Vapm. Dossantos et al., RELEVANCE OF RHEOLOGICAL PROPERTIES OF GEL BEADS FOR THEIR MECHANICALSTABILITY IN BIOREACTORS, Biotechnology and bioengineering, 56(5), 1997, pp. 517-529
The mechanical stability of biocatalyst particles in bioreactors is of
crucial importance for applications of immobilized-cell technology in
bioconversions. The common methods for evaluation of the strength of
polymer beads (mostly force-to-fracture or tensile tests) are, however
, not yet proven to be relevant for the assessment of their mechanical
stability in bioreactors. Therefore, we tested fracture properties of
gel materials and investigated their relevance for abrasion in biorea
ctors. Abrasion of gel beads was assumed to be a continuous fracturing
of the bead surface. At first, three rheological properties were cons
idered: stress at fracture; strain at fracture; and the total fracture
energy. If stress at fracture is the most important property, beads h
aving a similar fracture energy, but a smaller stress at fracture, wou
ld abrade faster in a bioreactor than beads with a larger stress at fr
acture; if fracture energy the determining factor, beads that require
less energy to fracture would abrade faster than those having a larger
fracture energy for the same fracture stress. To determine this, bead
s of kappa-carrageenan and agar (at two different polymer concentratio
ns) were tested for abrasion in four identical bubble columns under th
e same operating conditions. Agar beads were expected to abrade faster
than those of carrageenan because agar had either a lower stress at f
racture or a lower fracture energy. However, no correlation between fr
acture properties and abrasion rate was found in any of the combinatio
ns tested. Carrageenan beads abraded faster than those of agar in all
combinations. Furthermore, both the stress and strain at fracture of a
gar and carrageenan beads decreased during the run and those of carrag
eenan decreased faster, suggesting that the gels are liable to fatigue
in different ways. This hypothesis was confirmed by oscillating exper
iments in which gel samples were subjected to repeated compressions be
low their fracture levels. Their resistance to compression clearly dec
reased with the number of oscillations. Fatigue is probably related to
the development of microcracks and microfracture propagation within t
he material. We concluded that: (a) the use of tests based on bead rup
ture do not provide relevant information on the mechanical stability o
f gel beads to abrasion; and (b) abrasion of polymer beads is likely t
o be related to fatigue of the gel materials. (C) 1997 John Wiley & So
ns, Inc.