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Mechanism of spillage and excessive boiling ofwater during vacuum cooling

Xiao-yan Song, Bao-lin Liu*, Ganesh K. Jaganathan, Lan Chen

Institute of Cryobiology and Food Freezing, University of Shanghai for Science and Technology, 516 Jungong Road,

Shanghai 200093, PR China

a r t i c l e i n f o

Article history:

Received 19 January 2015

Received in revised form

10 March 2015

Accepted 12 April 2015

Available online 20 April 2015

Keywords:

Boiling

Bullet bubble

Spillage

Vacuum cooling

Volumetric displacement

* Corresponding author.E-mail address: blliuk@163.com (B.-l. Liu)

http://dx.doi.org/10.1016/j.ijrefrig.2015.04.0090140-7007/© 2015 Elsevier Ltd and IIR. All rig

a b s t r a c t

Immersion vacuum cooling is a novel method for cooling meat products. This method has

notable advantages including lower water loss rate of products during the cooling process.

However, excessive solution boiling and spillage during immersion vacuum cooling pro-

cess are considered as the serious problems limiting its wide-spread application. In this

study, the mechanism of water boiling and spillage during vacuum cooling was studied by

capturing the images of boiling phenomena with a high speed camera. Results show that

the growth of bullet bubble is a major reason for more than 42% of water loss during

boiling, because the diameter of a bullet bubble can increase to the diameter value of the

test tube in 0.36 s. Our results also show that using moderate volumetric displacement of

vacuum pump (for instance 0.0012 m3 s�1 in this paper) and controlling the chamber

pressure in the range of 10e2 kPa can weaken the intensity of boiling and spillage of water.

These results are discussed in the context of 'classical pool boiling' theory.

© 2015 Elsevier Ltd and IIR. All rights reserved.

M�ecanisme de d�eversement et d'�ebullition excessive de l'eaudurant le refroidissement sous vide

Mots cl�es : Ebullition ; Bulle en balle ; D�eversement ; Refroidissement sous vide ; D�eplacement volum�etrique

1. Introduction

Vacuum cooling is widely used for cooling food products with

a high water content and large porosities, due to its efficacy in

losing water from both within and outside the products

.

hts reserved.

(Augusto et al., 2012; Cepeda et al., 2013; Ozturk and Ozturk,

2009; Rinaldi et al., 2014). The increasing use of this tech-

nique in storing various agricultural, horticultural and ready-

to-eat products such as fruits (He et al., 2013), bakery products,

celery, bamboo shoots (Cheng, 2006), cabbage (Cheng and

Hsueh, 2007), lettuce (Ozturk and Ozturk, 2009), mushrooms

Fig. 1 e Schematic diagram of the cooling equipment,

including: 1 Vacuum chamber 2 High-speed camera 3

Electromagnetic valve 4 Pressure sensor 5 PC 6

Thermocouple 7 PLC 8 Frequency convertor 9 Vacuum

pump 10 Cold trap 11 Samples 12 Thermocouple.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 5 6 ( 2 0 1 5 ) 3 7e4 238

(Singh et al., 2010) and cookedmeats (Huber et al., 2006) can be

attributed to its significant advantages including clean, low

energy consumption and rapid cooling rate (Feng et al., 2012).

However, vacuum cooling has inherent disadvantages

limiting its wide scale application in the food industry. One

of the important problems is high moisture loss rate of the

products during the cooling process. Several methods have

been attempted to overcome this problem especially for

meat products. These include pre-wetting prior to vacuum

cooling (McDonald and Sun, 2000); brine solution injection

(Desmond et al., 2002); vacuum cooling of small meat pieces

in soup (Houska et al., 2003); pulsed immersion vacuum

cooling (Cheng and Sun, 2006); combined vacuum cooling

and air blast cooling (Jackman et al., 2007); integration of

cooking and vacuum cooling (Rodrigues et al., 2012;

Schmidt et al., 2010; Schmidt and Laurindo, 2014); vacuum

cooling followed by immersion vacuum cooling (Dong et al.,

2012); immersion vacuum cooling of large meat products

(Feng et al., 2013); pressure control (Feng et al., 2014; Song

and Liu, 2014). Of these methods, immersion vacuum

cooling can drastically reduce moisture loss rate (Feng

et al., 2012), because most of water used to refrigerate

comes from the surrounding hot soup or cold water. How-

ever, herein lies an important limitation. Because this pro-

cess can result in excessive solution boiling and spillage

(Feng et al., 2014), the work required to clean the vessel

after cooling process is tedious, and the splashing nutrient

contents such as the extremely small meat powder can

provide an optimal condition for the microorganism growth

in the place where is difficult to clean or easy to adsorb

particles. Consequently, avoiding the excessive solution

boiling and spillage is important for the immersion vacuum

cooling equipment.

McDonaldandSun (2000) foundthatmanuallyadjusting the

pressure drop rate inside the vacuum chamber following the

pressurecurveof saturatedvapor could result in lower levels of

moisture loss. However, accurately following the saturated

vaporcurveneedsahigherequipment investmentanda longer

processing period, which are somewhat impractical on in-

dustrial scale. Thus, selectively controlling the volumetric

displacement of vacuumpump insteadof pressuredrop rate in

one or more pressure ranges seems to be a more meaningful

approach. However, to date, the rationale behind how con-

trolling pressure can avoid violent boiling and spillage of the

cooling media during immersion vacuum cooling is unclear.

Thus, the main aim of this study was to reveal the

mechanism of the violent boiling and spillage of water

during immersion vacuum cooling with a high speed cam-

era. Then the feasibility of selectively controlling volu-

metric displacement of vacuum pump in the key range of

pressure to reduce the intensity of boiling and spillage was

verified.

2. Materials and methods

2.1. Experimental setup

The vacuum cooling apparatus designed for experimental

purpose is depicted in Fig. 1. The vacuum chamber was made

of organic glass with a volume of 0.045 m3. A high speed

camera (SVSi, Giga View, USA) with a full resolution of 532 fps

was used to acquire the video of the boiling process of water

during the vacuum cooling process. The vacuum pump (Ley-

bold, D8C, Germany) and the frequency converter (SINAMICS,

V10, Germany) were combined to reduce the total pressure

which was detected by a pressure sensor (Testo, 435-4, Ger-

many) and to stabilize the pressure at 1000 ± 50 Pa with the

help of an electromagnetic bleeding valve until the vacuum

was released. A controller (SINAMICS, S7-224, Germany) and a

frequency converter supplied the vacuum pump with various

powers according to the set rotational frequency values to get

different volumetric displacements. The cold trap condensed

the water vapor from the vacuum chamber. Both the control

task and data collectionwere performed by the controller with

the help of a programming software (STEP 7-MicroWin

V4.0.8.06).

2.2. Experimental conditions

The phenomenon of excessive boiling and spillage of cooling

media appears mainly near “flash point”, which refers to the

saturated pressure corresponding to the temperature of prod-

uct surface (Cheng and Lin, 2007). So, dividing the whole pro-

cess of pressure dropping into three stages (before boiling,

during boiling and after boiling) by the “flash point” was

thought to be beneficial for designing the experimental

schemes. In thispaper, thestageof “duringboiling” refers to the

pressure dropping process in the pressure range of 10e2 kPa.

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 5 6 ( 2 0 1 5 ) 3 7e4 2 39

In our experiment, there were four schemes. They were

performed as follows: (a) seven test tubes containing water

were placed inside the vacuum chamber. Water used in the

experiment was first boiled and allowed to cool to 32 ± 0.5 �C.Each test tube was filled with 100 mL volume of water; (b) into

one of the test tubes, a thermocouple was embedded at 1 cm

below the water surface to measure the temperature change

during the cooling process; (c) the chamber door was closed

tightly; (d) images were taken at a frame rate of 100 frames s�1

by the high speed video camera during the whole vacuum

cooling process; (e) when the cold trap was cooled to - 6 �C,both the vacuum pump and frequency convertor were

switched on, and the pressure of vacuum chamber was

reduced to 1000 Pa and maintained at this range until the

completion of experiment. The volumetric displacement of

vacuum pump was 0.002 m3 s�1 at stages of “before boiling”

and “after boiling”. However, different volumetric displace-

ments of vacuumpumpwere used (0.002m3 s�1, 0.0016m3 s�1,

0.0012 m3 s�1 and 0.0008 m3 s�1 corresponding to Schemes.

1e4 respectively) at the stage of “during boiling” (Fig. 2); (f)

when the temperature measured by the thermocouple

reached 10 �C, the vacuum cooling process was stopped.

2.3. Water loss rate and statistical analysis

The water loss rate was calculated as follows:

A ¼ ðW0 �W1Þ=W0*100% (1)

Where, A is the water loss rate; W0 is the water mass before

vacuum cooling, kg; and W1 is the water mass after vacuum

cooling, kg.

SPSS (Version 18.0) was used to evaluate the effect of

controlling the volumetric displacement in the pressure range

of 10e2 kPa on the water loss rate and cooling time of water

during the vacuum cooling process in a Analysis of variance

(One-Way ANOVA) at p < 0.05 (N ¼ 6; note that test tube

containing thermocouple was excluded from any analysis

Fig. 2 e Relationship between volumetric displacement of

vacuum pump and real-time pressure of vacuum chamber

during different cooling schemes.

because the presence of solid substance in the water could

affect the bubble formation).

3. Results and discussion

3.1. Growth and behavior of bubbles during the vacuumcooling

3.1.1. Growth and behavior of bubbles in water before boilingOur observations with high speed video camera reveal that

there were different stages of bubble formation during the

vacuum cooling process, which agrees with a previous study

(Cheng and Lin, 2007). At the beginning of the vacuum cooling

process, the drop in chamber pressure lead to the formation of

few small bubbles on the sidewalls andwater surface (Fig. 3a).

This is because of the pressure of water surface reached

the saturated pressure corresponding to the water surface

temperature, and the water adhered to the side wall was held

in a superheated state, thereby resulting in the formation of

bubbles from the tube wall where existed manymicrogrooves

serving as nucleation sites (Ahmadi et al., 2014; Wang and

Wang, 2014; Yabuki and Nakabeppu, 2014). Because most of

the products applied to vacuum cooling have porous struc-

tures (Feng et al., 2012), there aremany cracks andmicro voids

on their surfaces serving as nucleation sites. Such a phe-

nomenon is very common in practical immersion vacuum

cooling technique implied on an industrial scale. When the

chamber pressure dropped further, the size of bubbles form-

ing increased (Fig. 3b). Then, some of these bigger bubbles

moved to the water surface, due to the action of the buoyancy

and gathered together at the top (Fig. 3c and d).

The classical “pool boiling” theory explaining the forma-

tion of bubbles in normal cooking process does not fully

explain the bubble formation during vacuum cooling (Kim and

Kim, 2006). This is because of three reasons: a) during the

Fig. 3 e Growth and behavior of isolated bubbles before

“boiling” during Scheme 1.

Fig. 4 e Growth and behavior of bubbles near the water

surface at the boiling flow stage in Scheme 1.

i n t e r n a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 5 6 ( 2 0 1 5 ) 3 7e4 240

vacuum cooling process, degree of superheat is fundamen-

tally caused by the pressure drop; b) the increasing of bubble

size depends largely on the pressure drop from the tube bot-

tom towater surface during immersion vacuum cooling; c) the

bubbles at the tube bottom are more difficult to grow up than

the upper ones, which is different from the phenomenon in

cooking process (Ahmadi et al., 2012). Consequently, control-

ling the pressure drop rate seems to be one of the fundamental

methods to improve the effect of vacuum cooling.

3.1.2. Growth and behavior of bubbles in water during theboiling processThe boiling process during the vacuum cooling can be divided

into two stages: boiling flow stage and slug flow stage.

3.1.2.1. Boiling flow stage. When the chamber pressure

dropped further, the size of the bubble continued to increase,

especially once it reached the water surface (Fig. 4aec). With

more bubbles reaching the water surface, the water surface

Fig. 5 e Growth and breakup of bullet bub

fluctuated randomly (Fig. 4d). If the bubble has a high growth

intensity, it can be used to reduce the weight loss of products

during the practical immersion vacuum cooling process,

because the bubble expansion can provide benefit to thewater

penetration (Drummond et al., 2009; Schmidt et al., 2010).

However, the random fluctuation of water surface was not

expected, because it could lead to spillage and excessive

boiling of coolingmedia (Feng et al., 2014; Feng and Sun, 2014).

3.1.2.2. Slug flow stage. The characteristics of bubbles formed

in slug flow stage were different from the bubbles formed in

boiling flow stage. At the boiling flow stage, the growth of a

singlebubblecanbeeasilyobserved (Fig. 4aed).However,at the

slug flow stage, a small bubble that had generated at the tube

bottom developed into a big bullet bubble (see also Ahmadi

et al., 2014) and moved towards the water surface within

0.36 s (Fig. 5aef). Thedurationof this stagewas relatively short,

thus for a single bubble its thermal behavior was very difficult

to study qualitatively (Gorenflo et al., 2014). Consequently,

moststudiesaboutboilingof liquidhavebeenperformedby the

pressure control (Ahmadi et al., 2014; Swain and Das, 2014).

With the further growth of this bullet bubble, its lengthwas

prolonged due to the restraint of the tube wall, which caused

that the above liquid to extrude out of the tube in the liquid

form without any refrigeration contribution (Fig. 5geh). Later,

more bullet bubbles appeared and more liquid was extruded

out by them. In our experiments, we estimated at least 42

percent of lost water was caused by spillage due to the bullet

bubbles (Fig. 6). So, the growth and breakup of the bullet

bubbles are the major reasons for the excessive boiling and

splitting during the vacuum cooling process.

3.2. Effect of volumetric displacement on the water lossrate during the vacuum cooling

One of the questions we sought to answer in this work was

whether controlling the volumetric displacement of vacuum

pump could help improve the bubble formation, thereby

reducing the boiling and spilling of water during vacuum

cooling process. Thus, we tested different volumetric dis-

placements of vacuum pump (0.002 m3 s�1, 0.0016 m3 s�1,

bles at slug flow stage in Scheme 1.

Fig. 6 e Rate of water loss at four different schemes used in

this study.

i n t e rn a t i o n a l j o u r n a l o f r e f r i g e r a t i o n 5 6 ( 2 0 1 5 ) 3 7e4 2 41

0.0012 m3 s�1 and 0.0008 m3 s�1 respectively) in the pressure

range of 10e2 kPa during the vacuum cooling.

Our results show that using a volumetric displacement of

0.0012 m3 s�1 reduced the water loss rate significantly

(p < 0.05) compared to 0.0016 m3 s�1 and 0.002 m3 s�1 (Fig. 6).

This was because of reducing volumetric displacement of

pump during the vacuum cooling process not only reduced

the accelerated velocity of pressure drop at the water surface

but also provided enough time to conduct heat from product

surface to its inside (Cheng and Hsueh, 2007; Song and Liu,

2014), thereby resulting in both the growth and breakup of

bullet bubbles. However, therewas no significant difference in

the weight loss rates with the volumetric displacements of

0.0012 m3 s�1 and 0.0008 m3 s�1 (p > 0.05; Fig. 7). To further

choose an optimal volumetric displacement from

0.0012 m3 s�1 to 0.0008 m3 s�1, the time to cool the point

located at 1 cm below the water surface from 32 to 10 �C was

used as the evaluation criteria. Fig. 7 shows that there was no

significant difference in the cooling times with volumetric

displacements of 0.002m3 s�1, 0.0016m3 s�1 and 0.0012m3 s�1

(p > 0.05). However, cooling time with a volumetric displace-

ment of 0.0008 m3 s�1 was too much longer than that with a

volumetric displacement of 0.0012 m3 s�1 (p < 0.05; Fig. 7).

Fig. 7 e Time to cool water surface from 32�C to 10

�C with

different schemes employed in this study (measurements

made at 1 cm below the water surface using a

thermocouple).

To summarize, the volumetric displacement of vacuum

pump of 0.0012 m3 s�1 was an optimal compromise proposal

for vacuum cooling the water on the precondition that the

volumetric displacement was controlled only in the pressure

range of 10e2 kPa in this paper.

4. Conclusions

We used a high speed camera to study the mechanism of split-

ting and excessive boiling of water during the vacuum cooling

process. The results show that the growth and behavior of

bubbles are the major reasons for the water loss during the

vacuumcooling. Fromour results, it appears that theboiling and

spillage of water in a vacuum cooling process occurs in two

stages: (1) boiling flow can become obvious so that the water

surface is sheared and destroyed by the bubbles; (2) then, the

growthandbreakupofbulletbubblesextrude itsabove liquidout

of the tube violently, which is the major reason resulting in the

excessive water loss. In order to weaken or avoid the excessive

splittingofwater,usinganoptimalvolumetricdisplacement (for

instance 0.0012 m3 s�1 in this study) during a certain pressure

range (for instance 10e2 kPa in this study) could be an efficient

way. During the practical immersion vacuum cooling process,

there always exist many food powders in the cooling pool.

Because thepresenceofporoussolidhavea significant influence

on the bubble formation, the mechanism of violently splitting

and excessive boiling of water in the presence of porous solid

should form the premise of future study.

Acknowledgements

This work was supported by the National Science-technology

Support Plan of China (2013BAD19B01) and China Postdoctoral

Science Foundation funded project (2014M561491).

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