Geometry, cooling rates and weight losses during pig chilling

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UETTE_RWQRTH I N E M A N

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0140-7007(95)00039-9

Int. J. RejHg. Vol. 18, No. 7, pp. 456- 464, 1995 Copyright ~; 1995 Elsevier Science Ltd and IIR Printed in Great Britain. All rights reserved 0140-7007/95/$10.00 +.00

Geometry, cooling rates and weight losses during pig chilling S. Coulter and Q. T. Pham School o f Chemical Engineering and Industrial Chemistry, University o f New South Wales, Sydney, Australia 2052

I. McNeil F o r m e r l y o f C S I R O M e a t Research Laboratory, C a n n o n Hill, Queensland, Australia 4170

N. G. McPhail Australian Meat Technology Pty Ltd, C a n n o n Hill, Queensland, Australia 4170 Received 19 July 1994; revised 15 April 1995 Experiments were carried out to study the cooling rate and weight loss from whole, split and quartered pig carcasses under typical Australian air-chilling conditions. Simple geometric and heat and mass transfer models were compared with data. The cooling rate was primarily influencedby the type of carcass (whole, split or quartered), which was in turn determined by carcass weight and fatness. A simple geometricmodel, using a sphere, underpredicted the cooling times with an average error of 6%. There were no statistically significant variations in weight loss due to air velocity. (Keywords: pig; chilling; cooling; weight loss; geometry; modelling)

Geometrie, taux de refroidissement et pertes de masse lors du refroidissement de la viande porcine On a effectu~ des exp#riences pour ~tudier les taux de refroidissement et de perte de masse dans les carcasses de porcs entibres, en moiti~s et en quarts sous les conditions de refroidissement ~ l'air qui sont typiques de l'Australie. On a compar~ les donnOes obtenues avec les pr~visions de simples modbles g~om~triques et de transfert de chaleur et de masse. C'est le type de carcasse (entibre, en moitids ou en quarts) qui a eu le plus grand effet sur le taux de refroidissement; le type de carcasse a Ot~ d#termin~, ~ son tour, par sa masse et par son niveau de graisse. Un simpl# modble g~ometrique h base d'une sphbre a sous-estim~ les temps de refroidissement en moyenne de 6%o. On n "a constat~ aucune variation significative de perte de masse provoqu~e par la vitesse de l'air. (Mots cl~s: pore; rrfrigrration; refroidissement; perte de masse; g~om&rie; simulation)

Meat chilling should be carried out under conditions that minimize microbial activity and evaporative losses, while avoiding excessive cooling rates, which are conducive to high energy consumption and meat toughening. It is essential that the time-temperature curve complies with regulations. For example, European Community regulations require the carcass temperature to fall below 7 °C prior to transportation. To design chilling processes that satisfy the above criteria, we must have accurate data in order to be able to predict the heat transfer rate under various conditions. Currently, there are few data relating air temperature and velocity to chilling for pig carcasses. This can lead to poor refrigeration system design and under- or overchilling. As refrigeration systems and refrigerated * To whom all correspondence should be addressed.

facilities can constitute up to 70% of the capital and electricity consumption in an abattoir, the cost of not using optimal conditions can be considerable. Pig carcasses undergo a processing regime very different from that of beef and lamb carcasses. In particular, they are scalded in water at 60-63 °C for several minutes then dehaired, and may be singed in a flame before being eviscerated, instead of being skinned. The present investigation aims to (a) determine geometric data for the leg, shoulder and loin sections of a pig carcass that could be later used in a finite difference program to calculate product heat transfer, (b) determine weight loss and temperature-time curves for various air velocities, and (c) check whether timetemperature changes could be predicted using a simplified numerical model.

456

Geometry, cooling rates and weight losses during pig chilling

457

Nomenclature

Cp F h H k kg p q T t V W x

Specific heat (J kg- I K - I ) Fat thickness at P2 site Surface heat transfer coefficient (W m -2 K) Air enthalpy (J kg i air) Thermal conductivity (W m- 1 K - 1) Mass transfer coefficient (kg s-1 m -2 Pa -l) Partial pressure of water vapour (Pa) Heat flux ( W m -2) Temperature Time (s) Air velocity (m s-l) Weight (kg) Space coordinate (m)

Previous work Carcass chill&g rates

Carcasses entering chillers after slaughter and evisceration have been reported 1 as having body temperatures in the range 32.8-41.1°C, with surface temperatures varying between 22.2 and 28.9 °C. However, it is not until 1 - 2 h post mortem that the bodies reach their highest temperatures due to metabolic activities 2, rises of up to 3 °C on post-mortem temperatures being reported for pork 3. Conventional batch chilling is performed at temperatures of 0-4 °C and air velocities of up to 3 m s -1 . During loading, the air temperature can rise to 19°C before being pulled down to the design temperatures 4'5. Experimental measurements of carcass chilling rates have been carried out on pork 1'4 6' , goat 7 and beef 8 9' . The effect of air velocity on the rate of chilling is most prominent at low velocities ( 0 - 1 m s - l ) . At higher velocities, the decrease in chilling time associated with the increase in air velocity is probably not justifiable in most practical situations 9. James and Bailey9 also stated that increased fat cover decreases beef chilling rates (over a limited range of carcasses: 140kg, in air at 0°C and 0.5ms I). Brown and James l° give average cooling curves and upper approximate limits as to the weights of pig that can be effectively chilled to 7 °C within 16 h. Weight losses

For beef carcasses, an increase in air velocity from 0.75 to 3 m s -1 can increase weight loss by up to 0.2% of the total weight 9 (a typical weight loss is 2-2.5% of total body weight) when cooled for 18 h, but when a specific target meat temperature is required, there is no significant difference in weight loss (<0.1% for cooling to deep leg temperature of 10 °C, air velocities of 0 . 5 1 3.0 m s- ). Similarly, air temperature does not significantly influence weight loss when chilling to a set deep leg temperature, but weight loss increases as temperature decreases if chilling for a set time. For goat, there is no significant difference in weight loss 7 over a period of 23 h for velocities varying from 0.5 to 3.0ms ~. Weight losses in pork carcasses under conventional chilling regimes have been reported as being between 1.9

Y

Absolute humidity (kg water vapour/kg dry air)

Greek letters e Emissivity ~r Stefan-Boltzmann constant = 5.667 × 10-8Wm-2 K -4 T20 Time for the difference between deep leg and air temperatures to fall to 20% of initial value (h) Subscripts a Ambient conditions s Saturation value

and 2.1%, depending on how the carcass is cut before entering the chiller 1°'tl (initial weighing 30-50 min after slaughter, final weighing 14-24h). Air temperatures were 0-4 °C and air velocities 0.25-0.5 m s -~ . Modelling of temperature changes

Pflug et al. 12 presented an approximate analytical method for developing time-temperature cooling curves for spheres, infinite cylinders and infinite slabs/ plates. These were based on the exact solutions of heat transfer equations and the f and j factors developed by Ball 13. Ramaswamy 14 gives approximate equations for determining transient temperatures for foods (infinite slabs/ plates, infinite cylinders and spheres) undergoing convective heat transfer. His equations are applicable in the range 0.02 < Bi < 200, Fo < 0.2. Various approximate methods have been developed to calculate centre and mean temperature vs time for more complex shapes, such as Lin et al.'s 15. Finite difference schemes have been successfully used to model heat transfer processes in meat carcasses. Lovett 16 used Lee's three-level method of solving finite difference equations to determine the chilling rate of beef. A slab was used to model the side, with thickness a function of carcass weight. Calculated deep butt temperatures are within 3 °C of measured temperatures for an air velocity of 0.5 m s-l . Rapid changes in thermal properties and large time steps can give inaccurate predictions. When sub-zero temperatures are used, the problem of stepping over the latent heat peak around the freezing point can be avoided by using Pham's 17 enthalpy-temperature correction technique in the finite difference calculation. Pham et al. 18 monitored spot temperatures (deep leg and shoulder) in beef carcasses and compared the results with a finite difference program. The predictions were reasonable for deep leg temperatures, but surface temperature predictions were less satisfactory, owing to the geometric simplification of carcass parts to a sphere (leg) and slab (shoulder). Finite element methods (FEM) may perform better than finite difference methods for irregular geometries, complex boundary conditions and heterogeneous materials. However, FEMs are computationally

458

S. Coulter et al.

expensive. A modified FEM technique was used by Pham et al. 19 to determine approximate heat transfer coefficients of lamb loins.

and that fat has a lower emissivity than meat. The average emissivity at 9-16 °C was 0.92. However, Sevsik and Sunderland 36 obtained a value for emissivity of 0.8.

Modelling weight losses

Levy2°'2~ attempted to model the weight loss from a split carcass by assuming it had a simple shape (a slab of rectangular cross-section). The mean thickness was a function of the largest thickness of the carcass or the dressed weight. He developed a relationship between the evaporation rate and the combined heat flow due to convection, radiation and evaporation. From this, the actual mass of water lost could be calculated. He also introduced an evaporation resistance coefficient, which acts to lower the heat transfer if the surface is not liquid and so does not allow free evaporation. Radford et al. 22 proposed a model based on meat slabs consisting of a series of thin layers. Evaporation and heat transfer by convection occur only on the outermost layer with water brought to the surface by Fickian diffusion. Some parameters used in the heat and mass transfer equations were determined experimentally. Their model was capable of simulating results similar to those obtained for meat in a wind tunnel. Califano and Calvelo23 developed an analytical lumped parameter model, independent of carcass shape and time taken for cooling. It allows for internal temperature gradients and various percentages of meat, fat and bone. However, it assumes a constant air temperature. When compared with ovine and bovine experimental data there was, on average, a difference of 15% between the theoretical and experimental weight loss. Similar differences were obtained by Gac and Tuplin 24. Pham ~5 developed these psychrometric principles further, developing a model for maximum percentage weight loss. It is based on the difference in the air enthalpy and absolute humidity between the beginning and conclusion of the process and the specific heat of the product. Results can be obtained using psychrometric charts or by integration. The method assumes that the product temperature is initially uniform and that the surface is covered by ice or water. For lamb and beef there was a mean difference of 4 and 15% respectively between experimental and calculated results. The larger error for beef could be attributed to the longer times for chilling and the higher Biot number. Thermal properties

Morley26'27 reviewed thermal property data for meat, fat and bone. Hill et al. 28 reviewed thermal conductivity data. Pham 29 compared various methods for calculating the thermal conductivity of meat from composition and found that the best were Levy, s30 model and the effective medium theory or EMT model 3~. The Levy model was more accurate than the EMT model, but the latter had a better physical justification. Calorimetric properties of various types of meat and fat have been experimentally determined by Riede132, Fleming33 and Pham et al. ~ among others. The surface emissivity of carcasses has been studied by Gurrassimov and Rumyantsev35. They found that the lower the surface temperature, the higher the emissivity,

Theory

Cooling rates

Heat conduction is governed by Fourier's Law: p C p ~ t = V(kVT)

(1)

which for one-dimensional geometries can be written OT

1 0 {x"k OT)

pCp Ot - x" Ox \

-O~x]

(2)

(n = 0 for slabs, 1 for infinite cylinders, 2 for spheres.) To solve the equation, the boundary conditions must be known. For food chilling this is usually a Newtoniantype condition, where the ambient temperature and surface heat transfer coefficients are specified. The latter is found by combining the effect of radiation, forced convection, free convection, evaporative heat transfer, and the thermal resistance of any wrapping: 1

1

hTot -- hRad + hconv + hEvap ~-Rwrap

(3)

In the above equation, hconv represents the combined effect of free and forced convection. Equations for each type of convection are readily available from standard texts 36'37, at least for simple geometries such as plates, cylinders and spheres. The following equations will be used in the carcass chilling model of this work: Free convection: NUFree = 0.555(Pr.Gr) 1/4

(4)

the Grashoff number being based on the length of the carcass, estimated at about 1.5 m. Forced convection: NUForced = 0.22Re °'6

(5)

The effect of combined free and forced convection in the regions of interest to meat chilling is much less well documented. The following equation is often used for combined free and forced convection situations39: /,3 /1/3 hconv = (h3ree Jr-"Forced] (6) The above equations are only approximate. Equation (5) does not take into account the precise shape of the carcass, although trial calculations using different idealized shapes showed that the effect of shape is negligible (at 1 m s-1 , a sphere correlation and a cylinder in cross-flow correlation predict heat transfer coefficient values that differ by only 2%). The ensuing errors are likely to be swamped by even more drastic simplifications in the representation of the pigs by very simple shapes (such as cylinders and spheres), as well as by the uncertainty of the effect of evaporation (see below). Radiation heat transfer can be determined from Kirchoff's law: q = a 8 A ( T 4 - T 4)

(7)

459

Geometry, cooling rates and weight losses during pig chilling from which the radiative heat transfer coefficient hRa d can be determined: hRad =- (re(ra2 + rs2)(ra + rs) ~ 4aer3m

(S)

The effect of evaporative heat transfer is difficult to determine because it varies considerably during chiUing 4°. Initially, the meat surface is saturated and hot, so there is a high evaporative heat loss component. The meat surface then dries off as moisture movement from the inside is unable to keep up with evaporation. As the surface temperature nears the air's wet bulb temperature, the evaporation rate slows down and the surface may re-wet, at least for some time. The driving force in evaporation is the water partial pressure difference between the surface and the surrounding air. The mass transfer rate (water loss) can be written as dm 0--7 = Akg(psAws - Pa)

approach using a simple geometric model will be followed, as a reasonable compromise between flexibility and ease of implementation.

Weight loss The mass transfer equation, Equation (9), can be solved together with the heat transfer equation to yield the weight loss, provided kg, Aws, Pa and complete geometric data are known. For complex geometries a rigorous solution is difficult to obtain. Pham z5 showed that an estimate of the maximum weight loss for a chilling process can be obtained by assuming a completely wet product surface and uniform product temperature: Aw"'2 -- 1.1 [ 2Ys - Y

m

Jl Hs

-~cpdT~

(10)

(9)

where Aws is a factor to account for incompletely wetted surface 41.

Assuming that the surface heat transfer coefficient, htot, c a n be determined, the heat conduction equation can be solved by various methods: 1. Analytical methods42: used for simple geometries and constant ambient conditions only. 2. Analytical methods in combination with empirical shape factor15: applicable to a variety of shapes as long as ambient conditions are reasonably constant. 3. Finite difference methods: although this method can be adapted to complex geometries by means of coordinate transformation, they are normally used in conjunction with a simple geometric model. Their big advantage is the ability to handle variable ambient conditions. 4. Finite element methods: these methods are very flexible and can deal with composite materials, arbitrary geometries and variable ambient conditions. They are, however, costly to implement.

Materials and method

Sixty carcasses (mostly Large White X Landrace) were cooled in a chiller of a commercial abattoir, specially modified for the experiments. The air flowed vertically down over the carcasses through gaps in a plastic false ceiling above the rails (Figure 1). The fan speed could be adjusted manually or by a programmable logic controller. The carcasses were removed from the production line approximately 30 min after slaughter. They were of three types: whole (young pigs or 'porkers'), split (mediumsized pigs or 'baconers') and quartered (older pigs used for smallgoods and sausage production).

In the context of animal chilling, the complex geometry and composition mean that only the finite element method can hope to yield accurate answers 19. In the modelling part of this work, however, the finite difference

Slotted

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3u I d c r -

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~surements

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Figure I Experimental set-up Figure 1 Montage experimental

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Figure 2 Geometric measurements Figure 2 Mesures g~omdtriques

460

S. Coulter et al.

Each carcass was weighed on entering the chiller using a Sensortronics model 60048- I k-0108 load cell connected to a Toledo Electronic Digital Indicator with a maximum resolution o f + 3 g, a display error of up to 0.07% and a calibration error of 1.3% (over a range o f 50kg). Calibration tests in the laboratory were conducted with a resolution o f 20g, but for the work situations a resolution o f 50 g was used, owing to the effect o f vibrations. The carcasses were then weighed hourly (on the spot) for the following 4 h, where possible. The animals were chilled for a minimum of 16 h, with all carcasses weighed at the experiment's conclusion. Air and carcass temperatures were measured with type T (copper-constantan) thermocouples and recorded every 15min using a Datataker-500 datalogger. The thermocouples had a reading error of + 0.5 °C. Room temperature and temperatures above the carcasses were monitored. The air temperatures were approximately +5 °C in 32 tests and - 5 °C in 28 tests. The thermocouples that were used to measure carcass temperature were inserted using a stainless steel probe. The leg temperatures were measured at the largest leg diameter, as visually determined. The deep leg temperature was measured at half the diameter's distance into the leg, and the halfway temperature at a quarter of the diameter. The surface temperature was measured between the skin and subcutaneous fat layer. Air velocities were measured manually using a handheld TSI Velocicalc constant-temperature hot-wire anemometer. It had a maximum error of + 3.5% of the reading as calibrated by the manufacturer, and had temperature compensation. Velocities were measured 10 mm above and parallel to the carcass surface, at the Table 1

leg, loin and shoulder near the thermocouple insertion points. The readings were averaged automatically over a period of 1 rain. Although the anemometer was held by hand during the measurement, it is unlikely that shaking would have a significant effect, as velocities as low as 0.1 m s -1 were read. On the other hand, it is not known whether the low temperature could have introduced a significant error, in spite of the temperature-compensation mechanism. Geometric measurements of the carcasses were at locations shown in Figure 2. The locations of the measurements were all determined visually, and the dimensions determined with callipers. The surface fat layer of the carcasses was measured at the loin, shoulder and largest leg diameter sites using an Introscope (which shows the boundary between the fat and muscle layers) near the thermocouple insertion points. Results

Geometry Using a statistical analysis package, regression equations were found to correlate the important dimensions of the leg, loin and shoulder sections to carcass weight (IV) and fat thickness or P2 measurement (F). All dimensions are in mm. Measurement locations were determined visually. The following regression equations were obtained. The leg can be modelled as two truncated elliptical cones with a common base, with total length L = 37.3 W°'271F0"°57 R 2 = 43.2%

re0 for all experiments (first column of each cell contains weights in kg, second column r20 in hours)

Tableau 1

T20 pour toutes les experiences (la premikre colonne contient les masses en kg," la seconde colonne contient le 7-20en heures)

(a) Air temperature = +5 °C Air velocity groupings Carcass

0 . 1 0 - 0 . 2 9 m s -I

0.30-0.65 m s -l

0 . 8 0 - 1 . 3 0 m s -1

> 1.30ms -j

Split

50.30 53.60 61.95 62.10 70.85 76.05

7.00 7.50 8.50 9.00 9.25 8.75

70.75 70.85 73.00 76.80 82.70

9.75 7.25 9.00 9.00 11.50

54.05 61.85 73.15 74.65 80.45

6.50 7.50 8.00 8.50 8.00

64.95 68.25 73.60 78.60 82.65 83.10

9.00 7.75 8.00 7.50 8.25 11.50

Whole

59.85

8.50

44.55 45.25

6.00 7.25

79.80

9.25

72.00

9.50

Quartered

106.9

12.75

130.0

12.25

134.3

8.75

145.8 167.6

10.75 9.75

(b) Air temperature = - 5 °C Air velocity groupings Carcass

0 . 1 0 - 0 . 2 9 m s -l

0.30-0.65 m s -1

0.80-1.30m s -1

> 1.30ms -l

Split

65.50 66.75 77.05 79.30

10.00 11.50 10.00 11.00

68.20 61.25 77.35 73.60

8.50 7.25 9.75 8.25

55.10 69.20 78.45 73.45

7.00 9.00 9.00 9.25

61.75 63.9 73.15 79.10

7.75 7.25 8.25 9.00

Whole

43.80

8.50

42.95

8.50

44.55

6.50

46.90

7.25

Quartered

147.6

15.50

15.45

175.3 124.8

12.75 10.50

130.1

12.50

187.8

Geometry, coofing rates and weight losses during pig chilling 15

'

461

D3maj = 4 3 . 8 W 0'219

I

x whole

split

R 2 = 13.5%

• quartered

D3 rain -- 21.8W°'319

e-

R 2 -- 42.4%

"10

The loin can be modelled as a slab with

10

x~zS

xx

E

W i d t h = 66.0 W ° 3 ° F °°44

t'N

R 2 = 72.0% Thickness = 37.3 W 0'271F0057 i

5

r

I

i

1

I

I

i

lo

5

R 2 = 43.2%

i

5

T h e shoulder c a n be modelled as a slab with

1"20 (regression), h

W i d t h = 66.0W°338F 0°45

Figure 3

Measured cooling time 7-20vs regression values

Figure 3

Temps de refroidissement en fonetion des valeurs de r6gression

R 2 = 77.3% Thickness = 46.1 W °e72

a n d cross-sectional d i m e n s i o n s (see Figure 2 for symbols)

R 2 = 34.7%

D1 maj = 42.9W°'~°5 R 2 = 69.4%

Cooling rates

Dl min = 100.0W0"143

A m e a s u r e of the cooling rate is the time T20 for the difference between deep leg t e m p e r a t u r e a n d air t e m p e r a t u r e to fall to 20% of its initial value. This is s h o w n in Table 1. There was n o significant difference between air velocity g r o u p i n g s or carcass cuts. The following regression e q u a t i o n was o b t a i n e d (Figure 3):

R 2 = 28.2% D2maj -- 40.9 W °387 R 2 = 43.8% 0 2 rain -- 68.7 W°'I55F 0"059

~-20 = 5.48 + 0 . 0 3 7 9 W + 0.0902F - 0.869V

R 2 = 15.7%

R 2 = 70.9% A n analysis o f variance ( A N O V A ) test indicates that at the 5% significance level there was n o significant

30

1

--Brown .....................

I

I

15

& James

x whole

Thi

•

split quartered

20 0 04 Z--

"13

10

z~

t,

X

10

E

~

~: ~ X

0

0

I

i

I

40

80

120

Dressed

weight,

A

160

kg

Figure 4

Cooling times 7-20 from this work and from Brown and 0 I James upper curve, 0.5 m s- middle curve, 1.0 m s- ; bottom curve, 3.0ms -1

Temps de refroidissement 7-20 obtenu par le present travail, ainsi que par Brown et James 10 (courbe sup~rieure, 0,5 ms- 1 ; courbe moyenne, 1,0 ms- / ; courbe inf~rieure, 3.0 ms-l ) Figure 4

X

5

10

"r20 (sphere),

5

h

Figure 5 Measuredcooling time vs finite differencepredictions using a sphere model Figure 5 Temps de refroidissement mesur6 en fonction des differences finies pr~vues dans un modble sph~rique

S. Coulter

462

e t al.

Table 2 Weight losses (first column of each cell contains carcass weights in kg, second column % weight loss)

Tableau 2 Portes de masse (la premibre colonne de ehaque cellule indique les masses en kg; la seconde colonne indique [e % de la perte de masse) (a) Overnight loss at +5 ° C nominal air temperature Air velocity groupings Carcass

0.10-0.29ms -l

0.30-0.65ms -1

0.80-1.30ms 1

> 1.30ms

Split

50.30 53.60 61.95 62.10 70.85 76.05

2.14 2.15 1.45 1.61 1.91 1.91

70.75 70.85 73.00 76.80 82.70

1.70 1.83 1.78 1.82 1.81

54.05 61.85 73.15 74.65 80.45

1.94 1.78 1.64 1.81 1.86

64.95 68.25 73.60 78.60 82.65 83.10

1.54 1.90 1.77 1.72 1.88 1.74

Whole

59.85

1.92

44.55 45.25

1.80 1.44

79.80

2.13

72.00

1.74

Quartered

106.9

(b) Weight losses to 7

°C

1.54

130.0

1.92

134.3

1.94

145.8 167.6

2.43 1.85

deep leg for - 5 °C controlled experiments Air velocity groupings

Carcass Split

Whole Quartered

0.10-0.29 m s- l

0.30-0.65 m s- l

0.80-1.30 m s- 1

65.50 66.75 77.05 79.30

2.20 1.90 1.90 1.85

68.20 61.25 77.35 73.60

1.95 1.50 2.10 1.70

55.10 69.20 78.45 73.45

1.30

61.75

2.00 2.00 1.35

63.90 73.15 79.10

1.13 1.80 1.60 1.95

43.80

2.20

42.95

2.25

44.55

1.50

46.90

1.55

147.6

1.30

130.1

1.95

187.8 2.5

difference b e t w e e n the different air velocity groups. H o w e v e r , there was a significant difference with r e g a r d to the t y p e o f carcass used, with their m e a n values a n d s t a n d a r d d e v i a t i o n s as follows: Unsplit

7.89 -4- 1.2 h

Split

8.65 + 1.3 h

Quartered

> 1.30m s

12.10 + 2 . 2 h

'

'

- - a - - ' W f 1 6 7 ' . 5 5 kg ' ....x.-.. Wt 7 3 . 6 0 kg -o-

2.0

Wt 59.85

kg

--~ |

"I

1.5 0 .__l ~,

T h e regression e q u a t i o n was c o m p a r e d with the c h a r t s o f B r o w n a n d J a m e s 1°. T h e r e is r e a s o n a b l e a g r e e m e n t for lighter carcasses, b u t this w o r k finds c o o l i n g times m u c h s h o r t e r t h a n B r o w n a n d J a m e s ' s for larger carcasses (Figure 4).

1.0

0.:5

0.0 0.0

Cooling rates." comparison with f i n i t e difference calculations

F i n i t e difference c a l c u l a t i o n s were c a r r i e d o u t to see w h e t h e r a simplified g e o m e t r i c m o d e l c o u l d be used to calculate t e m p e r a t u r e v a r i a t i o n s in p i g carcasses. The leg was m o d e l l e d b y either a sphere o r an infinite c y l i n d e r with d i a m e t e r given b y the d i m e n s i o n Dm ( m e a n o f m a j o r a n d m i n o r axes at the largest cross-section, as c a l c u l a t e d f r o m the regression e q u a t i o n given earlier), m a d e u p o f a h o m o g e n e o u s m a t e r i a l with t h e r m a l c o n d u c t i v i t y v a r y ing f r o m 0 . 3 8 5 W m -1 K -1 at 0 ° C to 0.405 W m - 1 K -a at o 29 3 1 34 40 C and volumetric heat 2.83MJmK-. The surface h e a t t r a n s f e r coefficient hto t was c a l c u l a t e d by c o m b i n i n g the forced c o n v e c t i o n , n a t u r a l c o n v e c t i o n a n d r a d i a t i o n c o m p o n e n t s as d e s c r i b e d in the t h e o r y section (the e v a p o r a t i v e c o m p o n e n t was ignored). T h e c o o l i n g time ~-20was c a l c u l a t e d for each run using b o t h a sphere a n d the c y l i n d e r m o d e l . In c a l c u l a t i n g these times, the t e m p e r a t u r e s at the leg surface, leg centre

1.45

2.5

L:

1

i

i

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I

4.0

8.0

12.0

16.0

20.0

i

i

I

I

2.0

A

- - Wt 8 0 . 4 5 ....x - W, 7 3 . 0 0 -oWt 5 9 . 8 5 --

kg kg kg

24.0

I

1.5 o .d 1.0

0.5

0.0 0.0

I

I

I

I

I

4.0

8.0

12.0

16.0

20.0

Time

hrs

Typical weight loss curves Figure 6 Courbes typiques de pertes de masse

Figure 6

24.0

Geometry, cooling rates and weight losses during pig chilling and midway to leg centre, as measured at chiller entry, were used to construct the 'initial' temperature profile. Therefore, r is the time from chiller entry and not from slaughter. On average, the sphere model underestimated the cooling time r20 by 6% whereas the cylinder model overestimated it by 53%. The sphere model is clearly better than the cylinder model and is capable of giving reasonable predictions (Figure 5). It must be remembered, however, that evaporative cooling, which is a significant factor during the early stages of cooling, has not been taken into account. If it was, then the advantage of the sphere model over the cylinder model would not be as clear.

Weight losses Most of the weight loss occurs in the first few hours of chilling. At 5 °C nominal air temperature, for example, half the loss occurred in the first 2 h and 70% in the first 4 h (Figure 6). Table 2 shows overnight weight losses for carcasses cooled at 5 °C (which would be approximately the same as weight loss at 7 °C deep leg) and weight loss at 7 °C deep leg for carcasses chilled at - 5 °C. Mean and standard deviations of weight losses were: At +5 °C air temperature: Whole

1.81 5: 0.25%

Split

1.80 • 0.17%

Quartered

1.93 ±0.32%

At - 5 °C air temperature: Whole

2.41 :t: 0.27%

Split

2.66 ~ 0.24%

Quartered

1.86 ± 0.24%

463

The higher weight losses obtained at - 5 °C compared with +5°C are somewhat unexpected, because it is widely believed that quick chilling tends to reduce weight loss. A possible explanation is that the mean meat temperature that is attained at 7 °C deep leg is much lower when operating at - 5 °C: leg surface, shoulder surface and most of the loin region would be close to the air temperature, - 5 °C, and the mean meat temperature would have been closer to - 5 ° C than +7°C. When operating at +5 °C, on the other hand, the mean meat temperature at 7 °C deep leg would have been between 7 °C and 5 °C. As the amount of cooling is larger, the amount of weight loss would also have been larger. Yet another possible explanation is that, at - 5 ° C , the cooling coil had to use a larger refrigerant-to-air temperature difference and thus caused the relative humidity to be lower. A possible source of variation was the lack of control over the length of time between slaughter/hot water scalding (which occurs approximately 5min after slaughter) and entry into the chiller. For beef, Gigiel et al. 43 reported that if there is a time difference of 30 min in the time from slaughter to chilling (30-60 rain), weight loss from beef carcasses can be changed by up to 0.1% of hot weight. For pork, Cooper 11 reported a difference of 0.4% for the same difference in initial weighing times. However, the variance due to this factor is likely to be small in the present experiments, as the time to reach the chiller was approximately 40 :t: 10 rain for all tests. An attempt to correlate measured weight losses with theoretical maximum values as predicted by Pham 25 was disappointing. The measured weight loss was on average 14% higher than predicted and the correlation coefficient between the two was only 0.098, indicating that there were other factors at work, which could not be taken into account.

For - 5 °C nominal air temperature, regressing weight loss against weight, P2 fat thickness and velocity gave % weight loss (at 7°C deep leg) = 3.12 - 0.0039W - 0.0213F - 0.131V R 2 =

81.0%

In other words, the percentage weight loss to a given deep meat temperature decreased with weight, fat cover and air velocity, as could be expected from basic physical reasoning. Attempts to do a similar regression for carcasses cooled at +5 °C nominal air temperature unfortunately produced no statistically significant correlation. The weight losses at +5 °C are similar to those reported by other workers l°'ll, who obtained weight losses of 1.98% for split and 2.11% for whole carcasses (average carcass weight of 72.2kg, air temperature of 0-4 °C, average velocity of 0.5ms 1), and 1.9% (60kg carcass, air temperature of 0.5 oC and air speed of 0.25 m s 1 with initial weighing 30 rain after slaughter) respectively. The quartered sides tended to have the thickest fat layer (average P2 values were 22.7mm for quartered carcasses, 13.0 mm for split carcasses, and 11.0 mm for whole carcasses), which slowed down water movement to the surface. However, they also had the largest surface area for mass transfer. Cut surfaces, which had no fat cover, lose water more quickly. The two effects tended to counteract each other.

Conclusions

For the range of conditions studied in this work, the cooling rate of pig carcasses is primarily determined by the type of carcass (whole, split or quartered), which was in turn determined by carcass weight and fatness. A simple geometric model, using a sphere with diameter given by the mean of major and minor axes at the largest cross-section, predicted the cooling times with an average error of only 6%. Better accuracy could be obtained by using an empirical correction factor for the effective diameter and by taking evaporation and initial temperature profile into account. However, the fact that reasonable predictions were obtained when using no specially introduced empirical factor is very promising. Measured cooling rates were shorter than those plotted by Brown and James I°, particularly for heavy carcasses. The disagreement is probably less serious than it appears, however; Brown and James's charts are design charts: i.e. they were meant to cover the worst case, while our results represented the average case. Weight losses agreed with previous results for chilling at 5 °C but were higher for - 5 °C, which is contrary to expectations. This study found no statistically significant variations in weight loss during chilling due to air velocity. This is in agreement with previous findings for goat carcasses but disagreed with previous beef data.

S. Coulter et al.

464

Acknowledgments This work was carried out under a research grant by the Australian Pig Research and Development Corporation. The authors thank David MacFarlane for his contributions. References 1 2 3 4 5 6 7 8 9 10 11 12

13 14 15

16

17 18

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