Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1999-06-10
2001-03-06
Padmanabhan, Sreeni (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S366000, C568S377000, C568S379000, C426S600000
Reexamination Certificate
active
06198004
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an improved process for converting alpha acids and isoalpha acids into tetrahydroisoalpha acid for use as hop flavors. According to the process, tetrahydroisoalpha acids are produced in high yield and high purity from alpha acid and isoalpha acid solutions containing low to very high levels of sulfur without any pretreatment.
BACKGROUND OF THE INVENTION
Hops have been used for centuries for the purpose of contributing a clean bitter taste to beer. Analysis of hop constituents has demonstrated that the most important bittering compounds derived naturally from hops are a group of analogous compounds generally known as alpha acids (humulones).
In a conventional brewing process, hop cones are boiled with sweet wort in a copper kettle for about one to two hours, and then the resulting wort mixture is filtered and allowed to cool. Isoalpha acids are produced by isomerization of alpha acids during boiling of the wort. The isomerization yield in the boiling process, however, is low. Such low yields have led to the development of commercial processes which extract the active compounds, such as, alpha acids and beta acids, from hops and convert them to the desired hop flavors, e.g., isoalpha acids, rhoisoalpha acids, tetrahydroisoalpha acids and hexahydroisoalpha acids, at relatively high yields. The commercially produced hop product may then be added post-fermentation to maximize utilization and provide consistent hop flavor.
It is well known in the brewing industry that isoalpha acids are susceptible to degradation upon exposure to light. This degradation in beer produces a highly undesirable skunk-like odor.
It is also well known in the art that hydrogenating the isoalpha acids (or the precursor alpha or beta acids) will increase the light stability of the beer. This hydrogenation requires a catalyst, such as palladium on carbon (Pd/C).
The hydrogenation of normal homolog isoalpha acids with platinum oxide is described by P. M. Brown, G. A. Howard and A. B. Tatchell, J. Chem. Soc. 545 (1959). However, the process results in low yields of tetrahydroisoalpha acid. E. Byrne and S. J. Shaw, J. Chem. Soc. (C), 2810 (1971) describes the reduction of tetrahydroisoalpha acid to deoxygenated products by hydrogenation with palladium on carbon in methanol at a pH of about 3. A process for isomerizing and hydrogenating alpha acids to produce tetrahydroisoalpha acid (THIAA) or hexahydroisoalpha acid (HHIAA) is described in U.S. Pat. No. 5,013,571, the disclosure of which is herein incorporated by reference. Further, U.S. Pat. No. 5,600,012 describes a single step process for making pure tetrahydroisoalpha acids from pure iso-alpha acids by a controlled hydrogenation process.
When alpha, beta, or isoalpha acids are hydrogenated, compounds which are referred to as “catalyst poisons,” and which are associated with the hop extract, foul the catalyst. Such fouling of the catalyst causes the hydrogenation of the alpha, beta, or isoalpha acids to proceed slower than normal and eventually cease before all of the alpha, beta, or isoalpha acids are hydrogenated. In some instances, the catalyst becomes so quickly fouled that it must be replaced during the middle of the hydrogenation process in order for the hydrogenation to proceed to completion. Simple replacement or changeout of the catalyst is an unsuitable solution to the poisoning problem for at least two reasons. First, the substitution of a new catalyst midway through the hydrogenation process can promote the formation of overreacted products. Second, the catalyst is quite expensive, and frequent replacement of the catalyst greatly adds to the cost of production. Accordingly, there remains a need for a process for producing tetrahydroisoalpha acids hop flavors from alpha acids that is simple, inexpensive, and high yielding.
SUMMARY OF THE INVENTION
It has been discovered that when a noble metal catalyst is added incrementally or continuously during the hydrogenation step, alpha acids and isoalpha acids can be converted to tetrahydroisoalpha acids at lower pressures, lower temperatures, and shorter reaction times than previously known. It has also been discovered that the incremental or continuous addition of noble metal catalyst allows for the conversion of alpha acid containing high levels of sulfur in exceptional yield.
Accordingly, the invention relates to a process for converting alpha acid and isoalpha acids to tetrahydroisoalpha acid. The process comprises isomerizing alpha acid to isoalpha acid and hydrogenating the isoalpha acid in the presence of a noble metal catalyst wherein, the noble metal catalyst is added incrementally or continuously throughout the hydrogenation step.
In another embodiment of the invention, the tetrahydroisoalpha acid may be further reduced to produce hexahydroisoalpha acid.
Additional objects and advantages of the invention are discussed in the detailed description which follows, and will be obvious from that description, or may be learned by practice of the invention. It is to be understood that both this summary and the following detailed description are exemplary and explanatory only and are not intended to restrict the invention.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to a process for converting alpha acids and isoalpha acids to tetrahydroisoalpha acid. In the process, alpha acid is isomerized to isoalpha acid. The isoalpha acid is then hydrogenated in the presence of a noble metal catalyst to form tetrahydroisoalpha acid (THIAA). The noble metal catalyst is added incrementally or continuously throughout the hydrogenation step. The process of the invention may be demonstrated by the following reaction scheme:
wherein R is a lower alkyl, preferably, a C
1
-C
6
alkyl, more preferably isopropyl, isobutyl and sec-butyl.
Accordingly, the invention relates to a process of converting isoalpha acid to tetrahydroisoalpha acid comprising the step of hydrogenating the isoalpha acid in the presence of a noble metal catalyst, wherein the noble metal catalyst is added continuously or incrementally throughout hydrogenation.
The Isomerization Step
According to the process of the invention, alpha acid is isomerized to isoalpha acids. As discussed above, alpha acids are derived from hops where they exist as different homologs. Typically, the primary homologs are the isopropyl, isobutyl and sec-butyl homologs. The alpha acids can be isolated from hops by a process in which the organic components of the crushed hop cones are extracted by liquid CO
2
as taught, for example, by U.S. Pat. Nos. 5,523,489 and 4,590,296, the disclosures of which are herein incorporated by reference. The alpha acids are then separated from the hop extract using an aqueous extraction at a pH of about 8 to about 8.3. The process of the invention is applicable to alpha acids having the formula:
wherein R is a lower alkyl, preferably a C
1
-C
6
alkyl, more preferably isopropyl, isobutyl and sec-butyl. The process according to the invention can be used to convert alpha acids having a high sulfur content, i.e., ranging from about 400 to about 650 ppm.
The isomerization step of the process of the invention comprises the step of boiling a protic solvent solution containing from about 10% to about 40% by weight alpha acid at a temperature ranging from about 90° C. to about 105° C. for about 2.0 to about 4 hours. Any protic solvent system that results in the desired end product may be used. Exemplary protic solvent systems include, but are not limited to, water, alcohol, such as C
1
-C
3
alcohols, including isopropanol, ethanol, and methanol and mixtures thereof. Preferably, the solvent system is reaction inert. The expression “reaction inert solvent” as used in the context of the invention refers to a solvent which does not interact with starting material, reagents, intermediates or desired product in a manner which adversely affects the yield of the desired product. In addition, nonprotic solvents can be used in combination with protic solvent. These include chlorinated solvents such as me
Gauthier George
Maye John Paul
Weis Scott W.
Xu Jianping
Haas Hop Products, Inc.
Morgan & Lewis & Bockius, LLP
Padmanabhan Sreeni
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