Comparative Phytonutrient Analysis of Broccoli By-Products: The Potentials for Broccoli By-Product Utilization

Abstract

The phytonutrient concentrations of broccoli (Brassica oleracea var. italica) florets, stems, and leaves were compared to evaluate the value of stem and leaf by-products as a source of valuable nutrients. Primary metabolites, including amino acids, organic acids, and sugars, as well as glucosinolates, carotenoids, chlorophylls, vitamins E and K, essential mineral elements, total phenolic content, antioxidant activity, and expression of glucosinolate biosynthesis and hydrolysis genes were quantified from the different broccoli tissues. Broccoli florets had higher concentrations of amino acids, glucoraphanin, and neoglucobrassicin compared to other tissues, whereas leaves were higher in carotenoids, chlorophylls, vitamins E and K, total phenolic content, and antioxidant activity. Leaves were also good sources of calcium and manganese compared to other tissues. Stems had the lowest nitrile formation from glucosinolate. Each tissue exhibited specific core gene expression profiles supporting glucosinolate metabolism, with different gene homologs expressed in florets, stems, and leaves, which suggests that tissue-specific pathways function to support primary and secondary metabolic pathways in broccoli. This comprehensive nutrient and bioactive compound profile represents a useful resource for the evaluation of broccoli by-product utilization in the human diet, and as feedstocks for bioactive compounds for industry.

Keywords: antioxidant activity; broccoli; by-products; carotenoid; chlorophyll; glucosinolates; mineral; phenolic compounds; vitamins E and K.

Figure 1

Broccoli (“Gypsy” cultivar, grown at the West Virginia Agronomy farm 2016 with conventional practice) individual tissue biomass (fresh weight) percentage to total biomass. The data collected from seven individual mature broccoli plants. Average of total biomass was 776 g per plant.

Figure 2

Primary metabolites in different tissues of broccoli (amino acids, fatty acids, organic acids, and sugars) in heatmap (A) and the top nine selected biomarkers based on variable importance in projections (B); F, L, and S indicate floret, leaf, and stem, respectively. The number after each tissue code (F, L, or S) indicates biological replication.

Figure 3

Nitrile formation is shown as the relative ratio of nitrile to the total hydrolysis product formed (sum of isothiocyanates and nitriles). Data are presented as the mean concentration ± standard deviation (n = 3). Different letters above the error bar indicate significant differences between different tissues within the same glucosinolate substrate by Student’s t-test at p ≤ 0.05. One unit was defined as 1 μmol of hydrolysis products of glucosinolates released per min.

Figure 4

Top 15 significantly expressed genes (glucosinolate biosynthesis, myrosinase, cofactors genes) among the different broccoli tissues. The gene expression values were normalized based on mean value of floret and then transformed to logarithmic scale.

Figure 5

Oil soluble vitamin concentrations in different tissues of broccoli. Tocopherol (vitamin E) (A) and phylloquinone (vitamin K1) (B); Different letters above the error bars indicate significant differences among the different broccoli tissues by LSD at p ≤ 0.05.

Figure 6

Total phenolic content (gallic acid equivalent (GAE) mg/g DW) (A) and 2,2-diphenyl-1-picryl-hydrazyl-hydrate (DPPH) antioxidant activity (B) in different tissues of broccoli extract (12.5 mg DW/mL of 70% methanol). Vitamin C was used as positive control (125 µg/mL).

Figure 7

Representative image of various broccoli tissues for one biological replication. The black scale bar at the right upper corner of each picture is 10 cm. (A) Broccoli floret; (B) broccoli stem; (C) broccoli leaf.