Chemical and physical means

Several chemical microbicidic agents are effective in eliminating bacteria and fungi. Fungicides are occasionally applied in the field in order to protect the barley against plant pathogens. However, they may not be applied after the harvest. Recently, considerable research effort has been directed towards the development of novel compounds for plant disease control in order to minimize the use of chemicals and to reduce the resistance risk of field fungi (Gullino et al. 2000). It should be noted that application of fungicides during the growth period of barley may also have an impact on the microbial communities during malting and on the subsequent malt quality. Havlova et al. (2006) reported that application of some fungicides during barley cultivation increased the gushing

potential of the final malt. In addition, suppression of field fungi with fungicides increased pentosan and β-glucan levels in malts, with negative impacts on wort and beer properties. As a consequence, research has been directed towards developing more natural means for balancing the microbial communities in barley and during malting.

The treatments carried out during malting should not significantly influence the seed vigour. Various common practices are routinely applied to reduce adverse effects of microbes during malting, especially during the steeping phase, such as changing the steeping water in order to remove microbes and leached nutrients, balancing the temperature or modifying aeration (Briggs & McGuinness 1993).

Furthermore, steep water must be warm enough to allow rapid water uptake and germination of the grains, but cool enough to avoid extensive microbial growth.

Therefore, steeping is normally carried out at 10–20 °C (Pyler & Thomas 2000).

It is also important to provide sufficient aeration and to pulse the circulation throughout the immersion period in order to keep the grains moving and to avoid anaerobic, hot pockets in the grain bed which would lead to increased microbial activity and poor grain germination (Davies 2006).

Doran and Briggs (1993) reported that by acidifying the grain in the first steep the adverse effects of aerobic microbial growth could be greatly reduced.

Papadopoulou et al. (2000) suggested that fungal proliferation could be restricted by adding hop beta-acids into the malting process. Moreover, they demonstrated that the growth of fungi was inhibited by washing barley first with sodium hypochlorite (alkaline wash) followed by an acid wash with hypochloric acid. However, the feasibility and safety of acid treatments in large scale remains to be confirmed. Disinfectants have occasionally been added to steeping water in order to intensify the washing effect during steeping. Although different additives may effectively improve processing, their use in industrial processes is often limited by the legislation. Furthermore, the industry has a strong emphasis towards natural processing without chemicals (Olkku et al. 1992)

It is a well-known fact that several filamentous fungi, especially field fungi such as fusaria, are sensitive to heat. High-temperature treatments have been shown to effectively reduce the viable fungi on cereal grains, although it does not eliminate the preformed mycotoxins (Kristensen et al. 2005). Olkku et al. (2000) reported an invention in which the mould contamination of barley was

effectively reduced by exposing grains to heat (60–100 °C) for 0.5–3 s prior to the malting process. Heat treatment of barley notably decreased the Fusarium contamination without influencing grain germination. Moreover, it significantly reduced mycotoxin production during the malting process and alleviated the gushing tendency (Olkku et al. 2000).

Kottapalli et al. (2003) studied hot-water treatments for reducing fusaria in malting barley. They showed that soaking barley with water at 45 °C for 15 min resulted in a significant reduction of Fusarium contamination without influencing grain germination. The same effect was obtained by soaking at 50 °C for 1 min.

A great advantage of hot water treatments was that some water-soluble mycotoxins could be washed out of the grain. Briggs (2004) reported that short exposures to hot water, even at 100 °C for 5 s were advantageous with respect to microbiological safety and grain germination. Thus grains would be washed and surface-sterilized prior to steeping.

In addition to heat treatments, electron-beam irradiation of barley has been reported to be an effective, non-chemical means for reducing fusaria. For dry Fusarium-infected barley, an irradiation dose of > 4 kGy was required to obtain Fusarium reduction (Kottapalli et al. 2003, 2006). Although irradiation had no effect on the preformed mycotoxins, it greatly reduced the mycotoxin production during malting (Kottapalli et al. 2006). The advantage of irradiation was that the grain treatment could be carried out either prior to or after the storage. It could also help in the maintenance of quality by eliminating insect infestation. Insects such as beetles are the principal vectors of microbes in plant ecosystems (Suh &

Blackwell 2004). Grain germination was not significantly influenced up to a dosage of 8 kGy (Kottapalli et al. 2003).

Residues and undesirable reaction products in germinating barley and in the subsequent malt are of concern especially with chemical treatments, since they may have a negative impact on malt properties and yeast fermentation performance. Furthermore, precautions must be taken as some of the antimicrobial treatments in sublethal doses may stimulate the production of harmful metabolites such as gushing factors and mycotoxins. Malt-derived microbes, especially lactic acid bacteria and certain fungi, offer a potential alternative as natural, food-grade biocontrol agents. Natural biocontrol agents are attractive as they have a better public image and they could potentially be

used as starter cultures in bioprocesses in which the use of chemicals is considered undesirable. Some of the potential LAB and fungal strains studied in malting applications are listed in Table 4.