Industrial biotechnology and fermentation questions

Question 1

Describe how the conditions inside a stirred-tank fermenter are held at the optimum for the production organism, naming the part of the fermenter responsible in each case.

Accepted Answer

B1 a dense culture respires hard and respiration releases heat, so water circulating through the cooling jacket carries that heat away and holds the optimum temperature B1 metabolism produces carbon dioxide and organic acids, so a pH probe monitors the broth and acid or alkali is metered in to hold the optimum pH B1 an aerobic culture uses oxygen faster than it dissolves, so sterile air is forced in through the sparger near the bottom of the vessel B1 the impeller breaks up the rising bubbles and keeps organisms, nutrients, heat and oxygen evenly mixed, so no part of the tank becomes a stagnant corner running anaerobic

Question 2

Compare batch fermentation with continuous fermentation, referring to the closed-culture growth curve in your answer.

Accepted Answer

B1 batch culture is closed, with nothing added after inoculation, whereas continuous culture is open, with sterile medium flowing in and culture flowing out at matching rates B1 a batch culture passes through the whole curve and is harvested once, usually early in the stationary phase, whereas a continuous culture is held permanently in the exponential phase and never reaches stationary, because nutrients never run out and wastes never accumulate B1 batch production is slower overall, because the vessel spends time in lag phase, harvesting and cleaning, whereas a continuous vessel is always at peak output and runs for weeks B1 contamination costs a batch process one run, whereas it costs a continuous process a long production run

Question 3

A company cultures a bacterium that secretes a new antibiotic only once the culture stops dividing. It also sells the bacterial biomass itself as animal feed. Suggest which fermentation method suits each product, and suggest why.

Accepted Answer

B1 the antibiotic is a secondary metabolite, made only as growth slows and the culture enters the stationary phase B1 it must therefore be made in batch culture, because a continuous culture is held in the exponential phase and never experiences the conditions that switch production on, so it would grow bacteria and make almost no antibiotic B1 the biomass is the product of growth itself, a primary product that accumulates fastest while the culture is growing exponentially B1 it therefore suits continuous culture, with cells drawn off as fast as they are replaced, so the vessel stays at peak output instead of losing time to harvesting and cleaning

Question 4

A local authority proposes to replace part of the meat served in its school meals with mycoprotein. Evaluate this proposal.

Accepted Answer

B1 in favour: mycoprotein is produced in days in a small building all year round, on cheap substrates, and uses far less land and water per kilogram of protein than livestock, with no animal reared or slaughtered B1 in favour: the hyphae of Fusarium venenatum give a fibrous texture close enough to muscle to stand in for meat, and the product is high in protein and fibre, low in fat and free of cholesterol B1 against: the product has little taste of its own so flavour and colour must be manufactured into it, the cells are rich in nucleic acid whose breakdown product uric acid can cause gout so this must be reduced during processing, and some people are unwilling to eat a fungus grown on industrial glucose B1 a judgement supported by those points, for example that the nutritional and environmental case is strong provided the nucleic acid content is reduced and the food is labelled honestly, while accepting that the sterile, precisely controlled plant is expensive to build and a contaminated batch is a food-safety failure

Question 5

Describe how mycoprotein is produced from Fusarium venenatum, from culture to finished food.

Accepted Answer

B1 the fungus is grown in continuous culture on glucose syrup, with ammonia supplied as the nitrogen source B1 the fungal hyphae are harvested continuously as the culture grows B1 the harvested biomass is heat-treated, partly to reduce its nucleic acid content B1 it is then pressed into a textured food product, the fibrous hyphae giving it a texture close enough to muscle to stand in for meat

Question 6

Describe the four phases of the closed-culture growth curve that a batch fermentation passes through, in order.

Accepted Answer

B1 a lag phase, during which the population adjusts to the new medium and synthesises the enzymes it requires, so numbers barely rise B1 an exponential, or log, phase, during which nothing yet limits growth and the population doubles at a constant rate B1 a stationary phase, when a nutrient runs short or a waste product accumulates, so the rate of division comes to balance the rate of death and numbers level off B1 a death phase, when deaths exceed divisions and the population falls

Question 7

A continuous fermenter has a working volume of 8000 dm³ and is run at a dilution rate of 0.15 per hour. Calculate the flow rate of fresh medium into the fermenter in dm³ per hour, and calculate how long it takes for a volume of medium equal to the whole vessel to pass through.

Accepted Answer

M1 dilution rate = flow rate ÷ culture volume, rearranged to flow rate = dilution rate × volume A1 0.15 × 8000 = 1200 dm³ per hour M1 time for one working volume to pass through = volume ÷ flow rate A1 8000 ÷ 1200 = 6.7 hours, which is about 6 hours 40 minutes

Question 8

A small start-up proposes cutting costs by skipping the steam sterilisation of its fermenter medium, relying only on filtering the incoming air. Evaluate this proposal.

Accepted Answer

B1 for: filtering the incoming air does remove airborne bacteria and spores from that one contamination route, so some protection against contamination would remain B1 against: the growth medium itself, and the vessel and pipework, are further routes by which contaminating organisms can enter, and skipping medium sterilisation leaves those routes open B1 against: a contaminating organism can compete for the substrate, out-grow or degrade the product, and in a food or medicine renders the whole batch unsellable, so the saving from skipping one step could easily be outweighed by the cost of losing an entire batch B1 judgement: the proposal is not defensible as it stands, since the saving is small and the failure mode is a complete batch loss, though validating a shortened rather than an eliminated sterilisation cycle could be worth investigating

Question 9

Explain why a fermenter must be run under aseptic conditions, and explain how those conditions are achieved at industrial scale.

Accepted Answer

B1 a contaminating organism competes for the substrate and may out-grow the production strain or destroy the product, and in a food or a medicine it renders the whole batch unsellable B1 the vessel is sterilised with pressurised steam between runs and the medium is sterilised before it enters B1 the incoming air is passed through filters fine enough to remove bacteria and spores, and every other inlet is treated as a contamination route and sterilised

Question 10

A fermenter is inoculated with 5.0 × 10⁴ bacteria of a strain whose doubling time under these conditions is 20 minutes. Calculate the number of bacteria present after 4 hours of exponential growth.

Accepted Answer

M1 the number of divisions is the time divided by the doubling time, so 240 ÷ 20 = 12 M1 the number present is the starting number multiplied by 2 raised to the number of divisions, so 5.0 × 10⁴ × 2¹² A1 2¹² is 4096, giving 2.05 × 10⁸ bacteria

Question 11

Explain why microbial cells intended as human food have their nucleic acid content reduced during processing.

Accepted Answer

B1 microbial cells are proportionately much richer in nucleic acid than the cells of a plant or animal food source, because they are small and each one holds a full genome relative to its size B1 nucleic acid consumed in the diet is broken down, and one breakdown product is uric acid B1 a high intake of uric acid can cause gout, so the nucleic acid content of the harvested biomass must be lowered before it is sold as food

Question 12

Compare the generation time of a bacterium such as Escherichia coli with that of a fungus and with that of a farm animal such as a cow, and compare the consequence each has for industrial production.

Accepted Answer

B1 a bacterium such as Escherichia coli can double its numbers in about twenty minutes given warmth and nutrients, whereas a fungus doubles over hours and a cow doubles its numbers over years B1 industrial biotechnology exploits this difference by culturing microorganisms rather than farming plants or animals, since a much larger yield can be obtained from a given starting culture in a much shorter time B1 generation time is one of several distinct reasons for using microorganisms, working alongside low-cost substrates, conditions set inside the vessel rather than by season, no animal-welfare objections and ease of genetic modification, rather than being the whole explanation on its own

Question 13

Explain why the temperature and pH probes in a fermenter are described as providing negative feedback.

Accepted Answer

B1 each probe continuously measures a condition, temperature or pH, so that a departure from the optimum set point is detected B1 the fermenter's control system responds by correcting the departure, for example running more coolant through the jacket if the temperature rises, or metering in alkali if the pH falls B1 the correction acts to return the condition towards its original set point rather than push it further away, which is what makes the control negative feedback

Question 14

Apart from their rapid growth, state two reasons why industry grows microorganisms rather than farming a plant or an animal for the same product.

Accepted Answer

B1 they are cheap to feed, because many grow on waste from other industries such as whey, molasses or straw B1 production is independent of climate and season, because the environment is made inside the vessel; or there are no welfare objections; or the genome is small and easily altered, so a strain is quickly improved

Question 15

Name the fungus used to produce mycoprotein, and name the two substances supplied as its main carbon and nitrogen sources during culture.

Accepted Answer

A1 the fungus Fusarium venenatum A1 glucose syrup as the carbon source and ammonia as the nitrogen source