Evolution of microbial populations during malting

The microbial ecology of barley changes during malting. Before entering the malting process, barley is cleaned and graded in order to remove foreign material, dust, and small and broken kernels. Cleaning procedures also diminish the microbial load. However, malting conditions are extremely favourable for microbial growth in terms of available nutrients, temperature, moisture content and gaseous atmosphere. Figure 5 illustrates the growth of bacteria and yeasts in the industrial malting ecosystem (Wilhelmson et al. 2003). Steeping of barley leads to leakage of nutrients into steeping water and rapidly activates the dormant microbes present in barley grains (Kelly & Briggs 1992). Although some of the microbes and soluble nutrients are washed away along with steep water draining, the viable microbial numbers increase markedly during the steeping period (Briggs & McGuinness 1993, Douglas & Flannigan 1988, Flannigan et al. 1982, O’Sullivan et al. 1999, Petters et al. 1988). The steeping vessel and the water remaining at the bottom of the tank between steeps are known to serve as inocula for the next batches (O’Sullivan et al. 1999). Steeping is generally regarded as the most critical step in malting with respect to microbiological safety (Noots et al. 1999).

Microbial activity remains high throughout the germination period. Furthermore, microbial growth is accelerated during the first hours of kilning (Wilhelmson et al. 2003). The kilning regime has been identified as a significant factor in controlling microbial communities (Stars et al. 1993). Although high temperatures effectively restrict the growth and activity of microbes, kilning appears to have little effect on the viable counts of bacteria and fungi. The viable counts of microbes are generally higher in the finished malt than in native barley (Noots et al. 1999). Barley dries progressively from the bottom to the top of the grain bed, and the time that barley is exposed to each temperature depends on its

location in the kiln. Reduction of microbial activity depends on the moisture level and the length of time before the temperature breakthrough in the grain bed (Wilhelmson et al. 2003). Furthermore, the microbial community is also significantly influenced by the malthouse operations, and it has been shown that a specific microbial community develops in each malting plant (O’Sullivan et al.

1999, Petters et al. 1988). The microbial community of final malt reaching the brewery or distillery is naturally influenced by the handling and storage operations after the malting process as well as during transport of malt (Angelino & Bol 1990).

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

log cfu/g

Aerobic heterotrophic bacteria Pseudomonas spp. LAB, 30°C

LAB, 45°C Bacillus spp. Yeasts

Steep Germination Kilning

detection limit

< 50 cfu/g

Figure 5. Growth of aerobic heterotrophic bacteria, Pseudomonas spp., mesophilic (LAB, 30 °C) and thermophilic lactic acid bacteria (LAB, 45 °C), aerobic spore-forming bacteria and yeasts during industrial scale malting. The counts are mean values obtained from different industrial malting experiments (Wilhelmson et al. 2003).

Enterobacteria and Pseudomonas spp. are the predominant bacteria during malting, reaching 10⁸–10⁹ cfu/g during germination (Douglas & Flannigan 1988, Haikara et al. 1977, Noots et al. 1999, O’Sullivan et al. 1999, Petters et al.

1988). Lactic acid bacteria (LAB) constitute only a small minority of the bacterial community in native barley. However, their numbers increase

significantly to 10⁶–10⁸ cfu/g during the steeping process (Booysen et al. 2002, Haikara et al. 1977, O’Sullivan et al. 1999, Petters et al. 1988, van Waesberghe 1991). Malting equipment has been shown to act as a reservoir of additional LAB (O’Sullivan et al. 1999). The LAB population is dominated by heterofermentative leuconostoc species during steeping, whereas lactobacilli begin to dominate during germination (Booysen et al. 2002, O’Sullivan et al.

1999, van Waesberghe 1991). However, great variation in species diversity has been observed between different malting houses.

High numbers of yeasts and yeast-like fungi have been observed during the malting process (Bol & Huis in’t Veld 1988, Douglas & Flannigan 1988, Flannigan et al. 1982, Flannigan 2003, Haikara et al. 1977, O’Sullivan et al.

1999, Petters et al. 1988, Wilhelmson et al. 2003). Traditionally yeasts in the malting ecosystem have been roughly divided into white and pink yeasts based on the colony colour (Flannigan 2003). Previously, 10 ascomycetous and 6 basidiomycetous yeasts species were reported from barley and malting samples (Douglas & Flannigan 1988, Flannigan 1969, Flannigan & Dickie 1972, Flannigan et al. 1982, Flannigan 2003, Kottheimer & Christensen 1961, Noots et al. 1999, Petters et al. 1988, Tuomi et al. 1995, Tuomi & Rosenqvist 1995).

Furthermore, a yeast-like fungus Aureobasidium pullulans is commonly encountered in pre- and post-harvest barley samples (Clarke & Hill 1981, Flannigan 1969, Flannigan et al. 1982, Hoy et al. 1981). However, the diversity and the role of yeasts in the malting ecosystem are still largely unknown.

The genus Fusarium is the most important group of filamentous fungi related to barley and malting. The species of fusaria are adapted to different ecological niches all over the world as saprophytes and plant pathogens with a wide range of host plants. Currently, over 70 species are included in this genus (Leslie &

Summerell 2006). The malting environment is extremely favourable for Fusarium fungi (Douglas & Flannigan 1988, Haikara et al. 1977). As seen from Figure 6, intensive Fusarium growth has been observed during steeping, even when the original barley had only a low level of Fusarium contamination (Laitila et al. 2002). Approximately 30–50% higher Fusarium counts were measured after the steeping stage compared with the original contamination of barley. The levels of other field fungi such as Alternaria and Cladosporium usually decline during germination (Douglas & Flannigan 1988, Haikara et al.

1977). However, great variation in fungal communities has been observed due to

the differences in malting practices in different locations (Ackermann 1998, Douglas & Flannigan 1988, Flannigan 2003). Certain heat-resistant fungi, such as Rhizopus and Mucor, are frequently encountered at the end of germination and they continue to grow during the early hours of kilning (Douglas &

Flannigan 1988, Haikara et al. 1977).

Figure 6. Fusarium fungi in laboratory scale maltings of Kymppi barley. The data were collected from malting experiments carried out in 1991–1997 (Laitila et al. 2002).

1.3 Impact of microbes on grain germination