Optimization of stepwise blanching of frozen-thawed ... - Springer Link

Eur Food Res Technol (1999) 210 : 102–108

Q Springer-Verlag 1999

ORIGINAL PAPER

M.D. Alvarez 7 W. Canet

Optimization of stepwise blanching of frozen-thawed potato tissues (cv. Monalisa)

Received: 3 February 1999 / Revised version: 12 April 1999

Abstract Response surface methodology was used to compare the effect of temperature and time of the first step of blanching on compression, shear, tension and stress-relaxation parameters of frozen-thawed potato tissues. A central, composite rotatable design was used to study the effects of variation in levels of temperature (52.93–67.07 7C) and time (15.86–44.14 min) on rheological parameters. Blanching temperature was the most important factor affecting the mechanical properties tested. The models fitted for the apparent modulus of elasticity in compression, maximum tension force, and relaxed force in the first cycle (Fr1); all had R 2 1 0.85 (P^0.01) and were used for doing predictions. Optimum conditions were with in the ranges of temperature (60–65 7C) and time (25–35 min) used for each factor. In the experimental verification of the models at 65 7C during 30 min, the lowest percentage residual between experimental and predicted values was obtained for Fr1 (0.644), which was therefore the most appropiate parameter for detecting the firming effect that the pectinesterase activity produced on frozen potato tissues as a consequence of stepwise blanching under these conditions. Key words Potato 7 Rheological parameters 7 Blanching 7 Freezing 7 Response surface methodology

Introduction Understanding and minimizing the effects of each stage in the production of frozen vegetables, particularly blanching and frozen storage, can optimize their quality [1]. It is well-known that blanching leads to a loss of M.D. Alvarez (Y) 7 W. Canet Department of Plant Food Science and Technology, Instituto del Frío-CSIC, Ciudad Universitaria s/n, E-28040 Madrid, Spain e-mail: ifrat446if.csic.es

nutrients and other product quality characteristics such as texture, flavour and colour. Recrystallization and surface drying are accelerated by temperature fluctuations during the frozen storage of vegetable products, causing more mechanical damage when the temperature fluctuation is large and/or the storage temperature is high [2]. In addition, the effects of freezing depend on whether the tissue has been blanched. Rigorous blanching accentuates blanching accentuates the damage caused by freezing, producing structural changes which are even detectable after cooking [3, 4]. The freezing process itself causes damage to cell structures, but more appropiate methods can be used in order to optimize quality. Alvarez et al. [5] showed that when potato tissue was cooled at a low temperature (3 7C) for a long period (30 min) prior to freezing, the mechanical strength of the tissue increased for the different freezing rates tested. Lasztity et al. [6] reported that when blanched, then frozen, potato tissue was no longer capable of maintaining differences in concentration and pressure in its cells owing to the damage to the cell walls. Fuchigami et al. [7] found that the freezing rate had the same effect on firmness of blanched as of fresh carrots, although this effect was less significant in the blanched product because there was already structural damage. Therefore, it is obvious that the optimization of vegetable blanching alone requires a “just sufficient” heat treatment to inactivate enzymes responsible for deleterious changes during freezing and frozen storage [1]. Once the blanching is carried out under a tighter protocol, other aspects such as the application of programmed freezing to suit the characteristics of each product may be investigated [8], in order to decrease tension built up in the product during this process. One of the procedures used to decrease the negative effects of blanching is stepwise blanching. The literature gives different optimum blanching conditions for different potato-processing operations. Brown and Morales [9] recommended 80 7C, 15 min for the first step and 95 7C, 1 min for the second for the blanching of po-

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tatoes prior to frying. Canet et al. [10] compared blanching at boiling point (2 min) of cylindrical potato specimens with both one-step blanching (80 7C, 6 min) and stepwise blanching at 50 7C, 60 7C, and 70 7C, followed by cooling and a second step at boiling point (2 min); they concluded that stepwise blanching at 60 7C and 70 7C clearly improved the texture of frozen potatoes when cooked. Many theories to account for the effect of the first blanching step on the firmness of vegetable tissues attribute it to pectinesterase activity [11]. According to Bartolome and Hoff [12], this pretreatment causes a loss of membrane selective permeability, giving rise to diffusion of cations to the cell wall. This activates the enzyme, leading to the de-esterification of pectins and facilitates the formation of divalent bridges between residues of galacturonic acid belonging to adjacent pectic chains. The divalent ion-pectin complex thus formed acts as an intercellular cement to give firmness to tissues. The objective measurements used for heat-processed potatoes are of mechanical properties determined in compression, tension or shear tests, or a combination of these (e.g. compression, shear and stress-relaxation tests [10], Kramer shear cell [13], shear [14] and double direct shear [15], and texture profile analysis (TPA) [16]). The discrepancies among authors results as regards optimum conditions and correlations between mechanical properties and structural changes at the level of the cell may be due partly to the different methods used [17]. Because the combinations of levels of the independent variables that are optimum for one rheological parameter may be quite different for other rheological parameters, response surface methodology (RSM) is a statistical technique particularly suited to the analysis of this data. RSM commines planned and efficient experimental designs with least squares modelling to identify optimum conditions for the process response. RSM was especially useful in this study since 19 rheological parameters were considered. In the present research, the freezing process through stepwise blanching was carried out with the temperature and time of the second blanching step fixed at boiling point during 2 min and the freezing and thawing rates fixed at –2 7C/min and c0.5 7C/min, respectively. RSM was then used to investigate the effects of the first blanching step under different temperature/time conditions on several rheological parameters measured in frozen-thawed potato tissues, to determine which are the optimum temperature/time conditions for this step of the freezing process as a function of the rheological parameter studied. We also sought to find and develop a method for optimizing the mechanical properties of frozen potatoes.

Materials and methods Test material. The potatoes (Solanum tuberosum L., cv. Monalisa) came from Segovia, Spain and had weights (g) within the con-

fidence interval (153.83^m^186.56) and specific weights (g/cm 3) within the confidence interval (1.0635^m^1.0796); P^0.01. Just one trial of potatoes was needed. The potatoes were stored at 4 7C and 85% relative humidity during the whole experiment. Tests were carried out over a 2-week period. Cylindrical specimens (diameter 25.4 mm) were cut from the central region of potatoes with a 1-inch-diameter cork borer and then trimmed to a height of 10 mm by parallel-mounted microtome blades. For tension tests, bone-shaped specimens (5 mm thick, 75 mm long, 20 mm wide at the retaining ends and 8 mm wide at the neck region) were taken from the potatoes. Thermal treatments. Stepwise blanching of the samples under the different temperature/time combinations was carried out in a Hetofrig CB60VS waterbath (–30 7C to c110 7C) with a constant product weight:water volume ratio of 1 : 20. After the first blanching step in the temperature/time combinations shown in Table 1, the product was cooled down in iced water to achieve a temperature of 20 7C throughout the tissue. The second step was performed for 2 min at boiling point. According to Canet [3], this treatment is sufficient to deactivate peroxidase. The samples were frozen by blasting with liquid nitrogen vapour in an Instron programmable chamber (model 3119-05, –70 7C/c250 7C), at –60 7C (–2 7C/min) until the temperature at the thermal centre reached –18 7C. The product was thawed in the same chamber by blasting with air at 20 7C (c0.5 7C/min). Air and product temperatures were monitored by K-type thermocouples using hardware and software systems which permitted real-time data gathering, storage and calculation of freezing, thawing and heating rates [18]. Rheological parameters. Uniaxial compression, shear and tension tests were carried out on an Instron food testing instrument model 4501 (Instron, Canton, Mass.) using a 5-kN load cell and Instron series IX software. Ten replicates of each test were done. Cylindrical specimens were compressed between parallel plates at a constant deformation rate of 200 mm/min to determine the maximum compression force (Fc), apparent modulus of elasticity (Ec) and compression energy required for breaking per unit of volume (Uc). The shear test was performed using a shear cell [3] at a constant deformation rate of 400 mm/min to give the maximum shear force (Fs), modulus of rigidity (Gs) and shear energy required per unit of volume (Us). The tension test was performed at a constant deformation rate of 100 mm/min, using a cell consisting of two compressed-air clamps (1.5 bar) fitted to the specimen necks over filter paper to prevent slipping and cracking. This gave the maximum tension force (Ft), apparent modulus of elasticity (Et), energy required for breaking per unit of volume under tension (Ut) and maximum deformation (Dt). In the tensile specimen, a length of 30 mm in the neck region between the retained ends was taken as the main part of the specimen for which a uniform stress could be considered [19]. Stress relaxation tests were performed using an ANAME model TA-HD250 texturometer. Five replicates were examinated by the stress-relaxation test. Cylindrical specimens were compressed to a deformation of 2 mm

Table 1 Levels of independent variables in the experiment Blanching temperature ( 7C)

Blanching time (min)

Uncoded

Coded

Uncoded

Coded

55 55 65 65 67.07 52.93 60 60 60

–1 –1 1 1 1.41 –1.41 0 0 0

20 40 20 40 30 30 44.14 15.86 30

–1 1 –1 1 0 0 1.41 –1.41 0

104 (20%) between parallel plates, at a deformation rate of 400 mm/ min. The deformation was then held constant and the specimens were allowed to relax for 1 min following deformation. On the basis of previous studies [20, 21], the relaxed force parameter (Fr) was calculated as follows:

rotated to axes corresponding to the principal axes of the contour system, giving the fitted model:

Frp(F0PFi)/F0

where y0 is the estimated response at the stationary point and l1, l2, ..., lk are constants. The variables W1, W2, ..., Wk are called canonical variables. We solved the following quadratic equation:

where F0 is the maximum compression force for deformation of 2 mm and Fi the force recorded after 1 min of relaxation. The relaxation gradient is the slope of the straight line joining the maximum-compression-force and relaxed-force points after 1 min. The relaxation residual area is the area under the curve force versus time. Each specimen was compressed and allowed to relax 3 times, making a total of three successive cycles of stress relaxation. Experimental design and modelling. Optimisation was carried out by using multivariate statistical analysis. Based on previous works and literature sources with respect to stepwise blanching in potato tissues [3, 4, 10, 12], the two independent variables considered to be the most important factors affecting rheological parameters of blanched potatoes were temperature (x1) and time (x2). In the process of production of frozen potatoes, the second step of the stepwise blanching step was considered as a fixed factor and performed at boiling point for 2 min, as well as the freezing rate (–2 7C/min) and the thawing rate (c0.5 7C/min). Five levels of each of two independent variables were chosen for study. Thirteen combinations (including five replicates of the centre point) were performed in random order according to a central composite rotatable experimental design configuration for two factors [22, 23]. For each independent variable studied, the central value and interval between the levels were chosen on the basis of preliminary studies. The coded and uncoded values of the two independent variables are shown in Table 1. Nineteen dependent variables (or responses) were considered to evaluate the effect of stepwise blanching of frozen-thawed potato tissues. Statistical analysis. The differences in rheological parameters for the different temperature/time conditions were previously studied by one-way analysis of variance, using the least significant difference test with a 99% confidence interval for the comparison of the test means. Then, data were analysed by multiple regressions using the method of least squares to fit the following secondorder equation to all dependent variables: K

K

ip1

ip1

ypb0c A bi xic A bii x 2i cA bij xi xj i~j

where b0, bi, bii, bij are the equation parameter estimates (constant b0, parameter estimate for linear terms bi, parameter estimate for quadratic terms bii, parameter estimate for interaction terms bij), xi, xj are the levels of the factors and k the number of independent factors. If the value of b is large, it indicates the importance of its associated x value. Conversely, if the value of b is small, it indicates the lack of importance of its corresponding x value. In multiple linear regression problems, certain tests of hypotheses about the model parameters are helpful in measuring the uselfulness of the model. The variation found was broken down into three components, namely, the variation due to regression, the variation due to pure error (as evidenced by the variability of the five centre points), and the residual variation, which measures the inadequacy or lack-of-fit of the model. The significance of the equation parameters for each response variable was assessed by F-test. The fitted regression equation is particularly useful for calculating contours of equal response, with the axes representing the pair of design variables, e.g. xi and xj. The values (stationary points) which give optimum combinations for each rheological parameter were calculated. In order to determine the nature of each optimum, in a third working phase, canonical analysis of the response system was applied [24]. In this type of analysis, the second-order response surface model in matrix notation is translated to a new centre, namely the stationary point, and

k

ypy0c A li W 2i ip1

l 2–l(b11cb22)c(b11b22–1/4b12 2)p0 with its two associated solutions (eigenvalues) l1 and l2. The signs of the ls determine the nature of the stationary point, and their relative magnitude enables a better understanding of the response system. So, if both are negative, the stationary point is a point of maximum response; if both are positive, the stationary point is a point of minimum response. If one is positive and one is negative, then the stationary point is not of practical significance (saddle point a maximum point when one seeks a point of minimum response) or the stationary point is remote from the design region. It a saddle point is obtained further investigations are needed. Moreover, surface slope increases in the direction of Wi in which FliF is higher. Once the regression equation associated with the response surface has been established, a visual presentation of the response to the variable inputs can be prepared for the two-variable case. Statgraphics software version 5.0 (STSC, Rockville, M.D.) was used for the multiple regression involved in the modelling. Response surfaces and contour plots were drawn to highlight the main effects of independent variables on the rheological parameters.

Results and discussion Stepwise blanching significantly affected compression, shear and tension properties, Fr in three cycles and the residual area corresponding to the first stress relaxation cycle (comparisons of means not shown). The highest compression and shear values occurred in the first step (65 7C, 20 min); with the exception of Us, these were significantly different from the parameter values determined in the control samples subjected to conventional blanching at boiling point for 2 min. Stepwise blanching increased mechanical tension properties in thawed tissues for all the temperature/time combinations tested. However, the highest rheological tension values, with the exception of Dt, occurred with the combination 65 7C, 40 min. The highest Dt occurred with the combination 60 7C, 30 min. Fr values were highest in the second and third cycles; here, the conventionally blanched tissue was found to be less firm. The lowest Fr values in all three cycles occurred with the central combinations (60 7C, 30 min). Fourteen rheological parameters reflected the firming effect of stepwise blanching on frozen/thawed tissue, confirming the findings of Canet [3] and Canet et al. [10] for potato tissues; Canet and Espinosa [25], Fuchigami et al. [26] and Quintero-Ramos et al. [27] for carrot; and Bourne [28] for carrot and green bean. The pectinesterase activities determined by Alvarez [4] confirmed the theory of these latter researchers, which attributed the firming effect of stepwise blanching on vegetable tissues to pectinesterase activity.

105

The analysis of the coefficient estimates for the regression models (Table 2), showed that both independent variables had an effect on the rheological parameters of blanched, frozen-thawed potato tissues, although in different ways, depending on the mechanical test and the parameter considered. As regards majority-response

variables, the effect of blanching time was mainly linear (first order). The response increased or decreased continuously within the experimental field (Fig. 1), although time had a significant quadratic effect on Ft, Et and Fr (Fig. 2a, b, d), indicating the existence of an optimum level within the time range tested (15.86–44.14 min). For blanching temperature, on the other hand, there was a significant quadratic (secondorder) effect on the compression, shear and stress-relaxation parameters. This indicated the existence of an optimum level within the temperature range tested (52.93–67.07 7C). The linear coefficients of temperature for the tension parameters were highly significant; temperature had no quadratic effect on any of these parameters, whose values increased constantly within the experimental temperature range (Figs. 2a–c). Temperature had a highly significant quadratic effect on all three relaxed forces (Fig. 2d–f). Fr was the only parameter on which temperature and time had a significant quadratic effect in the first blanching step. According to significance tests on estimated coefficients, temperature influenced the mechanical parameters more strongly than did blanching time. Table 3 shows optimum points for those response variables which exhibited a significant quadratic effect on either or both independent variables and determination coefficients (R 2 1 0.75). The optimum points for the compression and shear parameters were within the region defined by the independent variable ranges considered. For these parameters, l1 and l2 values were negative, and hence were considered points of maximum response (Fig. 1). The optimal points for Gs and Ec lay outside the respective time and temperature ranges. For these parameters the values of l1 were close to zero (descending response surface) and the values of l2 were negative (ascending response surface). Further research is required before any inferences can be derived from these stationary points. A point where Ft was at its maximum was found although this was close to the limit of the region considered. The optimum points for

Table 2 Regression coeficients (R 2) and analysis of variance of the second-order polynomials a for 12 response variables. Fc Maximum compression force, Ec apparent modulus of elasticity, Uc

shear energy, Fs maximum shear force, Et apparent modulus of elasticity, Ut tension energy, Fr1 first-cycle relaxed force, Fr2 second-cycle relaxed force, Fr3 third-cycle relaxed force

Statistical analysis and response surfaces Coefficients and analysis of variance of the second-order models fitted with 12 dependent variables (or responses) from the 13 combinations used in the central composite rotatable design are shown in Table 2. The main results of this multiple regression analysis were developed for each of the nineteen dependent variables, with corresponding coefficients of determination (R 2). The models for Dt, residual areas and slopes of the three relaxation cycles are not included, as these showed very low percentages of variability explained. According to Henika [29], R 2~0.75 indicates a significant lack of fit, thus a careful inspection of residual errors was made. These indicated that a higher order model would be required to better describe the effects of independent variables with these responses, and particularly blanching temperature. The models for Fc (R 2p81.98), Uc (R 2p75), Fs (R 2p75.20), Gs (R 2p84.75), Us (R 2p78.58), Et (R 2p77.44), Ut (R 2p83.10), second-cycle relaxed force (R 2p79.28) and third-cycle relaxed force (R 2p77.84) showed low percentages of variability explained (0.75~R 2~0.85) and in consequence were used only for trend analysis. Models fitted for Ec (R 2p85.17), Ft (R 2p85.67) and first-cycle relaxed force (Fr1; R 2p88.48), were considered adequate with satisfactory R 2 values ( 1 0.85) and significant F values (probability of F~0.01). The relationships between independent and dependent variables are represented by three-dimensional response surfaces (Figs. 1 and 2). Mechanical properties

Uc (mJ/mm 3)

Fs (N)

1.0302

125.4070

27.4833

4.3540

89.5742

6.1756

0.5397

27.7741* –19.2118

0.0907 –0.1162*

11.4941* –11.4092*

1.9956* –1.4190

0.4023* –0.3242*

7.7810* –4.0852

0.7227** 0.5754*

0.0417** 0.0129

–45.5809** –10.8307

–0.1819** –0.1027

–12.9924** –1.2168

–2.8190* –1.1377

–0.5370** –0.1420

–9.8797** –3.5854

Coefficients

Fc (N)

B0 Linear B1 B2 Quadratic B11 B22 Interactions B12 Variability explained (R 2) F Probability of F

258.4240

Ec (MPa)

Gs (KPa)

Us (mJ/mm 3)

Ft (N)

Et (MPa)

Ut (mJ/mm 3)

Fr1 (%)

Fr2 (%)

Fr3 (%)

27.4833

81.9920

80.1300

77.6880

–0.8457 –0.8736

–0.5571 –0.7000

–0.7385 –0.4415

4.1186** 2.4354*

–0.3217 –0.3731*

–0.0170 –0.0303*

–0.4135 –0.3406

2.5940** 1.1490*

2.9525** 1.4225

3.1110** 1.2110

–3.0900*

–0.0985

4.0982

–1.1500*

–0.2600

–1.4262

0.0920

0.0216

0.8437

0.6550

0.8200

0.5100

81.98 6.3729 0.0154

85.17 7.6634 0.0095

75.00 3.9860 0.0496

75.20 4.2458 0.0428

84.75 7.7841 0.0089

78.58 5.1370 0.0269

85.67 7.7331 0.0090

77.44 4.8081 0.0317

83.10 6.8848 0.0125

88.48 10.7621 0.0035

79.2800 5.3593 0.0241

77.8400 4.9188 0.0299

**P^0.01, *P^0.05 a Model in which x1pblanching temperature, x2pblanching time

106

Fig. 1 Response surfaces for the effect of blanching temperature and time on the compression (a-c) and shear (d-f) rheological parameters

stress relaxation were within the region considered, corresponding to minimum response, with solutions l1 and l2 positive. The tension parameters showed increases in structural firmness of bone-shaped specimens at times and temperatures of the first blanching step greater than those obtained with cylindrical specimens. This might appear contradictory, since the thickness of each bone-

Table 3 Optimum point and nature of the rheological parameters. For abbreviations, see Table 2

shaped specimen was exactly one-half that of the cylindrical specimens, and therefore heat transfer was expected to be faster, and hence enzyme activation more significant in a shorter processing time. However, a possible explanation can be found for this if we consider that the activity of an enzyme may depend on its location within the tuber of potato. Andersson et al. [17] pointed out that the highest level of enzyme activity is located in the tip (where metabolic activity is greatest), while the lowest level of activity is in the inner phloem (perimedullar zone). Because of their different shapes, the proportions of the tissues making up cylindrical and bone-shaped specimens are likewise different. The in-

Optimum point

Nature of the optimum

Rheological parameter

Temperature ( 7C)

Time (min)

Fc (N) Ec (MPa) Fs (N) Gs (KPa) Us (mJ/mm 3) Ft (N) Et (MPa) Fr1 (%) Fr2 (%) Fr3 (%)

61.6780 62.3110 62.6800 64.1800 62.2000 66.2776 68.7830 60.5900 60.3130 60.5270

20.6500 22.1290 21.0530 10.9300 23.4256 39.2500 38.4087 33.4610 32.2790 31.6000

Maximum Maximum Maximum Downward slope Maximum Maximum Upward slope Minimum Minimum Minimum

107

Fig. 2 Response surfaces for the effect of blanching temperature and time on the tension (a-c) and stress (d-f) relaxation rheological parameters

ner phloem accounts for approximately 75% of a potato’s volume and has abundant parenchymatic storage cells [30]. Bone-shaped specimens contain a high percentage of this kind of tissue, and higher temperatures will therefore be required to attain a level of enzyme activity like that found in cylindrical specimens. The latter contain more inner medulla, and the results indicated that enzyme activity was higher in the cells of this kind of tissue. Optimization In order to optimize stepwise blanching with respect to all the dependent variables considered, the contour plots of Ec, Ft and Fr1 were overlaid (Fig. 3). These mechanical properties were chosen to narrow down the optimum zone by means of the significance of the second-order models fitted for these parameters. The optimum response for each parameter did not lie in exactly the same region. Ec and Fr set the limits on the optimum zone because of their importance for the study,

Fig. 3 Overlay of contour plots for apparent modulus of elasticity in compression (Ec), maximum tension force (Ft) and first-cycle relaxed force (Fr1). The shaded area represents optimum conditions

and Et could be optimized within the band demarcated by the other two parameters. The shaded area defines the optimum ranges of blanching temperature (60–65 7C) and time (25–35 min). The models fitted for these values proved adequate for making predictions, and the predicted values for the combination (65 7C, 30 min) were verified

108 Table 4 Experimental data and values predicted by the models for 65 7C and 30 min Response variable

Experimental value

Predicted value

Percentage residual

Ec (MPa) Ft (N) Fr1 (%)

0.8900 7.1223 84.2798

0.9390 6.5766 83.7403

5.2183 8.2976 0.6442

experimentally. Table 4 shows the experimental values and the values predicted by the models of these three rheological parameters. Ft was the parameter showing the largest gap between experimental and predicted values. By considering rheological parameters derived from other mechanical tests, it was possible to define a narrower optimum temperature range for the first blanching step than had been considered originally. However, as a consequence of the fact that the effect of the process duration on the compression and shear parameters was less significant than the effect of temperature, and the number of different optimum times which were identified (Table 3), it was not possible to confine optimum duration of the stepwise blanching to a narrow range which would be valid for objective texture measurements in the various mechanical tests. Process time is a function of the rheological parameter considered to measure the mechanical strength of a tissue. The differences in optimum conditions for stepwise blanching proposed by different researchers have been attributed to the levels of enzyme activity occurring in the different potato varieties used, which is affected by the maturity of the potato and the season [31]. Rheological parameters from various mechanical tests detected an increase in the structural firmness of the frozen tissue in response to pectinesterase activity in various process time/temperature conditions; their values only coincided approximately for compression and shear, which may be yet another factor contributing to the discrepancies found. Our results have shown that, in accordance with Andersson et al. [17], it is advantageous and perhaps essential to evaluate the mechanical properties of food products by more than one mechanical test. Since most frozen potato tissues are usually heated before consumption, and the mechanical properties resulting from freezing can be changed considerably by the heat treatment applied, futher studies to investigate the properties of thawed products after heat treatments (cooking and/or frying) are needed in order to obtain information on the optimization of potato-tissue processing. Acknowledgements We are indebted to the CICyT for financial support (project ALI 98-1055) and to the CSIC of Spain for the fellowship awarded to M.D. Alvarez. This work is part of her PhD thesis.

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