ELSEVIER

Int J. Refrig. Vol. 19, No. 2, pp. 79-86, 1996 Copyright © 1996 Published by Elsevier Science Ltd and IIR

Printed in Great Britain. All rights reserved SO 140-7007(96)00006-0 0140-7007/96/$15.00

Chilling of recipe dish meals to meet cook-chill guidelines

Judith Evans, Steven Russell and Stephen James F o o d Ref r ige ra t ion and Process Engineer ing Resea rch Cen t re ( F R P E R C ) , Univers i ty o f

Bristol, Churchi l l Building, Lan g fo rd , Bristol BS 18 7DY, U K

Within Europe the most severe cooling guidelines for cook chill products are those produced by the Department of Health in the United Kingdom. These guidelines recommend that 80-mm trays of product should be chilled to below 10°C in 2.5 h and that the smaller 40- and 10-mm trays should be chilled below 3°C in 1.5 h. Trials were carried out to determine whether 10-, 40- or 80-mm deep trays of bolognese sauce could be cooled to below 3°C within the guidelines. Assuming that surface freezing was to be avoided and a simple single-stage operation used, only the 10-mm tray could be chilled within these limits. A computer model was developed that produced predictions that were in good agreement with the experimental data. The program demonstrated that other foods, such as beef curry and chicken Italian, had similar cooling responses to those experimentally measured for bolognese sauce. (Keywords: food product; cook-chill; cooking; chilling; simulation; regulations; test; sauce; Europe)

Refrigeration de plats prepares conformement aux directives sur les produits cuits et refrigeres

De tousles pays europOens, c'est au Royaume-Uni que les directives, OdictOes par le MinistOre de la SantO, sur les produits cuits et rOfrigOrOs sont les plus sOv~res. Ces directives recommandent que des barquettes de plats prOparOs, d'une profondeur de 80 mm, soient rOfrigOrOes h une tempOrature infOrieure it IO°C en 2,5 h, et que des barquettes plus petites de 40 et 10 mm soient rOfrigOrOes h une tempOrature infOrieure h 3°C en 1,5 h. Des essais ont OtO effectuOs afin de dOterminer si des barquettes de sauce bolognaise, d'une profondeur de 10, 40 or 80 mm, pouvaient dtre rOfrigOrOes h une tempOrature infOrieure h 3°C, tout en respectant les directives. Si l'on considkre que la congOlation superficielle devait Otre OvitOe et qu'un fonctionnement simple monoOtagO devait gtre utilisO, seule une barquette de lO mm pouvait Otre utilisOe, si l'on voulait se confirmer h la rOglementation. On a mis au point un modOle par ordinateur qui a permis d'obtenir des provisions qui Otaient en accord avec les donnOes expOrimentales. Le programme a d~montrO que d'autres aliments, tels que le curry de boeuf et le poulet h l'italienne, prOsentaient des rOponses similaires h celles expOrimentalement mesurOes pour la sauce bolognaise. (Mots cl6s: produit alimentaire; plat pr~par+; cuisson; refroidissement; simulation; r6glementation; essai; sauce: Europe)

A cook-chil l system is defined 1 as 'a catering system based on the full cooking of food followed by fast chilling and storage in controlled low temperature conditions above the freezing point (usually 0-3°C) '.

The aim of the cooking process in the cook-chil l system is to ensure destruction of vegetative stages of any pathogenic micro-organisms. Normally, the minimum recommended temperature requirements are to achieve 70°C for not less than 2 min in the centre of the food. This should cause a 106-fold destruction of Listeria monocytogenes 2. There is always the possibility that some micro-organisms that produce spores will not be killed by the cooking process. Therefore, the temperature of the product should be rapidly reduced between 60 and 7°C to prevent multiplication. Further reduction to 3°C is required to reduce growth of spoilage bacteria. In addition to microbiological factors, rapid reduction in product temperature aids retention of nutrients which is vital in a system often used for the preparation of meals for the old, infirm and young people.

In the United Kingdom, the Department of Health

Cook-chil l Guidelines published in 19891 recommend maximum cooling regimes and that special equipment is used to reduce product temperatures rapidly after cooking. Many other countries in Europe have similar guidelines or recommendations for the cooling of cooked products (Table 1 ). However, there appears to be little, if any, published data on the conditions required to achieve these times when cooling different cooked products.

The U K Guidelines recommend that joints of meat or packs of food should not exceed 2.5kg or 100mm in thickness or height. It is also advised that containers have lids to help prevent contamination and to minimise dehydration during cooling. The Guidelines also state that the actual chilling process should commence as soon as possible after completion of cooking and certainly within 30 min of leaving the cooking process (this allows for portioning of meals). Smaller portions (less than 50- mm deep) should be chilled to between 0 and 3°C within 90min and larger portions to 10°C within 2.5h after removal from the cooking process.

The equipment used to chill products should have

79

80 J. Evans et al.

Table 1 Cooling requirements for cooked products in Europe

Tableau 1 Exigences de la rrfrigrration des plats prrparrs en Europe

Country Temperature range (°C) Maximum time allowed (h) Subsequent temperature ('C)

Denmark 65-1 3 < 5 France 70-10 2 0-3 West Germany 80-15 2

15-2 24 < 2 Sweden 80 8 4 < 3 UK 70-3 1.5 0-3

performance specifications showing that it is capable of reducing the temperature of a 50-mm-thick layer of food from 70 to 3°C (or less) in a period not exceeding 90 min when fully loaded. This paper investigates the conditions required to cool products to comply with these and other EU (European Union) recommendations. Cool- ing times of a cooked product of different thicknesses were determined experimentally under a range of cooling regimes and these data extended to cover more conditions using predictive modelling techniques.

Experimental procedure Bolognese sauce was used as a test dish in the investigation. The sauce was prepared using ingredients in the quantities listed in Table 2. A total of 1261 of bolognese sauce was cooked from fresh products in one batch in an industrial pressure cook, vacuum cool vessel (Guisti). After cooking and cooling the sauce was divided into smaller portions and sealed in vacuum bags before being frozen and stored at -20 + 2°C until required.

Before each experimental run, a portion of sauce was thawed overnight in a room operating at 3 + 2°C. The sauce was placed in a large cooking vessel and reheated until all the product was above 70°C. During reheating, the sauce temperature was measured with a hand-held digital instrument (Solomat, accuracy+0.5°C). The product was then transferred to a steel tray and cooled in an experimental wind tunnel 3 operating at 0 ± 0.1 °C and at either 0.5 or 3 m s -l . To study the influence of an initial ambient cooling stage, some trays of sauce were allowed to cool for 0.5 h in an ambient of 20°C and an air velocity of 0.5 m s -1 .

The trays used in the investigation were designed to be large enough to prevent heat transfer in the centre of the product being unduly influenced by edge effects. Steel trays with depths of 10, 40 and 80mm and lateral

Table 2 Constituents in bolognese sauce

Tableau 2 Ingrrdients de la sauce bolognaise

Ingredients Quantity

Mince beef 37.50 kg Onion 12.50 kg Canned tomato 26.40 kg Tomato puree 3.75 kg Oil 6.25 kg Water/stock (50/50) 32.50 kg Flour 3.13 kg Garlic (crushed) 0.63 kg Red wine 2.751 Salt 0.56 kg Pepper 0.06 kg Herbs (mixed) 0.01 kg Added water 2.88 kg

dimensions of 600 x 600 mm were used to investigate cooling rates. All trays were used with and without a close-fitting steel lid.

To measure temperatures during cooling, two multi- point copper-constantan thermocouple probes (accuracy +0.4°C) were inserted vertically into the product, one at the geometric centre of the tray and the other in a leading comer (Figure 1 ). The sensor spacing on the thermocouple probe was dependent on the tray depth with three thermocouples spaced 5 mm apart in the 10-mm tray, five thermocouples 10mm apart in the 40-ram tray and nine thermocouples 10 mm apart in the 80-mm tray. Air temperature was measured with four single thermocouples placed 100mm from the comers of the tray in the wind tunnel. Before each trial the air velocity was measured at three positions across the base of the empty tunnel at a height of 100mm using an air vane anemometer (Edra 5, accuracy 3% of reading). The speed of the fan was then adjusted until the average air speed was either 0.5 or 3ms -1.

All temperatures during cooling were recorded by a Magus Data Logger (Measurement Systems Ltd). During the first 30min of cooling, air and product temperatures were recorded every 45 s, after this period temperatures were recorded every 3 min.

Data were analysed by transforming the temperature data via a logarithmic transformation into straight-line data using the following function and plotted against the corresponding cooling time:

r T s - T a] In [Tsi - Ta]

Plan view

v

Air flow

0

T 6~mm 1

Elevation ~ To data logger

J f I Figure 1 Position of thermocouple probes in trays during cooling

Figure 1 Position des thermocouples darts les barquettes au cours de la rdfrigdration

Meeting cook-chill guidefines 8 1

A i r t e m l ~ r a t u r e (*C)

-30 -20 -10 -5 -2

,., 50 50 o 30 / ~0 40 40

2 0 30 : [ ,o ,o ~0 ~o

.~ 20

- 10 ~ to

i

0

~ 5

F i g u r e 2

Figure 2

0 +2 . . . . . , ' ' . . . . ' . . . . " ' " " ' 0 . 0

~o ~o ~ . ~ 50 50 '~ . . . . . . . --0.5 , o o

30 30 ~ ", \ "~ -.~_"• . . . . . . . . . . -I.0

'..,\ ",,:-,,, , _ . • ~ lOmmu~ywithlid . -1.5 ".., \ \-'-.. . ~ . , . , ..... ~:

I . . \ , N._ " . . . . . . - - , ~ , ~ n ~ . . , h , ~ 11 "~ ,0,0 /\ - \ ",,"-. \ \ N . " , - - ~mm 1my -2.0 [..,

' ' ' - 3 . 0

I _ \ \ . . . -35

\\ - - - - ' ~ J ' - ' - 4 . 5

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

T i m e (hours)

1 T i m e requi red to cool centre o f bolognese sauce in an air veloci ty of 0.5 m s

Temps requis pour r~frig&er le centre de la sauce bolognaise, avec une vitesse de l'air de 0,5 m s -~

where T~ = temperature of sauce, Ta = temperature of air, and Tsi = initial temperature of sauce.

From this data transformation, cooling times under other air temperatures could be predicted.

Mathematical model

A mathematical model was developed to allow cooling times to be predicted for different foods and cooling regimes. In the model an infinite slab represented the tray of food. The slab was divided into i - 2 layers of thickness dx, whose uniform temperature at time j was Tq. In addition there were boundary layers at the top

and bot tom of the slab with thicknesses of dx/2. In the computer program, a value of i = 21 was chosen to give reasonable accuracy and run time; Tii, (i = 1,21) was the temperature distribution inside the product and T22j the air temperature at time j. The temperature distribution was therefore an array which was calculated at each time j.

Inside the slab:

OT dx 02T pCp Ot = A ~ dx

where p is the density, Cp the specific heat and A the enthalpy of the food.

O

e ~

o o o

o

o "d

F i g u r e 3

Figure 3

A i r t e m p e r a t u r e (°C)

-30 -20 -10 -5 -2 0 +2

, ... \ I \ ,g--=--a,=--.

I - , - - - . . - - a -

I , \ I . L _ - - . , . - - . - , - i \

0 2 4

' 0 . 0

'-41.5

i -1.0

"-, I . . . . . ~*=*~ [..._........ . . .~..2:~ ................................ 4 ~ * " " y ' ~ "~ " x • , / - - a 0 , ~ y -2.o

~ . -25 "g - . . - • - 3 . 0

6 8

T i m e ( h o u r s )

10 12 14

T i m e requi red to cool centre o f bolognese sauce in an air velocity o f 3 m s 1

Temps requis pour r~frig&er le centre de la sauce bolognaise, avec une vitesse de Fair de 3 m s -1

-3.5

-4.0

_~,-4.5 16

82 J. Evans et al.

Air temperature (°C)

-30 -20 -10 -5 -2 0 +2

50 50 50 ,- 30 30 ,to !40 40 40 40 1 o 20 30

10 20 30 30 30 30 :~ I 20 20 3 o 20 ~ - 20 = 3 10 20

!

- I 5 10 lO

~ 3 5

N 3 5

- I

F i g u r e 4

Figure 4

\ " ~ " ;,~, •

tnly \ ~ " • - - - - lOmm troy wire lid

*'. \ ~ " . . . . . . . 40ram ny

I ', \ N " , , - - - - . - . . , . , . , .

g. i - , ..... \ \ ..... \ .... '-,::, 2 4 6 8 10 12 14 16 18 20 22 24 26 28

T i m e (hours)

Time required to cool surface of bolognese sauce in an air velocity of 0.5 m s - l

Temps requis pour rkfrig&er la surface de la barquette de sauce bolognaise, avec une vitesse de l'air de 0,5 rn s I

I 0.0

4).5

- I .0

'-1.5 ~z

'-2.0

'-2.5 "~ _=

'-3.0

• - 3 . 5

!-4.o

'---4.5 30

The numeric implicit translation for the computer This translated numerically to: model was:

pnCp, T i j+l - T i j T i + l j + l - 2 T i j + l + T i _ l j + 1 Top : p l C p l T I j + I A t - T l j -2A1T2jAX 2 - T I j

A t - A. A x 2

i E [2, 20] + 2h T22j -- T I j Ax

The boundary conditions were given by:

OT dx A OT pCp - ~ ~ - Ox + h(Ta - T)

where h is the surface heat-transfer coefficient and T the surface temperature.

Bottom : P21Cp2 ~ T21 j+ 1 - T2I j

At - - 2 ~ 2 1 T2°j - r21 j

A x 2

+ 2h T22j - T21j A x

-3O

.. 2o

o

i

i

F i g u r e 5

Figure 5

Air temperature (oc)

-20 -10 -5 -2 0 +2

50 50 50 50 511 30 40 40 40 40

3O 2O 3O 30 30 i 30' I 0 20 20

i

_.3 • 1o 120 20 20+

5 10 10

3 I0 - I 5 10

3 5 -1

3 5

3 5

- I 3

I I ~. \ \ '% ' ----t0mmeaywimlid

I I " \ N ", m=,t , .y

l l ' . "'., i i •

0 2 4 6 8 I0 12 14

T i m e (hours)

' 0 . 0

Time required to cool surface of bolognese sauce in an air velocity of 3 m s-1

Temps requis pour r~frig&er la surface de la barquette de sauce bolognaise, avec une vitesse de l'air de 3 m s -1

"-0.5

' - I . 0

' -1 ,5 ~11 j=

I [..,

' -2 .5 ~

' - 3 . 0

' - 3 . 5

' - 4 . 0

- 4 . 5 16

Meeting cook-chill guidelines 83

Air temperature (“C)

Figure 6 --- Experimental; - - ~ - model

Figure 6 Vitesse de [‘air de 0,s m s-’ pour une barquette avec couvercle

These equations provided a system given by:

(Aj)(Tij+l) = (bj), fori E [1,21]

with:

(bj) = ~

(I bz

where (given F = dt/dx2):

Uii__l = aji+l = AZF

Pn. cp,

vi 6 r&201 a,, = 1 + 2 hLF II PX,”

bi = Tij

Table 3 Time required to cool bolognese sauce from 70 to 10 or 3’C

Tableau 3 Temps requis pour rPfrig&w la sauce bolognaise de 70 6 10 ou 3°C

-2 0 2

Air temperature (“C)

Figure 7 ~ Experimental; ~ ~ - model

Figure 7 Vitesse de lhir de 0,s m SF’ pour une harquette sans couvercle

and

arr = 1+2%+253 PI Pi

i=l* &F al2 = -2-

Pl.C*,

bl = Tlj+2 h F dx T22j

Pl.C,,

X21F a2120 = -2---

P21.Cpz,

i=21*

I

X21F a2121 = 1 + 2--- hdxF

P21.Cpz, +2------

P21.Cpz,

bZ1 = T2lj+2 h F dx T22j

P21.Cpzl

Solving this system provided the temperature distribu- tion inside the product, at the time j + one time increment.

Thermophysical properties for the bolognese sauce were calculated using data on the initial freezing point and nutritional composition of the product. The intial freezing point was obtained experimentally from the mean of three

Air velocity (m s-‘)

Time from 70 to 10°C (h)

Air temperature (“C) -2 0 +2

Time from 70 to 3°C (h)

Air temperature (“C) -2 0 +2

10 mm with lid 0.5 1.1 1.5 1.6 10 mm without lid 0.5 0.9 1.0 1.1 10 mm with lid 3 0.7 0.8 0.9 10 mm without lid 3 0.4 0.5 0.6

40 mm with lid 0.5 5.4 40 mm without lid 0.5 3.9 40 mm with lid 3 2.7 40 mm without lid 3 1.8

80 mm with lid 0.5 10.8 80 mm without lid 0.5 8.5 80 mm with lid 3 5.9 80 mm without lid 3 5.0

Cooling times complying with UK cook-chill guidelines are underlined.

6.0 6.7 4.3 4.6 2.9 3.3 2.0 2.2

11.9 13.1 9.4 10.3 6.4 7.1 5.4 6.0

1.9 1.4 iYG m -

8.3 5.8 4.0 2.8

16.1 13.0 8.9 7.5

2.2 3.0 1.7 2.2

1.6 0.9 -

9.8 13.1 6.9 9.3 4.7 6.4 3.3 4.3

19.3 26.0 15.2 20.3 10.5 14.0 8.9 11.9

8 4 J. Evans et a l.

Table 4 Tempera tu re o f sauce af ter coo l ing for 0.5 h in amb ien t o f 20°C and air ve loc i ty of 0.5 m s -1

Tab l eau 4 Temperature de la sauce aprbs r~frig~ration pendant 0,5 h la temperature ambiante de 20°C et ~ une vitesse de l'air de 0,5rns -1

Tempera tu re (°C)

80 m m wi th lid 66.6 80 m m wi thou t l id 65.7

4 0 r a m wi th lid 64.3 40 m m wi thou t lid 60.9

1 0 m m wi th lid 40.3 10 m m wi thou t l id 38.4

replicate samples of bolognese sauce frozen in air at -20°C and air velocity of 3 m s- I. Chemical analysis was carried out on three replicate samples to obtain the mean nutritional composition (water, protein, fat, carbohy- drates, minerals). These data were then used in the COSTHERM program 4 to obtain values over a tempera- ture range between -20 and 90°C for conductivity, specific heat, density and enthalpy.

Surface heat-transfer coefficients were experimentally derived using the method described by Cowell and Namor 5. The heat-transfer coefficients at the surface of the trays were measured with and without lids for air velocities of 0.5 and 3 m s-1 and the relevant values used as input in the program.

A further investigation was carried out to predict if other meat-based products would have similar cooling times to that of bolognese sauce. Initial freezing points and nutritional data were experimentally evaluated for beef curry and chicken Italian and the thermal properties obtained from the COSTHERM program. These data were input into the model to predict cooling times and compare them to cooling rates for bolognese sauce.

Results

Experimental

Data transformed via the logarithmic transformation into straight line data are shown in Figures 2 (centre, 0.5 m s-X), 3 (centre, 3ms-l) , 4 (surface, 0.5ms -l) and 5 (surface, 3 m s-l). The time required to cool the centre of the bolognese sauce from 70 to 10 and 3°C are presented in Table 3.

It is clear that all four factors investigated, presence or absence of lid, depth of product, air velocity and air temperature have a substantial effect on the rate of cooling and consequently the cooling time.

In trays without lids the longest cooling time to 10°C was 10.3h. As would be expected this was produced when cooling the thickest depth of product (80mm)

o I under the least severe (2 C, 0.5 m s- ) regime. With a lid

Table 5 Expe r imen ta l l y der ived values for hea t - t rans fe r coefficients for top surface o f t r ay

Tab l eau 5 Valeurs exp~rimentalement cl~rivdes des coefficients de transfert de chaleur ~ la surface de la barquette

0 . 5 m s -1 3 m s - l ('~Vm -2 °C) ( W m - 2 ° C )

W i t h l id 6.5 13.5 W i t h o u t l id 14.4 33.3

Isoo I

6°°i ~ 14001

1200 ~" 2

I00o 80mm tray

o 800 ~

600 ,.'.1~- _ . qCq.. m . , ---- --'~ 40ram tray

4 0 0 - - _ j _ m . ~ - - - -

200! " . . . . . . . . ~ " ' IOmmtray,

-2 0 2

Ai r t empera tu re (°C)

Figure 8 - - Exper imenta l ; - - - mode l

F igure 8 Vitesse de l'air de 3 rn s -I pour une barquette avec couvercle

this time was increased by 2.8 h to 13.1 h, an increase of 27%. At the other extreme the thinnest unlidded product (10mm) under the most severe conditions (-2°C, 3 m s -1) could be cooled in 0.4 h. Adding a lid increased the time by 0.3 h - - a 75% increase in cooling time. The effect of adding a lid when cooling to 3°C was the same but the magnitude of the increase was larger.

Increasing the product thickness from 10 to 40 mm in trays without lids produced an approximately 4-fold increase in cooling time at both air velocities. Increas- ing the thickness from 10 to 80mm produced cooling times that were up to 10-fold longer at 0.5ms -l and up to 15-fold longer at 3ms -~.

With product thickness of up to 40 mm in unlidded trays reducing the air velocity from 3 to 0.5 m s -1 more than doubled the cooling time. In 80-mm thick products, the factor was reduced to approximately 1.7. In lidded trays the effect of air velocity was similar for all trays with a change in air velocity from 0.5 to 3ms 1, increasing cooling times by, on average, a factor of 1.9.

Reducing the air temperature produced significantly shorter cooling times in all cases. The effect was particularly noticeable when cooling to 3°C. Average

1800

2600

,~ 1400

1200 9

1000

,~ 800 ¸

9 o 400

i: 200

Figure 9

Figure 9

80ram tray

' " 41~m tray

r i . . . .

0 r -2 0 2

A i r t empera tu re (*C)

- - Exper imenta l ; . . . . mode l

Vitesse de l'air de 3 m s -1 pour une barquette sans couvercle

Meeting cook-chill guidelines 8 5

1800'

~" 1600

,,ooi ? ¢~ 1200: 2

1000' E 0

~. 800

"6 6oo

400

80mm tray

r

40ram tray

10ram tray 200 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0 ! -2 0 2

Air temperature (°C)

Figure 10 Air velocity 0 .5ms -] tray with lid. • Beef curry; chicken Italiene; × bolognese sauce

Figure 10 Vitesse de l'air de O ,5 m s 1 pour une barquette avec couvercle

cooling times to 3°C using an air temperature of -2°C were approximately 60% less than those produced when the air temperature was 2°C.

After 0.5h at 20°C in the 10-mm trays the centre temperature cooled substantially to 40.3 and 38.4°C for lidded and unlidded trays respectively (Table 4). How- ever, in the 80-mm trays of sauce the centre temperatures had dropped by less than 5°C to 66.6 and 65.7°C respectively.

Surface freezing can occur when air temperatures below the initial freezing point of the sauce are used. The straight-line transformation of cooling data can only be used to extrapolate cooling above the freezing point. The experimental determinations showed that the initial freezing point of the bolognese sauce was -1.2°C.

An examination of the cooling curves at the surface (Figures 4 and 5) show that using air at -2°C freezing would start to occur at the surface of lidded 10-mm trays after approximately 1.8h at 3.0ms 1 and after 2.4 h at 0.5 m s 1. Data from the figures can similarly be used to indicate when surface freezing is likely to occur at the other thicknesses and air temperatures.

1800

80mm tray -

1600 . . . . . . e ,

"001 ' ~

~ t ~ . '"'

i 600 [ .,Omm !ray ( .)

2 '00' 200 1 10mm tray

0: -2 0

Air temperature (°C)

Figure 11 Air velocity 0.5 m s-I tray without lid. • Beef curry; ~ O ~ chicken Italiene; × bolognese sauce

Figure 11 Vitesse de Fair de 0,5 m s - I pour une barquette sans couvercle

1800'

,60o:

1400'

t2oo 1

0

t 0 0 0 ' 80ram mty

O

9. ,Omm tray

IOmm tray o i I' " t

-2 0 2

A i r temperature ( °C)

Figure 12 Air velocity 3 m s -1 tray with lid. - • Beef curry; chicken ltaliene; >< -- bolognese sauce

Figure 12 Vitesse de l'air de 3 rn s z pour une barquette avec couvercle

Mathematical model

The chemical analysis of the contents of the sauce showed that it consisted of 72.5% water, 13.2% protein, 6.6% fat, 6.7% carbohydrate and 1% minerals. These values were therefore used in the COSTHERM program together with the initial freezing point of -1.2°C to provide the required thermal data for the predictive model. Measured surface heat-transfer coefficients on the top of the product are shown in Table 5. These values were also used in the model.

The cooling times from 70 to 3°C predicted by the model and those obtained experimentally are compared in Figures 6-9. Cooling times predicted by the model were all within + 10% of experimental cooling times.

Further cooling data were obtained from the model for beef curry and chicken Italian. The initial freezing point was found to be -2.1°C for the beef curry and -1.2°C for the chicken Italian. Composition of beef curry was 70.3% water, 9.4% protein, 5.7% fat, 13.4% carbohydrate and 1.2% minerals. The composition of the chicken Italian was 85.8% water, 8.5% protein, 1.7% fat, 3.8% carbohydrate and 0.2% minerals. Cooling times for

1800

,,~ 1600 ._,=

14oo

P en 1200 0

o iooo

2 80o

6oo 0 o 400

20O

L _ 80ram tray.

, 40ram tray

I Ontm tray i

0 2 -2

Air temperature (°C)

Figure 13 Air velocity 3 m s i tray without lid.

Figure 13 Vitesse de l'air de 3 rn s I pour une barquette sans couvercle

86 J. Evans et al.

both products from 70 to 3°C and a comparison with cooling of bolognese sauce are shown in Figures 10-13.

The cooling times for all three products were predicted by the model to be very similar, with overall differences between the cooling times for all the products being less than 6%. Although all very similar, the beef curry cooled slightly faster than the bolognese sauce, which cooled slightly faster than the chicken Italian.

The model predicted that beef curry, chicken Italian and bolognese sauce could be cooled to 3°C within 1.5 h in the 10-ram trays if the air velocity was 3 m s -1 and the air temperature was below 2°C. In the lower air velocity (0.5 m s -1) the 10-mm trays could only be cooled within this time period if the tray did not have a lid and the air temperature was -2°C.

Discussion

These investigations have produced data on the cooling time of 10-, 40- and 80-ram-thick layers of bolognese sauce under a range of cooling conditions. Figures produced using a transform of the cooling data can be used to predict cooling times for any air temperature providing the product remains unfrozen. Data on the rate of temperature reduction at the surface of the sauce can be used to predict when freezing occurs. In many chilled cooked products, freezing can cause quality problems such as separation and should be avoided.

A computer model has been developed that produces predictions that are in good agreement with the experimental data. The program has shown that other foods, such as beef curry and chicken Italian, will have a similar cooling response to those produced for bolognese sauce. It is therefore likely that the cooling response of most meat-based convenience meal mixtures would be similar to that of bolognese sauce. In many commercial operations the operators would like to use simple, single- stage batch chilling systems. These data therefore cover most of the conditions and many of the products that are likely to be of interest to a commercial operator.

It is interesting to compare the cooling data obtained in these investigations with some of the European cooling requirements for cooked products shown in Table 1. Currently the most severe cooling guidelines are those produced by the Department of Health in the United Kingdom. These guidelines recommend that 80-ram-thick trays of product should be chilled to below 10°C in 2.5 h and that the smaller 40- and 10-ram trays should be chilled below 3°C in 1.5 h. Assuming that surface freezing is to be avoided and a simple single-stage operation used, only the 10-mm tray could be chilled within these limits. This required air temperatures to be below 0°C and an air velocity of at least 3 m s - l if cooled with a lid. If cooled without a lid an air temperature of 2°C and an air velocity of 3 m s - t , or - 2 ° C and 0.5 m s -1 were required.

The 40-mm tray could not be chilled below 3°C in the required time, the fastest time to 3°C being in air at - 2 °C and air speed of 3 m s I when cooling could be achieved in 2.8h (1.3h more than recommended). The 80-mm trays required at least 5h to cool to 10°C, which was double the recommended cooling period.

The Department of Health guidelines allow 30min before chilling should commence. If the trays were cooled in an ambient air temperature of 20°C and air velocity of 0.5 m s I the predicted reduction in centre temperature would be between 3.4°C for the 80-mm tray with lid and 31.6°C for the 10-mm tray without a lid (Table 4). Surface temperatures would be much lower and a significant amount of heat would have been lost from the food. An initial ambient cooling stage would therefore reduce the initial product load that has to be extracted in the chilling system. However, even allowing for a 0.5 h ambient cooling period, it would still not be possible to cool a product thickness of 40 mm within the time specified in these guidelines.

The cooling requirements in France and Germany are slightly less severe and using air at 0°C and 3 m s - l , 40- ram-thick trays without lids of products could be cooled in the required time. However, more severe conditions than -2°C, 3 m s - l , would be required to cool lidded trays in the required time. These results indicate that even with 80-mm-thick lidded trays of product the least demanding Swedish requirement of 80 to 8°C in 4 h would be difficult to achieve.

Further reductions in cooling times could be achieved if air temperatures were reduced or air velocities increased. However, as already stated if air temperature were reduced product freezing would be likely to occur. Initial freezing points of the products examined were between -1 .2 and -2.1 °C and therefore air temperatures of much below -2°C could result in some degree of freezing. Alternatively air velocities could be increased, a two-stage cooling system used, or an alternative cooling method with a higher rate of heat transfer considered.

References

1 Department of Health Chilled and frozen Guidelines on Cook- Chill and Cook-Freeze Catering Systems (1989)

2 European Chilled Food Federation Guidelines to chilled food production (1994)

3 James S. J., Creed P. G., Bailey, C. The determination of freezing time of boxed meat blocks Proceedings oflnstitute of Refrigeration (1979)

4 Miles C. A., van Beck G., Veerkamp C. H. Calculation of the ther- mal properties of foods in Physical Properties of Foods (R. Jowitt, F. Escher, B. Hallstrom, H. F. Th. Meffert. W. E. L. Speiss, G. Vos, eds) Applied Science Publishers, New York, 269-311 (1983)

5 Cowell N. D., Namor, M. S. S. Heat transfer coefficients in plate freezing: the effect of packaging materials Annexe IIR Commissions B1, C1 and C2, Bressanone (1974)