Modeling of drying kinetics of the non-pomace residue of red grape (V-labrusca L.) juices: Effect on the microstructure and bioactive anthocyanins

The clarification of grape juices produces a colored non-pomace residue that is continuously discarded. This study evaluated the drying kinetics of the non-pomace residue of organic and conventional grape juices (V. labrusca L.) and assessed the effect of drying process on the anthocyanins content and microstructure of this residue. Residue samples were subject to convective drying and the process kinetics was modeled using four semi-empirical models. The Page model was selected as the best-fit model obtaining the lowest chi-square values ((2) < 0.000733). The effective diffusivity values ranged from 9.08 x 10(-4) to 1.52 x 10(-3) m(2)/s and the activation energies values were 23.06 and 20.89 kJ/mol for organic and conventional residues, respectively. The drying rate was augmented by increasing temperature with maximum values at 65 degrees C, for which the micrographs revealed the formation of smaller particles in samples when compared with drying at 45 and 55 degrees C. The conventional residue showed the highest levels of anthocyanins, which were consistently decreased with temperature as function of time, indicating that those are critical parameters for technological applications of this polyphenol-rich matrix. Practical applicationsThe clarification process is an important technological stage for processing of fruit juices. Grape juices are widely appreciated worldwide and the accumulation of viticulture by-products is a current concern in many producing regions. There is a global demand for reducing organic waste in food industries and alternatives for reutilization of plant materials emerge as a rewarding alternative to produce food ingredients. The non-pomace residue of grape juices is a solid and highly-colored material rich in polyphenols that is continuously discarded and currently unexplored. The study of its chemical properties as well as the use of drying processes to allow its technological application provides an important solution in order to achieve waste reduction in viticulture and discovering new matrices as sources of bioactive polyphenols.