|Khanal, Ramesh -|
|Howard, Luke -|
Submitted to: Journal of Food Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 27, 2009
Publication Date: August 15, 2009
Citation: Khanal, R.C., Howard, L.R., Prior, R.L. 2009. Procyanidin content of grape seed and pomace, and total anthocyanin content of grape pomace as affected by extrusion processing. Journal of Food Science. 74(6):H174-H182. Interpretive Summary: By-products of grape juice processing, namely grape seed and pomace (i.e., the material remaining after pressing the juice our of grapes), are a rich source of procyanidins, compounds which may afford protection against coronary heart disease, type II diabetes, and obesity. However, procyanidins are generally large molecules and as such are not readily absorbed in the gastrointestinal tract. This study was undertaken to develop methods whereby these large molecules could be broken down into forms that are more easily absorbed. Extrusion was selected as a method to study wherein the food material is forced through a small die under high pressure and temperature. Optimal extrusion conditions (i.e., temperature and pressure) for degradation of the large molecules were identified. Extrusion of both grape seed and pomace was found to increase the amount of small molecules and to decrease the large. Extrusion processing can be effectively used to increase the smaller more absorbable compounds in grape seed and grape pomace. This process may be beneficial in converting these by-products into useful dietary components.
Technical Abstract: Grape juice processing by-products, namely grape seed and pomace, are a rich source of procyanidins, compounds that may afford protection against coronary heart disease, type II diabetes, and obesity. This study was undertaken to identify optimal extrusion conditions to enhance the contents of monomers and dimers at the expense of large molecular weight procyanidin oligomers and polymers in grape seed and pomace. Extrusion variables, temperature (160, 170, and 180 deg C in grape seed, and 160, 170, 180, and 190 deg C in pomace) and screw speed (100, 150, and 200 rpm in both) were tested using mixtures of grape seed as well as pomace with decorticated white sorghum flour at a ratio of 30:70 and moisture content of 45%. Samples of grape seed and pomace were analyzed for procyanidin composition before and after extrusion, and total anthocyanins were determined in pomace. Additionally, chromatograms from diol and normal phase high performance liquid chromatography were compared for the separation of procyanidins. Extrusion of both grape seed and pomace increased the biologically important monomer and dimer contents considerably across all temperature and screw speeds. Highest monomer content resulted when extruded at a temperature of 170 deg C and screw speed of 200 rpm, which were 120 and 80% higher than the unextruded grape seed and pomace, respectively. Increases in monomer and dimer contents were apparently the result of reduced polymer contents, which declined by 27-54%, or enhanced extraction facilitated by disruption of the food matrix during extrusion. While diol phase usually produced a single peak for each of the procyanidins up to decamers and the polymer, normal phase produced multiple peaks during early stages of HPLC separation up to pentamers, but the peaks beyond hexamers were practically nonexistent. Extrusion processing reduced total anthocyanins in pomace by 18 to 53%. Extrusion processing can be effectively used to increase procyanidin monomer and dimer contents in grape seed and pomace.