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AN AMYLASE FROM YEAST |  | |||||||
 | The revolution that didn't happen The text on the left is taken from the original 1993 booklet. Little did I realise when writing it that within a few years, genetically-modified yeasts, able to break down starch, would be used for beer production. Nor did I realise that we'd be running workshops for journalists explaining how the technology worked and allowing them to sample the product. The beer was, however, only a research product and never made it to the shops. You can read more about it on the GM foods pages. | | ||||||
Ordinary brewer's yeast (Saccharomyces cerevisiae) is able to utilise a variety of carbohydrates as an energy source. These include glucose, sucrose and maltose. While sucrose is readily-available (as cane or beet sugar) glucose and the other sugars must be prepared by enzymatic or chemical hydrolysis of starch. This is the main purpose of malting in traditional beer brewing. All these sugar feedstocks are relatively expensive. Unlike S. cerevisiae, the yeast S. diastaticus is able to grow on starch and dextrins. This is because it makes an extra-cellular enzyme, glucoamylase (or amyloglucosidase - AMG), which catalyses the hydrolysis of alpha-1,4 glycosidic bonds in starch. It does this by progressively chopping off single glucose molecules from the ends of amylose chains. (Some glucoamylases can also attack the branching alpha-1,6 bonds of amylopectin, but at a much slower rate than alpha-1,4 bonds.) Great interest has focused in recent years on transferring the gene for glucoamylase into S. cerevisiae by sexual hybridisation with S. diastaticus. Such hybrids can grow on cheaper substrates and better utilise the carbon present in conventional feedstocks. This increases the yield of ethanol and allows the yeast to out-compete any bacterial contaminants which might lead to off-flavours in alcoholic drinks. (S. diastaticus can not be used directly for brewing, as it produces a compound which gives beer a spicy phenolic flavour.) This investigation of glucoamylase production by S. diastaticus uses an NCBE Bioreactor, although it may easily be adapted for use with other fermenters. Alternatively, the yeast can be used for a simple investigation of amylase production on a starch agar plate. | ||||||||
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Copyright © National Centre for Biotechnology Education, 2006 | www.ncbe.reading.ac.uk | ||
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