Spores of thermophilic bacilli are a common concern during the manufacture of milk powder. Spores are believed to occur in high numbers in milk powder due to their ability to survive pasteurisation, attach to stainless steel surfaces, germinate, grow as biofilms and subsequently enter the product stream and thereby contaminate the final product. In this study, thirty one thermophilic bacilli isolates were obtained from a New Zealand milk powder production line and identified as either Anoxybacillus flavithermus or Geobacillus spp. using random amplified polymorphic DNA (RAPD) and species-specific PCR. Sporulation media and a polyethylene glycol two-phase separation system were modified to produce high yields of spores free from debris. The spores of four Geobacillus spp. isolates (CGT-8, D4, E7 and E11) were characterised in terms of structure (electron microscopy), surface charge (zeta potential), hydrophobicity (contact angle and microbial adhesion to hexadecane) and attenuated total reflectance infrared spectroscopy (ATR-IR). Spores from three of the four isolates possessed an exosporium while the fourth did not. However the integrity of the exosporium varied over time. The spores were negatively charged (-10 to -20 mV) at neutral pH and high ionic strength (0.1 M KC1). Both hydrophobicity assays revealed that the spores of the four isolates were relatively hydrophilic while ATR-IR revealed the spores' surfaces consisted of protein and polysaccharides. The influence of these spore characteristics on adhesion to a variety of substrata under high flow rates was examined using the extended Derjaguin, Landau, Verwey and Overbeek (XDLVO) theory. Spores generally attached in higher numbers to hydrophobic surfaces compared to hydrophilic surfaces, however this observation was more prevalent for isolate D4. This result indicated that a single mechanism could not describe the adhesion of spores from different strains. A series of glass surfaces with modified characteristics were produced in order to test the antifouling properties on the adhesion of D4 spores. Spores suspended in a high ionic strength medium (0.1 M KC1) attached in greater numbers (1 Log₁₀ CFU cm⁻�) to positively charged and hydrophobic surfaces compared with negatively charged and hydrophilic surfaces. A clean in place (CIP) procedure, reduced spore numbers on hydrophobic and hydrophilic surfaces by 1.5 and by 2.0 Log₁₀ CFU cm⁻�, respectively. When spores were suspended in milk, there was little difference in the number of spores attaching to the different surfaces (ie. 3.5 to 3.8 Log₁₀ CFU cm⁻�), and spore removal from surfaces via a CIP regime was unchanged (1.5 to 2.0 Log₁₀ CFU cm⁻� reduction) compared with spores that attached in simple 1:1 electrolyte media. The effects of a caustic wash on spore surface characteristics and adhesion was determined. There was a significant reduction in spore viability (2 Log₁₀ CFU mL⁻�) after a 30 min caustic wash at 65 �C in the current study, however surviving spores displayed a greater propensity to attach to stainless steel. Surface characterisation results revealed an increase in hydrophobicity and a greater negative charge on the spores' surface after treatment with NaOH. Surviving spores could potentially recontaminate sections of the plant which are cleaned with this recycled caustic wash solution, thereby seeding surfaces with spores at the beginning of the next processing run. In conclusion, while surfaces that reduce spore adhesion and enhance removal can be produced, exposure to complex solutions such as milk can reduce the anti-fouling effectiveness of such surfaces to spore adhesion.

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Agglomerated milk powders are produced to give improved properties such as flowability, dispersibility, reduced dustiness and decreased bulk density. A key function of these powders is to dissolve "instantly" upon addition to water and because of this they are also called "instant milk powders". They are produced by agglomerating the undersized fines that are returned to the top of the spray drier with milk concentrate droplet spray. Interaction occurs in a collision zone, often with multiple sprays and fines return lines. Agglomeration can be a difficult process to control and operators find it hard to fine tune the process to produce specific powder properties. This work aimed to understand the effects of key droplet and fines properties on the extent of agglomeration to allow a mechanistic understanding of the process. Three scales of spray drier were investigated in this study with different rates of evaporation; a small scale drier (0.5 - 7 kg water h-1), a pilot scale drier (80 kg water h-1) and a range of commercial production scale driers (4 - 15 000 kg water h-1). A survey of operators of commercial scale driers showed that control of instant milk powder production to influence bulk density is highly intuitive. Fines recycle rates were expected to be important in control of agglomeration processes and were estimated on a specific plant by using the pressure drop measured in the fines return line. A model based on pressure drop along a pneumatic pipeline under-predicted the experimental values for pressure drop due to solids, which means a calibration curve should be generated for each specific drier. Fines recycle rates were predicted to be significantly higher at 95 to 130 % of production rates compared to those expected by operators of 50%. Experimental measurements agreed with existing models for the effect of temperature on the density and viscosity of milk concentrates. Experimental results showed that the surface tensions of concentrated milks were within the same range as literature values for standard milks below 60°C, but were significantly higher for milk above 60°C. This is thought to be linked to the mechanism of skin formation due to disulphide cross linking at high temperatures and concentrations. Powder properties were also established for selected products produced on the commercial scale driers. These powders were then used in experiments on the two smaller driers. Because collision frequency depends on the velocity and droplet size of sprays; these properties were measured for the small scale drier and estimated, where possible, for the pilot and commercial driers. The small scale agglomerating spray drier was configured to alter droplet and particle properties when interacting a vertical fines particle curtain with a horizontal spray sheet. An extensive design and improvement process was carried out to ensure the system consistently delivered these streams in a controllable manner. The processes of collision and adhesion occur very quickly inside the spray drier. In order to assess the extent of agglomeration that has occurred, the feed streams must be compared to the final product stream. An ideal way to do this is to use an agglomeration index which compares the particle size distributions of the feed (fines recycle and spray streams) and the particle size distribution of the product stream (the agglomerated powder). The index described changes between these steams across the particle size distribution and is called an agglomeration efficiency, ξg. However, it was found that the presence of fines in the product of the one-pass design obscured the agglomerates formed. The agglomeration efficiency, ξg, was modified to become ξh which subtracted the fines stream from the agglomerated product distribution. In this way ξh models industrial operation where the fines are recycled, by effectively just comparing the spray and product streams entering and leaving the process. The small scale drier was used for an experimental study on natural and forced agglomeration, where the drier was operated with spray only, then with spray and fines. For natural agglomeration, SEM images of the product powder indicated that little agglomeration occurred between spray droplets. The product yield was unacceptably low (~ 40%) due to adhesion of spray droplets to the drying chamber wall opposing the horizontal spray. When the fines curtain was introduced in the forced agglomeration experiments, product yield increased above 50% because the fines acted as collectors for the spray droplets. However, the agglomeration performance of the modified spray drier was lower than expected. The equipment design was then optimised by considering three key issues; fines dispersion, droplet dispersion and stickiness, and agglomerate breakdown. Final experiments studied agglomeration at low fines to spray mass flux ratios and showed that increasing the fines size had a positive effect on agglomeration efficiency,ξh. The agglomeration study at pilot scale identified the effect of key variables, total solids, concentrate and fines flow rate, and fines size on the agglomeration efficiency. A dimensionless flux approach was used to explain the experimental results. The fines to spray mass flux ratio and the projected area flux ratio (at constant concentrate flow rate) were found to be the most suitable to represent the physical processes during agglomeration. Experimental results showed that a higher dimensionless flux resulted in more agglomeration and as well as small fines size and atomising low solids concentrate. The critical Stokes number highlighted the importance of particle size and collision velocity on the outcome of the collision as well as the importance of stickiness on adherence following the collision. A statistical analysis established a relational model for predicting the agglomeration efficiency based on fines size, total solids and the fines to spray mass flux ratio. This thesis has gained insight into agglomeration processes during spray drying and knowledge about how to define the extent of agglomeration. Practical findings from this research can have a significant impact on successful spray drying operation for instant powders. There are some practical steps to be taken industrially to promote the control of agglomerating spray driers. The first step is to measure and control the flow of fines recycled to the top of the spray drier. The next step is to validate the findings at industrial scale and link the agglomeration index to the bulk powder properties. However, there are many challenges that remain to be tackled in the area of milk powder agglomeration. Milk powder agglomeration at the top of the spray drier is a complex process involving many different variables. A more detailed study of the micro processes that occur during agglomeration will give increased understanding of the relationships between key operating variables and agglomerate properties.