lactose content of freshly milled lactose sample.
After one week's equilibration time, the remaining samples were tested for strength using the multi-point penetrometry method. Figure 5 . 1 3 shows the results compared with the strength of liquid bridged purely crystalline lactose material after ten days
equilibration time (from Section 5.5.2).
0% 20% • • .a. • • 400'0 60% 80% 1 00% R elative humidity
Figure 5. 1 3 - Freshly milled lactose strength compared with purely
crystalline powder strength stored over saturated salt sol utions for one week.
This graph demonstrates that there is some extra strength in the freshly milled lactose
samples compared with the conditioned powders. In the week's equilibration time, the
samples equilibrated above 0,45 11w crystallised and thereby released moisture, The
therefore to explain why the low water activity samples showed greater strength than the conditioned lactose samples. One possible explanation is that fast adsorption of moisture to the surface of the amorphous layer could have occurred during transfer of the powder to the petri dishes. This could have enabled sticking of the particles before storage above the salt solutions.
Because the glass transition temperature was not exceeded in these samples the amorphous lactose surface would not have been capable oftlow, and therefore amorphous bridging should not have occurred. If amorphous lactose flow and
crystallisation, did occur in the samples containing 4% amorphous lactose, it would have been expected to see a dramatic increase in strength at approximately 0 . 5 a..v' No such change was observed.
It should be noted that even though the amorphous lactose containing samples showed greater strength than the crystalline samples, the strength measured was not high. No sample showed any signs of bulk powder strength. There was no signs of lumping or caking in the samples. This can be seen more clearly if the results are plotted with the measured strengths of lactose samples equilibrated for fifty days as shown in Figure 5 . 14. ro Q.. � (1J � (1J :!: c ::J ... Q) � .E " . � Q. a. « 1 000 800 600 400 200 0 0% 20% SpecIal Dense + 100# )( 40% 60% Relative h u midity • 200II 300# ... .. x 80% 1 00% )( FresHy mlled
Figure 5.14 - Strength of freshly milled lactose compared with strength of crystalline lactose stored for fifty days over saturated salt solutions.
It is clear from this graph and Table 5 . 1 which shows a qualitative description of the scale of the multi-point penetrometry method, that no significant strength occurred due to amorphous lactose flow in conditions above the glass transition temperature. This finding is consistent with that reported for freshly milled icing sugar by Roth ( 1 976).
This suggests that the role of amorphous lactose in the occurrence of caking in bulk lactose is due to increasing the moisture holding capacity of the powder and the subsequent release of this moisture if crystallisation proceeds. Roth ( 1 976) also concluded that amorphous sucrose influences caking phenomena in icing sugar by this mechanism. This may be due to the amorphous sugar layer being too thin for flow to occur toward the inter-particle contact points.
5.6.4 RELEVANCE OF AMORPHOUS LACTOSE TO CAKING I N BULK LACTOSE
It has been shown that there was a small increase in strength due to the presence of amorphous lactose. This extra strength was not significant with respect to lump
formation or obvious signs of caking. If these strength measurements are compared with samples exhibiting even weak lumps, they become insignificant. This suggests that amorphous l actose flow does not contribute to caking in bulk lactose in the low amorphous lactose content range.
Amorphous lactose is important, because of its moisture binding ability and the possible release of this moisture if conditions are favourable for crystallisation. This will cause high local moisture contents which result in lumping by liquid bridge formation as described in Section 5 . 5 . This moisture will be free for diffusion into other regions of the bulk lactose.
5.1 CLOSURE
This work has showed that the main cause of caking is due to the formation of inter particle crystalline bridges. Such bridges give the powder significant strength. These crystalline bridges occur though the formation of liquid bridges which have intermediate strength. Over time, the strength of these bridges increase dramatically. The mechanism for this is not clear although it is thought to be due to the dissolution of small inter particle asperities allowing shrinkage of the bulk powder. This will also result in closer liquid bridges and possibly the formation of crystalline bridges due to the growth of larger crystals in the region of the liquid bridge. This mechanism is a function of the geometry and packing of the particles and is too complex to allow accurate modelling Such modelling was not required to achieve the purpose of this work.
The one day liquid bridge strength, although not high, is an indicator of the flIture formation of caked product if high water activity conditions are maintained. As a general rule, product with a water activity greater than 0.9 <lv, is at high risk.
Amorphous lactose is important with respect to moisture binding and release during crystallisation and therefore was be included in the transport model to allow i nvestigation into possible ways to avoid caking in industrial applications. The next chapter outlines the inclusion of these caking mechanisms into the overall transport model