Yeast

Question 1

Why isn’t realistically easy to use genetic recombination or selective crosses to improve strain productivity in industrial yeast strains?

Answer 1

Genetically complex
Non haploid
Low mating
Low sporulation
Retention of full competitiveness in engineered strain?
Legal/ethical/moral issues especially with food
Containment issues with wine yeasts

Question 2

Describe the steps in beer production.

Answer 2

Malting: Produce amylases and proteases for endosperm cell wall glucan breakdown; kilning to arrest respiration of barley.
Wort production: Mashing (extraction hydrolysis), separation (filtering or lautering), boiling (with hops and sugars), cooling and removal of precipitates.
Fermentation: Sugar to ethanol+CO2 with acetaldehyde intermediate via pyruvate decarboxylase and alcohol dehydrogenase.
Downstream processing: Main concern is to lower diacetyl in lager below taste threshold. Solution could be to add alpha-acetolactate decarboxylase (expensive) to convert alpha-acetolactate directly to acetoin rather than via diacetyl or engineering alpha-ALDC into yeasts (some tests successful but problem of consumer attitudes). - Then found non-traditional heating at 90dc for 10 minutes completes step and some of next step – yeast would die so must remove them first. Maturation with immobilised yeast in a continuous bioreactor.

Question 3

What are the factors influencing fermentation efficiency?

Answer 3

Genetic characteristics
Strain physiology (stress resistance, fermentation rate, flocculation)
Nutrients
Physical paramters

Question 4

Describe using lignocellulosic hydrolysates (plant biomass) as a 2nd gen biofuel?

Answer 4

Use something cheap of high abundance that can be fermented e.g. straw.
Cellulases yield mostly glucose and xylose (pentose) from plant biomass.
Can improve xylose fermentation by INtroducing additional pentose metabolic pathways (e.g. XYL1/2 genes from Pichia to S. cerevisiae) OR IMprove ethanol yield in organisms that can already ferment pentoses.
Further refinements still required to improve commercial viability vs bioethanol from sucrose/starch fermentation