Physical and Rheological Properties of Pomegranate Juices and Concentrates

Abstract

The present work was designed to extract the pomegranate juices of Egyptian varieties Manfalouty and wonderful and to evaluate physical and thermal Properties of their concentrates obtained by rotary evaporator under vacuum. The results showed that Manfalouty juice contained higher °Brix, sugar/acid-ratio than those of Wonderful variety, while the later was characterized by higher content of total phenolics, flavonoids, tannins, anthocyanins and ascorbic acid as well as higher antioxidant activity. Rheological properties of the pomegranate concentrates showed Newtonian behaviour until a concentration of 52% and then shifted to non-Newtonian flow pattern with yield stress and thixotropic content at higher concentration (68% to 78%). Apparent viscosity values were reduced as temperature was increased from 5°C to 40°C. The reduction in viscosity was more remarkable for high °Brix-Concentrates (68% -78%) rather than for low or medium °Brix-Concentrates. Values of activation energy of flow ranged between 13.72 to 79.84 kj/mol. °k, where the higher Ea -values were related to concentrates with high °Brix-values. Rate of viscosity increase was slowly until 68 °Brix, and then it changed almost vertically at higher concentration. Therefore, concentration of 68 °Brix could considered as the inflection point in viscous behaviour of pomegranate juice concentrates. Freezing curves of 72% pomegranate juice concentrates showed neither super-cooling temperature nor starting freezing temperature. The concentrates started to freeze as an eutectic mixture at temperature range of -22 to-24°C. Specific heat values of pomegranate concentrate (72%) was measured to be 2.675 kj/kg. °C at 30°C and it was increased to 4.083 kj/ kg. °C as the temperature of the concentrates was increased to 60°C. The obtained data are important for designing Pumping, handling and thermal processing juices of Egyptian pomegranate fruits.