By Harmonie Bettenhausen, Ph.D., Hartwick Center for Craft Food and Beverage
As producers (and producer-adjacent) of malted grains, we are all quite aware that barley malt provides critical starches and enzymes to the brewing process, in exchange for saccharides and nutrients to be consumed by yeast and the creation of some tasty beverage. We are learning just how much of a contribution both variety and environment (terroir) have on our grains, malts, and beer. Recently, a novel study was done to assess the genetic basis of barley contributions to beer flavor: Genetic basis of barley contributions to beer flavor.
To understand the importance of this study and the impacts it will have, we have to first understand what happened in the study. Just who led this fine effort in investigative science? Maria Munoz Amatriain and Brooke Sayre-Chavez at Colorado State University led the charge, with Pat Hayes and the Barley World team at Oregon State University providing resources and data, and the Colorado State metabolomics team finishing it off.
The team looked at what is called a biparental population of Doubled Haploids (DHs) derived from the crosses of the cultivars Golden Promise and Full Pint (Oregon Promise). DHs are barley plants that are derived from a single pollen grain and whose chromosome complement is spontaneously doubled to form homozygous diploids (a diploid is a cell that has paired chromosomes, one from each parent).
A method called Quantitative Trait Locus (QTL) analysis was then utilized to map quality traits, beer sensory descriptors, and chemical compounds on this population. Quantitative trait loci (QTL) refer to a specific region on chromosomes, which harbors gene(s) controlling the traits. Barley is the owner of seven chromosomes, named 1H through 7H.
QTL analysis is subsequently the statistical method that provides a link between the phenotype (what you can see) and genotype (genetic makeup) of the barley to explain the genetic variations in different complex traits, such as disease resistance, drought resistance, dormancy, enzymes involved in starch breakdown, and perhaps now, flavor.
I know what you’re thinking. What? Why?
What prompted this study? For a long time, we have been looking at barley malting quality traits, as well as their genetics, to answer questions such as: How can we breed high-quality barley using this technology? How can we understand and analyze the association between quality traits and agronomic improvements? How does the degree of dormancy drive traits that affect malting? 
We put this study together to move the research further – into integrating malting quality, beer sensory, and chemical investigation of small molecules (metabolomics) through the understanding of the determinant genes and QTLs of one barley population. This important step (as does each step) moves us ever closer to understanding how each decision we make when we breed, grow, malt, brew, distill, and drink has rippling effects.
There were three main questions we set out to answer in this extensive study.
- Do barley genotypes differ in their contributions to beer flavor?
- Is there a genetic basis for these contributions?
- Are differences in sensory attributes and metabolite profiles due (or not due) to the degree of modification of malt and/or differences in beer analytics?
What did we do to answer these questions?
We genetically characterized 236 doubled haploids from the Oregon Promise population, using a high-density genotyping array, (Barley 50k iSelect SNP array;  and integrated – via biparental QTL mapping – the genotype data with phenotype data on malting quality, beer sensory, and beer metabolomics.
What did we find out?
Out of the 236 lines evaluated from the Oregon Promise population, all malting quality traits except one (wort clarity) had significant QTLs. Meaning? In Figure 2, we have a Manhattan Plot. This shows us each chromosome (1H through 7H on the X-axis in different colors), and the dots you see represent a single nucleotide polymorphism (SNP) as a strong association with that trait. SNPs are the form of genetic variation that could induce changes we see in the phenotype and help us locate the QTL, which is on the chromosome. The Y-axis is the significant P-values (a measure of the probability that an observed difference has occurred by chance, represented by -log10 value).
21 QTLs (Remember, QTL is just a location on a chromosome, similar to a town inside of a state) for 14 malt quality traits (kernel weight, plump, color, barley color, malt extract, wort color, barley protein, wort protein, S/T, DP, AA, BG, FAN) were identified on chromosomes 1H, 2H, 3H, 5H, and 7H. In Figure 2, we see that Chromosome 5H is our hotspot for association (the most significant SNPs) for the malt extract and wort color.
Sensory analysis was conducted by a trained panel to further elucidate flavor attributes. A total of eight QTLs for six sensory traits were identified on chromosomes 2H, 3H, 5H, and 7H. One of the sensory attributes that overlapped with our malting quality traits on 5H was “toasted.”
We used metabolomics, a tool that is used to better understand the volatile chemical composition of these barley malts and beer [3, 4]. There were eight QTLs for the accumulation of five flavor metabolites on Chromosomes 2H, 3H, 5H, 6H, and 7H. In Figure 3, we see two of the flavor metabolites identified, acetic acid, 2-phenylethyl ester, and 2-methoxy-4-vinylphenol that have significant SNPs, indicating QTLs on our hotspot 5H chromosome.
What does this all mean and why do we care about things like Chromosome 5H?
We look at these genes as “putative candidate genes for QTL clusters” for breeding purposes. Our hotspot on 5H happens to include a famous gene associated with the degree of dormancy. This gene and genes for morphological traits (e.g. semidwarf growth habit) may have profound downstream effects on malting quality, beer flavor, and metabolite abundance.
This was the first comprehensive look at the genetic basis of barley contributions to beer flavor, together with metabolomic compounds in beer. The sensory and metabolite data sets are firmly embedded in the malting quality data set, that being: malt precedes beer and it is the malting process that leads to these differences. Any analysis of the contributions of barley genotype to beer flavor is inseparably confounded by the style of malt, and how each variety responds to the malting protocol that was used to make the malt. The abundance of certain flavor compounds and corresponding aroma or flavor may be due to the interactions of the malt with other components of the finished beer.
- Fang Y, Zhang X, Xue D. Genetic Analysis and Molecular Breeding Applications of Malting Quality QTLs in Barley. Frontiers in Genetics. 2019;10:352.
- Bayer MM, Rapazote-Flores P, Ganal M, Hedley PE, Macaulay M, Plieske J, et al. Development and evaluation of a barley 50k iSelect SNP array. Frontiers in plant science. 2017;8:1792.
- Bettenhausen HM, Barr L, Broeckling CD, Chaparro JM, Holbrook C, Sedin D, et al. Influence of malt source on beer chemistry, flavor, and flavor stability. Food Res Int. 2018;113:487-504. Epub 2018/09/10. doi: 10.1016/j.foodres.2018.07.024. PubMed PMID: 30195545.
- Frank T, Scholz B, Peter S, Engel KH. Metabolite profiling of barley: Influence of the malting process. Food Chemistry. 2011;124(3):948-57. doi: 10.1016/j.foodchem.2010.07.034. PubMed PMID: WOS:000283459700035.
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