Abstract
Polyphenolic compounds, plant secondary metabolites essential for plant survival, are known for their high antioxidant and anti-inflammatory activity. In addition, several polyphenols, such as phloretin, also have potential antiviral effects, making these compounds potential ingredients of biofunctional foods. A promising source for the extraction of phloretin is a by-product of apple production—apple tree leaves. Focusing on green technologies, the first aim of the present study was to optimize the direct ultrasound-assisted extraction conditions to gain the maximum yield of phloretin from air-dried apple leaves. For the optimization of process parameters, we applied the response surface method with Box–Behnken design. The optimal extraction conditions were extraction time 14.4 min, sonication amplitude 10% and 10 g of sample per 100 mL solvent (70% ethanol, w/w). Using these conditions, we assessed the content of individual and total polyphenolic compounds along with antioxidant activity in the leaves of different autumn and winter apple cultivars grown in Estonia. The analyses were carried out with chromatographic (HPLC-DAD-MS/MS) and spectrophotometric methods. The phloretin concentration ranged from 292 to 726 µg/g and antioxidant activity from 6.06 to 11.42 mg GA eq./g, these being the highest in the local winter cultivars ‘Paide taliõun’ and ‘Tellissaare’, respectively.
Highlights
There are over 8000 polyphenolic compounds found in plant-derived material [1]
Ultrasound-assisted extraction (UAE) was selected for the extraction of phloretin from apple leaves, as this technique combined with the cooling of the sample allows one to conduct extractions at relatively low temperatures, which is favorable to enhance the extraction yield of heat-sensitive components
The minimum and maximum levels of these factors for the extraction of phenolic compounds from leaves were established during the preliminary experiments, so we did not expand the design space and used the Box–Behnken design for the optimization of these factors for the ultrasound-assisted extraction (UAE) process
Summary
There are over 8000 polyphenolic compounds found in plant-derived material [1]. The polyphenolic compounds are known for their high antioxidant and anti-inflammatory activity, which favorably affects health risks for metabolic disorders [2]. Natural polyphenols have been used in the form of infusions and extracts for treating ailments of digestive, vascular, urinary, and respiratory systems, in dermatology and for anticancer therapy [5]. Their main role is considered to consist in counteracting oxidative stress at the cellular level [6]. The health effects of these compounds are dependent on their amount in daily intake and their bio-availability [2]
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