HVG Germany: Hopfen

Here the terms "specific" and "unspecific" should be briefly explained. The result of an analysis can be considered specific when it is guaranteed that only the one desired substance has actually been recorded.
Therefore a separation process ( = chromatography) must generally be used. Figure 1 shows for example a HPLC chromatogram of the bitter acids of a fresh hop. With the alpha- and beta-acids two peaks can be seen in each case, i.e. co-alpha and n- + ad-alpha-acids as well as co-beta- and n- + ad-beta-acids. 

analytik6_1 analytik6_2

Fig. 1: Fresh Hallertauer Hersbrucker          Fig. 2: Aged Hallertauer Hersbrucker

The height of the co-peaks compared with the other two (n- and ad-peak) is lower in the case of aroma hops such as Hersbrucker, Tettnanger and Spalter than with bitter hops. 

Therefore with the HPLC we can be sure that we only measure alpha- or beta-acids.

As a comparison Figure 2 shows the chromatogram of a badly aged hop. The losses of alpha- and beta-acids can be seen immediately. In addition to this some new peaks are visible Ð substances which have resulted from separating the bitter acids.

On the other hand unspecific methods such as conductometry or spectral-photometry do without a separation (chromatography). They make use of specific characteristics of the alpha-acids Ð the spectral-photometric method e.g. the optic characteristics of the alpha-acids. On the other hand the conductometry (measuring conductibility) uses the property of the alpha-acids to bond with a salt. Besides the alpha-acids other hop components also possess the properties mentioned (light absorption in the spectral-photometry and salt formation in the conductometry). Consequently these components are also in the measurement. Therefore a somewhat higher measurement is obtained with unspecific methods than with specific ones.  

Principle of the conductometry

Humulone + lead acetate = humulate

The tannins extracted from the sample are titrated conductimetrically with Pb(AcO)2 (lead acetate).

The humulones react with the lead acetate and form lead salts. When all the humulones (or iso-humulones) have reacted, the conductometer value increases. What makes the analysis unspecific is the fact that other hop components can also cause interference during measuring.

The use of lead acetate is entered in a formula with which the alpha-acid content of the sample is calculated as a percentage.
In the case of fresh hops the results correlate with the actual alpha-acid content of the sample.

If on the other hand the hops or the hop product are old and oxidized, the aging components distort the result of the analysis.

Reaction curve EBC 7.5 
Reaction principle