Disease pressure in the Midwest also limits the yield of many two-row cultivars. On the other hand, midwestern six-row cultivars (with a wider range of adaptation) transplanted to the West have yields comparable to two-row varieties. Consequently, some midwestern varieties are grown under contract in the western United States; the contracting of barley in the Midwest, however, is limited.
Canadian barley is grown almost exclusively under dryland conditions, but these conditions are not necessarily equivalent to those of the American Midwest; growing conditions (rainfall, length of season, temperature, and so forth) vary between, and even within, provinces, allowing the growth of both two- and six-row barley. Western Canadian yields are, on average, lower than those for irrigated western U.S. barley.
Only a small amount of Mexican barley is irrigated.
Although breeding programs have minimized the differences between two- and six-row barley, differences remain in terms of kernel size/uniformity, grain protein content, and malt enzyme levels (3). Kernel size and protein directly influence the manner in which six- and two-row barleys are malted by affecting the rate of water uptake, germination, and modification (see Tables I and II).
Kernel size and uniformity: The central kernel of six-row barleys is symmetrical, but the two lateral kernels are slightly twisted and also tend to be slightly shorter and thinner (4). Two-row barley kernels, by contrast, tend to be symmetrical in shape, more uniform in size, and plumper because only one kernel/rachis node develops (see box, "The Anatomy of a Barley Spike"). Because of the irregularities in kernel size, maltsters often separate each lot of six-row barley into several kernel size fractions for more uniform germination and modification. Plumper fractions are reblended upon completion of malting and used as brewer's malt. The thinner malt kernels may be sold as distiller's malt, where it is preferred for its high enzymatic activity. The thinnest barley kernels are removed and sold as feed. Two-row barleys often don't require such extra handling because their kernel size is more uniform.
A major advance came in 1961 with the release of the six-row cultivar Larker (5). Larker significantly reduced the size differential between large kernels in six- and two-row cultivars. The name Larker, in fact, was coined from the words "large kernels." Although this variety is no longer used for malting (having been replaced by newer, improved cultivars), kernel plumpness in six-row cultivars released since that time has generally continued to increase. Nevertheless, the plumpness of two-row cultivars still tends to be greater, particularly when grown under irrigation in adapted environments.
Kernel plumpness serves as a moderate indicator of malt extract yield (3). Plumper kernels are thought to have a higher starch content, which is the principal contributor to extract. Before the breeding breakthroughs of the 1970s, the extract from six-row malts was as much as 4% below those of two-row malts. The release of the cultivar Morex (so-named because it has "more extract") in 1978 marked a trend toward higher extract levels for six-row barley (5). Currently, six-row malts are only 1-2% lower.
Protein levels: Another important distinction between six- and two-row barley cultivars is in the average level of grain protein (3). A high protein level often indicates a thinner kernel with less starch available for conversion to malt extract. Acceptable six-row malting barleys may range from 12 to 13.5% protein, whereas two-row cultivars range from 11 to 13%; barleys with greater than 13.5% protein are rarely used for malt. The high temperatures and moisture stress frequently encountered in dryland conditions (under which most six-row barley is grown) can limit the amount of grain fill (starch synthesis) and thus result in higher protein contents.
The protein content differential is also related to genetic differences in how each cultivar accumulates protein during grain development. Total protein content is defined as nitrogen content x 6.25. Because the net loss of nitrogen during malting is minimal, the total protein content does not change greatly in the process. Much of the barley protein, however, is converted into a soluble form by proteolytic enzymes; a portion of this is further broken down into amino acids and peptides in the wort.
Six-row malts tend to yield higher levels of wort-soluble protein. The ratio of soluble protein to total protein is an indication of the extent of protein breakdown (modification) during malting: 40-45% is considered acceptable.
Higher protein malting barleys are generally believed to inversely reduce the level of malt extract in the kernel. In addition, high protein content can lengthen steeping time, cause erratic germination (especially if grain traders blend low- and high-protein barleys to meet protein limits), increase malting losses, and increase enzymatic activity and, ultimately, the level of dimethyl sulfide. High soluble protein levels can sometimes result in brewing or beer-quality problems.
Malt modification time: While most six-row barley cultivars require four-and-a-half to five days of germination to achieve proper malt modification, traditional North American two-row cultivars generally require an additional one to two days of germination time (3). Harrington, however, a two-row cultivar released in 1981, modifies in only four days. Because Harrington is currently the predominant two-row cultivar in North America, particularly in Canada, it can safely be stated that modern two-row barleys generally require less malting time than six-row barleys -- a testimony to the success of modern breeding programs. This advantage represents a major economic consideration for maltsters. This change of tendency for two-row cultivars has represented a major advancement achieved through barley breeding.
Malt enzymes: Six-row malts traditionally (that is, before recent breeding advances) yielded higher levels of the desirable starch-degrading enzymes a-amylases and greater diastatic power (DP). a-amylases are the enzymes that convert starch to dextrins, reduce mash or cooker viscosity, and increase the susceptibility of starch to ß-amylase attack (7,8). DP is a measure of the activity of the malt enzymes that break down complex carbohydrates into reducing sugars (principally ß-amylase, the key saccharifying enzyme responsible for converting starch to fermentable maltose and for further breaking down large dextrins). The modern two-row cultivar Harrington, however, has levels of a-amylases equal to or slightly greater than those of current six-row malting cultivars. Despite the recent advances in favor of more a-amylases, two-row malts continue to have considerably lower levels of DP -- a potentially limiting factor in some applications, such as when high levels of unmalted grains are used as adjuncts.
ß-glucans. The ß-glucan content of most barley cultivars falls between 4 and 7% of the total grain weight (9). In general, the ß-glucan content of six-row barleys is slightly lower than that of two-row barleys. ß-glucans are usually extensively degraded by malt ß-glucanase enzymes during germination, meaning that little will be extracted into wort. Undegraded ß-glucans contribute to viscosity and can cause wort separation and beer filtration problems (10). Both two- and six-row North American malts tend to be well modified; ß-glucan-related problems are not often encountered but are more likely when undermodified malt or high levels of umalted barley are used.
Husk content: Husk content provides one other difference between two- and six-row barley. A thin, tightly adhering husk is desirable in all malting cultivars because the husk protects the germinating grain during malting and plays an important role in lautering. Six-row barleys are generally believed to have a higher husk content because they tend toward thinner kernels, but husk content varies with growth environment (11). High husk content barley can mean more phenolics end up in the wort, thereby contributing an astringent flavor to beer. Oxidizable polyphenolic substances react with proteins and may contribute to haze formation (8). Care must be taken in the brewing process to avoid extraction of these compounds from the husk and to promote their precipitation in the wort (the hot break).
Implications for Brewing Practice
Protein and DP: In terms of brewing performance, the most apparent differences between two-and six-row malts relate to their levels of grain protein and diastatic power. The higher protein and enzyme levels of adapted six-row cultivars allow for the widespread use of cereal adjuncts in major North American breweries and the double-mash* system for precooking them (4).
*The double mash system is used with rice or corn grits. A portion of the malt bill can be replaced (usually no more than 40%) with rice or corn. The rice or corn grits are first "cooked" with a small portion of the malt in a separate vessel known as a cereal cooker. Most of the malt will be mashed in the main mash vessel. As the temperature rises in the cereal cooker, the adjunct starch is gelatinized, which makes it susceptible to enzymatic hydrolysis by the amylases contained in the malt. Eventually, the cooker temperature will reach boiling, after which the cereal mash is transferred to the main mash tun. This transfer usually occurs at the end of the main mash protein rest and raises the main mash temperature to saccharification temperature.
Uniformity and size: The more uniform kernel size distribution of two-row malt helps brewers, at least those using two-roller mills, obtain a proper grind at the beginning of the brewing process (12). Kernel size differences, however, are likely to be less significant when using more sophisticated six-roller mills with screening systems, such as those used by the major breweries. In terms of the type of wort separation method used, a larger grist particle size distribution is extremely important in lautering, and virtually unimportant with the modern mash filters used by some large-scale brewers. Mash filters are able to handle smaller particles because they use filter cloth, a lower bed depth, and higher pressures.
Extract yield: Two-row barley yields malts with 1-2% greater theoretical extract (13). Extract is a major economic concern for many large-scale brewers because the amount of brewhouse extract obtained determines the amount of beer that can be produced from a given amount of malt. Small-scale brewers, however, are generally less concerned about extract yield and may not consider this as important a criterion in their malt choice. Large-scale brewers must weigh the higher extract levels of two-row malts against higher cost and often lower diastatic power.
Soluble protein: During the malting and brewing processes, approximately 38-45% of the malt protein is converted to wort-soluble protein in the form of various nitrogenous substances, including peptides and amino acids (3,7,8,13). The balance of these components in the wort is important because they contribute to beer foam and mouthfeel, beer color and flavor, and yeast metabolism.
Some soluble protein is essential. Problems can arise, however, when levels become excessive in wort or beer. This level depends on the process and product, but problems might be expected when wort-soluble protein exceeds 5.5%. High levels of protein, like those found in six-row malts, can lead to too much color development during wort boiling, filtration problems, and the risk of haze formation.
Proteins and adjuncts. The widespread use of unmalted cereal adjuncts (corn, rice, etc.) by North American brewers developed, in part, to compensate for the higher soluble protein levels of six-row malts and, later, because adjuncts are cheaper. It is generally accepted that 150-170 ppm amino nitrogen (component of soluble protein) is required in the wort to support adequate yeast metabolism and fermentation (12). A high-protein six-row malt will provide levels far in excess of these values. Because the protein in corn or rice adjuncts is largely insoluble, it is possible to replace a portion of the malt with adjunct and thus dilute the overall level of wort-soluble nitrogen. Cereal adjuncts can be used to replace up to 40% of six-row malt grist without adversely affecting fermentation performance. Two-row malt typically allows for less adjunct use because of its lower soluble nitrogen levels and lower diastatic power.
The use of cereal adjuncts began as an innovative response to available malt quality and was born of concern for quality. Now, with improved North American malt strains available, it is no longer necessary but is now both economically advantageous and traditional for those breweries' beers.
Proteins and DMS. Protein levels also increase the potential for dimethyl sulphide (DMS) formation in beer. The precursor of DMS, S-methyl methionine (SMM), is formed through protein breakdown during malting (14,15). Much of the SMM is converted to DMS through thermal decomposition during kilning and wort boiling. DMS formed during kilning and wort boiling is lost to the atmosphere. Pale malts generally have higher levels of SMM than do darker, highly kilned malts. When the length or vigor of boiling is inadequate to convert all residual SMM, DMS may continue to form as the wort cools. This DMS may persist into the beer. Although some DMS is desirable in lager beers, levels in excess of 50 ppb are thought to contribute a cooked or sweet corn flavor. Six-row malts contain higher levels of the DMS precursor SMM, presumably because of their higher protein content.
Malt enzymes: Higher protein levels are somewhat positively correlated with malt enzyme levels and six-row cultivars tend to have higher levels of DP than do two-row cultivars (3,14). Levels of a-amylase are roughly equal.
Because the ratio of DP to a-amylase is greater in six-row malts, one might expect conversion to fermentable sugars to proceed more rapidly. This may be of importance when throughput (brews/day) is a concern. For the home brewer, it may provide some leeway when high mash-in temperatures are used because more conversion would take place. ß-amylase, the major component of DP, is much more temperature-sensitive than a-amylase and is inactivated earlier in the mash.
Syrup adjuncts. Although the higher level of DP in six-row malts also allows brewers to use more cereal adjuncts (see "Proteins and adjuncts," above), the situation with syrup adjuncts is somewhat different. Syrups are prepared through the enzymatic hydrolysis of corn starch to fermentable sugars. Because this adjunct is added in a fermentable form, excess malt enzyme is not needed for fermentation. In fact, some brewers have reported problems with the high enzyme and soluble protein levels of certain modern six-row cultivars.
A World of Choices
Many differences distinguish two- and six-row malt, but these differences have become less pronounced over the past 20 years as new varieties have been bred. The high protein and enzyme content of six-row barley makes it unlikely that a brewer producing an all-malt beer would wish to use exclusively six-row malt. Supplementing two-row malt with some six-row malt, however, might serve to increase extraction, conversion time, and fermentability, especially with high proportions of adjunct. Although most craft brewers don't normally use corn and rice, other unmalted grains such as wheat, barley, and oats are becoming increasingly common.
On a final note, it should be mentioned that every barley cultivar, whether six-row or two-row, can have distinct effects on the organoleptic (flavor, aroma, color) characteristics of beer (3). Two-row malts are generally believed to yield a mellower flavor, but these differences are very difficult to quantify. Malting barley and malt are marketed on the basis of cultivar, and, thanks to modern breeding practices, brewers have a world of options when choosing which cultivar best meets their processing and beer quality requirements.
The authors thank Sherman Chan, Scott Heisel, John Holt, Norman Kendall, Mauro Zamora Diaz, John Mittleider, and William Wilson for their valuable input. Statistics on barley production in Canada and the United States were from reports provided by the American Malting Barley Association (Milwaukee, Wisconsin) and the Brewing and Malting Barley Research Institute (Winnipeg, Manitoba, Canada).