Molar ratio

Ca:PO4

UHT preheat treatment UHT preheat treatment Milk _composition Deposit Molecular _weight

75 °C, 11 s 95 °C, 147 s 75 °C, 11 s 95 °C, 147 s Ca 40 0.10 0.21 Ortho-PO4 95 0.06 0.15 FWM (Milk Nos. 8-10) P 31 0.07 0.15 1.7 1.4 Ca 40 0.06 0.16 Ortho-PO4 95 0.03 0.10 RCB (Milk Nos. 8 and 2b) P 31 0.04 0.12 2.0 1.6 Ca 40 0.11 0.14 Ortho-PO4 95 0.06 0.10 Recon (Milk Nos. 8 and 2b) P 31 0.08 0.10 1.8 1.4 1

Moles of each mineral component in the deposit = the weight of each mineral component in the deposit / molecular weight of that component (e.g. Moles of Ca in FWM, PH 75 °C, 11 s = 3.8 / 40 = 0.10 mole). Note: Total phosphorus (P) in milk is dominated by ortho-PO4 and the phosphoserine in the caseins.

Contributions from the phosphorus-containing components, such as phospholipids, are generally negligible (Walstra & Jenness, 1984).

Table 7.11 Moles of Ca, PO4 and P in the deposit from RCB at the

75 °C, 2 s and 95 °C, 155 s evaporator preheat treatments (Milk Nos. 8 and 2b).

Moles1 Molar ratio

Ca:PO4

Evaporator preheat

treatment Evaporator preheat treatment Milk _composition Deposit Molecular _weight

75 °C, 2 s 95 °C, 155 s 75 °C, 2 s 95 °C, 155 _s Ca 40 0.06 0.11 Ortho-PO4 95 0.03 0.07 RCB P 31 0.04 0.08 2.0 1.6 1

Moles of each mineral component in the deposit = the weight of each mineral component in the deposit / molecular weight of that component (e.g. Moles of Ca in FWM, PH 75 °C, 11 s = 2.4 / 40 = 0.06 mole). Note: Total phosphorus (P) in milk is dominated by ortho-PO4 and the phosphoserine in the caseins.

Contributions from the phosphorus-containing components, such as phospholipids, are generally negligible (Walstra & Jenness, 1984).

Tables 7.10 and 7.11 show that the moles of Ca in the deposits was greater than the moles of both PO4 and P in the deposits for both low and high preheat treatments and for all milk

Chapter 7 Effect of preheat treatments on the total weight and composition of fouling deposits 7-21

types. The molar ratio of Ca:PO4 for both UHT and evaporator preheat treatments is in the

range of 1.4-2.0.

In most milk preparations, the moles of PO4 and P for a given preheat treatment were not

equal. The moles of PO4 is always less or equivalent to the moles of P for both low and

high preheat treatments.

7.9 Discussion

There were a clear positive relationships between the individual weights of fat, protein and ash in the deposits from FWM, RCB and Recon and fouling rate over than ranges of UHT and evaporator preheat treatments studied (Figures 7.1 and 7.2). This result showed that the measurement of temperature difference (milk outlet temperature - water inlet temperature in the high-temperature heater) reflected the deposit weight despite (totally expected) non-uniform deposition in the high-temperature heater. The increase in the extent of fouling with increasing temperature was very evident when the distribution of deposit was inspected in the high temperature heater (Figure 7.1 (c)).

Effect of UHT preheat treatment on the deposit composition

The total deposit weight and the weights of fat, protein and ash in the deposits from FWM, RCB and Recon generally increased with the intensity of UHT preheat treatment by a factor of two to four (Table 7.1). The variation in total deposit weight corresponded to the variation in fouling rates. The similar trend among the three whole milks suggested that there was a common basic fouling mechanism for FWM, RCB and Recon.

Ash constituted the largest proportion of the deposits from FWM, RCB and Recon for all UHT preheat treatments (Table 7.1). This indicated that the deposit was type B although it was not grey in colour, brittle and gritty as reported by Lyster (1965), Burton (1968), Lalande et al. (1984), Tissier et al. (1984) and Patil and Reuter (1988). The deposition of minerals can be due to the reverse temperature-dependence of the solubility of calcium phosphate in milk (Burton, 1968; Lalande et al., 1984; Visser & Jeurnink, 1997).

Chapter 7 Effect of preheat treatments on the total weight and composition of fouling deposits 7-22

The weight of fat in deposits increased to a greater extent than did the weight of protein for all milk types when UHT preheat treatment became greater (Table 7.1). These deposits were white and greasy because of the high levels of fat.

The deposition of fat in the high-temperature heater (Table 7.1) seemed to have been dependent on in the nature of the proteins covering the fat globules (differences in the MFGMs of FWM, RCB and Recon were described in Chapter 5). When the fat deposited in the high temperature heater, it probably deposited as fat-bound protein rather than as fat; i.e., the fat globules acted as protein particles. The results of Chapter 6 show that the amount of fat-bound whey protein increased with the intensity of UHT preheat treatment.

Increases in the weights of protein and fat in the deposit from RCB, compared with reconstituted skim milk (Table 7.5), showed the deposition of fat-bound protein. Furthermore, the deposition of protein from the milk plasma of reconstituted skim milk was lower than the deposition of protein and fat-bound protein for RCB.

The level of fat in the deposit from homogenization then UHT preheated FWM was greater than that in the deposit from UHT preheated then homogenized FWM (Table 7.4). From the results of Chapters 5 and 6, the thicker covering of protein around the fat globules of homogenized then UHT preheated FWM corresponded to a greater fouling rate. Thus, the thicker membrane of fat globules gave a greater level of fat in the deposit and a higher fouling rate.

Effect of evaporator preheat treatment on deposit composition

The increasing weight of protein in the deposit from RCB with increasing intensity of evaporator preheat treatment (Table 7.3) showed that the deposition of protein in the high temperature heater was related to evaporator preheating intensity. On the other hand, the weights of fat and ash in the deposit did not show a similar trend to the weight of protein when evaporator preheat treatment was increased from 85 °C, 155 s to 95 °C, 155 s. This result suggests that only protein has a role in the deposition toward an increase of total deposit weight and of fouling rate (Chapter 5).

Chapter 7 Effect of preheat treatments on the total weight and composition of fouling deposits 7-23

Comparison of fouling deposits with changes in milk composition during UHT processing

Depletion in the weights of ash, PO4, calcium, and total phosphorus between the feed and

the UHT milk were consistently 1-2 times greater than the corresponding amounts

recovered in the deposits in the high-temperature heater for all milk types (Tables 7.6-7.9). Although the ash constituted the greatest proportion of the deposit in the high-temperature heater, not all ash lost from the milk was recovered in the deposit. This may be due to the deposition of ash in other parts of the milk plant: preheater, holding tube, UHT holding tube, or cooling section (Lalande et al., 1984; Tissier et al., 1984; Burton, 1988; Patil & Reuter, 1988; Visser & Jeurnink, 1997), which this was not covered in this present study.

Depletion of fat and protein between the feed and the UHT milk for FWM, RCB and Recon was very low and was not be detectable, compared with the deposition of fat and protein in the high-temperature heater (Tables 7.6-7.9).

Molar ratio of Ca:PO4

The molar ratio of Ca:PO4 in the deposits of FWM, RCB and Recon varied in the range

1.4-2.0 and decreased with the intensity of UHT and evaporator preheat treatments (Tables 7.10 and 7.11). The forms of minerals in the deposit from fresh whole milk were reported by Burton (1968) and Lyster (1965) to be in the form of β-Ca3(PO4)2, in which the

ratios of Ca:P and Ca:PO4 are 1.5:1. This was transformed into hydroxyapatite

(Ca5OH(PO4)3), giving a ratio of Ca/P of 1.6, when the precipitate was subjected to

prolonged heating in the heat exchanger. Visser et al. (1997) reported that the deposit from FWM was a mixture of calcium phosphate dihydrate (CaHPO4.2H2O), which has a ratio of

Ca:P of 1:1, and octacalcium phosphate (Ca8H2(PO4)6.5H2O), which has a ratio of Ca:P of

1.3:1. The range of the molar ratio of Ca:PO4 found in this study suggests that the ash in

Chapter 7 Effect of preheat treatments on the total weight and composition of fouling deposits 7-24