Effect of Radio Frequency Cooking on the Texture, Colour And

8/14/2019 Effect of Radio Frequency Cooking on the Texture, Colour And

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Effect of radio frequency cooking on the texture, colour andsensory properties of a large diameter comminuted meat product

L. Zhang, James G. Lyng *, Nigel P. Brunton

Department of Food Science, Faculty of Agriculture, University College Dublin, Belfield, Dublin 4, Ireland

Received 8 February 2004; received in revised form 15 March 2004; accepted 15 March 2004

Abstract

Radio frequency (RF) cooking is a form of dielectric heating similar to microwave heating. In this study an optimised cooking

protocol was developed for pasteurising 1 kg cased meat emulsion samples, which were immersed in 80 C circulating water during

cooking. Subsequently, selected quality attributes of RF pasteurised samples were compared to steam pasteurised samples, by in-

strumental and sensory methods. Instrumental assessments show that RF heated meat batters had a greater ability to hold water,

were significantly harder, chewier and gummier (P < 0:001), while having less cook colour development than their steam cookedcounterparts. Differences were also detected by sensory methods. In conclusion, while differences were detected, it is possible that

these could be eliminated by adjusting the cooking protocol to produce similar cook values in RF samples to those in products

cooked by steam.

2004 Elsevier Ltd. All rights reserved.

1. Introduction

Traditional methods for cooking of comminuted

meat products involve heating the product using hot

water or steam. While cooking meats in this way, the

outer surfaces of the product heat first with heat sub-

sequently transferring to the colder interior predomi-

nantly by conduction. This in turn can lead to

overheating of the outer surfaces while waiting for the

interior to reach an appropriate temperature. In contrast

using dielectric forms of heating, polarizing electro-

magnetic radiation (EMR) allows volumetric heating of

the product such that all parts of the product in prin-

ciple heat at the same rate. In practice, however, the

frequency of the incident EMR limits the depth to which

it can penetrate with lower frequency EMR being re-

quired for greater penetration depths. Thus for products

such as large diameter comminuted meats, low fre-

quency EMR such as that encountered in RF heating

(i.e., 1300 MHz (Risman, 1991)) would be required to

provide efficient and uniform heating. In addition, these

larger diameter products while generally encased duringcooking (which reduces weight loss) are more suscepti-

ble to heat damage as their larger size means their outer

surfaces may spend much longer periods at high tem-

peratures than smaller sized products. Furthermore,

since RF heating involves placing the product between

two parallel electrodes between which the RF field is

generated, to achieve uniform heating within the prod-

uct it must have uniform shape (for ease of slicing)

which is a characteristic of most processed large diam-

eter meats which are traditionally cooked in uniformly

shaped casings or moulds. Casing or packaging meats

prior to cooking also fulfils an important role in the

prevention of post-process contamination of meats fol-

lowing cooking.

Some information is available regarding RF dielectric

properties of meat batters which govern their interaction

with RF energy (Zhang, Lyng, Brunton, & McKenna, in

press). In addition work has also been published on the

quality of RF heated non-cased meats. Houben,

Schoenmakers, Van Putten, Van Roon, and Krol (1991)

evaluated RF as a method for the pasteurisation of

uncased sausage meat emulsions in a sealed system.

Largely due to the findings of these workers, this ap-

proach found commercial application with the devel-

*Corresponding author. Tel.: +353-1-716-7710; fax: +353-1-716-

1147.

E-mail address: [email protected] (J.G. Lyng).

0309-1740/$ - see front matter 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.meatsci.2004.03.011

Meat Science 68 (2004) 257268

MEATSCIENCE

www.elsevier.com/locate/meatsci
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opment of a method for the continuous production of

cooked meats by Tulip International AS Denmark and

APV. This method involved pumping meat between the

electrodes of an RF generator, followed by flow pack-

aging into aluminised films while still hot, though it is no

longer used today. More recently Laycock, Piyasena,

and Mittal (2003) examined the use of RF for cookingground, comminuted and whole beef products. In this

study the products were cooked without casings in a

specially designed applicator cylinder. Future applica-

tions of RF to meats are likely to involve a different

approach to that taken by Houben et al. (1991). Aside

from Bengtsson and Green (1970), no other study has

evaluated the use of RF for cooking of cased meat

products. If RF cooking is to find application in a

commercial setting, packaging of the product will most

likely be required to prevent post-process contamination

and therefore RF studies and systems should be

designed with this in mind.

In view of the lack of information with regard to RF

cooking of cased meat products, a principal objective of

the present study was the development of a method,

which permits the application of RF heating to a com-

minuted cased meat products and which could be easily

integrated with current methods for meat pasteurisation.

In addition, a number of quality attributes of the RF

heated products were assessed and compared to prod-

ucts pasteurised by conventional methods (steam cook-

ing). The quality attributes assessed were texture and

colour measured using an Instron and a chromameter,

respectively. In addition, a sensory comparison of a RF

pasteurised and conventionally produced comminutedmeat product was carried out.

2. Materials and methods

2.1. Meat handling

Lean pork shoulder and pork back fat were obtained

from a local producer (Galtee meats, Cork, Ireland).

Lean tissue was ground through a plate with 3.5 mm

diameter holes, while fat was ground through a 10 mm

plate using a mechanical mincer (Model No. TS8E,

Tritacarne, Omas, Italy). About 2 kg lots of ground

tissue were then placed in polyethylene bags, vacuum-

packaged using a Webomatic packaging system (Model

No. 021ODC681, Webomatic, Bochum, Germany) and

stored at )18 C until required for product manufac-

ture. Suitable amounts of the frozen muscle and fat were

air-tempered at 5 C for 24 h prior to batter preparation.

2.2. Luncheon roll manufacture

The recipe used in preparation of the luncheon roll

(LR) meat batter is given in Table 1, while Table 2

outlines the manufacturing protocol. Processing of

batters involved blending thawed minced meat and fat

with the remaining ingredients in a Manica bowl chop-

per (Model No. CM22, Equipaimentos Carnicos, Bar-

celona, Spain). During preparation, the temperature of

the batters did not exceed 15 C. After blending the LR

batter was filled into casings (Walsrode K-Plus, Case-

tech, GMBH, Germany) using a mechanical filler

(Model No. EM-12, Equipaimentos Carnicos, Barce-

lona, Spain) to a weight of 1.0 kg (0.005 kg) and sealedwith plastic cable-ties (Maplin electronics, Dublin,

Ireland).

2.3. RF cooking protocol experiment

An orthogonal experiment described in Section 2.11

was conducted to determine the optimum conditions for

Table 1

Ingredients and suppliers used in the manufacture of luncheon roll

Ingredient Weight

(kg)

Supplier

Lean pork 4.4 Galtee meats, Cork, Ireland

Pork fat 2.6 Galtee meats

Onion National Food Ingredients,

Limerick, Ireland

Seasoninga 0.3 National Food Ingredients,

Iced water 3.1

Rusk meal William Blakes Ltd., Dublin,

Ireland

Superfine rusk 0.6 National Food Ingredients

Potato starch 0.6 National Food Ingredients

Cure solutionb 0.2

500E (soya protein

isolate)

0.2 National Food Ingredients

Total 12.0

aSalt, spices, onion powder, stabiliser E450, E451, rusk (made from

wheat flour, salt, E503), preservative E221, flavour enhancer E621,

colour E128, antioxidant E301 and flavourings.b Cure solution consisted of water (81%), salt (1%), STPP (2.0%),

sodium ascorbate (0.25%) and sodium nitrite (0.1%).

Table 2

Procedure used in the preparation luncheon roll meat batters

Step No. Description

1 Lean pork (2.8 kg) and pork fat (1.9 kg) fat placed in

bowl

2 500E evenly distributed into bowl

3 Half the water added (1.55 kg)

4 Cure solution added

5 Chopped for 90 s at knife and bowl speed 1

6 Remainder of lean pork (1.6 kg) and pork fat (0.7 kg)

added

7 Superfine rusk, seasoning, potato starch and the

remainder of the water (1.55 kg) added

8 Chopped for 30 s at knife and bowl speed 1

9 Chopped for a further 90 s at knife and bowl speed 2

10 Batter was filled into casings in 1 kg lots and tied with

plastic cable ties

11 Stored under refrigerated temperatures until required

258 L. Zhang et al. / Meat Science 68 (2004) 257268


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RF cooking. Four factors consisting of cooking time

(min), power (W), holding water temperature (C) and

holding time (min) were examined at three levels as in-

dicated in Table 3. Endpoint temperature measurements

for each treatment were evaluated for each cooked LR

by transferring the product immediately following

cooking to a specially constructed thermocouple jig.

Fig. 1 provides an illustration of the jig, which was

manufactured from rectangular blocks of wood with a

hollowed out area for insertion of the LR. Temperaturesat 15 points within the LR were monitored and recorded

using type T thermocouples (Industrial Temperature

Sensors, Dublin, Ireland) and a Grant Squirrel data

logger (Model No. 1600, Grant Instruments Ltd., Bar-

rington, Cambridge CB2 5QZ, England). Maximum

temperature (MxT), minimum temperature (MnT),

mean temperature (X T) and temperature difference (DT)

of the endpoint temperature were measured and

recorded.

2.4. RF cooking protocol of luncheon roll

All RF heated products were processed in a custo-

mised RF oven (Capenhurst Technologies Ltd.,

Capenhurst Technology Park, Capenhurst, Chester

CH1 6ES, England). The maximum power output of the

unit was 0.6 kW. A photograph of the cell positioned

within the RF oven is shown in Fig. 2 and an illustration

of the cell used is provided in Fig. 3. To ensure products

were sufficiently pasteurised, timetemperature profiles

were recorded during RF cooking using a Fluoroptic

thermometer (Model No. 790, Luxtron Corporation,

2775 North Western Parkway, Santa Clara, CA).

2.5. Steam cooking of LR

For steam processing, samples were prepared as de-

scribed in Section 2.2 but were cooked in a thermo-

statically controlled KERRES smoke-air steam oven

(Type CS 350, Raicher-und-Kochanlagen, D-71560

Sulzbach-Murr, Germany) set at 80 C for 150 min.

Product and oven temperatures were recorded at three

points within the roll at 30 s intervals using a Grant

Squirrel logger (Grant Instruments Ltd.) and type T

thermocouples (Radionics). The temperature at 15

points within the product was monitored at the end of a

run using the thermocouple jig described above.

2.6. Calculation of pasteurisation unit and cook values

From the timetemperature data recorded during RF

and steam cooking, pasteurisation units (PU60) and

cook values (Cs100) were calculated. For PU60 values, a z

value for pasteurisation (zp) value of 5.5 C (Listeria

monocytogenes) in meat products (Institute of Food

Technologists (2001)), a reference temperature (hp) of

60 C and Eq. (1) was used,Ztp

0

dt 10Thp=zp

Ztp

0

dt 10T60=5:5

pasteurisation units PU60; 1

where T is the temperature at the measurement point at

any time during the heat process, dt is the duration of

time at a particular temperature and tp is the time at the

end of the heating process. Cs100 values were calculated

using the procedure of Mansfield (1962) and the fol-

lowing equation:

Table 3

Orthogonal experimental design for optimisation of radio frequency

cooking

Level A B C D

Circulating water

temperature (C)

Power

(W)

Cooking

time (min)

Holding

time (min)

1 74 450 25 12 77 500 30 2

3 80 550 35 3

1/6

1/4 1/4

16 cm

Wood jig

9 cm

Luncheon roll

Thermal Couples

Fig. 1. Schematic side profile of thermocouples jig for end point temperature measurement in luncheon rolls.

L. Zhang et al. / Meat Science 68 (2004) 257268 259


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Ztp

0

dt 10Thc=zc

Ztp

0

dt 10T100=33

cook value Cs100; 2

where Cs100 is the Cs value calculated using a reference

temperature (hc) of 100 C and a z-value for cooking (zc)

of 33 C. T, dt and tp are as previously defined.

2.7. Textural measurements

Texture profile analysis (TPA) was conducted using

the methods of Bourne (1978) as described by Colmen-

ero, Barreto, Mota, and Carballo (1995). An Instron,

Universal Testing Machine (Model No. 5544, Instron

Corporation, High Wycombe, England), was used and

200 mm

Rubber

Washer

140 mm

118 mm

5 mm

15 mm

Water out

Water in

50 mm

Fig. 3. Illustration of polyethelyene cell used for cooking luncheon roll in the radio frequency oven.

Fig. 2. Photograph of polyethylene cell (showing water circulation pipes) within the radio frequency oven.



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data were interpreted using the accompanying Instron

Merlin software package (Version 22055). Attributes

analysed included hardness (H1 and H2), cohesion force

(CF), cohesion energy (CE), springiness (SP), chewiness

(CW) and gumminess (GM). Kramer shear analysis was

conducted using crosshead speed of 200 mm min1 with

2.5 cm 6.0 cm samples being cut using a template.From the graph of load vs. crosshead extension, the

peak load and the energy used were calculated.

2.8. Expressible fluid test

Expressible fluid (EF) were measured using the

method of Lee, Whiting, and Jenkins (1987). A pre-

weighed sample (89 g) was placed between three layers

of pre-dried, pre-weighed 7 cm Whatman filter paper

(Whatman International Ltd., Maidstone, England) on

the Instron and was compressed 10 mm along 20 mm

axis of the sample at a rate of 50 mm min1. The papers

were reweighed to determine total expressible fluid

(TEF) and subsequently dried at 105 C and reweighed

to determine total expressible moisture and expressible

suspended solids.

2.9. Colour measurement

Samples for colour measurement was prepared as

described above. A chromameter (Model CR-300 Mi-

nolta, Minolta (UK) Limited, Milton Keynes, England)

was used to determine Hunter L (lightness), a (redness/

greenness) and b (yellowness/blueness) from which the

hue angle (H) and the saturation (S) were calculatedusing the following expressions:

H tan1b

a; 3

Sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2 b2

p: 4

The chromameter was calibrated for internal light

(D65) before carrying out colour measurements.

2.10. Sensory analysis

Sensory evaluation of LRs cooked by RF or steam

oven was carried out by Triangle Test according to the

method of Meilgaard, Civille, and Carr (1991) to de-

termine whether or not panellists could differentiate

between the two treatments. Forty eight untrained

panellists were recruited to carry out the evaluation.

Prior to each session, samples were cut into 2 mm thick

slices. The slices were served on plastic trays with codes

to six panellists per session. A balanced incomplete

block design was used to allocate three samples to each

panellist at each time. Sample codes were created ran-

domly. Panellists assessed samples for overall texture,

juiciness, flavour and other factors. Each panellist was

given three samples (two identical and one different) of

cooked LR and asked to record the number of the odd

sample. The tests were conducted in individual tasting

booths with controlled lighting.

2.11. Statistical analysis

All instrumental texture and colour data were sub-

jected to one-way analysis of variance (ANOVA) with

cooking protocol as a factor using SAS/STAT (1990)

(Version 8.2, Statistical Analysis Systems, Cary, NC).

Where ANOVA indicated significant differences be-

tween samples, a Tukey pairwise comparison of the

means was conducted. Sensory analysis data were

analysed using a sequential test (Meilgaard et al., 1991).

Multiple comparisons of the means were performed

using the StudentNeumannKeuls test.

An L9 (34) orthogonal experiment (Geramita & Se-

berry, 1979) was designed for optimisation of the RF

cooking method. Four factors including cooking time,

output power, holding water temperature and holding

time, each containing three levels are displayed in

Table 3. A one-way ANOVA was preformed to deter-

mine if differences existed between MxT, minimum

MnT, X T and DT of the endpoint temperature by SAS

GLM program.

3. Results and discussion

3.1. Optimisation of RF cooking protocol

Results of an orthogonally designed experiment on the

effect of selected processing parameters on the MxT,

MnT, DT and XT of LR following RF cooking are pre-

sented in Fig. 4(a)(d). MxT, MnT and X T were all sig-

nificantly dependent on RF power (P < 0:005) and RFheating time (P< 0:001). However, DT was not signifi-cantly affected by any of the factors assessed (PP 0:05).Not surprisingly, MxT, MnT and X T of the LRs in-

creased as power (P < 0:05) and cooking time (P < 0:01)increased. Furthermore whilst a power of 550 W gave the

highest (P < 0:05) MxT, MnT and XT for the LR arcingand burning was a problem at this power and thus this

power level was not used in future work.

The objective in developing a cooking protocol in the

current study was to heat all measured points of the

product to a temperature in excess of 72 C and to hold

all measured areas of the product above that tempera-

ture for a minimum of 2 min. Cooking times in excess of

30 min produced higher end point temperatures (EPT)

than those required for effective pasteurisation with

temperatures up to a maximum of 90 C being attained

(Fig. 4(a)). In order to produce products, which had

received a similar heat treatments to steam cooked

products, excessively high product EPTs were avoided



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by keeping the RF heating time at 30 min. In addition, a

holding time of 2 min resulted in the lowest average DT

(10 C) within the samples though this effect was not

significant (PP 0:05). Therefore, the optimised RFcooking protocol used for all subsequent experiments

consisted of an RF power of 500 W, an RF heating time

of 30 min with circulating water at a temperature of

80 C throughout this time followed by a holding time of

2 min with no RF but with water circulating at 80 C.

3.2. Comparison of RF and steam cooked samples

3.2.1. Cooking time

A typical timetemperature profile for an RF heated

LR cooked using the optimised conditions described

above is compared to the timetemperature profile for a

steam cooked sample heated at 80 C in Fig. 5(a) and

(b). For steam cooked samples, a total cooking time of

150 min was required to ensure that the samples were

heated above 72 C for 2 min similar to the heat treat-

ment received in RF cooked samples. Thus, a 79% re-

duction in cooking time could be achieved using the

optimised conditions described previously. Fig. 5(a) and

(b) also illustrates the characteristic linear heating pro-

file of an RF heated sample (r2 0:999) followed by aplateau during the 2 min holding time. In contrast, the

timetemperature profile of a steam cooked sample

(Fig. 5(a) and (b)) has the characteristic curvilinear

timetemperature profile typical in a product where heat

transfer takes place primarily by conduction. Previous

work (Laycock et al., 2003) has indicated that for

cooked comminuted beef samples, RF samples had a90% reduction in cooking time (i.e., cooked tenfold

faster) when compared to samples cooked by immersion

in hot water between 65 and 85 C. However, the

maximum output power of the RF generator used in the

former study was 1.5 kW and the beef samples were

processed at 695 W as compared to 500 W in the present

study. The RF power used in the present study was

limited by the maximum power output of the RF gen-

erator used (600 W) and by the occurrence of product

arcing at RF powers greater than 500 W.

3.2.2. EPT distribution

Typical EPT distributions for LRs cooked by RF and

steam methods are illustrated in Fig. 6(a) and (b). As

expected for steam cooked samples, the coldest point (72

C) was located at the geometrical centre of the samples.

In contrast, for RF cooked samples, the cold point (74

76 C) was located at the part of the LR nearest the

bottom electrode (see Fig. 1). Surprisingly, given the

volumetric nature of RF heating, the temperature dif-

ferential between the coldest and hottest points was

considerably greater than for steam cooked samples

(10 vs. 5 C). Whilst in theory RF heating is volumetric

in nature with all parts of the product receiving equal

MnTMxT

75

80

85

90

WT P CT HT

74 77 80 450 500 550 25 30 35 1 2 3

WT P CT HT

74 77 80 450 500 550 25 30 35 1 2 3

WT P CT HT

74 77 80 450 500 550 25 30 35 1 2 3

WT P CT HT

74 77 80 450 500 550 25 30 35 1 2 3

NS * ** NS

65

70

75

80

NS * ** NS

TX

_

T

70

75

80

85

NS * ** NS

0

5

10

15

NS NS NS NS

(a) (b)

(d)(c)

Fig. 4. Results of orthogonal experiments on factors (water temperature (WT); power (P); cook time (CT); holding time (HT)) affecting: (a) maximum

temperature (MxT), (b) minimum temperature (MnT), (c) mean temperature (XT) and (d) temperature differential (DT) of a comminuted meat

product.



7/12

heat treatments, temperature differentials can occur for

a number of reasons. First, whilst penetration depths for

RF cooking are greater than for microwave (MW)

cooking, for large diameter products penetration depths

for RF still may not be sufficient to achieve uniform

heating of the product. In addition, and perhaps more

importantly, a lack of dielectric uniformity caused by

areas of products with high capacities for RF absorption

can lead to runaway heating and even product arcing in

these areas (Reuter, 1993).

The magnitude of the temperature differentials in the

current study was substantially less than those reported

by Laycock et al. (2003), where temperature differentials

of 19.7 9.3 C were reported in RF cooked commi-

nuted beef. This may be related to the fact that the

samples in the current study were cooked while in cir-

culating hot water whilst for Laycock et al. (2003),

samples were cooked in air which was unheated.

Therefore, it is possible that the findings of Laycock

et al. (2003) are due to the fact that samples could have

continually lost heat to the surroundings due to the

lower temperature of the surrounding air. However, it is

also possible that the meat emulsion batter in the cur-

rent work was more finely comminuted than the com-

minuted muscle of Laycock et al. (2003). Widely varying

temperature differentials imply the necessity to heat re-

gions of the product to temperatures way in excess of

those required for effective pasteurisation, which in turn

Fig. 5. Timetemperature, cook value and pateurisation unit profile for radio frequency and steam cooked luncheon roll products.



8/12

can lead to a reduction in product quality in the over-

heated regions.

3.3. Cook loss and expressible fluids

Table 4 shows results of cook loss (%) and total ex-

pressible fluid (TEF) (%), which in turn consists of ex-

pressible moisture (EM) and expressible suspended

solids (ESS). No significant difference in cook losses was

noted between RF and steam cooked samples

(PP 0:05). Furthermore cook losses were relatively low

for both samples, which was not surprising given that

samples were cooked and cooled whilst sealed in plastic

casings. Cook loss is an index of great importance to

food processors in that it relates to the final weight of

the cased end product following cooking. Since cook

loss values for both RF and steam cooked samples were

similar, it could be assumed that the encased RF and

steam cooked samples had similar moisture contents

prior to TEF measurement. In contrast, TEF is a very

different index, which is more closely related to the

juiciness of the uncased final product. TEF, EM and

ESS values were significantly lower for RF cooked vs.

steam cooked samples (P < 0:001). TEF suggest thatRF cooked LR samples had higher water-holding ca-

pacities. This observation is in contrast with the water-

holding capacity results of Laycock et al. (2003) for

comminuted meats and to those of Bengtsson and Green

(1970) who reported that RF heated hams were juicier

than water bath cooked samples. However, the findings

reported here are in agreement with those of Laycock

et al. (2003) for whole muscle and ground beef. These

workers reported that RF heated whole muscle and

ground beef samples had higher WHCs than their water

bath heated counterparts. Similar observations to those

Table 4

Cook loss and expressible fluids of luncheon roll by and steam oven

methods

Attribute Cooking method ANOVA

Steam Radio

frequency

Cook loss (%) 0.84 0.78 NS

Total expressibl e fluid (TEF) (%) 1.48 1.06 ***

Expressible moisture (%) 1.33 0.95 ***

Expressibl e suspended soli ds (%) 0.15 0.11 ***

Peak load values during TEF test 59.34 69.10 ***

(a)

Width (cm)

Height(cm)

4

2 3 4 5 6 72 3 4 5 6 7

6

8

10

12

72.072.573.073.574.074.5

(b)

Width (cm)

Height(cm)

4

6

8

10

12

78798081

8283

Luncheon roll edge

Fig. 6. Typical end-point temperature distribution profile of radio frequency and steam cooked luncheon roll: (a) steam cooked and (b) radio

frequency cooked.



9/12

of the present study were reported by Shirsat, Brunton,

Lyng, and McKenna (in press) who found that meat

emulsions which were cooked by rapid ohmic heating

methods had significantly lower TEF and EM values

than steam cooked samples. Lower TEF values in RF

cooked samples may be in some way related to the ex-

tent of protein denaturation which may be less in RFcooked samples because of the significantly shorter

cooking time (32 min for RF vs. 150 min for steam).

However, further work is needed to verify this theory.

Peak load values (N) measured during expressible

fluid analysis and fluid loss for RF cooked samples were

significantly higher than for steam cooked samples

(P < 0:001). While the expressible fluids test is not amethod specifically intended for measuring texture,

higher peak values measured during this test indicate

that RF cooked samples were slightly less compressible

than their steam cooked counterparts.

3.4. TPA and Kramer shear values

TPA and Kramer shear values for RF and steam

cooked LR are presented in Table 5. No significant

differences in springiness and cohesiveness between RF

and steam cooked samples were found (P > 0:05).Similarly Laycock et al. (2003) reported no significant

differences in springiness between RF and water bath

cooked ground, comminuted and entire muscle beef

products though these workers also reported no signif-

icant difference in chewiness between RF and water bath

cooked samples. In the present study, Hardness 1 and 2,

chewiness and gumminess of RF cooked samples weresignificantly higher than steam cooked samples

(P < 0:001). This is in agreement with the findings ofVan Roon, Houben, Koolmees, and Van Vilet (1994)

who reported that dynamic rheological measurements

demonstrated that RF cooked meat doughs had higher

storage and loss moduli and that the samples fractured

at higher stress values indicating that the samples were

more firm. In contrast, Laycock et al. (2003) reported

that Hardness 1 and 2, springiness and gumminess val-

ues for RF cooked comminuted beef were significantly

lower when compared to water bath cooked samples

(P < 0:05). The reason for this discrepancy between theresults reported in the present study and those of Lay-

cock et al. (2003) is unclear. However, it is worth notingthat the batter under examination in the present study

would be classified as a meat emulsion and had the in-

gredients added representative of what would be used in

such a product, whereas in the study of Laycock et al.

(2003) the meats examined had no added ingredients

(other than 2% salt in the case of their comminuted

meat) and merely differed in the degree of comminution.

In addition, it is worth noting that product examined in

the study of Van Roon et al. (1994) was much more

closely aligned with the type of product examined in the

present study and these authors did report that RF

cooked samples were firmer than their conventionally

cooked counterparts. In addition, the higher Kramer

shear compression load values (N) recorded in RF

cooked samples (P < 0:001) (Table 5) were broadly inagreement with the TPA hardness results (N) and also

peak load values (N) recorded during TEF analysis.

Thus, it would appear that for the products examined in

the present study RF samples were significantly firmer

than steam cooked samples which contrasts with the

observations of Laycock et al. (2003).

3.5. Instrumental colour

Results of instrumental colour analysis are presentedin Table 6. RF cooked LR samples had significantly

lower Hunter L, b and hue angle values (P < 0:01)than their steam cooked counterparts. In contrast,

Laycock et al. (2003) reported that L and a values for

RF cooked ground, comminuted and entire beef were

not significantly different to samples cooked in a water

bath. Similar to the current study, Shirsat, Brunton,

Table 5

Textural parameters of luncheon roll cooked by radio frequency and steam oven methods

Measured variable Units Abbreviation Steam (at 80C) Radio frequency ANOVAA

Texture profile analysis (TPA)

Hardness 1 N H1 25.16 31.88 ***

Hardness 2 N H2 17.04 22.37 ***

Energy 1 J A1 0.094 0.122 ***

Energy 2 J A2 0.042 0.055 ***

Springiness mm S 7.96 7.80 NS

Cohesion energy A2/A1 0.45 0.45 NS

Gumminess N GH1(A2/A1) 11.32 14.35 ***Chewiness N mm GS 90.11 111.93 ***

Kramer shear

Compression load N 49.79 59.81 ***

Extension mm 12.00 11.92 NS

A ANOVA, analysis of variance; NS, not significant PP 0:05; *, P < 0:05.



10/12

Lyng, McKenna, and Scannell (submitted) reported that

meat emulsion batters cooked very rapidly using ohmic

heating had significantly (P < 0:05) lower hue angles.However, these workers also found higher a values in

ohmically cooked samples (P < 0:001). No significantdifference in a values was found in the current study. It

has been shown that higher mean redness values (i.e., a

values) are indicative of a less well done colouration

(Lien et al., 2001a, 2001b). This is because heating is

accompanied by the denaturation of myoglobin and loss

of its characteristic bright red colour. One possible ex-

planation for the differences in a results between Shirsat

et al. (in press) and the present study is differences in the

rate of heating to temperatures in excess of 72 C. For

example, in Shirsat et al. (in press) ohmically heated

samples (showing significant differences in a values)

were heated to temperatures in excess of 73 C in 2.54.3

min, whereas in the present study the time to similar

EPTs for RF cooked samples was substantially longer at

32 min. It is very likely therefore that the very shortheating times in the study of Shirsat et al. (in press)

resulted in less heat denaturation of myoglobin. A more

likely explanation for the discrepancy is that the meat

emulsion recipe used by Shirsat et al. (in press) differed

from that used in the current study. The seasoning used

in the present study contained an artificial colouring

(E128) known as Red 2G, which was not included in the

recipe of Shirsat et al. (in press). This colouring im-

parted a pinkish red colour to the meat batter and may

have served to mask any differences in redness between

RF and steam cooked samples. This difference in the

redness of the samples is dramatically illustrated if a

values for the present study (average a 17:045) arecompared to average a values of Shirsat et al. (in press)

(average a 5:745). In the absence of Red 2G, it ispossible that differences in redness between RF and

steam cooked samples would have been noted in the

present work.

It has also been shown that rising L values correlate

with the development of cooked colour in red meats

(Ledward, 1992), where heating of samples is accompa-

nied by an increase in L value, which in turn indicates an

increase in lightness. In the present case although L

value differences were small they were significantly lower

(P < 0:01) in RF cooked samples, which in turn indicatesslightly less cook colour development in these samples.

This correlates well with Cs values for RF vs. Steam

cooked samples (Fig. 5(b)). The lower Cs values for RF

cooked samples reflect the rapid rate of heating in RF

cooking, which increased sample temperatures to the

appropriate pasteurisation temperature in a muchshorter time than an equivalent pasteurisation in a steam

cooked sample. This relationship between Cs and mi-

crobial destruction was discussed at length by Holds-

worth (1985) who described four possible outcomes for a

heat treatment. These outcomes redefined in the context

of the present pasteurisation treatment could be de-

scribed as uncooked unpasteurised, cooked unpasteur-

ised, uncooked pasteurised and cooked pasteurised.

Given the low Cs values for RF samples and their rela-

tive lack of colour development compared to a steam

cooked samples, it is possible that RF cooked samples

are more closely aligned to the uncooked pasteurised

state than their steam cooked counterparts. This is not

surprising since RF samples were cooked at five times the

rate of steam cooked samples and the reactions which

leading to colour change on heating are dependent on the

rate of heating (Palombo & Wygaards, 1990). However,

a possible solution to this problem would be to extend

the holding time after application of RF which in turn

would increase the Cs value. Alternatively higher Cs

values could be obtained in RF cooking by allowing

product temperatures to increase to higher levels than

the maximum encountered in the present study.

3.6. Sensory evaluation

A similarity triangle test using 48 panellists indicated

that panellists could distinguish at the 1% level between

RF and steam cooked LR samples. Results for the

sensory sequential test are shown in Fig. 7. In keeping

with results for instrumental value for texture and col-

our, panellists attributed the differences to texture and

colour. However, a significant number of panellists also

reported that they could discern differences in flavour

between the samples. Van Roon et al. (1994) also re-

ported that sensory evaluation of core samples from RF

and waterbath cooked meat batters indicated that RF

cooked samples were firmer than their water bath

cooked counterparts. In contrast, in a triangle test

similar to that of the present study, Shirsat et al. (in

press) reported no significant difference (PP 0:05) be-tween steam and ohmically cooked samples. It is worth

noting, however, that instrumental textural analyses of

ohmically cooked samples in the study of Shirsat et al.

(in press) indicated that the samples differed only from

steam cooked samples with regard to their springiness

values whereas in the present case a number of textural

parameters differed between RF and steam cooked

samples.

Table 6

Colour attributes of luncheon roll cooked by radio frequency and

steam oven methods

Attribute Cooking method ANOVA

Steam Radio frequency

L 60.09 59.09 **

Hue angle () 22.62 19.89 **Saturation 18.41 18.17 NS

a 17.01 17.08 NS

b 6.96 6.19 **



11/12

4. Conclusion

Using the optimised cooking protocol developed in

this work, a 79% reduction in pasteurisation time of

encased luncheon roll meat was obtained compared to

equivalent steam cooked samples. Temperature differ-

entials were twofold higher within the RF cooked

sample relative to its steam cooked counterparts. TEF,

EM and ESS values were significantly lower for RF vs.

steam cooked samples indicating that RF samples had

higher capacity to hold water following cooking. In-

strumental texture analysis indicated that RF cooked

samples were harder, gummier and chewier than theirsteam cooked counterparts. RF cooked samples ap-

peared to have less colour development than their steam

cooked equivalents as indicated by their lower L, b and

hue angle values, though differences in a values may

have been masked due to the inclusion of a red col-

ouring in the formulation. This is most likely related to

the substantially lower Cs values for RF cooked sam-

ples, which indicates that RF samples while sufficiently

pasteurised may require extended heat treatment to

produce characteristics which are more comparable with

the steam cooked samples. In agreement with instru-

mental values for texture, colour and expressible fluids,results of a sensory similarity test indicated that panel-

lists could distinguish between RF and steam cooked

samples.

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05 10 15 20 25 30 35 40 45

number of trials

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http://dx.doi.org/doi:10.1016/j.meatsci.2004.02.009http://dx.doi.org/doi:10.1016/j.meatsci.2004.02.009http://dx.doi.org/doi:10.1016/j.meatsci.2004.02.009http://dx.doi.org/doi:10.1016/j.meatsci.2004.02.009

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Effect of Radio Frequency Cooking on the Texture, Colour And