RECOLECCIÓN, AISLAMIENTO E IDENTIFICACIÓN DE CEPAS

The physical properties of a particular milk powder are largely determined by the spray drying process. The particle size and shape are determined by the droplet size from the atomiser and the temperature of the drier. At high temperatures, the droplets rapidly form a hard shell of dried material on the surface. As moisture continues to evaporate from the droplet, bubbles of vapour are created within the droplet, forming voids within the particles and resulting in a low particle density (Nijdam & Langrish, 2006). At lower temperatures, the exterior of the droplet dries more slowly. Outward diffusion of moisture from the interior keeps the shell soft, and as the droplet dries the shell collapses inward, resulting in a convoluted particle surface. In addition, segregation of the various components in the milk occurs during drying, with fat and protein migrating toward the particle surface while lactose accumulates in the centre of the particle. Slower drying allows more time for this segregation to occur, resulting in greater differences between the bulk and surface composition (Kim, Chen, & Pearce, 2009; Nijdam & Langrish, 2006).

2.4.2Milk Powder Stickiness

Stickiness and caking in dairy powders can cause significant problems in industry and have been the focus of much research. The stickiness of dairy powders depends on the powder composition, with both amorphous lactose and fat components having a significant effect on the cohesiveness of the powder (Fitzpatrick, Barry, et al., 2007). Stickiness also depends on storage conditions, due to phase transitions that occur within the powder at specific temperature and moisture levels.

In powders with a high fat content, such as whole milk powder (WMP) and cream powder, cohesiveness is thought to be primarily due to the fat melting and causing liquid bridging between particles (Fitzpatrick, Barry, et al., 2007). Dairy powders contain many different fats with melting points ranging from -40°C to +40°C (Kim, Chen, & Pearce, 2005b), so fat melting is significant over a wide range of temperatures. Dairy baghouses are typically operated at 70-80°C, so the fat is liquid during filtration. Furthermore, in spray dried dairy powders, fat tends to accumulate on the surface of the particles (Kim, Chen, & Pearce, 2002; Nijdam & Langrish, 2006), so even low levels of bulk fat can have significant effects on the particle interactions. At high temperatures, the inter-particle bonds formed by fat bridging are

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relatively weak; however as the powder cools, the liquid bridges solidify, causing much stronger bonding (Foster, Bronlund, & Paterson, 2005a).

In low fat, high lactose powders, such as SMP, stickiness is primarily due to mobility of amorphous lactose and is dependent on both temperature and humidity (Fitzpatrick, Iqbal, Delaney, Twomey, & Keogh, 2004). Cohesiveness due to lactose is generally defined by two important points, the glass transition and the sticky point.

The glass transition is a phase transition related to the mobility of the lactose molecules in the powder. Spray dried dairy powders typically contain lactose in an amorphous form, as the rapid drying process occurs too quickly for crystallisation to take place. Amorphous lactose, like many other amorphous solids, undergoes a glass transition, which is a phase transition from a rigid, amorphous solid state, to a softened state resembling a highly viscous liquid (Hogan, Famelart, O’Callaghan, & Schuck, 2010). The temperature at which this transition occurs is known as the glass transition temperature (Tg). With lactose, the glass transition temperature is dependent on the water activity. Lactose is highly hygroscopic and readily absorbs moisture from the surroundings. Water acts as a plasticiser, causing a reduction in Tg. The effect of moisture on Tg is described by the Gordon-Taylor equation (Gordon & Taylor, 1952):

, ,

(2.1)

where kG is a constant, ws and Tg,s are the mass ratio and glass transition temperature of the substance in question, and ww and Tg,w are the mass ratio and glass transition temperature of water. Tg,w is thought to be about 136 K, however there is some disagreement on this, with a range of values reported in the literature. Tg has been closely studied for pure lactose (Haque, Kawai, & Suzuki, 2006; Thomsen, Jespersen, Sjostrom, Risbo, & Skibsted, 2005) and for many lactose containing powders (Haque & Roos, 2004b; Hogan et al., 2010; Vuataz, 2002). Below Tg, the amorphous lactose is rigid and does not contribute to powder cohesion. Above Tg, molecular mobility is higher, and caking and lactose crystallisation can occur with time. Caking is primarily due to softened lactose forming bridges between particles, causing sintering. Given sufficient time, this can form a very rigid powder cake, which requires substantial force to break up and makes further handling and processing of the powder difficult. Caking occurs more rapidly at temperatures further above Tg (Fitzpatrick,

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O’Callaghan, & O’Flynn, 2008). Caking is therefore commonly described in terms of the temperature offset from the glass transition temperature, T-Tg (Fitzpatrick, Hodnett, et al., 2007; Fitzpatrick et al., 2008). It should also be noted that the glass transition is a property of amorphous solids, and the caking mechanisms described above do not apply to crystallised lactose (Fitzpatrick et al., 2008; Vuataz, 2002). Nevertheless, Silalai and Roos (2010) found that partial pre-crystallisation of the lactose increased cohesion, as the pre-existing crystals helped to seed further crystal growth. In most commercial spray-dried powders, the lactose is almost exclusively in amorphous form, as moisture loss and solidification during spray drying occur too rapidly for substantial crystallisation to occur. For example, Listiohadi, Hourigan, Sleigh, and Steele (2005) calculated the water of crystallisation in a non- instantised SMP powder, and found none detectable, indicating that the lactose was entirely in amorphous form.

Caking processes can also be slowed by mass transfer constraints. When a powder is suddenly exposed to warm, humid air, it can take some time for the powder to absorb moisture and equilibrate to the new conditions. Moisture sorption data for pure lactose is readily available (Bronlund & Paterson, 2004), however sorption in other milk powders can be more complex, due to the presence of protein, fat, and minerals. Moisture sorption data is available for some specific powders, including SMP (Kockel, Allen, Hennigs, & Langrish, 2002). Foster, Bronlund, and Paterson (2005b) developed a technique for predicting moisture sorption from powder composition, with an accuracy of around 10% for most powders. Nevertheless, direct measurement remains the most reliable method of determining the moisture sorption isotherm for any specific dairy powder.

The sticky point is defined as the point at which the particles become noticeably sticky as measured by a mechanical test. Above the sticky point, the adherence of particles to each other and to hard surfaces is essentially instantaneous. Sticky behaviour therefore affects dynamic processes with moving particles, such as the bulk flow of powder through processing equipment, or the adhesion of suspended particles to the inside surfaces of spray driers and ducts. This is in contrast to the caking behaviour described above, which applies only to static particles, for example in storage. Stickiness is therefore more relevant to active processing such as spray drying and baghouse collection of powders, where the powder is generally not held static for sufficient time for caking to become a problem. The sticky point is related to the lactose glass transition, and generally follows a curve similar to the Tg curve,

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but shifted up in temperature (Ozmen & Langrish, 2002). Consequently, the sticky point for a powder is often reported in terms of (T-Tg)crit, the critical temperature offset from the glass transition above which sticky behaviour occurs. Measurements of (T−Tg)crit depend on the method used, with reported values for SMP ranging from 23.3°C using a stirrer method (Hennigs, Kockel, & Langrish, 2001) to 37.9°C using a particle bombardment method (Paterson, Bronlund, Zuo, & Chatterjee, 2007). This difference is most likely due to differences in the time scales of the adhesive interactions being measured, as well as differences in the shearing and inertial forces produced by different methods. The sticky point measured by a particular method is the point at which the bonds formed between particles become significant relative to the forces applied to the particles by the test method. Greater contact time between particles allows for stronger bonds to form, so bond strength reaches measureable levels at a lower T-Tg. In addition, Boonyai, Bhandari, and Howes (2004) observed that for some measurement methods the sticky point may depend on the exposure time above Tg. The particle bombardment method used by Paterson et al. (2007) results in a short exposure time and very short contact times between the particles and the impacted surface, and thus results in a high sticky point of 37.9°C above Tg for SMP. In contrast, the stirrer method used by Hennigs et al. (2001) allows for slightly longer contact times between particles in the stirred flask, and thus detects sticky behavior at a lower T-Tg level of only 23.3°C. Nevertheless, for both of these methods, the typical interaction time scale is only a fraction of a second. In baghouses, interactions occur at a much longer time scale, with particles in the filter cake remaining static for up to 5 minutes (the period between pulses). It is therefore highly likely that softening of particles and increased adhesion to the filter become important at T-Tg levels much lower than the sticky point values measured in any of the above studies. Throughout the rest of this thesis, wherever a single value for the sticky point is required, the (T-Tg)crit of 23.3°C reported by Hennigs et al. (2001) will be used, as this is at the lower end of the range reported in the literature, and therefore closer to the range expected to affect baghouse performance. The sticky point is also affected by other components of the milk powder, and is therefore dependent on the powder composition. For pure lactose, Paterson, Brooks, Bronlund, and Foster (2005) report a sticky point of 25°C above Tg, while for high protein MPC powders, the sticky point may be as much as 90°C above Tg (Hogan & O’Callaghan, 2010).

Particle stickiness may have a range of effects on different aspects of baghouse performance. Sticky particles colliding with surfaces can adhere and cause fouling or blockages. Murti,

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Paterson, Pearce, and Bronlund (2010) found that sticky particles readily adhere to a range of surfaces, with surface characteristics such as elasticity and texture making little difference. It is therefore likely that at conditions above the sticky point particles adhere more strongly to the filter fabric, making the filter cake more difficult to remove. Measures to compensate for this, such as increasing the compressed air pressure to the pulse, have associated costs. On the other hand, research from other industries indicates that more cohesive particles tend to form a more porous filter cake structure, with a lower resistance to air flow (Miller & Laudal, 1992). In addition, particle cohesion encourages agglomeration of particles, which reduces the penetration of particles into the filter and encourages gravitational settling. Both of these effects lower the long term pressure differentials, thus reducing the energy cost. Optimising baghouse performance therefore requires finding a balance between different factors.

The sticky point relies on the properties at the particle surface, and does not require the powder moisture content to equilibrate with the surrounding air conditions (Murti, Paterson, Pearce, & Bronlund, 2009). Consequently, particles become sticky almost immediately when exposed to humid conditions. Whether or not a particular particle adheres to a surface depends on a range of variables, including the particle size, and impact velocity. Walmsley, Walmsley, Atkins, Neale, and Sellers (2014) showed that smaller particles exhibited a lower (T-Tg)crit, which may exacerbate the effect of stickiness on milk powder baghouses. As larger particles tend to settle out in the bottom of the spray drier and do not reach the filter, the mean particle size in the filter cake is generally lower than in the spray drier. While spray driers and milk powder baghouses are generally operated just below the estimated sticky point to minimise fouling, the combination of the smaller particle size at the filter surface and the relatively long time for which particles remain in contact with the filter surface may mean that sticky behaviour affects the filtration process at T-Tg levels lower than otherwise expected. Murti et al. (2010) found that the velocity of particles impacting a surface affected the adhesion, with high velocity resulting in a higher (T-Tg)crit. They concluded that high velocities create greater inertial forces and reduce the time scale of the particle-particle and particle-surface interactions. Interestingly, these authors found that the angle of impact between a particle and a surface had no effect on the adhesion, which suggests that the time scale of the impact is much more important than the inertial forces involved.

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2.5

Modelling