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Assuring the microbial safety and quality of Australian malt and barley

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Kaur, M (2010) Assuring the microbial safety and quality of Australian malt and barley. PhD thesis, University of Tasmania.

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Full text restricted
Available under University of Tasmania Standard License.

Abstract

Australia produces approximately 32% of the world's traded malting barley, ranking
number one in the world for malting barley export. In the international export market,
Australia has traditionally had a reputation for producing clean and bright barley/malt,
which is presumably due to low microbial loads, resulting from dry conditions
occurring during maturation and harvest.
The diverse microbial communities naturally colonizing barley grains greatly influence
malt quality, and subsequently other products in the malt value chain, in particular beer.
The objective of this thesis is to comprehend microbial diversity associated with barley
and malt, thereby leading to a better understanding of cause of factors (such as
microorganisms) that produce mycotoxins and cause premature yeast flocculation which
impact brewing efficiency and beer quality.
Microbial fingerprinting of Australian malt and barley grown in different regions was
benchmarked against malting barley grown internationally by using terminal restriction
fragment length polymorphism {TRFLP) analysis. This approach was supported by
cloning and sequencing techniques to assess microbial population composition. The
TRFLP approach was considered the most appropriate because it is comparatively
rapid, cost efficient and that microbial profiles from a large number of samples can be
assessed. The TRFLP approach uses amplification of generic primers for the bacterial
16S rRNA gene and Dl/D2 domain of the fungal 26/28S rRNA gene. Both qualitative
and quantitative differences were observed in bacterial and fungal communities
associated with malts produced in different geographical regions. The TRFLP and
cloning approaches identified a greater diversity in yeast and filamentous fungi
associated with barley malts than previously reported. Presumably this is the result of
TRFLP being a culture independent approach, compared to traditional "wet plate"
culture techniques which can bias towards the selective enrichment of fast growing
microorganisms adapted to high substrate concentrations that can potentially represent a
minor fraction of the resident microbial community.
Considerable differences in terms of bacterial and fungal populations were observed
between Australian barley samples and their corresponding malts. The malts produced
in different malt houses were dissimilar in terms of fungal community structure. Fungal
clone libraries of different barley and malt samples demonstrated the absence of
Fusarium graminearum, Aspergillus and Penicillium spp., the sources of
deoxynivalenol (DON) and ochratoxin A (OTA). The absence of these mycotoxins in
Australian malt was verified by testing malt samples for DON and OTA.
A new procedure for the regeneration of DON and OTA immunoaffinity columns was
developed and used for detecting these mycotoxins in Australian barley and malts. This
new regeneration method reduces the cost of screening for these commonly tested
mycotoxins. None of the samples were found to contain detectable levels of either of
DON or OTA. This outcome was attributed to the typically dry to hot climatic
conditions in the Australian barley growing regions during the period from heading to
maturity of the barley crop. In addition temperatures below 20°C at anthesis avoid the
optimal conditions required for the infection of DON-producing Fusarium
graminearum strains. Furthermore, the dry harvest conditions result in dry barley with
moisture content (in this study, average 10.9% with a range of 8.7 - 12.4%) <13% for
storage, which is well below the minimum moisture content (>14%) that is conducive
for the growth of OTA-producing Aspergillus and Penicillium spp.
Premature yeast flocculation (PYF) is an intermittent brewing fermentation problem
that results in incomplete wort fermentation, and is a significant problem for some
breweries. The traditional approach to avoiding and solving this problem has been to
detect PYF positive malts by using a small scale fermentation test. These fermentation
tests are time consuming, expensive, sometimes inconsistent and difficult to transfer
between testing laboratories. Research has also been directed at identifying the causal
wort components (pectin/arabinoxylan or protein) of PYF. Neither of these approaches
has been particularly successful over the past 40+ years. Consequently the problem was
approached from a different and novel perspective. That was to use molecular finger
printing as a step to identify the microbial taxa that cause PYF by comparing positive
and negative malts using TRFLP, cloning and sequencing. A significant breakthrough
has been made with this approach and a concept developed identifying substantial differences between PYF positive and negative malts in their TRFLP (Haelll digestion)
fungal fingerprints using the generic primers for the Dl/D2 domain of the fungal 26/28S
rRNA gene. This analysis indicates that more than one taxon of fungi are associated
with PYF which perhaps indicates why previous researchers have had difficulty
identifying the causal microbial taxa and causal agent/s.

Item Type: Thesis (PhD)
Keywords: Barley, Malt
Copyright Holders: The Author
Copyright Information:

Copyright 2010 the author

Additional Information:

Available for use in the Library and copying in accordance with the Copyright Act 1968, as amended. Introduction and aims of the study -- Improving the cost efficiency of quality assurance screening for mycotoxins in malting barley -- Microbial diversity of barley malt grown under different environmental conditions, in diverse geographic locations -- Investigation of premature yeast flocculation using TRFLP and clone libraries --Microbial community changes during malting - study of Australian barley and malt. Thesis (PhD)--University of Tasmania, 2010. Includes bibliographical references

Date Deposited: 19 Dec 2014 02:54
Last Modified: 11 Mar 2016 05:53
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