| Date | November 2020 | Marks available | 2 | Reference code | 20N.3.HL.TZ0.10 |
| Level | Higher level | Paper | Paper 3 | Time zone | TZ0 / no time zone |
| Command term | Explain | Question number | 10 | Adapted from | N/A |
Question
Citric acid was produced in a fermenter using the pulp of cassava (Manihot esculenta) and the fungus Aspergillus niger. This process was carried out at room temperature for 6 days and citric acid was then collected. The apparatus was cleaned and set up for a new fermentation. The process is shown in the diagram.
[Source: Prado, F.C., Vandenberghe, L.P.S., Woiciechowski, A.L., Rodrígues-León, J.A. and Soccol,
C.R., 2005. Citric acid production by solid-state fermentation on a semi-pilot scale using different percentages of
treated cassava bagasse. Brazilian Journal of Chemical Engineering, 22(4), pp.547–555.]
State in which numbered part of the process you would find the probes to detect changes in pH.
Explain the possible causes of these changes in pH.
Suggest, with a reason, whether this is a batch or a continuous fermentation.
State one use of the citric acid produced.
Markscheme
2
a. pH decreased by citric acid;
b. (pH lowered by) production of CO2 produced (by the fungus) in respiration;
c. (pH lowered by) production of carbonic acid by CO2 dissolving in water;
batch fermentation because the citric acid is collected at the end of the fermentation
OR
batch fermentation because the process was carried out over 6 days then re-set up
OR
diagram does not show constant supply of nutrients so it must be a batch;
preservative/flavour
Accept other verified use, e.g. buffering.
Examiners report
Most could correctly identify part 2, others wrote part 1.
Most stated that the pH changes were due to acidic conditions, but fewer could explain how the acidic conditions were generated.
Most suggested that it was a batch fermentation, but some omitted to provide a reason; a few others had incorrect answers.
Good candidates correctly stated flavour enhancer, others didn't know.
