Lesson 8 - Quality Management End of Unit SAQ

***The marketing department in your brewery are very excited about the new craft brewing craze for struck match and butterscotch lager which is trending on Twitter. They want to launch a version. The pilot brewery has produced five prototypes for the new beer which is brewed to have higher levels of diacetyl and SO₂.

a)    Explain the sensory tests you would use to screen panellists and then compare beers with reference to these attributes and panel preference.

b)    Describe what analytical techniques you would use to determine the levels of diacetyl and SO₂ in the beers.

Response:

a) The two compounds of interest are SO₂ and diacetyl. The panel should therefore be screened on the basis of their ability to detect and describe these compounds. As SO₂ is an allergen it would be essential to remove any assessors who are sensitive to SO₂. Difference tests like Triangle or Tetrad tests would be used, with spiked samples as the sample which is different. A lager with a similar abv, SG, and colour should be used as the spiked and unspiked sample. The spiked sample should be spiked at just above the recognition threshold for the compound of interest. The assessors would then be asked which samples are different to the others. Tests should be repeated a sufficient number of times to eliminate the possibility that the different samples are selected by chance. Any panellists failing to select the different sample(s) in any test should be excluded. Threshold tests to establish the difference threshold for SO₂ and diacetyl for each panellist could also be undertaken to determine the sensitivities of the panellists to the compounds.

In order to compare the beers, ranking tests should be used. In the ranking test the assessors would be asked to sniff the samples and make a provisional ranking and then rank them in terms of either attribute. The test would then be repeated for the other attribute and then preference. To avoid sensory fatigue the tests would be carried out on separate days or by separate panels. SO₂ detection is significantly affected by fatigue so the number of samples the panellists assess should be limited to eight per session. The panel should be made up of at least five assessors, one of whom should be a sensory expert. If consumer preference ranking is required, at least 100 assessors are needed. A simpler approach for preference testing would be the paired comparison test. This could be undertaken on the two highest ranked samples from the ranking tests to pick the prototype to take forward into the scaled-up production. This time 300 assessors of the target demographic in two differing geographical locations would be required.

b) Diacetyl can be measured by gas chromatography (GC) or by colourimetry. For measurement in a gas chromatograph, diacetyl is generated from its precursors by aeration of the sample followed by incubation at 60 °C for 90 minutes. Two forms of GC analysis are used in approved methods: headspace chromatography and gas liquid chromatography (GLC). In headspace chromatography the beer is saturated with ammonium sulphate in a sealed vial and volatiles in the sample are allowed to come into equilibrium with the headspace. The gas from the headspace is then injected into the GC column and the components separated by their affinities to the column lining. Compounds leaving the column are measured using an electron capture detector (ECD) which measures the change in the resistance between the electrodes as the diacetyl molecules pass through.

In GLC the stationary phase in the column is a layer of liquid on an inert solid matrix. The operation of the column is essentially the same as using a solid stationary phase. Both methods quantify diacetyl by comparison with results based on curves drawn from data from running external standards.

Gas chromatograph ionic mobility spectrometry (GC-IMS) claims to produce reliable results for VDK concentration without any sample preparation in 20 minutes. This method is not yet approved by EBC, ASBC, or MEBAK.  

For colourimetric analyses the beer sample is distilled for 8-10 minutes and the distillate mixed with either o-phenylenediamine and acid or α-naphthol in alkaline solution. The absorbance at 335 nm and 530 nm respectively are then read and the concentration of diacetyl is calculated. The method using α-naphthol is less accurate because it measures total vicinal diketones and not just diacetyl. A more rapid version of the o-phenylenediamine and acid method is available from Hach who supply a tube to which you add the distillate. This reduces the test time to around 10 minutes.  

SO₂ can be measured by distillation and titration. In the Monier-Williams method, SO₂ is removed from beer acidified with orthophosphoric acid by distilling with steam before oxidising with hydrogen peroxide into sulphuric acid. The sulphuric acid is then titrated with sodium hydroxide. The total SO₂ is calculated from the amount of sodium hydroxide used to neutralise the sulphuric acid.

In another distillation method, vapour from a steam distillation is passed into an iodine solution. The iodine solution is then titrated with a standard solution of sodium thiosulphate with starch as an indicator. This method is more accurate than the Monier-Williams method at lower levels of SO₂. 

SO₂ can also be measured by colourimetry. In one method the sample is mixed with r-rosaniline and formaldehyde and reacts to form a coloured compound. The absorbance of the solution is measured at 550 nm in a spectrophotometer.  

In another method the SO₂ is stripped from the acidified beer using nitrogen and absorbed in a buffered solution of 5,5-dithiobis (2-nitrobenzoic acid) (DTNB). The absorbance of the DTNB solution is then measured at 415 nm by a spectrophotometer. An adapted version of this method is automated in the Thermo Scientific Gallery Beermaster.