The growth of thermophilic bacteria in a milk powder plant and the formation of spores in biofilms of the dairy thermophile Anoxybacillus flavithermus : a thesis presented in partial fulfilment of the requirements for the degree of Masters of Science in Microbiology at Massey University

Abstract

Dairy production makes up 20% of New Zealand's export earnings, with whole milk powder being the number one dairy export. However, contamination of milk powder by spores of thermophilic bacteria is an ongoing problem in the manufacture of milk powder. These spores survive manufacturing processes, contaminate final product and have potential to spoil foods manufactured with milk powder. The main thermophilic organisms that cause concern in the New Zealand dairy industry are Geobacillus spp. and Anoxybacillus flavithermus. The vegetative forms of these organisms are able to grow within biofilms, but there is very little information as to the origin of their spores found in the product and the conditions under which they are produced. We have now monitored the dynamics and location of spore formation in the industrial and controlled laboratory settings. A survey was undertaken at the Pahiatua milk powder manufacturing plant to determine the origin and rate of spore formation. The predominant sites of spore formation were the plate heat exchanger and evaporator. Spores began to develop approximately 11 h into an 18 h manufacturing run. The spores were identified as Anoxybacillus flavithermus and Geobacillus species. To examine the dependence of spore formation on the development of the A. flavithermus biofilm under controlled laboratory conditions, a continuous flow reactor was used. The release of spores and vegetative cells into the milk was measured using change in impedance. Impedance change confirmed the presence of both vegetative cells and spores on stainless steel sample tubes. At the end of an 8.5 h run at 55°C, using the continuous flow reactor, the total number of thermophilic bacteria released into the milk reached up to 106 cells mL -1. At least 10 % of cells attached to the stainless steel surface were spores. These results indicate that spores form readily in biofilms of A. flavithermus believed to colonise the surface of the manufacturing plant. When the temperature of the continuous flow reactor was decreased to 48°C no spores were detected within the biofilm. The results from this study have provided key information about where thermophilic spores form in a milk powder manufacturing plant and how biofilms of one of the typical thermophilic bacteria, Anoxybacillus flavithermus, develop. This knowledge will help the dairy industry to design strategies to prevent spore formation.