The effect of chemical cleaning of WPC from milk has been largely characterised in the literature as uneven. The cleaning process has three distinct phases seen by many independent researchers (For example Bird (1992), Gillham (1997), Grasshoff (1997), Tuladhar (2001) and Christian (2004)):
(i) Swelling — alkali solution contacts the deposit and causes swelling, forming a protein matrix of high void fraction.
(ii) Erosion — uniform removal of deposit by shear stress forces and diffusion. There may be a plateau region of constant cleaning rate, but this depends on the balance between swelling and removal.
(iii) Decay — the swollen deposit is thin and no longer uniform, so that removal of isolated islands occurs by shear stress and mass transport.
Many authors quote 0.5% NaOH to be optimal for WPC removal from stainless steel although the existence of cleaning optima has not been categorically proved. Bird and Fryer (1991) found that increasing the NaOH concentration to 2% can produce a deposit with a less open (dissolved) structure than at 0.5% lengthening the swelling phase. Plett (1985) reported that a maximum cleaning rate occurs when cleaning with detergent. The contribution of flow rate is hard to determine in chemical cleaning because both shear stress imposed on the deposit and mass flow to the deposit are dependent on the flow rate. In general the higher the flow rate, the shorter the cleaning time. Timperley and Smeulders (1988) found that the cleaning time of a PHE decreased with increasing flow velocity from 0.2 to 0.5 m s-1. There are arguments supporting higher flow
71 rates which create turbulent conditions. This is because turbulent conditions are known to disrupt the boundary layer in cleaning. However Bird and Fryer (1991) found there was no significant change in cleaning rate when moving from laminar to turbulent flow. Disruption of the boundary layer is further discussed in Section 2.7. Generally increasing the temperature decreases the cleaning time. Gillham et al., (1999) found that removal of whey protein deposits from stainless steel pipes was strongly dependent on temperature (less so the swelling phase).
Sweet condensed milk (SCM) is an intermediate in the manufacture of some confectionary
products, made by evaporating water from milk and adding sugar to lower the water activity of the product.SCM has 70 – 74% total solids of which 40 - 45% is sucrose (Fisher and Rice, 1924) leaving 29 - 30 % milk solids. In the study of Othman et al., (2010) SCM was cooked for 4 h at 85-90°C on stainless steel coupons and removed by chemical cleaning in a flow cell. It was found that increasing the flow velocity from 0.25 to 0.5 m s-1 decreased the cleaning time at all
temperatures. An increase in temperature from 40 to 60 to 80°C decreased the cleaning time linearly. Interestingly, the authors found that increasing the NaOH concentration from 0.5 to 1.5
% did not significantly affect the cleaning time at each temperature. This agrees with findings for
WPC cleaning that quote 0.5 % NaOH as the optimum concentration. It was the increase in temperature rather than the increase in chemical concentration that decreased cleaning time.
Cleaning time vs. Re was plotted for SCM at 1 % NaOH (40, 60 and 80°C) by Othman et al., (2010) and at 0.1, 0.5 and 1% NaOH (at 30, 50, 70°C) for WPC by Christian et al., (2004) given in Figure 2.13 (a) and (b). For WPC the range of Re investigated was around 800 to 4840. There were separate groups of data at each temperature that could not be plotted on one master curve.
72 This suggests temperature was the dominant parameter in controlling cleaning time. Christian (2004) concluded an increase in Re was only beneficial to cleaning time at low concentration. Jennings et al., (1957) suggested the existence of a threshold Re of 25,000 for cleaning a pipe surface of dry milk deposit before an increase in Re resulted in increased cleaning rate. For SCM the Re range investigated was much higher, from 6500 to 27,000. All the data collapsed onto one curve. As the Re increased the cleaning time decreased, suggesting Re was the dominant parameter controlling cleaning time. Othman et al., (2010) did find however that the effect of Re on cleaning time became less significant as the temperature was increased. Gillham et al., (1999)
found that τw = Re-n where n was in the range 0.2 - 0.35 for 0.5% NaOH, where τw is the wall
73 Re 1000 2000 3000 4000 5000 Tim e to c lea n ( s) 0 2000 4000 6000 8000 10000 0.1 wt% 0.5 wt% 1.0 wt% (a) 0 50 100 150 200 250 300 350 400 450 0 5000 10000 15000 20000 25000 30000
Reynold number (Re)
T im e to c le a n (s ) 40 C 60 C 80 C (b)
Figure 2.13: Re vs. visual cleaning time of (a) SCM at 40, 60 and 80°C using 1% NaOH (from Othman et al., 2010) and (b) WPC at 30, 50, 70°C, using 0.1, 0.5 and 1% NaOH (from Christian, 2003).
° ° °
74 The cleaning of egg albumin was characterised by Aziz (2008). Generally an increase in temperature decreased the cleaning time. However at 1% NaOH, cleaning time was faster at 50°C than at 70°C. Deposit was not removed at 30°C at any flow velocity or NaOH concentration investigated. An increase in NaOH concentration from 0.25 to 3% NaOH decreased the cleaning time (at 50°C, 2.3 l min-1); however increasing the flow rate had a less significant impact on cleaning time at higher chemical concentration. The author concluded a high temperature, mid to high flow velocity and mid-range chemical concentration appeared to be the optimum, similarly to WPC cleaning optima. For egg albumin the range of Re investigated was 1090 to 4840. There were separate groups of data at 50 and 70°C that could not be plotted on one master curve. This suggests temperature was the dominant parameter in controlling cleaning time similarly to WPC.
In Christian (2003) Rd profiles were measured in WPC cleaning experiments. These were
conducted using 0.5% NaOH at 30, 50, 70°C and 0.7, 1.5, 2.3 l min-1. R
d (the fouling resistance)
is a measure of resistance to the flow of heat to the sensor. Rd measured at the same flow rate
reduced Rd more rapidly as the temperature was increased from 30 – 70°C. An increase in flow
rate from 1.5 to 2.3 l min-1 revealed similar R
d profiles, suggesting temperature dictated the
cleaning time in this case.
For all type 3 deposits detailed here, temperature seems to be the dominant contributor to cleaning time at both low and high flow velocity and low and high concentration. This perhaps indicates a reaction rate event being the rate limiting step.
75