Lesson 5 - Hop Chemistry SAQ

** Explain how the hop aroma in finished beer would change if the following three scenarios were adopted. Assume that all other parameters are the same, and ignore the impact on bitterness; detail what hop components would cause changes – if any:

a)    Hops added at the start of boil only.

b)   Hops added halfway through the boil only.

c)     Hops added in the whirlpool only.

Response:

Hop aroma primarily comes from the hops’ essential oils. The essential oils in hops (hop oils) make up around 0.5-3% of the total dry weight of the hops and so their overall quantity in the beer is small. On top of this, they are volatile compounds and are thus easily lost through the brewing process. Due to these properties, it is highly likely that the three beers would have different hop aromas.

Hop oil is subdivided into hydrocarbons, oxygenated compounds and sulphur compounds. They have the following general characteristics:

Hydrocarbons – very volatile. They make up the majority of the hop oil, with β-myrcene + α-humulene + β-caryophyllene making up 80-90% of the total essential oil in hops.  Myrcene is the most volatile and makes up 30-60% of the total hop oil. If they survive in the final beer, they tend to have green, resinous, pine, wood-like aromas.

Oxygenated components – polar chemicals, thus more likely to survive in the beer. A complex category, with a multitude of aromas that can change during fermentation through biotransformation by the yeast. The full spectrum of hop aromas can be achieved – from green/grassy/herbal through to floral//fruity.

Sulphur compounds – only trace amounts present in hops but have potent aromas. Only small amounts are needed in the beer to have a big impact on aroma. These compounds can add negative aromas such as onion, garlic, cooked vegetable or cheesy notes. Yet they can also add desirable aromas, such as tropical fruit and wine-like notes. They are very volatile, and therefore easily removed in the brewing process.

a)    Due to the volatile nature of the hop oils, it is unlikely that this beer would have any discernible hop aroma. Hydrocarbons and sulphur compounds are very likely to be removed, and only oxygenated compounds could potentially remain in the beer.

b)   Hydrocarbons are unlikely to survive. Some oxygenated compounds could make it into the final beer and it is possible that the more flavour active sulphur compounds might be present. Although this is still unlikely. Any hop aroma present is likely to be minimal. As it is likely to be coming only from oxygenated compounds, any existing aroma can come from anywhere on the aroma spectrum (green, grassy, floral, spicy, citrus, fruit).

c)     It is still likely that the hydrocarbon myrcene will be lost, but others might survive, such as humulene and caryophyllene. This would impart a woody and pine-like aroma, but probably at a low level. The oxygenated compounds are likely to remain in the beer at a noticeable level and again, the full range of hop aromas is available, depending on the hop variety. Some sulphur compounds are also likely to make it into the final beer, so careful hop variety selection is necessary to ensure that the more pleasant tropical fruit (and not the undesirable aromas such as vegetable sulphur) are present.