To date, around 10,000 flavoring substances have been identified in foods and beverages. Ester compounds are the largest group of aromas. Next are sulfur-containing aroma substances, summarized under the technical term "thiols". The term is derived from the Greek word "theion", meaning sulfur. Thiols are small sulfur aroma compounds that are subject to constant change through oxidation, isomerization, or rearrangement due to their high reactivity. They can contain alcohols, aldehydes, or esters as functional groups. Thiols are found in many foods and beverages, including wine, coffee, cheese, olive oil, fruits, and vegetables.
We have known for almost 20 years that thiols are important in hops and beer, ever since the first thiol was identified in the American hop variety Cascade. Since then, we have found them in their free form in all new cultivars, which not only have citrus-like fruit aromas (which can also be found in landraces) but above all have various sweet or tropical fruit aromas. To date, over 40 different thiols have been identified in hopped beers, but only some of them have a fruity, tropical character. Many of these sulphur compounds in beer are described as grilled, nutty, plastic-like, sweaty, diesel-like, light-struck, or a cat-like aroma. Thiols are particularly exciting because the aroma quality of the same compound can be described using a whole range of different descriptors, depending on the concentration and sensitivity of the smeller. 4MMP (4-mercapto-methyl-pentan-2-one), for example, is described as boxwood, passion fruit, blackcurrant, cat-like or sweaty, depending on concentration and sensitivity. Analysing thiols is difficult in itself, but analysing thiols in hops or beer is particularly challenging. Therefore, analyses are only feasible in selected laboratories and are usually very expensive. It is therefore by no means a routine analysis suitable for everyday use that breweries could easily establish.
In recent years, it has become clear that, although free thiols are found in almost all new cultivars that stand out due to their fruity aromas, precursor structures containing bound thiols are present in high concentrations in every hop variety. These bound thiols are found in various forms, such as cysteine or glutathione, a tripeptide. Therefore, brewers rightly ask how they can release the bound thiols during the brewing process to impart more fruity flavors to the beer.
Only a few nanograms per liter (ng/L) are required for thiols to impart a fruity flavor to beer. In very hop-intensive beers, concentrations of up to 300 ng/L can be detected. Free thiol concentrations in hop varieties can reach 100 µg/kg, while precursor structure concentrations can exceed 30,000 µg/kg. The potential therefore seems enormous. However, the largest quantities of precursor substances are bound to 3MH (3-mercaptohexan-1-ol), a thiol that tends to smell like grapefruit and offers less aromatic diversity than 4MMP. Unfortunately, the concentration of precursor substances around 4MMP is much lower and often undetectable.
A few years ago, a team of Belgian researchers developed a theoretical way of determining which enzymes are required from yeast to release the thiols. First, glutathione is enzymatically degraded into cysteine. Then, the thiols are released from the cysteine through beta-lyase activity. However, experimental fermentations showed very low release compared to wine fermentation. Since then, many yeast marketers have examined the beta-lyase activity of their yeasts, revealing which strains can release thiols. At the same time, new yeast strains have been bred using the CRISPR CAS method to strongly express the genes responsible for thiol release. These GMO yeasts are permitted in the USA and are effective. However, they sometimes produce very strong fruity aromas that are no longer associated with the hops used.
The precursor structures of thiols can be found in spent hop pellets, which comprise of the remaining particles pressed into pellets – after the hops have been distracted. In its own trials, the BarthHaas Brewing Solutions team demonstrated that by using spent hop pellets intense fruity aromas can be achieved with classic ale and thiol yeasts through dry hopping during fermentation. Beers brewed with thiol yeast produce a wider range of aromas than those brewed with classic ale yeast, depending on the amount of spent hop pellets used. Additionally, we demonstrated that mash hopping slightly increases the concentration of free thiols.
An important yet often overlooked topic in connection with thiols is the interaction of aromas. Every food and beverage contains several hundred, if not a thousand, different aroma substances. In beer, these include original aroma substances from the raw materials, as well as substances created during the brewing process. Each aroma substance has a threshold value, which is the concentration at which it can be recognized in water or another medium. However, the presence of a second flavoring agent renders this threshold value obsolete! Binary mixtures can have a much higher or lower threshold value. This means that flavorings can have a masking, synergistic, additive, or subtractive effect. Unfortunately, little research has been conducted on this topic. However, it's clear that these aroma interactions are strong, and thiols act synergistically with monoterpene alcohols, such as linalool and geraniol. One need only give it a moment's thought to realize how complex this topic is and how impossible it is to study the exact interactions of the thousands of different aroma compounds in beer.
Therefore, brewers have many tools at their disposal to tease out fruity aromas from hops by adjusting process parameters and yeast. The last few years have brought many groundbreaking findings in this area. However, thiols are particularly fragile in finished beer. You must ask yourself if it's worthwhile to put in so much effort for aromas that disappear after just a few weeks, leaving consumers to enjoy only a fraction of them.
