Before I commenced my postdoc at RMIT University in July 2008, I applied to The University of Queensland in 2007 for a postdoctoral fellowship, which I was unsuccessful for. The text in this blog post formed part of my research proposal.
Bacteria from the genus Bacillus and Geobacillus are ubiquitous (Chen et al., 2004) and therefore can enter the raw milk supply from many sources (Meer et al., 1991). However, Geobacillus, being thermophilic with an optimal growth temperature of 55-65 °C (Nazina et al., 2001), are more likely to be present on heated milk processing equipment, particularly as biofilms (Flint et al., 2001). Although they are present in freshly drawn raw milk at low levels (Flint et al., 2001), they may be present in long-life dairy products (Westhoff & Dougherty, 1981), and are very important due to their potential involvement in spoilage of those products. This involvement has been recognised for a long time (Grinstead & Clegg, 1955). High levels of vegetative cells may cause spoilage of reconstituted milk powder (Flint et al., 2001) and G. stearothermophilus in particular associated with flat sour spoilage of low-acid foods, including milk (Ito, 1981), especially evaporated milk (Kalogridou-Vassiliadou et al., 1989). Furthermore, in milk powder, spores from thermophiles are mostly present up to approximately 5.0 x 102/g while a typical concentration for their vegetative cells is up to approximately 104/g (Luck & Jordaan, 1978; Chopra et al.,1984). Some species are also important spoilers of acidic food products and for some applications, the mere presence of spores from these organisms is cause for concern (Jenson et al., 2001). In addition to spoilage, corrosion of stainless steel and reduced manufacturing efficiency can result from the presence of biofilms (Parkar et al., 2004). While G. stearothermophilus is considered non-pathogenic (Ito, 1981), the involvement of B. lichinformis in human diseases of various locations within the body has been reported (Santini et al., 1995).
The persistence of these organisms in the dairy processing environment is cause for concern as they readily form biofilms, which contribute vegetative cells or spores to the raw milk. In addition, as various Bacillus spp. have been found in long-life milk products (Mostert et al., 1979), it is likely that thermophilic (such as G. stearothermophilus) or thermotolerant (such as B. licheniformis) species in particular, may survive heat treatment of milk, even UHT processing. Even if the cells do not survive, their spores will. They will then germinate under favourable conditions, and growth of vegetative cells can result in flat sour spoilage (Kalogridou-Vassiliadou, 1992). It is widely acknowledged that these spores are highly resistant to many physicochemical treatments, such as heat, drying, radiation and chemicals.
Processing of long-life dairy products involves eliminating all microbes, therefore, effectively sterilising the product. While sterility is the aim, in practice, commercial sterility is achieved, which refers to a particular failure rate or particular percentage of non-sterile units of product. Ideally, heat processing of long-life products should eliminate vegetative cells and spores. Due to the heat resistance of spores, moist heat, at temperatures in excess of 121°C are generally required to inactivate them. This is especially the case with G. stearothermophilus, whose spores are regarded as one of the most heat resistant of all aerobic microorganisms (Watanabe et al. 2003). Even at 124°C, a heating time of 63 sec. is still required to reduce the G. stearothermophilus spore count by one log (Feeherry et al., 1987). As a result of this extreme heat resistance, inactivation of G. stearothermophilis spores is generally regarded as effective heat treatment to result in a sterile product.
A disadvantage of high temperature heat treatment of food products is the alteration that can potentially result to the physical, chemical and/or nutritional composition of the food. This is particularly true with milk, which is relatively heat-sensitive, as distinctive flavours and aromas result after UHT processing. This flavour and aroma has been a major problem in some countries as it has directly prevented the acceptance of UHT milk. One of those countries is Australia, with UHT milk sales comprising a small percentage of all fluid milk products sold (Perkins & Deeth, 2001). Therefore, an increase in UHT milk sales could be generated in Australia by its production without the accompanying characteristic aroma and flavour which many people find unpleasant. An approach could be the use of means other than heat to sensitise the spores. These injured spores may then be completely inactivated by more gentle heat treatments, such as 115-120°C, stated by Craven et al. (2001) as the typical temperature of processing for high heat milk powder. In addition to providing a means to produce long-life milk without severe heat treatment, much information regarding the alteration of the physiology and structure of spores could be determined.Thermotolerant B. licheniformis and G. stearothermophilis are amongst the most often isolated species on dairy farms, compared to other related species (Scheldman et al., 2005). In one study of Bacillus spp. in the dairy environment, B. lichinformis was the most frequently isolated from raw milk and environmental sources and its spores were dominant in milk powder (Crielly et al., 1994). Therefore, they are always going to be present in milk production/processing areas and will almost always be present in raw milk, especially because there is no real seasonal variation in occurrence of vegetative cells of these species in raw milk, with low and high occurrence reported at all times of the year (Phillips et al., 1986). Some of these bacteria, including those responsible for flat sour spoilage, may grow at temperatures as low as 30°C which is why strict temperature control of UHT milk during storage and distribution is imperative (Olson & Sorrells, 1992). Moreover, the pathogenic species are thermotolerant mesophiles and therefore they are able to grow well at these temperatures.
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