The Com­plex Case of Thi­ols

Craft Beer & Brewing Magazine - - Contents - By Stan Hierony­mus

Thi­ols make up less than 1 per­cent of the es­sen­tial oils in a hops cone, but Stan Hierony­mus re­ports they might hold a key to the fash­ion­able trop­i­cal fla­vors that are in de­mand these days.

Thi­ols, also known as mer­cap­tans, are sul­fur-con­tain­ing or­ganic com­pounds with a sul­fur atom bound to a hy­dro­gen atom. Thi­ols make up less than one per­cent of the es­sen­tial oils in a hops cone but might hold a key to the fash­ion­able trop­i­cal fla­vors that are in de­mand these days.

“Here I am in the 21st cen­tury I have to say it ain’t as cool as I hoped it would be No man on the moon, no­body on Mars Where the hell is my fly­ing car?” —From “21st Cen­tury Blues” by Steve Earle

WAIT­ING FOR TECH­NOL­OGY THAT

turns brew­ing re­search into in­for­ma­tion use­ful to brew­ers can feel a lit­tle like wait­ing for a fly­ing car. At the In­ter­na­tional Brew­ers Sym­po­sium on Hop Fla­vor and Aroma in Beer last sum­mer at Ore­gon State Uni­ver­sity, five of the dozen pre­sen­ta­tions em­pha­sized the role thi­ols play in cre­at­ing the bold, of­ten trop­i­cal, fla­vors cur­rently fash­ion­able in beer. Sci­en­tists in France, Bel­gium, Ja­pan, the United States, and Ger­many are all in­volved in re­lated re­search.

The po­ten­tial seems so ob­vi­ous that Nyséos, a spe­cialty lab­o­ra­tory in France that fo­cuses pri­mar­ily on wine, has cre­ated a Thiol Po­tency (TP) in­di­ca­tor. Lau­rent Da­gan, di­rec­tor at Nyséos, ex­plains that dis­cov­er­ies about the role thi­ols play in de­vel­op­ing fla­vor and the TP in­di­ca­tor are his “call” to brew­ers. “Hey, we have new in­for­ma­tion. It is com­plex, but we are try­ing to find in­di­ca­tors use­ful for you. What do you think about that?” he asks.

The key word is com­plex be­cause thi­ols make up less than 1 per­cent of the es­sen­tial oils in a hops cone, and there are un­counted hun­dreds of com­pounds that con­trib­ute to hops odor that our brains con­vert into hops aroma.

What Are Thi­ols?

Thi­ols, also known as mer­cap­tans, are sul­fur-con­tain­ing or­ganic com­pounds with a sul­fur atom bound to a hy­dro­gen atom. Sci­en­tists first iden­ti­fied them in hops in the early 2000s, fo­cus­ing on three that wine­mak­ers have known about for decades. One, 4-mer­capto-4-methylpen­tant-2-one (4MMP), smells and tastes of box tree, black cur­rant, and ribes. It is also known as 4-methyl-4-sul­fanylpen­tan-2-one (4MSP). An­other, 3-mer­cap­to­hexan-1-ol (3MH), is of­ten de­scribed as ex­otic, smelling of rhubarb and cit­rus. And the third, 3-mer­cap­to­hexyl ac­etate (3MHA), is rem­i­nis­cent of pas­sion fruit and guava.

These com­pounds are all prom­i­nent in Sau­vi­gnon blanc, Ries­ling, and other

They are formed dur­ing fermentation wines, al­though they do not oc­cur in from pre­cur­sors present in grape grapes. must. How­ever, free thi­ols have been iden­ti­fied in many hops va­ri­eties along with two dif­fer­ent bound forms (pre­cur­sors) in both hops and bar­ley malt. “Lit­tle is known about how these an­a­lytes de­velop in malt and hops over time or what process changes di­rectly re­sult in the in­crease of these an­a­lytes,” says Scott La­fontaine, a grad­u­ate re­search as­sis­tant at Ore­gon State Uni­ver­sity. In­cluded are not only the thi­ols found in wine, but sev­eral oth­ers.

An­a­lyz­ing thi­ols is chal­leng­ing be­cause of their low con­cen­tra­tion and high re­ac­tiv­ity. The stan­dards and in­stru­men­ta­tion that are needed to mea­sure them are very ex­pen­sive, and few lab­o­ra­to­ries have the equip­ment needed. “Part of the rea­son [this is chal­leng­ing] is that thi­ols have very low odor-de­tec­tion thresh­olds, and there­fore, very small con­cen­tra­tions of these an­a­lytes can po­ten­tially have a large im­pact on beer fla­vor,” La­fontaine says. “In terms of con­cen­tra­tions, we are look­ing at parts per tril­lion. To give you an idea of what we are try­ing to find, it is like look­ing for the top of a pin in an Olympic-size swim­ming pool.”

Whoa, There’s Never a Sin­gle Marker

Stop and take a breath. Thi­ols are not a magic bul­let for cre­at­ing trop­i­cal fla­vors. The hops aroma/fla­vor ma­trix re­mains a prod­uct of syn­ergy. Re­mem­ber the les­son

“Part of the rea­son [this is chal­leng­ing] is that thi­ols have very low odor-de­tec­tion thresh­olds, and there­fore, very small con­cen­tra­tions of these an­a­lytes can po­ten­tially have a large im­pact on beer fla­vor,” La­fontaine says. “In terms of con­cen­tra­tions, we are look­ing at parts per tril­lion. To give you an idea of what we are try­ing to find, it is like look­ing for the top of a pin in an Olympic-size swim­ming pool.”

learned with linalool, a monoter­pene. In 1981, Val Pea­cock and as­so­ciates at Ore­gon State had sug­gested the im­por­tance of linalool to hops aroma. They de­vel­oped a model to pre­dict the amount of flo­ral-hops aroma likely in a beer based on the amount of linalool, geran­iol, and ger­anyl es­ters in the es­sen­tial oils. This led to re­search by other sci­en­tists. Pea­cock no­ticed that “be­cause of the amount of at­ten­tion given to linalool dur­ing re­cent decades, its per­ceived im­por­tance has been el­e­vated far be­yond its true rel­e­vance.” As a re­sult, he con­cluded, “this dis­torts brew­ers’ un­der­stand­ing of the na­ture of hops aroma in beer. Hops have more to con­trib­ute to the aroma of beer than just linalool.”

Thi­ols in Ac­tion

As re­ported in “Hops Oils & Aroma: Un­charted Waters” (Craft Beer & Brew­ing Mag­a­zine®, March/april 2016), a team at Sap­poro Brew­eries in Ja­pan con­ducted a study in 2012 in which they com­pared the com­po­si­tions of monoter­pene al­co­hols in var­i­ous hops, ex­am­in­ing the be­hav­ior of geran­iol and cit­ronel­lol un­der var­i­ous hop­ping and hops-blend con­di­tions. Among other things, their ex­per­i­ments demon­strated that the co­ex­is­tence of linalool, geran­iol, and cit­ronel­lol could in­crease the per­cep­tion of “cit­rus” in a model so­lu­tion. Anec­do­tally, us­ing this in­for­ma­tion, ad­ven­tur­ous brew­ers com­bined hops high in linalool (such as Bravo or Nugget) with oth­ers rich in geran­iol (for in­stance, Chi­nook) to brew beers they de­scribed as fruity, some­times trop­i­cal. But it turns out linalool, geran­iol, and cit­ronel­lol may not have been to­tally re­spon­si­ble for those fla­vors. Fol­low­ing ad­di­tional re­search, the team from Sap­poro has a new hy­poth­e­sis. In their ex­per­i­ments, sen­sory pan­elists rated var­i­ous aroma at­tributes of a so­lu­tion dosed with 4MMP, scor­ing it two (out of four) for “trop­i­cal.” Sim­i­larly, a so­lu­tion dosed with a mix­ture of linalool, geran­iol, and cit­ronel­lol scored less than two for trop­i­cal. How­ever, when all four were blended, the trop­i­cal score jumped to a full four. (That may be what hap­pens when brew­ers use Chi­nook be­cause not only is it geran­iol-rich, it also con­tains 4MMP.)

The Thiol Po­tency In­di­ca­tor

The for­mula Nyséos pro­poses to mea­sure thiol po­tency gen­er­ates a TP in­di­ca­tor, but the lab also pro­vides in­for­ma­tion about bound and free thi­ols. For in­stance, Ca­lypso has a TP of 112, but 95 per­cent of that is bound 3MH po­tency and 5 per­cent bound 4MMP po­tency. To free bound thi­ols, Nyséos sug­gests us­ing hops in the ket­tle. Ci­tra, on the other hand, has a lower TP (57), but 33 per­cent is free 4MMP, 8 per­cent bound 4MMP, 1 per­cent free 3MH, and 58 per­cent bound 3MH, mak­ing it suit­able for use in the ket­tle or dry hop­ping.

Un­for­tu­nately, Ci­tra doesn’t ac­tu­ally have a sin­gle TP. Its TP varies from one lot to the next. For in­stance, Nyséos an­a­lyzed two dif­fer­ent lots of Ci­tra dur­ing its re­search. One con­tained two-thirds the amount of free 3MH as the other, but 50 per­cent more free 4MMP.

Dur­ing the aroma and fla­vor con­fer­ence at Ore­gon State, Martin Stein­haus pre­sented in­for­ma­tion from the Ger­man Re­search Cen­ter for Food Chem­istry about 4MMP in forty-six va­ri­eties from five coun­tries. Not sur­pris­ingly, the group con­cluded that va­ri­ety was the largest fac­tor in de­ter­min­ing 4MMP con­cen­tra­tions. How­ever, the re­search also found vari­a­tions re­lated to har­vest year and prove­nance. The study did not take into ac­count other fac­tors—such as har­vest ma­tu­rity, post-har­vest pro­cess­ing, and hops stor­age—that other re­searchers have re­ported may have a sig­nif­i­cant im­pact on the com­po­si­tion of es­sen­tial oils, in­clud­ing thi­ols.

That’s why the pos­si­bil­ity of a TP in­di­ca­tor num­ber printed on a pack­age of hops, along with a list of other com­po­nents, is so ap­peal­ing. It would mea­sure the fi­nal prod­uct. For now, the cost of tech­nol­ogy makes this look like a fly­ing car, but Da­gan is an op­ti­mist.

“Thiol man­age­ment is tricky in wine,” Da­gan says. “But af­ter twenty years, wine­mak­ers are able to pro­duce great Sav­i­gnon blanc wine with bet­ter man­age­ment of thi­ols. So, I think that will be the same for beer in five years.”

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