American Scientist

Beer has been made quite possibly for as long as people have cultivated cereals—that is, for a very long time. Archaeological evidence of grain processing might indicate that beer or beer-like beverages originated not long after humans figured out that grinding or pounding plant material yielded better tasting, sweeter food—a practice that goes back tens of thousands of years.

This beverage might have originated in an even earlier era, before stone tools, because the chewing of grain (as is still done in making Andean chicha) adds salivary enzymes that convert starches into sugar, ready for fermentation. On this basis, beer could conceivably have been made in some form right back to the point at which our species began behaving in the modern manner, around 100,000 years ago.

Many grains, including rice, millet, corn, and sorghum, are used to make beers in different areas of the world, but the key grain used in brewing western-style beers is barley. This prevalence is not just a matter of historical coincidence: Barley has what you might call an enzymatic toolbox that makes it the perfect brewing ingredient.

Like most grasses, barley has a fairly simple anatomy. For brewers, the spike at the top is the important part of the barley plant, because it is where the seeds sit. The structure of the spike varies significantly among different strains of barley, and those different structures are keenly relevant to brewing beer. Spikes can vary in the number of rows of seeds they bear, in multiples of two: two, four, and six. And although more might intuitively seem better, six is not necessarily the preferred row number. Indeed, European brewers overwhelmingly prefer two-row barley.

The barley seed is layered, a property that is important for understanding why it is the preferred cereal for making beer. And it is the tiny sliver of seed tissue called the aleurone layer that is critical in brewing.

During the normal life cycle of a barley plant, the endosperm of the seed develops a large reserve of starch, destined to power later development when the seed starts to germinate. In its original form this starch is not directly available to the seed for growth, but the aleurone layer contains a reserve of enzymes that are released when germination starts. Those enzymes promptly begin to break down the endosperm boundary, exposing the starch granules inside to other aleurone enzymes that break them down into sugars, primarily maltose.

Although other grains have an aleurone layer in their seeds, none has quite the capacity that barley does to break open the endosperm and turn starch into sugar. Accordingly, a brewer making beer primarily with rice or wheat will usually also add some barley. The process of getting the sugars out of the barley seed by starting germination is known as malting. Maltsters soak and aerate the seeds to stimulate sprouting, then dry them to stop the sprouting process before the resulting sugars are consumed. The dormant sugars can then be exposed to the tender mercies of the yeast whenever required, allowing the maltsters to hijack nature’s system by keeping the barley seeds from germinating until they want to make their malt. At that point, germination is artificially induced.

Although other grains have an aleurone layer, none has quite the capacity that barley does to turn starch into sugar.

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The seed arrangements of six-row, four-row, and two-row barley varieties vary according to the degree to which the spike is twisted. This twisting governs the number of kernels per row. Two-row barley is completely untwisted, so that all the kernels are symmetrical and straight, one row per side. Six-row has a two-thirds twist to it, and four-row has a half twist. Most beer outside the United States is brewed from two-row barley, whereas New World brewers incline toward the six-row forms. A question of taste may be involved here, because there are flavor differences between the two kinds.

Barley can be cultivated in both the spring and the winter, a major difference being that winter barleys require a process called vernalization (basically, cold exposure) to stimulate flowering during the late fall. If vernalization does not occur, winter plants will fail to produce a seed head. Most cultivated barley strains (known as landraces) fare better as spring crops than as winter crops, and right up until the 1960s most malting in Europe was done with two-row spring barley.

There are literally thousands of barley cultivars. Barley growers have kept good breeding records over the past century or two, so that the pedigrees of many of these cultivars are well known. Not all varieties are used in brewing, and many are used exclusively in livestock feed production. But modern maltsters and brewers make use of many of them, and each year in the United States, the American Malting Barley Association (AMBA) informs maltsters which strains are going to be the best for that year. In Europe, Euromalt serves as the clearinghouse for information about barley strains and malting, and in Australia, Malt Australia performs the same service. The recommendations of these associations differ from country to country. For instance, in 2017 Malt Australia accredited 27 landraces, of which Bass, Baudin, Commander, Flinders, La Trobe, and Westminster were listed as the major players. Like Europe, Australia focuses mostly on two-row barley strains for malting and brewing. In the United States, AMBA listed 28 accredited landraces for 2017, including both two-row and six-row forms. Among six-row barleys, Tradition and Lacey appeared to be the most sought-after for 2017, whereas the two-row landraces most in demand were ABI Voyager, AC Metcalfe, Hockett, and Moravian 69.

The Genetics of Cultivated Barley

Rice, barley, corn, and wheat are all very similar in their basic anatomical structures. After all, they are all grasses, and quite closely related. Grasses are monocots, members of one of two major branches in the plant tree of life. During plant development, a region of the plant embryo called the cotyledon develops into the very first leaves of the plant. Monocots are the flowering plants that have only one such cotyledon region (members of the other great flowering plant lineage, dicots, have two). The monocots are very diverse, and together with grasses, they include lilies, palms, tulips, onions, agave, bananas, and several other major groups. Along with grasses, lemongrasses, sedges, and bromeliads, cereals like barley, rice, wheat, and oats belong to the division of the monocots called Poales.

Poales can be further divided into more than 40 groups that include maize, barley, rice, and lawn grass. These grasses are all members of the family Poaceae, and, within this family, barley is in the genus Hordeum. Depending on which expert you believe, the barley genus contains anywhere from 10 to more than 30 species. The name Hordeum derives from the Latin horreo, for bristle, referring to the pointy spikes of the plant. The barley used to make most beer is from the species H. vulgare, another Latin name that means common. The wheat and rice also often used in brewing are members of the family Poaceae as well, and have the genus and species names Triticum aestivum and Oryza sativa, respectively.

In 2015, Jonathan Brassac and Fred Blattner of the Leibniz Institute of Plant Genetics and Crop Plant Research in Germany used genome-level DNA sequence data to look at how the 30-odd species of barley are related to one another. It was clear that H. vulgare and two other species, H. bulbosum and H. murinum, form a group quite distinct from the other 30 or so species in the genus Hordeum. This analysis confirmed the traditional morphological grouping of these species together in their own subgenus. But doubt continues to hover over one entity that is classified as its own species by some taxonomists, and as a mere subspecies by others. This is (to call it by its subspecies name) H. vulgare spontaneum, a form considered to be the wild counterpart of all the cultivated landraces of H. v. vulgare. There is still no agreement on whether this wild barley—the closest thing we know to the common ancestor of the landraces—is its own independent species, or whether all the domesticated forms remain conspecific with it.

Because the landraces of Hordeum vulgare have gone through what plant breeders call domestication syndrome, we should expect that some of the traits in the domesticated strains will differ from their counterparts in the wild strains. And it turns out that in the landraces of barley the spikes are much less brittle than in the wild forms. The brittleness of wild barley spikes enhances the dissemination of the seeds under natural conditions, but for human barley growers the calculation is very different. You don’t want the seeds to fall off when you harvest your barley, and ancient barley breeders seem to have indulged in a rudimentary form of genetic engineering by selecting plants that had a particularly strong spike structure holding the kernels together during harvesting.

The obvious question to ask now is, “Where did the barley landraces come from?” But before you can figure that out, you need to know whether barley was domesticated only once, or independently on several occasions, from multiple wild strains. Several studies have looked at the population structure of wild barley and the cultivated landraces with a view to answering this question.

Barley geneticists have tried to standardize their efforts by setting up what is called the Wild Barley Diversity Collection. This collection is made up of 318 wild barley strains (called accessions), selected both to represent the broadest possible array of non-landrace strains, and to represent as much as possible of the ecological diversity within which barley flourishes. Most accessions are from the Fertile Crescent, the area of the Near East where most scientists think barley was first domesticated, but some are from Central Asia, North Africa, and the Caucasus region between the Black and Caspian seas. The landrace counterpart collection of barley used for comparison is from a center called the International Center for Agricultural Research in the Dry Areas, which contains 304 worldwide accessions. Some studies use this collection exclusively, but others also include a broader sampling of cultivated strains in order to cover as much geographic and genetic diversity as possible.

To make analysis of the genomes of these many strains easier, researchers exploited certain reproductive characteristics of the barley plant. Individuals of barley and other grains can mate with themselves, and indeed have found that this is the best way to reproduce. They breed with other individuals occasionally, but their preferred mode of reproduction is with themselves. This selfing mode of reproduction means that they behave a little—but not exactly—like clones of themselves. It also makes it easier to trace their genetics and to reconstruct their origins than it would be with a sexually reproducing species such as ours—for as we all know, sex complicates everything. To make the barley study as easy as possible, the accessions used were forced to reproduce with themselves for three generations before being harvested and processed.

Geographic Spread of Barley Strains

Several groups of researchers examined the genetic dispositions of varieties within the species Hordeum vulgare. Joanne Russell of the James Hutton Institute in Scotland, Martin Mascher of the Leibniz Institute of Plant Genetics and Crop Plant Research, and their colleagues looked at the barley landraces using a technique called whole exome sequencing. This technique obtains genome sequences from regions of the genome that code for proteins. Their analysis, published in 2016, showed that all of the landraces are more similar to one another than they are to the wild strains (H. v. spontaneum).

Ana Poets, Zhou Fang, and Peter Morrell at the University of Minnesota and Michael Clegg of the University of California, Irvine, examined a larger collection of barley landraces (803 of them) to see if there is any clustering within the landraces. And they found a lot of it, with six major clusters. More surprisingly, in two-dimensional space those clusters can be overlain on a map of where the landraces are found.

These studies are interesting because they indicate that landraces tend to stick to certain geographic regions. As Poets and her colleagues observe, “Despite extensive human movement and admixture of barley landraces since domestication, individual landrace genomes indicate a pattern of shared ancestry with geographically proximate wild barley populations.” Such research can also help us estimate the number of clusters, or populations, of barley landraces and wild strains.

Russell, Mascher, and colleagues delved deeper into their Fertile Crescent findings and analyzed a data set including 91 wild and 176 landraces. The scientists narrowed the geographic range of their analysis because they were primarily interested in the genetics of five special accessions. They separated the landrace individuals from the wild accessions, and each of the wild accessions was then assigned to one of five ancestral populations. The wild strains fall into two recognizable clusters, suggesting that they come from two well-defined ancestral populations. The geographic break between the two clusters appears to be between a group of accessions mostly from Israel, Cyprus, Lebanon, and Syria, and those from Turkey and Iran.

Once they had obtained the detailed picture of wild strains, the researchers analyzed the landraces. They found that there are at least three ancestral patterns for landrace barley from this region. The five special accessions mentioned earlier are included in this analysis; and they are as special as accessions get, because they consist of 6,000-year-old barley kernels, found in Israel, that are believed to represent cultivars that humans used all that time ago. And they appear to be very similar to modern landraces. More specifically, these cultivars show close affinity to current landraces from Israel and Egypt. This result is spot on with the idea that the domestication of barley was initiated in the Upper Jordan Valley. Close examination of the ancestral components of these five samples suggests that the Israeli landraces grown today have not changed much in 6,000 years, despite some occasional mating with wild strains.

Genome-level information is instructive not only about the ancestry of barley, but also about the genes that might have been involved in its domestication. We have already discussed the major outward difference that distinguishes wild accessions from landraces—the brittle spike. But other traits were certainly also selected for by barley breeders over the past 10,000 years. Indeed, Russell, Mascher, and colleagues used their data set to identify the kinds of genes that have been, and continue to be, under selection in landraces. Among the traits they showed to be under breeding selection over the past several millennia are days to flowering, and height as response to temperature and dryness. Both traits are important in the adaptation of cultivated barleys to their domestic circumstances. But as the scientists point out, there are doubtless many factors still to be uncovered. More genomics work will help us discover what they are.

What about the brittle spike trait that we have seen was perhaps the most important genetic change during domestication? It turns out that the trait is under quite simple genetic control. Two genes are involved, Btr1 and Btr2, whose protein products interact with each other. When these two gene products interact properly, the central stem, or rachis, is brittle; but if there is an abnormal interaction as the result of gene mutation, the rachis stays strong, and no shattering occurs. Other domestic grains, such as rice and wheat, also have strong rachises, raising the question of whether breeders of rice, wheat, and barley selected for this trait in these grains via the same genetic pathways. Mohammad Pourkheirandish and Takao Komatsuda of the National Institute of Agrobiological Sciences in Japan settled this question by showing that the brittle rachis trait in barley is in fact unique: The rice and wheat systems do not involve the Btr1/Btr2 interaction. Clearly, there is more than one way to achieve the same rachis qualities. This theme is a common one in evolutionary biology, so it is hardly surprising that plant breeders have also stumbled onto the same principle using artificial selection.

The Future of Plant Breeding

In the first sentence of his 2015 review of barley biology, Robin G. Allaby, of the University of Warwick in England, summed up the understanding of the domestication history of barley in eight words: “Barley did not come from any one place.” This shrewd observation is important, because most researchers have long assumed that domestication is necessarily a singular event. Allaby clarifies our interpretation of the genomic data by pointing out that every single landrace of barley so far examined has genomic remnants of the four or five ancestral wild accessions, and he raises a key question—is barley the exception among domesticated forms, or is it the rule? The answer is that barley might well illustrate the rule. Domestication—which in the case of barley seems to have taken place over the general region of the Fertile Crescent—was evidently not a simple process.

With the rise of genomic technology, a very different approach to barley breeding has now become possible, using cheaper and faster techniques.

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In the past, the breeding of landraces of barley possessing the most desirable traits for agriculture was a trial-and-error affair. Six thousand years ago, barley farmers knew nothing of formal genetics, but they were smart and clearly knew enough about their plants to achieve the results they wanted. Breeders continue to grapple with the same two major kinds of traits: yield and quality. Yield traits include features such as numbers of seeds set, capacity to breed multiple times a year, or the brittle spike character that, if mutated, allows for more efficient harvesting. Quality traits are those that effect the protein content, oil content, or any other phenotype concerned with the nutritive content of the plant. During the 20th century, barley breeders were still using their knowledge of classical genetics to facilitate breeding in a tedious and labor-intensive process. With the rise of genomic technology, and the ease with which it can be applied to large numbers of lines and landraces, a very different approach to barley and other grain breeding has now become possible, using cheaper and faster techniques.

Genome-based plant breeding uses a concept called genomic prediction that relies on the predictive abilities of traits. It requires genome-level sequencing of large numbers of landraces, as well as abundant data on the traits that might be targeted (such as seed size, protein content, and protein yield). Prior to the use of this approach, barley breeding experiments were massive and costly. Now, using genomic prediction, barley breeders can get a more precise, quicker, and cheaper idea of how easy it will be to breed for certain traits. Several such studies have already been directed at the assessment of quality traits that are important in brewing.

Malthe Schmidt of the German plant-breeding company KWS SAAT and his colleagues analyzed the predictive abilities of 12 malting characteristics of spring and winter barleys. By ranking those 12 desirable malting traits, they showed that winter barley would be easier to work with. Another study demonstrated the feasibility of the genomic remnants in improving seed quality traits. Nanna Hellum Nielsen of the Danish company Nordic Seed and her colleagues examined features such as seed weight, protein content, protein yield, and ergosterol levels (generally thought to be an indicator of resistance to fungi and bacteria), showing how genomics could predict the efficacy of breeding programs for these traits too. So, although it is still early days, genomic approaches have already demonstrated their ability to facilitate improvement in the efficiency, yield, and quality of barley cultivation. Still, it is quite likely that the future of barley will lie in an even more cutting-edge technique: direct gene editing using the CRISPR technology and the newly developed prime editing approach. However the story plays out, one thing is certain: Molecular biology holds huge promise for improving the raw materials of maltsters and brewers.

This article is excerpted and adapted from A Natural History of Beer, © Rob DeSalle and Ian Tattersall. Reprinted with permission from Yale University Press.