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Ultra-high-temperature processing of chocolate flavoured milkSangeeta Prakash, Thom Huppertz1, Olena Karvchuk, Hilton Deeth*School of Land, Crop and Food Sciences, University of Queensland, Brisbane 4072, Australiaarticle infoArticle history: Received 4 March 2009 Received in revised form 10 June 20...

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Journal of Food Engineering 96 (2010) 179–184

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Journal of Food Engineering j o u r n a l h o m e p a g e : w w w .e l s e v i e r . c o m / l o c a t e / j f o o d e n g

Ultra-high-temperature processing of chocolate flavoured milk Sangeeta Prakash, Thom Huppertz 1 , Olena Karvchuk, Hilton Deeth * School of Land, Crop and Food Sciences, University of Queensland, Brisbane 4072, Australia

a r t i c l e

i n f o

Article history: Received 4 March 2009 Received in revised form 10 June 2009 Accepted 10 July 2009 Available online 15 July 2009 Keywords: Milk UHT Chocolate Fouling Carrageenan

a b s t r a c t Chocolate milk with different carrageenans ( jappa and lambda) and sugar concentrations was heat treated indirectly at 145 °C for 6 s using a bench-top UHT plant. The temperature of the milk in the preheating and sterilizer sections, and the milk flow rate were determined to evaluate the overall heat transfer coefficient (OHTC) for monitoring fouling during UHT processing. Kappa-carrageenan was more effective than lambda-carrageenan in providing stability against fouling during UHT processing. By optimizing concentrations of j-carrageenan and sugar, fouling could be minimized during UHT processing. The apparent viscosity and sedimentation of UHT-processed chocolate milk increased with increasing concentration of carrageenan and sugar. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Ultra-high-temperature processing is the preferred way of heattreating chocolate flavoured milk as it enhances the flavour of the chocolate without making it harsh or bitter, provides an overall smoothness and favourable mouth-feel and extends its shelf-life (Bixler et al., 2001). Chocolate milk is formulated with milk, cocoa powder, sugar and hydrocolloids. The final composition, physical and sensory properties of the chocolate milk largely depend on the levels of the ingredients including fat, the type of cocoa and type of hydrocolloid (Yanes et al., 2002a). It is common practice to add a hydrocolloid to all UHT flavoured milk to increase the creaminess of the final product and impart a more lasting taste (Anonymous, 2000). The largest dairy application for kappa-carrageenan (j-car) is in hot-processed chocolate milk as it imparts a favourable mouth-feel to the milk and provides long-term suspension of the cocoa particles (Bixler et al., 2001). The favourable mouth-feel results from the enhanced apparent viscosity of the carrageenan–casein network (Bixler et al., 2001). Heat treatment significantly increases the shelf-life of flavoured beverages and also aids the hydration of the hydrocolloid. It has also been shown that heating milk containing carrageenan at UHT temperatures increases the strength of the carrageenan gels and improves the long-term stability of the product. This has been attributed to the displacement of carrageenan complexed to j-casein by the denatured b-lactoglobulin, thus increasing the * Corresponding author. Tel.: +61 7 3346 9191; fax: +61 7 3365 1177. E-mail address: [email protected] (H. Deeth). 1 NIZO food research, P.O. Box 20, 6710 BA, Ede, The Netherlands. 0260-8774/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jfoodeng.2009.07.008

availability of carrageenan for carrageenan–carrageenan interactions which are largely responsible for formation of the weak gel network (Sedlmeyer and Kulozik, 2007; Tijssen et al., 2007). However, during sterilization, problems may arise due to the increased viscosity achieved by the hydrocolloids and also due to the interactions between the constituents of the flavour and milk that result in flocculation, coagulation and sediment formation (Ramesh et al., 1993; Tziboula and Horne, 2000). If the amount and type of stabilizer are not satisfactory, the finished product can exhibit a number of undesirable characteristics such as flocculation and coagulation (Anonymous, 2000). Traditionally j -car has been used as a stabilizer in protein systems, particularly dairy applications (Langendorff et al., 2000). However data from differential scanning calorimetry show that heat-induced aggregation of b-lactoglobulin (b-Lg) decreases in solutions containing low amounts of sulfate-containing polysaccharides like k-carrageenan (k-car) (Zhang et al., 2004), suggesting the potential importance of k-car in dairy processing. Studies related to chocolate milk have mainly concentrated on its sensory (Folkenberg et al., 1999), rheological and optical properties (Yanes et al., 2002b). The effect of heat treatment on the stability of chocolate milk has revealed three types of instability: sedimentation of cocoa particles, formation of large flocs and formation of light- and dark-coloured layers (Yanes et al., 2002b) that arise due to interactions between the ingredients of the chocolate and milk components (van den Boomgaard et al., 1987). Fouling or deposit formation is an every-day concern of the dairy industry and is a very common problem during UHT processing of chocolate flavoured milk. No work has been reported on fouling that takes place during UHT treatment of chocolate milk. The main aim of

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the present work was to study UHT processing of chocolate milk with different concentrations of j-car, k-car and sugar. 2. Materials and methods 2.1. Chocolate milk preparation Reconstituted whole milk was prepared by mixing whole milk powder into water to a final concentration of 120 g per L at 35 °C using a heavy duty mixer (van den Boomgaard et al., 1987). Standard chocolate milk was prepared by adding cocoa powder (1.5%), caster sugar (7%, 9% and 11%) and carrageenan (j -car or k-car) (0.00%, 0.01%, 0.03%, 0.05% or 0.10%) to the reconstituted milk. Commercial-grade cocoa powder was supplied by Frutex Australia (New South Wales, Australia). j -car (WG80 M) was supplied by Woods & Woods Pty Ltd. (NSW, Australia) and k-car (Seakem CM611) was supplied by Swift Company Ltd. (Victoria, Australia). 2.2. UHT processing A bench-top UHT plant, similar to the one described by Wadsworth and Bassette (1985), was used in the experimental trials. The holding times in the preheating and high-temperature sections were 8 s at 95 °C and 6 s at 145 °C, respectively. After heat treatment the milk was cooled to 6 35 °C in 25 s in a water-jacketed cooler. The flow rate of the milk was maintained at 120 mL/min. The Reynolds Number (Re) of the flow was calculated as 4.03  104, which falls in the turbulent flow regime (Holman, 2002). The bench-top plant was instrumented with thermocouples located at the inlet and outlet of the pre-heater and the outlet of the sterilizer and high-temperature holding sections. The thermocouple at the outlet of preheater was at the inlet of the sterilizer section. All thermocouples were connected to a data logger (PCLD8115 supplied by Advantech, Company Ltd., Taiwan) which measured and recorded inputs. The system used a PC-based data acquisition system, VISIDAQ, that allowed real-time monitoring of data and generated data logs which were used for data management Ò in MS-Excel . Milk flow rate was determined by measuring the time required to collect a known volume of milk. Three measurements were recorded for every 5 min of the entire run and their average represented the flow rate for the 5 min period. 2.3. Fouling measurement Fouling was monitored by changes in overall heat transfer coefficient (OHTC) which was measured using the following equation from Kastanas (1996):

GC D h OHTC ¼ p AD T lm

ð1Þ

where G = mass flow rate of the milk in kg/s; Cp = specific heat of chocolate milk in J/(kg°C) – this was calculated taking into account the specific heat capacity and mass fraction of each component (milk powder, specific heat capacity = 3750 J/(kg°C) Walstra et al. (2005)); cocoa powder, specific heat capacity = 1200 J/(kg°C) (personal communication Dr. Ulrich Krause); sugar specific heat capacity = 1250 J/(kg°C), Walstra et al. (2005). The specific heat capacity of carrageenan was not included as a negligible amount of carrageenan was added to the mixture); D h = temperature difference between the inlet and outlet of the UHT section, in °C; A = heat exchanging surface area of the tubing = 1.627  105 m 2; D Tlm = logarithm mean temperature difference (LMTD) in °C:

D Tlm ¼

ðT o  T moÞ  ðT o  T mi Þ ln½ðT o  T moÞ=ðTo  T mi Þ

ð2Þ

where To is the temperature of oil in the high-temperature section in °C; Tmo and T mi are the temperatures of milk at the inlet and outlet of the high-temperature section, respectively (Kastanas, 1996). The bench-top UHT plant was operated and cleaned as described by Kastanas (1996). The experiment was stopped when the temperature at the outlet of the high-temperature section dropped below 120 °C or earlier if deposits blocked the channel or if the back-pressure could not be maintained at 0.4 MPa. The indicator of the pressure gauge fluctuated from its set point of 0.4 MPa with fouling, and the fluctuation became more pronounced as deposit built up on the walls of the tubes. All the experimental trials were carried out in duplicate. 2.4. Apparent viscosity The apparent viscosity of the UHT-processed chocolate milk was measured at room temperature using a Brookfield Viscometer (Model DV-I Viscometers, Brookfield Engineering Laboratories, INC., USA) fitted with a UL adaptor. A spindle speed of 30 rpm was used. 2.5. Sediment measurement Sedimentation in chocolate milk is defined as the settling of particles, including cocoa particles, under gravity. Sediment in the sterilized chocolate milk was measured estimated by a centrifugation method, using calibrated centrifuge tubes (50 mL). The weight of the centrifuge tubes was recorded and milk was weighed (40 ± 0.1 g) into them. The samples were placed in a centrifuge (Model 5702 R, Eppendorf) at 3000g for 15 min. After centrifugation, the solid sediment was separated from the supernatant by decanting. The tubes were then placed in an oven at 120 °C for 36 h and the dry weight of the sediment was measured. For each sample, duplicate analyses were carried out. The results were expressed as g/100 g of milk. 2.6. Statistical analysis Ò

All statistical analyses were performed using MINITAB 15 statistical software (Minitab Inc., Chicago, USA, 2007). The general linear model analysis was performed with the GLM ANOVA tests procedure. When the main ANOVA tests were significant, multiple comparisons of treatments were further conducted with the Tukey test implemented in the procedure. The p-values of specific comparisons are reported in the text. 3. Results 3.1. Effect of j-car, k-car and sugar concentration on UHT processing of chocolate milk The variation in OHTC with time of operation for chocolate milk with 1.5% cocoa powder, 7% sugar and different concentrations of j -car (0%, 0.01%, 0.03%, 0.05% and 0.1%) is shown in Fig. 1. The control sample, i.e. the sample to which no j-car was added, showed considerable fluctuation in temperature and back-pressure right from the start which became more pronounced towards the end of the run. The cocoa particles settled to the bottom of the balance tank and interfered with the running of the bench-top UHT plant as the back-pressure could not be maintained. The OHTC dropped rapidly with time of operation. The fluctuation in OHTC is a result of the fluctuation in temperature.

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1000 0.03% 0.01%

600

400 0.10%

0.05%

κ-carrageenan α-carrageenan

4000

Average run-time (s)

OHTC (kW/m2 0C)

800

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0%

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5000

0.00

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0.04

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0.06

0.08

0.10

Carrageenan concentration (%)

Fig. 1. Variation in OHTC with time of operation during UHT processing of chocolate milk with different concentrations of added j-car.

UHT processing of chocolate milk with 0.01% added j -car showed an improvement in run-time (average run-time, 4020 s) (SD = 60 s); however there was fluctuation in both temperature and back-pressure, and towards the end of the runs the fluctuation was so severe that the bench-top UHT plant had to be shut down. The amount of cocoa particles settling to the bottom of the balance tank was also less than in the run with no added carrageenan. Processing chocolate milk with 0.03% j-car greatly improved the run-time (average run-time 5000 s) (SD = 20 s) of the plant compared with that obtained with 0 and 0.01% j-car. The fluctuations in temperature and back-pressure were minimal and the amount of cocoa particles that settled at the bottom of the balance tank was also less than in the runs with 0% and 0.01% j -car. Addition of 0.05% and 0.1% of j-car markedly reduced the runtime of the plant (Fig. 1). There was no induction period but a steep drop in OHTC. The back-pressure was constant throughout the run; however the temperature at the outlet of the sterilizer section dropped rapidly. Fouling was very severe with 0.1% added j -car and the tubes were almost blocked before the run could be terminated. The run-times obtained with UHT processing of chocolate milk with 1.5% cocoa powder, 7% sugar and different concentrations (0.01%, 0.03%, 0.05% and 0.1%) of k-car are shown in Fig. 2.

Fig. 3. Interaction plot for run-times at various concentrations of carrageenan (SE = 37.26 s).

j- and k-

The run-times at the lower concentrations of k-car (0.01% and 0.03%) were shorter than those with similar concentrations of j car (Fig. 3) (P < 0.001). There was no significant difference in the run-times of chocolate milk with 0%, 0.01% and 0.03% k-car. An observation made during processing which is also evident in Fig. 2 was the fluctuation in temperature and back-pressure even at 0.03% k-car which was not observed at similar concentrations of j -car. At higher concentrations (0.05% and 0.1%) the run-times were short, as was observed with similar concentrations of j -car (Fig. 3). The interaction plot of run-time at 0–0.1% concentrations of j car and k-car is presented in Fig. 3. Tukey simultaneous tests were performed for all pair-wise comparisons of j -car and k-car. The results showed that the run-times for k-car were significantly shorter than those for j-car at 0.01%, 0.03% and 0.05% (P < 0.05), with no significant difference at 0.1% k-car and j -car. The UHT-processed chocolate milk, when collected in a glass beaker, showed distinct dark and light layers with 0.05% and 0.1% j -car and k-car but not with 0%, 0.01% and 0.03% j-car and k-car. The appearance of these layers is often associated with the type of cocoa powder, the sterilization time or percentage of stabilizers (van den Boomgaard et al., 1987). Considering the same cocoa powder and the same sterilization time were used for all the trials in our study, the layers are attributable to the higher percentages of j-car and k-car.

1200 0% 0.01% 0.03% 0.05% 0.10%

1000

1000

7%

20

OHTC (kW/m C)

OHTC (kW/m2 0C)

800

800

600

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400 7% 9% 11%

9%

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11%

0

0

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500

1000

1500

2000

2500

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3500

Time of operation (s) Fig. 2. Variation of time of operation of the UHT system with different concentrations of added k-car.

0

500

1000 3500

4000

4500

5000

Time of operation (s)

Fig. 4. Variation of time of operation of the UHT system with different concentrations of added sugar.

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Standard chocolate milk has a sugar concentration of 7% (van den Boomgaard et al., 1987). The role of sugar during UHT processing of chocolate milk was investigated by increasing the concentration to 9 and 11% and keeping the concentrations of carrageenan (0.03%) and cocoa powder (1.5%) constant. j-Car at 0.03% was used as it gave the longest run-time. The variation of OHTC with time of operation is shown in Fig. 4 (some data points between 1000 and 3500 s have been omitted in the graph to facilitate display of the short and long runs). Higher sugar concentration (9% and 11%) fouled the plant very quickly.

of carrageenan (P < 0.001). However there was no significant difference in amount of sediment obtained at similar concentrations in the range 0.01–0.03% of the different types of carrageenan. The addition of sugar at 9% and 11% increased the apparent viscosity of the milk and markedly reduced the run-time of the system (Fig. 4). There was no significant difference in run-time of the plant during UHT processing of chocolate milk with 9% and 11% added sugar but these run-times were significantly different from those with 7% added sugar (P < 0.001) (Fig. 4). The observed viscosities and processing parameters, as shown in Table 2, suggest that the higher apparent viscosity resulting from the higher sugar contents caused shorter run-times and more sedimentation. There was no observable induction period and the decline phase was steep for the two higher sugar levels (Fig. 4). Analysis was performed on the log-transformed data for the amount of sediment and apparent viscosity obtained with different concentrations of sugar. There was a significant difference in the apparent viscosity and amount of sediment obtained between 7% and 9% added sugar (P < 0.001 and 0.05), between 7% and 11% added sugar (P < 0.001 and 0.01), and between 9% and 11% added sugar (P < 0.01 and 0.01). The results (Tables 1 and 2) suggest that sediment and apparent viscosity both increase with increasing carrageenan concentration.

3.2. Effect of j -car, k-car and sugar on apparent viscosity and sedimentation The initial OHTC and product outlet temperature (OHTC and outlet temperature at commencement of the experiment), apparent viscosity and sediment data of UHT-processed samples of chocolate milk with different concentrations of j-car and k-car are shown in Table 1. The addition of j- and k-car significantly influenced the apparent viscosity of the processed chocolate milk (P < 0.001). There was a steady increase in apparent viscosity with increase in j- and kcar concentration with 0.1% markedly increasing the apparent viscosity of the UHT-processed chocolate milk. The increase in apparent viscosity at 0.05% and 0.1% was accompanied by a reduction in run-time of the plant, with 0.1% fouling the plant in less than 9 min (Figs. 1 and 2). The decreased initial product outlet temperature and OHTC at higher j - and k-car concentration could be due to increasing apparent viscosity (thus decreasing turbulence in the tubular heat exchangers) during heating (Table 1). A similar increase in viscosity was observed by Kastanas (1996) during fouling of concentrated milk. A Tukey simultaneous test was performed for all pair-wise comparisons of apparent viscosity. The apparent viscosity of UHT-treated chocolate milk at 0.1% carrageenan was significantly different from the apparent viscosity obtained at lower concentration (0.01–0.05%) with a significant difference (P < 0.05) in apparent viscosity obtained with similar concentrations of different carrageenans. Analysis was performed on log-transformed data for the amount of sediment obta...