Fun-guys: The Problem With Mycotoxins in Malted Grains

Fungi in a nutshell (or grain husk)

This weeks’ issue

Dear brew enthusiasts

In this weeks episode, we will step into the world of plant pathology. We will look at a paper by the groups of Robert Brueggeman and Paul Schwarz, who have devised a new method to visualize pathogenic fungi in grains. Their work is important since it illustrates how some fungi can play hide-and-seek in grains and impact grain quality in the malting process.

Images of Barley (BC, non-infected), Infected barley (BI), Wheat (WC, non-infected), and infected wheat (WI). Adapted from Figure 2 in Jin et al. (2020).

Plants such as Barley, Wheat, Oat and Rye, provide the essential ingredients required for beer production. These plants, much like all other crops, are grown in a hostile environment where microbes can colonize them. Most interactions are harmless, whereas others can have a genuine and negative impact on quality.

While the vast majority of microbes are harmless and never seem to affect a given crop, others can infect and colonize host tissues and cause disease. Fortunately, research has identified a range of crop protection strategies that keep epidemics in check and limit yield losses. Yield, and loss, however, are slightly more complex concepts than most people would assume.

Although yield is a simple measure (tonnes per hectare), it does not describe quality. We already know that in the case of malting, barley (or wheat) grain has to meet specific quality criteria (e.g. starch and protein content). But one other important factor, independent of characteristics that influence beer profiles, is mycotoxin levels.

What are Mycotoxins?

In short, mycotoxins are metabolites that Fungi produce. As the name suggests, these fungal compounds are toxic and concern when found in (malted) grains. Because of their impact on human health, farmers not only have to prevent crop losses, they also need to ensure that Fungi, capable of producing mycotoxins, are kept at bay. Disease management, or suppressing the buildup of pathogen populations in the field, is critically important. Luckily, the availability of fungicides has helped farmers achieve just that.

Most grains destined for malting have very low amounts of mycotoxins. Storage and milling of grain often reduce their levels, although maltsters observe sharp increases in some cases. These observations have raised questions about the origin and nature of the fungal infection and high mycotoxin concentrations seen after malting. The question then is, how could it be?

That's what the authors of this paper set out to find out.

Some pathogens can grow in stealth mode, meaning that you can't see any phenotype while tissues are getting infected. Pathologists or anyone else interested in malt quality needs a robust methodology to "see" them. To find out if and where a pathogen is hiding, microscopy is the best option. But microscopy requires some practice and knowledge, especially when you try to find a needle in a haystack (meaning, small fungal cells growing on top of or between cells in grains). The authors, therefore, set out to devise a staining method that could selectively label fungal cells and visualize these structures, now emitting fluorescent light, under a confocal microscope. They used this method to (i) assess where fungal cells locate in barley grains and (ii) validate their observations by comparing their results to Scanning Electron microscopy (SEM) based analyses.

Figure 4M from Jin et al. (2020): Hyphae within the husk, pericarp (P), testa (T), aleurone layer (AL), and endosperm of barley grains, evidenced by distinct yellow-green staining of grain compartments.

Their results were exciting and sobering (no pun intended) at the same time. Microscopic analyses revealed that consistent with the idea that washed and scrubbed grains could contain mycotoxins; fungal biomass was present within the husk, the pericarp (the outer layer of the grain without husk), the testa (part of the seed coat), the aleurone layer (top of the endosperm), the endosperm and even the embryo! Interestingly, levels of growth were greater in wheat than in barley, fitting with the observation that wheat beers tend to have higher levels of mycotoxins then “regular” ales or beers.

These results show that at least in some grain batches, Fungi play hide-and-seek inside the seeds. Because of this, most if not all treatments can rid grains from these pathogens once they have established. How do these stealthy pathogens impact malting and malt quality?

To further establish the impact of infection and mycotoxins during malting, the authors conducted another informative experiment. They took grains, removed the husks and pericarp (seed coat), and used the de-coated grains for malting. They used barley, wheat, rye and triticale grains for this experiment and took some from each batch before and after malting. They then extracted DNA from these samples and quantified mycotoxin levels, expressed as mg of mycotoxin per kg of (malted) grain. The results were quite revealing.

Figure 3 from Jin et al. (2020). Changes in Mycoflora, Fusarium and mycotoxin levels associated with malting of barley, wheat, rye, and triticale. B: Barley; MB: Malted barley; W: Wheat; MW: Malted wheat; R: Rye; MR: Malted rye; T: Triticale; MT: Malted triticale. The percentage of DNA reads of F. graminearum, and other (Fusarium) species detected.

As you can see from the graph and despite removing the outer layer from grains, there still is a low level of Fusarium in barley, wheat, rye and triticale. Importantly and perhaps not surprisingly, the conditions used for germinating seeds (humidity, room temperature) are also ideal for fungal growth. A drastic increase in fungal biomass, along with toxin levels, thus accompanies seed germination. This is particularly true in malted rye (MR) and malted triticale (MT), where 10 and 25 mg of mycotoxin (deoxynivalenol or DON) were found present, respectively. Notably, malting drastically increased fungal biomass in wheat when compared to barley, again consistent with the observation that wheat grains carry higher fungal biomass then barley before malting.

What does this all mean?

In a nutshell (or more appropriately, “in a grain husk”), we should not rely on grain quality information as a predictor of malt quality. Especially when making your own malts, regular testing and inspection of your malted grains may be needed.

Should we worry if there is a little mycotoxin in our malted grains? The answer to that question is: it depends on contamination levels. A robust boil, fermentation and storage will get rid of some toxic potential, but not a lot. Mycotoxins can be rather stable. The best approach, therefore, would be to malt your grains in batches, to test each batch for mycotoxin content and adhere to defined quality standards.

If you purchase malted grains (dried), make sure to store them in dry (and cool) conditions to avoid further growth of unwanted fungi. Most (if not all) malted grains will have been tested and must pass rigorous quality control. The key is to deny pathogens the opportunity to grow and cause problems down the road.

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Cheers,

Edgar, The Beerologist

Edgar Huitema is a Scientist, Brewer & Scientific Consultant at ExtrAnalytics. Subscribe to my free newsletter to get the latest advances in science. Contact me if you wish to discuss your needs and our research.