One of the aspects of beer flavor that has interested me the most are what are called “glycosides”. The reason they are so interesting to me is because it seems they add a whole new level of complexity to hop flavors beyond the traditionally accepted sources such as hydrocarbons, oxygenated compounds, and sulfur-containing compounds. Glycosides are compounds which contain two parts: a sugar component which is in its cyclic (ring) form, and another component attached to the sugar at the number 1 (or “glycosidic” carbon). Some of these companion molecules can also be sugars, so sucrose, being a 2-sugar molecule (disaccharide) made of one glucose unit and one fructose unit attached through the appropriate carbon, is an example of one of these types of glycosides. The glycosides which appear to influence beer flavor are found in hops and have aromatic compounds bonded to the carbohydrates. In their combined state, the glycosides have no flavor; no sweetness and no aroma. However, once this glycosidic bond is broken the aromatic compound is free to volatilize into the headspace of the beer and ultimately into your nose.
Much of the initial research into hop glycosides has come from the research labs at Miller Brewing Company. Miller is in a unique position among many brewers for this type of research because of the way they use hops: much of their beer is hopped with extracts rather than with the vegetative hop material itself. For example, Miller Genuine Draft and Miller High Life are hopped with “reduced hop extracts” which make them resistant to the UV degredation processes that lead to lightstruck/skunky flavors which arise from using clear and green bottles (more on that another time), and since these chemically reduced extracts only impart bitterness then they also must use hop aroma extracts to achieve the desired levels of hop aroma. These processes gives Miller a unique perspective on the influence of hop extracts and beer flavor, and one thing they’ve noticed is while beers hopped with extracts can be “good”, they acknowledge that there is something different about these beers, whether it is for better or worse.
Hops can be extracted in a number of ways, but modern methods use liquid or super-critical CO2. This extracts the hydrophobic portion of hops, which includes the oils and resins that are apparently the main sources of hop flavor – aroma and bitterness. What is left over is the water-soluble portion which is made up of carbohydrates, proteins, polyphenols, and inorganic material and totals about 20-25% of the original weight. This material is called “spent hop powder” and even though the oil-based hop flavors have been removed Miller’s research has shown a surprising potential for it to contribute to the “kettle hop” flavor of beer.
Part of the reason for the focus on glycosides in hop flavor is because wort is an aqueous matrix; it’s mostly made of water. Most of the native aromatic compounds in hops are hydrophobic (meaning they’re similar to oil in that they’d rather be in an organic solvent than in water), and they tend to be quite volatile as well. So, what happens when volatile hydrophobic compounds come in contact with the boiling aqueous solution? It’s hard to imagine many of them going into solution considering the two sizable obstacles of temperature and solubility. As is commonly understood, most of these volatile oils evaporate during the boil and are lost out the top of the kettle. This is why brewers tend to add hops early in the boil if they want to impart bitterness to the beer, and they add hops late to the boil to add aroma (but not much bitterness, since there isn’t much time to isomerize).
As I mentioned, Miller has done some research to identify why some of their beers brewed only with extracts have different hop flavors than those brewed with “real” hops. They took some of this spent hop powder and extracted it with an aqueous/alcohol mixture then further refined their extraction with various bench-top chromatography techniques and other purification steps. Once this extract was finished, they added beta-glucosidase, which is an enzyme that breaks the glycosidic bonds. After this treatment, they analyzed the aromatic results with GC/MS and found a number of interesting aromatic compounds, including: benzaldehyde (almond, maraschino cherry), vanillin (vanilla), raspberry ketone, geraniol (floral, rose), linalool (floral), phenylacetaldehyde (honey, floral), and many other primary alcohols, ketones, and aldehydes which are also aromatic.
Some of the research I have performed involved aqueous extracts of spent hop material as well (although for different purposes), and I believe I have encountered some of these glycosides and seen how the flavors they can produce. After extracting and concentrating my spent hop extract I was pleasantly surprised to note that it had quite a lot of aroma, especially considering that it should have all been extracted by the original super-critical CO2 extraction process. The aromas I found were fig-like, blueberry, fruity-type aromas and were quite pleasing. I immediately thought of this glycoside research.
Identifying these glycosides is tricky, since in their combined form they are not volatile, so gas chromatography doesn’t work (it works well for analyzing the volatile hydrolysis/breakdown products, however). LC-MS or HPLC-MS must be used to detect the original glycosides, but it’s still difficult to obtain the desired sensitivity. From what I’ve seen, there is still a lot of work that can be done to characterize exactly how these compound influence beer flavor, but there are a number of ideas as to how this might happen. For example, the glucoside of linalool (a glycoside of glucose and linalool) has been identified in hops (as have glycosides of linalool with other, more complex carbohydrates), and linalool itself has been found in beer. Are the linalool glycosides biologically stored in hops where linalool might be released during aging? Is linalool released during the kettle boil where the glycosidic bond can be hydrolyzed with acid? Some even think glycosides might survive the brewing process and the aromas are released in the mouth.
I find glycosides an interesting aspect of beer flavor since there hasn’t been much focus on them historically, as researchers and brewers traditionally focus on the oil fraction of hops, which makes a bit of sense since it’s the source of the potent aroma of the fresh hops themselves (and don’t get me wrong, hop oils play a profound role in beer flavor when used after boiling, particularly during dry hopping). I find them interesting because they seem to add a new layer of complexity to hop aroma, an area with plenty of complexity and mystery already. And I find them interesting because I think I ran across them in my own research in a rather serendipitous manner. Hopefully more answers come soon so brewers can continue to refine their approach to hop aroma.
Goldstein, H., Ting, P., Navarro, A., Ryder, D., Water-soluble hop flavor precursors and their role in beer flavor, EBC Congress, 1999, 53-61.