Biochemical and microbiological changes in sugarcane stalks during a simulated harvest-to-crush delay.
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Date
2008
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Abstract
Post-harvest cane deterioration in the South African sugar industry results in significant
revenue loss that is estimated to be in the region of ZAR 60 million per annum. Despite
these large losses, precise biological data relating to the process of cane deterioration
under South African conditions is limited. Severity of deterioration is influenced by a
number of factors, including the length of the harvest-to-crush delay (HTCD), ambient
temperature and harvesting practices. For example, burning of cane prior to harvest
may result in rind splitting, which provides entry for microbes, particularly Leuconostoc
mesenteroides that may exacerbate deterioration. The effect of these factors on
deterioration was examined by quantifying the biochemical and microbiological changes
that occur in sugarcane stalks after harvest, with the influence of length of HTCD,
degree of L. mesenteroides infection and ambient temperature receiving attention. The
primary novelty of the work resides in the analysis of deterioration under tightly
regulated temperatures, which were designed to reflect diurnal variations typically
experienced during summer and winter in the South African sugar belt. In addition,
inoculation of mature internodes with a consistent titre of L. mesenteroides was used as
a means to mimic a consistent level of infection of harvested stalks by the bacterium.
Metabolites selected for analysis were those both native to the stalk and produced as
by-products of microbial metabolism, viz. sucrose, glucose, fructose, ethanol, lactic
acid, dextran and mannitol. Simulated HTCDs under summer temperatures resulted in
increasing glucose and fructose levels with time, which contrasted to the approximately
constant levels of these hexose sugars under winter conditions. Commonly referred to
as ‘purity’ in an industrial context, precise determination of the concentration of these
hexoses in cane consignments could potentially indicate the extent of deterioration.
Despite the detection of a basal concentration of lactic acid in unspoiled cane, the
observed increase in concentration of this organic acid over the simulated summer
HTCD suggests that this metabolite could also potentially serve as an indicator for postharvest
deterioration. In contrast, the investigation indicated that ethanol was an
unsuitable biochemical marker for deterioration of L. mesenteroides infected cane. An
inability to detect dextran and mannitol in the samples, combined with consistent
sucrose levels and variable mill room data, suggest that extreme proliferation of L. mesenteroides is facilitated primarily by in-field practices, particularly the manner in
which cane is prepared prior to harvest and transport to the mill.
Bacterial proliferation and infection by L. mesenteroides of inoculated stalks were
monitored by standard selective culturing techniques. Despite the limited detection of L.
mesenteroides-associated metabolites, culture-based analyses revealed that the
bacterium was the dominant bacterial species within the samples. A number of other
bacterial species were isolated and identified, however the extent to which the total
number of microorganisms proliferated was limited to a maximum of 1 x 105 colony
forming units per gram of fresh tissue. In conjunction with these analyses, a molecular
approach known as Polymerase Chain Reaction-Mediated Denaturing Gradient Gel
Electrophoresis (PCR-DGGE) was undertaken to investigate the bacterial diversity
patterns associated with deteriorating sugarcane stalks throughout the delay period. In
contrast to the results obtained by means of the culture-based assays, PCR-DGGE
revealed that L. mesenteroides was not the dominant bacterial population, and showed
that the level of bacterial diversity was relatively consistent across the differing
treatments and with time. The use of complimentary culture-dependent and cultureindependent
analyses thus permitted the detection of this discrepancy and indicated the
utility of PCR-DGGE in the determination of bacterial community structure of postharvest
sugarcane tissue.
The biology of post-harvest deterioration of green sugarcane stalks is highly complex,
even under rigorously controlled temperature and infection regimens. The results of this
study emphasize the important effects that harvest method and environmental
conditions have on post-harvest sugarcane deterioration. Towards the formulation of
industry-relevant recommendations for combating post-harvest deterioration, future
work will strive to mimic the effects that harsh harvesting and transport practices have
on the severity of the problem.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Pietermaritzburg, 2008.
Keywords
Sugarcane--Postharvest physiology--South Africa., Sugarcane--Postharvest losses--South Africa., Temperature., Leuconostoc--South Africa., Theses--Biochemistry.