Nucleation, growth and breakage phenomena in agitated wet granulation processes: a review

Texture and mouthfeel arising from the consumption of food and beverages are critical to consumer choice and acceptability. While the food structure design rules for many existing products have been well established, although not necessarily understood, the current drive to produce healthy consumer acceptable food and beverages is pushing products into a formulation space whereby these design rules no longer apply. Both subtle and large scale alterations to formulations can result in significant changes in texture and mouthfeel, even when measurable texture-related quantities such as rheology are the same. However, we are only able to predict sensations at the initial stages of consumption from knowledge of material properties of intact food. Research is now on going to develop strategies to capture the dynamic aspects of oral processing, including: from a sensory perspective, the recent development of Temporal Dominance Sensation; from a material science perspective, development of new in vitro techniques in thin film rheology and tribology as well as consideration of the multifaceted effect of saliva. While in vivo, ex vivo, imitative and empirical approaches to studying oral processing are very insightful, they either do not lend themselves to routine use or are too complex to be able to ascertain the mechanism for an observed behaviour or correlation with sensory. For these reasons, we consider that fundamental in vitro techniques are vital for rational design of food, provided they are designed appropriately to capture the important physics taking place during oral processing. We map the oral breakdown trajectory through 6 stages and suggest a dynamic multi-scale approach to capture underlying physics. The ultimate goal is to use fundamental insights and techniques to design new food and beverages that are healthy yet acceptable to consumers.

Oral processing, texture and mouthfeel: from rheology to tribology and beyond

Process control and scale-up of pharmaceutical wet granulation processes: a review.

A regime map of granule growth behavior is proposed based on granule deformation during collision and the granule liquid content measured as the maximum pore saturation. The granule deformability on collision is represented by a deformation number, which is a ratio of granule impact energy to the plastic energy absorbed per unit strain. Granule growth regimes such as steady growth, induction, nudeation, crumb, and slurry are defined. Ttus regime map qualitatively explains the variations in granulation behavior. Laboratory drum granulation experiments were used to test the regime map. Experiments were performed in a 0.3-m-dia. drum using three sizes of glass ballotini (19, 31, and 60 μm) with water andglycerol as liquid binders. Increasing granule yield stress by decreasing particle size and increasing binder viscosity caused the system to move from steady growth to induction behavior as predicted by the regime map. Preliminary validation with literature data was also encouraging. More work, however, is required to better quantify the boundaries bet\veen different growth regimes and to investigate the effect of process agitation intensity. Tfiis regime map has great potential to help design and control granulation systems, because it is based on properties of the powder/binder system that can be measured or estimated without performing any granulation tests.

Growth regime map for liquid-bound granules

The discrete modelling and understanding of the particle dynamics in fluidized bed apparatuses, mixers, mills and others are based on the knowledge about the physical properties of particles and their mechanical behaviour during slow, fast and repeated stressing. In this paper model parameters (modulus of elasticity, stiffness, yield pressure, restitution coefficient and strength) of spherical granules (γ-Al2O3, zeolites 4A and 13X, sodium benzoate) with different mechanical behaviour have been measured by single particle compression and impact tests. Starting with the elastic compression behaviour of granules as described by Hertz theory, a new contact model was developed to describe the force-displacement behaviour of elastic-plastic granules. The aim of this work is to understand the energy absorption during compression (slow stressing velocity of 0.02 mm/s) and impact (the impact velocity of 0.5–4.5 m/s) of granules. For all examined granules the estimated energy absorption during the impact is found to be far lower than that during compression. Moreover, the measured restitution coefficient is independent of the impact velocity in the examined range and independent of the load intensity by compression (i.e. maximum compressive load). In the case of repeated loading with a constant load amplitude, the granules show cyclic hardening with increasing restitution coefficient up to a certain saturation in the plastic deformation. A model was proposed to describe the increase of the contact stiffness with the number of cycles. When the load amplitude is subsequently increased, further plastic deformation takes place and the restitution coefficient strongly decreases.

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S.M. Iveson

Papers

Growth regime map for liquid-bound granules

Fundamental studies of granule consolidation Part 1: Effects of binder content and binder viscosity

Fundamental studies of granule consolidation. Part 2 : Quantifying the effects of particle and binder properties

Liquid-bound granule impact deformation and coefficient of restitution