This 2019 USDA study investigated representative broccoli cooking methods for their impact on kaempferol and quercetin levels:

“Understanding cooking effects on flavonoids is crucial to accurately estimate their daily intake and further investigate their health benefits. The purpose of this study was not to compare different conditions of each cooking methods, but to focus on retentions of individual flavonoids under common cooking conditions in the US:

• For boiling, we chose to use 85 seconds.
• A 5-minute steaming time was used.
• Microwave treatment was carried out in a 1200W microwave at full power for 1 minute.

Seven kaempferol (Km) glycosides and one quercetin (Qn) glycoside were identified and quantified in raw and cooked broccoli by HPLC-MS:

Boiling resulted in significant loss of all flavonoids, while steaming and microwaving led to minor losses or even increases of flavonoids.

Microwaving without water or with small amount of water tended to retain or increase total phenolics and/or flavonoids. When a large amount of water was added during microwaving, to some extent resembling boiling, total phenolics and/or flavonoids decreased.

Different agriculture practices may affect flavonoids’ existence and their interactions with other compounds, which in turn alter their sensitivity to heat treatments.

Retention of nutrients in cooked foods can be calculated as apparent retention (AR) based on dry form, or true retention (TR) based on fresh/wet form. TR represents actual consumption forms and also takes weight change after cooking into consideration.

The possible explanation for TR over 100% is that thermal processing may increase extractability and/or release from binding to other compounds as a result of matrix softening.”

https://www.cell.com/heliyon/fulltext/S2405-8440(19)30568-7 “Effects of domestic cooking on flavonoids in broccoli and calculation of retention factors”

The Material and methods section didn’t state that heated products’ temperatures were measured. So there wasn’t sufficient evidence for a solely thermal explanation of only microwaving achieving percentages over 100 per:

“The possible explanation for TR over 100% is that thermal processing..”

A more plausible explanation similar to Microwave broccoli to increase sulforaphane levels may account for microwaving’s increased percentages:

“Microwave treatment causes a sudden collapse of cell structure due to the increase in osmotic pressure difference over vacuole membrane.

We didn’t expect this result, and think microwave irradiation might help to release more conjugated forms of glucosinolates and then get hydrolyzed by released myrosinase.”

I immerse 3-day-old broccoli sprouts in 100 ml distilled water, then microwave them on 1000W full power for 35 seconds to ≤ 60°C (140°F). After microwaving I transfer broccoli sprouts to a strainer, and allow further myrosinase hydrolization of glucoraphanin and other glucosinolates into sulforaphane and other healthy compounds.

Myrosinase deactivation above 60°C wasn’t a consideration, since boiling, steaming, and a 1200W microwave on full power for one minute may have produced temperatures above 60°C. I’ll guess that an active enzyme wasn’t a requirement for flavonoid contents of broccoli purchased in a Beltsville, Maryland, grocery store.

The microwave tests used:

“Broccoli florets (150 g) were put in a microwave safe bowl with a 1 tablespoon [15 ml] of water.”

I use:

• A lesser weight of 3-day-old broccoli sprouts;
• A greater volume of distilled water;
• A less powerful microwave operated on full power for a lesser duration.

Before microwaving, I would expect a estimated 134 mg total flavonoids from eating 3-day old broccoli sprouts every day. This study’s findings lead me to expect that current practices with microwaving would improve flavonoid levels.