What are terpenes?
Terpenes are aromatic compounds that form an important part of the essential oils of plants. In hops, the lupulin glands contain an oily substance that is rich in terpenes. There are thousands of different terpenes structures that vary by the length of their hydrocarbon chain or the presence of different functional groups (acid, aldehyde, alcohol). Terpenes can have diverse flavor impacts (citrus, floral) and higher levels of terpenes are associated with greater overall hop aroma intensity (OHAI).
Many terpenes have a pleasant sensory profile described as floral, citrussy, and sweet (e.g. geraniol, linalool, beta-citronellol, limonene). Other terpenes have more polarising characteristics such as ‘woody’, pine, and resinous (e.g. caryophyllene, pinene, and myrcene).
Sources of terpenes
In brewing, terpenes are typically introduced to the wort stream or beer through hops. These terpenes can be present in ‘free’ aromatic form, or ‘bound’ forms, the latter of which is glycosylated (bonded to a sugar molecule).
Other brewing ingredients may also contain terpenes. For example, citrus peel is rich in limonene, and coriander seed contains geraniol and linalool.
Want to know which hop varieties are high in either free or bound terpenes? Check out this resource.
Terpene Biotransformation
Some brewing yeast strains can produce the β-glucosidase enzyme can cleave the terpene-sugar bond, freeing a sugar molecule for the yeast to consume, as well as a highly sensory active terpene molecule. The yeast secretes β-glucosidase enzyme outside the cell, so the reaction takes place in the beer itself. Additionally, specific terpenes can be modified by yeast metabolism to produce different compounds with unique aromas (i.e. geraniol conversion to beta-citronellol).
Two Types of Terpene Biotransformation
Modification
- Yeast imports terpene compound
- Internal yeast enzymes modify terpene compound
- (a) NADPH dehydrogenase enzyme reduces geraniol to β-citronellol
- (b) Esterase enzyme forms terpene ester
- Yeast exports modified terpene compound
Release
- Yeast secretes β-glucosidase enzyme
- β-glucosidase enzyme cleaves sugar from terpenyl glycoside precursor
- Aromatic terpene released into beer
The terpene biotransformation potential of a yeast strain is related to the expression level of β-glucosidase and other enzymes that interact with terpenes, as well as the quantity of bound precursor terpene in the wort stream.
We have characterized Lallemand Brewing yeast strains for their terpene biotransformation potential. A summary of the most up to date strain characterization for biotransformation potential is available here.
Terpene flavor impact and sensory threshold
Terpenes have a diverse range of sensory characteristics such as citrus, floral, pine, or other flavors depending on the compound. Terpenes have very low sensory thresholds, meaning that they are perceptible at very low concentrations. As a point of reference, geraniol is sensory active at <100 μg/L, whereas acetaldehyde is sensory active at 10,000-20,000 μg/L. Even at very low concentrations, terpenes contribute to overall hop intensity and enhance the perception of other positive sensory attributes such as ‘tropical’ from thiols.
| Sensory Attributes | Sensory Threshold | Natural Sources | ||
| Geraniol |  |
Citrus, floral (rose), fruity | ≥ 6 μg/L | Above threshold in orange peel, strawberry, rose oil, geranium, raspberry, cherry, peach, plum, mango, passionfruit. Considered a high-impact aroma compound in cherry, peach, apricot, grapes, orange, and passionfruit |
| Nerol |  |
Floral (rosewater, citrus blossom) | 500 μg/L (in beer) | Grape, apricot, plum, rosewater |
| Linalool |  |
Floral (lavender), sweet, fruity, spicy | ≥ 5 μg/L | Orange peel, lavender, bay leaf, basil, coriander |
| α-Terpineol |  |
Woody, pine, floral, lime | 450 μg/L (model solution) | Passionfruit, peach, plum, apricot, mango |
| β-Citronellol |  |
Lime, lemon, floral | ≥ 8 μg/L | Lemon, grape, rose oil |
| β-Myrcene |  |
Resinous, citrus, spicy, pepper | ≥ 7-15 μg/L | Mango, orange, apricot, grapefruit |
How to optimize terpene biotransformation
Terpene biotransformation can be optimized through careful consideration of the hop variety, yeast strain, and brewing process.
- Choose hop varieties that are rich in terpene glycosides for addition in the whirlpool or early in fermentation. This allows for bound precursors to be extracted into the wort or beer and for the yeast to produce beta-glucosidase enzymes to release free aromatic terpenes. Early hop additions also allow for the yeast to consume the glucose released from the beta-glucosidase reaction.
- Hops rich in geraniol can also be added to whirlpool or early fermentation to encourage conversion into beta-citronellol by the yeast during fermentation.
- Select a yeast strain with good terpene biotransformation potential. Specific yeast strains are known to express higher levels of beta-glucosidase enzymes, or modify specific terpene compounds. Download the Lallemand strain table here.
- Add exogenous beta-gluosidase enzymes such as ABV Aromazyme™ to increase the release of aromatic terpenes from glycoside precursors.
The following chart is a guide to help brewers optimize terpene biotransformation during the brewing process in order to achieve specific hop aroma profile. Hop recommendations are not exhaustive, contact your hop supplier for recommendations for other varieties with similar characteristics. Yeast recommendations may be updated based on current R&D and commercial feedback. For a more detailed discussion about optimizing biotransformation in the brewing process, visit our Process Considerations page.
Learn more
Want to know more about the latest in terpene biotransformation research? Check out our R&D resources page.