Yeast cell immobilisation on carbon nanotubes for fermentation processes
Date
2011-05-11
Authors
Mamvura, Tirivaviri Augustine
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Abstract
Fermentation and maturation are the most time consuming steps in the production
of beer, the duration of which is typically between 5–7 and 7–30 days,
respectively. The continuous fermentation process based on immobilised yeast
cell technology allows producing an acceptable end product within as little as 2–3
days. In spite of the economic advantages that continuous beer fermentation
offers, difficulties of technical and economic origin have retarded the
implementation of the process at industrial scale so far. For example, the total
investment costs depend significantly on carrier costs and applied technology.
Thus the use of cheap carrier materials in a suitably designed bioreactor could
favour the economics of the immobilised process, inspire researchers and
encourage brewing engineers. Among the available yeast cell immobilisation
techniques, flocculation of micro-organisms, due to its simplicity and low cost, is
very attractive because there are no complex mechanical devices needed as well
as any supporting material in this technique. This can be an advantage over other
immobilisation techniques since it is well known that a support represents a major
cost in immobilisation procedures.
Flocculation of yeast cells usually observed at the end of fermentation and is of
great importance in beer brewing. It can occur naturally or it can be artificially
induced by different agents. The focus of the study was to investigate the
immobilisation of yeast cells onto Carbon Nanotubes (CNTs) using flocculation
method. CNTs; long, thin cylinders of carbon; can be used as artificial agents to
induce flocculation of yeast cells because they are increasingly being recognised
as promising materials for catalysis, either as catalysts themselves, as catalyst
additives or as a catalyst support. CNTs are inert and positively charged which
enable them to attract negatively charged yeast cells to form flocs. The use of
CNTs to improve yeast flocculation for fermentation processes has not been reported yet in literature. CNTs were synthesised by Swirled Floating Catalyst Chemical Vapour
Deposition (SFCCVD) method. The optimum conditions required to synthesise
the best samples of CNTs were temperature of 800 oC, acetylene flowrate of 844
ml/min for a reaction time of 20 minutes. The synthesised CNTs were
characterised by Transmission Electron Microscopy (TEM) and Raman
Spectroscopy to obtain the type of nanotubes, morphology and their purity.
The yeast cells (Saccharomyces cerevisiae strain NRRL Y2084, a dry brewer’s
yeast obtained from National Food Products, Emmarentia, Johannesburg) were
immobilised onto CNTs by flocculation method to produce a immobilised cells.
The flocculation process was measured by two methods: a qualitative process of
using the naked eye to rank the flocs as either -, +, ++ or +++ and a quantitative
method of measuring the floc weight recovered using a centrifuge and dried in an
oven at 40 oC for 24 hours. The flocculation of the immobilised cells was
compared with a control experiment which had free cells. The immobilised cells
and free cells were both recovered and dried using a freeze dryer for analysis and
use in fermentation. Conditions required for the flocculation process were an
agitation speed of 110 rpm, pH 5.60, a temperature of 30 oC and concentration of
53.57 μg/ml of CNTs. Addition of calcium ions at 5.49 mM resulted in good
flocculation but the presence of glucose delayed onset of flocculation by 4 – 5
days. The flocculated cells were characterised by Scanning Electron Microscopy
(SEM) and Optical Microscopy. The flocculation conditions used in the study
were comparable with those in literature.
The immobilised cells and the free cells were introduced into malt extract for
fermentation at 15 and 30 oC. The fermentation rates observed were compared
with those found in literature by comparing the final ethanol concentration
observed. Free cells produced more ethanol, 2.49 % (v/v) than the immobilised
cells, 1.56 % (v/v) at 15 oC but these values were not comparable to literature with 5.50 % (v/v) for free cells and 5.20 % (v/v) using the immobilised cells. Free
cells produced higher alcohol content (0.52 % v/v) than the biocatalyst (0.39%
v/v) at 30 oC but these values were not comparable to the reported literature
values of 6.20 % (v/v).