A Laboratory Scale Liquid Nitrogen FreezerAuthor(s): T. R. GormleySource: Irish Journal of Agricultural Research, Vol. 14, No. 2 (Aug., 1975), pp. 214-217Published by: TEAGASC-Agriculture and Food Development AuthorityStable URL: http://www.jstor.org/stable/25555774 .

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214 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 14, NO. 2, 1975

REFERENCES

1. Anon., Food Processing Industry 40 (479): 51, 1971. 2. Lovelidge, B., The Grower 78 (6): 239, 1972. 3. Anon., The Grower 79 (2): 85, 1973. 4. Longmore, A. P., Comml Grow. No. 4078: 359, 1974.

Received January 29,1975

A LABORATORY SCALE LIQUID NITROGEN FREEZER

Abstract: A laboratory scale liquid nitrogen freezer for food and other biological samples is described. In the apparatus, liquid nitrogen is forced under pressure through a sprayhead fitted to a plastic pipe.

The sample for freezing is inserted into the pipe under the liquid nitrogen sprays. Freezing times for strawberries, mushrooms and tomato slices were 6, 4 and 2 min respectively with this system.

Introduction

Cryogenic freezing of food in liquids such as liquid nitrogen (LN) generally results in a high quality product due to the rapid rate of freezing (1). In commercial LN freezers the food is conveyed through a tunnel and meets progressively colder nitrogen gas which causes crust freezing. It finally passes under LN sprays which completely freeze the food.

Many laboratories cannot afford the high capital cost of an LN freezer and test

freezing must be done either by simply dipping the food directly in LN or by availing of a commercial LN freezer?if there is one in the locality. The practice of freezing by direct immersion in LN may be undesirable since it can cause the product to crack due to the cold shock. For these reasons it was decided to build a low-cost tunnel

system fitted with spraying devices for applying the LN. It should be stressed that this

system was not built to simulate a commercial LN freezer, but only as an improvement on the dipping technique. Use can also be made of the cold nitrogen gas (as a precool) with this system.

Apparatus

The system is shown in Fig. 1. The tunnel is a 3-m length of'Wavin' plastic pipe (diam. 230 mm) fitted with 13 mm wide aluminium rails which run the entire length ofthe tunnel and protrude at each end. The rails are joined by cross ties and will only fit in the pipe when inserted diametrically, i.e., the rails are almost 230 mm apart.

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COMMUNICATIONS TO THE EDITOR 215

PLAN .Clip 23cm i _

,.,.ijii.nirr-r-"- *-.- -L-.T-.T- j ,, M 3 ^MtBH^SSSsSsSSBSSa T * _ft t i? T t' 1 By???ll 'Ag,,, .? ?T3 *T2 ^

!> [ II II

^_3m__^ \ ^ IT" ' ~~~

Aluminium rails

T^ ,12mm i.d.^Allman

No.l spray nozzle /

Freezing tray Ti ^

^ { h Tl Rubber flaps. / J 'Jfllllw^

^^^^ "

N2gas =J =OPressure gauge ^"^ toP ELEVATION

/ \ |44cm / \ i

Tj?4 =4 thermocouple points

Liquid N2 / \ A container ?30cm

1 * 44cm

Fig.l : Diagrammatic representation of the freezing apparatus

The tunnel is slightly inclined to cause the cold nitrogen gas to flow from left to right. The freezing tray framework, made with 13-mm wide aluminium and covered with

stainless steel wire mesh, slides on the rails and is kept from falling off by the sides of

the tunnel. Two semicircular rubber flaps were fitted at each end of the tunnel to

restrict air movement, thereby minimising temperature fluctuations. Thermocouple

attachment points were drilled at four places along the length of the tunnel (0.44,1.00, 1.56 and 2.96 m from the right hand side) and are referred to as thermocouples 1, 2, 3

and 4 respectively. The copper-constantan wires were connected to a Honeywell

multipoint recorder calibrated in the range plus 50? to minus 50?C. The sprayhead consists of 5 x No. 1 brass Allman spray nozzles fitted to a 12-mm

diameter copper tube. The tube is held in position by two clips attached to the tunnel

and the five nozzles are accommodated through five holes drilled in the tunnel. Each

nozzle gives a fan shaped spray of LN across the diameter of the tunnel. The spray head is attached by a flexible copper hose (20 mm external diameter) to a Cryovac LN

container. There is a T junction at the mouth of the container consisting of a thread

which fits into the neck of the LN container, a stainless steel pipe which takes LN

from the bottom of the container, a pressure gauge, and an entry point for nitrogen

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216 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 14, NO. 2, 1975

gas. The rate of consumption of LN was followed by placing the LN tank on a Berkel scales during an actual run.

Operation and performance It is important to prepare all the material for freezing before starting up the system

in order to avoid wastage of LN. Operation is commenced by pressurising the LN in the tank (20,684 Nm~ i)* with nitrogen gas from a cylinder. This forces the LN up the tube and out through the spray nozzles into the tunnel. This operating pressure is

maintained throughout the run. The rate of temperature drop in the tunnel at the four thermocouple points is shown in Fig. 2. It was fastest in the vicinity of the spray nozzles (thermocouples 3 and 4). After 10 min the tunnel is sufficiently cold and food

samples can be introduced. The freezing tray will accommodate 1 kg of produce as a

single layer. The food tray plus food can be moved to and fro under LN sprays by means of a copper rod attached to the tray. Alternatively, the tray can be introduced to the tunnel at the right hand side (Fig. 1) and pulled against cold nitrogen gas and

finally under the spray nozzles.

Thermocouple

20-^x^a a 1

-1"1 * i * i i \ "' '

10 20 30 40

Time (min)

Fig. 2 : Rate of temperature drop at four points in the tunnel

3 psi

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COMMUNICATIONS TO THE EDITOR 217

TABLE 1: Freezing time and drip loss for strawberries, mushrooms and tomato slices frozen in liquid nitrogen (LN)

Freezing time Drip loss (%) Drip loss (%) Produce (min)%* LN LN Blast freezing

Strawberries (Cambridge Vigour) 6 29 41 Mushrooms 4 5 27 Tomato slices 2 22 ?

J Temperature change +18* to ~50?C b Product moved to and fro under sprays

Table 1 shows the results for freezing time and drip loss for strawberries, mush rooms and tomato slices (1 kg samples) frozen in LN. Comparative values for blast

freezing are also shown. As expected, drip loss was lower for the LN frozen samples and the appearance of the products after thawing was good; this applied especially to the tomato slices.

The rate of use of LN was constant throughout the freezing run at 0.45 kg/min. Since the LN tank contains 20 kg the freezing tunnel can be kept in operation for 45

min with one charge of LN. If account is taken ofthe 10 min cooling down time, this

leaves 35 min of actual freezing time which represents about 6 kg of strawberries or

9 kg of mushrooms or 18 kg of tomatoes based on the data in Table 1. Two freezing trays are required for this throughput. When one is removed from the tunnel the other is already loaded with product and can be put in without delay.

Conclusion

The freezing tunnel proved satisfactory under the operating conditions tested and

has applications for freezing test samples of food or other biological materials.

Acknowledgment The author is grateful to Mr. Sean Egan for technical assistance.

T. R. Gormley

An Foras Taluntais, Kinsealy Research Centre, Malahide Road, Dublin 5

REFERENCE

1. Gormley, T. R., Technology Ireland 3 (8): 13,1971.

Received January 29,1975

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