Pectinmethylesterase affects the flow behavior of orange pulp

In the orange juice industry, pulp (ruptured juice sacs) is separated from the juice after extraction and pasteurized separately before blending back with juice or sold for other food applications. However, pulp is not always pasteurized immediately after extraction and the flow behavior is affected by endogenous pectinmethylesterase (PME). This is particularly important because of the high power required to pump orange pulp in industrial pasteurizers. This study characterized the effect of PME on the flow behavior of citrus pulp during storage at 4 °C using rotational rheometry and during pulp storage at 22 °C using capillary rheometry. PME activity was 0.011 ± 0.001 PEU for the pulp used in rotational rheometry and 0.030 ± 0.002 PEU for the pulp used in capillary rheometry. Rotational rheometry was used to assess the effect of PME on the Power Law parameters at shear rates ( $$ ) below those that cause slip. There were no significant differences among storage time on the flow behavior index (n). However, the consistency coefficient (K) increased with storage time. Significant differences were found after 12-hr storage increasing from 120 ± 30 Pa·sⁿ to 160 ± 15 Pa·sⁿ (16.7%) after 24 hr. Capillary rheomtery was used to assess the effect of PME on the pressure drop in a flow system. Significant differences were found at or after 12-hr storage. The mean pressure drop increased by 34% after 24 hr, for a flow rate of 50 × 10^-6 m³ /s (0.8 GPM) in an 8.9-m long (29.2 ft), 0.022-m (1-in) diameter pipe. PRACTICAL APPLICATION: Design and optimization of processing equipment and industrial handling systems of orange pulp require detailed knowledge of its rheological (flow) properties. Citrus pulp is rich in pectinmethylesterase, an enzyme that causes thickening of the product when not inactivated during pasteurization. Understanding how fast PME affects the flow properties of orange pulp is important for citrus processors to decide how long pulp can be stored before pasteurizing it.