Coefficiency of Pasteurization and Incubation Systems of Prototype In-Line-Stirred Yogurt Machine for Small Scale Industry

Wanphen Jitjaroen1+, Arkhom Suvannakita1, Aphinan Jitjaroen2 and Orathai Bunthawong1 1Department of Agro industry, Rajamangala University of Technology Lanna Lampang, Lampang 52000

Thailand 2Department of Engineering, Rajamangala University of Technology Lanna Lampang, Lampang 52000

Thailand

Abstract. The purpose of the study was to test the coefficiency of pasteurization and incubation systems of a prototype in-line stirred yogurt machine in order to use in small scale dairy plant. The pasteurization unit consisted of a shell and tube heat exchanger for heating milk to 72 oC with a milk flow rate of 806 ml/min. During the pasteurization process, this holding temperature was maintained for a period of 67 sec. Thereafter, the pasteurized milk was cooled down to 42 oC by cold water and then transferred to a 50-litre incubation tank. The tank consisted of two jackets. The outer layer was insulated, the middle one was a cooling channel, and the inner one had an agitator. These jackets could keep the milk at 40-45 oC for three hours of incubation time. Thereafter, the yogurt curd was stirred using an agitator at 20 rpm. whilst water at 1oC was flowed at 20 liters/minute into the middle jacket to reduce yogurt temperature to 20-25 oC. Results showed that the prototype was consistently able to maintain the pasteurized and incubated temperatures. The phosphatase enzyme was not found in the pasteurized milk. The yogurt had unchanging the acidity of 0.76% throughout the cooling period. This indicated that the integrating of the systems could work together and produced a good quality yogurt.

Keywords: stirred yogurt, pasteurization, incubation, shell and tube heat exchanger

1. Introduction Thailand has total market value of soft, drinking and culture yogurts of around 12,000 MB in 2009.[1]

This is growing continuously every year. But there are only 3-4 manufacturers nowadays. This is a big market gap to open an opportunity for small scale industries. However, the big barrier for small industries is importing the machines which are expensive. Jitjaroen et al. (2005) [2] have invented a prototype in-line stirred yogurt production machine, to use in small industries and educational institutes (Fig.1). The machine consisted of a milk mixing, pasteurization and incubation systems. These were installed together on a portable-basement sized 2.8 m2.

This research studied the coefficiency of pasteurization and incubation systems for yogurt production. After integrating both systems together, they should be able to cooperate continuously to make a standardized product.

2. Material and Methods To calculate fluid flow in the tube and flat sheet, we used “Reynold number” which is one of the factors

from the fluid heat exchanger theory, and “Nusselt number” to calculate the heat transfer coefficient.[3],[4],[5]

+ Corresponding author. E-mail address: Wanphenjit@hotmail.com

2011 2nd International Conference on Agricultural and Animal Science IPCBEE vol.22 (2011) © (2011) IACSIT Press, Singapore

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Fig. 1: Layout and image of a prototype in-line stirred yogurt machine system.

2.1 Stirred yogurt production by prototype machine The milk was heated from 4 oC to pasteurized temperature of least 72 oC within 15 seconds. After that, it

was cooled down to 40-45 oC, then transferred to the incubation tank. The milk was incubated with yogurt starter for 3-4 hours until it curded. The curd yogurt was stirred to a cream, together with reducing the temperature down to 20-25 oC, before the packing process.[6],[7]

2.2 Designing and making the pasteurization system A forced draft burner (Fig. 2) was the equipment used for heating up the milk. It was connected with two

sets of shell-and-tube heat exchangers, called “pre-heater” and “heater”, to increase the temperature to pasteurized level (Fig.3). These allow the hot milk to cool down by exchanging the heat with the cold milk flowing in from the opposite direction.

Control panel

Thermocouple

Safety shut - off valve

Pressure regulator

Pressure gauge

Air pressure switch

Motor Air Fan with Air Dumper

Burner

Flame

Fig. 2: Diagram of a force draft burner system.

A holding tube was used to delay the time, to ensure the full pasteurizing period. The proper temperature of milk related to the period of time holding for each particular temperature.[8] A thermocouple was installed within the system, in a proper location involved with the flowing system, to measure the temperature.

The efficiency of pasteurization was measured from the relationship between the proper temperature and time, and measured the level of phosphatase enzyme which indicates the number of microorganism in the pasteurized milk.[6],[8]

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(a) Pre-heater unit

(b) Heater unit

Fig. 3: Structure and image of the shell and tube heat exchanger accessory

2.3 Designing and making the Incubation system The incubation tank (Fig. 4) was designed in three layers of stainless steels no. 304, cylinder shape. The

outer layer was shielded with 1 inch thickness of polyurethane, to insulate the temperature while being in the incubation process. The middle layer was a circulate channel for 1 ๐C water. The inner layer contained pasteurized milk. The tank was of 50 litres capacity, with a lid on top, and installed a 1/4 hp. 1:60 motor gear. The incubation tank started to drive the agitator to stir the yogurt at 20 rpm. as well as the cold water was pumped to circulate in the middle layer, until the yogurt temperature was reduced to 20-25 ๐C. This indicated that the packing process can be begin.[9]

The efficiency of yogurt incubation system was measured from the milk temperature while incubated in the tank, and from the total acidity level while reducing the temperature of yogurt.[6]

3. Results and Discussion

3.1 Pasteurization system testing Pasteurization system prototype machine can eliminate the microorganism as expected, by monitoring

the milk temperature coming into the system in the different period of time and location. Figure 5 shows that the milk has some variation of pasteurization temperature (T3) within the first 5 minutes of milk pumping process. Then the temperature was gradually adjusted to the balance situation (Line B).

Milk pasteurization at the temperature higher than 72 oC, flow rate 806 ml/min., showed no phosphatase. This means there is no pathogenic microorganism, which is very dangerous to human.

The major heat exchanger can exchange the heat faster than what we expected by 30%. This means the quality to increase the milk temperature was very high. On the other hand, the heat exchange rate to reduce

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the temperature of the milk, which took place near the pre-heater unit and the temperature reducing unit before the incubation process, was slower than expected. This was the reason for extending the heat exchanging area by increasing the temperature of the cold-water tank and cold-water tube inside the tank, to interface more with cold water. This helps control the incubation temperature to 42 oC.

However, the graph shows that the pre-heat temperature (T5 post-cooling) was at approximately 42 oC throughout the process. This indicated the accuracy of the pasteurization process.

Fig. 4: Inside structure and image of incubation tank of in-line-stirred yogurt.

3.2 Incubation tank testing In terms of Engineering, the efficiency of incubation tank testing was examined from the capability to

keep the temperature of yogurt steady, and reducing temperature in a given period of time, by measuring the temperature at the center of the tank every 15 minutes.

Figure 6 shows the temperature throughout the incubation for 3-4 hours was around 42 oC constantly and cooling periods was able to reduce the temperature of yogurt lower than 25 ๐C in an hour, with constant 0.76% of total acidity. This indicated that the incubation tank shield was sufficient to keep the temperature steady, and yogurt temperature reducing process was effective as expected.

Fig. 5: Relationship between temperature and time of pasteurization (T1=33 oC, flow rate = 806 ml/min, Tb=210 oC).

4. Conclusions

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The prototype machine can pasteurize the milk within a period of time given, cool down the pasteurized milk to qualify for the incubation process, keep the temperature in the tank steady while incubation process until the milk curded, and reduce the temperature of yogurt before packing. The stirred yogurt reached the biochemical standard, and the prototype machine can also be applied for yogurt milk pasteurization industries.

Fig. 6: Yogurt temperature during incubation and cooling periods

5. Acknowledgment Our research team, supported by Rajamangala University of Technology Lanna, Lampang, would like to

express our gratitude to our supervisors Prof. Somchai Jomduang and Prof. Satean Sriboonruang from Chiangmai University for their help, and to the Department of Agro-industry of RMUTL for their permission to install the machine in their facilities.

6. References [1]. Oknation. Marketing share of Drinking yogurt. July 2010. Available online.

http://www.oknation.net/blog/print.php?id=473184. [20.07.2010].

[2]. Jitjaroen W., Suvannakita A., Bunthawong , O., Jitjaroen A., 2005. The Construction of a Prototype In- Line-Stirred Yogurt Machine, Controlled by an Electronic and Computer System, and Designed for Use by SMEs and Educational Institutes. University of Technology Lanna Lampang. Thailand, 245p.

[3]. Yunus A. Cengel, 2003. Heat Transfer a Practical Approach. 2nded, McGraw-Hill, Singapore, 932p.

[4]. Toledo, T., 1991. Fundamentals of Food Process Engineering. 2nded, Von Nostrand Reinhold, New York, 602p.

[5]. Holman. J. P., 1997. Heat Transfer. 8thed, McGraw-hill, New York, 696p.

[6]. Alfa-Laval Dairy and Food Engineering, 1980. Dairy Handbook. Division Sweden, 301p.

[7]. Jitjaroen W., 1999. Principle of Analysis and Calculation of Dairy Products. University of Technology Lanna Lampang, Thailand, 149p.

[8]. Jitjaroen W., Suvannakita A., Bunthawong, O., Jitjaroen A., 2008. A Pasteurization System of Prototype In-Line-Stirred Yogurt Machine, Designed and Constructed for Use by SMEs. Journal of Industrial Education, 4 (2) : 57-67.

[9]. Jitjaroen W., Suvannakita A., Jitjaroen A., Bunthawong O., 2011. Designed and Constructed an Incubation Unit of Prototype In-Line-Stirred Yogurt Machine. The 4th National Conference on Technical Education, July 7-8, 2011, Engineeering and Technical Education. Thailand, King Mongkut’s University of Technology North Bangkok, 174-178 p.

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