Although the grain disease called Fusarium head blight has posed little serious threat to the North American brewing industry in the recent past, reports of the disease's recent resurgence in certain regions may merit some concern among growers, maltsters, and brewers alike.

Any crop is potentially susceptible to the ravages of pests or disease, and barley is no exception. One fungal disease has been appearing in certain production areas where climate and soil conditions are conducive to the fungus's growth. The disease, known as Fusarium head blight (FHB, or scab), affects barley and wheat and has created many problems for farmers and livestock. This article presents what we know and don't know about the disease's effect on the malting and brewing industries.

Why We Should Worry about It
The Fusarium fungus itself can diminish grain quality and make swine and other livestock ill. The economic pain that farmers feel from production losses may well be passed on to end users. But the fungus is only the beginning of the trail of hazards wrought in fields infected with Fusarium. The fungus produces resilient toxic by-products that are able to survive the malting and kilning process and creep into the finished beer.

Studies have shown that high doses of these toxins can be lethal to laboratory animals, although the threat to humans from the typically low levels that might be found in beer and other cereal food products has yet to be clearly established. Even in the absence of a clearly defined health threat, however, brewers may wish to consider public perceptions about health and quality when they select their malt. In addition, the disease is linked to certain brewing-related problems, including gushing in packaged beer. For these reasons, it well worth examining and understanding FHB.

Where It Strikes
FHB is most common among grain crops in the humid and semihumid production areas of the world, where the climate is favorable to fungus development. Because of the disease's dependence on warm, moist weather, its occurrence and severity vary greatly from year to year. FHB has been known to strike with considerable magnitude in some of the barley production areas of North America, Europe, China, Korea, and Japan.

FHB is not a new disease; it was recorded as early as 1890, when Indiana wheat crops were reportedly infected with severe scab. Scab of barley hit the eastern and central U.S. corn belt so hard during the first half of this century that it essentially eliminated barley production (1,2). More recently, FHB epidemics erupted in German crops in 1987 and 1991 (3). In North America, the fungus has hit the Midwest the hardest. The disease has been widespread enough to have an economic impact on the wheat and barley industry in the midwestern United States and central Canada (Manitoba) since 1993.

Of concern to brewers buying malt, then, is the six-row barley typically grown in the Midwest and all European barley and wheat (midwestern wheat is not typically used for malting).

Fighting the Blight
Unfortunately, the options for controlling Fusarium-related problems are currently limited as long as weather patterns and modern minimum-tillage practices continue to give Fusarium the perfect environment to grow and spread (see box, "The Root of the Problem"). All current barley varieties are highly susceptible to FHB. American barley growers typically disdain the use of fungicides because of their poor efficacy and high cost.

Maltsters are limited in their options for control. Gravity separation can be effective for removing infected wheat kernels, but cleaning is not an effective tool for barley, neither technologically nor economically. DON levels in barley can be reduced only 10-20% by cleaning, if at all.

Chemical control of Fusarium during the malting process is not really an option either because of concerns over residues and reduced malt quality. Biologically controlling Fusarium in the malthouse by inoculating the barley with other microorganisms (yeasts and bacteria, for example) has shown some promise in European studies (4). But the most common means of control is simply to avoid using infected barley. Most maltsters and large breweries in North America routinely screen for FHB and/or the mycotoxins it produces and accept only barley or malt with undetectable to minimal levels of these toxins.

Effects on Barley and Animals
Fusarium head blight rears its ugly head in several ways. Infected grain may develop floret sterility or produce light, shriveled kernels. Visual signs of infection are not always apparent, however, particularly on barley. A salmon-pink to reddish dustlike growth may be evident on heavily infected kernels or spikelets. Developing kernels are often shrunken and grayish brown. Wheat kernels are particularly susceptible to infection, where FHB manifests in extremely shriveled berries called "tombstones," which are discarded during harvesting or the grain-cleaning process.

Fusarium Head Blight at a Glance

FHB is a worldwide fungal disease of cereal crops, including barley and wheat. Epidemics have been reported in Europe, Asia, and, most recently, in North America. FHB infection can result in inferior grain quality and mycotoxin contamination. DON (deoxynivalenol) is the most common mycotoxin on grains in North America. DON causes feeding problems in animals, particularly swine. Because vomiting or feed refusal (aversion) may be encountered at high levels, the compound is also known as vomitoxin. The effects of chronic exposure to DON on human health are not known. FHB infection of barley and the DON it produces can result in reduced malt quality and problems in malting and brewing, including gushing in packaged beer. Many maltsters and brewers routinely test for the presence of the fungus or associated mycotoxins. Although Fusarium is killed during kilning, DON can survive into the finished beer. Worldwide surveys have shown minimal incidence of DON in commercial beers.

In addition to grain quality degradation, toxins produced by the fungus can cause illness in many animals, particularly swine. One compound in particular, deoxynivalenol (DON, sometimes referred to as vomitoxin), is the most common mycotoxin associated with small-grain production in North America (5) and the most common toxin occurring on barley in the upper midwestern United States (6). Interestingly, DON can be detected in seemingly healthy kernels that lack obvious FHB symptoms.

The effect of DON toxicosis on humans is not yet clear from research. DON is comparably less toxic than some of the other compounds produced by Fusarium that can have harsher effects on livestock, but DON nonetheless can cause problems by prompting livestock to either refuse to eat or, at higher levels, to vomit (thus the toxin's common name). The accompanying box, "The Root of the Problem," tells more about the mycotoxins produced by Fusarium and their effects on livestock.

Effects on Malt and Finished Beer
As previously mentioned, the problem for brewers is that the Fusarium fungus thrives on the conditions inherent in the malting process. While the fungus itself is killed during kilning, the heat-resistant mycotoxins can persist into the finished beer.

The transfer of a mycotoxin from its barley host into the beer is largely dependent upon the solubility and temperature stability of the specific toxin. One study using naturally infected barley to evaluate the fate of DON during the malting and brewing process (7) resulted in the following observations.

In the malthouse: The DON mycotoxin is water soluble and is rinsed from infected barley during steeping. In the first phase of malting, barley is steeped in water until it reaches about 45% moisture. In modern practice, steeping actually involves a number of cycles of water immersion followed by drain or air-rest stages.

Fusarium present on infected grain, however, persists through steeping. Thus, although infected barley is typically free of DON at steep-out, the subsequent four- to five-day germination stage provides ideal temperature and relative humidity for further fungus growth and therefore further DON production.

Kilning then kills the Fusarium, halting further toxin production. The amount of DON produced during the malting of infected grain is highly variable, and probably depends on the individual sample as well as on malting conditions. In most samples of malted barley analyzed in the study (7), DON levels in malt were lower than in the original infected barley.

In the finished beer: Only small amounts of DON present on the malt are later detected on the spent grains; most of the DON (80-95%) is extracted during mashing. Because of its high temperature tolerance, DON will persist through the boil and into the finished beer. One must keep in mind, however, that even though much DON is extracted, it will undergo an approximate 8- to 10-fold dilution during brewing.

In a survey (10) of six studies on mycotoxin contamination in beers worldwide (the studies covered 327 commercial beers from the United States, Canada, and Europe), DON was typically absent from the beers analyzed, suggesting that either FHB was not present at all in the malt or that the malting industry was effective in its screening for the disease. The highest level of DON reported was 570 parts per billion (ppb) in a German wheat beer; that beer, however, was part of a study on gushing beer, and was therefore not randomly selected. Most beers that tested positive in random testing had much lower levels, on average 5-20 ppb, if detected at all.

At these low levels, DON probably presents no concern for acute toxic reaction. An individual would need to consume several hundred liters of beer in one sitting to accumulate a lethal dose of DON. The FDA's advisory level is a maximum of 1 ppm in wheat products for human consumption, or 1,000 ppb (11); no advisory levels have yet been set for malting barley. The effects of low-level chronic exposure (long-term exposure) are not yet known.

Whatever the facts turn out to be, craft brewers rely heavily on the public's perception of the purity of their products, and public concern may well be exacerbated by DON's common name, vomitoxin.

Brewhouse Performance
Effects on brewing: Possible health concerns are just part of the picture. The malting and brewing performance of barley or wheat also may be affected by Fusarium. All of the effects of FHB/DON infection on grain metabolism are not yet known, but one study reported malt quality losses and increased extract, alpha-amylase, and wort nitrogen when barley samples were artificially infected with Fusarium during malting (12). These observations make sense based on what we do know. DON acts as an inhibitor of protein synthesis, so it can potentially affect germination and other metabolic processes in the germinating grain. The Fusarium fungus itself produces amylolytic and proteolytic enzymes and also gibberellins, which promote enzyme synthesis in the barley. Excessive enzyme activity can make conversion difficult to control.

Fermentation: When applied to wort, DON has been reported to retard yeast growth and attenuation (13), but the concentration required for this effect (50 ppm in wort) would mean an initial malt contamination of several hundred ppm, which is not likely to be encountered. Further studies are needed to evaluate the effects of lower levels of wort DON (<100 ppb) on fermentation before realistic conclusions can be drawn.

Off-flavors: Molds such as Fusarium are known to produce off-flavors in beers -- one reason that brewers have long avoided moldy barley even before anything was known about mycotoxins.

Gushing: Perhaps the best known effect of FHB on malting and brewing quality is the propensity of Fusarium-infected malts to cause gushing problems in packaged beer. Gushing has been defined in the following way: "Immediately after opening the bottle, that is, by removing the excess pressure above the beer, a very great number of fine bubbles are formed throughout the volume of beer and ascend very quickly, creating foam which flows out of the bottle or, in severe cases, actually spurts from the bottle. Usually, the violent action ceases after a few seconds" (14).

Gushing in action: This beer was prepared in a laboratory trial from a malt sample that was heavily infected with Fusarium. A compound produced by the Fusarium fungus has been found to cause such gushing, though the effect can also be caused by other brewing and packaging process factors.

The type of gushing associated with the use of fungus-infected barley is referred to as primary gushing, and although most commonly associated with Fusarium, it can also be caused by other fungi such as Aspergillus, Rhizopus, Penicillium, and Nigrospora. The actual compound (or compounds) that cause gushing is unknown, but research has suggested that it may be composed of protein (15) that provides additional nucleation sites on which carbon dioxide bubbles form. Its exact nature eludes determination; few people have conducted long-term research, and the activity is difficult to isolate. What is known is that this compound appears to increase with the level of Fusarium on barley and malt, and the amount of gushing observed would thus appear to increase with the severity of the Fusarium infection on malt (16). Little or no overfoaming is observed when infection levels are low to nil, whereas one-half or more of the bottle contents may erupt when the malt has been heavily infected.

Not all gushing, of course, is necessarily related to fungal infection. Secondary gushing may be caused by a number of processing factors. Assuming the malt presents no visible signs of infection, brewers should rule out the following possible problems before suspecting FHB: contamination of packaging material with foreign particulate matter, rough bottle surface, overcarbonation, calcium oxalate crystals (otherwise known as beerstone, problems may occur when it precipitates in the finished beer), hop oxidation products, excessive haze, and filtration breakthroughs. A few other possibilities exist as well (14). One possible clue is that gushing problems due to process factors might be expected to affect bottles in a batch of beer sporadically, whereas FHB infection would, of course, affect an entire batch of beer.

What you can do in the brewery: As mentioned before, visual signs of infection are unfortunately not necessarily apparent in barley, though heavily infected barley may well result in discolored and/or malformed malt. Although black lights are effective for aflatoxin detection, they do not work for Fusarium and DON. Test kits of varying accuracy and price are available for conducting your own malt analyses, and some testing services will evaluate both malt and beer for toxins (see the "For More Information" section). Asking the right questions of your malt supplier may be your best line of defense.

Implications for Growers, Maltsters, and Brewers The rigid standards set by large U.S. breweries and maltsters have helped to lessen the impact of this problem over the years, but any increase in the extent of infected crops raises the potential number of lot rejections. This could adversely affect the grower as well as the malting company; if the grain becomes highly contaminated it may not even meet livestock feed requirements, resulting in a financial loss for farmers and a grain deficit that might raise the price of subsequent crops. The economic effects may be felt all the way down the line.

Because the Fusarium problem has primarily affected the regions of the United States and Canada producing six-row barley, brewers who exclusively use U.S. or Canadian two-row malt may be able to breathe a bit more easily. But brewers purchasing malt from Europe, where conditions for disease development in barley and wheat producing areas are more commonly found than in the United States, may wish to exercise some caution. The fact that an extremely high level of DON was measured in a German wheat beer should not be ignored.

The full effect that beer prepared from barley infected with FHB could have on brewers and consumers may not be known for some time, but public perceptions alone could be enough to cause concern in the industry. This article is intended to help educate brewers about the facts of Fusarium infection. By asking the right questions and knowing what to watch for, anyone purchasing malt can decrease the chance of using diseased malt.

For More Information
Much cooperative research is being conducted by government and industry groups. The Canadian Grain Commission maintains a website with up-to-date information on FHB, including maps, photographs, and available publications (www.cgc.ca). The United States does not yet have a central repository for information on the disease. Some information can be found, however, at http://www.rrtrade.org/smallgrains/research/research.htm, and a search will find some other scattered information sources as well. The references cited in this article do, however, embody an excellent survey of the available literature. DON test kits are available from several sources, including the companies named in the "Products and Services" section of this publication (see "Malt Testing Kits," p. 159). Many companies also offer malt testing services; some may also be able to test finished beer for DON infection. Dr. Howard Caspar of North Dakota State University, for one, will test commercial malt or beer samples for a fee (tel. 701/231-7529), and the Canadian Grain Commission also offers a testing service (tel. 204/983-2770).

  References

1. G. Bai and G. Shaner, "Scab of Wheat: Prospects for Control," Plant Disease 78, pp. 760-766 (1994).

2. D.E. Mathre, Ed., "Scab or Head Blight," in American Phytopathological Society Compendium of Barley Diseases (APS Press, St. Paul, Minnesota, 1982), pp. 42-43.

3. L. Niessen, M. Böhm-Schrami, H. Vogel, and S. Donhauser, "Deoxynivalenol in Commercial Beer -- Screening for Toxin with an Indirect Competitive ELISA," Mycotoxin Research 9, pp. 99-109 (1993).

4. A. Haikara, H. Uljas, and A. Suurnëkki, "Lactic Starter Cultures in Malting -- A Novel Solution to Gushing Problems," in European Brewery Convention, Proceedings: #24 (IRL Press Ltd., Oxford, UK), pp. 164-172.

5. T. Tanaka, A. Hasegawa, S. Yamamoto, U.S. Lee, Y. Sugiura, and Y. Ueno, "Worldwide Contamination of Cereals by the Fusarium Mycotoxins Nivalenol, Deoxynivalenol, and Zearalenone," Journal of Agricultural and Food Chemistry 36, pp. 979-983 (1988).

6. P.B. Schwarz, H.H. Casper, and J.M. Barr, "Survey of the Natural Occurrence of Deoxynivalenol (Vomitoxin) in Barley Grown in Minnesota, North Dakota and South Dakota During 1993," Master Brewers Association of America Technical Quarterly 32 (4), pp. 190-194 (1995).

7. P.B. Schwarz, H.H. Casper, and S. Beattie, "Fate and Development of Naturally Occurring Fusarium Mycotoxins During Malting and Brewing," Journal of the American Society of Brewing Chemists 53 (3), pp. 121-127 (1995).

8. C.J. Mirocha, S.V. Pathre, and C.M. Christensen, "Mycotoxins," in Advances in Cereal Science (American Association of Cereal Chemists, St. Paul, Minnesota, 1980), pp. 159-225.

9. Vicam, company brochure (Vicam, Watertown, Massachusetts).

10. P.M. Scott, "Mycotoxins Transmitted into Beer from Contaminated Grains During Brewing," Journal of the Association of Official Analytical Chemists International 79, pp. 875-882 (1996).

11. U.S. Department of Agriculture, Program Bulletin 96.2, "Deoxynivalenol (DON) Advisory Levels," USDA, Grain Inspection, Packers, and Stockyards Administration, Federal Grain Inspection Service (15 February 1996).

12. W. Sloey and N. Prentice, "Effects of Fusarium Isolates Applied During Malting on Properties of Malt," in American Society of Brewing Chemists, Proceedings: 1962 (American Society of Brewing Chemists, St. Paul, Minnesota, 1962), pp. 24-28.

13. R. Ryman, "The Effect of Natural Contamination of Grain by Fusarium and Associated Mycotoxins in Malting and Brewing," malting science thesis (Heriot-Watt University, Edinburgh, Scotland, 1994).

14. G.P. Casey, "Examination of Mechanistic Differences Between Primary and Secondary Gushing and Assay Procedures Used to Assess Malt/Beer Gushing Potential," Master Brewers Association of America Technical Quarterly 33 (4), pp. 229-235.

15. K. Kitabatake, S. Fukushima, I. Kawasaki, and M. Amaha, "Gushing-Inducing Peptides in Beer Produced by Penicillium Chrysogenum," in Peptide Chemistry, H. Yonehara, ed. (Protein Research Foundation, Osaka, Japan, 1979), pp. 7-12.

16. P.B. Schwarz, S. Beattie, and H.H. Casper, "Relationship Between Fusarium Infestation of Barley and the Gushing Potential of Malt," Journal of the Institute of Brewing 102, pp. 93-96 (1996).

Neil Gudmestad is professor of plant pathology at North Dakota State University (NDSU) in Fargo. He has been interested in barley since the mid- to late-1970s when he first worked on barley diseases. His most recent interest in the plant centers on his hobby of home brewing.

Raymond Taylor is head brewer at the Great Northern Restaurant & Brewery in Fargo, North Dakota. His interest in barley diseases arises from his formal training in the Department of Plant Pathology at NDSU as well as his current job and home brewing hobby.

Paul Schwarz is associate professor in the Department of Cereal Science at NDSU, where he specializes in research on the biochemistry and use of barley and malt.