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Welcome to YeastBytes |
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| Like a five year old who has just been refused those must have black chunky shoes, here we are kicking and screaming with YeastBytes no.4. Despite the unrelenting pressure, we have stuck to our task to deliver a late summer bumper edition with some more finely honed pieces in our bimonthly homage to all things Saccharomyces. |
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| The lead article in this issue is something of a party political broadcast on behalf of the 'Yeast Supply' party, often seen as failing to punch its weight and grabbing too few votes. This piece focuses on the benefits of yeast supply or management through the eyes of a 'party worker'. Unable to resist the pun, 'freeze dying' hits home on the perils of using dried yeast from commercial collections for yeast supply. For the first time in YeastBytes' young life we take a deep breath and consider the thorny subject of the 'generation game' and the pros and cons of culling and replacing yeast. Next up Cara new boy, Pieter Swanepoel introduces ('passport control') some of the amazing developments in molecular methods that have made strain fingerprinting a quick and cost-effective monitoring tool. YeastBytes regular Chris Giles mulls over the joys of the QA of microbiological media. Finally, in the bizarre corner that is 'did you know?', YeastBytes catches up on the spread of Australian yeast extract in art! |
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passport control |
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| You may have noticed but we don?t miss the chance to humanise yeast in YeastBytes. Reflecting on advances in yeast identification via snazzy molecular approaches triggered the thought that checking yeast ID is a bit like going through passport control. However there is one small point of difference! I could be wrong but I?m not convinced that Mr Yeast respectfully waits at the line and then steps forward, with a good intentioned smile whilst inwardly praying ?not me?! |
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| Keeping your yeast identity true can be a bit of a struggle. The yeast ?supply chain? from propagator through fermenter and yeast recovery and back again can ? with some strains ? be a little taxing. But, as a lot of brewers have found in the past, your yeast will change its behaviour over time whether you take care of it or not. This is because the genome sequence will change over time as mutations accumulate. Even though this is an increasingly accepted issue, strain typing is still regarded by many as a costly and time consuming exercise. However the good news is that yeast strain identification is now a quick, cost effective and revealing value?adding activity |
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| Past techniques for detecting and identifying brewery yeast include colony morphology, fermentation performance, sugar fermentation tests, tolerance to various stresses (eg ethanol tolerance) phenotypes with functional relevance (eg flocculation) nutritional requirements (eg oxygen), and resistance and sensitivity levels of cycloheximide. Regrettably these methods have many disadvantages, including slow turnaround time, failure to differentiate between different strains of yeast, and lack of reproducibility. |
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| As brewing yeast strains are polyploid, these tests were generally inadequate as dominant mutations have the greatest physiological effects. These are often the more serious mutations because they cause specific loss or gain of function. However, these changes are often not recognised as ? unless they impact on flocculation ? they are likely to missed in the ?noise? around fermentation. Such changes whilst often subtle can contribute to a drift in product quality that may or may not be accepted by Consumers! |
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| So what has changed? Recent developments in molecular biology and protein chemistry have provided new methods for identifying microorganisms, including DNA restriction fragment length polymorphisms, chromosome fingerprinting, and protein electrophoresis patterns. Such techniques have been used over the last 10-15 years to identify and characterise changes in brewing yeast. Although these techniques enabled identification of strains with small changes in genome sequence they were still expensive and, from a brewing perspective, painfully slow (1-2 weeks). However, with the advent of the polymerase chain reaction (PCR) and subsequent standardization of the technique, yeast characterisation has become less bound by cost and time. The big news being that PCR enables rapid confirmation of the identity of a yeast strain which hitherto would have been slow and ambiguous via ?traditional? methods. |
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| PCR based DNA fingerprinting is now routinely used to discriminate between yeast strains that have similar physiological characteristics. By using primers specific for regions of the yeast genome that are more prone to mutations the same approach can be used to detect subtle changes in target sequence. The target sites chosen are so called mutation ??hot spots?? which have a much higher mutation frequency than the rest of the genome. This means that, when a fingerprint obtained from brewery yeast is compared to that of its original reference culture, it is easy to determine if the strain has mutated from its original form. In the event of any changes, a new yeast line can be introduced. However, a more proactive approach is to use this insight to set the number of generations for a yeast strain before culling (see ?the generation game? in this issue). |
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| As well as strain stability, DNA fingerprinting also offers the ability to monitor the relative levels of yeast strains within a mixed culture of Saccharomyces cerevisiae or Saccharomyces pastorianus. This is important to brewers who may feel that the particular characteristics of their product rely upon maintenance of a minority yeast strain population within their culture. Once each individual strain has been identified and fingerprinted, any deviation from a diagnostic pattern in terms of proportion or type can be readily detected. This also means that introduction of a wild yeast strain into a culture can be detected with equal ease, allowing the brewer to intervene much more swiftly than previously possible. By using the PCR fingerprint of the wild yeast the cause of the contamination can be swiftly traced. This quality control opportunity cannot be over-emphasized. Currently, microbiological testing of a fermentation process is often an after-the-event exercise designed only to pinpoint the batch where a particular problem may have arisen. With the present technology, same day results are now standard. Near real-time surveillance of yeast is now a reality. |
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| To conclude, for any brewer who is serious about product quality (are there any out there who are not?), PCR fingerprinting opens a ?new door? in the assurance of yeast identity and quality. Not only a key step in the assurance of yeast supply but a near real-time (five hour) validation of yeast identity in fermenter or storage tank. |
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| Pieter Swanepoel |
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