Polymerization of lactose to polylactose by twin-screw extrusion

Abstract Text: Our objective is to create a value-added product from lactose. We successfully polymerized lactose into polylactose, a mixture of oligosaccharides via extrusion. Previously, we evaluated one extruder feed rate and two citric acid catalyst concentrations. We hypothesized that a lower residence time in the extruder would achieve similar fiber contents while reducing caramelization side-reactions. The objectives of this project were to further evaluate acid catalyst concentrations and extruder conditions on fiber yield. We hypothesized that concentrations above 2% citric acid would yield more fiber. We tested the hypotheses by extruding lactose with 2, 4 and 6% citric acid, and 20% glucose. Extruder feed rate was at 15 and 30kg/hour for all formulas. Product was extruded on a Buhler twin-screw 44mm extruder and the screw configuration, rpms and temperature profile were kept constant between runs. Process (temperature, motor torque and specific mechanical energy (SME)) and product responses (color, dietary fiber, degree of polymerization (DP), and residual lactose) were measured in response to the formula and process changes. Color and brown pigments were determined by HunterLab (10° observer and D65 illuminant) and spectrophotometry (420 nm) of an aqueous extrudate solution. Liquid chromatography (LC)-evaporative light scattering detector analysis was used to quantify dietary fiber by the AOAC integrated dietary fiber method (2009.01).   DP was determined by LC-MassSpectrometry with positive electrospray ionization. Residual lactose was quantified enzymatically using a commercial test kit. The higher feed rate resulted in lower SME and higher motor torque, indicating the material had higher viscosity. The higher feed rate resulted in less brown pigmentation, lower b-values, higher L-values and more residual lactose and glucose. These results indicate that at the lower feed rate more lactose and glucose were converted to caramelization products. Citric acid concentration effects were more pronounced at the lower feed rate with higher concentrations leading to more browning and higher b-values. Residual lactose decreased with higher citric acid concentrations. Dietary fiber profiles were similar between formulas with DP’s of generated oligosaccharides ranging from 3 – 5. In contrast to the higher feed rate, the HPLC profiles at the lower feed rate of 6% citric acid formulas had lower oligomer peak areas when compared to 2% citric acid.  This was not the case for the high feed rate. Overall, the higher feed rate resulted in more favorable processing parameters, less caramelization, and maintained dietary fiber concentrations.

Keywords: extrusion; lactose; oligosaccharides; dietary fiber