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Milk pH as a Function of CO₂ Concentration, Temperature, and Pressure in a Heat Exchanger¹

Northeast Dairy Foods Research Center, Department of Food Science, Cornell University, Ithaca, NY 14853

Corresponding author: D. M. Barbano; e-mail address: dmb37{at}cornell.edu.

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
Raw skim milk, with or without added CO₂, was heated, held, and cooled in a small pilot-scale tubular heat exchanger (372 ml/min). The experiment was replicated twice, and, for each replication, milk was first carbonated at 0 to 1°C to contain 0 (control), 600, 1200, 1800, and 2400 ppm added CO₂ using a continuous carbonation unit. After storage at 0 to 1°C, portions of milk at each CO₂ concentration were heated to 40, 56, 72, and 80°C, held at the desired temperature for 30 s (except 80°C, holding 20 s) and cooled to 0 to 1°C. At each temperature, five pressures were applied: 69, 138, 207, 276, and 345 kPa. Pressure was controlled with a needle valve at the heat exchanger exit. Both the pressure gauge and pH probe were inline at the end of the holding section. Milk pH during heating depended on CO₂ concentration, temperature, and pressure. During heating of milk without added CO₂, pH decreased linearly as a function of increasing temperature but was independent of pressure. In general, the pH of milk with added CO₂ decreased with increasing CO₂ concentration and pressure. For milk with added CO₂, at a fixed CO₂ concentration, the effect of pressure on pH decrease was greater at a higher temperature. At a fixed temperature, the effect of pressure on pH decrease was greater for milk with a higher CO₂ concentration. Thermal death of bacteria during pasteurization of milk without added CO₂ is probably due not only to temperature but also to the decrease in pH that occurs during the process. Increasing milk CO₂ concentration and pressure decreases the milk pH even further during heating and may further enhance the microbial killing power of pasteurization.

Key Words: carbon dioxide concentration • pH in heat exchanger • temperature • pressure

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
To achieve a long shelf life of pasteurized milk, the fluid milk industry typically applies a heat treatment with a time and temperature combination substantially higher than the legal minimum (72°C for 15 s) specified by the Pasteurized Milk Ordinance (PMO, 1999). For example, in New York State, the typical temperature and time combinations used by fluid milk processors are 76.7 to 77.2°C for 30 to 35 s (Ma et al., 2000). Recently, Loss and Hotchkiss (2000) reported that the addition of CO₂ at concentration of about 1500 ppm decreased the thermal resistance of Pseudomonas fluorescens and suggested that dissolved CO₂ could be used as a processing aid to enhance microbial kill during pasteurization and thus reduce the extent of heating needed to achieve longer shelf life of milk. Using CO₂ as a processing aid is promising because, if CO₂ is undesirable in the final product, CO₂ can be removed by applying a vacuum after its desirable impact has been achieved. More importantly, CO₂ is a GRAS (generally recognized as safe) food additive (Code of Federal Regulations, 2001).

Only a few studies were focused on the chemical changes in milk with added CO₂ during pasteurization (Calvo and de Rafael, 1995; Ruas-Madiedo et al., 1998). An important change that occurs in milk upon carbonation at low CO₂ levels and pressures is a decrease of milk pH (Ma et al., 2003). Although the antimicrobial effect of CO₂ is not pH dependent (King and Mabbitt, 1982; Ma et al., 2003), many dairy processing steps are pH dependent (Corredig and Dalgleish, 1996; Singh et al., 1996; Oldfield et al., 2000). This is particularly true when heating is involved. The effect of heating on the quality and functional properties of dairy foods is highly pH dependent (Corredig and Dalgleish, 1996; Oldfield et al., 2000).

During heating of milk without added CO₂, milk pH decreased linearly with increasing temperature, and the decrease was related to shifts in degree of association of calcium phosphate (Dixon, 1963; Chaplin and Lyster, 1988; Fox and McSweeney, 1998). Using the linear coefficients of -0.0073 pH unit/°C reported by Chaplin and Lyster (1988), the pH of a milk without added CO₂ at 80°C would be about 0.58 pH units lower than the pH of the same milk at 0°C. This is a very dramatic decrease in milk pH caused simply by heating. The change in milk pH with temperature is usually reversible if the heating does not exceed 100°C and there is no degradation of milk protein or lactose during heating (Fox and McSweeney, 1998). The measurement of pH in previous studies (Chaplin and Lyster, 1988) was conducted while milk was mixed and heated under a nitrogen atmosphere in a water bath. No data are available on the pH of milk under the conditions of heating and pumping in a heat exchanger, but the effect of temperature on milk pH would be expected to be the same. In addition, no information is available on the extent of pH reduction in milk with added CO₂ during heating in a heat exchanger.

The objective of this study was to measure the pH of raw skim milk, without and with added CO₂, directly inline as milk was heated in a tubular heat exchanger at temperatures of 40, 56, 72, and 80°C and under pressures of 69, 138, 207, 276, and 345 kPa (i.e., 10, 20, 30, 40, and 50 psi). The unique feature of this study is the measurement of milk pH directly inline as milk was being heated in a heat exchanger at 72 and 80°C. The measured pH represented the pH values that a milk without and with added CO₂ would experience in an HTST pasteurizer at various pressures.

	MATERIALS AND METHODS

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Experimental Design and Statistical Analysis
The experiment was replicated two times using different batches of fresh raw skim milk. Each replication was completed in 3 d. On the first day, milk was carbonated at 0 to 1°C to contain 0 (control), 600, 1200, 1800, and 2400 ppm added CO₂ using a continuous tubular carbonation unit (230 ml/min). After overnight storage at 0 to 1°C, subportions of milk at each CO₂ concentration were heated to 40, 56, 72, and 80°C, held at the desired temperature, and cooled to 0 to 1°C in a tubular heat exchanger at a flow rate of 372 ml/min. The 40 and 56°C processing was done on the second day, and the 72 and 80°C processing was done on the third day. At each temperature five pressures were applied: 69 (control without added pressure), 138, 207, 276, and 345 kPa. The pH of skim milk at each carbonation level, each temperature, and each pressure was measured directly inline at the end of the holding section as milk was being pumped through the heat exchanger.

Because it is known that milk pH decreases with increasing temperature (Chaplin and Lyster, 1988), data from the experiment would be best presented by focusing on how milk pH changed during heating as a function of CO₂ concentration and system pressure. The entire dataset was analyzed as four independent split-plot designs by setting temperature at a fixed level (i.e., at 40, 56, 72, or 80°C). The whole plot factor was CO₂ concentration (five levels) and the subplot factor was pressure (five levels). The ANOVA model is listed in Table 1. Analyses were done using SAS (Version 8.02, 1999–2001).

Inline pH Measurement and Pressure Control
Milk pH was measured inline as milk was heated to 40, 56, 72, and 80°C in a laboratory-scale countercurrent stainless steel tubular heat exchanger (i.d. = 0.5 cm). The system consisted of 19 straight pieces of tube that were each 105 cm long, interrupted by eighteen 13-cm diameter U-turns with flow entering at the lowest point in the system and having an upward pitch on successive loops of tubes until reaching the system exit. This was done to promote turbulent flow. The heat exchanger consisted of eight sections, they were sequentially from inlet to exit: 1) a milk feed reservoir; 2) a peristaltic pump, which fed the milk (0 to 1°C) into the heat exchanger at a flow rate of 372 ml/min; 3) a heating section, which heated the milk to the target temperature; 4) a holding section, which kept the milk at the target temperature for a fixed time period; 5) a sanitary pressure gauge (Anderson Instrument Company, Inc., Fultonville, NY) that registered the system pressure at the end of the holding section; 6) pH probes (model HA 405 DXK-58/120 combination pH probe; Mettler Toledo, Columbus, OH) that were inserted perpendicular to the milk flow through Tee-fittings to measure milk pH continuously inline at the end of the holding section; 7) a cooling section, circulated with ice water that cooled the milk to 0 to 1°C for collection at the exit; and 8) a needle valve that was opened and closed for decreasing and increasing of system pressure. Throughout the system, several temperature probes were inserted inline to monitor the inlet, heating, holding, and exit temperature of the milk.

The heating and holding sections were circulated with hot water to achieve the desired temperature targets. For both replications, each of the four target temperatures was achieved and well maintained in the holding section for each of the five CO₂ concentrations and the five pressures. For the first replication, the average temperatures (n = 25) were 40.2 ± 0.2°C, 56.0 ± 0.2°C, 72.2 ± 0.1°C, and 80.4 ± 0.2°C and for the second replications, temperatures were 40.5 ± 0.2°C, 56.5 ± 0.4°C, 72.1 ± 0.2°C, and 80.3 ± 0.2°C. For the 40, 56, and 72°C treatments, it took the milk 19.5 s (heating time) to reach the target temperature, and the milk was maintained at the target temperature for 31.2 s (holding time) before being cooled. For the 80°C treatment, the heating time and holding time were 29.6 and 20.9 s, respectively. The 72°C for 31.2 s and 80°C for 20.9 s heating conditions produced a comparable degree of heat denaturation of whey proteins as the conditions used for milk pasteurization in the fluid milk industry in New York (Ma et al., 2000).

Both a pressure gauge and pH probe were inline next to each other and were both at the end of the holding section, just before the cooling section. Pressures were set at 69 (control, without added pressure and with the needle valve completely open), 138, 207, 276, and 345 kPa. Increasing pressure up to 345 kPa did not change the flow rate of milk (i.e., 372 ml/min).

The pH probe was calibrated first with buffers (Fisher Scientific, Fair Lawn, NJ) before being inserted inline. To obtain accurate pH measurement, the pH probe and the calibration buffers were tempered to the temperature at which the pH of the milk was measured. This tempering procedure ensured more rapid response and more stable readings of the pH probe. In addition, the appropriate reference pH for each calibration buffer at each temperature was used. The pH of the buffers were 6.97 and 4.03 at 40°C, 6.98 and 4.08 at 56°C, 6.98 and 4.14 at 72°C, and 6.98 and 4.16 at 80°C.

Sample Analysis
Raw milk was tested for percentage fat (Mojonnier method, AOAC, method number 989.05; 33.2.26), TS (AOAC, method number 990.20; 33.2.44), total nitrogen (AOAC, method number 991.20; 33.2.11), and NPN (AOAC, method number 991.21; 33.2.12). Percentage true protein was calculated as (total nitrogen-NPN) x 6.38. The CO₂ concentration in milk before heating on each of the processing days (i.e., second and third day) was measured (Ma et al., 2001). To verify that no loss of CO₂ occurred in the heat exchanger, the CO₂ concentration of milk collected at the exit of the heat exchanger after heating and cooling was also determined at the four temperatures (i.e., 40, 56, 72, and 80°C) and three selected pressures (69, 207, and 345 kPa).

	RESULTS

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Milk Composition, Carbonation Level, and Initial Pressure
Mean (n = 2) TS, fat, and true protein for the two batches of raw skim milk were 8.85, 0.07, and 3.16% respectively. Before heating, calculated mean (n = 5) CO₂ concentrations and standard deviations of the control and carbonated milks were 121 ± 12, 648 ± 35, 1187 ± 12, 1761 ± 18, and 2352 ± 48 ppm. After heating and cooling, no significant change in milk CO₂ concentration was observed at each of the heating temperatures at 69, 207, and 345 kPa [Table 2, only data for the lowest (40°C) and highest (80°C) temperatures are shown]. Similar values were obtained at the intermediate temperatures and pressures (data not shown). There was no loss of CO₂ in the heat exchanger as milk was being pumped, heated, and cooled.

pH as a Function of CO₂ Concentration and Pressure
At each temperature level, the ANOVA found a significant effect for CO₂, pressure, and CO₂ x pressure (Tables 3 and 4). The effect of CO₂ was expected because it has been shown in many previous studies (Ma et al., 2001; Ma and Barbano, 2003a, 2003b) that increasing CO₂ decreases milk pH (Figures 1 to 4). The effects of pressure and CO₂ x pressure are reflected as the differences in the pressure dependence of milk pH at various carbonation levels (Figures 1 to 4).

View larger version (12K): [in this window] [in a new window]   Figure 1. Mean (n = 2) milk pH at the end of the holding section of a tubular heat exchanger as a function of CO2 concentration and pressure during heating at 40°C. The five CO2 concentrations are control (), 600 ppm (), 1200 ppm (•), 1800 ppm (), and 2400 ppm (). At the same CO2 concentration, values with no common letter are significantly different (P < 0.05, lsd = 0.017).

View larger version (13K): [in this window] [in a new window]   Figure 4. Mean (n = 2) milk pH at the end of the holding section of a tubular heat exchanger as a function of CO2 concentration and pressure during heating at 80°C. The five CO2 concentrations are control (), 600 ppm (), 1200 ppm (•), 1800 ppm (), and 2400 ppm (). At the same CO2 concentration, values with no common letter are significantly different (P < 0.05, lsd = 0.025).

pH at 56°C.
In general, the pH of milk at each carbonation level, without or with added CO₂, was lower at 56°C (Figure 2) than at 40°C (Figure 1). Similar to the observations at 40°C, the pH of milk without added CO₂ (6.47) and milk with 600 pm (6.17) did not change with increasing pressure up to 345 kPa. At 1200 ppm of CO₂, increasing pressure from 69 to 138 kPa decreased milk pH from 6.00 to 5.98 and further increase in pressure up to 345 kPa did not affect milk pH in the heat exchanger. At both 1800 ppm and 2400 ppm of CO₂, milk pH decreased with increasing pressure up to 207 kPa and a further increase in pressure up to 345 kPa did not further decrease milk pH (Figure 2). At pressures of 69 and 138 kPa, even though their CO₂ concentrations in the starting milk were different, milks with 1800 and 2400 ppm of CO₂ had similar pH (P > 0.05). Difference in their pH was only observed between 207 and 345 kPa.

View larger version (13K): [in this window] [in a new window]   Figure 2. Mean (n = 2) milk pH at the end of the holding section of a tubular heat exchanger as a function of CO2 concentration and pressure during heating at 56°C. The five CO2 concentrations are control (), 600 ppm (), 1200 ppm (•), 1800 ppm (), and 2400 ppm (). At the same CO2 concentration, values with no common letter are significantly different (P < 0.05, lsd = 0.027).

View larger version (13K): [in this window] [in a new window]   Figure 3. Mean (n = 2) milk pH at the end of the holding section of a tubular heat exchanger as a function of CO2 concentration and pressure during heating at 72°C. The five CO2 concentrations are control (), 600 ppm (), 1200 ppm (•), 1800 ppm (), and 2400 ppm (). At the same CO2 concentration, values with no common letter are significantly different (P < 0.05, lsd = 0.030).

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
pH of Milk Without Added CO₂
Because at 40, 56, 72, and 80°C, the pH of milk without added CO₂ was independent of pressure (Figures 1 to 4), its mean (n = 5) pH at 40, 56, 72, and 80°C averaging over the five pressure levels for each replication were calculated (Figure 5). The pH of milk without added CO₂ decreased linearly with increasing temperature, and the least square linear regression line is Y = 6.90 - 0.0078X (R² = 0.99), where Y is pH and X is the temperature in °C. The linear coefficient (-0.0078 pH unit/°C) obtained in the current study for milk without CO₂ in a tubular heat exchanger was consistent with the coefficient (-0.0073 pH unit/°C) reported by Chaplin and Lyster for milk stirred with a nitrogen atmosphere in a water bath (1988). During heating in a heat exchanger, we found that the pH of milk without added CO₂ decreased linearly with increasing temperature in the temperature range from 40 to 80°C.

View larger version (11K): [in this window] [in a new window]   Figure 5. pH of milk without added CO2 at the end of the holding section of a tubular heat exchanger during heating from 40 to 80°C. The solid squares () and open circles () represent data from replication 1 and 2, respectively. The line is the least square linear regression line.

Regardless of pressure and temperature, the CO₂ concentration of milk remained the same before and after the milk was processed in the heat exchanger (Table 2); therefore, the change in pH with pressure must be related to the degree of interaction of CO₂ with milk during heating at the various pressures. The solubility of CO₂ decreases with increasing temperature (Fogg and Gerrard, 1991). In the pasteurizer, without added pressure, the originally dissolved CO₂ might not stay dissolved in milk especially when the initial carbonation level and heating temperature were high. Increasing pressure in the pasteurizer increased the amount of dissolved CO₂ and resulted in a decrease in milk pH. Increasing the applied pressure decreased the pH to a minimum value that was a function of the total CO₂ in the milk. Once the minimum pH was reached for a constant CO₂ concentration, a further increase in pressure did not decrease milk pH further. The stabilization of milk pH probably indicated that at a fixed CO₂ concentration and temperature, the CO₂ interaction with milk had reached equilibrium condition at that temperature and pressure.

For milk without added CO₂, milk pH decreased with increasing temperature in the heat exchanger (Figure 5). However, for milk with added CO₂, especially at high CO₂ concentration, an increase in temperature does not necessarily mean a decrease in milk pH in the heat exchanger as one would expect for the milk without added CO₂. For example, when the heating temperature was increased from 72 to 80°C (Figures 3 and 4), there was very little difference in the pH of milk with 1800 or 2400 ppm of CO₂ at 69, 138, and 207 kPa.

Increasing CO₂ concentration from 0 to 2400 ppm at atmospheric pressure has been shown to decrease milk pH, but that pH change was influenced by both milk composition and temperature (0 to 40°C) at which the CO₂ was injected (Ma et al., 2001; Ma and Barbano, 2003a, 2003b). However, when milk with added CO₂ was heated in a heat exchanger, increasing CO₂ concentration does not necessarily mean a decrease in milk pH either. For example, at 80°C and pressures from 69 to 276 kPa, increasing CO₂ concentration from 1800 to 2400 ppm did not decrease milk pH significantly (Figure 4). It was only when pressure was increased to 345 kPa that a difference in pH of milks with 1800 and 2400 ppm of CO₂ became apparent.

Implication of the Current Study
Under HTST pasteurization conditions in the current study, even for the milk without added CO₂, there was a substantial decrease in milk pH as it was being pasteurized at 72 to 80°C in the heat exchanger. The change in milk pH with temperature is typically reversible if the heating does not exceed 100°C and there is no degradation of milk protein or lactose during heating (Fox and McSweeney, 1998). Therefore, the decrease in milk pH during pasteurization is usually "invisible" because milk pH at the exit of the pasteurizer after cooling to 0 to 1°C is the same as milk pH at 0 to 1°C at the inlet of the pasteurizer. Therefore, thermal death of bacteria during pasteurization of milk without added CO₂ is probably due not only to temperature but also to this "invisible" decrease in pH in the pasteurizer. By adding CO₂ to milk and increasing pressure, the pH of milk can be decreased even further during pasteurization. At 80°C and a pressure of 345 kPa, the pH of milk with 2400 ppm of CO₂ can be decreased to as low as 5.63 versus 6.26 for milk without added CO₂. This decrease in pH may further enhance the microbial killing power of pasteurization.

As CO₂ continues to find applications other than its antimicrobial effect (Hotchkiss and Lee, 1996; Hotchkiss et al., 1999) in the dairy industry, knowing the pH of milk with added CO₂ during heating, a common dairy processing step, may be important. The extent of pH reduction during heating will affect the properties of milk after heating. If a decrease in milk pH is needed to achieve a certain change in a dairy food system, then the conditions during heating, such as pressure, need to be defined and properly set so that the effect of added CO₂ on milk component interactions can be optimized.

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 
The pH of milk in a heat exchanger depended on CO₂ concentration, temperature, and pressure. During heating of milk without added CO₂, pH decreased linearly as a function of increasing temperature but was independent of pressure. A linear coefficient of -0.0078 pH unit/°C in the temperature range from 40 to 80°C was obtained. Thermal death of bacteria during pasteurization of milk without added CO₂ is probably due not only to temperature but also to this "invisible" decrease in pH in the pasteurizer. In general, the pH of milk with added CO₂ decreased with increasing CO₂ concentration and pressure. For milk with added CO₂, at a fixed CO₂ concentration, the effect of pressure on pH decrease was greater at a higher temperature. At a fixed temperature, the effect of pressure on pH decrease was greater for milk with a higher CO₂ concentration. Decreased milk pH at high CO₂ concentration and high pressure may further enhance the microbial killing power of pasteurization.

	ACKNOWLEDGEMENTS

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The authors thank Tom Burke, Maureen Chapman, Bob Kaltaler, Laura Landolf, Joanna Lynch, and Pat Wood for technical assistance. We also thank the Northeast Dairy Foods Research Center (Ithaca, NY) and the New York State Milk Promotion Board (Albany, NY) for financial support.

	FOOTNOTES

 
¹ Use of names, names of ingredients, and identification of specific models of equipment is for scientific clarity and does not constitute any endorsement of products by the authors, Cornell University, or the Northeast Dairy Foods Research Center.

Received for publication December 27, 2002. Accepted for publication April 29, 2003.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION CONCLUSIONS ACKNOWLEDGEMENTS REFERENCES

 


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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Ma, Y. Articles by Barbano, D. M. Search for Related Content PubMed PubMed Citation Articles by Ma, Y. Articles by Barbano, D. M.