Design and Optimization of a Semicontinuous Hot-Cold Extraction of Polyphenols from Grape Pomace Jeana K. Monrad, †,‡ Keerthi Srinivas, § Luke R. Howard, † and Jerry W. King* ,§ † Department of Food Science, University of Arkansas, 2650 North Young Avenue, Fayetteville, Arkansas 72704, United States § Department of Chemical Engineering, University of Arkansas, 3202 Bell Engineering Center, Fayetteville, Arkansas 72701, United States ABSTRACT: Grape pomace contains appreciable amounts of polyphenolic compounds such as anthocyanins and procyanidins which can be recovered for use as food supplements. The extraction of these polyphenols from the pomace is usually accomplished at slightly elevated temperatures, frequently employing hydroethanolic solvents. Due to governmental regulations and the cost involved in using ethanol as a solvent, as well as the loss in polyphenolics due to thermal degradation, improved extraction techniques are required. In this study, a semicontinuous extraction apparatus employing only water was developed to maximize the recovery of anthocyanins and procyanidins from red grape pomace (Vitis vinifera). Water is preheated prior to its entry to the extraction cell containing the grape pomace sample, where it is allowed to then flow continuously through the unheated extraction vessel prior to its collection at ambient conditions. Extraction variables that impacted the polyphenolic recovery included pomace moisture content (crude or dried), sample mass, water flow rate, and extraction temperature. A response surface method was used to analyze the results from the extraction, and the optimal conditions were found to be 140 °C and 9 mL/min water flow rate. These conditions can produce an extract containing 130 mg/100 g DW of anthocyanins and 2077 mg/100 g DW of procyanidins. Higher yields of polyphenolics were observed using crude (wet) rather than dried pomace, hence avoiding the need to dry the pomace prior to extraction. The described semicontinuous extraction method using only water as the extraction solvent under subcritical conditions allowed the efficient extraction of polyphenols from red grape pomace without the attendant loss of polyphenolic content due to having to heat the extraction vessel prior to commencement of extraction. KEYWORDS: anthocyanins, grape pomace, hot-cold extraction, procyanidins, subcritical water ■ INTRODUCTION Flavonoids, such as anthocyanins and procyanidins, have been investigated for their purported health benefits and are extensively used in food, cosmetic, and nutraceutical indus- tries. 1 Such compounds have been traditionally extracted from natural products using organic solvents such as hexane, methanol, or acetone 2 as well as with hydroalcoholic solvents. 3 There have been a number of these reported studies on extracting such polyphenolic compounds from grape pomace obtained as a residue in grape and wine industry after juice removal. 3,4 Traditional extraction methods including liquid-liquid and solid-liquid extraction can involve long extraction times accompanied by organic solvent uptake into the remaining pomace. 5 Pressurized liquid extraction using water and ethanol or mixtures thereof have been investigated and have been shown to provide a rapid, selective, and benign method for the extraction of flavonoids from natural products. 6-9 Such studies have confirmed the efficient extraction of the targeted compounds at higher temperatures due to increased solubility of polyphenolics in the solvent and concomitant decrease in the solvent viscosity and dielectric constant. 6 These extraction processes involve the use of solvents at temperatures above their boiling point under pressure and are called subcritical fluids. 10,11 The optimal yield of the polyphenolics is obtained by adjusting the extraction temper- ature, whereas pressure has a negligible effect on the extraction efficiency. 12 Previous studies conducted in our research group used pressurized hydroethanolic solvent mixtures to extract flavonoids from grape pomace, and it was found that the maximum yield of anthocyanins and procyanidins occurred between 80 and 120 °C. 13-15 Such studies were performed by allowing solvent flow into a batch reactor containing the grape pomace followed by an increase in the temperature or pressure on the extraction vessel. However, it was observed that as the vessel was heated up to the higher extraction temperatures, the oxygen radical absorbance capacities (ORAC) were lower due to the thermal degradation of the glucoside linkages in the anthocyanins. 13 Such results were supported by other studies cited in the literature. 16-18 One such study 19 indicated that there was significant thermal degradation of quercetin diglucosides when extracted from red onions using pressurized water at 110 °C within the first 8 min of the extraction cycle. Similarly, in another study involving the extraction of silymarins from milk thistle, it was found that the thermal degradation of these compounds occurred during extraction and can be modeled using first order kinetics. 20 With such adverse effects, it can be seen that there is still a need for an improved extraction method that can reduce degradation of Received: February 9, 2012 Revised: April 28, 2012 Accepted: May 11, 2012 Published: May 11, 2012 Article pubs.acs.org/JAFC © 2012 American Chemical Society 5571 dx.doi.org/10.1021/jf300569w | J. Agric. Food Chem. 2012, 60, 5571-5582