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.).

Millicent G Managa,Dharini Sivakumar,Yasmina Sultanbawa

doi:10.3390/molecules25061326

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.

Highlights

Chinese cabbage (Brassica rapa L. subsp. chinensis) and Nightshade (Solanum retroflexum Dun) are popular vegetables among rural and peri-urban households in Southern Africa
Nightshade (Solanum retroflexum Dun.) and Chinese cabbage (Brassica rapa L. subsp. chinensis) leaves were obtained from Tshiombo irrigation scheme in Venda, Limpopo, South Africa
The antioxidant activity of the vegetables was maintained during freeze-drying, it is expensive to be implemented in the sub-Saharan rural regions, solar drying, which is regarded as a more hygienic way to dry leafy vegetables with shorter drying time, can be recommended to the farmers due to the moderate retention of antioxidant activity, chlorophyll carotenoid retention, protein, dietary fibre and reduced colour difference (∆E)

Summary

Introduction

Chinese cabbage (Brassica rapa L. subsp. chinensis) and Nightshade (Solanum retroflexum Dun) are popular vegetables among rural and peri-urban households in Southern Africa. Chinensis) and Nightshade (Solanum retroflexum Dun) are popular vegetables among rural and peri-urban households in Southern Africa. The Ca, Fe, glucosinolates and β-carotene contents were reported as 1020 g kg−1 FW, 26 g 36 kg−1 FW and 26 g 36 kg−1 DW respectively, and the kempferol content varied from 0.2002 to 0.25 g kg−1 on dry weight basis [1,2]. The raw leaves of Brassica rapa L. subsp. Chinensis contain kaempferol-sophoroside-O-hexoside, kaempferol-dihexoside, kaempferol-sophoroside, kaempferol hexoside, ferulic acid and myrectin-O-arabinoside. Sinigrin was reported as the highest glucosinolate. Molecules 2020, 25, 1326 in freshly harvested Chinese cabbage [3]. The Nightshade leaves contained higher levels of

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