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

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, which catalyses the hydrolysis of

a-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

a-1,6 bonds of amylopectin, but at a much slower rate than a- 1,4 bonds.)
 
 

Great interest has focussed in recent years on transferring the gene for glucoamylase into S.

cerevisiae by sexual hybridization with S. diastaticus. Such hybrids can grow on cheaper

substrates and better utilize 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.

 
 
   Material

    Practical details