Impact of different disruptive pre-treatments on Lactiplantibacillus plantarum growth and antioxidant properties in broccoli stalks (Brassica oleracea var. italica).

In this study, we tried to assess the effects of four different pre-treatments and subsequent fermentation on broccoli stalks, which are a commonly discarded portion of the broccoli flower. The aim was to observe how antioxidant compounds content and antioxidant activity, as well as the probiotic properties of the matrix evaluated through microbial growth, were affected by the pre-treatments and fermentation. The perspective of reconsidering broccoli stalks as a high valued by-product might have an industrial application, reducing food waste and increasing the utility of a vegetable portion that would otherwise be disposed. Broccoli stalks, which can at times be used in households, constitute a big volume of organic waste coming from companies that transform the raw and whole broccoli into frozen ready-to-cook products or fresh ready-to-eat items. The positive health effects that may come with the reutilization of this product, given by the probiotic properties and antioxidant activity, can offer an even more interesting perspective. The analysis were carried out on fresh broccoli collected the same morning of the beginning of the laboratory procedures from a local supermarket. The first part of the study was to assess the best microwave pre-treatment, and in particular the most efficient combination of time and power, based on the final phenolic content and microbial growth. Power-time combinations were picked as suggested by Conesa et Al. (2016). Four powers were tested: 2W/g, 4W/g, 6W/g and 9W/g. Each one of those was then applied for four different amounts of time: 4, 5, 6 and 7 minutes, exception made for the 9W/g which was only tested for 4, 5 and 6minutes, since unwanted browning was observed after the treatment. The samples were tested for their content of soluble solids (refractometry), reducing sugars (DNS colorimetric assay) and total phenolic content. The total phenolic content was repeated after 24 hours of fermentation carried out by Lactobacillus plantarum (strain CECT:749), reactivated in MRS broth, together with the microbial growth assessed as ufc/g on MRS agar. Looking at the data obtained by all the series of experiments we finally selected the combination of 4W/g, 5 minutes. The four different combinations of pre-treatments were the following ones: pasteurization, autoclave pre-treatment (121°C, 5 minutes), freezing and pasteurization, pasteurization and microwave pre-treatment. All treatments were characterized for their effect on soluble solids (refractometry) and reducing sugars (DNS colorimetric assay). To evaluate the antioxidant activity, the total phenolic compounds content, the ABTS and DPPH scavenging activity and the total flavonoid compounds concentration have been evaluated. Antioxidant compounds, antioxidant activity (ABTS and DPPH assays) and microbial growth were evaluated at 0 hours (exception made for microbial growth, instead, the first day microbial count was operated on the inoculum), 24, 48, 72 and 96 hours.