Effects of Different Drying Methods on Untargeted Phenolic Metabolites, and Antioxidant Activity in Chinese Cabbage (Brassica rapa L. subsp. chinensis) and Nightshade (Solanum retroflexum Dun.)

Abstract

Chinese cabbage (Brassica rapa L. subsp. chinensis) and Nightshade (Solanum retroflexum are popular traditional leafy vegetables consumed predominantly by rural Africans. Sun drying is adopted as a traditional method of postharvest preservation to store theses leaves during off seasons. The influence of different types of postharvest processing treatments, such as conventional oven drying, solar cabinet drying, sun drying and freeze drying, on the changes on colour properties and antioxidant components were investigated. Freeze-drying retained the ascorbic acid content, antioxidant activities, total chlorophyll content, green colour by reducing the colour difference (∆E). With regard to Chinese cabbage and Nightshade leaves, sun and microwave drying respectively had the most negative impact on all the identified phenolic compounds. The OPLS-DA and the UPLC–QTOF/MS and chemometric approach showed kaempferol-3-O-sophoroside, kaempferol-3-sophorotrioside-7-glucoside and hydroxyoctadecenedioic acid as the markers responsible for the separation of sun-dried samples from the other drying treatments in Chinese cabbage. Sinapoyl malate was not detected in sun-dried samples. Caffeoylmalic acid was identified as the marker compound to separate the other drying treatments from the microwave dried samples of Nightshade leaves. Trihydroxyoctadecadiene derivative and hydroxyoctadecanedioic acid were detected in microwaved samples. Due to the cost effectiveness, solar dryer cabinet treatment was recommended for drying both vegetables. The proximate analysis of solar dried functional powder of Chinese cabbage and Nightshade vegetables demonstrated higher contents of protein and dietary fibre.

Effect of Lactic Acid Fermentation on Color, Phenolic Compounds and Antioxidant Activity in African Nightshade

“…All measurements took place on day 3 after fermentation. Total color difference (∆E) was determined using a colorimeter (CR-400 Chroma Meter), according to Managa et al (2020) [ 1 ]. The L* (lightness), a* (greenness) b* (yellowness) color components determined the ∆ E of each composite sample using the following formula: (1) Fresh leaf samples’ color coordinate values are L 1 * , a 1 * , b 1 * and the samples from fermentation treatments with LAB strains represent L 2 * , a 2 * , and b 2 * …”

“…The irrigation scheme in Thulamela Municipality, Venda, Limpopo Province, harvested the nightshade leaves during the winter season in 2019, as described by Managa et al (2020) [1]. The leaves were detached from the stem, rinsed in tap water, disinfected (5 min) with NaOCl 5%, rinsed further with distilled water to remove the excess chlorine from the leaves, blanched in a water bath at 95 • C for 5 min and subsequently placed in screw cap bottles (75 g of leaves per bottle) to cool to 30 • C before adding 0.8% NaCl sterile solution.…”

“…Proximate analysis used standard methods, as reported by Managa et al (2020) [1] without any modifications. Leaf samples (100 g) in three replicate samples were used for proximate analysis without modifications.…”

“…is a popular food in Venda, Limpopo Province, South Africa. Nightshade leaves are a rich source of the minerals Ca (199 mg/100 g), Mg (92 mg/100 g) and Fe (7.2 mg/100 g), and the raw leaves are rich in rutin [1]. Generally, the consumption of nightshade leaves is in fresh form after cooking, but they have a limited shelf life due to their highly perishable nature.…”

Assessment of the Processing Effects of Fresh Solanum Anguivi Berries on Biochemical Contents and Functional Properties of Powder

“…and Managa, Sultanbawa, and Sivakumar (2020) using sun-drying on onion powder, fig and Solanum retroflexum Dun respectively.…”