Polyphenol Extraction from Humulus lupulus (Hop) Using a Neoteric Glycerol/L-Alanine Deep Eutectic Solvent: Optimisation, Kinetics and the Effect of Ultrasound-Assisted Pretreatment

Abstract

The investigation presented herein had as its scope the development of an integrated process for the efficient extraction of polyphenols from hop. For this purpose, a novel, natural deep eutectic solvent (DES) was synthesised, composed of glycerol and L-alanine, and the process was optimised by deploying a response surface methodology based on a Box-Behnken design. The variables considered were the DES/water proportion, the liquid-to-solid ratio and the stirring speed. Under the optimised conditions, the yield in total polyphenols achieved was 118.97 ± 8.27 mg gallic acid equivalents per g of dry mass. Ultrasonication, incorporated into the process as a pretreatment step, was shown to significantly change the kinetic pattern of polyphenol extraction and contributed to attaining higher yields only at 80 • C, whereas at lower temperatures a supressing effect was observed. Furthermore, increasing temperature was negatively correlated with the second-order extraction rates, evidencing a slow-down of the extraction rate at elevated temperatures.

A Green Extraction Process for Polyphenols from Elderberry (Sambucus nigra) Flowers Using Deep Eutectic Solvent and Ultrasound-Assisted Pretreatment

“…Recent evidence pointed out that UAE may not be very effective as a standalone method, but high extraction yields could be achieved by a combination of UAE and stirred-tank extraction [12]. The following studies concurred with these findings, yet the effect of ultrasonication as a pretreatment step was somewhat unclear, exhibiting dependence on temperature [13,14]. Thus, additional information is necessary to further clarify this issue and bring out the usefulness of ultrasonication in sample pretreatment.…”

“…The pretreatment was delivered as described elsewhere [13], with some minor modifications. An Elma D-78224 Singen HTW heated ultrasonic bath (Elma Schmidbauer GmbH, Singen, Germany), operated at a frequency of 50 Hz and a power of 550 W, was fed with 7.3 L deionized water to provide an acoustic energy density of 75.3 W L −1 .…”

“…Details of the experimental design employed have been described elsewhere [13]. Briefly, the experimental set-up was based on a Box-Behnken design with three central points.…”

“…The selection of an appropriate R D/A mol is important in the synthesis of DES because the molar proportion between HBD and HBA may critically affect DES extraction performance [13,14]. Earlier investigations outlined that DES composed of L-lactic acid (LA) and glycine (Gly) were not stable at R D/A mol ≤ 3 and tended to form plastic solid at room temperature [21].…”

Saffron Processing Wastes as a Bioresource of High-Value Added Compounds: Development of a Green Extraction Process for Polyphenol Recovery Using a Natural Deep Eutectic Solvent

“…The mode chosen was a Box-Behnken experimental design with three central points. Key extraction variables including the concentration of DES in aqueous mixtures (C DES ), the liquid-to-solid ratio (R L/S ) and the stirring speed (S S ) [15] were taken into account and termed X 1 , X 2 , and X 3 , respectively ( Table 1). Yield in total polyphenols (Y TP ) was the screening response and the three independent variables were coded between −1 (lower limit) and 1 (upper limit).…”