Hop Assignment

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T

he female hop plant produces the α-acids or humulones which give beers their characteristic bitterness and aromas. The lupulin glands of the female plant synthesise three closely related types, namely, humulones, cohumulones and adhumulones. They impart their flavour only after boiling in the wort, when subtle chemical changes to the molecular structure produce the active isomerised forms. The transformation is incomplete but the methods used to determine bitterness cannot distinguish between bitter and non-bitter compounds. β-acids are reported in the literature of most suppliers but they can be largely ignored contributing little to the final product.

Whole hops are purchased directly on the spot market or by contract with the supplier ensuring a constant supply. This is important in maintaining product characteristics and quality.

Whole hops are preferred by many brewers and their customers for their traditional, authentic appeal. The British Hop Association lists 27 varieties with α-acid content up to 14.5%, higher levels are seen in some American varieties, going up to 17%, with oil content of 3.5% for CTZ ideal for the North American IPA style.

Whole hops added to the boil are active against gram positive bacteria preventing beer spoilage, they also contribute to the organoleptic and foaming qualities and prevent oxidation of the beer (Roberts 2016). For aroma, many breweries add the hops late to the boil, this reduces evaporation of the volatile oils from the wort retaining the hop aromas. Brewers also use dry hopping post fermentation this gives a cold aqueous extract. The hops are often added directly into the cask, which gives different results as there is no interaction between the yeast and the hops during fermentation. Aroma comes from the hop oils, each variety has its distinct range of volatile low molecular weight compounds which give a unique nose, this does not translate from the field to the glass as the volatile oils are lost at each stage of the brewing process (Schönberger & Kostelecky 2011). Dry hopping is popular in the USA with extremely high hopping rates often exceeding 1kg/hl added directly to the cask. This presents an opportunity for spoilage organisms as the hops are not sterile.

What are the drawback to using whole hops?

Hops vary from year to year with climate, location, and horticultural practice. Kishimoto et. al. (2008) observed an example of this with black currant aromas in beers made with American hop varieties which produced 4-mercapto-4-methylpentan-2-one (4MMP), a component of wine bouquet (and cat urine), this molecule was not found in European hops due to the different cultural practices employed.

Using whole hops is an inefficient way of extracting the flavour compounds and the isomerisation reaction does not proceed optimally in wort conditions.

Storage is a problem, hops deteriorate, bales are bulky and cold storage is required.

And the advantages.

Brewing credibility.

Doing things right.

Improved filtration through the plant material.

Improved foaming.

Hop polyphenols precipitate protein, reducing haze formation.

Hop Products.

Vacuum packed hops reduce storage requirements, deterioration, and supply problems for the brewer. Pellets are the product of choice for many brewers, these are available as type 90, which are cleaned and compressed, the type 45 offers a range of varieties sourced from around the world which contain less plant material giving a higher percentage of acids and oils.

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Pre-isomerised (Figure 1) hops are now available to the brewer, their use can make more efficient use of the available α-acids, most of which are lost in traditional wort boiling procedures. Wilson et. al. (2001) reported near doubling of the α-acid yield. This is a major consideration for all commercial breweries where costs are constantly under review. Clearly there are opportunities for savings with a lower input of hops required and the period of wort boiling may be cut. Savings can be made as a shorter boil means less energy is required for each brew.

Hop extracts and pre-isomerised extracts are prepared by supercritical CO2 extraction. Supercritical extraction is an ideal method of producing additives for the food and drinks industries as no solvents are employed in the extraction and the CO2 evaporates easily and is recycled.

Pre-isomerised aroma hops are added late to the kettle, the isomerised acids are extracted efficiently and the aroma characteristics are preserved. Pre-isomerised hop pellets and extracts can be added directly to the wort allowing further reactions to take place within the brew kettle, this gives a higher percentage recovery of isomerised acids in the finished beer. There is potentially a loss of aroma as the isomerisation is carried out at high temperature which allows the low molecular weight terpenes to volatilise (Wilson et. al., 2001). [pic 5]

Isomerised Kettle Extract (IKE) and the water soluble potassium salts (PIKE) are prepared from CO2 extracts by catalytic conversion employing magnesium as catalyst.

Products containing pre-isomerised α-acids can be added following fermentation only after the oils and waxy components of the extract have been separated. This further reduces costs as no losses due to boiling, yeast interaction or hot break result. Extracts of α-acids cannot be used at this stage as they require boiling in the wort to develop bitterness. [pic 6]