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. 


Chen, L., Coolbear, T., Daniel, R.M. (2004). Characteristics of proteinases and lipases produced by seven Bacillus sp. isolated from milk powder production lines. International Dairy Journal 14 : 495-504.

Chopra, A.K. & Mathur, D.K. (1984). Isolation, screening and characterization of thermophilic Bacillus species isolated from dairy products. Journal of Applied Bacteriology 57 : 263-271.

Craven, H.M., Broome, M.C., Chandler, R.E. & Jensen, N. (2001). Dairy Products. In C.J. Moir (Ed. In Chief), Spoilage of Processed Foods: Causes and Diagnosis. (pp. 147-163). Sydney, NSW: Australian Institute of Food Science and Technology.

Crielly, E.M., Logan, N.A. & Anderton, A. (1994). Studies on the Bacillus flora of milk and milk products. Journal of Applied Bacteriology 77 : 256-263.

Feeherry, F.E., Munsey, D.T. & Rowley, D.B. (1987). Thermal inactivation and in jury of Bacillus stearothermophilus spores. Applied and Environmental Microbiology 53 : 365-370.

Flint, S., Palmer, J., Bloemen, K., Brooks, J. & Crawford, R. (2001). The growth of Bacillus stearothermophilus on stainless steel. Journal of Applied Microbiology 90 : 151-147.

Grinstead, E. & Clegg, L.F.L. (1955). Spore-forming organisms in commercial sterilized milk. Journal of Dairy Research 22 : 178-190.

Ito, K.A. (1981). Thermophilic organisms in food spoilage: Flat-sour aerobes. Journal of Food Protection 44 : 157-163.

Jenson, I., Jensen, N. & Hyde, M. (2001). Gram Positive Aerobic Sporeforming Rods. In C.J. Moir (Ed. In Chief), Spoilage of Processed Foods: Causes and Diagnosis. (pp. 271-294). Sydney, NSW: Australian Institute of Food Science and Technology.

Kalogridou-Vassiliadou, D. (1992). Biochemical activities of Bacillus species isolated from flat sour evaporated milk. Journal of Dairy Science 75 : 2681-2686.

Kalogridou-Vassiliadou, D., Tzanetakis, N. & Manolkidis, K. (1989). Bacillus species isolated from flat sour evaporated milk. Lebensmittel Wissenschaft und Technologie 22 : 287-291.

Luck, H. & Jordaan, I. (1978). Bacterial content of milk powder. South African Journal of Dairy Technology 10 : 91-96.

Meer, R.R., Baker, J., Bodyfelt, F.W. & Griffiths, M.W. (1991). Psychrotrophic Bacillus spp. in fluid milk products: A review. Journal of Food Protection 54 : 969-979.

Mostert, J.F., Luck, H. & Husmann, R.A. (1979). Isolation, identification and practical properties of Bacillus species from UHT and sterilized milk. South African Journal of Dairy Technology 11 : 125-132.

Nazina, T.N., Tourova, T.P. Poltaraus, A.B. Novikova, E.V., Grigoryan, A.A. Ivanova, A.E., Lysenko, A.M., Petrunyaka, V.V., Osipov, G.A., Belyaev, S.S. & Ivanov, M.V. (2001). Taxonomic study of aerobic thermophilic bacilli: descriptions of Geobacillus subterraneus gen. nov., sp. nov. and Geobacillus uzenensis sp. nov. from petroleum reservoirs and transfer of Bacillus stearothermophilus, Bacillus thermocatenulatus, Bacillus thermoleovorans, Bacillus kaustophilus, Bacillus thermodenitrificans to Geobacillus as the new combinations G. stearothermophilus, G. thermocatenulatus, G. thermoleovorans, G. kaustophilus, G. thermoglucosidasius and G. thermodenitrificans. International Journal of Systematic and Evolutionary Microbiology 51 : 433-446.

Olson, K.E. & Sorrells, K.M. (1992). Thermophilic Flat Sour Sporeformers. In C. Vanderzant & D.F. Splittstoesser (Eds.), Compendium of Methods for the Microbiological Examination of Foods. (pp. 299-307) (3rd ed.). Washington, D.C., United States: American Public Health Association.

Parkar, S.G., Flint, S.H. & Brooks, J.D. (2004). Evaluation of the effect of cleaning regimes on biofilms of thermophilic bacilli on stainless steel. Journal of Applied Microbiology 96 : 110-116.

Perkins, M.L. & Deeth, H.C. (2001). A survey of Australian consumers’ attitudes towards UHT milk. The Australian Journal of Dairy Technology 56 : 28-34.

Phillips, J.D. & Griffiths, M.W. (1986). Factors contributing to the seasonal variation of Bacillus spp. in pasteurised dairy products. Journal of Applied Bacteriology 61 : 275-285.

Scheldeman, P., Pil, A., Herman, L., De Vos, P. & Heyndrickx, M. (2005). Incidence and diversity of potentially highly heat-resistant spores isolated at dairy farms. Applied and Environmental Microbiology 71 : 1480-1494.

Watanabe, T., Furukawa, S., Hirata, J. Koyama, T., Ogihara, H. & Yamasaki, M. (2003). Inactivation of Geobacillus stearothermophilus spores by high-pressure carbon dioxide treatment. Applied and Environmental Microbiology 69 : 7124-7129.

Westhoff, D.C. & Dougherty, S.L. (1981). Characterization of Bacillus species isolated from spoiled ultrahigh temperature processed milk. Journal of Dairy Science 64 : 572-580.