The Effect of Frozen Storage Preparation Method on Sulforaphane Content in Kale

Abstract

BackgroundHigh amounts of glucosinolates are found in certain cruciferous vegetables, such as broccoli, cabbage, and kale. When these plants are damaged, the enzyme myrosinase hydrolyzes glucosinolates. One glucosinolate hydrolysis product, sulforaphane, has potential therapeutic applications for cancer and neurodegenerative disorders. However, further research regarding this chemical's stability under common food storage methods is needed.ObjectiveWe investigated the availability of sulforaphane in curly kale (Brassica oleracea var sabellica) under two frozen storage methods. Frozen storage methods were chosen to reflect non‐commercial, household kitchen practices to elucidate the effects of typical consumer habits on sulforaphane content in kale.MethodsFresh kale was procured from a grocery store in Philadelphia in whole form. Samples were roughly chopped to activate the hydrolysis of glucoraphanin to sulforaphane through the enzyme myrosinase. Kale was either frozen at −18°C in a domestic, upright freezer immediately after chopping, or was blanched in 90°C water for 10 seconds. Blanched kale was immediately cooled in an ice bath and dried with paper towels before storage in the freezer. Sulforaphane content of fresh, chopped kale was measured to use as a baseline reference. Frozen samples were measured 12 days after freezing. Ten gram samples were liquefied with 100 mL of acetone in a Waring blender and centrifuged in 50 mL conical vials for 10 minutes at 2800 rpm. A Silica Sep‐Pak cartridge was placed on a vacuum manifold and conditioned with 5 mL of acetone. The Silica Sep‐Pak cartridge was then loaded with 10 mL of the acetone supernatant. After vacuuming, the acetone sample solvent was discarded. The cartridge was then loaded with 10 mL methylene chloride solvent and vacuumed. The eluent was collected in a 15 mL glass test tube and evaporated to dryness in a hot water bath at 60°C under a stream of nitrogen. A boiling chip was added to the test tube to avoid bumping. The residue was then re‐suspended in 1 mL acetonitrile and transferred to a 2 mL high‐performance liquid chromatography (HPLC) auto‐sampler vial. The sample extract was analyzed with a UV/Vis detection system at 260 nm wavelength.ResultsSulforaphane concentration in the blanched, frozen sample increased by 489.8% (1.736 to 8.501 μg/g). Sulforaphane in the non‐blanched, frozen sample increased by 174.4% (1.736 to 3.027 μg/g). For additional reference, the average serving size of cooked kale is 85 g.ConclusionsThis experiment suggests that low‐temperature blanching increases the sulforaphane concentration in kale, likely through increased activation of myrosinase through tissue damage via moderate heat. It is possible that other glucosinolates found in cruciferous vegetables and their metabolites are similarly affected. Although a clinically significant intake of sulforaphane has not yet been established, those looking to increase sulforaphane yields from foods may benefit from including low‐temperature blanching. Further investigation is needed to determine at what time and temperature boiling or blanching has a negative effect on glucosinolate conversation or its derivative's stability in cruciferous vegetables and whether prolonged freezing moderates these effects. It would also be useful to evaluate the effects of low‐temperature, rapid blanching on other vitamins and minerals present in kale and cruciferous vegetables.