ARTICLE IN PRESS Journal of Food Engineering xxx (2005) xxx–xxx www.elsevier.com/locate/jfoodeng Effects of capillary condensation on the caking of bulk sucrose S.W. Billings, J.E. Bronlund, A.H.J. Paterson * Institute of Technology and Engineering, Massey University, P.O. Box 11-222, Palmerston North, New Zealand Received 31 January 2005; accepted 2 August 2005 Abstract Caking in sucrose is a major problem for the sugar industry, especially during transportation. Liquid bridging in sucrose is the first step towards caking. This paper demonstrates, by measuring the strength of the liquid bridges and then the strength of the subsequently formed solid bridges under different water activity conditions that the critical water activity for initiation of caking is about 0.8. The identification of this point allows the conditions under which bulk sucrose is stored and transported to be specified to prevent the occur- rence of water activities going above this critical level to eliminate caking. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Capillary condensation; Kelvin radius; Sucrose; Caking 1. Introduction the contact points of sugar crystals in a packed bed. In these capillaries, the pressure is lower than the surrounding Within the sucrose industry, caking of bulk sugar often atmosphere, which results in the saturated vapour pressure occurs in bulk sucrose during transport and storage. In above the liquid surface being less than the vapour pressure environments where there is a high daily temporal shift, of the bulk air at the same temperature. If the pressure is or where the temperature of the sucrose being placed into lowered enough to cause the vapour pressure to be greater storage is warmer than the ambient stored temperature, than the saturated partial pressure above the capillary sur- temperature gradients exist and subsequently create areas face, then condensation will occur, even at humidity levels of higher relative humidity (Bagster, 1970). In the areas below total saturation in the bulk air. of higher relative humidity, liquid bridges can form and This condensation leads to surface dissolution and upon a change in humidity, the liquid bridges can re-crys- liquid bridging between the sugar particles. When the mois- tallise to form solid bridges. The strength of the solid ture is removed (due to an effect such as a temperature bridge can be strong enough so that sometimes the forces change) a solid bridge is formed between the particles. encountered during the transportation process are not en- ough to break them (Ludlow & Aukland, 1990). 2. Kelvin radius This paper investigates the mechanism by which these li- quid bridges form. The vapor pressure required for condensation to occur The pores formed between adjacent particles allow cap- is related to the size of the capillary, the Kelvin radius illary condensation to occur at a critical water activity. (rk) [m], the wetting angle (h) and surface tension (r). These Capillary condensation can be described as the process are all related by the Kelvin equation (Adamson, 1963). by which surface tension effects cause the direct condensa- P À2r cos hV 0 tion of moisture in the pores or ‘‘capillaries’’, formed by v rkRðT þ273:15Þ Aw ¼ ¼ e ð1Þ P w * Corresponding author. Tel.: +64 6 350 5241; fax: +64 6 350 5604. where Aw is water activity, Pv is the water vapor pressure of E-mail address: [email protected] (A.H.J. Paterson). the material [Pa], Pw is the vapor pressure of pure water 0260-8774/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2005.08.031 ARTICLE IN PRESS 2 S.W. Billings et al. / Journal of Food Engineering xxx (2005) xxx–xxx 45 40 35 m) 30 µ 25 20 15 Capillary Radius ( 10 5 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Water Activity (RH/100) Fig. 1. Capillary radius versus water activity at 20 °C. [Pa] and V0 the volume that 1 mol of a saturated sucrose along with their equilibrium relative humidity values at solution occupies (61.3 · 10À6 m3/mol). 25 °C(Greenspan, 1977). The Kelvin equation can be used to predict the Kelvin Two methods of caking strength measurement were used radius where capillary condensation will occur for a range on the basis of their ease of use and accuracy. These were a of water activities (Fig. 1). Capillaries of smaller radius multi point penetrometer developed by Bronlund (1997) than the Kelvin radius will be full of water/sugar solution. and a blow tester (Paterson, Bronlund, & Brooks, 2001). At 20 °C, the maximum sucrose solubility was calcu- These methods also have the advantage that replicate mea- lated to be 66.7% by weight, and the surface tension for surements can be made on a sample, unlike methods such a saturated sucrose solution was estimated from sucrose as friability or angle of repose, where the sample is poured surface tension data as 80.2 N/m (Mathlouthi & Resier, out of the container or onto a surface. From the moisture 1995). Also, perfect particle wetting was assumed so a wet- sorption isotherms (Bakhit & Schmidt, 1993; Iglesias & ting (contact) angle of zero radians was used. Chirife, 1982; Roth, 1976), and results from the Kelvin Fig. 1 shows that the critical capillary radius increases equation, it was foreseen that at the higher water activities exponentially from a water activity of between 0.75 and the sugar might cake to the point where flow would not oc- 0.8 and this implies that significant liquid bridging will oc- cur, negating the possibility of accurate results from tech- cur between the particles from this point onwards. The niques that were based on the samples flow. exponential change in capillary radius will then effect cak- The liquid and solid bridge strengths were measured ing, which should also increase exponentially from a water after one, two, and three days, as three days is typically activity of about 0.8 onwards. the maximum period of time that produced sugar will be An experiment to test this theory was carried out by stored prior to transportation. measuring the equilibrium caking strength at a range of humidities, to create a range of degrees of caking in sugar samples. Table 1 Saturated salt solutions and their relative humidities at 20 °C 3. Caking strength measurement Saturated salt solution Relative humidity (%) Potassium hydroxide 9 Based on the observation that significant capillary Magnesium chloride 33 condensation between particles, (and hence liquid bridge Sodium bromide 59 Sodium chloride 75 formation), would occur above a relative humidity of 75– Ammonium chloride 79 80%, saturated salt solutions were prepared for use to Ammonium sulfate 81 provide constant relative humidity environments, with the Potassium bromide 84 main focus on the >75% RH region. The saturated salt Potassium chloride 87 solutions used in this work are listed in Table 1 below, Potassium nitrate 94 ARTICLE IN PRESS S.W. Billings et al. / Journal of Food Engineering xxx (2005) xxx–xxx 3 4. Liquid bridge strength experimental observed, the breakthrough point was the point where the penetrometer made a large shift in penetration depth, To measure the liquid bridge strength, sugar samples with the pins completely buried in the sample. The weight were placed in standard laboratory petri dishes, approxi- of water required for breakthrough was recorded and the mately 12 mm deep. The samples were then placed in air- breakthrough stress calculated by dividing by the total tight containers containing one of the saturated salt probe area. solutions above, and left to equilibrate for one, two and three days. The samples were then removed and the liquid 4.2. Blow tester bridge strength measured. Measurements for the blow test data were carried out by 4.1. Penetrometer resting the portable blow tester on the surface of the sample and holding it in place with a clamp stand. The blow tester Fig. 2 shows the penetrometer set up developed by was simply a 1 mm tube that was placed 3 mm above the Bronlund (1997). bed at an angle of 45° to the horizontal. A needle valve Penetrometer measurements were made by setting the was then used to control the airflow from a bottle of com- counter weight balance so that an empty container sat pressed air into the blow tester, with the flow rate increased weightless above the sample to be measured. Weight was steadily until the point where the airflow was sufficient to then added to the container by slowly adding water to begin dislodging sugar particles from the surface of the the container until the ‘‘breakthrough’’ point was sample. This flow rate was recorded as the break through flow rate. This flow rate was then plotted against the water activity that the samples were equilibrated at, for the differ- ent equilibration times. Further details of the blow test measurement apparatus and technique are given in Paterson et al. (2001). 5. Results The weight of the water required for breakthrough was used to calculate the stress imposed at breakthrough on the bed and has been plotted against the water activity that the sample was equilibrated to for the differing equilibration times in Fig. 3. Fig. 2. Penetrometer used in bridge strength testing. 140 120 ) 2 100 80 60 40 Breakthrough Stress (kN/m 20 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Water Activity (RH/100) 1d equilibration 2d equilibration 3d equilibration Fig. 3. Penetrometer data for liquid bridge strength test. ARTICLE IN PRESS 4 S.W. Billings et al. / Journal of Food Engineering xxx (2005) xxx–xxx 90 80 ) 70 2 y = 2.4936x + 9.3982 R2 = 0.9936 60 y = 0.75x + 35.984 R2 = 0.977 50 Intercept at a radius of 15.2 µm 40 30 Breakthrough Stress (kN/m 20 10 0 0 5 10 15 20 25 30 35 Kelvin Radius (µm) Fig.