccoli” search, but not in any Part 2 of Do broccoli sprouts treat migraines? I3C combination searches. A plain “I3C” search term like how I search PubMed weekly on “sulforaphane” would have found it.

2. Don’t understand why blenders 1-5 makes and models weren’t stated in the study. Using blender 1 made a significant difference in TPC in the above graphic, but that was the effect. What could have been the cause? Aren’t researchers obligated to provide such explanations?

And why didn’t the study text support the graphic and address all TPC results with microwaved broccoli? Microwaving produced neither significant TPC differences among blenders 2-5 broccoli samples, nor in any of the kale samples.

3. Also don’t understand why these researchers didn’t microwave chopped broccoli samples and measure them for TPC and I3C. Maybe that wouldn’t have produced anything for TPC if phenolics aren’t produced from the myrosinase hydrolysis chain of events. But I3C is a myrosinase hydrolysis product. Testing microwaved chopped samples for I3C may have changed the above bolded statement to:

“Microwaving significantly elevated the extractable amount of I3C from broccoli regardless of the blenders used. [food preparation method.]

4. One broccoli treatment was blending 100 grams broccoli in 200 ml water, halving the purée, then microwaving half on 700W power for 30 seconds. The study didn’t say what temperature was achieved, but it was probably < 60°C because that’s similar to what I do. Twice every day I microwave an average 65.5 grams of 3-day-old broccoli sprouts in 100 ml water on 1000W full power for 35 seconds to achieve up to but not exceeding 60°C.

5. My son encouraged me to try blending microwaved broccoli sprouts this summer. I stopped after two weeks as I consistently had trouble swallowing them.

6. The study design required microwaving broccoli sprouts after blending. I don’t think people would do it in this order at home.

It would be too messy to scrape a broccoli sprout purée out of a blender and into a microwavable dish, while maintaining a desired water volume for microwaving. The reverse order is easier – measure the desired water volume into the broccoli sprouts dish, microwave, then plop microwaved broccoli spouts into a blender.

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