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882 JOURNAL OF FOOD SCIENCEVolume 63, No. 5, 1998

Heat Distribution in Industrial-scaleWater Cascading (Rotary) Retort

C. SMOUT, A. VAN LOEY and M. HENDRICKX

ABSTRACT

A heat distribution study was performed in an industrial-scale4-basket water cascading rotary retort under fully loadedoperating conditions. Detailed analysis of the temperaturedistribution basket-by-basket was followed by an evaluationof the temperature distribution throughout the retort (at 0, 4and 7 rpm). Finally, a heating rate distribution study, usingsilicone elastomer food-simulants, was carried out to evalu-ate heat transfer uniformity throughout the retort. Resultsrevealed that for static processes the slowest heating zone

was located at the bottom of the basket at the back of theretort, whereas for rotary processes the center of this bas-ket was heated slowest. Heating rate distribution results in-dicated uniform heat transfer throughout the retort.

Key Words: water cascading retort, heat distribution, tem-perature, heating rate

INTRODUCTIONSTEAM-HEATEDWATERWITHAIR-OVERPRESSUREANDSTEAM/AIR

mixtures are commonly used to sterilize food in glass containers and

in flexible and semi-rigid plastic packages. The air-overpressure is

used to promote uniform heat transfer and protect package integrity.

However, such heating media exhibit lower surface heat transfer co-

efficients than saturated steam (Adams et al., 1983; Ramaswamy et

al., 1983; Tung et al., 1984).

Thermal processing of foods should result in safe and high-qual-

ity products uniformly throughout the retort. High quality retention

is sought by applying minimum lethality values to guarantee prod-

uct safety. Heating medium agitation (circulation), container layout,

can rotation etc. will influence the heat distribution in a retort with

direct effects on product safety and quality.

Heat distribution and heat penetration studies are part of food

process design. Heat distribution studies are performed to ensure

adequate lethality at any point, especially to identify the position of

lowest lethality. Heat penetration data must be collected for a prod-

uct positioned in the least heated zone of the retort. Such experi-

ments can be carried out on a pilot retort if the least heated zone of

the production retort can be simulated.If the initial conditions (e.g. product and retort temperature) are

uniform, the accomplished lethality in the product after processing

is dependent only on the retort temperature and heat transfer to and

within the product. If temperature and heat transfer rates at any point

within the retort are uniform, then all products processed in the re-

tort would receive the same degree of lethality. Such condition is not

achievable in practice. Therefore, a complete heat distribution deter-

mination, including a temperature distribution study followed by

assessment of uniformity of heat transfer to the containers, is re-

quired. Differences in product heating as a result of processing con-

ditions (heating medium) can be determined by measuring tempera-

tures inside food products and calculating the distribution of heating

rates (fh-values) among containers processed.

It is a common food industry practice to limit heat distribution

studies to the measurement of temperature distribution of the heat-

ing medium (Adams and Hardt-English, 1990; Park et al., 1990).

Published studies (Tung et al., 1989; Tung et al., 1990; Ramaswamy

et al., 1991; Campbell and Ramaswamy, 1992; Tung and Britt, 1992)

have indicated that for air-overpressure retorts, heating medium tem-

perature measurements alone were not sufficient to assure consis-

tent heating of products at all points. Analysis of food product heat-

ing rate or lethality distribution among processed containers may be

required. Nevertheless, temperature uniformity remains an impor-

tant and necessary criterion for evaluation of retort performance (Ra-

maswamy et al., 1991).

Our objective was to evaluate heat delivery to containers inside

an industrial-scale 4-basket water cascading rotary retort (operating

in both static and rotary modes) by measuring heating medium tem-

perature and simulated food product heating rate distributions.

MATERIALS & METHODS

Retort system

Heat distribution was evaluated in a 1400 mm diameter industri-

al-scale 4-basket water cascading rotary retort (Barriquand Steriflow

retort, France) (Fig. 1). Water cascading retorts are heated by a sprayof superheated water showered on top layers and flowing from top

to bottom. From the bottom, the water is recycled passing through a

steam-supplied heat exchanger (at the back of the retort). For cool-

ing the heat exchanger is supplied with cold water instead of steam.

Pressure is regulated by introduction of pressurized air. The retort

process is controlled by a microprocessor, which independently con-

trols time, temperature and pressure.

The retort was fully loaded with 400 mL cans (300405), filled

with water in order to generate a high heat demand during the retort

come up. Each basket (width height depth = 0.80 0.80 0.85m)

contained 7 layers of 110 containers, separated by perforated pads

(thickness = 4 mm, cross sectional open area = 35%).

JOURNAL OF FOOD SCIENCE

ENGINEERING/PROCESSING

The authors are affiliated with the Laboratory of Food Technology, Dept. of Food &

Microbial Technology, Faculty of Agricultural & Applied Biological Sciences,Katholieke Universiteit Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Bel-gium. Address inquiries to Dr. M. Hendrickx.

Fig. 1Layout of the 4-basket water cascading rotary retort (A =heat exchanger; B = water inlet; C = water outlet; D = pump; E = airinlet).


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Volume 63, No. 5, 1998JOURNAL OF FOOD SCIENCE 883

Product-simulants

To measure heat transfer into any container within a retort, prod-

uct-simulants with thermal properties similar to foods were used in

order to minimize variations in thermal properties within containers

or changes in heating characteristics with repeated use. In order to

minimize any change in the flow profile of the heating medium, prod-

uct-simulants with the same dimensions and shape as that of the prod-

uct containers were developed. Silicone elastomer (Sylgard 170, Dow

Corning, CCMP, Belgium) cylinders were constructed to simulate

strict conduction-heating products.

The simulants were calibrated three times in a pilot steam heated

retort to determine the heating response under conditions of near

infinite surface heat transfer coefficients. The temperature was mon-

itored in the center of each cylinder and the location of each cylinder

in the retort was changed for each run. The calibration indicated no

difference (p>0.05) in heating rates (fh-values) between calibration

runs, but a difference (p


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ginning of holding and the temperature difference between the high-

est-reading and lowest-reading thermocouple at the beginning of

holding, and after 1, 3 and 5 min. holding were summarized (Table

2). Standard deviations and temperature differences were largest in

basket 1 for static and rotary processes. The deviation and difference

values for basket 2, basket 3 and basket 4 were slight.The reported temperature distribution in one basket of a 4-basket

water cascading static retort (1300 mm diameter, loaded with 211212

cans filled with water; Adams and Hardt-English, 1990), resulted in

temperature differences between the highest-reading and lowest-read-

ing thermocouple of 2.2C during the first minute, 1.0C by the third

minute and 0.8C by the fifth minute. With the loading configuration

and under the processing conditions we used, the temperature distri-

bution of basket 1 confirmed their reported results. The temperature

distribution of the other baskets was more uniform.

Temperature distribution throughout the retort

The temperature-time data for the retort indicated that for static

processes, the thermocouple at the bottom of basket 1 (back of the

retort) was heating (and cooling) slowest. For rotary processes the

thermocouple in the center of basket 1 was heating (and cooling) the

slowest. These results seemed to reflect the water and/or air inlet.

Superheated water was brought into the retort through a distribution

manifold near basket 1 in the direction of the front of the retort.

Because of this arrangement basket 1 could partly miss the water

spray. Cold air was also injected into the retort near basket 1.

The standard deviations of temperature for all thermocouple lo-

cations and the spread of the thermocouple readings at each time

interval were used to evaluate the temperature distribution through-

out the retort. The standard deviation at the beginning of holding

and the temperature difference between the highest-reading and low-est-reading thermocouple at the beginning of holding, and after 1, 3

and 5 min. holding were compared (Table 3). Rotation slightly im-

proved temperature distribution: the standard deviations and tem-

perature differences at the beginning of the holding phase were not

different (p>0.05).

We concluded that the standard deviation and temperature differ-

ences calculated for basket 1 (Table 2) were the same order of mag-

nitude as the standard deviation and temperature differences calcu-

lated for the whole retort (Table 3).

NFPA (1985) advises that all points in a steam retort should be at

or above the desired process temperature within one minute after the

retort reaches the process temperature and the holding phase is start-

ed. In addition, all thermocouple readings after the first minute should

have a maximum range 1.7C and should be within 0.8C of thereference temperature device. The loading configuration and the dif-

Hea t D i s t r i b u t i o n S tudy i n a Cascad i ng Re t o r t . . .

Table 2 Temperature distribution basket by basket

Std dev T T after T after T afterbegin HT begin HT 1 min. HT 3 min. HT 5 min. HT

Basket (C) (C) (C) (C) (C)

0 rpm 1 0.84 3.4 2.5 1.0 0.72 0.18 1.0 1.0 0.5 0.83 0.26 1.0 0.5 0.5 0.64 0.13 0.5 0.6 0.6 0.6

7 rpm 1 0.69 2.5 1.9 0.9 0.52 0.25 1.1 0.8 0.6 0.6

3 0.28 1.0 0.8 0.8 0.54 0.26 1.1 0.9 0.4 0.4

aStandard deviation (std dev) at the beginning of holding time (HT) and temperaturedifference ( T) between highest-reading and lowest-reading thermocouple at thebeginning of holding, and after 1, 3 and 5 min holding time.

Table 3 Temperature distribution throughout the retort

Std dev T T after T after T afterbegin HT begin HT 1 min HT 3 min HT 5 min HT

Run (C) (C) (C) (C) (C)

0 rpm 1 0.83 3.8 2.5 1.3 0.72 0.91 4.1 2.6 1.2 0.63 0.98 4.6 3.0 1.6 0.8

4 rpm 1 0.76 3.2 3.0 1.5 0.82 0.66 2.7 2.3 1.9 1.1

3 0.77 3.1 2.0 1.6 0.77 rpm 1 0.76 2.9 1.9 0.9 0.42 0.76 3.2 2.0 0.7 0.33 0.76 3.0 2.4 1.3 0.5

aStandard deviation (std dev) at the beginning of holding time (HT) and temperaturedifference ( T) between highest-reading and lowest-reading thermocouple at thebeginning of holding, and after 1, 3 and 5 min. holding time.

Fig. 3Thermocouple (and simulant) layout for (a) temperature distribution test and (b) heating rate distribution test throughout theretort. Side-view of positioning of baskets in the retort.


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Volume 63, No. 5, 1998JOURNAL OF FOOD SCIENCE 885


ferent processing conditions used in the test showed that tempera-

tures of the retort did not meet NFPA recommendations.

For each thermocouple location, the mean and standard devia-

tion of the temperature during holding were calculated to enable de-

tection of cold spots and measurement of temperature stability at

each location. For static processes (Fig. 4a) the coldest zone was

situated at position 1, the center of the bottom layer of basket 1. For

rotary processes (Fig. 4b) the coldest zone was situated at position 2,

the center of the middle layer of basket 1. This trend was exhibited

for all runs. The temperature stability seemed to be better (smaller

standard deviations) for the static process than for the rotary pro-

cess. This could be attributed to small temperature fluctuations at

each thermocouple location as a result of periodical interruptions of

the continuous water flow by rotation.

The spread of mean temperatures at the different locations dur-

ing holding (15 min) and the overall mean temperatures and stan-

dard deviations during holding were compared (Table 4). The spread

varied between 0.5C and 1.2C and the overall standard deviation

ranged from 0.14C to 0.32C.

A temperature distribution study was reported for a standard 1-

basket (0.810.810.81m) water cascading retort (Ramaswamy et

al., 1991). The maximum spread of mean temperatures at various

locations during holding (27 min. at 121C) was 0.6C. The maxi-

mum overall standard deviation in retort temperature during holdingwas 0.71C.

Heating rate distribution throughout the retort

The resulting fh-values at each position for each experimental

run were calculated and compared (Table 5). The fh,HD/fh,calibration-

values were only slightly higher than 1, indicating that the fh-values

in a water cascading retort were only slightly higher than those for a

condensing steam retort. There were almost no limiting heat transfer

conditions when superheated water was used as heating medium.

The difference in fh-values was


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an appropriate measure to detect the lowest lethality zone. These

results describe a protocol for testing retort systems to assure mini-

mum lethality. Although such data may vary from one retort to an-

other, it provides a guide as to where to expect the slowest heating

zones and the extent of variations that might be expected.

REFERENCESAdams, H.W. and Hardt-English, P.K. 1990. Determining temperature distribution in

cascading water retorts. Food Technol. 44(12): 110-112.Adams, J.P., Peterson, W.R. and Otwell, W.S. 1983. Processing of seafood institution-

al-sized retort pouches. Food Technol. 37(4): 123-127.Campbell, S. and Ramaswamy, H.S. 1992. Distribution of heat transfer rate and lethal-

ity in a single basket water cascade retort. J. Food Process Eng. 15: 31-48.NFPA. 1985. Guidelines for thermal process development for foods packaged in flex-

ible containers. National Food Processors Association, Washington, DC.Park, D.J., Cabes, L.J. Jr. and Collins, K.M. 1990. Determining temperature distribu-

tion in rotary, full-immersion, hot-water sterilizers. Food Technol. 44(12): 113-118.

Ramaswamy, H.S., Tung, M.A. and Stark, R. 1983. A method to measure surface heattransfer from steam/air mixtures in batch retorts. J. Food Sci. 48: 900-904.

Ramaswamy, H., Campbell, S. and Passey, C. 1991. Temperature distribution in a stan-dard 1-basket water-cascade retort. Can. Inst. Food Sci. Technol. J. 24(1/2): 19-26.

Tung, M.A. and Britt, I.J. 1992. Heat transfer efficacy of overpressure media in rota-tional thermal processes for shelf stable foods in plastic packages. Ch. 12 in Ad-vances in Food Engineering, R.P. Singh and M.A. Wirakartakusumah (Ed.), p. 207-220. CRC Press, Boca Raton, Ann Arbor, London and Tokyo.

Tung, M.A., Ramaswamy, H.S., Smith, T. and Stark, R. 1984. Surface heat transfercoefficients for steam/air mixtures in two pilot scale retorts. J. Food Sci. 49: 939-943.

Tung, M.A., Morello, G.F. and Ramaswamy, H.S. 1989. Food properties, heat transferconditions and sterilization considerations in retort processes. In Food Properties

and Computer-aided Engineering of Food Processing Systems, R.P. Singh and A.G.Medina (Ed.), p. 49-70. Kluwer Academic Publishers.

Tung, M.A., Britt, I.J. and Ramaswamy, H.S. 1990. Food sterilization in steam/airretorts. Food Technol. 44(12): 105-109.

Ms received 7/29/97; revised 3/30/98; accepted 4/23/98.

This research has been supported by the European Commission, Project AIR2-CT94-1017.

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