Prevention of fungal growth during storage and malt production
The environmental conditions prevailing during long-term storage of brewing cereals are essential for growth of fungi and for their metabolic activity. Availability of water and oxygen are the most important growth-limiting factors alongside temperature, pH, and availability of nutrients (Christensen and Kaufmann, 1965; Lacey, 1989; Doohan et al., 2003; Magan and Aldred, 2007). Also, the production of mycotoxins during storage of cereals is influenced and limited by environmental factors (Magan et al., 1984, 2010; Miller, 1995; Schrodter et al., 2004). Reduction of mycotoxin levels during storage of barley and other cereals has been achieved by controlling environmental conditions, including water activity (Magan and Aldred, 2007), and by addition of natural substances such as essential oils (Paster et al., 1995; Juglal et al., 2002; Chulze, 2010) or microbial starter cultures. Yeast strains (Bjornberg and Schntirer, 1993; Petersson and Schntirer, 1995; Adel-Druvefors and Schntirer, 2005), bacteria (Frandberg and Schntirer, 1995) or even fungi ( Jensen et al., 2000) have been added to stored grain and were demonstrated to have a reducing or at least preserving effect on the fungal community.
Malting is a process in which a complex ecosystem evolves due to the prevailing moisture and temperature conditions, thus allowing contaminating microorganisms to develop and to have a negative influence on the quality of malt (Lai- tila, 2007; Laitila et al., 2007; Noots et al., 1999; Wolf-Hall, 2007; Raulio et al., 2009). The fungal community and other microorganisms growing during malting compete with grain metabolism for oxygen and may therefore considerably reduce grain germination during malting (Doran and Briggs, 1993; Noots et al., 1999). The steeping step as part of the malting process strongly promotes growth of bacteria, yeasts, and fungi with the establishment of stable biofilms on the grain surface (Laitila et al., 2007; Raulio et al., 2009). Beside fungal growth, significant increases in levels of several mycotoxins can occur during malting (Vegi et al., 2011; Oliveira et al., 2012). As shown in Fig. 8.1, several Fusarium species were demonstrated to proliferate from steeping through germination until early stages of kilning (Oliveira et al., 2012; Sarlin et al., 2005b; Vegi et al., 2011). Fusarium mould depletes grain nutrients, such as starch and protein, and colonizes its interior via exo-proteolytic and cellulolytic enzymes (Kang and Buchenauer, 2000; Oliveira et al., 2012, 2013), which can result in malting losses. Prevention of fungal growth during malting can be accomplished with chemical, physical, and biological methods. Addition of antifungal chemicals or general microbiocides is possible and has some inhibitive effect on the malt fungi. Presence of residues from the treatment will compromise the quality and safety of the malt produced and are therefore not recommend (Wolf-Hall, 2007). However, methods using gaseous ozone or hydrogen peroxide may be promising since they do not leave any residues in the malt and have been shown to have some effect in reducing fungal growth during malt production (Kottapalli et al., 2005). Also, non-degrading physical methods such as electron-beam irradiation (Kottapalli et al., 2006) or microwave treatment (Akaranuchat et al., 2008) have been demonstrated to show some reductive effect on fungi during malt production. Growth reduction by addition of appropriate starter cultures has been achieved in analogy to grain storage by addition of starter cultures during malt production (Boivin et al., 1997; Linko et al., 1998; Laitila et al., 2002; Wolf-Hall, 2007). Addition of starters was either done during steeping or by spraying during soaking or germination. Special regulations may exist in national law of individual states or the EU. According to German national law any additives to the malting process are prohibited unless the malt is intended for export.