- Email: [email protected]
An examination of factors affecting radio frequency heating of an encased meat emulsion
MEAT SCIENCE Meat Science 75 (2007) 470–479 www.elsevier.com/locate/meatsci
An examination of factors affecting radio frequency heating of an encased meat emulsion James G. Lyng
a,*
, Denis A. Cronin a, Nigel P. Brunton b, Wenqu Li a, Xiaohong Gu
c
a
c
School of Agriculture, Food Science and Veterinary Medicine, College of Life Sciences, University College Dublin, Belfield, Dublin 4, Ireland b Ashtown Food Research Centre, Teagasc, Ashtown, Dublin 15, Ireland Key Laboratory of Food Science and Safety, Ministry of Education, Southern Yangtze University, Wuxi, Jiangsu 214036, China Received 19 November 2005; received in revised form 9 June 2006; accepted 27 July 2006
Abstract The potential of radiofrequency (RF) heating for rapid cooking of a cased comminuted meat emulsion (white pudding) to a pasteurisation temperature of 73 C was examined. Immersion of the product in water was essential in order to prevent thermal damage to the casings by electrical arcing effects during heating. Using a polyethylene heating cell with non-circulating water the applied RF power, primary electrode distance as well as the mineral content, temperature and volume of the surrounding water all influenced the efficiency of the RF heating. Under optimised conditions maximum/minimum temperature gradients (DT) across the products in excess of 15 C were observed. These could be reduced to around 6 C by heating the white puddings in a cell operating with recirculating hot water (80 C). Using an oven power output of 450 W a 4.3-fold reduction in cooking time compared to conventional steam oven cooking could be achieved. 2006 Elsevier Ltd. All rights reserved. Keywords: Radio frequency; Cooking protocol development; Meat emulsion
1. Introduction RF heating involves application of an alternating electromagnetic field to a foodstuff at frequencies from 1 to 300 MHz (Risman, 1991). The method differs from other dielectric methods of heating such as microwave (MW) heating as the product is placed between two parallel electrodes and an RF field is generated in a directional fashion at right angles to the surface of the electrodes. In addition, the mechanism of heating in an RF field is different. MW heating occurs mainly via frictional heat generated from the dipolar rotation of free water molecules whereas the predominant mechanism of heating at RF is via the depolarisation of solvated ions. *
Corresponding author. Tel.: +353 1 7167710; fax: +353 1 7161147. E-mail address: [email protected] (J.G. Lyng).
0309-1740/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2006.07.022
The earliest application of RF technology dates back to the 1940s when the method was used to cook meat products, heat bread and dehydrate and blanch vegetables (Kinn, 1947; Moyer & Stoltz, 1947). Unfortunately, the high capital cost required for installation of the equipment prevented uptake of the technique by industry. Later the focus turned to the use of RF for thawing fish and meat blocks (Bengtsson, 1963; Sanders, 1966), which resulted in several commercial production lines (Jason & Sanders, 1962a, 1962b). More recently, the technology has found application in the post bake drying of cookies and snack foods (Mermelstein, 1998; Rice, 1993) and this is perhaps the most widely used current application of this technique. By contrast, though several studies have examined the use of RF for pasteurisation of meat products including cured hams (Bengtsson & Green, 1970) sausage emulsions (Houben, Schoenmakers, van Putten, van Roon, & Krol, 1991)
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
and whole and comminuted beef products (Laycock, Piyasena, & Mittal, 2003), the method is not widely used commercially for this purpose. In fact, the only industrial application for RF pasteurisation of meats involved a joint venture between APV and Tulip International where attempts were made to use RF for the pasteurisation of uncased meats which were cooked while being pumped continuously through a low loss tube. However, while this system was widely acclaimed commercially it is not currently in use. Recent work at UCD Dublin focused on the application of RF energy to encased meat products, which is a different approach to that taken by APV. Results have shown RF cooked cased meats, including the product examined in the present study, to be largely comparable in quality to conventionally cooked products (Brunton et al., 2005; Tang, Cronin, & Brunton, 2005; Tang, Lyng, Cronin, & Durand, 2006; Zhang, Lyng, & Brunton, 2004; Zhang, Lyng, & Brunton, 2006). Because quality considerations were the main focus of the published work, details of the actual development of an RF cooking procedure, which was not straightforward, were never reported. The objective of the current paper is to outline the experimental strategies used in the development of efficient methods to cope with special problems associated with the cooking of meats contained within plastic casings. Such information could be potentially of use to other researchers involved in the development and optimisation of RF cooking procedures for a more diverse range of packaged food products. 2. Materials and methods 2.1. Meat product manufacture A commercial meat product (as opposed to a model food system) was the preferred material for this study. Work was conducted using a pork white pudding (WP) formulation which, because of its highly homogenous nature and relatively small size, was a convenient and cost effective product on which to carry out large numbers of investigative cooking experiments. 2.1.1. Preliminary handling of meat Lean pork shoulder and pork back fat were obtained from a local producer (Galtee meats, Cork, Ireland). A mechanical mincer (Model No. TS8E, Tritacarne, Omas, Italy) was used to grind lean tissue through a plate with 3.5 mm diameter holes while fat was ground through a 10 mm plate. The ground tissue (2 kg lots) was 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 manufacture. 2.1.2. WP manufacture The ingredients, recipe and manufacturing protocol used in the preparation of a comminuted meat batter for-
471
mulated to the specification of a fairly standard WP were given in Brunton et al. (2005). 2.2. Cooking equipment 2.2.1. Conventional cooking of WP For steam processing, samples were cooked in a thermostatically controlled Kerres smoke-air steam oven (Type CS 350, Raicher-und-Kochanlagen, D-71560 Sulzbach-Murr, Germany) set at 80 C. Other oven settings include having the turbine at 2800 rpm, turning the heating switch to ‘‘high’’ and humidification in the on position (to produce a 100% RH environment in the oven chamber). 2.2.2. RF cooking of WP The RF oven used was a custom built 50X system built by C-Tech Innovation (Chester, UK) using a Coaxial Power System Ltd low power RF generator (Model No. RFG 600-27, Spectrum House, Finmere Road, Eastbourne, East Sussex, UK) and a complementary automatic impedance matching network and controller (Model No. AMN 60027). The system had an operating frequency of 27.12 MHz and a maximum power output of 600 W. The oven consisted of a chamber with electrically insulated walls, while the RF generated energy was applied via a set of parallel rectangular steel electrodes. The upper electrode (29 · 20 cm) (Fig. 1) was fixed in position while the lower electrode (46 · 24.5 cm), was in the form of a shelf, which could be moved up and down within the oven. These primary electrodes could be spaced up to 24.5 cm apart, though the oven was fitted with a rack which allowed the shelf to be moved up and down in 7 cm intervals, while fine tuning within these intervals was by way of a series of nuts and bolts which were attached to rack and shelf. Additional fine adjustment of the space between electrodes was achieved with aid of a fabricated perforated steel plate (20 · 15 cm) screwed to the lower shelf with adjustable metal spacers (Fig. 1). 2.3. Temperature distribution within products following cooking Endpoint temperature measurements for each treatment were evaluated for every cooked pudding by transferring the latter immediately after cooking to a specially constructed thermocouple jig. Fig. 2 provides an illustration of the jig, which was manufactured from rectangular blocks of wood with a hollowed out area for insertion of the pudding. In order to avoid cooling the outer part of the pudding by contact with the jig, the latter was preheated by immersion in a water bath set at 80 C for a few minutes just prior to making a measurement. Temperatures at nine points within the sample were monitored and recorded using 1.2 mm diameter Type T thermocouples (Industrial Temperature Sensors, Dublin, Ireland) and a Grant Squirrel data logger (Model No. 1600, Grant Instruments Ltd., Barrington, Cambridge CB2 5QZ, England). Maximum temperature (MxT), minimum temperature (MnT), mean temperature ð xTÞ and tem-
472
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
Cell containing product placed in this region
Primary electrodes
Fig. 1. Schematic diagram of RF oven interior showing electrode positions.
Fig. 2. Schematic side profile of thermocouple jig for end point temperature measurement in white puddings.
perature difference (DT) of the endpoint temperature was extracted from the measured data.
in air. For reasons to be discussed later this was found to be unsatisfactory and was abandoned.
2.4. Developing a method for RF cooking of cased meatsz 2.4.1. RF cooking of cased products in air Preliminary studies on the application of RF heating to the cooking of cased meats involved cooking the products
2.4.2. RF cooking of cased products submerged in water Subsequent trials were all conducted by cooking the puddings while they were submerged in water and this involved the development of holding cells in which the
Table 1 Semi-crystalline high density polyethylene cells used for RF cooking of white puddings in the present study Dimensions
Cell design A
B
C
External dimensions – L · B · H (mm) Internal dimensions – L · B · H (mm) Upper electrode dimensions – L · B · H (mm) Lower electrode dimensions – L · B · H (mm) Shape of internal cross section Circulating water Water temperature (C)
205 · 111 · 51 184 · 61 · 46 183 · 81 · 1.5 178 · 61 · 1.5 Rectangular No 20–80
210 · 111 · 51 187 · 61 · 46 207 · 102 · 1.5 184 · 61 · 1.5 Rectangular Yes 80
251 · 111 · 61 188 · 61 · 49 202 · 77 · 1.5 188 · 60 · 1.5 Rectangular Yes 80
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
473
Fig. 3. Images of polyethylene cells developed in the current study.
meats could be cooked under either static (non-circulating water) conditions or under water circulation. Three cells, the dimensions of which are given in Table 1, and which are illustrated in Fig. 3 were manufactured from semi-crystalline high density polyethylene (Alperton Engineering Ltd., Dublin Industrial Estate, Glasnevin, Dublin, Ireland) with each cell being designed to accommodate a single 200 g encased pudding during cooking.
2.4.2.1. Developing a cell to allow RF cooking under noncirculating water 2.4.2.1.1. Cell Design A. The initial basic cell design was based on that outlined by Bengtsson and Green (1970) and therefore was fitted with secondary electrodes, one of which (bottom electrode) was a removable stainless steel plate which was placed internally on the base of the cell. These authors claimed that secondary elec-
trodes helped to focus the RF field on the product and to maximise efficiency. After placing a pudding in the cell the latter was filled with water at a specified temperature and a steel plate which also functioned as the top secondary electrode was immediately placed on top. Two short polythene screws were used diagonally opposite to each other to hold the top electrode in position. This cell was placed on the lower electrode of the RF oven during cooking. In the present study a standard size of WP was used which was submerged in water. However, the volume of water surrounding this WP could be reduced stepwise from approximately 300 ml to 150 ml by inserting an appropriate number of rectangular polyethylene spacers (70 mm · 10 mm · 5 mm) into the cell before addition of the water required to cover the WP during cooking. The volume of the immersion water was determined by draining the latter into a graduated cylinder before refilling for cooking.
Table 2 Factors evaluated in initial experiments on RF cooking of white puddings in a static cell (Cell Design A) Factor examined
RF output power (ROP) Initial water temp. (ITW) Water vol. in cell (VWC) Prim. electrode gap (PEG) Water used in cell (H2O) CT: Cooking time.
Levels examined
200, 300, 400, 500, 600 20, 40, 60, 80 150, 180, 210, 240, 270, 300 55, 65, 75, 85, 95 Deionised (D), Tap (T)
Levels of controlled factors CT (min)
ROP (W)
ITW (C)
VWC (ml)
PEG (mm)
H2O
12 12 12 12 12
– 400 400 400 400
20 – 20 20 20
180 180 – 180 180
65 65 65 – 65
T T T T –
474
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
2.4.2.1.2. Optimisation of RF cooking conditions in a static cell. In order to assess the impact of a range of variables on the efficiency of RF cooking of WPs of uniform size (200 g), five separate experiments was conducted (Table 2). The factors examined in these experiments included (a) RF output power (200–600 W), (b) initial temperature of surrounding water (20–80 C), the latter was varied by introducing water of different temperatures into the RF cell immediately prior to cooking, (c) volume of water in RF cooking cell (150–300 ml), (d) distance between primary RF electrodes (55–95 mm) and (e) the use of deionised or tap water to surround the samples. While an individual factor such as water volume or type (deionised or tap water) was being examined all the other parameters were kept constant at the following levels: RF power 400 W, initial water temperature, 80 C, electrode distance, 65 mm. MxT, MnT, xT and DT data for all RF cooked samples was collected. Apart from the experiment on water type where 10 replicates were used, the results presented are the mean values of four replicates.
2.4.2.2. Developing a cell to allow RF cooking under circulating water. In this work, the RF oven was set at 450 W as preliminary investigations with recirculating water indicated that the power level of 450 W appeared to give slightly higher xT but had comparable reproducibility in replicate cooking experiments to the 400 W used with the static cell. Other cooking conditions such as water type, primary electrodes distance etc. were set up on the basis of the optimised operating parameters established with the static cell, details of which are presented in Section 3.2. 2.4.2.2.1. Cell Design B. Cell B was similar in dimensions to Cell Design A but was fitted with polyethylene inlet and outlet ports to allow water circulation during cooking. Sealing the top electrode onto the cell was by way of screws and a countersunk rubber gasket. Fig. 3b provides an image of this cell while dimensions are given in Table 1. 2.4.2.2.2. Cell Design C. This cell incorporated inlets and outlets for circulating water (Fig. 3c). It also included a side mounted screw type cap which enabled product to be introduced through the side of the cell. A leak proof system was achieved by securing the steel plate comprising the top electrode to the block with the aid of 12 short (1 cm) evenly spaced steel screws. The cell was manufactured from 60 mm polyethylene which allowed for the use of optimum primary electrode spacing (Table 1). A second series of experiments was carried out to see if further improvements in performance could be achieved using the optimised circulating cell design (Cell C). Compared with the first set of experiments, in which each factor was examined individually because they were not interdependent, in the second series of experiments the influence of continuous water flow at different flow rates as well as the effect of continuous/interrupted flow at a fixed flow rate was examined using a factorially designed experiment to
assess the level of interaction between these factors. In the continuous mode the three levels of water flow rate examined were 250, 450 and 800 ml min1. Using a flow rate of 450 ml min1 and a water bath temperature of 80 C the periods of interrupted flow during an 8 min RF cooking cycle were 7, 3 and 0 min. MxT, MnT, xT and DT data were monitored and recorded as for the previous experiments. 2.5. Statistical analysis All ANOVA analyses were carried out using the SAS statistical analysis software (Version 8.2, Statistical Analysis Systems, Cary, NC, USA). Where ANOVA indicated significant differences between samples a Tukey pairwise comparison of the means was conducted. 3. Results and discussion A principle objective of the present study was the development of an RF cooking protocol which facilitated fast and uniform heating of a comminuted pork meat product as represented by WP. To this end a number of different factors were examined in order to determine the best conditions to achieve efficient RF cooking. 3.1. RF cooking of cased meats in air RF cooking of the cased meats in air would have been the most convenient and simple method of applying the RF technology to products of this type. However, from the outset it became apparent that this was not a viable option for cased products as the casings and cable ties showed signs of overheating accompanied by frequent and unpredictable ignition during the cooking process. This was completely unsatisfactory since under these conditions the migration of potentially toxic compounds from the packaging into the product was a possibility, not to mention the adverse effect on product appearance. A variety of strategies were examined in an attempt to alleviate this problem which included: (a) Changing independent variables within the RF heating system (such as power level and electrode spacing). (b) Evaluating a range of commercially available casings and cable ties (in case the ignition problem was packaging material specific). (c) Evaluating a range of meat products (in case the problem was specific to a particular product formulation/composition). (d) Covering the bottom electrode with PTFE overlays (to evaluate whether or not ignition was being induced through contact with a metal surface). (e) Increasing the humidity of the oven cavity (to see if the presence of condensed water on the product could prevent ignition).
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
(f) Increasing/decreasing the temperature within the oven cavity. (g) Suspending the cased meat products between the electrodes (in an attempt to see if avoiding contact with a surface could prevent ignition). None of the above strategies was completely successful in preventing thermal damage to casings during RF heating of the puddings. However, it was found that if cased products were submerged in a beaker of water during RF cooking, the problem of casing ignition could be largely eliminated. The overheating effect appeared to be due to excessive focussing of RF energy on specific positions within the casings where, during the heating process, localised pools of fluid with a high ionic concentration and dielectric potential may have collected. The beneficial effect of surrounding the product with water appeared then to be due to the ability of the latter to dissipate this heat before it caused thermal damage to either product or casing. Preventing excessive focussing of RF energy should help to minimise field distortions and this has been claimed to be one of the benefits of the use of surrounding water in RF heating (Houben et al., 1991; Sanders, 1966). 3.2. RF cooking under non-circulating water – Cell Design A Since immersion in water appeared to be a prerequisite for safe RF heating of cased meat products, it was then necessary to examine the conditions under which the procedure could be carried out with both efficiency and convenience. Thus, a simple cooking cell fitted with secondary Table 3 Effect of RF power on temperature profile of white puddings RF powerA (W)
xT (C)
MxT (C)
MnT (C)
DT (C)
TCW (C)
200 300 400 500 600
58.1a 73.2b 87.0c 85.7c 86.0c
61.3a 80.3b 94.2c 95.0c 92.4c
54.2a 66.4b 78.0c 73.2c 76.8c
7.1a 13.8b 16.2bc 21.8c 15.6b
47.5a 56.8b 66.2c 69.0cd 71.3d
xT: Mean temp.; MxT: maximum temp.; MnT: minimum temp.; DT: temp. rise pre- to post-cooking. a–d Means in the same column with unlike letters are different, P < 0.05. A See Table 2 for levels of controlled factors. Table 4 Effect of electrode distance on temperature profile of white puddings Electrode distanceA (mm)
xT (C)
MxT (C)
MnT (C)
DT (C)
TCW (C)
55 65 75 85 95
79.0b 87.0a 72.8c 61.7d 50.8e
89.5a 94.2a 82.4b 70.2c 54.4d
67.1b 78.0a 64.9b 54.2c 47.5d
22.4a 16.2b 17.5ab 16.0b 6.9c
66.8a 66.2a 56.5b 48.4c 39.5d
xT: Mean temp.; MxT: maximum temp.; MnT: minimum temp.; DT: temp. rise pre- to post-cooking. a–e Means in the same column with unlike letters are different, P < 0.05 A See Table 2 for levels of controlled factors.
475
Table 5 Effect of initial cell water temperature on post-cooking temperatures in white puddings cooked in Cell Design A Water temp.A (C)
xT (C)
MxT (C)
MnT (C)
DT (C)
xTCW (C)
20 40 60 80
79.3a 80.3a 81.6a 84.4b
83.3a 86.2ab 87.2b 88.1b
74.7a 75a 76.1a 79.8b
8.6a 11.2a 11.1a 8.3a
63a 68.7b 70bc 73.1c
xT: Mean product temp.; MxT: maximum product temp.; MnT: minimum product temp.; DT: product temp. rise pre- to post-cooking; xTCW: Mean Cooking Water Temp. a–c Means in the same column with unlike letters are different, P < 0.05. A See Table 2 for levels of controlled factors. Table 6 Effect of surrounding water volume on post-cooking temperatures of white puddings cooked in Cell Design A Water volumeA (ml)
xT (C)
MxT (C)
MnT (C)
DT (C)
xTCW (C)
150 180 210 240 270 300
87a 87a 84.4a 81.5b 79.2bc 76.9c
93.6a 94.2a 90ab 89.8ab 87.8bc 83.6c
79.2a 78a 77.5a 73.2b 72.7b 71.3b
14.4a 16.2a 12.5a 16.6a 15.1a 12.3a
72.9a 66.2b 66.2b 64.3b 61c 57.4d
xT: Mean temp.; MxT: maximum temp.; MnT: minimum temp.; DT: temp. rise pre- to post-cooking. a–d Means in the same column with unlike letters are different, P < 0.05. A See Table 2 for levels of controlled factors. Table 7 Effect of water-type on temperature profile of white puddings Water typeA
xT (C)
MxT (C)
MnT (C)
DT (C)
TCW (C)
Deionised Tap
83.6a 87b
95.4a 94.5a
70.9a 77.6b
24.5a 16.9b
54.9a 66.6b
xT: Mean temp.; MxT: maximum temp.; MnT: minimum temp.; DT: temp. rise pre- to post-cooking. a–b Means in the same column with unlike letters are different, P < 0.05. A See Table 2 for levels of controlled factors.
stainless steel electrodes to focus the RF energy, was designed for cooking 200 g WPs. The purpose of this cell was to evaluate the kind of performance that might be expected under batch cooking conditions where products would be transferred to an RF heating cell, covered with water and the RF power turned on. The time for steam cooking of the WPs to a temperature of 73 C at the coldest point (centre) was 33 min. In order to examine a range of cooking variables under RF conditions it was decided to arbitrarily aim for a cooking time that was 2.5–3 times faster than steam cooking. Thus, the data presented in Tables 3–7 were, unless otherwise stated, measured using an RF cooking time of 12 min (i.e. 2.75 faster). Before looking at the effect of other variables on the performance of RF in the cooking of WPs, it was useful to first establish the optimal instrumental operating parameters for the particular RF oven arrangement used in the present study. The other variables could then be examined at the appropriate optimised oven settings. Of the latter the two most impor-
476
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
tant were the oven power and the distance between the primary electrodes. 3.2.1. RF power and electrode distance Post-cooking temperature data for WPs cooked in water (180 ml, initial temperature 20 C) with RF power levels in the range 200–600 W are listed in Table 3. xT, MxT and MnT temperatures increased with increasing RF power (P < 0.05) up to 400 W above which no significant change (P P 0.05) was observed suggesting a plateau in the power range 400–600 W. The data in Table 3 clearly shows very little increase in heating performance using power settings above 400 W. In addition 400 W afforded maximum stability in relation to tuning of the system and was therefore used in subsequent experiments carried out with static cell A. Temperature data on WPs cooked at 400 W when the distance between the electrodes was varied from 55 to 95 mm are given in Table 4 and show clearly that the optimum distance was 65 mm. 3.2.2. Initial water temperature Post-cooking temperature data for RF cooked WPs with starting water temperatures of 20–80 C are listed in Table 5. Not unexpectedly, the values of xT and MnT appeared to increase with increasing initial water temperature. However, the values for xT and MnT were only significantly different (P < 0.05) for the samples cooked at initial water temperatures of 20–60 vs. 80 C. The relatively modest temperature differentials between the hottest and coldest spots (DT) ranging from 8.3 to 11.2 C for the different starting water temperatures, were not significantly different (P P 0.05). In this work the target to achieve sufficient microbial kill was to ensure that all areas of the product were heated to a temperature of in excess of 73 C for 2 min which is well above the Irish guideline of 70 C for 2 min for such products. Under RF cooking in a static cell, this could be achieved by using RF energy to heat the product to >73 C and subsequently allowing the product to stand in the surrounding water with RF energy off for 2 min. Success in this depended upon having the surrounding water at sufficiently high temperature (i.e. minimum of >73 C, throughout the holding period). In this experiment it must be highlighted that although the specified starting temperature of the water at addition to the cell ranged from 20– 80 C, the introduction of a refrigerated pudding (200 g) (at 4 C) would have had a substantial cooling effect on the volume of water used (180 ml). In the experiment described a 12 min cooking time only raised the mean temperature of the surrounding water to a suitable holding value (73.1 C) in the cooking run where the water initially added was at 80 C. Even under these conditions there was a problem since at the end of the RF heating period a temperature differential of 5-10 C was observed between the surrounding water at the top side of the WP and the much hotter water underneath it. Possible solutions to this could include (a) physical agitation of the cell and its contents at
the end of RF heating to rapidly equilibrate the surrounding water temperature (b) no agitation, but extended RF heating to ensure the water were heated in excess of 73 C at its coldest point. 3.2.3. Volume of water in RF cell Post-cooking temperature data for RF cooked comminuted meat samples with starting water volumes of 150–300 ml are listed in Table 6. On average, for all three parameters listed (xT, MxT and MnT) final product temperatures generally increased with decreasing volume of surrounding water though significant differences were only noted when the extremes of volume are compared (P < 0.05). Prior to cooking the temperature of the smaller volumes of 20 C water would have been reduced to a somewhat greater degree by the addition of the refrigerated pudding (5 C). However, possible explanations for the higher final temperatures observed in both product and surrounding water could include: (a) during cooking simple heat loss from the white pudding would increase the temperature of a smaller volume of water to a greater extent; (b) the mass of potentially dielectrically active material was reduced due to the partial replacement of water in the cell by dielectrically inactive polyethylene spacers. 3.2.4. Effect of deionised water Both final xT and MnT for WPs were significantly higher for samples surrounded in tap water as opposed to deionised water (Table 7) (P < 0.05). Although there appears to be an apparent contradiction to this, with MxT for samples surrounded by deionised water appearing on average higher, this effect was not significant (P P 0.05). It has been suggested elsewhere that surrounding the product in deionised water would serve to increase the absorption of RF power by the product (Houben et al., 1991). However, in the present case both xT and MnT were significantly higher when the product was surrounded by tap water. The most important effect however, was that use of deionised water resulted in significantly higher DT in samples (P < 0.05) most likely due to a greater cooling influence of the surrounding deionised water. This cooling effect would have arisen due to the deionised water (with its very low content of ions) absorbing less RF energy. Heat would subsequently flow from the RF heated pudding into this cooler water which in turn would increase the DT. (Note: Electrical conductivity (lS cm1) was used as an index of the ionic content of water and values for the deionised, distilled and tap water used in the present study were 0.7, 3–12 and 166, respectively.) The greater DT could result in notable variation in product quality within a given sample not to mention causing greater difficulties in the validation of the heat process. It should be noted however that although the product was surrounded in a casing some seepage could have occurred during RF cooking thus the water was not free of ions by the end of the cooking time.
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
3.3. RF cooking under circulating water Based on the observations above, the RF cooking of products in a static water environment would appear to have the following shortcomings: (a) relatively high product DT’s (as shown in Tables 3–7), (b) temperature differentials within the cooking water itself, (c) significant cooling of the added water on introduction of refrigerated WPs. Therefore, another series experiments were conducted to examine the effect of recirculating hot water around the product. The circulation of hot water also allowed holding periods in circulating water after RF heating, thereby reducing uncertainties in relation to cooling of the water/ product during the holding period. 3.3.1. Optimisation of cell design for RF cooking with circulating water 3.3.1.1. Cell Design B. This design was based on Cell Design A but proved very difficult to get a water tight seal and was therefore abandoned as it was problematical in terms of leaks during cooking.
3.3.1.2. Cell Design C. Leaks were not a problem with this cell design and it was also easy to install and remove the puddings. However, an intermittent problem of burning was encountered in this circulating water cell and also in Cell Design A as shown in Fig. 4a. The image illustrates that two points on the upper side of the pudding were burned, and similar burn marks were found at corresponding positions on the top secondary electrode indicating that burning could sometimes occur if the pudding moved and
477
touched the top secondary electrode during cooking. To avoid this problem and to prevent movement of the pudding within the cell a square of thin perspex of identical cross section to the interior of the cell was prepared with a hole in the centre (Fig. 4b). The purpose of this opening was to allow the tie at the end of the casing to pass through thereby anchoring the pudding in position. Some smaller holes were also incorporated to allow water to circulate around the end section of the pudding. A series of holes were also drilled in a 30 ml plastic weighing cup which was placed over the other end of the pudding in such a way that when the sealing cap was tightened on the cell the pudding was prevented from moving. This strategy eliminated all problems of arcing with this system. 3.3.2. Effect of water flow conditions on temperature distribution within the product MxT, MnT, xT and DT data for WPs cooked by RF in combination with continuous/interrupted hot (80 C) water flow at different flow rates are presented in Fig. 5i– iv. No significant difference in xT (Fig. 5i) was observed using either continuous or interrupted water flow (P P 0.05), nor when the water flow rate was increased from 450 to 800 ml min1 (P P 0.05), though a significant increase in xT was noted when the water flow rate was increased from 250 to 800 ml min1 (P < 0.05). The results suggest that considerable variations in water flow rates may occur before differences in xT will be encountered. DT values for WPs cooked by RF with hot water circulation are presented in Fig. 5iv. Significant differences were observed due to flow time, with continuous flow showing lower DT values compared to products around which water
Fig. 4a. Image showing an intermittent burning problem encountered circulating water cells where the pudding contacted the upper secondary electrode during cooking.
Fig. 4b. Schematic cross section through cell showing modifications to prevent contact between secondary electrodes and the product.
478
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
(ii) MxT 80
75
75
a1
ab
b
a
a
a
70
Temperature (oC)
Temperature (oC)
(i) x T 80
65
60
ab
a
b
b
65
Flow Rate (ml min-1) 250 450 800
1
60
Flow time (min) 5 Continuous
Flow Rate (ml min-1) 250 450 800
(iv)
(iii) MnT 20
75
15
70
a
b
b
a
a
a
60
Temperature (oC)
a Temperature (oC)
a
70
80
65
a
b
ab
1
Flow time (min) 5 Continuous
ΔT a
ab
b
10
5
0
Flow Rate (ml min-1) 250 450 800
1
Flow time (min) 5 Continuous
Flow Rate (ml min-1) 250 450 800
1
Flow time (min) 5 Continuous
Fig. 5. (i)–(iv) Mean ð xTÞ, maximum (MxT) and minimum (MnT) temperatures and also temperature differentials (DT) in products RF heated in circulating water cells under a range of flow rates and flow time.
was circulated for only 1 min (P < 0.05). However, no significant difference in DT was noted between products under continuous flow and those which had water circulating around them for 5 min (P P 0.05). Results suggest that continuous flow or reasonably prolonged circulation of water with interruptions is required to minimise product DT. Results for flow rate show an increase in xT (significant between 250 vs. 800, P < 0.05) which can be rationalised on the basis of a greater energy input into the product from the larger volumes of 80 C water circulating around it per unit time. Although the reasons for the DT differences as a function of flow rate (Fig. 5iv) are not entirely clear a possible explanation is as follows. The higher DT at 800 ml min1 may possibly be due to greater heating of the outer regions of the product due to the higher volumes of hot water circulating around it per unit time. At the lowest flow rate (250 ml min1), the higher DT may be due to insufficient water circulating to remove the excess heat generated by the absorption of RF power at the bottom of the product, which could be similar to the temperature gradients observed between water at the bottom vs. top of the cell noted under non-circulating water flow conditions (Section 3.2.2). For the current system the optimum flow rate which produced the smallest DT values occurred when the water flow rate was 450 ml min1. Table 8 shows the combined effect of water flow rate and flow duration on DT within WP’s. A continuous flow rate of 450 ml min1 produced a DT value of 6.3 C which was
Table 8 Combined effect of water flow rate and flow duration on DT within white puddings Flowing time (min)
Flow rate (ml min1)
1 5 Continuous
250 15.9 17.8 12.8
450 13.4 13.5 6.3
800 16.3 12.5 13.7
DT: Temp. rise pre- to post-cooking.
the lowest DT achieved in all experiments with static and continuous flow cells. A DT of 6.3 C is low relative to the magnitude of DT values which have been reported elsewhere for RF heated meats (Bengtsson & Green, 1970; Laycock et al., 2003). However, the cooking conditions, RF applicators and product types used in these studies are not directly comparable to those used in the current study. Zhang et al. (2004) published a study on a larger RF cooked comminuted product (1.2 kg and 9 cm diameter vs. 0.2 kg and 4.2 cm diameter in the present study) in a circulating water cell and reported DT values of approximately 10 C which is larger than that obtained in the present study. This difference can possibly be attributed to larger sample size and differences in the design of the RF cooking cell between the studies. Finally, when using the optimised conditions involving an RF oven power output of 450 W and recirculating hot water (80 C) at a flow rate of 450 ml min1, the 200 g
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
white puddings could be heated to a minimum temperature of 73 C in 7.7 min compared to 33 min using a steam oven. This represented a 4.3-fold decrease in cooking time. 4. Conclusions This study has shown that the cooking of encased meat products in air is not a viable proposition because of an unacceptable risk of thermal damage to the casing due to arcing. Surrounding the product with water (static or circulating) largely alleviates this problem though it is important to prevent the product from making physical contact with the upper surfaces within cooking cells. Such contact may result in the formation of a localised static air/vapour/liquid pool which can occasionally lead to arcing and burning of the casing. Efficient RF cooking in static water was achieved through the optimisation of RF power, electrode spacing and initial water temperature/volume/type. However, recirculating hot water around the product during RF cooking was found to give greater overall temperature control and reduced temperature gradients within the product. Acknowledgement This research has been part-funded by grant aid under the Food Institutional Research Measure, which is administered by the Department of Agriculture and Food, Ireland. References Bengtsson, N. E. (1963). Electronic defrosting of meat and fish at 35 and 2450 Mc – a laboratory comparison. Food Technology, 17(10), 92–100. Bengtsson, N. E., & Green, W. (1970). Radio frequency pasteurisation of cured hams. Journal of Food Science, 35, 681–687.
479
Brunton, N. P., Lyng, J. G., Li, W., Cronin, D. A., Morgan, D., & McKenna, B. (2005). Effect of radio frequency heating on the texture, colour and sensory properties of a comminuted pork meat product. Food Research International, 38, 337–344. Houben, J., Schoenmakers, L., van Putten, E., van Roon, P., & Krol, B. (1991). Radio frequency pasteurisation of sausage emulsions as a continuous process. Journal of Microwave Power and Electromagnetic Energy, 26(4), 202–205. Jason, A. C., & Sanders, H. R. (1962a). Dielectric thawing of fish. Experiments with frozen herrings. Journal of Food Technology, 16(6), 101–106. Jason, A. C., & Sanders, H. R. (1962b). Dielectric thawing of fish. Experiments with frozen White fish. Journal of Food Technology, 16(6), 107–112. Kinn, T. P. (1947). Basic theory and limitations of high frequency heating equipment. Food Technology, 1, 161–173. Laycock, L., Piyasena, P., & Mittal, G. S. (2003). Radio frequency cooking of ground, comminuted and muscle meat products. Meat Science, 65(3), 959–965. Mermelstein, N. H. (1998). Microwave and radiofrequency drying. Food Technology, 52(11), 84–86. Moyer, J. C., & Stoltz, E. (1947). The blanching of vegetables by electronics. Food Technology, 1, 252–257. Rice, J. (1993). RF technology sharpens baker’s edge. Food Processing, 6, 18–24. Risman, P. O. (1991). Terminology and notation of microwave power and electromagnetic energy. Journal of Microwave Power, 26, 243– 250. Sanders, H. R. (1966). Dielectric thawing of meat and meat products. Journal of Food Technology, 1, 183–192. Tang, X., Cronin, D. A., & Brunton, N. P. (2005). The effect of radio frequency heating on chemical, physical and sensory aspects of quality in turkey breast rolls. Food Chemistry, 93(1), 1–7. Tang, X., Lyng, J. G., Cronin, D. A., & Durand, C. (2006). Radio frequency heating of beef rolls from biceps femoris muscle. Meat Science, 72, 467–474. Zhang, L., Lyng, J. G., & Brunton, N. P. (2004). Effect of radio frequency cooking on the texture, colour and sensory properties of a large diameter comminuted meat product. Meat Science, 68(2), 257–268. Zhang, L., Lyng, J. G., & Brunton, N. P. (2006). Quality of radio frequency heated pork leg and shoulder ham. Journal of Food Engineering, 75, 275–287.
An examination of factors affecting radio frequency heating of an encased meat emulsion James G. Lyng
a,*
, Denis A. Cronin a, Nigel P. Brunton b, Wenqu Li a, Xiaohong Gu
c
a
c
School of Agriculture, Food Science and Veterinary Medicine, College of Life Sciences, University College Dublin, Belfield, Dublin 4, Ireland b Ashtown Food Research Centre, Teagasc, Ashtown, Dublin 15, Ireland Key Laboratory of Food Science and Safety, Ministry of Education, Southern Yangtze University, Wuxi, Jiangsu 214036, China Received 19 November 2005; received in revised form 9 June 2006; accepted 27 July 2006
Abstract The potential of radiofrequency (RF) heating for rapid cooking of a cased comminuted meat emulsion (white pudding) to a pasteurisation temperature of 73 C was examined. Immersion of the product in water was essential in order to prevent thermal damage to the casings by electrical arcing effects during heating. Using a polyethylene heating cell with non-circulating water the applied RF power, primary electrode distance as well as the mineral content, temperature and volume of the surrounding water all influenced the efficiency of the RF heating. Under optimised conditions maximum/minimum temperature gradients (DT) across the products in excess of 15 C were observed. These could be reduced to around 6 C by heating the white puddings in a cell operating with recirculating hot water (80 C). Using an oven power output of 450 W a 4.3-fold reduction in cooking time compared to conventional steam oven cooking could be achieved. 2006 Elsevier Ltd. All rights reserved. Keywords: Radio frequency; Cooking protocol development; Meat emulsion
1. Introduction RF heating involves application of an alternating electromagnetic field to a foodstuff at frequencies from 1 to 300 MHz (Risman, 1991). The method differs from other dielectric methods of heating such as microwave (MW) heating as the product is placed between two parallel electrodes and an RF field is generated in a directional fashion at right angles to the surface of the electrodes. In addition, the mechanism of heating in an RF field is different. MW heating occurs mainly via frictional heat generated from the dipolar rotation of free water molecules whereas the predominant mechanism of heating at RF is via the depolarisation of solvated ions. *
Corresponding author. Tel.: +353 1 7167710; fax: +353 1 7161147. E-mail address: [email protected] (J.G. Lyng).
0309-1740/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2006.07.022
The earliest application of RF technology dates back to the 1940s when the method was used to cook meat products, heat bread and dehydrate and blanch vegetables (Kinn, 1947; Moyer & Stoltz, 1947). Unfortunately, the high capital cost required for installation of the equipment prevented uptake of the technique by industry. Later the focus turned to the use of RF for thawing fish and meat blocks (Bengtsson, 1963; Sanders, 1966), which resulted in several commercial production lines (Jason & Sanders, 1962a, 1962b). More recently, the technology has found application in the post bake drying of cookies and snack foods (Mermelstein, 1998; Rice, 1993) and this is perhaps the most widely used current application of this technique. By contrast, though several studies have examined the use of RF for pasteurisation of meat products including cured hams (Bengtsson & Green, 1970) sausage emulsions (Houben, Schoenmakers, van Putten, van Roon, & Krol, 1991)
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
and whole and comminuted beef products (Laycock, Piyasena, & Mittal, 2003), the method is not widely used commercially for this purpose. In fact, the only industrial application for RF pasteurisation of meats involved a joint venture between APV and Tulip International where attempts were made to use RF for the pasteurisation of uncased meats which were cooked while being pumped continuously through a low loss tube. However, while this system was widely acclaimed commercially it is not currently in use. Recent work at UCD Dublin focused on the application of RF energy to encased meat products, which is a different approach to that taken by APV. Results have shown RF cooked cased meats, including the product examined in the present study, to be largely comparable in quality to conventionally cooked products (Brunton et al., 2005; Tang, Cronin, & Brunton, 2005; Tang, Lyng, Cronin, & Durand, 2006; Zhang, Lyng, & Brunton, 2004; Zhang, Lyng, & Brunton, 2006). Because quality considerations were the main focus of the published work, details of the actual development of an RF cooking procedure, which was not straightforward, were never reported. The objective of the current paper is to outline the experimental strategies used in the development of efficient methods to cope with special problems associated with the cooking of meats contained within plastic casings. Such information could be potentially of use to other researchers involved in the development and optimisation of RF cooking procedures for a more diverse range of packaged food products. 2. Materials and methods 2.1. Meat product manufacture A commercial meat product (as opposed to a model food system) was the preferred material for this study. Work was conducted using a pork white pudding (WP) formulation which, because of its highly homogenous nature and relatively small size, was a convenient and cost effective product on which to carry out large numbers of investigative cooking experiments. 2.1.1. Preliminary handling of meat Lean pork shoulder and pork back fat were obtained from a local producer (Galtee meats, Cork, Ireland). A mechanical mincer (Model No. TS8E, Tritacarne, Omas, Italy) was used to grind lean tissue through a plate with 3.5 mm diameter holes while fat was ground through a 10 mm plate. The ground tissue (2 kg lots) was 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 manufacture. 2.1.2. WP manufacture The ingredients, recipe and manufacturing protocol used in the preparation of a comminuted meat batter for-
471
mulated to the specification of a fairly standard WP were given in Brunton et al. (2005). 2.2. Cooking equipment 2.2.1. Conventional cooking of WP For steam processing, samples were cooked in a thermostatically controlled Kerres smoke-air steam oven (Type CS 350, Raicher-und-Kochanlagen, D-71560 Sulzbach-Murr, Germany) set at 80 C. Other oven settings include having the turbine at 2800 rpm, turning the heating switch to ‘‘high’’ and humidification in the on position (to produce a 100% RH environment in the oven chamber). 2.2.2. RF cooking of WP The RF oven used was a custom built 50X system built by C-Tech Innovation (Chester, UK) using a Coaxial Power System Ltd low power RF generator (Model No. RFG 600-27, Spectrum House, Finmere Road, Eastbourne, East Sussex, UK) and a complementary automatic impedance matching network and controller (Model No. AMN 60027). The system had an operating frequency of 27.12 MHz and a maximum power output of 600 W. The oven consisted of a chamber with electrically insulated walls, while the RF generated energy was applied via a set of parallel rectangular steel electrodes. The upper electrode (29 · 20 cm) (Fig. 1) was fixed in position while the lower electrode (46 · 24.5 cm), was in the form of a shelf, which could be moved up and down within the oven. These primary electrodes could be spaced up to 24.5 cm apart, though the oven was fitted with a rack which allowed the shelf to be moved up and down in 7 cm intervals, while fine tuning within these intervals was by way of a series of nuts and bolts which were attached to rack and shelf. Additional fine adjustment of the space between electrodes was achieved with aid of a fabricated perforated steel plate (20 · 15 cm) screwed to the lower shelf with adjustable metal spacers (Fig. 1). 2.3. Temperature distribution within products following cooking Endpoint temperature measurements for each treatment were evaluated for every cooked pudding by transferring the latter immediately after cooking to a specially constructed thermocouple jig. Fig. 2 provides an illustration of the jig, which was manufactured from rectangular blocks of wood with a hollowed out area for insertion of the pudding. In order to avoid cooling the outer part of the pudding by contact with the jig, the latter was preheated by immersion in a water bath set at 80 C for a few minutes just prior to making a measurement. Temperatures at nine points within the sample were monitored and recorded using 1.2 mm diameter Type T thermocouples (Industrial Temperature Sensors, Dublin, Ireland) and a Grant Squirrel data logger (Model No. 1600, Grant Instruments Ltd., Barrington, Cambridge CB2 5QZ, England). Maximum temperature (MxT), minimum temperature (MnT), mean temperature ð xTÞ and tem-
472
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
Cell containing product placed in this region
Primary electrodes
Fig. 1. Schematic diagram of RF oven interior showing electrode positions.
Fig. 2. Schematic side profile of thermocouple jig for end point temperature measurement in white puddings.
perature difference (DT) of the endpoint temperature was extracted from the measured data.
in air. For reasons to be discussed later this was found to be unsatisfactory and was abandoned.
2.4. Developing a method for RF cooking of cased meatsz 2.4.1. RF cooking of cased products in air Preliminary studies on the application of RF heating to the cooking of cased meats involved cooking the products
2.4.2. RF cooking of cased products submerged in water Subsequent trials were all conducted by cooking the puddings while they were submerged in water and this involved the development of holding cells in which the
Table 1 Semi-crystalline high density polyethylene cells used for RF cooking of white puddings in the present study Dimensions
Cell design A
B
C
External dimensions – L · B · H (mm) Internal dimensions – L · B · H (mm) Upper electrode dimensions – L · B · H (mm) Lower electrode dimensions – L · B · H (mm) Shape of internal cross section Circulating water Water temperature (C)
205 · 111 · 51 184 · 61 · 46 183 · 81 · 1.5 178 · 61 · 1.5 Rectangular No 20–80
210 · 111 · 51 187 · 61 · 46 207 · 102 · 1.5 184 · 61 · 1.5 Rectangular Yes 80
251 · 111 · 61 188 · 61 · 49 202 · 77 · 1.5 188 · 60 · 1.5 Rectangular Yes 80
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
473
Fig. 3. Images of polyethylene cells developed in the current study.
meats could be cooked under either static (non-circulating water) conditions or under water circulation. Three cells, the dimensions of which are given in Table 1, and which are illustrated in Fig. 3 were manufactured from semi-crystalline high density polyethylene (Alperton Engineering Ltd., Dublin Industrial Estate, Glasnevin, Dublin, Ireland) with each cell being designed to accommodate a single 200 g encased pudding during cooking.
2.4.2.1. Developing a cell to allow RF cooking under noncirculating water 2.4.2.1.1. Cell Design A. The initial basic cell design was based on that outlined by Bengtsson and Green (1970) and therefore was fitted with secondary electrodes, one of which (bottom electrode) was a removable stainless steel plate which was placed internally on the base of the cell. These authors claimed that secondary elec-
trodes helped to focus the RF field on the product and to maximise efficiency. After placing a pudding in the cell the latter was filled with water at a specified temperature and a steel plate which also functioned as the top secondary electrode was immediately placed on top. Two short polythene screws were used diagonally opposite to each other to hold the top electrode in position. This cell was placed on the lower electrode of the RF oven during cooking. In the present study a standard size of WP was used which was submerged in water. However, the volume of water surrounding this WP could be reduced stepwise from approximately 300 ml to 150 ml by inserting an appropriate number of rectangular polyethylene spacers (70 mm · 10 mm · 5 mm) into the cell before addition of the water required to cover the WP during cooking. The volume of the immersion water was determined by draining the latter into a graduated cylinder before refilling for cooking.
Table 2 Factors evaluated in initial experiments on RF cooking of white puddings in a static cell (Cell Design A) Factor examined
RF output power (ROP) Initial water temp. (ITW) Water vol. in cell (VWC) Prim. electrode gap (PEG) Water used in cell (H2O) CT: Cooking time.
Levels examined
200, 300, 400, 500, 600 20, 40, 60, 80 150, 180, 210, 240, 270, 300 55, 65, 75, 85, 95 Deionised (D), Tap (T)
Levels of controlled factors CT (min)
ROP (W)
ITW (C)
VWC (ml)
PEG (mm)
H2O
12 12 12 12 12
– 400 400 400 400
20 – 20 20 20
180 180 – 180 180
65 65 65 – 65
T T T T –
474
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
2.4.2.1.2. Optimisation of RF cooking conditions in a static cell. In order to assess the impact of a range of variables on the efficiency of RF cooking of WPs of uniform size (200 g), five separate experiments was conducted (Table 2). The factors examined in these experiments included (a) RF output power (200–600 W), (b) initial temperature of surrounding water (20–80 C), the latter was varied by introducing water of different temperatures into the RF cell immediately prior to cooking, (c) volume of water in RF cooking cell (150–300 ml), (d) distance between primary RF electrodes (55–95 mm) and (e) the use of deionised or tap water to surround the samples. While an individual factor such as water volume or type (deionised or tap water) was being examined all the other parameters were kept constant at the following levels: RF power 400 W, initial water temperature, 80 C, electrode distance, 65 mm. MxT, MnT, xT and DT data for all RF cooked samples was collected. Apart from the experiment on water type where 10 replicates were used, the results presented are the mean values of four replicates.
2.4.2.2. Developing a cell to allow RF cooking under circulating water. In this work, the RF oven was set at 450 W as preliminary investigations with recirculating water indicated that the power level of 450 W appeared to give slightly higher xT but had comparable reproducibility in replicate cooking experiments to the 400 W used with the static cell. Other cooking conditions such as water type, primary electrodes distance etc. were set up on the basis of the optimised operating parameters established with the static cell, details of which are presented in Section 3.2. 2.4.2.2.1. Cell Design B. Cell B was similar in dimensions to Cell Design A but was fitted with polyethylene inlet and outlet ports to allow water circulation during cooking. Sealing the top electrode onto the cell was by way of screws and a countersunk rubber gasket. Fig. 3b provides an image of this cell while dimensions are given in Table 1. 2.4.2.2.2. Cell Design C. This cell incorporated inlets and outlets for circulating water (Fig. 3c). It also included a side mounted screw type cap which enabled product to be introduced through the side of the cell. A leak proof system was achieved by securing the steel plate comprising the top electrode to the block with the aid of 12 short (1 cm) evenly spaced steel screws. The cell was manufactured from 60 mm polyethylene which allowed for the use of optimum primary electrode spacing (Table 1). A second series of experiments was carried out to see if further improvements in performance could be achieved using the optimised circulating cell design (Cell C). Compared with the first set of experiments, in which each factor was examined individually because they were not interdependent, in the second series of experiments the influence of continuous water flow at different flow rates as well as the effect of continuous/interrupted flow at a fixed flow rate was examined using a factorially designed experiment to
assess the level of interaction between these factors. In the continuous mode the three levels of water flow rate examined were 250, 450 and 800 ml min1. Using a flow rate of 450 ml min1 and a water bath temperature of 80 C the periods of interrupted flow during an 8 min RF cooking cycle were 7, 3 and 0 min. MxT, MnT, xT and DT data were monitored and recorded as for the previous experiments. 2.5. Statistical analysis All ANOVA analyses were carried out using the SAS statistical analysis software (Version 8.2, Statistical Analysis Systems, Cary, NC, USA). Where ANOVA indicated significant differences between samples a Tukey pairwise comparison of the means was conducted. 3. Results and discussion A principle objective of the present study was the development of an RF cooking protocol which facilitated fast and uniform heating of a comminuted pork meat product as represented by WP. To this end a number of different factors were examined in order to determine the best conditions to achieve efficient RF cooking. 3.1. RF cooking of cased meats in air RF cooking of the cased meats in air would have been the most convenient and simple method of applying the RF technology to products of this type. However, from the outset it became apparent that this was not a viable option for cased products as the casings and cable ties showed signs of overheating accompanied by frequent and unpredictable ignition during the cooking process. This was completely unsatisfactory since under these conditions the migration of potentially toxic compounds from the packaging into the product was a possibility, not to mention the adverse effect on product appearance. A variety of strategies were examined in an attempt to alleviate this problem which included: (a) Changing independent variables within the RF heating system (such as power level and electrode spacing). (b) Evaluating a range of commercially available casings and cable ties (in case the ignition problem was packaging material specific). (c) Evaluating a range of meat products (in case the problem was specific to a particular product formulation/composition). (d) Covering the bottom electrode with PTFE overlays (to evaluate whether or not ignition was being induced through contact with a metal surface). (e) Increasing the humidity of the oven cavity (to see if the presence of condensed water on the product could prevent ignition).
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
(f) Increasing/decreasing the temperature within the oven cavity. (g) Suspending the cased meat products between the electrodes (in an attempt to see if avoiding contact with a surface could prevent ignition). None of the above strategies was completely successful in preventing thermal damage to casings during RF heating of the puddings. However, it was found that if cased products were submerged in a beaker of water during RF cooking, the problem of casing ignition could be largely eliminated. The overheating effect appeared to be due to excessive focussing of RF energy on specific positions within the casings where, during the heating process, localised pools of fluid with a high ionic concentration and dielectric potential may have collected. The beneficial effect of surrounding the product with water appeared then to be due to the ability of the latter to dissipate this heat before it caused thermal damage to either product or casing. Preventing excessive focussing of RF energy should help to minimise field distortions and this has been claimed to be one of the benefits of the use of surrounding water in RF heating (Houben et al., 1991; Sanders, 1966). 3.2. RF cooking under non-circulating water – Cell Design A Since immersion in water appeared to be a prerequisite for safe RF heating of cased meat products, it was then necessary to examine the conditions under which the procedure could be carried out with both efficiency and convenience. Thus, a simple cooking cell fitted with secondary Table 3 Effect of RF power on temperature profile of white puddings RF powerA (W)
xT (C)
MxT (C)
MnT (C)
DT (C)
TCW (C)
200 300 400 500 600
58.1a 73.2b 87.0c 85.7c 86.0c
61.3a 80.3b 94.2c 95.0c 92.4c
54.2a 66.4b 78.0c 73.2c 76.8c
7.1a 13.8b 16.2bc 21.8c 15.6b
47.5a 56.8b 66.2c 69.0cd 71.3d
xT: Mean temp.; MxT: maximum temp.; MnT: minimum temp.; DT: temp. rise pre- to post-cooking. a–d Means in the same column with unlike letters are different, P < 0.05. A See Table 2 for levels of controlled factors. Table 4 Effect of electrode distance on temperature profile of white puddings Electrode distanceA (mm)
xT (C)
MxT (C)
MnT (C)
DT (C)
TCW (C)
55 65 75 85 95
79.0b 87.0a 72.8c 61.7d 50.8e
89.5a 94.2a 82.4b 70.2c 54.4d
67.1b 78.0a 64.9b 54.2c 47.5d
22.4a 16.2b 17.5ab 16.0b 6.9c
66.8a 66.2a 56.5b 48.4c 39.5d
xT: Mean temp.; MxT: maximum temp.; MnT: minimum temp.; DT: temp. rise pre- to post-cooking. a–e Means in the same column with unlike letters are different, P < 0.05 A See Table 2 for levels of controlled factors.
475
Table 5 Effect of initial cell water temperature on post-cooking temperatures in white puddings cooked in Cell Design A Water temp.A (C)
xT (C)
MxT (C)
MnT (C)
DT (C)
xTCW (C)
20 40 60 80
79.3a 80.3a 81.6a 84.4b
83.3a 86.2ab 87.2b 88.1b
74.7a 75a 76.1a 79.8b
8.6a 11.2a 11.1a 8.3a
63a 68.7b 70bc 73.1c
xT: Mean product temp.; MxT: maximum product temp.; MnT: minimum product temp.; DT: product temp. rise pre- to post-cooking; xTCW: Mean Cooking Water Temp. a–c Means in the same column with unlike letters are different, P < 0.05. A See Table 2 for levels of controlled factors. Table 6 Effect of surrounding water volume on post-cooking temperatures of white puddings cooked in Cell Design A Water volumeA (ml)
xT (C)
MxT (C)
MnT (C)
DT (C)
xTCW (C)
150 180 210 240 270 300
87a 87a 84.4a 81.5b 79.2bc 76.9c
93.6a 94.2a 90ab 89.8ab 87.8bc 83.6c
79.2a 78a 77.5a 73.2b 72.7b 71.3b
14.4a 16.2a 12.5a 16.6a 15.1a 12.3a
72.9a 66.2b 66.2b 64.3b 61c 57.4d
xT: Mean temp.; MxT: maximum temp.; MnT: minimum temp.; DT: temp. rise pre- to post-cooking. a–d Means in the same column with unlike letters are different, P < 0.05. A See Table 2 for levels of controlled factors. Table 7 Effect of water-type on temperature profile of white puddings Water typeA
xT (C)
MxT (C)
MnT (C)
DT (C)
TCW (C)
Deionised Tap
83.6a 87b
95.4a 94.5a
70.9a 77.6b
24.5a 16.9b
54.9a 66.6b
xT: Mean temp.; MxT: maximum temp.; MnT: minimum temp.; DT: temp. rise pre- to post-cooking. a–b Means in the same column with unlike letters are different, P < 0.05. A See Table 2 for levels of controlled factors.
stainless steel electrodes to focus the RF energy, was designed for cooking 200 g WPs. The purpose of this cell was to evaluate the kind of performance that might be expected under batch cooking conditions where products would be transferred to an RF heating cell, covered with water and the RF power turned on. The time for steam cooking of the WPs to a temperature of 73 C at the coldest point (centre) was 33 min. In order to examine a range of cooking variables under RF conditions it was decided to arbitrarily aim for a cooking time that was 2.5–3 times faster than steam cooking. Thus, the data presented in Tables 3–7 were, unless otherwise stated, measured using an RF cooking time of 12 min (i.e. 2.75 faster). Before looking at the effect of other variables on the performance of RF in the cooking of WPs, it was useful to first establish the optimal instrumental operating parameters for the particular RF oven arrangement used in the present study. The other variables could then be examined at the appropriate optimised oven settings. Of the latter the two most impor-
476
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
tant were the oven power and the distance between the primary electrodes. 3.2.1. RF power and electrode distance Post-cooking temperature data for WPs cooked in water (180 ml, initial temperature 20 C) with RF power levels in the range 200–600 W are listed in Table 3. xT, MxT and MnT temperatures increased with increasing RF power (P < 0.05) up to 400 W above which no significant change (P P 0.05) was observed suggesting a plateau in the power range 400–600 W. The data in Table 3 clearly shows very little increase in heating performance using power settings above 400 W. In addition 400 W afforded maximum stability in relation to tuning of the system and was therefore used in subsequent experiments carried out with static cell A. Temperature data on WPs cooked at 400 W when the distance between the electrodes was varied from 55 to 95 mm are given in Table 4 and show clearly that the optimum distance was 65 mm. 3.2.2. Initial water temperature Post-cooking temperature data for RF cooked WPs with starting water temperatures of 20–80 C are listed in Table 5. Not unexpectedly, the values of xT and MnT appeared to increase with increasing initial water temperature. However, the values for xT and MnT were only significantly different (P < 0.05) for the samples cooked at initial water temperatures of 20–60 vs. 80 C. The relatively modest temperature differentials between the hottest and coldest spots (DT) ranging from 8.3 to 11.2 C for the different starting water temperatures, were not significantly different (P P 0.05). In this work the target to achieve sufficient microbial kill was to ensure that all areas of the product were heated to a temperature of in excess of 73 C for 2 min which is well above the Irish guideline of 70 C for 2 min for such products. Under RF cooking in a static cell, this could be achieved by using RF energy to heat the product to >73 C and subsequently allowing the product to stand in the surrounding water with RF energy off for 2 min. Success in this depended upon having the surrounding water at sufficiently high temperature (i.e. minimum of >73 C, throughout the holding period). In this experiment it must be highlighted that although the specified starting temperature of the water at addition to the cell ranged from 20– 80 C, the introduction of a refrigerated pudding (200 g) (at 4 C) would have had a substantial cooling effect on the volume of water used (180 ml). In the experiment described a 12 min cooking time only raised the mean temperature of the surrounding water to a suitable holding value (73.1 C) in the cooking run where the water initially added was at 80 C. Even under these conditions there was a problem since at the end of the RF heating period a temperature differential of 5-10 C was observed between the surrounding water at the top side of the WP and the much hotter water underneath it. Possible solutions to this could include (a) physical agitation of the cell and its contents at
the end of RF heating to rapidly equilibrate the surrounding water temperature (b) no agitation, but extended RF heating to ensure the water were heated in excess of 73 C at its coldest point. 3.2.3. Volume of water in RF cell Post-cooking temperature data for RF cooked comminuted meat samples with starting water volumes of 150–300 ml are listed in Table 6. On average, for all three parameters listed (xT, MxT and MnT) final product temperatures generally increased with decreasing volume of surrounding water though significant differences were only noted when the extremes of volume are compared (P < 0.05). Prior to cooking the temperature of the smaller volumes of 20 C water would have been reduced to a somewhat greater degree by the addition of the refrigerated pudding (5 C). However, possible explanations for the higher final temperatures observed in both product and surrounding water could include: (a) during cooking simple heat loss from the white pudding would increase the temperature of a smaller volume of water to a greater extent; (b) the mass of potentially dielectrically active material was reduced due to the partial replacement of water in the cell by dielectrically inactive polyethylene spacers. 3.2.4. Effect of deionised water Both final xT and MnT for WPs were significantly higher for samples surrounded in tap water as opposed to deionised water (Table 7) (P < 0.05). Although there appears to be an apparent contradiction to this, with MxT for samples surrounded by deionised water appearing on average higher, this effect was not significant (P P 0.05). It has been suggested elsewhere that surrounding the product in deionised water would serve to increase the absorption of RF power by the product (Houben et al., 1991). However, in the present case both xT and MnT were significantly higher when the product was surrounded by tap water. The most important effect however, was that use of deionised water resulted in significantly higher DT in samples (P < 0.05) most likely due to a greater cooling influence of the surrounding deionised water. This cooling effect would have arisen due to the deionised water (with its very low content of ions) absorbing less RF energy. Heat would subsequently flow from the RF heated pudding into this cooler water which in turn would increase the DT. (Note: Electrical conductivity (lS cm1) was used as an index of the ionic content of water and values for the deionised, distilled and tap water used in the present study were 0.7, 3–12 and 166, respectively.) The greater DT could result in notable variation in product quality within a given sample not to mention causing greater difficulties in the validation of the heat process. It should be noted however that although the product was surrounded in a casing some seepage could have occurred during RF cooking thus the water was not free of ions by the end of the cooking time.
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
3.3. RF cooking under circulating water Based on the observations above, the RF cooking of products in a static water environment would appear to have the following shortcomings: (a) relatively high product DT’s (as shown in Tables 3–7), (b) temperature differentials within the cooking water itself, (c) significant cooling of the added water on introduction of refrigerated WPs. Therefore, another series experiments were conducted to examine the effect of recirculating hot water around the product. The circulation of hot water also allowed holding periods in circulating water after RF heating, thereby reducing uncertainties in relation to cooling of the water/ product during the holding period. 3.3.1. Optimisation of cell design for RF cooking with circulating water 3.3.1.1. Cell Design B. This design was based on Cell Design A but proved very difficult to get a water tight seal and was therefore abandoned as it was problematical in terms of leaks during cooking.
3.3.1.2. Cell Design C. Leaks were not a problem with this cell design and it was also easy to install and remove the puddings. However, an intermittent problem of burning was encountered in this circulating water cell and also in Cell Design A as shown in Fig. 4a. The image illustrates that two points on the upper side of the pudding were burned, and similar burn marks were found at corresponding positions on the top secondary electrode indicating that burning could sometimes occur if the pudding moved and
477
touched the top secondary electrode during cooking. To avoid this problem and to prevent movement of the pudding within the cell a square of thin perspex of identical cross section to the interior of the cell was prepared with a hole in the centre (Fig. 4b). The purpose of this opening was to allow the tie at the end of the casing to pass through thereby anchoring the pudding in position. Some smaller holes were also incorporated to allow water to circulate around the end section of the pudding. A series of holes were also drilled in a 30 ml plastic weighing cup which was placed over the other end of the pudding in such a way that when the sealing cap was tightened on the cell the pudding was prevented from moving. This strategy eliminated all problems of arcing with this system. 3.3.2. Effect of water flow conditions on temperature distribution within the product MxT, MnT, xT and DT data for WPs cooked by RF in combination with continuous/interrupted hot (80 C) water flow at different flow rates are presented in Fig. 5i– iv. No significant difference in xT (Fig. 5i) was observed using either continuous or interrupted water flow (P P 0.05), nor when the water flow rate was increased from 450 to 800 ml min1 (P P 0.05), though a significant increase in xT was noted when the water flow rate was increased from 250 to 800 ml min1 (P < 0.05). The results suggest that considerable variations in water flow rates may occur before differences in xT will be encountered. DT values for WPs cooked by RF with hot water circulation are presented in Fig. 5iv. Significant differences were observed due to flow time, with continuous flow showing lower DT values compared to products around which water
Fig. 4a. Image showing an intermittent burning problem encountered circulating water cells where the pudding contacted the upper secondary electrode during cooking.
Fig. 4b. Schematic cross section through cell showing modifications to prevent contact between secondary electrodes and the product.
478
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
(ii) MxT 80
75
75
a1
ab
b
a
a
a
70
Temperature (oC)
Temperature (oC)
(i) x T 80
65
60
ab
a
b
b
65
Flow Rate (ml min-1) 250 450 800
1
60
Flow time (min) 5 Continuous
Flow Rate (ml min-1) 250 450 800
(iv)
(iii) MnT 20
75
15
70
a
b
b
a
a
a
60
Temperature (oC)
a Temperature (oC)
a
70
80
65
a
b
ab
1
Flow time (min) 5 Continuous
ΔT a
ab
b
10
5
0
Flow Rate (ml min-1) 250 450 800
1
Flow time (min) 5 Continuous
Flow Rate (ml min-1) 250 450 800
1
Flow time (min) 5 Continuous
Fig. 5. (i)–(iv) Mean ð xTÞ, maximum (MxT) and minimum (MnT) temperatures and also temperature differentials (DT) in products RF heated in circulating water cells under a range of flow rates and flow time.
was circulated for only 1 min (P < 0.05). However, no significant difference in DT was noted between products under continuous flow and those which had water circulating around them for 5 min (P P 0.05). Results suggest that continuous flow or reasonably prolonged circulation of water with interruptions is required to minimise product DT. Results for flow rate show an increase in xT (significant between 250 vs. 800, P < 0.05) which can be rationalised on the basis of a greater energy input into the product from the larger volumes of 80 C water circulating around it per unit time. Although the reasons for the DT differences as a function of flow rate (Fig. 5iv) are not entirely clear a possible explanation is as follows. The higher DT at 800 ml min1 may possibly be due to greater heating of the outer regions of the product due to the higher volumes of hot water circulating around it per unit time. At the lowest flow rate (250 ml min1), the higher DT may be due to insufficient water circulating to remove the excess heat generated by the absorption of RF power at the bottom of the product, which could be similar to the temperature gradients observed between water at the bottom vs. top of the cell noted under non-circulating water flow conditions (Section 3.2.2). For the current system the optimum flow rate which produced the smallest DT values occurred when the water flow rate was 450 ml min1. Table 8 shows the combined effect of water flow rate and flow duration on DT within WP’s. A continuous flow rate of 450 ml min1 produced a DT value of 6.3 C which was
Table 8 Combined effect of water flow rate and flow duration on DT within white puddings Flowing time (min)
Flow rate (ml min1)
1 5 Continuous
250 15.9 17.8 12.8
450 13.4 13.5 6.3
800 16.3 12.5 13.7
DT: Temp. rise pre- to post-cooking.
the lowest DT achieved in all experiments with static and continuous flow cells. A DT of 6.3 C is low relative to the magnitude of DT values which have been reported elsewhere for RF heated meats (Bengtsson & Green, 1970; Laycock et al., 2003). However, the cooking conditions, RF applicators and product types used in these studies are not directly comparable to those used in the current study. Zhang et al. (2004) published a study on a larger RF cooked comminuted product (1.2 kg and 9 cm diameter vs. 0.2 kg and 4.2 cm diameter in the present study) in a circulating water cell and reported DT values of approximately 10 C which is larger than that obtained in the present study. This difference can possibly be attributed to larger sample size and differences in the design of the RF cooking cell between the studies. Finally, when using the optimised conditions involving an RF oven power output of 450 W and recirculating hot water (80 C) at a flow rate of 450 ml min1, the 200 g
J.G. Lyng et al. / Meat Science 75 (2007) 470–479
white puddings could be heated to a minimum temperature of 73 C in 7.7 min compared to 33 min using a steam oven. This represented a 4.3-fold decrease in cooking time. 4. Conclusions This study has shown that the cooking of encased meat products in air is not a viable proposition because of an unacceptable risk of thermal damage to the casing due to arcing. Surrounding the product with water (static or circulating) largely alleviates this problem though it is important to prevent the product from making physical contact with the upper surfaces within cooking cells. Such contact may result in the formation of a localised static air/vapour/liquid pool which can occasionally lead to arcing and burning of the casing. Efficient RF cooking in static water was achieved through the optimisation of RF power, electrode spacing and initial water temperature/volume/type. However, recirculating hot water around the product during RF cooking was found to give greater overall temperature control and reduced temperature gradients within the product. Acknowledgement This research has been part-funded by grant aid under the Food Institutional Research Measure, which is administered by the Department of Agriculture and Food, Ireland. References Bengtsson, N. E. (1963). Electronic defrosting of meat and fish at 35 and 2450 Mc – a laboratory comparison. Food Technology, 17(10), 92–100. Bengtsson, N. E., & Green, W. (1970). Radio frequency pasteurisation of cured hams. Journal of Food Science, 35, 681–687.
479
Brunton, N. P., Lyng, J. G., Li, W., Cronin, D. A., Morgan, D., & McKenna, B. (2005). Effect of radio frequency heating on the texture, colour and sensory properties of a comminuted pork meat product. Food Research International, 38, 337–344. Houben, J., Schoenmakers, L., van Putten, E., van Roon, P., & Krol, B. (1991). Radio frequency pasteurisation of sausage emulsions as a continuous process. Journal of Microwave Power and Electromagnetic Energy, 26(4), 202–205. Jason, A. C., & Sanders, H. R. (1962a). Dielectric thawing of fish. Experiments with frozen herrings. Journal of Food Technology, 16(6), 101–106. Jason, A. C., & Sanders, H. R. (1962b). Dielectric thawing of fish. Experiments with frozen White fish. Journal of Food Technology, 16(6), 107–112. Kinn, T. P. (1947). Basic theory and limitations of high frequency heating equipment. Food Technology, 1, 161–173. Laycock, L., Piyasena, P., & Mittal, G. S. (2003). Radio frequency cooking of ground, comminuted and muscle meat products. Meat Science, 65(3), 959–965. Mermelstein, N. H. (1998). Microwave and radiofrequency drying. Food Technology, 52(11), 84–86. Moyer, J. C., & Stoltz, E. (1947). The blanching of vegetables by electronics. Food Technology, 1, 252–257. Rice, J. (1993). RF technology sharpens baker’s edge. Food Processing, 6, 18–24. Risman, P. O. (1991). Terminology and notation of microwave power and electromagnetic energy. Journal of Microwave Power, 26, 243– 250. Sanders, H. R. (1966). Dielectric thawing of meat and meat products. Journal of Food Technology, 1, 183–192. Tang, X., Cronin, D. A., & Brunton, N. P. (2005). The effect of radio frequency heating on chemical, physical and sensory aspects of quality in turkey breast rolls. Food Chemistry, 93(1), 1–7. Tang, X., Lyng, J. G., Cronin, D. A., & Durand, C. (2006). Radio frequency heating of beef rolls from biceps femoris muscle. Meat Science, 72, 467–474. Zhang, L., Lyng, J. G., & Brunton, N. P. (2004). Effect of radio frequency cooking on the texture, colour and sensory properties of a large diameter comminuted meat product. Meat Science, 68(2), 257–268. Zhang, L., Lyng, J. G., & Brunton, N. P. (2006). Quality of radio frequency heated pork leg and shoulder ham. Journal of Food Engineering, 75, 275–287.