yeast in brewing

Question 1

why are yeasts used (6)

Answer 1

they are fast growing, have small genomes, are easy to transform, have DNA which is easy to isolate, have shuttle plasmids and can exist as diploid or haploid

Question 2

how do yeast cells express foreign genes

Answer 2

they are integrated into the genome, or transported in by vectors, and the yeast translation machinery expresses the genes to produce the protein product

Question 3

what are constitutive promoters

Answer 3

promoters which carry out transcription continuously. they are often glycolytic enzymes due to the fact that glycolysis happens continuously

Question 4

what are regulatable promoters

Answer 4

promoters which can be turned on and off. this is good if the protein product of the foreign gene is toxic to the yeast cell

Question 5

what are the yeast’s methods of post-transcriptional modification and are these beneficial

Answer 5

yeast may carry out glycosylation or proteolysis, but the level of modification varies depending on the species of yeast. this may not be beneficial to the desired protein product, in which case secretion is promoted

Question 6

how is secretion of the protein products promoted

Answer 6

by integrating the secretion sequence into the foreign gene so it becomes incorporated into the protein product

Question 7

what is one way in which the way proteins are secreted is manipulated

Answer 7

secretion can be manipulated to that the protein product is implanted into the cell wall. an example of this is antigen presentation

Question 8

what are the issues with manipulating industrial yeast strains (5)

Answer 8

they have more complex genomes, the engineered strains are often less competitive than natural strains, there are legal/moral/ethical issues, containment issues and pride

Question 9

what does physical manipulation entail

Answer 9

optimising incubation conditions in order to give a higher cell density

Question 10

what type of yeast is used in beer brewing

Answer 10

typically a Saccharomyces species. they often use complex hybrids of multiple species to allow the specificity used in craft beer

Question 11

what is the process of lager brewing (3 steps)

Answer 11

1. malting to produce the amylase/protease to break down the barley grain 2. wort production to remove insoluble compounds 3. fermentation to convert sugars to ethanol and CO2

Question 12

what are the factors that determine the efficiency of yeasts in brewing (4)

Answer 12

genetics, physiology (tolerance), nutrient requirements and physical parameters (O2 requirements, temperature etc)

Question 13

how has DNA recombination been used to optimise flocculation in beer brewing

Answer 13

they put the FLO1 (flocculation) gene under the control of HSP30 promoters. these are only active at high ethanol and low sugar/ nitrogen which are the conditions at the end of fermentation, when flocculation is desired

Question 14

what are the two stages of downstream processing in beer brewing

Answer 14

primary fermentation which gives most of the flavour, and secondary fermentation which has less effect on flavour but takes much longer

Question 15

why is secondary fermentation essential in beer brewing, how is it sped up and why is this a favourable method

Answer 15

it is needed to decrease the amount of diacetyl. at higher temperatures (90C) the alpha acetolactate is converted to diacetate much more quickly, which can then be converted to acetoin by yeast. this method is favourable because its not classed as GM

Question 16

what is inverse engineering

Answer 16

identifying the phenotype and then determining which genes determine it through sequencing

Question 17

why are non-GM methods favoured and what do they entail

Answer 17

they are favoured because GM has a negative public opinion. non GM methods involve inducing mutations and then selecting for favourable mutants that increase efficiency

Question 18

how has wine fermentation changed

Answer 18

makers used to use wild microflora in fermentation but now often use S. cerevisiae started culture for speed and better reproducibility

Question 19

how is bioethanol produced

Answer 19

ethanol is made from the fermentation of high sugar crops such as sugar cane

Question 20

what are the different substrates different yeast species can use in bioethanol production

Answer 20

lactose, amylose and pentose

Question 21

what is a second generation biofuel

Answer 21

uses cellulases to yield glucose and xylose from plant biomass which is then fermented to ethanol

Question 22

what are the two ways by which ethanol yield from xylose can be improved

Answer 22

by introducing new metabolic pathways by gene transfer between yeast species or by increasing ethanol yield in species which already ferment xylose

Question 23

in what instances is yeast biomass valuable in itself

Answer 23

in bakers or brewers yeast

Question 24

what is the crabtree effect

Answer 24

excess sugar is converted to ethanol due to a respiratory bottleneck. this is avoided by batch feeding the cultures so as not to exceed metabolic capacity

Question 25

which yeast cell components can be valuable products

Answer 25

proteins and RNA. they can be extracted and degraded with products of degradation being used as flavour enhancers, gravy granules etc