Atomization of fruit juice with fibres as drying aid : nozzle : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Chemical and Process Engineering at Massey University, Palmerston North, New Zealand. EMBARGOED until 1 July 2021.
Spray drying of fruit juices is desirable as it produces dry powders which extend the shelf-
life, reduce storage and transport costs, and produce a free-flowing powder which makes it
easier to blend as an ingredient. Commercially, maltodextrin is added to the juices as a drying aid to increase the efficiency of the spray drying process. In this project, pomace fibres were investigated as an alternative drying aid. The main attraction of pomace fibres as a drying aid is the pomace fibres are originally derived from the fruits itself. This study
explores the rheological behaviour of juice-fibre suspensions inside the spraying device, specifically the nozzle, to ensure high efficiency powder production by enabling atomization of the mixtures. This study also sought to determine the type of
nozzle and operating conditions for efficient atomization of the juice-fibre suspensions inside the spray dryer. Flow-fields inside a nozzle consists of shear and extensional flows. Previous studies on the shear rheology of fibre suspensions revealed the addition of fibres creates a non-Newtonian
shear-thinning liquid. The studies on extensional rheology of fibre suspensions, however, were absent. It is widely known the atomization of liquids with both shear and extensional resistances, require additional energy for atomization when compared to Newtonian liquids or viscous non-Newtonian liquids of a similar intrinsic viscosity. In this work, four types of fibres with different aspect ratios were investigated. Some of the significant and notable methods achieved during the study include 1) the use of capillary viscometer to examine the shear rheology of fibre suspensions at shear rates up to 20 000 s⁻¹, which represents the calculated shear rate experienced during atomization, 2) the building of a portable capillary breakup extensional rheometer to accurately characterize the extensional rheology of the fibre suspensions at high extensional strain rates and 3) the use of flash photography technique to capture the atomization patterns. Overall, the methodology used in this thesis provides a systemic means of investigating the suitability of an atomization method for the spraying of a non-Newtonian fluid with extensional rheological properties.--Shortened abstract