Abstract
Horseradish (Armoracia rusticana) leaves pomace, which contains high-value bioactive compounds, is the product resulting from pressing horseradish leaves for juice production. The aim of the current research was to investigate the effect of convective, microwave-vacuum and freeze-drying on the content of bioactive compounds in horseradish leaves pomace. Convective hot air-drying was performed at 40, 60 and 80 °C. The total phenolic content (TPC), total flavonoid content (TFC), total flavan-3-ol content, total phenolic acid content, total flavonol content, chlorophylls and total carotenoids, and antioxidant activity were determined by spectrophotometric methods. Individual profiles of phenols and organic acids are estimated by high-performance liquid chromatography (HPLC), but volatile compounds are estimated by gas chromatography (GC). Totally, 14 individual phenolic compounds, 8 organic acids, and 49 volatile compounds were analysed in the studied samples. The main phenolic compound identified in horseradish leaves pomace was rutin (3231 mg/100 g DW), among organic acids—quinic and malic acids, and volatile compounds—allyl isothiocyanate, 3-butenenitrile and benzyl alcohol. In the drying process, the content of some (total flavan-3-ols, total carotenoids content) compounds increased, but others (TPC, total organic acids content) decreased, and it was drying method-dependent. Freeze-drying caused the reduction of TPC by 29%, whereas convective drying by 53–59%. Fresh pomace contains such isothiocyanates as allyl isothiocyanate and butyl isothiocyanate, which were completely lost in the drying process. Freeze-drying allowed the best retention of various phenolic and volatile compounds in horseradish leaves pomace.
Highlights
Horseradish (Armoracia rusticana) leaves pomace is the product resulting from pressing horseradish leaves for juice production
total phenolic content (TPC) in fresh horseradish leaves pomace (HRLP) was 4539 mg GAE/100 g dry weight (DW) (Table 1), being higher than TPC in fresh horseradish leaves determined in our previous study
Compared to all drying methods studied in the current research, FD better preserved phenolic compounds, namely, FD caused the reduction by 29%, whereas convective drying by 53–59%
Summary
Horseradish (Armoracia rusticana) leaves pomace is the product resulting from pressing horseradish leaves for juice production. Pomace is considered to be a production waste, which gains a strong interest due to the environmental aspects related to the waste disposal. It is well documented that production waste, such as peels, seeds, and pomace, contains high-value bioactive. Convective drying is one of the most popular drying methods, which is based on the moisture removal by hot air. As a result of thermal effect, volatile compounds may be lost during air-drying [5]. Due to cell wall damage, some compounds may be released and become more extracted. As a result of oxidative reactions or glycosidic hydrolysis of new compounds, which are not present in fresh samples, may be formed [6]
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