Today in 1968, US Patent 3418135 A was issued, an invention of Peter D. Bayne, assigned to Schlitz Brewing Co., for his “Light-Insensitive Malt Beverage and Process of Producing the Same.” There’s no Abstract, although in the description it includes this summary:
This invention relates to a process of producing a malt beverage such as beer, and more particularly to a process of producing a malt beverage which is insensitive to light.
Malt beverages such as beer and ale are not stable to light. When subjected to light, the beer develops an odor and flavor characterized as skunky. To prevent the development of the sunstruck or skunky odor, beer is generally packaged in colored bottles which minimizes exposure to sunlight. Even though packaged in colored bottles or in cans or kegs, the beer can nevertheless develop the sunstruck odor by virtue of the short exposure to sunlight in drinking glasses.
It has been found that the presence of isohumulones are responsible for the development of the sunstruck odor in beer. The hops, which are boiled with the wort, contain resins and oils which are contained in the lupulin. The resins include the alpha and beta resins, with the alpha resins containing a bitter acid called humulone and the beta resins containing an acid called lupulone. The alpha acids provide the bitter flavor for the beer while the beta acids have low solubility in the Wort and do not appreciably enter into the brewing process.
During the brewing-process, the humulone fraction is isomerized to the corresponding isohumulones. The sunstruck odor substance in beer has been identified as a mercaptan,3-Inethyl-2-butene-l-thiol, which is formed by the photolysis of the siX-membered side chain on the isohumulone molecule. The free radical formed by the intervention of sunlight splits Off carbon monoxide, forming the 3-methyl-2-butenyl radical. This product, in turn, reacts with hydrogen sulfide, which is formed from proteins or amino acids by photochemical action, to form 3- methyl-Z-butene-l-thiol. This mercaptan is considered to be the main component of the sunstruck odor in malt beverages.
To prevent the sunstruck odor, it has been proposed to convert a carbonyl group of the isohumulones into a secondary alcohol by reduction. In this reduced form, the molecule becomes insensitive to sunlight and there is no formation of odor on exposure to sunlight. This conversion has been carried out in the past by use of sodium borohydride as disclosed in Patent No. 3,079,262, but the use of sodium borohydride has not been completely successtul in that the reaction is difiicult to control. The contact time between the sodium borohydride and the isohumulone must be accurately controlled in order to effect a complete reduction of the isohumulones In addition, the sodium borohydride is a relatively costly material which adds to the overall cost of the beer.
The present invention is directed to a process for producing a light-insensitive malt beverage by reducing the isohumulones by use of an alkali metal dithionite. More specifically, the process consists of initially extracting ground hops with a solvent, such as hexane. The solvent is then evaporated, leaving a dark resinous oil which is isomerized in a dilute alkaline solution. After the isomerization, the material is neutralized to pH of 6.0 to 7.0 with a mineral acid and the lupulone, wax and chlorophyll are removed by further extractions with a solvent. The aqueous phase is then further acidified to a pH in the range of 1.0 to 2.0 and the isohumulones extracted with a solvent such as diethyl ether. The reduction is then accomplished by contacting the ether-isohumulone extract -with an aqueous solution of sodium dithionite.
After the reduction, the reduced isohumulone extract is separated from the dithionite solution and washed with brine. The ether is then removed by distillation to produce the reduced isohumulone concentrate.
The reduced isohumulone when added to a standard unhopped wort or beer produces a beer which does not develop the characteristic sunstruck odor when subjected to sunlight and at the same time allows precise control of the isohumulone bitterness levels which was hitherto impossible by traditional hopping methods.
