Hop breeding: Better hops for a bitter tomorrow

Date:10-29-2019


When you lift a glass of IPA to your nose and smell the complex, floral aroma, you probably aren’t thinking about botany or genetics.

However, those aromas—and the hops varieties that they come from—are the results of generations of hops breeding. New hops varieties appear all the time, and more are in the pipeline. Here’s how they come into being.

Let’s start at the beginning to get the big picture. We all know that beer is made from malt, hops, water, and yeast. Brewers yeast (Saccharomyces cerevisiae) is a fungus and water (H2O) is a molecule. Malt is most often made from barley (Hordeum vulgare), which is a flowering plant (or angiosperm). Like all grasses (including all cereal grains), barley is a monocot (its seed contains only one embryonic leaf), one of the two types of angiosperms.

That’s Classified

The hops plant (Humulus lupulus) is also a flowering plant, but it is a dicot (or eudicot), the more abundant type of angiosperm. Its seed has two embryonic leaves. Of the more than forty orders of dicots, hops plants belong to the Order Rosales, which comprises roses and their relatives—including apples, strawberries, and peaches. Of the nine families within Order Rosales, the hops plant belongs in the Family Cannabaceae, along with hackberries and—of course—hemp and cannabis. There are three species of hops—Humulus yunnanensis, H. japonicus, and H. lupulus—the last of which is the species used in brewing.

Let’s Talk About Sex

Hops plants are dioescious, meaning they have male and female flowers on separate male and female plants. Hops plants don’t produce showy flowers with large petals. Instead the flowers are small spiky-looking structures.

Wild hops varieties are diploid, meaning they have two sets of chromosomes—one inherited from the mother and one from the father. A few commercial cultivars are triploid, meaning they have three sets of chromosomes. Hops plants have ten chromosomes, nine of which are autosomes (“regular” chromosomes). The final chromosome is the sex-determining chromosome.
As in humans, hops have sex chromosomes labeled X and Y, and an XX individual is female and an XY is male. (Note: Although labeled as X and Y chromosomes, hops chromosomes are not homologous to human chromosomes.)

In the wild, hops reproduce sexually. Pollen from the male plant—carrying one copy of each of its ten chromosomes—lands on the female cone and fertilizes an egg. The egg, of course, carries one copy of each of its ten chromosomes. The fusion of the pollen and the egg creates a zygote that contains two copies of each of its ten chromosomes.

This diploid zygote is encapsulated in a seed and the seed grows into a seedling (either a male or female plant) and the cycle continues. Hops also produce rhizomes (underground storage stems that can sprout both roots and stems) and can also spread asexually via lateral rhizome growth.

Let’s Talk About Cloning

Commercially, hops plants are made to reproduce asexually (clonally), and only female plants are grown for brewery use. Female plants yield the seed cones (or strobiles) containing the lupulin glands that contain both the alpha and beta acids, plus the essential oils used in brewing. Males lack cones and their presence near female plants will additionally cause the female cones to bear seeds, which reduces their brewing value.

Hops plants are perennials and, once established, sprout each spring from rhizomes. In a commercial setting, the first set of bines is cut back each spring, and the second set to emerge is trained to trellis wires when they are tall enough. A bine is a type of vine that lacks tendrils. In the wild, hops bines grow on whatever support is available—trees, fences, etc. Hops plants can grow up to thirty feet vertically. In the late summer, they flower and then produce cones.

Typically, the density of cones is greatest at the top of the bine. It generally takes three years for a newly planted hops clone to begin yielding a worthwhile crop of cones.

On the Farm

On a hops farm, female hops plants are planted as rhizomes. The rhizomes come from splitting existing rhizomes into pieces. At planting, the rhizome pieces can be as small as the length and width of a human finger. But they grow in size each year.

I grew hops a few years ago and found that, after 3 years, the underground hops rhizome was a roughly basketball-sized tangle of rhizomes. These large masses of rhizomes are called crowns. Many new hops plants can be established by breaking up a single crown.

On a commercial hops farm, the rows of hops trellises can stretch almost from horizon to horizon. When you stand in a hops field where a single variety is being grown, you are surrounded by a single hops plant that emerged from a single seed many years ago. In the case of some classic hops varieties, that seed might have sprouted centuries ago.

In addition, you are surrounded by a cultivar of hops represented by a single female and for which no male exists.

Name any variety of hops used in brewing, and there is no male of that variety. (There are, of course, males that share part of the cultivar’s genetics, but never all of it. For starters, the father of any brewing cultivar shares 50 percent of its genetics with that cultivar.)

Hops Breeding

Although there are more than 100 hops varieties available today, breeders continue to produce more. Brewers are always on the lookout for new varieties with appealing brewing characteristics. Growers are always looking for better yields, better disease resistance, higher vigor, and high alpha-acid levels (because some growers get paid by alpha, not weight of the hops). In addition, new hops varieties have to work well with the existing machines that harvest the plants and separate the cones from the bines.

“Storageability” is also an important characteristic; you can breed the best brewing hops in the world, but if its alpha is mostly gone after a few months, no brewery will want to use it.

Hops plants have a high level of heterozygosity (a measure of genetic variance). Diploid organisms carry two copies of every gene. Each of these copies is called an allele. If an organism has two different alleles for one gene, it is heterozygous for that gene. (Or heterozygous at that locus, using the terminology of geneticists. A locus is a location.)

If the organism had two of the same alleles at that locus, it would be homozygous.

As an example, let’s take the human blood type gene. In the population, there are three alleles, A, B, and O. If an individual were AA (or BB or OO), (s)he would be homozygous. Individuals who were AB, AO, or BO would be heterozygous. Heterozygosity is a measure of what percentage of loci (genetic locations, or genes) are heterozygous in an organism or population.

New hops varieties begin with the cross of a female hops plant (almost always a known brewing variety) and a male plant with a desirable characteristic. When the female flowers emerge, they are bagged to block airborne pollen. Pollen from male flowers is swabbed and applied to the female flowers, which results in seeds being produced. These seeds are planted in small planters—usually in a greenhouse—and germinated. An interesting observation about the seedlings produced from hops crosses is that none of the offspring match either of the parents exactly. For example, if a Cascade female were crossed with a male—even a male with some Cascade genetic material—none of the seeds would grow into a Cascade plant. Hops are heterozygous to the extent that each seed contains a novel mix of alleles. And, although all the seedlings are obviously hops, each is a unique hops variety.

Seedling Evaluation

For the first year, the seedlings are grown in a greenhouse. Plants with obvious defects in their agronomic properties are abandoned, and the remaining seedlings are then moved outdoors. After 2 years of outdoor growth, the hops plants are evaluated for disease resistance, vigor, and yield. An initial assessment of brewing chemistry (alpha-acid levels, etc.) is also made.

Multi-Hill Evaluation

Plants that advance to the next stage have their rhizomes broken up so that 15–30 clones of the seedling are produced. Each clone is a “hill,” in the lingo, because the rhizomes are planted in a mound of soil. These plants mature and are evaluated as before to ensure the performance of the single seedling wasn’t a fluke or influenced by local conditions. (For example, a seedling may do better than others because by chance it was planted in an unusually rich bit of soil in the field.) Analysis of the plants’ properties continues, and those that perform well are planted to successively larger plots.

Commercial Scale Trials

If any of the cultivars make it to this final stage, they are planted in a two-acre plot. This size plot can yield enough hops to cover the bottom of an oast (photo, right)—the hops drying “oven” (although it’s more like a building than an appliance) that commercial growers use. The grower can go through picking, cone separation, drying, and packaging to see whether this hops candidate is compatible with the machines and methods of hops growers. It’s also possible at this stage to conduct commercial-scale brewing trials. If everything works out, then the breeder needs to decide whether or not to release the cultivar. Given the large number of existing hops cultivars, a new hops variety needs something to set it apart from other cultivars in order to compete.

Comparison to Other Plant Breeders

The goals of hops breeders and of other plant breeders are in many ways the same. Both want to breed varieties with increased disease resistance, increased yield (for crop plants), better storage potential, etc. They also want to produce plants that are more enticing to both growers and consumers—the public in the case of crop breeders and brewers in the case of hops varieties. However, there are also some notable differences.

Most crop plants, and commercially grown flowers, are annuals. They grow from seed, flower, and produce seed—and fruit or other edible plant parts in crop vegetables—in one growing season. A breeder working on beans or petunias can cross two plants and evaluate the outcome of the cross several months later. With a well-equipped greenhouse, the breeder could grow and evaluate at least a couple of generations of an annual plant per year.

Most other plants that other breeders work on do not grow to 30 feet and require large trellises to grow. A lot of crop plants and flowers can be bred in relatively small greenhouses and field-tested in comparably small outside plots without any infrastructure. For crop plants bred to be grown commercially, some large-scale testing with harvesting equipment is needed.

Most plant breeders work from plant varieties that “breed true.” For example, seeds from one variety of tomato plant—the result of the male part of a tomato flower fertilizing the female part—yield seeds of that tomato variety. This is usually because, after an initial cross is made, the hybrid offspring are repeatedly backcrossed to the parental type. For example, let’s say you took a tomato variety—which I’ll call Early Red Boy—with desirable characteristics and crossed it with another tomato variety with a gene that confers resistance to the tobacco mosaic virus (TMV).

The result would be hybrid offspring that were genetically half Early Red Boy and half the other plant. If offspring containing the TMV resistance gene were crossed back to an Early Red Boy plant, these second generation offspring would genetically be 75 percent Early Red Boy and 25 percent the other plant.

If offspring from this back cross that carried the TMV resistance gene were backcrossed to Early Red Boy, the resulting offspring would be 87.5 percent Early Red Boy and 12.5 percent the other type. Repeated backcrossing, with TMV resistant plants selected each generation, would eventually result in a plant that was almost indistinguishable from Early Red Boy but with a TMV resistance gene added.

Hops, which are very heterozygous, do not breed true. However, since they can be propagated by cloning (breaking up and planting rhizomes), a field of any given hops variety will consist of identical plants even if the parent plants do not breed true.

Bract to the Future

So hops breeding entails crossing two highly heterozygous parent plants and sifting among all their offspring for one or more promising seedlings. Initially promising seedlings will be repeatedly cloned until they can undergo a full-scale test—with machine harvesting and cone separation.

By the time a hops variety is ready to be released, a decade may have passed since the initial seedling was produced, and the number of clones of that seedling may easily be into the tens of thousands. Each acre of hops, when planted with the standard spacing, has 889 hills.

For every new variety released, hundreds of hybrid seedlings were not. So the next time you lift a pint of IPA, know that a lot of work—in labs greenhouses, and fields—has gone into the hops profile.